Requirements that shall be part of the completed plan but retained in the permanent Department files for review by the Commission upon request include the following:
(1) Stormwater management and stormwater drainage computations, used in the design of pipe culverts, channels, inlets, ditches and other components of the stormwater management and erosion and sediment control systems.
(2) Location of project on Federal Emergency Management Agency flood maps and federal and State wetland maps, where appropriate.
(3) Design reports sealed by a qualified design professional and certified that the reports have been prepared in accordance with these regulations, standards, and specs.
(4) Additional information necessary for a complete project review may be required by the Commission as deemed appropriate. This additional information may include items such as public sewers, water lines, septic fields, wells, etc.
(5) A description of the predominant soil types on the site, as described by the appropriate soil survey information available through the Commission or the local Conservation District.
Specific requirements for the erosion and sediment control portion of the stormwater management and sediment control plan approval process include, but are not limited to, the following items. The Commission may modify the following items for a specific project or type of project.
(1) All plans shall include details and descriptions of temporary and permanent erosion and sediment control measures and other protective measures shown on the stormwater and sediment management plan.
Specifications for a sequence of construction operations shall be contained on all plans describing the relationship between the implementation and maintenance of sediment controls, including permanent and temporary stabilization and the various stages or phases of earth disturbance and construction. The specifications for the sequence of construction shall, at a minimum, include the requirements of Standard Specifications for Highway Construction, Edition of 1986, and standard drawings prepared by the Department.
Changes to the sequence of construction operations may be modified by the Department and do not constitute a violation unless measures to control stormwater runoff and sediment are not utilized.
(3) When work in a live waterway is performed, precautions shall be taken to minimize encroachment, control sediment transport and stabilize the work area to the greatest extent possible during construction.
(4) Vehicle tracking of sediments from land disturbing activities onto paved public roads carrying significant amounts of traffic (ADT of 25 vehicles/day or greater) shall be minimized.
(5) The Department shall initiate appropriate vegetative practices on all disturbed areas within seven (7) calendar days after the last activity at that area. In terms of phased construction, this requirement may require the Department to utilize temporary seeding or other appropriate measures to protect exposed soils.
Specific requirements for the permanent stormwater management portion of the stormwater management and sediment control plan approval process include, but are not limited to, the following items. The Commission may modify the following items for a specific project or type or project.
(1) It is the overall goal of the Commission to address stormwater management on a watershed basis to provide a cost effective water quantity and water quality solution to the specific watershed problems. These regulations will provide general design requirements that must be adhered to in the absence of Designated Watershed specific criteria.
(2) All hydrologic computations shall be accomplished using methods recommended by the American Association of State Highway Transportation Officials (AASHTO). The method used shall be based on the size of the drainage area, the land use, site conditions, and topography. Hydrologic computations are not required when run-off flows in sheet flow down grass covered slopes or when roadway run-off is collected in grass lined ditches constructed on non erodible slopes.
(3) If a highway project crosses several watersheds, the stormwater management requirements for the specific project shall be based on the impacts on each individual watershed. If the project is phased, the initial submittal shall control that area proposed in the initial phase and establish a procedure and obligation for total site control.
Water quantity control is an integral component of overall stormwater management. The following design criteria for flow control is established for water quantity control purposes, unless a waiver is granted based on a case-by-case basis:
(a) The impact of the proposed highway project on the outfall channel should be evaluated by: (1) Evaluate the capacity of the outfall for the 2-and 10-year peak discharges based on prior and post construction conditions. The evaluation should take into account the condition and capacity of existing structures downstream from the outfall point. (2) Compare the time of concentration of the outfall channel with the time of concentration of the proposed development. (3) If the outfall channel has insufficient capacity to carry the system design discharge, channel improvements will be implemented. The design discharge will be the 10-, 25-, or 50-year peak discharge depending on the road classification and the system or structure type. (4) If the 10-year peak discharge on the outfall is increased by the project and if there is development along the outfall that would be impacted by the increased discharge, stormwater management methods shall be used to reduce the 10-year peak discharge to predeveloped levels.
(b) The velocity for the design peak discharge at the outlet of hydraulic structures will be reduced to nonerosive velocity. Ditches and channels will be protected from erosion from the design discharge by the appropriate channel lining.
Water quality control is also an integral component of stormwater management. The following design criteria is established for water quality protection unless a waiver or variance is granted on a case-by-case basis.
(a) When ponds are used for water quality protection, the ponds shall be designed as both quantity and quality control structures. Sediment storage volume shall be calculated considering the clean out and maintenance schedules specified by the Department during the land disturbing activity. Sediment storage volumes may be predicted by the Universal Soil Loss Equation or methods acceptable to the Commission.
(b) Stormwater runoff that drains to a single outlet from land disturbing activities which disturb ten acres or more shall be controlled during the land disturbing activity by a sediment basin where sufficient space and other factors allow these controls to be used until the final inspection. The sediment basin shall be designed and constructed to accommodate the anticipated sediment loading from the land-disturbing activity and meet a removal efficiency of 80 percent suspended solids or 0.5 ML/L peak settable solids concentration, or 3600 cubic feet of sediment storage volume per disturbed acre that drains to a single point excluding off site flows, whichever is less. The outfall device or system design shall take into account the total drainage area flowing through the disturbed area to be served by the basin.
(c) Other practices may be acceptable to the Commission if they achieve an equivalent removal efficiency of 80 percent for suspended solids or 0.5 ML/L peak settable solids concentration, or 3600 cubic feet of sediment storage volume per disturbed acre that drains to a single point excluding off site flows, whichever is less. The efficiency shall be calculated for disturbed conditions for the 10-year 24-hour design event.
(d) Permanent water quality ponds having a permanent pool shall be designed to store and release the first 1/2 inch of runoff from the site over a 24-hour period. The storage volume shall be designed to accommodate, at least, 1/2 inch of runoff from the entire site.
(e) Permanent water quality ponds, not having a permanent pool, shall be designed to release the first inch of runoff from the site over a 24-hour period.
(f) Permanent infiltration practices, when used, shall be designed to accept, at a minimum, the first inch of runoff from all impervious areas.
(g) For activities in the eight coastal counties of Beaufort, Berkeley, Charleston, Colleton, Dorchester, Georgetown, Jasper and Horry, additional water quality requirements may be imposed to comply with the S.C. Coastal Council Stormwater Management Guidelines. If conflicting requirements exist for activities in the eight coastal counties, the S.C. Coastal Council guidelines will apply.
(6) Where ponds are the proposed method of control, the Department shall submit to the approving agency, when required in writing, an analysis of the impacts of stormwater flows downstream in the watershed for the 10-and 100-year storm events. The analysis shall include hydrologic and hydraulic calculations necessary to determine the impact of hydrograph timing modifications of the proposed land disturbing activity, with and without the pond. The results of the analysis will determine the need to modify the pond design or to eliminate the pond requirement. Lacking a clearly defined downstream point of constriction, the downstream impacts shall be established, with the concurrence of the Commission.
(7) Where existing wetlands are intended as a component of an overall stormwater management system, the approved stormwater management and sediment control plan shall not be implemented until all necessary federal and state permits have been obtained.
(8) Designs shall be in accordance with standards developed or approved by the Commission.
(9) Ease of maintenance must be considered as a site design component. Access to the stormwater management structure must be provided.
(10) A maintenance schedule shall be included in the plans and design report.
Infiltration practices have certain limitations on their use on certain sites. These limitations include the following items:
(a) Areas draining to these practices must be stabilized and vegetative filters established prior to runoff entering the system. Infiltration practices shall not be used if a suspended solids filter system does not accompany the practice. If vegetation is the intended filter, there shall be, at least a 20 foot length of vegetative filter prior to stormwater runoff entering the infiltration practice;
(b) The bottom of the infiltration practice shall be at least 0.5 feet above the seasonal high water table, whether perched or regional, determined by direct piezometer measurements which can be demonstrated to be representative of the maximum height of the water table on an annual basis during years of normal precipitation, or by the depth in the soil at which mottling first occurs;
(c) The infiltration practice shall be designed to completely drain of water within 72 hours;
(d) Soils must have adequate permeability to allow water to infiltrate. Infiltration practices are limited to soils having an infiltration rate of least 0.30 inches per hour. Initial consideration will be based on a review of the appropriate soil survey, and the survey may serve as a basis for rejection. On-site soil borings must be used to verify the actual site and seasonal high water table conditions when infiltration is to be utilized;
(e) Infiltration practices greater than three feet deep shall be located at least 10 feet from basement walls;
(f) Infiltration practices designed to handle runoff from impervious parking areas shall be a minimum of 150 feet from any public or private water supply well;
(g) The design of an infiltration practice shall provide an overflow system with measures to provide a non-erosive velocity of flow along its length and at the outfall;
(h) The slope of the bottom of the infiltration practice shall not exceed five percent. Also, the practice shall not be installed in fill material as piping along the fill/natural ground interface may cause slope failure;
(i) An infiltration practice shall not be installed on or atop a slope whose natural angle of incline exceeds 20 percent.
(j) Clean outs will be provided at a minimum, every 100 feet along the infiltration practice to allow for access and maintenance.
All stormwater management and sediment control practices shall be designed, constructed and maintained with consideration for the proper control of mosquitoes and other vectors. Practices may include, but are not limited to:
(1) The bottom of retention and detention ponds should be graded and have a slope not less than 0.5 percent.
(2) There should be no depressions in a normally dry detention facility where water might pocket when the water level is receding.
(3) Normally dry detention systems and swales should be designed to drain within three (3) days.
(4) An aquatic weed control program should be utilized in permanently wet structures to prevent an overgrowth of vegetation in the pond. Manual harvesting is preferred.
(5) Fish may be stocked in permanently wet retention and detention ponds.
(6) Any rutting of normally dry swales and detention ponds caused by maintenance activities shall be immediately filled and smoothed out.