Mississippi Administrative Code › Title 11 - Mississippi Department of Environmental Quality › Part 7 - Surface Water and Groundwater Use and Protection, Licensing of Water Well Contractors and Dam Safety Regulations › Chapter 3 - Mississippi Commission on Environmental Quality Dam Safety Regulations › Rule 11-7-3.5 - Design and Maintenance Requirements

Current through June 25, 2024
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.
Miss. Code Ann.
				  §§
										51-3-1,
				  et seq., 49-2-1, et seq. and 49-17-1, etseq.