7 Del. Admin. Code § 5101-11.0 - Post Construction BMP Standards and Specifications
11.1 Infiltration Practices
11.1.1 Infiltration practices are defined as
practices that capture and temporarily store the design storm volume before
allowing it to infiltrate into the soil over a two day period. Design variants
include:
11.1.1.1 Infiltration
Trench;
11.1.1.2 Infiltration
Basin; and
11.1.1.3 Underground
Infiltration .
11.1.2
Infiltration practices receive 100% retention volume credit (Rv) for the volume
stored and infiltrated by the practice.
11.1.2.1 The RPv runoff reduction performance
credit for stormwater infiltration is 100% of the retention storage.
11.1.2.2 The Cv runoff reduction performance
credit for stormwater infiltration is 100% of the retention storage.
11.1.2.3 The Fv runoff reduction performance
credit for stormwater infiltration is 100% of the retention storage.
11.1.2.4 The total nitrogen pollutant
reduction performance credit for stormwater infiltration is 100% of the load
reduction.
11.1.2.5 The total
phosphorus pollutant reduction performance credit for stormwater infiltration
is 100% of the load reduction.
11.1.2.6 The total suspended solids pollutant
reduction performance credit for stormwater infiltration is 100% of the load
reduction.
11.1.3
Infiltration Feasibility Criteria
11.1.3.1
Infiltration practices shall be located a minimum horizontal distance of 200
feet from down-gradient slopes greater than 20% unless slope stability
calculations demonstrate stable conditions.
11.1.3.2 A minimum vertical distance of two
feet must be provided between the bottom of the infiltration practice and the
seasonal high water table as determined by subsection 12.1 or bedrock layer.
The minimum vertical distance of two feet may be relaxed if a groundwater
mounding analysis or piezometer testing has been performed by a qualified
professional.
11.1.3.3 Native
soils, in proposed infiltration areas, must have a minimum infiltration rate of
one inch per hour. Designers must verify soil permeability by using the on-site
soil investigation methods provided in subsection 12.1 .
11.1.4 Infiltration Conveyance Criteria
11.1.4.1 Infiltration practices must be
designed to pass the maximum design storm event (Fv) if the Fv is being routed
through the practice rather than bypassing. An earthen emergency spillway
designed to convey the Fv shall be cut in natural ground or, if cut in fill,
shall be constructed and stabilized with methods to prevent erosion and
structural failure.
11.1.4.2
Infiltration basins constructed to meet regulatory stormwater management
requirements in the State of Delaware shall be designed and constructed in
accordance with the USDA NRCS Pond Code 378, as amended.
11.1.5 Infiltration Pretreatment Criteria
11.1.5.1 Every inlet into an infiltration
system shall have pretreatment .
11.1.5.2 Exit velocities from the
pretreatment shall be non-erosive during the largest design storm that is
routed through the facility .
11.1.6 Infiltration Design Criteria
11.1.6.1 Infiltration basin side-slopes shall
be no steeper than 4H:1V.
11.1.6.2
Stone , when used for infiltration trenches or underground infiltration systems,
shall consist of clean, washed aggregate with a maximum of 2.0% passing the
#200 sieve. Stone shall have a maximum diameter of 2.5 inches and a minimum
diameter of 0.5 inches. A porosity value of 0.4 shall be used in the design of
stone reservoirs, although a larger value may be used if underground retention
chambers are installed within the reservoir .
11.1.6.3 Infiltration trenches and
underground infiltration practices shall include an inspection port to
facilitate periodic inspection and maintenance .
11.1.6.4 Geotextile fabric, when used to
separate stone from native soil, in an infiltration trench or underground
infiltration system shall have a flow rate of 110 gallons per minute per square
foot (gal/min/sf) or greater.
11.1.6.5 For design purposes, the field
verified infiltration rate shall have a factor of safety applied in accordance
with subsection 12.1 to account for potential compaction during construction
and to approximate long term infiltration rates.
11.1.6.6 Infiltration practices shall be
designed so that the RPv infiltrates within 48 hours.
11.1.6.7 Infiltration practices shall be
designed so that they will:
11.1.6.7.1
Infiltrate the Fv within 72 hours, or
11.1.6.7.2 Dewater the Fv within 72 hours,
or
11.1.6.7.3 Manage the Fv on site
with no adverse impact.
11.1.6.8 All Infiltration practices must be
designed so as to be accessible for maintenance .
11.1.6.8.1 A maintenance right-of-way or
easement must extend to the Infiltration practice from a public or private
road.
11.1.6.8.2 Adequate
maintenance access must extend to the perimeter of the Infiltration practice
and outlet structure, if applicable.
11.1.6.8.3 Maintenance access must meet the
following criteria:
11.1.6.8.3.1 Minimum
width of fifteen feet;
11.1.6.8.3.2
Profile grade that does not exceed 10H:1V; and
11.1.6.8.3.3 Minimum 10H:1V cross
slope.
11.1.7 Infiltration Construction Criteria
11.1.7.1 During site construction, steps
shall be taken to prevent compaction and sedimentation of the infiltration
practice unless extensive design and construction methods are employed to
protect the infiltration practices' ability to infiltrate.
11.1.7.2 Construction reviews are required
during the following stages of construction , and shall be noted on the plan in
the sequence of construction :
11.1.7.2.1
Pre-construction meeting.
11.1.7.2.2 Initial site preparation including
installation of erosion and sediment controls and sensitive area protection
surrounding infiltration practice locations.
11.1.7.2.3 Construction of the embankment,
including installation of the principal spillway and the outlet structure. as
applicable for infiltration basins.
11.1.7.2.4 Excavation and grading including
interim and final elevations. Confirmatory infiltration testing and
verification must be completed prior to stone placement for infiltration
trenches and underground infiltration .
11.1.7.2.5 Implementation of required
stabilization.
11.1.7.2.6 Final
construction review including development of a punch list for facility
acceptance.
11.1.7.3 The
infiltration rate and separation from groundwater of the constructed
infiltration practice shall be verified prior to completion of construction in
accordance withsubsection 12.1 . The results shall be included with the Post
Construction Verification Documentation upon project completion.
11.1.7.4 Upon facility completion, the owner
shall submit post construction verification documents to demonstrate that the
infiltration practice has been constructed within allowable tolerances in
accordance with the approved Sediment and Stormwater Management Plan and
accepted by the approving agency. Allowable tolerances for infiltration
practices are as follows:
11.1.7.4.1 The
constructed top of bank elevation may be no lower than the design elevation for
top of bank.
11.1.7.4.2 The
constructed area of the infiltrating surface shall be no less than 90% of the
design surface area.
11.1.7.4.3 The
constructed volume of the infiltration practice surface storage shall be no
less than 90% of the design volume.
11.1.7.4.4 The constructed elevation of any
structure shall be within 0.15 foot of the design.
11.1.8 The infiltrating surface
shall never be covered by an impermeable material, such as asphalt or
concrete.
11.2
Bioretention
11.2.1 Bioretention is defined
as practices that capture and store stormwater runoff and pass it through a bed
of engineered soil media comprised of sand, lignin and organic matter, known as
biosoil. Filtered runoff may be collected and returned to the conveyance
system, or allowed to infiltrate into the soil. Design variants include:
11.2.1.1 Traditional Bioretention;
11.2.1.2 In-Situ Bioretention including Rain
Gardens;
11.2.1.3 Streetscape
Bioretention;
11.2.1.4 Engineered
Tree Boxes;
11.2.1.5 Stormwater
Planters; and
11.2.1.6 Advanced
Bioretention systems.
11.2.2 Bioretention Stormwater Credit
Calculations
11.2.2.1 The volume management
credit for bioretention practices shall be based upon the volume of runoff that
is either slowly released or infiltrated from the practice.
11.2.2.2 Advanced Bioretention Systems shall
be evaluated on a case-by-case basis and assigned performance credits as deemed
appropriate by the Department .
11.2.2.3 Bioretention with underdrain
11.2.2.3.1 The RPv , Cv and Fv runoff
reduction performance credits for bioretention with underdrain is 100% of the
detention storage.
11.2.2.3.2 The
total nitrogen pollutant reduction performance credit for bioretention with
underdrain is not less than 30% removal efficiency.
11.2.2.3.3 The total phosphorus pollutant
reduction performance credit for bioretention with underdrain is not less than
40% removal efficiency.
11.2.2.3.4
The total suspended solids pollutant reduction performance credit for
bioretention with underdrain is not less than 80% removal efficiency.
11.2.2.4 Bioretention with
infiltration
11.2.2.4.1 The RPv , Cv and Fv
runoff reduction performance credits for bioretention with infiltration is 100%
of the retention storage.
11.2.2.4.2 The total nitrogen, total
phosphorous and total suspended solids pollutant reduction performance credits
for bioretention with infiltration is 100% of the load reduction.
11.2.3 Bioretention
Feasibility Criteria
11.2.3.1 A minimum
vertical distance of two feet must be provided between the bottom of the
infiltrating bioretention practice and the seasonal high water table as
determined by the procedures in subsection 12.1 or bedrock layer. The minimum
vertical distance of two feet may be relaxed if a groundwater mounding analysis
or piezometer testing has been performed by a qualified professional.
11.2.3.2 Underdrains are required if the
permeability of the underlying soils does not have a minimum field-verified
infiltration rate of one inch per hour.
11.2.3.3 An impermeable bottom liner and an
underdrain system must be employed when a bioretention facility will receive
untreated hotspot runoff.
11.2.3.4
Infiltrating bioretention practices shall be located a minimum horizontal
distance of 200 feet from down-gradient slopes greater than 20% unless slope
stability calculations demonstrate stable conditions.
11.2.4 Bioretention Conveyance Criteria
11.2.4.1 Bioretention practices must be
designed to pass the maximum design storm event (Fv) if the Fv is being routed
through the practice rather than bypassing. An earthen emergency spillway
designed to convey the Fv shall be cut in natural ground or, if cut in fill,
shall be constructed and stabilized with methods to prevent erosion and
structural failure.
11.2.4.2
Bioretention practices constructed to meet regulatory stormwater management
requirements in the State of Delaware shall be designed and constructed in
accordance with the USDA NRCS Pond Code 378 as amended.
11.2.4.3 An overflow structure shall be
incorporated into on-line designs to safely convey larger storms through the
bioretention facility .
11.2.4.4 The
maximum design discharge velocity shall be checked for a non-erosive condition
at the outlet point. Outlet protection shall be provided as
necessary.
11.2.5
Bioretention Pretreatment Criteria
11.2.5.1
Every inlet into a bioretention practice shall have pretreatment .
11.2.5.2 Exit velocities from the
pretreatment shall be non-erosive during the largest design storm that is
routed through the facility .
11.2.6 Bioretention Design Criteria
11.2.6.1 Bioretention practices shall have
energy dissipation provided at all inlets.
11.2.6.2 Bioretention practices shall be
designed so that the RPv either infiltrates or discharges within 48
hours.
11.2.6.3 Bioretention
practices shall be designed so that they will:
11.2.6.3.1 Infiltrate the Fv within 72 hours,
or
11.2.6.3.2 Dewater the Fv within
72 hours, or
11.2.6.3.3 Manage the
Fv on site with no adverse impact.
11.2.6.4 Traditional and advanced
bioretention facilities and rain gardens shall be constructed with side slopes
above biosoil media of 3:1 or flatter.
11.2.6.5 Biosoil Media
11.2.6.5.1 The Biosoil-14 soil mixture shall
have the following volumetric composition:
11.2.6.5.1.1 60% coarse concrete sand having
a Fineness Modulus > 2.75
11.2.6.5.1.2 30% triple shredded hardwood
mulch
11.2.6.5.1.3 10% aged, STA
certified compost , meeting the requirements of Delaware Erosion and Sediment
Control Handbook Appendix A-6 Compost Material Properties.
11.2.6.5.2 Biosoil media must be obtained
from a Department approved vendor.
11.2.6.5.3 The design permeability rate for
biosoil media shall be 2.83 inches per hour.
11.2.6.5.4 The biosoil media bed depth shall
be a minimum of 24 inches for traditional bioretention and advanced
bioretention systems.
11.2.6.5.5
Gravel layers used for extending the bioretention facility into a more
permeable layer shall meet the same requirements as those for an underdrain
design.
11.2.6.6 A
surface cover shall be provided over the biosoil media.
11.2.6.6.1 Mulch, if used as a surface cover,
shall be triple shredded hardwood aged for a minimum of six months.
11.2.6.6.2 Use of alternative surface cover
shall be shown on the approved plan.
11.2.6.7 Underdrains
11.2.6.7.1 For bioretention designs that
require an underdrain, the underdrain shall be a minimum of 4-inch perforated
corrugated polyethylene pipe (CPP).
11.2.6.7.2 The underdrain shall be encased in
a layer of clean, washed nominal ¼ inch gravel with a maximum of 2.0%
passing the #200 sieve with a minimum of 3" of cover.
11.2.6.7.3 The gravel layer in traditional
bioretention shall be extended a minimum of 2' below the invert of the
underdrain.
11.2.6.8 All
traditional and advanced bioretention systems shall include at least one
inspection port or cleanout pipe.
11.2.6.9 Traditional sizing approaches using
design volume considering void ratio of the stone and biosoil media shall be
used when sizing bioretention.
11.2.6.10 All Bioretention practices must be
designed so as to be accessible for maintenance .
11.2.6.10.1 A maintenance right-of-way or
easement must extend to the Bioretention practice from a public or private
road.
11.2.6.10.2 Adequate
maintenance access must extend to the perimeter of the bioretention practice
and outlet structure.
11.2.6.10.3
Maintenance access must meet the following criteria:
11.2.6.10.3.1 Minimum width of fifteen
feet.
11.2.6.10.3.2 Profile grade
that does not exceed 10H:1V.
11.2.6.10.3.3 Minimum 10H:1V cross
slope.
11.2.7 Bioretention Landscaping Criteria
11.2.7.1 A planting plan shall be provided
for all bioretention facilities. Minimum elements of a planting plan include
the following:
11.2.7.1.1 The proposed
bioretention template to be used;
11.2.7.1.2 Delineation of planting
areas;
11.2.7.1.3 Size and spacing
of plant material; and
11.2.7.1.4
The planting sequence, including post-nursery care and initial maintenance
requirements.
11.2.7.2
Planting plans must be certified by a qualified professional.
11.2.8 Bioretention Construction
Criteria
11.2.8.1 When a bioretention system
is used as a sediment trap or basin during construction , the Sediment &
Stormwater Plan must include notes and graphic details specifying that:
11.2.8.1.1 The maximum excavation depth of
the trap or basin at the construction stage must be at least one foot higher
than the final invert or bottom of the facility , and
11.2.8.1.2 The bottom of the facility shall
be ripped, tilled or otherwise scarified upon final excavation.
11.2.8.2 The plan shall include
the proper procedures for converting the temporary sediment control practice to
a permanent bioretention facility , including dewatering, cleanout and
stabilization.
11.2.8.3 For
infiltrating bioretention systems, confirmatory infiltration testing and
verification must be completed prior to completion of construction in
accordance with subsection 12.1 . The results shall be included with the Post
Construction Verification Documentation upon project completion.
11.2.8.4 The final bottom elevation of any
bioretention facility shall not be traversed by construction
equipment.
11.2.8.5 Construction
reviews are required during the following stages of construction , and shall be
noted on the plan in the sequence of construction :
11.2.8.5.1 Pre-construction
meeting.
11.2.8.5.2 Initial site
preparation including installation of erosion and sediment controls, sensitive
area protection surrounding bioretention locations, and blockage of inlets to
bioretention locations
11.2.8.5.3
Excavation and grading including interim and final elevations. For infiltrating
bioretention systems, confirmatory infiltration testing and a verification must
be completed prior to gravel and biosoil media placement.
11.2.8.5.4 Construction of the underdrain,
including inspection ports and installation of the overflow structure, as
applicable.
11.2.8.5.5 Installation
of gravel and biosoil media.
11.2.8.5.6 Implementation of required
stabilization and planting plan.
11.2.8.5.7 Final construction review
including development of a punch list for facility acceptance.
11.2.8.6 Upon facility completion,
the owner shall submit post construction verification documents to demonstrate
that the bioretention practice has been constructed within allowable tolerances
in accordance with the approved Sediment and Stormwater Management Plan and
accepted by the approving agency. Allowable tolerances for bioretention
practices are as follows:
11.2.8.6.1 The
constructed top of bank elevation may be no lower than the design elevation for
top of bank.
11.2.8.6.2 The
constructed area of the bioretention surface shall be no less than 90% of the
design surface area.
11.2.8.6.3 The
constructed volume of the bioretention storage shall be no less than 90% of the
design volume.
11.2.8.6.4 The
constructed elevation of any structure shall be within 0.15 foot of the
design.
11.2.9 Supplemental fertilizer applications
shall consist of a 0% phosphorus formulation only as needed to maintain plant
vigor.
11.3 Permeable
Pavement Systems
11.3.1 Permeable Pavement
Systems are defined as paving surfaces that capture and temporarily store
stormwater by filtering runoff through voids in the pavement surface into an
underlying reservoir . Design variants include:
11.3.1.1 Porous Asphalt (PA)
11.3.1.2 Pervious Concrete (PC)
11.3.1.3 Permeable interlocking concrete
Pavers (PP) or Concrete grid Pavers (CP)
11.3.1.4 Plastic Grid Pavers (GP)
11.3.2 Permeable Pavement Systems
receive 100% retention volume credit (Rv) for the volume stored and infiltrated
by the practice.
11.3.2.1 The RPv runoff
reduction performance credit for permeable pavement is 100% of the retention
storage.
11.3.2.2 The Cv runoff
reduction performance credit for permeable pavement is 100% of the retention
storage.
11.3.2.3 The Fv runoff
reduction performance credit for permeable pavement is 100% of the retention
storage.
11.3.2.4 The total
nitrogen pollutant reduction performance credit for permeable pavement is 100%
of the load reduction.
11.3.2.5 The
total phosphorus pollutant reduction performance credit for permeable pavement
is 100% of the load reduction.
11.3.2.6 The total suspended solids pollutant
reduction performance credit for permeable pavement is 100% of the load
reduction.
11.3.3
Permeable Pavement Feasibility Criteria
11.3.3.1 Drainage Area . For parking lots and
roadways configurations:
11.3.3.1.1 The
contributing drainage area to permeable pavement shall not exceed five times
the surface area of the permeable pavement.
11.3.3.1.2 Pervious areas shall be diverted
from the permeable pavement area such that the total contributing drainage area
is at least 80% impervious.
11.3.3.2 Soils and Overdrains. Parking lot
and roadway configurations utilizing permeable pavement shall meet the
following requirements:
11.3.3.2.1
Infiltration testing in accordance with subsection 12.1 shall be
required.
11.3.3.2.2 Overdrains are
required if the permeability of the underlying soils does not have a minimum
infiltration rate of one inch per hour.
11.3.3.3 The surface slope shall be no
greater than 5.0%.
11.3.3.4 The
bottom slope of a permeable pavement installation shall be no greater than
1.0%.
11.3.3.5 If an overdrain is
not provided, a separation distance of two feet is required between the bottom
of the reservoir layer and the seasonal high water table as determined in
accordance with subsection 12.1 .
11.3.3.6 Permeable pavements shall not be
used to treat hotspot runoff .
11.3.4 Permeable pavement designs shall
include methods to safely convey the Cv and Fv.
11.3.5 Permeable Pavement Design Criteria
11.3.5.1 Permeable pavement shall be designed
according to DelDOT specifications or the product manufacturer's
recommendations as applicable.
11.3.5.2 Internal Geometry and Drawdown
11.3.5.2.1 For design purposes, the field
verified infiltration rate shall have a factor of safety applied in accordance
with subsection 12.1 to account for potential compaction during construction
and to approximate long term infiltration rates.
11.3.5.2.2 Permeable pavement practices shall
be designed so that the RPv infiltrates within 48 hours.
11.3.5.2.3 Permeable pavement practices shall
be designed so that they will:
11.3.5.2.3.1
Infiltrate the Fv within 72 hours, or
11.3.5.2.3.2 Dewater the Fv within 72 hours,
or
11.3.5.2.3.3 Manage the Fv on
site with no adverse impact.
11.3.5.3 Reservoir layer
11.3.5.3.1 The suitability of the soil
subgrade shall be determined by a qualified geotechnical engineer.
11.3.5.3.2 The reservoir layer shall be
composed of clean, washed gravel with a maximum of 2.0% passing the #200 sieve
and sized for both the maximum storm event to be managed and the structural
requirements of the expected traffic loading.
11.3.5.3.3 The depth of the reservoir layer
shall be a minimum of six inches.
11.3.5.3.4 If an overdrain is not provided, a
separation distance of two feet is required between the bottom of the reservoir
layer and the seasonal high water table as determined in accordance with
subsection 12.1 .
11.3.5.3.5
Overdrains shall be a minimum of four inches in diameter.
11.3.5.4 All parking lot and roadway
configurations shall include inspection ports.
11.3.5.5 The permeable pavement shall be
designed to support the maximum anticipated traffic load.
11.3.5.6 The reservoir layer shall be sized
to temporarily store and then infiltrate the RPv.
11.3.6 Permeable Pavement Construction
11.3.6.1 All permeable pavement areas shall
be fully protected from sediment intrusion by silt fence or construction
fencing to prevent construction traffic tracking.
11.3.6.2 During site construction, steps
shall be taken to prevent compaction of the underlying soil and sedimentation
of the permeable pavement practice.
11.3.6.3 The infiltration rate and separation
from groundwater of the constructed permeable pavement practice shall be
verified prior to completion of construction in accordance with subsection 12.1
. The results shall be included with the Post Construction Verification
Documentation upon project completion.
11.3.6.4 During construction , care shall be
taken to avoid tracking sediments onto any permeable pavement surface to avoid
clogging.
11.3.6.5 When locating a
sediment basin on an area intended for permeable pavement is unavoidable, the
invert of the sediment basin must be a minimum of one foot above the final
design elevation of the bottom of the reservoir course.
11.3.6.6 Permeable pavement shall be
installed according to DelDOT specifications or the product manufacturer's
recommendations as applicable.
11.3.6.7 Construction of the permeable
pavement shall only begin after the entire contributing drainage area has been
stabilized.
11.3.6.8 The proposed
permeable pavement area shall be kept free from sediment during the entire
construction process.
11.3.6.9
Construction reviews are required during the following stages of construction ,
and shall be noted on the plan in the sequence of construction :
11.3.6.9.1 Pre-construction
meeting;
11.3.6.9.2 Initial site
preparation including installation of erosion and sediment controls, sensitive
area protection surrounding permeable pavement locations;
11.3.6.9.3 Excavation and grading including
interim and final elevations. Observation of infiltrating surface and permeable
pavement practice verification must be completed prior to gravel
placement;
11.3.6.9.4 Construction
of the overdrain, including inspection ports and installation of the overflow
structure, as applicable;
11.3.6.9.5 Installation of gravel;
11.3.6.9.6 Implementation of required
stabilization; and
11.3.6.9.7 Final
construction review including development of a punch list for facility
acceptance.
11.3.6.10
Upon facility completion, the owner shall submit post construction verification
documents to demonstrate that the permeable pavement has been constructed
within allowable tolerances in accordance with the approved Sediment and
Stormwater Management Plan and accepted by the approving agency. Allowable
tolerances for permeable pavement are as follows:
11.3.6.10.1 The constructed permeable
pavement surface area shall be no less than the design permeable pavement
surface area.
11.3.6.10.2 The
contributing drainage area as constructed shall be no greater than the design
contributing drainage area .
11.3.6.10.3 The constructed storage volume of
the reservoir layer shall be no less than 90% of the design volume.
11.3.6.10.4 The constructed elevation of the
overdrain or any structure shall be within 0.15 foot of the design.
11.3.6.11 In the event that the
allowable tolerances are exceeded for permeable pavement surface area or volume
or structure elevations, supplemental calculations and provisions of adequate
maintenance must be submitted to the approval agency to determine if the
permeable pavement, as constructed, meets the design
requirements.
11.3.7
Activities that have the potential to clog the permeable pavement surface,
including but not limited to sanding, re-sealing , re-surfacing, storage of snow
piles containing sand, storage of mulch or soil material, or construction
staging, shall be prohibited.
11.4 Vegetated Roofs
11.4.1 Vegetated Roofs are defined as
practices on top of buildings that capture and store rainfall in an engineered
growing media, which is designed to support plant growth. A portion of the
captured rainfall evaporates or is taken up by plants, which helps reduce
runoff volumes, peak runoff rates, and pollutant loads on development sites.
Design variants include:
11.4.1.1 Extensive
Vegetated Roofs which contain shallow growth media with drought resistant
plants, such as Sedum.
11.4.1.2
Intensive Vegetated Roofs which contain deep growth media with a wide range of
plant varieties and typically include irrigation.
11.4.2 Vegetated Roofs receive annual runoff
reduction credit (RR) for the contributing roof area, along with associated
pollutant removals as follows:
11.4.2.1 The
RPv runoff reduction performance credit for Extensive Vegetated Roof is 50%
annual runoff reduction.
11.4.2.2
The Cv runoff reduction performance credit for Extensive Vegetated Roof is 5%
of the RPv allowance.
11.4.2.3 The
Fv runoff reduction performance credit for Extensive Vegetated Roof is 1% of
the RPv allowance.
11.4.2.4 The RPv
runoff reduction performance credit for Intensive Vegetated Roof is 75% annual
runoff reduction.
11.4.2.5 The Cv
runoff reduction performance credit for Intensive Vegetated Roof is 8% of the
RPv allowance.
11.4.2.6 The Fv
runoff reduction performance credit for Intensive Vegetated Roof is 2% of the
RPv allowance.
11.4.2.7 The total
nitrogen pollutant reduction performance credit for Extensive Vegetated Roof
and Intensive Vegetated Roof is not less than 0%.
11.4.2.8 The total phosphorus pollutant
reduction performance credit for Extensive Vegetated Roof and Intensive
Vegetated Roof is not less than 0%.
11.4.2.9 If the phosphorous (P) content of
mature growth media is 2.0 mg/l (Saturated Paste Extraction), or less, then the
Vegetated Roof will be assumed to be neutral with respect to P loadings. If the
P content of mature growth media exceeds 2.0 mg/l, a supplemental
phosphorus-reducing BMP, such as an activated alumina or hematite filter, will
be required.
11.4.3
Vegetated Roof Feasibility Criteria
11.4.3.1
The designer shall demonstrate that the building will be able to support the
additional live and dead structural load. Structural capability of the roof
must be assessed by a qualified licensed professional and included with
building permit documentation.
11.4.3.2 Safe access to the Vegetated Roof
shall be available to allow for delivery of construction materials and
performance of routine maintenance reviews and maintenance
operations.
11.4.3.3 A permanent
source of water shall be provided to all Vegetated Roof areas.
11.4.3.4 A minimum one foot wide
vegetation-free zone is required along the perimeter of all Vegetated Roofs and
around all roof penetrations.
11.4.3.5 The Vegetated Roof design must
comply with all federal, state and local building codes.
11.4.4 Vegetated Roof Conveyance Criteria
11.4.4.1 The Vegetated Roof drainage layer
shall convey flow from under the growth media layer to an outlet or overflow
system.
11.4.4.2 All drains and
scuppers shall be accessible through enclosures that include lids that are
level with the surface of the growth media layer.
11.4.4.3 Emergency drains or emergency
scuppers shall have inverts that are high enough above the waterproofing
surface to prevent discharge during the RPv event.
11.4.5 Vegetated Roof Design Criteria
11.4.5.1 All Vegetated Roof systems must
include an effective and reliable waterproof membrane to prevent water damage
to the building structure.
11.4.5.2
Protection Layer shall have a puncture resistance in accordance with ASTM D4833
>220 lbs.
11.4.5.3 Waterproof
membranes designed for burial in Vegetated Roofs do not require supplemental
root barriers; however root barriers are required in combination with some
waterproof membranes.
11.4.5.3.1 Chemical root
barriers or physical root barriers that have been impregnated with pesticides,
metals or other chemicals shall not be used as those chemicals can leach into
stormwater runoff and will slowly lose effectiveness over time.
11.4.5.3.2 To insure that a building is
adequately protected against damage from roots, the waterproof membrane or the
root-barrier shall be certified as root-resistant based on the two-year
ANSI/SPRI VR-1; Procedure for Investigating the Root Penetration Resistance of
Vegetated Roofs or the two-year European FLL root-security test
(Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V., Richlinien
fuer die Planung, Ausfuehrung und Pflege von Dachbegruenung). If certification
is not available at the time the system is permitted, an HDPE membrane, 30 mil
or thicker, with seams overlapped at least three inches and continuously
hot-air welded shall be provided as a root barrier.
11.4.5.4 A drainage layer shall underlie the
growth media to control the release of water that percolates into the Vegetated
Roof assembly.
11.4.5.5 Roof drains
and any emergency overflow shall be designed in accordance with state and local
building codes.
11.4.5.6 Growth
Media
11.4.5.6.1 The growth media shall be
the uppermost layer in a Vegetated Roof assembly.
11.4.5.6.2 The designer shall certify that
growth media meets or exceeds all required specifications prior to
placement.
11.4.5.6.3 The growth
media must maintain minimum 6.0% air filled porosity (ASTM E2399) to avoid
anoxic conditions when wetted.
11.4.5.6.4 The upper one inch of a growth
media profile shall consist of an engineered mineral soil. To minimize the
potential for clogging of fabrics and migration of fine particles, the
silt-size fraction (0.063mm) of engineered mineral soils shall not exceed 15%
and the clay fraction (2 micron) shall not exceed 5.0%.
11.4.5.6.5 Organic amendments used in
preparing growth media must be stable. The respiration rate of organic
ingredients must be 3.0 mg CO2-C/g OM/day (TMECC 05.08.B), or less.
11.4.5.6.6 For a Vegetated Roof to be
regarded as neutral with respect to phosphorous (P) loading in runoff , the P
content of the growth media must be controlled.
11.4.5.6.6.1 The initial available P content
of the growth media may not exceed 200 ppm dry weight (Mehlich III).
11.4.5.6.6.2 Chemical fertilizers containing
phosphorous may not be added to the growth media during blending or used
subsequently during maintenance unless a phosphorus deficiency in the plants
has been documented. Only nitrogen fertilizers may be used and these must be
applied according to soil test. If the P limit for the media layer cannot meet
compliance, the Vegetated Roof must be treated as a P source and a supplemental
water quality BMP must be introduced.
11.4.5.6.7 If trees are included in the
Vegetated Roof landscape plan, the growth media must be at least 30 inches
deep.
11.4.5.7 Plant
cover
11.4.5.7.1 Initial planting plan shall
be designed such that mature plant coverage within 24 months of initial
planting shall be:
11.4.5.7.1.1 Minimum 75%
warm season plant coverage for Extensive Vegetated Roofs.
11.4.5.7.1.2 Minimum 90% plant coverage for
turf intensive Vegetated Roofs.
11.4.5.7.1.3 Uniform cover with vigorous
plants conforming to the design plant density for non-turf Intensive Vegetated
Roofs.
11.4.5.7.2 The
planting plan for Vegetated Roofs must be certified by a qualified
professional.
11.4.5.8
Vegetated Roof sizing
11.4.5.8.1 The size of
the Vegetated Roof, both Extensive and Intensive, shall be a minimum 66% of the
total contributing drainage area .
11.4.5.8.2 No runoff reduction credit shall
be given for runoff from bare areas of the roof that do not come in contact
with root zone.
11.4.6 Vegetated Roof Landscaping Criteria
11.4.6.1 Irrigation is required for Vegetated
Roofs for the first year after planting.
11.4.6.2 The minimum plant coverage shall be
achieved 24 months after initial planting and maintained throughout the life of
the Vegetated Roof.
11.4.7 Vegetated Roof Construction
11.4.7.1 Construction reviews are required
during the following stages of construction , and shall be noted on the plan in
the sequence of construction :
11.4.7.1.1
Pre-construction meeting;
11.4.7.1.2 During placement of the waterproof
membrane, to ensure that it is properly installed and watertight;
11.4.7.1.3 During placement of the drainage
layer and drainage system, to prevent future ponding water;
11.4.7.1.4 During placement of the growing
media, to confirm that it meets the approved plan;
11.4.7.1.5 Upon installation of plants, to
ensure they conform to the planting plan; and
11.4.7.1.6 Final construction review
including development of a punch list for facility acceptance.
11.4.7.2 Post Construction
Verification Documentation
11.4.7.2.1 Upon
facility completion, the owner shall submit post construction verification
documents to demonstrate that the Vegetated Roof has been constructed within
allowable tolerances in accordance with the approved Sediment and Stormwater
Management Plan and accepted by the approving agency. Allowable tolerances for
Vegetated Roofs are as follows:
11.4.7.2.1.1
Growth media thickness within 15% of design thickness.
11.4.7.2.1.2 Plant density no less than
specified on the planting plan.
11.4.7.2.1.3 No less than 66% of the total
contributing drainage area shall be Vegetated Roof.
11.4.7.2.2 The post construction verification
shall confirm that temporary or permanent irrigation has been installed in
accordance with the approved plan.
11.4.7.2.3 Certification of growth media
shall be submitted with post construction verification.
11.4.8 Vegetated Roof Maintenance
Criteria
11.4.8.1 Maintenance Reviews
11.4.8.1.1 A minimum of one maintenance
review is required each year for Vegetated Roofs.
11.4.8.1.2 Maintenance reviews shall be
performed by a qualified reviewer.
11.4.8.1.3 The completed maintenance review
report shall be sent to the Department or the appropriate Delegated
Agency.
11.4.8.1.4 Vegetated Roofs
must be reviewed at the end of the first 24 months to confirm minimum vegetated
surface cover specified in the Operation and Maintenance Plan has been
achieved, and to look for leaks, drainage problems and any rooftop structural
concerns.
11.4.8.2
Growth media shall be routinely tested for P as part of the Operation and
Maintenance Plan. Phosphorous-removing BMPs (e.g., activated alumina or
hematite filters) must be installed for Vegetated Roofs, 24 months and older,
with phosphorous concentrations that are consistently higher than 2.0
mg/l.
11.5
Rainwater Harvesting
11.5.1 Rainwater
harvesting is defined as systems that intercept, divert, store and release
rainfall for future use. Rainwater that falls onto impervious surfaces is
collected and conveyed into an above- or below-ground cistern, where it can be
used for non-potable water uses and on-site stormwater disposal or
infiltration. Runoff collected and temporarily stored in more traditional
stormwater management practices constructed in accordance with the Post
Construction Stormwater BMP Standards and Specifications, such as wet ponds,
can also be used for irrigation purposes to achieve these same goals. Design
variants include:
11.5.1.1 Seasonal Rainwater
Harvesting Systems; and
11.5.1.2
Continuous Rainwater Harvesting Systems.
11.5.2 Rainwater Harvesting Stormwater Credit
Calculations
11.5.2.1 If not protected from
freezing, Rainwater Harvesting systems must be taken offline for the winter and
credited as seasonal systems.
11.5.2.2 Seasonal Rainwater Harvesting
Performance Credits
11.5.2.2.1 The RPv runoff
reduction performance credit for Seasonal Rainwater Harvesting systems is 50%
of the retention storage.
11.5.2.2.2 The Cv and Fv runoff reduction
performance credit for Seasonal Rainwater Harvesting systems is 0%.
11.5.2.2.3 The total nitrogen, total
phosphorus, and total suspended solids pollutant reduction performance credits
for Seasonal Rainwater Harvesting is 100% of the load reduction.
11.5.2.3 Continuous Rainwater
Harvesting Performance Credits
11.5.2.3.1 The
RPv runoff reduction performance credit for Continuous Rainwater Harvesting
systems is 75% of the retention storage.
11.5.2.3.2 The Cv and Fv runoff reduction
performance credit for Continuous Rainwater Harvesting systems is 0%.
11.5.2.3.3 The total nitrogen, total
phosphorus, and total suspended solids pollutant reduction performance credits
for Continuous Rainwater Harvesting is 100% of the load reduction.
11.5.3 Rainwater
Harvesting Feasibility Criteria
11.5.3.1
Designers and plan reviewers shall consult all applicable local, State and
Federal regulations to determine the allowable indoor uses and required
treatment for harvested rainwater.
11.5.3.2 Pipes and spigots using rainwater
must be clearly labeled as non-potable.
11.5.3.3 The final invert of the outlet pipe
from the cistern must be at an elevation that will not allow water from the
discharge point to backflow into the cistern.
11.5.3.4 In areas where a below-ground
cistern will be buried partially below the water table , buoyancy calculations
must be conducted for the empty cistern and special design features must be
employed, as applicable, to secure the cistern.
11.5.3.5 Cisterns must be installed according
to the manufacturer's specifications.
11.5.3.6 The bearing capacity of the soil
upon which the full cistern will be placed must be considered.
11.5.3.7 Cisterns shall be designed to be
watertight.
11.5.4
Rainwater Harvesting Conveyance Criteria
11.5.4.1 All conveyance pipes to the cistern,
including gutters and downspouts, must be kept clean and free of sediment ,
debris and rust.
11.5.4.2 Overflow
11.5.4.2.1 An overflow mechanism must be
included in the Rainwater Harvesting system design to handle flows that exceed
the capacity of the cistern.
11.5.4.2.2 Overflow pipes must have a
capacity equal to or greater than the total capacity of the inflow pipes and
have a diameter and slope sufficient to drain the cistern before it reaches
full capacity.
11.5.4.2.3 The
overflow pipe must be screened to prevent access to the cistern by rodents and
birds.
11.5.5
Rainwater Harvesting Pretreatment Criteria
11.5.5.1 Pretreatment is required to keep
sediment, leaves, and other debris from the system.
11.5.5.2 Small cistern systems of 2,500
gallons or less shall have leaf screens or gutter guards for pretreatment as a
minimum.
11.5.5.3 Large cistern
systems of greater than 2,500 gallons shall include a pretreatment system
capable of treating and conveying the flow rate generated by the RPv from the
contributing impervious surface drainage area without creating a backup or
bypass condition.
11.5.6
Rainwater Harvesting Design Criteria
11.5.6.1
The following factors that must be considered when designing a Rainwater
Harvesting system and selecting a cistern:
11.5.6.1.1 Rainwater Harvesting Systems shall
comply with all applicable local, State , and Federal regulations.
11.5.6.1.2 Above-ground cisterns must be
impact resistant or protected from impact using bollards or other physical
barriers.
11.5.6.1.3 Below-ground
cisterns must be designed to support the overlying soil and any other
anticipated loads.
11.5.6.1.4
Below-ground cisterns must have a standard size manhole or equivalent opening
to allow access for cleaning, inspection, maintenance , and repair
purposes.
11.5.6.1.5 Cisterns must
be screened to discourage mosquito breeding and reproduction.
11.5.6.1.6 A suitable foundation must be
provided to support the cistern when it is filled to capacity.
11.5.6.1.7 Dead storage below the outlet to
the distribution system and an air gap at the top of the cistern must be added
to the total volume.
11.5.6.1.8 Any
hookup to a municipal backup water supply must have a backflow prevention
device to keep municipal water separate from stored rainwater.
11.5.6.2 Distribution Systems
11.5.6.2.1 The Rainwater Harvesting system
must be equipped with an appropriately-sized pump, if necessary, that produces
sufficient pressure for all intended end-uses.
11.5.6.2.2 A backflow preventer is required
to separate harvested rainwater from the main potable water distribution
lines.
11.5.6.2.3 Distribution
lines for Continuous Rainwater Harvesting Systems must be buried beneath the
frost line.
11.5.6.2.4 A drain plug
or cleanout sump, also draining to a pervious area, must be installed to allow
the system to be completely emptied, if needed.
11.5.6.2.5 Above-ground outdoor pipes must be
insulated or heat-wrapped to prevent freezing and ensure uninterrupted
operation during winter.
11.5.6.2.6
Distribution lines and above ground outdoor pipes for Seasonal Rainwater
Harvesting Systems shall be drained or otherwise winter-proofed during the
non-operational period.
11.5.6.3 Rainwater Harvesting Material
Specifications
11.5.6.3.1 Cisterns must be
structurally sound, watertight, and sealed using a water-safe, non-toxic
material. Re-purposed tanks used to store rainwater for reuse must be
acceptable for potable water or food-grade products.
11.5.6.3.2 Above-ground cisterns must be UV
resistant and opaque to prevent the growth of algae in the tank.
11.5.6.3.3 Below-ground cisterns shall be
located below the frost line.
11.5.6.4 Sizing of Rainwater Harvesting
Systems
11.5.6.4.1 For seasonal rainwater
harvesting systems, weekly irrigation demand shall be at least 50% of the
stored volume.
11.5.6.4.2 For
Continuous Rainwater Harvesting Systems, a minimum of 50% of the demand shall
be met through non-irrigation needs, such as plumbing, process water, car
washing, or other uses that are present throughout the year.
11.5.6.5 All Rainwater Harvesting
Systems must be designed so as to be accessible for maintenance .
11.5.6.5.1 A maintenance right-of-way or
easement must extend to the Rainwater Harvesting System from a public or
private road.
11.5.6.5.2 Adequate
maintenance access must extend to all components of the Rainwater Harvesting
System.
11.5.6.5.3 Maintenance
access must meet the following criteria:
11.5.6.5.3.1 Minimum width of fifteen
feet.
11.5.6.5.3.2 Profile grade
that does not exceed 10H:1V.
11.5.6.5.3.3 Minimum 10H:1V cross
slope.
11.5.7 If the harvested rainwater is to be
used for irrigation, the design plan must include the delineation of the
proposed planting areas to be irrigated and quantification of the expected
water demand based upon the area to be irrigated.
11.5.8 Rainwater Harvesting Construction
11.5.8.1 Rainwater Harvesting system
components connecting to the internal plumbing system shall be installed by a
licensed plumber.
11.5.8.2
Construction reviews are required during the following stages of construction ,
and shall be noted on the plan in the sequence of construction :
11.5.8.2.1 Pre-construction
meeting;
11.5.8.2.2 Initial site
preparation including installation of erosion and sediment controls;
11.5.8.2.3 Excavation and grading including
interim and final elevations for cistern foundations;
11.5.8.2.4 Installation of cistern,
pretreatment system and conveyance system;
11.5.8.2.5 Implementation of required
stabilization; and
11.5.8.2.6 Final
construction review including development of a punch list for facility
acceptance.
11.5.8.3
Upon facility completion, the owner shall submit post construction verification
documents to demonstrate that the rainwater harvesting practice has been
constructed in accordance with the approved Sediment and Stormwater Management
Plan and accepted by the approving agency. Items to be checked and verified are
as follows:
11.5.8.3.1 Presence of a
pretreatment device.
11.5.8.3.2
Capacity of any cisterns matches the design plan.
11.5.8.3.3 For ponds , the constructed volume
shall be no less than 90% of the design volume.
11.5.8.3.4 For continuous systems, all pumps,
controls, and other appurtenances installed in accordance with the
plan.
11.5.8.3.5 For irrigation
systems, area of coverage is within 90% of that shown on the plan.
11.5.9 Operation and
Maintenance Plans shall clearly outline how Rainwater Harvesting Systems will
be managed taking into account seasonal variations and cistern
location.
11.6
Restoration Practices
11.6.1 Restoration
Practices include Regenerative Stormwater Conveyance Systems (RSCS), also known
as Coastal Plain Outfalls, and other practices that restore existing degraded
natural systems to their former functional condition. Streambank stabilization
is also included as a Restoration Practice.
11.6.2 Design variants for Restoration
Practices include:
11.6.2.1 Step Pool
RSCS
11.6.2.2 Seepage Wetland
RSCS
11.6.2.3 Streambank
Stabilization
11.6.3
Runoff reduction and pollutant reduction performance credits for Restoration
Practices shall be determined by the Department on a case-by-case
basis.
11.6.4 Upon facility
completion, the owner shall submit post construction verification documents to
demonstrate that the restoration practice has been constructed within allowable
tolerances in accordance with the approved Sediment and Stormwater Management
Plan and accepted by the approving agency.
11.7 Rooftop Disconnection
11.7.1 Rooftop Disconnection involves
managing runoff close to its source by intercepting, infiltrating, filtering,
treating, or reusing it as it moves from the rooftop to the drainage system.
Rooftop Disconnection can reduce the volume of runoff that enters the combined
or separate storm sewer systems.
11.7.2 Rooftop Disconnection that meets the
minimum full disconnection length shall receive the following annual runoff
reduction and pollutant reduction credits:
11.7.2.1 Rooftop Disconnection receives 100%
annual runoff reduction performance credit for the RPv.
11.7.2.2 The Cv runoff reduction performance
credit for Rooftop Disconnection is 10% of the RPv allowance.
11.7.2.3 The Fv runoff reduction performance
credit for Rooftop Disconnection is 1% of the RPv allowance.
11.7.2.4 The total nitrogen pollutant
reduction performance credit for Rooftop Disconnection is 100% of the load
reduction.
11.7.2.5 The total
phosphorus pollutant reduction performance credit for Rooftop Disconnection is
100% of the load reduction.
11.7.2.6 The total suspended solids pollutant
reduction performance credit for Rooftop Disconnection is 100% of the load
reduction.
11.7.3
Partial RPv runoff reduction credit shall be based on the ratio of the
disconnection length provided to the full disconnection length.
11.7.4 Rooftop Disconnection Feasibility
Criteria
11.7.4.1 If being used for RPv credit
in a residential subdivision, a Record Plan shall include a not identifying
Rooftop Disconnection as a BMP.
11.7.4.2 The disconnection area shall have a
maximum slope of 25%.
11.7.4.3 The
maximum impervious rooftop area treated may not exceed 1,000 square feet per
downspout.
11.7.4.4 Receiving area
shall be graded away from the structure per local requirements.
11.7.5 Rooftop Disconnection areas
shall be designed to safely convey all design storm events (Rpv , Cv, and Fv)
over the receiving area without causing erosion.
11.7.6 A downspout energy dissipater shall be
located at the discharge point of the downspout.
11.7.7 Rooftop Disconnection Design Criteria
11.7.7.1 Regardless of rooftop area collected
the available pervious disconnection area at the point of discharge for any
downspout must be at least 15 feet wide.
11.7.7.2 A five foot long transition section
from the downspout point of discharge shall be provided prior to the beginning
of the disconnection area.
11.7.7.3
The minimum full disconnection lengths shall be as follows:
11.7.7.3.1 For projects above the C&D
Canal, the full disconnection length shall be 75 feet.
11.7.7.3.2 For projects below the C&D
Canal, the full disconnection length shall be 60 feet.
11.7.7.4 Impervious areas shall not be
constructed within the area designated as the pervious rooftop disconnection
area.
11.7.7.5 The pervious rooftop
disconnection area must be stabilized with vegetation for a non-erosive
condition.
11.7.8 All
pervious disconnection areas receiving rooftop runoff shall be vegetatively
stabilized to prevent erosion or transport of sediment to receiving practices
or drainage systems.
11.7.9 The
post construction verification for Rooftop Disconnection shall visually verify
that no impervious surface exists within the rooftop disconnection
area.
11.7.10 The Sediment and
Stormwater Plan shall include the following operation and maintenance notes for
Rooftop Disconnection:
11.7.10.1 The rooftop
disconnection area shall be maintained in a stabilized vegetative
condition.
11.7.10.2 Ensure that
downspouts remain disconnected and pervious filtering or infiltrating areas are
not converted to impervious surface .
11.8 Vegetated Channels
11.8.1 Vegetated channels are open
conveyances planted with grass or other suitable vegetation and having a
shallow depth of flow to allow runoff to be filtered and recharged along the
length of the channel. Design variants include:
11.8.1.1 Bioswale
11.8.1.2 Grassed Channel
11.8.2 Vegetated channels receive the
following annual runoff reduction and pollutant reduction credits.
11.8.2.1 The RPv runoff reduction performance
credit for a Bioswale on A/B soil or compost amended C soil is 50% annual
runoff reduction.
11.8.2.2 The RPv
runoff reduction performance credit for a Grassed Channel on A/B soil or
compost amended C soil is 20% annual runoff reduction.
11.8.2.3 The RPv runoff reduction performance
credit for a Bioswale on C/D soil is 25% annual runoff reduction.
11.8.2.4 The RPv runoff reduction performance
credit for a Grassed Channel on C/D soil is 10% annual runoff
reduction.
11.8.2.5 The Cv runoff
reduction performance credit for Vegetated Channels is 10% of the RPv
allowance.
11.8.2.6 The Fv runoff
reduction performance credit for Vegetated Channels is 1% of the RPv
allowance.
11.8.2.7 The total
nitrogen pollutant reduction performance credit for Vegetated Channels is 100%
of the load reduction.
11.8.2.8 The
total phosphorous pollutant reduction performance credit for Vegetated Channels
is 100% of the load reduction.
11.8.2.9 The total suspended solids pollutant
reduction performance credit for Vegetated Channels is 100% of the load
reduction.
11.8.3
Vegetated Channel Feasibility Criteria
11.8.3.1 The bottom of vegetated channels
shall be above the seasonal high water table .
11.8.3.2 Approval from the applicable utility
company or agency is required if utility lines will run below the vegetated
channel.
11.8.4
Vegetated Channel Conveyance Criteria
11.8.4.1 The bottom width and slope of a
vegetated channel shall be designed such that the flow depth based on 50% of
RPv peak flow rate , does not exceed four inches.
11.8.4.2 Vegetated channels shall convey the
Cv and Fv peak flow rate at non-erosive velocities for the soil and vegetative
cover provided.
11.8.5
Every inlet into a vegetated channel system shall have pretreatment .
11.8.6 Vegetated Channel Design Criteria
11.8.6.1 Channel Geometry
11.8.6.1.1 The bottom width of a trapezoidal
channel shall be a minimum of two feet wide to ensure that an adequate surface
area exists along the bottom of the channel for filtering.
11.8.6.1.2 If a channel bottom will be wider
than eight feet, benches, check dams, level spreaders, or multi-level cross
sections shall be incorporated to prevent braiding and erosion along the
channel bottom.
11.8.6.1.3
Vegetated channel side slopes shall be no steeper than 3H:1V.
11.8.6.2 Check Dams
11.8.6.2.1 Check dams must be firmly anchored
into the side-slopes to prevent outflanking; check dams must also be anchored
into the channel bottom to prevent hydrostatic head from pushing out the
underlying soils.
11.8.6.2.2 Check
dams must be designed to pass the Cv design storm peak flow.
11.8.6.2.3 Check dams shall be composed of
wood, concrete, stone , or other non-erodible material.
11.8.6.2.4 Each check dam shall have a weep
hole or similar drainage feature, to allow for dewatering following a storm
event.
11.8.6.3 All
seeded vegetated channels require a minimum SSM-III biodegradable erosion
control matting conforming to Delaware Erosion and Sediment Control
Handbook.
11.8.6.4 The soil
amendments, when used, shall extend over the length and width of the channel
bottom, and the compost shall be incorporated to the depth as shown on the
approved plan.
11.8.6.5 Adequate
conveyance and treatment capacity shall be provided in accordance with the
following guidelines:
11.8.6.5.1 Hydraulic
capacity shall be verified using Manning's equation or an accepted equivalent
method, such as tractive forces and vegetal retardance.
11.8.6.5.2 Design storm flow depth based on
50% of RPv peak flow rate shall be maintained at four inches or less.
11.8.6.5.3 Manning's "n" value for vegetated
channels shall be 0.2 for flow depths up to four inches, decreasing to 0.03
above four inches of flow depth. If alternative vegetation is used to increase
the Manning's "n" value and decrease the resulting channel width, material
specifications and construction oversight shall be provided to ensure that the
denser vegetation is actually established.
11.8.6.5.4 Peak flow rates for the Cv and Fv
storms shall be non-erosive.
11.8.6.5.5 The Cv peak flow rate shall be
contained within the channel banks.
11.8.6.5.6 If the Fv storm event is not
contained within the channel, the area of inundation shall be shown.
11.8.6.5.7 The total peak discharge at the
outlet shall be used to calculate the depth of flow and velocity for the
channel unless lateral flow along the channel is calculated
incrementally.
11.8.6.5.8 Hydraulic
residence time is the time for runoff to travel the full length of the channel.
For both Bioswales and Grassed Channels hydraulic residence time is computed
based upon 50% of the RPv peak flow rate .
11.8.6.5.8.1 For Bioswales, the hydraulic
residence time shall be a minimum of nine minutes. If flow enters the channel
at several locations, a nine minute minimum hydraulic residence time shall be
demonstrated for each entry point.
11.8.6.5.8.2 For Bioswales, adjusted RPv
runoff reduction credit based on the ratio of the computed residence to the
minimum residence time shall be applied to Bioswales that meet the maximum
depth of flow criteria. The maximum adjusted RPv runoff reduction credit is 75%
for HSG A/B soils and 40% for HSG C/D soils. Adjusted RPv reduction credit
shall not be granted for computed residence times of less than five
minutes.
11.8.6.5.8.3 For Grassed
Channels, the hydraulic residence time for concentrated flow entering the
Grassed Channel shall be a minimum of five minutes.
11.8.6.5.8.4 Lateral flow entering the
Grassed Channel as sheet flow may be excluded from residence time calculations
but shall be accounted for in the channel depth and velocity
calculations.
11.8.6.5.8.5 For
Grassed Channels with in-line culverts, the proportion of grassed channel flow
length shall be a minimum of 80% of the total flow length.
11.8.6.6 All Vegetated Channels
must be designed so as to be accessible for maintenance .
11.8.6.6.1 A maintenance right-of-way or
easement must extend to the Vegetated Channel from a public or private
road.
11.8.6.6.2 Adequate
maintenance access must extend to the full Vegetated Channel length.
11.8.6.6.3 Maintenance access must meet the
following criteria:
11.8.6.6.3.1 Minimum width
of 15 feet.
11.8.6.6.3.2 Profile
grade that does not exceed 10H:1V.
11.8.6.6.3.3 Minimum 10H:1V cross
slope.
11.8.7 Vegetated Channel Landscaping Criteria
11.8.7.1 A planting plan must be provided
that indicates the methods used to establish and maintain vegetative
stabilization of the vegetated channel.
11.8.7.2 Vegetated channels shall be
established at such a density to achieve a 90% vegetated cover for project
completion.
11.8.7.3 All seeded
vegetated channels require a minimum SSM-III biodegradable erosion control
matting conforming to Delaware Erosion and Sediment Control Handbook.
11.8.8 Vegetated Channel
Construction
11.8.8.1 Construction reviews
are required during the following stages of construction , and shall be noted on
the plan in the sequence of construction :
11.8.8.1.1 Pre-construction
meeting;
11.8.8.1.2 Initial site
preparation including installation of erosion and sediment controls, sensitive
area protection surrounding vegetated channel locations, and blockage of inlets
to vegetated channels;
11.8.8.1.3
Excavation and grading including interim and final elevations;
11.8.8.1.4 Construction of check dams and
pretreatment practices, as applicable;
11.8.8.1.5 Implementation of required
stabilization and planting plan; and
11.8.8.1.6 Final construction review
including development of a punch list for facility acceptance.
11.8.8.2 Post Construction
Verification Documentation.
11.8.8.2.1 Upon
facility completion, the owner shall submit post construction verification
documents as follows to demonstrate that the vegetated channel has been
constructed within allowable tolerances in accordance with the approved
Sediment and Stormwater Management Plan and accepted by the approving agency:
11.8.8.2.1.1 Spot elevations of top of bank,
bottom of bank, and centerline of the vegetated channel every 25 feet
throughout the length of the channel;
11.8.8.2.1.2 Additional spot elevations that
demonstrate positive downstream drainage beyond the end of the vegetated
channel;
11.8.8.2.1.3 Cross section
of the vegetated channel at the midpoint; and
11.8.8.2.1.4 Photo documentation of the
vegetated channel depicting the channel bottom width and verification of
achievement of the required 90% vegetated cover.
11.8.8.2.2 The constructed slope, bottom
width, depth, and length of the vegetated channel shall be within 90% of the
design geometrics for those parameters.
11.8.8.2.3 In the event that the constructed
allowable tolerances are exceeded for the vegetated channel, supplemental
calculations shall be submitted to determine if the vegetated channel, as
constructed, meets the design requirements. The computed residence time rounded
to the nearest minute shall be no less than the minimum design residence
time.
11.8.8.2.4 Performance of a
vegetated channel shall be evaluated by the Department or Delegated Agency if
requested in writing to determine if reconstruction of a vegetated channel that
exceeds allowable tolerances is necessary.
11.9 Sheet Flow to Vegetated
Filter Strip or Vegetated Open Space
11.9.1
Vegetated areas can treat sheet flow delivered from adjacent impervious and
managed turf areas by slowing runoff velocities and allowing sediment and
attached pollutants to settle or be filtered by the vegetation. Vegetation can
consist of grasses, planted trees, or existing forest. Design variants include:
11.9.1.1 Sheet Flow to Grassed Filter
Strip;
11.9.1.2 Sheet Flow to
Afforested Filter Strip;
11.9.1.3
Sheet Flow to Forested Filter Strip;
11.9.1.4 Sheet Flow to Grassed Open
Space ;
11.9.1.5 Sheet Flow to
Afforested Open Space ; and
11.9.1.6
Sheet Flow to Forested Open Space .
11.9.2 Sheet Flow practices receive the
following annual runoff reduction and pollutant reduction credits.
11.9.2.1 The RPv runoff reduction performance
credit for a Vegetated Filter Strip on A/B soil or compost amended C soil is as
follows:
11.9.2.1.1 Grassed filter strips
receive 25% annual runoff reduction performance credit.
11.9.2.1.2 Afforested filter strips receive
30% annual runoff reduction performance credit.
11.9.2.1.3 Forested filter strips receive 40%
annual runoff reduction performance credit.
11.9.2.2 The RPv runoff reduction performance
credit for a Vegetated Filter Strip on C/D soil is as follows:
11.9.2.2.1 Grassed filter strips receive 10%
annual runoff reduction performance credit.
11.9.2.2.2 Afforested filter strips receive
15% annual runoff reduction performance credit.
11.9.2.2.3 Forested filter strips receive 20%
annual runoff reduction performance credit.
11.9.2.3 The RPv runoff reduction performance
credit for Sheet Flow to Vegetated Open Space on A/B soil or compost amended C
soil is as follows:
11.9.2.3.1 Grassed open
space receives 50% annual runoff reduction performance credit.
11.9.2.3.2 Afforested open space receives 60%
annual runoff reduction performance credit.
11.9.2.3.3 Forested open space receives 65%
annual runoff reduction performance credit.
11.9.2.4 The RPv runoff reduction performance
credit for Sheet Flow to Vegetated Open Space on C/D soil is as follows:
11.9.2.4.1 Grassed open space receives 20%
annual runoff reduction performance credit.
11.9.2.4.2 Afforested open space receives 30%
annual runoff reduction performance credit.
11.9.2.4.3 Forested open space receives 40%
annual runoff reduction performance credit.
11.9.2.5 The Cv runoff reduction performance
credit for all Sheet Flow practices is 10% of the RPv allowance.
11.9.2.6 The Fv runoff reduction performance
credit for all Sheet Flow practices is 1% of the RPv allowance.
11.9.2.7 The total nitrogen pollutant
reduction performance credit for all Sheet Flow practices is 100% of the load
reduction.
11.9.2.8 The total
phosphorous pollutant reduction performance credit for all Sheet Flow practices
is 100% of the load reduction.
11.9.2.9 The total suspended solids pollutant
reduction performance credit for all Sheet Flow Practices is 100% of the load
reduction.
11.9.3 Sheet
Flow Feasibility Criteria
11.9.3.1 To maintain
sheet flow through the practice, maximum slope for Filter Strips shall be 8.0%
unless additional calculations are submitted showing the maximum depth and
minimum residence time can be met.
11.9.3.2 Filter Strips shall not be used in
structural fill areas where material must be compacted to meet specific
structural criteria.
11.9.3.3 To
maintain sheet flow through the practice, maximum slope for Open Space shall be
3.0%.
11.9.4 Sheet Flow
Design Criteria
11.9.4.1 Stormwater shall
enter the filter strip or open space as sheet flow.
11.9.4.1.1 Sheet flow length from impervious
surfaces shall be limited to 150 feet.
11.9.4.1.2 A gravel diaphragm or other level
spreading device shall be provided for impervious sheet flow lengths greater
than 75 feet.
11.9.4.1.3 When the
inflow is from a pipe or channel, an engineered level spreader or other device
shall be used to convert the concentrated flow to sheet flow.
11.9.4.2 Vegetated Filter Strip
11.9.4.2.1 The maximum length of a Vegetated
Filter Strip shall be 100 feet.
11.9.4.2.2 Vegetated Filter Strips shall have
the following minimum lengths, measured in the direction of flow, unless
calculations are provided in accordance with the Computational Method of
Compliance.
11.9.4.2.2.1 Vegetated Filter
Strips having slopes less than 3.0% shall have a minimum length of 20
feet.
11.9.4.2.2.2 Vegetated Filter
Strips having slopes between 3.0% and 8.0% shall have a minimum length of 30
feet.
11.9.4.3 Vegetated Open Space
11.9.4.3.1 Vegetated Open Space shall have a
maximum slope of 3.0%.
11.9.4.3.2
The minimum area of the Vegetated Open Space shall be equivalent to the
impervious area of the contributing drainage area to the Vegetated Open
Space .
11.9.4.4
Computational Method of Compliance.
11.9.4.4.1 Vegetated Filter Strips using the
Computational Method of Compliance shall meet the following criteria in order
to receive RPv runoff reduction credits listed above:
11.9.4.4.1.1 The maximum depth of flow shall
be 0.5 inches (0.04 feet).
11.9.4.4.1.2 The minimum residence time shall
be 2.5 minutes.
11.9.4.4.2 Adjusted RPv runoff reduction
credit based on the ratio of the computed residence to the minimum residence
time shall be applied to Vegetated Filter Strips that meet the maximum depth of
flow criteria.
11.9.4.4.2.1 The maximum
adjusted RPv runoff reduction credit is 75% for HSG A/B soils and 30% for HSG
C/D soils.
11.9.4.4.2.2 RPv runoff
reduction credit shall not be adjusted for lengths greater than 100
feet.
11.9.4.5 Soil amendments, when used, shall
extend over the length and width of the Vegetated Filter Strip or Vegetated
Open Space , and compost shall be incorporated to the depth as shown on the
approved plan.
11.9.4.6 All
Vegetated Filter Strips and Vegetated Open Spaces must be designed so as to be
accessible for maintenance .
11.9.5 Sheet Flow Landscaping Criteria
11.9.5.1 Grassed Filter Strips and Grassed
Open Space shall be established at such a density to achieve a 90% vegetated
cover for project completion.
11.9.5.2 Afforested Filter Strips and
Afforested Open Space shall be planted in accordance with Afforestation
requirements.
11.9.5.3 Forested
Filter Strips and Forested Open Space shall have no grading or clearing of
native vegetation and shall have at least 80% tree canopy coverage.
11.9.5.4 All Vegetated Filter Strips and
Vegetated Open Spaces must be stabilized to prevent erosion or transport of
sediment to receiving practices or drainage systems.
11.9.5.5 A planting plan shall be provided
that indicates the methods used to establish and maintain vegetative
stabilization of the Vegetated Filter Strip or Vegetated Open Space .
11.9.6 Sheet Flow Construction
11.9.6.1 No clearing or grading shall take
place in Vegetated Open Space except temporary disturbances associated with
incidental utility construction, restoration operations, or management of
nuisance vegetation. The Vegetated Open Space area shall not be stripped of
topsoil.
11.9.6.2 Construction
reviews are required during the following stages of construction , and shall be
noted on the plan in the sequence of construction :
11.9.6.2.1 Pre-construction
meeting;
11.9.6.2.2 Initial site
preparation including installation of erosion and sediment controls and
sensitive area protection surrounding vegetated filter strip
locations;
11.9.6.2.3 Excavation
and grading including interim and final elevations;
11.9.6.2.4 Implementation of required
stabilization and planting plan; and
11.9.6.2.5 Final construction review
including development of a punch list for facility acceptance.
11.9.6.3 Post Construction
Verification Documentation.
11.9.6.3.1 Upon
facility completion, the owner shall submit Post Construction Verification
Documents at the discretion of the Department or Delegated Agency as follows to
demonstrate that the Vegetated Filter Strip or Vegetated Open Space has been
constructed within allowable tolerances in accordance with the approved
Sediment and Stormwater Management Plan and accepted by the approving agency.
The following items, as applicable, shall be included in the Post Construction
Verification Documentation for Sheet Flow Practices:
11.9.6.3.1.1 Dimensions of Vegetated Filter
Strips (length and width).
11.9.6.3.1.2 Area of Vegetated Open
Space .
11.9.6.3.1.3
Cross-slope.
11.9.6.3.1.4
Elevations of any structural components, such as gravel diaphragms or
engineered level spreaders.
11.9.6.3.1.5 Photo documentation of the
grassed filter strip or grassed open space providing verification of
achievement of the required 90% vegetated cover.
11.9.6.3.2 Constructed allowable tolerances
for vegetated filter strips and vegetated open spaces, if disturbed, shall be
within the tolerances of design geometrics for the following parameters:
11.9.6.3.2.1 Slope shall be no greater than
2.0% steeper than design slope;
11.9.6.3.2.2 Length shall be no less than 90%
of design length;
11.9.6.3.2.3
Width shall be no less than 90% of design width; and
11.9.6.3.2.4 Elevations of any structural
components shall be within 0.15 feet of design elevation.
11.9.6.3.3 In the event that the constructed
allowable tolerances are exceeded for the vegetated filter strip, supplemental
calculations shall be submitted to determine if the vegetated filter strip, as
constructed, meets the minimum residence time. The computed residence time
rounded to the nearest minute shall be no less than the minimum design
residence time.
11.9.6.3.4
Performance of a vegetated filter strip shall be evaluated by the Department or
Delegated Agency if requested in writing to determine if reconstruction of a
vegetated filter strip that exceeds allowable tolerances is
necessary.
11.10 Detention Practices
11.10.1 Detention Practices are storage
practices that are explicitly designed to provide stormwater detention for the
Conveyance Event, Cv (10-year) and Flooding Event, Fv (100-year). Design
variants include:
11.10.1.1 Dry Detention Pond
11.10.1.1.1 The RPv runoff reduction
performance credit for this variant is 0%.
11.10.1.1.2 The Cv runoff reduction
performance credit for this variant is 0%.
11.10.1.1.3 The Fv runoff reduction
performance credit for this variant is 0%.
11.10.1.1.4 The total nitrogen pollutant
reduction performance credit for this variant is not less than 5%.
11.10.1.1.5 The total phosphorous pollutant
reduction performance credit for this variant is not less than 10%.
11.10.1.1.6 The total suspended solids
pollutant reduction performance credit for this variant is not less than
10%.
11.10.1.2 Dry
Extended Detention (ED) Basin
11.10.1.2.1 The
RPv runoff reduction performance credit for this variant is 100%.
11.10.1.2.2 The Cv runoff reduction
performance credit for this variant is 1%.
11.10.1.2.3 The Fv runoff reduction
performance credit for this variant is 0%.
11.10.1.2.4 The total nitrogen pollutant
reduction performance credit for this variant is not less than 20%.
11.10.1.2.5 The total phosphorous pollutant
reduction performance credit for this variant is not less than 20%.
11.10.1.2.6 The total suspended solids
pollutant reduction performance credit for this variant is not less than
60%.
11.10.1.3
Underground Detention Facilities
11.10.1.3.1
Underground Detention Facilities not achieving 48-hour ED of the full RPv shall
have RPv , Cv, and Fv runoff reduction performance credits and total nitrogen,
total phosphorous, and total suspended solids pollutant reduction performance
credits in accordance with that of Dry Detention Pond.
11.10.1.3.2 Underground Detention Facilities
achieving 48-hour ED of the full RPv shall have RPv , Cv, and Fv runoff
reduction performance credits and total nitrogen, total phosphorous, and total
suspended solids pollutant reduction performance credits in accordance with
that of Dry Extended Detention (ED) Basin.
11.10.2 Full runoff reduction credit is given
for detention practices that provide 48-hour extended detention of the full
RPv.
11.10.3 Detention Practices
Feasibility Criteria
11.10.3.1 Depth-to-Water
Table and Bedrock.
11.10.3.1.1 Dry Detention
Ponds or Dry ED Basins shall not be allowed if the seasonal high water table or
bedrock will be within one foot of the floor of the pond.
11.10.3.1.2 Non-watertight Underground
Detention Facilities shall be no lower than the seasonal high water table and
two feet above bedrock.
11.10.3.1.3
For watertight Underground Detention Facilities, an anti-flotation analysis is
required to check for buoyancy problems in seasonal high water table
areas.
11.10.3.1.4 Subsection 12.1
shall be followed for testing.
11.10.3.2 Underground Detention Facilities
must meet structural requirements for bearing capacity, overburden support, and
traffic loading as determined by a licensed design professional, and based upon
manufacturer's recommendations where applicable.
11.10.4 Detention Practice Conveyance
Criteria
11.10.4.1 Principal Spillway
11.10.4.1.1 The principal spillway must be
accessible from dry land.
11.10.4.1.2 A structure-pipe spillway shall
be designed with anti-flotation, anti-vortex and trash rack devices on the
structure.
11.10.4.1.3 The outfall
pipe and all connections to the outfall structure shall be made watertight.
Soil tight only joints are not acceptable.
11.10.4.1.4 Anti-seep collars shall be used
in accordance with USDA NRCS Delaware Pond Code 378, as amended.
11.10.4.1.5 When the principal spillway is
composed of a weir wall discharging to a channel, the channel below the weir
must be reinforced with riprap or other acceptable material to prevent
scour.
11.10.4.2
Non-Clogging Outlet
11.10.4.2.1 For Dry ED
Basins, the control structure must include an outlet that will slowly release
the RPv over a 48-hour period.
11.10.4.2.2 When a low flow orifice is
specified, it must be adequately protected from clogging by either an
acceptable external trash rack or by internal orifice protection. Orifice
diameters shall not be less than three inches unless internal orifice control
is provided.
11.10.4.3
The design shall specify an outfall that can discharge the maximum design storm
event in a non-erosive manner at the project point of discharge.
11.10.4.4 Emergency Spillway .
11.10.4.4.1 Dry Detention Ponds and Dry ED
Basins must be designed to pass the maximum design storm event (Fv) if the Fv
is being routed through the Dry Detention Ponds and Dry ED Basins rather than
bypassing.
11.10.4.4.2 An earthen
emergency spillway designed to convey the Fv shall be cut in natural ground or,
if cut in fill, shall be constructed and stabilized with methods to prevent
erosion and structural failure.
11.10.4.5 Inflow Points.
11.10.4.5.1 Inflow points into the Dry
Detention Ponds and Dry ED Basins must be stabilized to ensure that non-erosive
conditions exist during storm events up to the conveyance event (Cv).
11.10.4.5.2 A forebay shall be provided at
each inflow location that provides 10% or greater of the total RPv inflow to
the Dry Detention Ponds and Dry ED Basins.
11.10.4.6 In the event that the embankment is
a regulated dam , the designer must verify that the appropriate Dam Safety
Permit has been approved by the Department 's Dam Safety Program.
11.10.4.7 For Underground Detention
Facilities, an internal or external high flow bypass or overflow shall be
included in the design to safely pass the Fv.
11.10.5 Detention Practices Pretreatment
Criteria
11.10.5.1 A forebay must be located
at each major inlet to trap sediment and preserve the capacity of the main
treatment cell. The following criteria apply to forebay design:
11.10.5.1.1 A major inlet is defined as an
individual storm drain inlet pipe or open channel conveying at least 10% of the
Dry Detention Pond's and Dry ED Basin's contributing RPv runoff
volume.
11.10.5.1.2 The forebay
shall be no deeper than three feet.
11.10.5.1.3 The forebay must be sized to
contain 10% of the volume of runoff from the contributing drainage area for the
Resource Protection event.
11.10.5.1.4 Discharge from the forebay shall
be non-erosive.
11.10.5.2 Every underground detention
practice shall have pretreatment mechanisms to protect the long term integrity
of the practice.
11.10.6
Detention Practices Design Criteria
11.10.6.1
Dry Detention Ponds and Dry ED Basins constructed to meet regulatory stormwater
management requirements in the State of Delaware shall be designed and
constructed in accordance with the USDA NRCS Delaware Pond Code 378 as
amended.
11.10.6.2 Detention
Practice Sizing.
11.10.6.2.1 For RPv
compliance, a Dry ED Basin or Underground Detention Facility must provide 48
hours extended detention for the RPv runoff volume.
11.10.6.2.2 Detention time shall be based on
the time of initial inflow to time of final outflow from the facility . In order
to simulate a baseflow condition to the extent practicable, the peak discharge
for the outflow hydrograph shall not exceed five times the average discharge
rate.
11.10.6.3 Earthen
side slopes shall be designed and constructed no steeper than 3H:1V.
11.10.6.4 Retaining walls around Dry
Detention Ponds and Dry ED Basins shall be limited to no more than 50% of the
pond perimeter based upon the peak elevation of the Cv. In order to maintain
the safety requirements, retaining walls shall be configured as follows:
11.10.6.4.1 The retaining wall shall have a
maximum height of three feet.
11.10.6.4.2 Any additional retaining walls
shall have a maximum height of two feet and provide a minimum 10-foot level
terrace from a lower retaining wall.
11.10.6.5 Safety Features
11.10.6.5.1 Any inflow opening 12 inches or
greater discharging to a closed drainage system shall include safety
grates.
11.10.6.5.2 The emergency
spillway must be located so that downstream structures will not be adversely
impacted by spillway discharges.
11.10.6.5.3 The emergency spillway exit
channel must be designed to direct runoff to a point of discharge without
adverse impact to downstream structures.
11.10.6.6 All Detention Practices shall be
designed so as to be accessible for maintenance .
11.10.6.6.1 Adequate maintenance access must
extend to the pretreatment, riser, and outlet structure. Adequate maintenance
access must also be provided for all Underground Detention
Facilities.
11.10.6.6.2 A
maintenance right-of-way or easement must extend to the Detention Practice from
a public or private road.
11.10.6.6.3 Maintenance access must meet the
following criteria:
11.10.6.6.3.1 Minimum
width of 15 feet.
11.10.6.6.3.2
Profile grade that does not exceed 10H:1V.
11.10.6.6.3.3 Minimum 10H:1V cross
slope.
11.10.6.7 Maintenance Set-Aside Area
11.10.6.7.1 The maintenance set-aside area
shall accommodate the volume of 50% of the collective forebay volume.
11.10.6.7.2 The maximum depth of the set
aside volume shall be one foot.
11.10.6.7.3 The slope of the set aside area
shall not exceed 5.0%.
11.10.6.8 Detention Vault and Tank Materials.
All construction joints and pipe joints shall be water tight. Cast-in-place
wall sections must be designed as retaining walls.
11.10.6.9 For watertight Underground
Detention Facilities, anti-flotation analysis is required to check for buoyancy
problems in the high water table areas. Anchors shall be designed to counter
the pipe and structure buoyancy by at least a 1.2 factor of safety.
11.10.7 Detention Practices
Landscaping Criteria
11.10.7.1 Woody
vegetation shall not be planted or allowed to grow within 15 feet of the
embankment and 10 feet on either side of principal spillway or inflow
pipes.
11.10.7.2 For Dry Detention
Ponds and Dry ED Basins, a planting plan shall be provided that indicates the
methods used to establish and maintain vegetative coverage within the Detention
Practice and its vegetated perimeter area. Minimum elements of a plan include
seed mixes by botanical and common names as well as percentages by weight or
volume.
11.10.8
Detention Practices Construction
11.10.8.1
Construction of proprietary Underground Detention Facilities must be in
accordance with manufacturer's specifications.
11.10.8.2 Underground Detention Facilities
must be inspected and cleaned of sediment after the site is
stabilized.
11.10.8.3 Use of Dry
Detention Pond or Dry ED Basin for Erosion and Sediment Control .
11.10.8.3.1 Approval from the Department or
the appropriate Delegated Agency must be obtained before any planned Dry
Detention Ponds and Dry ED Basins can be used as a sediment basin.
11.10.8.3.2 If a Dry Detention Pond or Dry ED
Basin serves as a sediment basin during project construction, the volume of the
sediment basin must be based on the more stringent sizing rule.
11.10.8.3.3 When the sediment basin is being
converted into a Dry Detention Pond or Dry ED Basin, the sediment basin shall
be dewatered in accordance with the approved plan and appropriate details from
the Delaware Erosion and Sediment Control Handbook prior to removing
accumulated sediment and regrading the pond bottom.
11.10.8.3.4 The Sediment and Stormwater Plan
must include conversion steps from sediment basin to permanent Dry Detention
Ponds and Dry ED Basins in the construction sequence.
11.10.8.3.4.1 The Department or Delegated
Agency must be notified and provide approval prior to conversion from sediment
basin to the final configuration of the Dry Detention Pond or Dry ED
Basin.
11.10.8.3.4.2 Appropriate
procedures must be implemented to prevent discharge of turbid waters when the
sediment basin is being converted into a Dry Detention Pond or Dry ED
Basin.
11.10.8.4 Construction reviews are required
during the following stages of construction , and shall be noted on the plan in
the sequence of construction :
11.10.8.4.1
Pre-construction meeting.
11.10.8.4.2 Initial site preparation
including installation of erosion and sediment controls.
11.10.8.4.3 Construction of the embankment,
including installation of the principal spillway and the outlet
structure.
11.10.8.4.4 For Dry
Detention Pond and Dry ED Basin, excavation and grading including interim and
final elevations.
11.10.8.4.5 For
Underground Detention, subgrade, placement of stone , system components in
accordance with manufacturer's recommendations and backfill.
11.10.8.4.6 Implementation of the planting
plan and vegetative stabilization.
11.10.8.4.7 Final inspection including
development of a punch list for facility acceptance.
11.10.8.5 Upon facility completion, the owner
shall submit post construction verification documents to demonstrate that the
Detention Practice has been constructed within allowable tolerances and in
accordance with the approved Sediment and Stormwater Management Plan and
accepted by the approving agency.
11.10.8.5.1
Allowable tolerances for Dry Detention Pond and Dry ED Basin are as follows:
11.10.8.5.1.1 The constructed top of bank
elevation may be no lower than the design elevation for top of bank.
11.10.8.5.1.2 The constructed volume of the
dry pond surface storage shall be no less than 90% of the design
volume.
11.10.8.5.1.3 The
constructed elevation of any structure shall be within 0.15 foot of the
design.
11.10.8.5.2
Allowable tolerances for Underground Detention Facilities are as follows:
11.10.8.5.2.1 Grate and invert elevations of
all structures, including weirs shall be within 0.15 foot of the
design.
11.10.8.5.2.2 Diameter of
all pipes or dimensions of chambers within underground detention facility shall
be as shown on the plan.
11.10.8.5.2.3 Dimension of any weirs shall be
within 10% of the design.
11.10.8.5.3 When the allowable tolerances are
exceeded for volume or structure elevations, supplemental calculations must be
submitted to the approval agency to demonstrate that the Detention Practice, as
constructed, meets the design requirements.
11.10.9 Detention Practices Maintenance
Criteria
11.10.9.1 Repair of critical
structural features such as embankments and risers shall be performed by
responsible personnel that have successfully completed the Department
Contractor Training Program.
11.10.9.2 Sediment removal in the Dry
Detention Pond or Dry ED Basin pretreatment practice must occur when 50% of
total forebay capacity has been lost.
11.11 Stormwater Filtering Systems
11.11.1 Stormwater Filter Systems are
practices that capture and temporarily store the design storm volume and pass
it through a filter media or material. Filtered runoff may be collected and
returned to the conveyance system, or allowed to partially infiltrate into the
soil. Design variants include:
11.11.1.1
Non-Structural Sand Filter;
11.11.1.2 Surface Sand Filter;
11.11.1.3 Three-Chamber Underground Sand
Filter; and
11.11.1.4 Perimeter
Sand Filter.
11.11.2
Stormwater Filtering Systems shall be combined with a separate facility to
provide stormwater detention.
11.11.3 Proprietary stormwater filtering
systems shall be verified by the Department for adequate performance, sizing,
and longevity.
11.11.4 Stormwater
Filtering Systems Stormwater Credits
11.11.4.1
Stormwater Filtering Systems receive no runoff reduction performance
credit.
11.11.4.2 Stormwater
Filtering Systems sized in accordance with the design criteria shall receive
the following pollutant reduction performance credits:
11.11.4.2.1 The total nitrogen pollutant
reduction performance credit for Stormwater Filtering Systems is not less than
40% removal efficiency.
11.11.4.2.2
The total phosphorus pollutant reduction performance credit for Stormwater
Filtering Systems is not less than 60% removal efficiency.
11.11.4.2.3 The total suspended solids
pollutant reduction performance credit for Stormwater Filtering Systems is not
less than 80% removal efficiency.
11.11.5 Stormwater Filtering Systems
Feasibility Criteria
11.11.5.1 Depth to Water
Table and Bedrock
11.11.5.1.1 A minimum
vertical distance of two feet must be provided between the bottom of the
non-structural sand filter or surface sand filter and the seasonal high water
table as determined by the procedures in subsection 12.1 or bedrock
layer.
11.11.5.1.2 The minimum
vertical distance of two feet may be relaxed if a groundwater mounding analysis
or piezometer testing has been performed by a qualified professional.
11.11.5.1.3 Three-chamber underground sand
filter and perimeter sand filter require no minimum separation to seasonal high
water table or bedrock.
11.11.5.2 Stormwater Filtering Systems shall
not be located on slopes greater than 6.0%.
11.11.6 Stormwater Filtering Systems
Conveyance Criteria
11.11.6.1 On-line
stormwater filtering systems' designs, shall demonstrate that the filter will
safely pass the largest design storm event to a stabilized water course without
resuspending or flushing previously trapped material.
11.11.6.2 All Stormwater Filtering Systems
shall be designed to drain or dewater within 48 hours after a storm
event.
11.11.7 Every
inlet into a Stormwater Filtering System shall have a pretreatment mechanism to
trap sediment, preserve the capacity of the main treatment area, and protect
the long term integrity of the practice.
11.11.8 Stormwater Filtering Systems Design
Criteria
11.11.8.1 Stormwater Filtering
Systems shall be designed to drain the design storm volume from the filter
chamber within 48 hours after each rainfall event.
11.11.8.2 Filter
11.11.8.2.1 The filter media shall consist of
clean, washed AASHTO M-6/ASTM C-33 medium aggregate concrete sand with
individual grains between 0.02 and 0.04 inches in diameter.
11.11.8.2.2 A minimum filter bed depth of 12"
is required.
11.11.8.3
Underdrain
11.11.8.3.1 The underdrain shall be
a minimum of four inch perforated corrugated polyethylene pipe (CPP).
11.11.8.3.2 The underdrain shall be encased
in a layer of clean, washed nominal ¼ inch gravel with a maximum of 2.0%
passing the #200 sieve with a minimum of three inches of cover.
11.11.8.3.3 When an underdrain is specified a
needled, non-woven, polypropylene geotextile having a flow rate (ASTM D4491)
[GREATER THEN OR EQUAL TO]110 gallons per minute per square foot (gal/min/sf)
and an apparent opening size (ASTM D4751) of US #70 or #80 sieve shall be
placed beneath the filter media and above the underdrain gravel
layer.
11.11.8.4 All
Stormwater Filtering Systems must be designed so as to be accessible for
maintenance .
11.11.8.4.1 A maintenance
right-of-way or easement must extend to the Stormwater Filtering System from a
public or private road.
11.11.8.4.2
Adequate maintenance access must extend to the perimeter of the Stormwater
Filtering System pretreatment area and the filter bed
11.11.8.4.3 Maintenance access must meet the
following criteria:
11.11.8.4.3.1 Minimum
width of fifteen feet.
11.11.8.4.3.2 Profile grade that does not
exceed 10H:1V.
11.11.8.4.3.3
Minimum 10H:1V cross slope.
11.11.8.4.4 Access to Underground Sand
Filters must be provided by manholes at least 30 inches in diameter, along with
steps to the areas where maintenance will occur.
11.11.8.5 The Stormwater Filtering System
including pretreatment shall be sized to contain a minimum of 75% of the RPv
prior to filtration .
11.11.9 Vegetative cover shall be established
over the contributing pervious drainage areas before runoff can be accepted
into the Stormwater Filtering System.
11.11.10 Stormwater Filtering Systems
Construction Criteria
11.11.10.1 Erosion and
Sediment Control .
11.11.10.1.1 No runoff shall
be allowed to enter the Stormwater Filtering System prior to completion of all
construction activities, including revegetation and final site
stabilization.
11.11.10.1.2
Construction runoff shall be treated in separate sedimentation basins and
routed to bypass the filter system. Should construction runoff enter the filter
system prior to final site stabilization, all contaminated materials shall be
removed and replaced with new clean filter materials before a regulatory
inspector approves its completion.
11.11.10.1.3 The approved Sediment &
Stormwater Plan shall include specific measures to provide for the protection
of the filter system before the final stabilization of the site .
11.11.10.2 Construction reviews
are required during the following stages of construction , and shall be noted on
the plan in the sequence of construction :
11.11.10.2.1 Pre-construction
meeting.
11.11.10.2.2 Initial site
preparation including installation of erosion and sediment controls, sensitive
area protection, and blockage of inlets to stormwater filtering system
locations.
11.11.10.2.3 Excavation
and grading to design dimensions and elevations.
11.11.10.2.4 Installation of the filter
structure, including the water tightness test as applicable.
11.11.10.2.5 Installation of the underdrain
and filter bed.
11.11.10.2.6 Check
that stabilization in contributing area is adequate to bring the stormwater
filtering system online.
11.11.10.2.7 Final construction review after
a rainfall event to ensure that it drains properly and all pipe connections are
watertight. Develop a punch list for facility acceptance.
11.11.10.3 Upon facility completion, the
owner shall submit post construction verification documents to demonstrate that
the stormwater filtering system has been constructed within allowable
tolerances in accordance with the approved Sediment and Stormwater Management
Plan and accepted by the approving agency. Allowable tolerances for stormwater
filtering systems are as follows:
11.11.10.3.1 The constructed surface area of
the filter bed shall be no less than 90% of the design surface area.
11.11.10.3.2 The constructed volume of the
surface storage shall be no less than 90% of the design volume.
11.11.10.3.3 Depth of filter media shall be
no less than 12 inches.
11.11.10.3.4 The constructed elevation of any
structure shall be within 0.15 foot of the design.
11.12 Constructed
Wetlands
11.12.1 Constructed Wetlands are
practices that mimic natural wetland areas to treat urban stormwater by
incorporating permanent pools with shallow storage areas. Constructed Wetlands
may provide stormwater detention for larger storms (Cv and Fv) above the RPv
storage. Design variants include:
11.12.1.1
Traditional Constructed Wetlands ;
11.12.1.2 Wetland Swales;
11.12.1.3 Ephemeral Constructed Wetlands ;
and
11.12.1.4 Submerged Gravel
Wetlands .
11.12.2
Constructed Wetland Stormwater Credits
11.12.2.1 Traditional Constructed Wetlands
Performance Credits
11.12.2.1.1 Traditional
Constructed Wetlands receive 100% runoff reduction credit for the
RPv.
11.12.2.1.2 Traditional
Constructed Wetlands receive not less than 1% of the RPv allowance for the
Cv.
11.12.2.1.3 Traditional
Constructed Wetlands receive not less than 0% runoff reduction credit for the
Fv.
11.12.2.1.4 Traditional
Constructed Wetlands receive not less than 30% pollutant removal efficiency for
total nitrogen reduction.
11.12.2.1.5 Traditional Constructed Wetlands
receive not less than 40% pollutant removal efficiency for total phosphorous
reduction.
11.12.2.1.6 Traditional
Constructed Wetlands receive not less than 80% pollutant removal efficiency for
total suspended solids reduction.
11.12.2.2 Wetland Swale Performance Credits
11.12.2.2.1 Wetland Swales receive the
following runoff reduction credits:
11.12.2.2.1.1 Wetland Swales located in HSG A
or B soils receive 15% annual runoff reduction for the RPv.
11.12.2.2.1.2 Wetland Swales located in HSG C
or D soils receive 10% annual runoff reduction for the RPv.
11.12.2.2.1.3 Wetland Swales receive not less
than 1% of the RPv allowance for the Cv.
11.12.2.2.1.4 Wetland Swales receive not less
than 0% runoff reduction credit for the Fv.
11.12.2.2.2 Wetland Swales receive 100% of
the load reduction plus not less than 20% pollutant removal efficiency for
total nitrogen reduction.
11.12.2.2.3 Wetland Swales receive 100% of
the load reduction plus not less than 30% pollutant removal efficiency for
total phosphorous reduction.
11.12.2.2.4 Wetland Swales receive 100% of
the load reduction plus not less than 60% pollutant removal efficiency for
total suspended solids reduction.
11.12.2.3 Ephemeral Constructed Wetland
Performance Credits
11.12.2.3.1 Ephemeral
Constructed Wetlands receive the following runoff reduction credits:
11.12.2.3.1.1 Ephemeral Constructed Wetlands
located in HSG A or B soils receive 40% annual runoff reduction for the
RPv.
11.12.2.3.1.2 Ephemeral
Constructed Wetlands located in HSG C or D soils receive 10% annual runoff
reduction for the RPv.
11.12.2.3.1.3 Ephemeral Constructed Wetlands
receive not less than 1% of the RPv allowance for the Cv.
11.12.2.3.1.4 Ephemeral Constructed Wetlands
receive not less than 0% runoff reduction credit for the Fv.
11.12.2.3.2 Ephemeral Constructed
Wetlands receive 100% of the load reduction plus not less than 20% pollutant
removal efficiency for total nitrogen reduction.
11.12.2.3.3 Ephemeral Constructed Wetlands
receive 100% of the load reduction plus not less than 30% pollutant removal
efficiency for total phosphorous reduction.
11.12.2.3.4 Ephemeral Constructed Wetlands
receive 100% of the load reduction plus not less than 60% pollutant removal
efficiency for total suspended solids reduction.
11.12.2.4 Submerged Gravel Wetland
Performance Credits
11.12.2.4.1 The RPv , Cv
and Fv runoff reduction performance credit for Submerged Gravel Wetlands is
100% of the detention storage.
11.12.2.4.2 Submerged Gravel Wetlands receive
not less than 30% pollutant removal efficiency for total nitrogen
reduction.
11.12.2.4.3 Submerged
Gravel Wetlands receive not less than 40% pollutant removal efficiency for
total phosphorous reduction.
11.12.2.4.4 Submerged Gravel Wetlands receive
not less than 80% pollutant removal efficiency for total suspended solids
reduction.
11.12.3 Constructed Wetlands shall not be
located within existing jurisdictional wetlands .
11.12.4 Constructed Wetland Conveyance
Criteria
11.12.4.1 Principal Spillway
11.12.4.1.1 The principal spillway must be
accessible from dry land.
11.12.4.1.2 A structure-pipe spillway shall
be designed with anti-flotation, anti-vortex and trash rack devices on the
structure.
11.12.4.1.3 The outfall
pipe and all connections to the outfall structure shall be made watertight.
Soil tight only joints are not acceptable.
11.12.4.1.4 Anti-seep collars shall be used
in accordance with Pond Code 378, as amended.
11.12.4.1.5 When the principal spillway is
composed of a weir wall discharging to a channel, the channel below the weir
must be reinforced with riprap or other acceptable material to prevent
scour.
11.12.4.1.6 When a low flow
orifice is specified, it must be adequately protected from clogging by either
an acceptable external trash rack or by internal orifice protection. Orifice
diameters shall not be less than three inches.
11.12.4.2 The design shall specify an outfall
that can discharge the maximum design storm event in a non-erosive manner at
the project point of discharge.
11.12.4.3 Constructed Wetlands must be
designed to pass the maximum design storm event (Fv) if the Fv is being routed
through the Constructed Wetland rather than bypassing. An earthen emergency
spillway designed to convey the Fv shall be cut in natural ground or, if cut in
fill, shall be constructed and stabilized with methods to prevent erosion and
structural failure.
11.12.4.4
Inflow Points
11.12.4.4.1 Inflow points into
the Constructed Wetland must be stabilized to ensure that non-erosive
conditions exist during storm events up to the conveyance event (Cv).
11.12.4.4.2 For Submerged Gravel Wetlands ,
the inflow volume shall enter the gravel substrate directly via a pipe manifold
or inflow chimneys or as sheet flow through connected gravel layer.
11.12.5 Constructed
Wetland Pretreatment Criteria
11.12.5.1 Every
inlet into a Constructed Wetland shall have pretreatment .
11.12.5.2 Exit velocities from the
pretreatment shall be non-erosive during the largest design storm that is
routed through the Constructed Wetland.
11.12.5.3 A forebay shall be located at each
major inlet to trap sediment and preserve the capacity of the main treatment
cell.
11.12.5.4 The following
criteria apply to forebay design:
11.12.5.4.1
A major inlet is defined as an individual storm drain inlet pipe or open
channel conveying at least 10% of the Constructed Wetland's contributing RPv
runoff volume.
11.12.5.4.2 The
forebay must be sized to contain 10% of the volume of runoff from the
contributing drainage area for the Resource Protection event.
11.12.5.4.3 Discharge from the forebay shall
be non-erosive.
11.12.6 Constructed Wetland Design Criteria
11.12.6.1 Constructed Wetlands constructed to
meet regulatory stormwater management requirements shall be designed and
constructed in accordance with the USDA NRCS Pond Code 378 as
amended.
11.12.6.2 Constructed
Wetlands shall be designed so that they will:
11.12.6.2.1 Dewater the Fv within 72 hours,
or
11.12.6.2.2 Manage the Fv on
site with no adverse impact. The extents of the Fv shall be clearly
delineated.
11.12.6.3
The lowest discharge elevation on the outlet device shall be located no lower
than the seasonal high groundwater table as determined by subsection 12.1.
11.12.6.4 Liners
11.12.6.4.1 All Traditional Constructed
Wetlands shall be evaluated for feasibility and ability to maintain permanent
pool, including the need for a liner, by a qualified, licensed geotechnical
engineer or geologist. If the design professional chooses not to follow the
recommendations of the geotechnical professional, a signed, sealed and dated
letter from the design professional providing justification for removal of the
liner from the design shall be provided to the Department or Delegated
Agency.
11.12.6.4.2 When the
geotechnical engineer recommends a liner , acceptable options include the
following:
11.12.6.4.2.1 A clay liner having a
minimum compacted thickness of six inches with an additional six inch layer of
engineered wetland soil mix containing a minimum of 35% organic material above
it. Clay used as a liner must meet the following specifications:
11.12.6.4.2.1.1 Permeability of 1x10-6
centimeters per second (cm/sec) using ASTM D-2434 procedure;
11.12.6.4.2.1.2 Plasticity index of not less
than 15% using ASTM D-423/424 procedures;
11.12.6.4.2.1.3 Liquid limit of not less than
30% using ASTM D-2216 procedure;
11.12.6.4.2.1.4 Clay particles passing not
less than 30% using ASTM D-422 procedure; and
11.12.6.4.2.1.5 Compaction of 95% of standard
proctor density using ASTM D-2216 procedure.
11.12.6.4.2.2 Other acceptable measures as
recommended by a qualified geotechnical professional.
11.12.6.5 Trash Racks.
11.12.6.5.1 Trash racks shall be provided for
low-flow pipes and for all riser structure openings.
11.12.6.5.2 All metal trash racks shall be
coated with a rust inhibitor to increase longevity of the device.
11.12.6.6 When a riser is used, it
must be located such that it is accessible from the side slope for the purposes
of inspection and maintenance .
11.12.6.7 Safety Features
11.12.6.7.1 Any opening 12 inches or greater
discharging to a closed drainage system shall include safety grates.
11.12.6.7.2 The emergency spillway must be
located so that downstream structures will not be adversely impacted by
spillway discharges.
11.12.6.7.3
The emergency spillway exit channel must be designed to direct runoff to a
point of discharge without adverse impact to downstream structures.
11.12.6.8 All Constructed Wetlands
must be designed so as to be accessible for maintenance .
11.12.6.8.1 Adequate maintenance access must
extend to the forebays, micropools, riser, and outlet structure.
11.12.6.8.2 A maintenance right-of-way or
easement must extend to the Constructed Wetland from a public or private
road.
11.12.6.8.3 Maintenance
access must meet the following criteria:
11.12.6.8.3.1 Minimum width of 15
feet.
11.12.6.8.3.2 Profile grade
that does not exceed 10H:1V.
11.12.6.8.3.3 Minimum 10H:1V cross
slope.
11.12.6.9 Maintenance Set-Aside Area
11.12.6.9.1 The maintenance set-aside area
shall accommodate the volume of 50% of the collective forebay volume.
11.12.6.9.2 The maximum depth of the set
aside area shall be one foot.
11.12.6.9.3 The slope of the set aside area
shall not exceed 5.0%.
11.12.6.10 Traditional Constructed Wetlands
11.12.6.10.1 The permanent pool volume, or
the volume below the normal water surface elevation, shall be equivalent to a
minimum of 50% of the RPv volume.
11.12.6.10.2 Traditional Constructed Wetlands
shall be sized so that the RPv has a maximum ponding depth of 12 inches above
the normal water surface elevation.
11.12.6.10.3 The RPv shall dewater within 48
hours.
11.12.6.10.4 The Cv maximum
ponding depth shall not exceed 12 inches above the normal water surface
elevation for more than 12 hours.
11.12.6.10.5 Flow Path
11.12.6.10.5.1 The total length of the flow
path compared to the linear length through the Traditional Constructed Wetland
shall be a minimum ratio of 2:1.
11.12.6.10.5.2 When an inlet is located near
the outlet , the ratio of the shortest flow path through the system to the
overall length shall be a minimum of 0.5:1.
11.12.6.10.5.3 The drainage area served by
any inlets located less than a 0.5:1 ratio shall constitute no more than 20% of
the total contributing drainage area .
11.12.6.10.6 Traditional Constructed Wetlands
shall be composed of the following zones:
11.12.6.10.6.1 Zone 1: Deep Pools.
11.12.6.10.6.1.1 The volume of water stored
in the deep pools, also referred to as micropools, shall be a minimum of 20% of
the RPv volume.
11.12.6.10.6.1.2 A
minimum of two deep pools in addition to the forebay shall be provided, one of
which shall be located prior to the outlet location to provide for additional
sediment deposition.
11.12.6.10.6.1.3 Deep pools shall range from
a minimum of 30 inches to a maximum of six feet in depth below the normal water
surface elevation and shall be designed to remain permanently
saturated.
11.12.6.10.6.1.4 The
deep pools shall be hydraulically connected within the water flow
path.
11.12.6.10.6.1.5 The deep
pools shall be designed with a side slope not steeper than 3:1.
11.12.6.10.6.1.6 A safety bench is required
for deep pool depths greater than four feet.
11.12.6.10.6.2 Zone 2: Transition Zone.
11.12.6.10.6.2.1 Zone 2 is a short transition
zone between the deeper pools and the low marsh zone, and ranges from a minimum
of six inches to a maximum of 30 inches below the normal water surface
elevation.
11.12.6.10.6.2.2 The
volume of water stored in the transition zone shall be a minimum of 20% of the
RPv volume.
11.12.6.10.6.2.3 The
transition zone shall have a maximum side slope of 3:1 from the deep pool to
the low marsh zone.
11.12.6.10.6.3 Zone 3: Low Marsh Zone.
11.12.6.10.6.3.1 The low marsh zone ranges
from a maximum of six inches below the normal water surface elevation to the
normal water surface elevation.
11.12.6.10.6.3.2 The volume of water stored
in the low marsh zone shall be a minimum of 10% of the RPv volume.
11.12.6.10.6.3.3 The side slope within the
low marsh zone shall not be steeper than 4:1.
11.12.6.10.6.4 Zone 4: High Marsh Zone.
11.12.6.10.6.4.1 The upper end of the marsh
zone is the high marsh zone, which ranges from the normal water surface
elevation to a maximum of 12 inches above the normal water surface elevation,
allowing the RPv to inundate to the top of the high marsh zone.
11.12.6.10.6.4.2 The side slope within the
high marsh zone shall not be steeper than 4:1.
11.12.6.10.6.5 Zone 5: Floodplain .
11.12.6.10.6.5.1 A low floodplain shall range
between a minimum of 12 inches and a maximum of 18 inches above the normal
water surface elevation and be planted with plants suited for infrequent to
temporary saturations.
11.12.6.10.6.5.2 The side slope within the
floodplain shall not be steeper than 4:1.
11.12.6.10.7 A minimum 10-foot-wide vegetated
perimeter around the wetland area shall be planted with appropriate grasses,
trees, and shrubs.
11.12.6.10.8 A
simple water balance calculation shall be performed to ensure that the deep
pools will not go completely dry during a 30-day summer drought.
11.12.6.11 Wetland Swales
11.12.6.11.1 Wetland Swale Sizing.
11.12.6.11.1.1 Wetland swales shall contain
the Cv event.
11.12.6.11.1.2 If the
Fv event is not contained within the Wetland swale top of bank, then the area
of inundation and discharge route shall be delineated.
11.12.6.11.1.3 The maximum RPv water surface
elevation shall be no greater than six inches above the normal water surface
elevation.
11.12.6.11.2
The average groundwater elevation as determined by subsection 12.1 shall be
below the bottom of the Wetland Swale. Only the seasonal high groundwater as
determined by the Soil Investigation Procedures may intersect the bottom of the
Wetland Swale.
11.12.6.11.3 Wetland
Swales shall not have side slopes steeper than 3:1.
11.12.6.11.4 The maximum longitudinal slope
shall be an average of 1.0%.
11.12.6.11.5 A minimum 10-foot-wide vegetated
perimeter on both sides of the wetland swale shall be planted with appropriate
grasses, trees and shrubs.
11.12.6.12 Ephemeral Constructed Wetlands
11.12.6.12.1 Ephemeral Constructed Wetland
Sizing.
11.12.6.12.1.1 The RPv event shall
pond a minimum of six inches and a maximum of 12 inches of water above the
ground surface of the Ephemeral Constructed Wetland.
11.12.6.12.1.2 The Fv water surface shall be
a maximum of 30 inches above the ground surface of the Ephemeral Constructed
Wetland.
11.12.6.12.1.3 The average
groundwater elevation as determined by the Soil Investigation Procedures shall
be below the wetland bottom of the Ephemeral Constructed Wetland. Only the
seasonal high groundwater as determined by subsection 12.1 may intersect the
bottom of the Ephemeral Constructed Wetland. If the seasonal high groundwater
intersects the bottom of the Ephemeral Constructed Wetland, the wetland shall
be modeled considering the elevation of the seasonal high
groundwater .
11.12.6.12.2 The side slopes of the buffer
area and within the wetland shall be 4:1 or flatter.
11.12.6.12.3 A minimum 10-foot-wide vegetated
perimeter around the wetland area shall be planted with appropriate grasses,
trees, and shrubs.
11.12.6.13 Submerged Gravel Wetlands
11.12.6.13.1 Submerged Gravel Wetland Sizing.
11.12.6.13.1.1 The maximum surface ponding
depth for the RPv shall not be greater than the tolerance depths of the wetland
plantings selected, or two feet, whichever is less.
11.12.6.13.1.2 The Submerged Gravel Wetland
shall store the RPv volume within the stone substrate and wetland soils and
above the soils in surface ponding.
11.12.6.13.1.3 Submerged Gravel Wetlands
shall have no minimum detention time.
11.12.6.13.2 Gravel substrate.
11.12.6.13.2.1 The gravel substrate shall be
a minimum of two feet and a maximum of four feet in depth.
11.12.6.13.2.2 The gravel substrate shall be
sized to contain a minimum of 25% of the RPv volume considering 40% void
ratio.
11.12.6.13.2.3 The gravel
substrate shall be composed of clean washed gravel, with a maximum of 2.0%
passing the #200 sieve. Gravel shall have a maximum diameter of 2.5 inches and
a minimum diameter of 0.5 inches. A porosity value of 0.4 shall be used for
areas of stone in the design of gravel substrate. Sand shall not be an
acceptable substitute for gravel.
11.12.6.13.3 An engineered wetland soil layer
containing a minimum of 15% organic material and a maximum of 15% clay content
shall be included on the surface of the Submerged Gravel Wetland. The wetland
soil layer shall be a minimum of eight inches thick.
11.12.6.13.4 A minimum four inch thick layer
of clean, washed nominal ¼ inch gravel with a maximum of 2.0% passing
the #200 sieve shall be installed between the gravel substrate and the wetland
soil layer.
11.12.6.13.5
Underdrain.
11.12.6.13.5.1 An underdrain
shall be provided at an elevation three inches above the invert of the gravel
substrate.
11.12.6.13.5.2 The
underdrain shall be a minimum of four inch perforated high density polyethylene
pipe (HDPE) or polyvinyl chloride pipe (PVC).
11.12.6.13.5.3 The underdrain shall connect
to the outlet structure. The discharge elevation shall be four inches below the
wetland soil surface.
11.12.6.13.6 There shall be a minimum of 15
feet separation distance between all gravel substrate inflow points and all
underdrain outlet points.
11.12.6.13.7 Side slopes above the gravel
substrate shall not be steeper than 3:1.
11.12.7 Constructed Wetland Landscaping
Criteria
11.12.7.1 A planting plan is required
for all Constructed Wetlands . Invasive species shall not be specified within
Constructed Wetlands .
11.12.7.2 The
planting plan shall be certified by a qualified professional with demonstrated
knowledge in wetland species.
11.12.7.3 Plants used in Constructed Wetlands
shall be supplied by a certified wetland nursery using plants selected for the
region.
11.12.8
Constructed Wetland Construction
11.12.8.1
Approval from the Department or the appropriate Delegated Agency must be
obtained before any planned Constructed Wetlands can be used as a sediment
basin. If a Constructed Wetlands serves as a sediment basin during project
construction, the volume of the sediment basin must be based on the more
stringent sizing rule.
11.12.8.2
The Sediment and Stormwater Plan must include conversion steps from sediment
basin to permanent Constructed Wetlands in the construction sequence.
11.12.8.2.1 The Department or Delegated
Agency must be notified and provide approval prior to conversion from sediment
basin to the final configuration of the Constructed Wetlands.
11.12.8.2.2 Appropriate procedures must be
implemented to prevent discharge of turbid waters when the sediment basin is
being converted into a Constructed Wetland.
11.12.8.3 Construction reviews are required
during the following stages of construction , and shall be noted on the plan in
the sequence of construction :
11.12.8.3.1
Pre-construction meeting;
11.12.8.3.2 Initial site preparation
including installation of erosion and sediment controls;
11.12.8.3.3 Construction of the embankment,
including installation of the principal spillway and the outlet structure as
applicable;
11.12.8.3.4 Excavation
and grading including interim and final elevations;
11.12.8.3.5 Construction of wetland features
including grading of the various zones, introduction of soil amendments and
staking of planting zones;
11.12.8.3.6 Construction of the underdrain,
installation of gravel substrate and wetland soils as applicable;
11.12.8.3.7 Implementation of the planting
plan and vegetative stabilization; and
11.12.8.3.8 Final inspection including
development of a punch list for facility acceptance.
11.12.8.4 All areas surrounding the
Constructed Wetlands that are graded or denuded during construction must be
planted with turf grass, native plantings, or other approved methods of soil
stabilization.
11.12.8.5 Outside of
optimum seeding and planting dates, temporary seed, such as annual rye or
winter wheat, may be used to stabilize the soil within the Constructed Wetland,
but permanent species shall then be planted or seeded during the next optimum
planting period.
11.12.8.6
Stabilization matting shall be utilized in Wetland Swales and in all areas of
concentrated flow or slopes 3:1 or steeper.
11.12.8.7 Upon facility completion, the owner
shall submit Post Construction verification documents to demonstrate that the
Constructed Wetlands has been constructed within allowable tolerances in
accordance with the approved Sediment and Stormwater Management Plan and
accepted by the approving agency. Allowable tolerances for Constructed Wetlands
practices are as follows:
11.12.8.7.1 The
constructed top of bank elevation may be no lower than the design elevation for
top of bank.
11.12.8.7.2 The
constructed volume of the Constructed Wetlands surface storage and any other
required volumes shall be no less than 90% of the design volume.
11.12.8.7.3 The constructed volume of the
gravel substrate storage for Submerged Gravel Wetlands shall be no less than
90% of the design volume.
11.12.8.7.4 The constructed elevation of any
structure shall be within 0.15 foot of the design.
11.12.8.8 When the allowable tolerances are
exceeded for Constructed Wetlands surface area or volume or structure
elevations, supplemental calculations must be submitted to the approval agency
to determine if the Constructed Wetlands , as constructed, meets the design
requirements.
11.12.9
Constructed Wetland Maintenance Criteria
11.12.9.1 During the first two years
following construction , the Constructed Wetland shall be reviewed twice each
year by a qualified professional with demonstrated knowledge of wetland
species, once in the spring and once in the fall after a storm event that
exceeds one-half inch of rainfall. The Operation and Maintenance Plan shall
outline a detailed schedule for the monitoring and possible reinstallation of
vegetation in the wetland and its buffer for the first two years of
establishment.
11.12.9.2 Repair of
critical structural features such as embankments and risers shall be performed
by responsible personnel that have successfully completed the Department
Contractor Training Program.
11.12.9.3 Project closeout shall not occur
until a minimum of 70% of the wetland area is permanently vegetated.
11.12.9.4 Sediment removal in the
pretreatment forebay shall occur when 50% of total forebay capacity has been
lost.
11.12.9.5 The Department or
the Delegated Agency shall be notified before a Constructed Wetland is
drained.
11.13
Wet Ponds
11.13.1 Wet Ponds are stormwater
storage practices that consist of a combination of a permanent pool, micropool,
or shallow marsh that promote a good environment for gravitational settling,
biological uptake and microbial activity . Wet Ponds are widely applicable for
most land uses and are best suited for larger drainage areas. Runoff from each
new storm enters the wet pond and partially displaces pool water from previous
storms. The pool also acts as a barrier to re-suspension of sediments and other
pollutants deposited during prior storms. When sized properly, Wet Ponds have a
residence time that ranges from many days to several weeks, which allows
numerous pollutant removal mechanisms to operate. Wet Ponds can also provide
storage above the permanent pool to help meet stormwater management
requirements for larger storms. Design variants include:
11.13.1.1 Wet Quantity Management Pond
11.13.1.1.1 The RPv runoff reduction
performance credit for this variant is 0%.
11.13.1.1.2 The Cv runoff reduction
performance credit for this variant is 0%.
11.13.1.1.3 The Fv runoff reduction
performance credit for this variant is 0%.
11.13.1.1.4 The total nitrogen pollutant
reduction performance credit for this variant is not less than 0%.
11.13.1.1.5 The total phosphorus pollutant
reduction performance credit for this variant is not less than 0%.
11.13.1.1.6 The total suspended solids
pollutant reduction performance credit for this variant is not less than
0%.
11.13.1.2 Wet
Extended Detention (ED) Pond
11.13.1.2.1 The
RPv runoff reduction performance credit for this variant having 48 hours of
extended detention is 100%.
11.13.1.2.2 The Cv runoff reduction
performance credit for this variant having 48 hours of extended detention is
1%.
11.13.1.2.3 The Fv runoff
reduction performance credit for this variant having 48 hours of extended
detention is 0%.
11.13.1.2.4 The
total nitrogen pollutant reduction performance credit for this variant having
48 hours of extended detention is not less than 30%.
11.13.1.2.5 The total phosphorus pollutant
reduction performance credit for this variant having 48 hours of extended
detention is not less than 55%.
11.13.1.2.6 The total suspended solids
pollutant reduction performance credit for this variant having 48 hours of
extended detention is not less than 60%.
11.13.2 Wet Ponds constructed to meet
regulatory stormwater management requirements shall be designed and constructed
in accordance with the USDA NRCS Pond Code 378 as amended.
11.13.3 Wet Pond Feasibility Criteria
11.13.3.1 Soil investigations must be
conducted in accordance with subsection 12.1 to determine the suitability of
the soils to meet recommended embankment and permanent pool criteria.
11.13.3.2 Locating Wet Ponds within perennial
streams will require all appropriate state or federal permits.
11.13.4 Wet Pond Conveyance
Criteria
11.13.4.1 Principal Spillway
11.13.4.1.1 The principal spillway must be
accessible from dry land.
11.13.4.1.2 A structure-pipe spillway shall
be designed with anti-flotation, anti-vortex and trash rack devices on the
structure.
11.13.4.1.3 The outfall
pipe and all connections to the outfall structure shall be made watertight.
Soil tight only joints are not acceptable.
11.13.4.1.4 Anti-seep collars shall be used
in accordance with Pond Code 378, as amended.
11.13.4.1.5 When the principal spillway is
composed of a weir wall discharging to a channel, the channel below the weir
must be reinforced with riprap or other acceptable material to prevent
scour.
11.13.4.1.6 When a low flow
orifice is specified, it must be adequately protected from clogging by either
an acceptable external trash rack or by internal orifice protection. Orifice
diameters shall not be less than three inches unless internal orifice control
is provided.
11.13.4.2
The design shall specify an outfall that can discharge the maximum design storm
event in a non-erosive manner at the project point of discharge.
11.13.4.3 Wet Ponds must be designed to pass
the maximum design storm event (Fv) if the Fv is being routed through the Wet
Pond rather than bypassing. An earthen emergency spillway designed to convey
the Fv shall be cut in natural ground or, if cut in fill, shall be constructed
and stabilized with methods to prevent erosion and structural
failure.
11.13.4.4 Inflow Points
11.13.4.4.1 Inflow points into the Wet Pond
must be stabilized to ensure that non-erosive conditions exist during storm
events up to the conveyance event (Cv).
11.13.4.4.2 A forebay shall be provided at
each inflow location that provides 10% or greater of the total RPv inflow to
the Wet Pond.
11.13.4.5
In the event that the embankment is a regulated dam , the designer must verify
that the appropriate Dam Safety Permit has been approved by the Department 's
Dam Safety Program.
11.13.5 Wet Pond Pretreatment Criteria
11.13.5.1 A forebay must be located at each
major inlet to trap sediment and preserve the capacity of the main treatment
cell.
11.13.5.2 The following
criteria apply to forebay design:
11.13.5.2.1
A major inlet is defined as an individual storm drain inlet pipe or open
channel conveying at least 10% of the Wet Pond's contributing RPv runoff
volume.
11.13.5.2.2 A safety bench
is required at the pond shoreline for forebay depths greater than three
feet.
11.13.5.2.3 The forebay must
be sized to contain 10% of the volume of runoff from the contributing drainage
area for the Resource Protection event.
11.13.5.2.4 Discharge from the forebay shall
be non-erosive.
11.13.6 Wet Pond Design Criteria
11.13.6.1 For RPv compliance, a Wet ED Pond
must provide 48 hours extended detention for the RPv runoff volume. Detention
shall be based on the time of initial inflow to time of final outflow from the
facility . In order to simulate a base flow condition to the extent practicable,
the peak discharge for the outflow hydrograph shall not exceed five times the
average discharge rate.
11.13.6.2
The minimum depth to prevent the permanent pool area from being overtaken by
undesirable vegetation is four feet.
11.13.6.3 The maximum depth of the permanent
pool shall not exceed eight feet for safety reasons.
11.13.6.4 Earthen side slopes for Wet Ponds
both above and below permanent pool shall be no steeper than 3H:1V.
11.13.6.5 Excluding areas containing
retaining walls, when Wet Pond side slopes above permanent pool are steeper
than 4H:1V, a ten foot wide safety bench shall be constructed one foot above
the permanent pool. The maximum slope of the safety bench shall be
5.0%.
11.13.6.6 A 10 foot wide
aquatic bench shall be provided one foot below permanent pool.
11.13.6.7 Retaining walls
11.13.6.7.1 Retaining walls around Wet Ponds
shall be limited to no more than 50% of the pond perimeter based upon the peak
elevation of the Cv.
11.13.6.7.2 In
order to maintain the safety requirements, retaining walls shall be configured
as follows:
11.13.6.7.2.1 The retaining wall
at the permanent pool shall have a maximum height of three feet above the
aquatic bench.
11.13.6.7.2.2 Any
additional retaining walls shall have a maximum height of two feet and provide
a minimum 10-foot level terrace from a lower retaining wall.
11.13.6.8 Liners
11.13.6.8.1 All Wet Ponds shall be evaluated
for feasibility and ability to maintain permanent pool, including the need for
a liner, by a qualified, licensed geotechnical engineer or geologist. If the
design professional chooses not to follow the recommendations of the
geotechnical professional, a signed, sealed and dated letter from the design
professional providing justification for removal of the liner from the design
shall be provided to the Department or Delegated Agency.
11.13.6.8.2 When the geotechnical engineer
recommends a liner , acceptable options include the following:
11.13.6.8.2.1 A clay liner having a minimum
compacted thickness of 12 inches with an additional 12 inch layer of compacted
soil above it. Clay used as a pond liner must meet the following
specifications:
11.13.6.8.2.1.1 Permeability
of 1x10-6 centimeters per second (cm/sec) using ASTM D-2434
procedure.
11.13.6.8.2.1.2
Plasticity index of not less than 15% using ASTM D-423/424
procedures.
11.13.6.8.2.1.3 Liquid
limit of not less than 30% using ASTM D-2216 procedure.
11.13.6.8.2.1.4 Clay particles passing not
less than 30% using ASTM D-422 procedure.
11.13.6.8.2.1.5 Compaction of 95% of standard
proctor density using ASTM D-2216 procedure.
11.13.6.8.2.2 A 30 mil poly-liner ;
or
11.13.6.8.2.3 Other acceptable
measures as recommended by a qualified geotechnical professional.
11.13.6.9 Trash racks
shall be provided for low-flow pipes and for all riser structure openings.
11.13.6.9.1 All metal trash racks shall be
coated with a rust inhibitor to increase longevity of the device.
11.13.6.9.2 The low flow extended detention
orifice shall be protected from clogging by an external trash rack.
11.13.6.10 When a riser is used,
it must be located such that it is accessible from the pond side slope or
safety bench for the purposes of inspection and maintenance .
11.13.6.11 All materials used in construction
of a Wet Quantity Management Pond or Wet ED Pond shall meet the material
specifications in USDA NRCS Pond Code 378 as amended.
11.13.6.12 Safety Features
11.13.6.12.1 Any opening 12 inches or greater
discharging to a closed drainage system shall include safety grates.
11.13.6.12.2 The emergency spillway and exit
channel must be designed to direct runoff to a point of discharge without
adversely impacting downstream structures.
11.13.6.13 All Wet Ponds must be designed so
as to be accessible for maintenance .
11.13.6.13.1 Adequate maintenance access must
extend to the pretreatment, safety bench, riser, and outlet
structure.
11.13.6.13.2 A
maintenance right-of-way or easement must extend to the Wet Pond from a public
or private road.
11.13.6.13.3
Maintenance access must meet the following criteria:
11.13.6.13.3.1 Minimum width of 15
feet.
11.13.6.13.3.2 Profile grade
that does not exceed 10H:1V.
11.13.6.13.3.3 Minimum 10H:1V cross
slope.
11.13.6.14 Maintenance Set-Aside Area
11.13.6.14.1 The maintenance set-aside area
shall accommodate the volume of 50% of the collective forebay volume.
11.13.6.14.2 The maximum depth of the set
aside area shall be one foot.
11.13.6.14.3 The slope of the set aside area
shall not exceed 5.0%.
11.13.7 Wet Pond Landscaping Criteria
11.13.7.1 Woody vegetation shall not be
planted or allowed to grow within 15 feet of the embankment and 10 feet on
either side of principal spillway or pipes.
11.13.7.2 A planting plan must be provided
that indicates the methods used to establish and maintain vegetative coverage
in the Wet Pond and its vegetated perimeter. Minimum elements of a planting
plan include the following:
11.13.7.2.1
Delineation of zones within both the Wet Pond and vegetated perimeter
area;
11.13.7.2.2 Selection of
corresponding plant species; and
11.13.7.2.3 Size and spacing of plant
material or application rate of seed mixes, as applicable.
11.13.7.2.3.1 Native plant material shall be
specified by botanical and common name.
11.13.7.2.3.2 Seed mixes shall be specified
by botanical and common names as well as percentages by weight or
volume.
11.13.8 Wet Pond Construction
11.13.8.1 Use of Wet Ponds for Erosion and
Sediment Control
11.13.8.1.1 Approval from the
Department or the appropriate Delegated Agency must be obtained before any
planned Wet Quantity Management Pond or Wet ED Pond can be used as a sediment
basin.
11.13.8.1.2 If a Wet Pond
serves as a sediment basin during project construction, the volume of the
sediment basin must be based on the more stringent sizing rule.
11.13.8.1.3 The Sediment and Stormwater Plan
must include conversion steps from sediment basin to permanent Wet Pond in the
construction sequence.
11.13.8.1.3.1 The
Department or Delegated Agency must be notified and provide approval prior to
conversion from sediment basin to the final configuration of the Wet Quantity
Management Pond or Wet ED Pond.
11.13.8.1.3.2 Appropriate procedures must be
implemented to prevent discharge of turbid waters when the sediment basin is
being converted into a Wet Pond.
11.13.8.2 Construction reviews are required
during the following stages of construction , and shall be noted on the plan in
the sequence of construction :
11.13.8.2.1
Pre-construction meeting.
11.13.8.2.2 Initial site preparation
including installation of erosion and sediment controls.
11.13.8.2.3 Construction of the embankment,
including installation of the principal spillway and the outlet
structure.
11.13.8.2.4 Excavation
and grading including interim and final elevations.
11.13.8.2.5 Implementation of the planting
plan and vegetative stabilization.
11.13.8.2.6 Final construction review
including development of a punch list for facility acceptance.
11.13.8.3 All areas surrounding
the Wet Pond that are graded or denuded during construction must be planted
with turf grass, native plantings, or other approved methods of soil
stabilization.
11.13.8.4 Upon
facility completion, the owner shall submit post construction verification
documents to demonstrate that the wet pond has been constructed within
allowable tolerances and in accordance with the approved Sediment and
Stormwater Management Plan and accepted by the approving agency. Allowable
tolerances for wet pond practices are as follows:
11.13.8.4.1 The constructed top of bank
elevation may be no lower than the design elevation for top of bank.
11.13.8.4.2 The constructed volume of the wet
pond surface storage shall be no less than 90% of the design volume.
11.13.8.4.3 The constructed elevation of any
structure shall be within 0.15 foot of the design.
11.13.8.5 When the allowable tolerances are
exceeded for wet pond surface area or volume or structure elevations,
supplemental calculations must be submitted to the approval agency to determine
if the wet pond, as constructed, meets the design requirements.
11.13.9 Wet Pond Maintenance
Criteria
11.13.9.1 Repair of critical
structural features, such as embankments and risers, shall be performed by
responsible personnel that have successfully completed the Department
Contractor Training Program.
11.13.9.2 The Department or the Delegated
Agency shall be notified before a Wet Pond is drained.
11.13.9.3 Sediment removal in the Wet Pond
pretreatment forebay must occur when 50% of total forebay capacity has been
lost.
11.14
Soil Amendments
11.14.1 Soil Amendment, also
called soil restoration , is a technique applied after construction to till
compacted soils and restore their porosity by amending them with compost. Soil
amendments reduce the generation of runoff from compacted urban lawns and may
also enhance the performance of impervious cover disconnections and grass
channels.
11.14.2 Soil Amendment
Stormwater Credit Calculations
11.14.2.1 Soil
Amendments receive the following runoff reduction performance credits. Runoff
reduction allowances are applied to the amendment area only.
11.14.2.1.1 The RPv runoff reduction
performance credit for Soil Amendments is based upon the hydrologic soil group
(HSG) of the existing soil:
11.14.2.1.1.1 HSG
A receives 48% annual runoff reduction credit.
11.14.2.1.1.2 HSG B receives 50% annual
runoff reduction credit.
11.14.2.1.1.3 HSG C receives 29% annual
runoff reduction credit.
11.14.2.1.1.4 HSG D receives 13% annual
runoff reduction credit.
11.14.2.1.2 The Cv runoff reduction
performance credit for soil amendments is 10% of the RPv allowance.
11.14.2.1.3 The Fv runoff reduction
performance credit for soil amendments is 1% of the RPv allowance.
11.14.2.2 The total nitrogen,
total phosphorus, and total suspended solids pollutant reduction performance
credit for soil amendments is 100% of the load reduction.
11.14.3 Soil Amendments shall not be applied
where:
11.14.3.1 The water table or bedrock is
located within two feet of the soil surface. Subsection 12.1 shall be followed
for determination of depth to the limiting layer.
11.14.3.2 Slope of soil to be amended exceeds
10%.
11.14.3.3 Soil to be amended
is saturated or seasonally wet.
11.14.4 Soil Amendment Design Criteria
11.14.4.1 Soil testing shall be conducted
during two stages of the Soil Amendment process.
11.14.4.1.1 The first test shall be performed
to determine soil properties to a depth one foot below the proposed soil
amendment area, with respect to saturation, bulk density, pH, salts, and soil
nutrients . The initial test shall determine what soil amendments are
needed.
11.14.4.1.2 The second soil
test shall be conducted at least one week after compost has been incorporated
into the soils to determine whether any further nutritional requirements, pH
adjustment, and organic matter adjustments are necessary for plant
growth.
11.14.4.2 When
Soil Amendments are used to either adjust the hydrologic soil group of the
amended area to lower the curve number of the site , or receive the annual
runoff reduction performance credits for the amendment area, the soil amendment
area shall receive no impervious cover runoff and shall place three inches of
compost into the soil amendment area to a minimum incorporation depth of six
inches using a tiller.
11.14.4.3
When Soil Amendments are used within the footprint of a BMP such as sheet flow
to filter strip, sheet flow to open space , or vegetated channels to adjust the
hydrologic soil group of the amended area and receive the runoff reduction
performance credits for those BMPs, the following criteria apply:
11.14.4.3.1 Soil amendment areas having a
contributing impervious cover (square feet) to surface area of compost
amendment (square feet) ratio of up to 0.5 shall place four inches of compost
into the soil amendment area to a minimum incorporation depth of eight inches
using a tiller.
11.14.4.3.2 Soil
amendment areas having a contributing impervious cover (square feet) to surface
area of compost amendment (square feet) ratio of 0.51 to 0.75 shall place six
inches of compost into the soil amendment area to a minimum incorporation depth
of 15 inches using an excavation and mixing method.
11.14.4.3.3 Soil amendment areas having a
contributing impervious cover (square feet) to surface area of compost
amendment (square feet) ratio greater than 0.75 shall place eight inches of
compost into the soil amendment area to a minimum incorporation depth of 20
inches using an excavation and mixing method.
11.14.4.4 Compost incorporation depths
greater than 12 inches require removal of the existing soil down to the
incorporation depth and physically mixing existing soil with compost.
11.14.4.5 Compost used for soil amendment
shall be STA certified compost , meeting the requirements of Delaware Erosion
and Sediment Control Handbook Appendix A-6 Compost Material
Properties.
11.14.5 Soil
Amendment Construction Criteria
11.14.5.1 For
compost incorporation depths up to 12 inches:
11.14.5.1.1 The proposed incorporation area
shall be deep tilled to a depth of two to three feet using a tractor and
sub-soiler. This deep-tilling step may be omitted when soil amendment is used
for filter strip widths of 20 feet or less in the direction of flow.
11.14.5.1.2 Existing soils shall be in dry
condition prior to incorporating compost .
11.14.5.1.3 The compost layer shall be placed
on surface of proposed amendment area to the depth specified and then
incorporated into the soil using a roto-tiller or similar equipment.
11.14.5.1.4 Conduct soil test to determine
whether any further nutrient requirements, pH adjustment, and organic matter
adjustments are necessary for plant growth.
11.14.5.2 For compost incorporation depths 12
inches or greater:
11.14.5.2.1 The proposed
amendment area shall be excavated to the required incorporation depth, as
follows:
11.14.5.2.1.1 Remove topsoil and
stockpile for later use.
11.14.5.2.1.2 Excavate subsoil working in
strips perpendicular to the slope and flow path using multiple lifts.
11.14.5.2.1.3 Separate and remove a minimum
of 25% of the subsoil, taking the most densely compacted soils for removal .
Stockpile remaining subsoil next to excavated area, separately from
topsoil.
11.14.5.2.1.4 Scarify
bottom of excavated area.
11.14.5.2.2 Amended soil shall be returned to
the soil amendment area as follows. The number of lifts may vary depending on
the capabilities of the equipment being used, but a minimum of two lifts is
required.
11.14.5.2.2.1 Replace subsoils by
loosening, aerating, and mixing subsoil.
11.14.5.2.2.2 Replace stockpiled
topsoil .
11.14.5.2.2.3 Incorporate
required layer of compost , such that compost is uniformly incorporated
throughout. Existing soils shall be in dry condition prior to incorporating
compost .
11.14.5.2.2.4 Repeat above
steps for each lift .
11.14.5.2.3 Rake to level and remove surface
woody debris and rocks larger than one inch.
11.14.5.2.4 The finished grade of the
combination of replaced subsoil, topsoil and compost shall be a minimum of four
inches above the existing grade to account for settlement, but must be adjusted
to account for field conditions and soil texture, such that a final settled
grade at three months post-installation matches the original grade.
11.14.5.2.5 Conduct soil test to determine
whether any further nutritional requirements, pH adjustment, and organic matter
adjustments are necessary for plant growth.
11.14.5.3 Construction reviews are required
during the following stages of construction , and shall be noted on the plan in
the sequence of construction :
11.14.5.3.1
Pre-construction meeting;
11.14.5.3.2 Initial site preparation
including installation of erosion and sediment controls;
11.14.5.3.3 Deep tillage using subsoiler or
excavation of existing subsoil;
11.14.5.3.4 Incorporation of compost
amendment into existing soil including verification of the depth of compost
amendment;
11.14.5.3.5
Implementation of required stabilization and planting plan; and
11.14.5.3.6 Final construction review
including development of a punch list for facility acceptance.
11.14.5.4 Upon project completion,
the owner shall submit Post Construction verification documents, including but
not limited to compost delivery tickets and photo documentation of
construction, to demonstrate that the soil amendment has been constructed
within in accordance with the approved Sediment and Stormwater Management Plan
and accepted by the approving agency.
11.15 Proprietary Practices
11.15.1 Proprietary Practices are
manufactured stormwater treatment practices that utilize settling, filtration,
absorptive/adsorptive materials, vortex separation, vegetative components, or
other appropriate technology to manage the impacts caused by stormwater
runoff.
11.15.2 Proprietary
Practices Stormwater Credit Calculations
11.15.2.1 Proprietary Practices receive no
runoff reduction credits unless approved by the Department .
11.15.2.2 Practices may receive pollutant
reduction credits as determined by the Department on a case-by-case
basis.
11.15.3 All
proprietary practices shall be designed to safely overflow or bypass flows from
larger storm events to downstream drainage systems.
11.15.4 Pretreatment shall be provided in
accordance with manufacturer's recommendations for individual Proprietary
Practices.
11.15.5 Proprietary
Practice Design Criteria
11.15.5.1 Design
criteria for Proprietary Practices shall be proposed by the manufacturer and
approved by the Department .
11.15.5.2 All Proprietary Practices must be
designed so as to be accessible for maintenance .
11.15.5.2.1 A maintenance right-of-way or
easement must extend to the Proprietary Practice from a public or private
road.
11.15.5.2.2 Adequate
maintenance access must extend to the all components of the Proprietary
Practice.
11.15.5.2.3 Maintenance
access must meet the following criteria:
11.15.5.2.3.1 Minimum width of fifteen
feet.
11.15.5.2.3.2 Profile grade
that does not exceed 10H:1V.
11.15.5.2.3.3 Minimum 10H:1V cross
slope.
11.15.6 Landscaping shall be provided in
accordance with manufacturer's recommendations for individual Proprietary
Practices.
11.15.7 Proprietary
Practice Construction Sequence
11.15.7.1
Construction and installation of Proprietary Practices shall be conducted in
accordance with manufacturer's recommendations for individual Proprietary
Practices.
11.15.7.2 Construction
reviews are required during the following stages of construction , and shall be
noted on the plan in the sequence of construction :
11.15.7.2.1 Pre-construction
meeting;
11.15.7.2.2 Initial site
preparation including installation of erosion and sediment controls;
11.15.7.2.3 Construction of the Proprietary
Practice in accordance with manufacturer's recommendations;
11.15.7.2.4 Implementation of required
stabilization and planting plan as applicable; and
11.15.7.2.5 Final construction review
including development of a punch list for facility acceptance.
11.15.7.3 Upon project completion,
the owner shall submit Post Construction verification documents to demonstrate
that the Proprietary Device has been installed in accordance with
manufacturer's recommendations.
11.15.7.4 All Proprietary Practices shall be
inspected and maintained in accordance with the manufacturer's instructions and
recommendations.
11.16 Source Controls
11.16.1 Source Control consists of measures
to prevent pollutants from coming into contact with stormwater runoff.
Preventing pollutant exposure to rainfall and runoff is an important management
technique that can reduce the amount of pollutants in runoff and the need for
stormwater treatment.
11.16.2
Design variants for Source Controls include:
11.16.2.1 Nutrient Management
11.16.2.2 Street Sweeping
11.16.3 Source Controls Stormwater
Credit Calculations
11.16.3.1 Source controls
receive no runoff reduction credits.
11.16.3.2 Nutrient Management receives the
following pollutant reduction credits:
11.16.3.2.1 The total nitrogen pollutant
reduction performance credit is not less than 17%.
11.16.3.2.2 The total phosphorous pollutant
reduction performance credit is not less than 22%.
11.16.3.2.3 The total suspended solids
pollutant reduction performance credit is not less than 0%.
11.16.3.3 Street Sweeping receives
the following pollutant reduction credits:
11.16.3.3.1 The total nitrogen pollutant
reduction performance credit is not less than 3%.
11.16.3.3.2 The total phosphorous pollutant
reduction performance credit is not less than 3%.
11.16.3.3.3 The total suspended solids
pollutant reduction performance credit is not less than 9%.
11.16.4 Source Controls
Design Summary
11.16.4.1 To receive nutrient
management pollutant reduction performance credits, sites must fully comply
with the requirements of the Delaware Nutrient Management Law through
implementation of a nutrient management plan.
11.16.4.2 To receive street sweeping
pollutant reduction performance credits, sites must submit to the Department or
Delegated Agency a plan documenting the street sweeping frequency. Annual
street sweeping tracking shall be submitted to the Department or Delegated
Agency.
11.17
Afforestation
11.17.1 Afforestation includes
practices that mimic the hydrologic benefits of a natural forest utilizing a
regeneration process within the landscape by selectively planting tree
seedlings (less than one inch DBH) or saplings (greater than one inch DBH).
Afforestation can be used as both a runoff reduction practice by converting
non-forested areas to forested areas as well as a mitigation practice for
offsetting the clearing of forested areas during the development process.
Design variants for afforestation include:
11.17.1.1 Afforestation. RPv credit based on
runoff reduction from open space (good) condition to wooded (good)
condition.
11.17.1.2 Urban Tree
Planting. For areas planted in trees that do not meet the design criteria for
afforestation, credit shall be an equivalent 1/200th of an acre per
tree.
11.17.2
Afforestation Design Criteria
11.17.2.1 The
minimum size of the afforestation area shall be 10,000 square feet with a
minimum width of 50 feet.
11.17.2.2
The proposed afforestation area shall be upland.
11.17.2.3 At the end of the second year there
shall be at least 200 live plants six inches or higher.
11.17.3 Afforestation Landscaping Criteria
11.17.3.1 The planting density shall account
for mortality, which over time can result in more random arrangement of the
trees.
11.17.3.2 Final
stabilization shall meet EPA requirements at the end of the second
year.
11.17.4
Afforestation Construction Sequence
11.17.4.1
The sizes and types of plantings shall be in accordance with the planting
schedule developed for the site .
11.17.4.2 Construction reviews are necessary
for the success of any phase of a project , including Pre-construction meeting;
Planting Phase (with designer and installation contractor); and Final Review
(punch list of corrections for acceptance).
11.17.5 Afforestation Maintenance Criteria
11.17.5.1 Following planting, a period of
maintenance and monitoring will begin. The afforestation planting will be
considered successful if the survival of trees at the end of the second year is
at least 200 combined live, planted or volunteer, trees per acre.
Notes
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No prior version found.