11 Miss. Code. R. 7-3.5 - Design and Maintenance Requirements

A. Any modification, alteration, enlargement, or major repair of an existing dam, whether requested by the owner or directed by the Commission, will be subject to the current design standards for the appropriate hazard classification as set forth in this regulation.

B. High Hazard dams must be capable of safely passing the runoff from a 24 hour duration 100-year rainfall event through the principal spillway without activating the auxiliary (emergency) spillway. The 100- year rainfall event is established by the National Weather Service Atlas 14 and varies for different areas of the state. The runoff from one hundred percent (100%) of the 24 hour Probable Maximum Precipitation (PMP), as defined in Rule 3.1. of this regulation, must be passed through the principal and auxiliary (emergency) spillway and/or stored in the reservoir without overtopping the dam unless an incremental consequence analysis as defined in Rule 3.5 E. of this regulation indicates a lesser inflow design flood is applicable. The appropriate PMP for each county as obtained from NOAA HMR 51 and the hyetograph (Natural Resource Conservation Service Spillway Emergency Distribution) to be used in design routings are provided in Appendix A of this regulation.

C. Significant hazard dams must be capable of safely passing and /or storing the runoff from at least fifty percent (50%) of the 24 hour PMP without overtopping the dam unless an incremental consequence analysis as defined in Rule 3.5 E. of this regulation indicates a lesser inflow design flood is applicable. The owner and any other persons responsible for the construction and operation of the dam shall assume all risks for future costs to upgrade a dam in the event the hazard classification change.

D. Low hazard dams must be capable of safely passing and/or storing the runoff from either the 24 hour duration 100-year rainfall for the dam location according to NOAA Atlas 14 or thirty-five percent (35%) of the 24 hour PMP without overtopping the dam. The owner and any other persons responsible for the construction and operation of the dam shall assume all risks for future costs to upgrade a dam in the event the hazard classification changes.

E. An inflow design flood based on Incremental Consequence Analysis (ICA) may be developed and submitted to MDEQ. The analysis shall be conducted in accordance with MDEQ Acceptable Procedures for Conducting an Incremental Consequence Analysis; a copy of which may be obtained by contacting the MDEQ Dam Safety Division. MDEQ will review the ICA to determine whether the spillway design criteria for high and significant hazard dams, stated at Rule 3.5 B. and C. of these regulations, may be modified based on the ICA. The range of inflow design floods that can be considered as part of an ICA are as defined in the table below.

Hazard Classification	ICA Inflow Design Flood
High	50-100% PMP
Significant	500 year - 50% PMP

Any future changes in downstream land use, development, or critical hydraulic structures will require a re-evaluation of the incremental consequences and could require additional increases in spillway capacity for the dam. The owner and any other persons responsible for the operation of the dam shall assume all risks for future costs to upgrade a dam in the event there is a change in incremental consequences. Consequences shall be re-evaluated when changes occur but no less frequently than once every 5 years during a formal inspection of the dam.

F. When a conduit is proposed to be used in a high or significant hazard dam, the professional engineer responsible for the project shall provide MDEQ with detailed hydraulic, hydrologic, and structural computations supporting selection of the size and type of pipe to be used. Detailed drawings and specifications relating to the installation of the pipe shall include, but not be limited to, construction measures that adequately address critical loading, bedding, backfill, compaction, and seepage precautions related to installation of the pipe.

G. All concrete structures shall be designed in accordance with the applicable design standards in place at the time of construction. Details, as necessary, shall be provided showing reinforcement, cut offs, under drains/filters, waterstops, construction joints, control joints, and any other details necessary to construct.

H. The use of corrugated metal pipe for any purpose is expressly prohibited for high and significant hazard dams, because corrugated metal pipe typically experiences severe corrosion and fails long before its expected design life.

I. The use of geotextiles in filters is expressly prohibited for high and significant hazard dams, because, geotextiles will plug with native soil material and make the filter inoperable.

J. The soils in an earthen auxiliary (emergency) spillway shall be capable of withstanding the water velocities generated when the auxiliary (emergency) spillway is activated without experiencing excessive erosion. In order to demonstrate this a SITES Stability and Integrity Analysis shall be performed for all new earthen spillways proposed for high and significant hazard dams as well as all existing high and significant hazard dam earthen spillways where major modifications are proposed to any part of the dam.

K. Side slopes of all dams shall be a minimum of three horizontal to one vertical (3:1).

L. Seepage and slope stability analysis may be required at the discretion of MDEQ.

M. Wave wash protection must be provided along the upstream face of the dam or engineering justification proving it is not necessary must be provided for the construction of all new high and significant hazard dams.

N. Owners of high hazard or significant hazard dams will be required to prohibit livestock grazing on the dam in order to prevent damage to the turf and to prevent erosion associated with establishment of animal trails.

O. Owners of earthen dams covered under this regulation shall establish and maintain a healthy turf on the exposed faces of the dam to prevent erosion, and shall mow frequently enough to prevent the encroachment of woody vegetation into the slopes of the dam embankment or within the prescribed limits (generally 50 feet from the toe) set by MDEQ.

P. Operation and Maintenance manuals may be required for high and significant hazard dams at the discretion of MDEQ.

Notes

11 Miss. Code. R. 7-3.5

Miss. Code Ann. §§ 51-3-1, et seq., 49-2-1, et seq. and 49-17-1, etseq.

Amended 7/13/2018