W. Va. Code R. § 38-4-7 - Design Requirements

7.1. General Hydrologic Requirements

7.1.a. Hydrologic Investigation: A survey shall be conducted to evaluate soil types, land use, and slope watershed area, runoff curve number, and any of the factors needed to establish watershed characteristics. A summary of all hydrologic and hydraulic data compiled in the initial site investigation and used in the analysis shall be included in table or figure form in the plan package.

7.1.b. A stream flow analysis shall be conducted to evaluate stream flow quantity and quality as it affects the dam and its appurtenances.

7.1.c. Design Storm Requirements:

7.1.c.1. All dams shall be designed to meet the following minimum hydrological criteria based upon hazard classification:

7.1.c.1.A. Class A impoundments shall be designed for a minimum of P100 + 0.12 (PMP-P 100) inches of rainfall in six (6) hours.

7.1.c.1.B. Class B impoundments shall be designed for a minimum of P100 + 0.40 (PMP-P 100) inches of rainfall in six (6) hours.

7.1.c.1.C. Class C impoundment shall be designed for a probable maximum precipitation (PMP) of a six (6) hour or greater event plus three feet of freeboard.

7.1.d. Antecedent Moisture Conditions: Where applicable to the development of a hydrograph, antecedent moisture condition II (AMC II) may be used unless a different condition class is required by the Secretary.

7.1.e. Flood Routings: An analysis shall be performed for the reservoir and spillways which includes inflow hydrographs, stage storage curves, stage discharge curves, and routings. The spillways must be able to safely discharge that portion of the design storm that is not stored in the reservoir. If a computer analysis is used, the input data and output results must be clearly labeled and identified. Trial calculations or intermediate results not relevant to the final results may be omitted from the plan package.

7.1.f. Specific Flood Routing and Storage Requirements:

7.1.f.1. Class A dams must be designed with an open channel spillway unless otherwise (90) percent of the stored portion of the design storm must be discharged or removed within ten (10) days after the storm event.

7.1.f.2. Class B dams must be designed with either an open channel spillway only, or with an emergency spillway and a principal spillway together. Ninety (90) percent of the stored portion of the design storm shall be discharged or removed within ten (10) days after the storm event.

7.1.f.3. Class C dams may be designed in one of three ways:

7.1.f.3.A. An impoundment designed without discharge structures shall be capable of storing a minimum of two (2) six (6) hour duration probable maximum storms. A system shall be designed to dewater the impoundment of the probable maximum storm in ten (10) days by pumping or by other means. The requirements of 25.14 shall also be met.

For existing structures exceeding the minimum 2 PMP volume requirement, the dewatering system shall be installed when the containment volume is reduced to 2 PMPs.

7.1.f.3.B. An impoundment designed with a decant or principal spillway only shall be capable of storing at least one (1) six (6) hour duration probable maximum storm Ninety (90) percent of the stored shall be discharged or removed within ten (10) days after the storm event.

7.1.f.3.C. An impoundment designed with either an open channel spillway only, or with an emergency spillway and principal spillway together shall be capable of discharging that portion of the six (6) hour duration probable maximum storm that cannot be safely stored in the impoundment. Ninety (90) percent of the stored portion of the storm shall be discharged or removed within ten (10) days after the storm event.

7.1.g. Surface Drainage Control: Surface drainage control devices (e.g., vegetated slopes, benches, groin ditches, and collection channels) shall be provided as necessary to protect the dam and its appurtenances from the effects of erosion. Riprap or other erosion protection measures shall be included where excessive velocity is anticipated or experienced. All surface drainage control devices must be designed to exit safely beyond the downstream toe of the embankment in a natural drain way and be capable of carrying the design flow without excessive erosion. Any open channel spillway designed for less than one hundred (100) percent probable maximum precipitation shall be provided with freeboard above the maximum water surface as determined by the equation 1 + 0.025 vd^1/3.

7.1.h. Hydraulic Considerations: Using standard engineering practices, a hydraulic analysis shall be performed for the spillways and surface drainage system. Typical cross section design techniques may be used where constant shapes are encountered. All hydraulic structures shall be designed to safely control the velocity of water in order to prevent excessive erosion. Accepted engineering practices shall be used to design riprap, non-flexible channel linings, bedding, and energy dissipaters.

7.1.h.1. Specific Hydraulic Requirements: Open channels, including open channel spillways, shall be analyzed for flow depth, velocity, non-uniform flow conditions, super-elevation, and hydraulic jumps.

7.1.h.1.A. Stage Discharge: Where an open channel is used as a spillway, a stage discharge rating shall be developed using standard engineering practices for the type and shape of the spillway. In developing the rating, increase in upstream water depth due to change in velocity head must be considered.

7.1.h.1.B. Water Surface Profiles: Where channel slopes or cross-sections vary and non-uniform flow conditions result, a water surface profile may be necessary in order to analyze the channel flow depths and the location of hydraulic jumps.

7.1.h.1.C. Hydraulic Jumps: Where hydraulic jumps will occur, channel sidewall height shall be sufficient to contain the jump. The channel lining shall be designed to withstand the hydraulic jump without damage.

7.1.h.1.D. Critical Flows: Channels shall be designed so that water will not flow at critical depth for extended distances. In channels of varying slope or cross-sections where non-uniform flow occurs, the transition through critical flow shall be as rapid as possible.

7.1.h.1.E. Super Elevation: Channel walls shall be designed to contain super elevated flows on curves.

7.1.i. Closed Conduit Systems: Closed Conduit Systems including principal spillways, risers and pipes shall be analyzed to determine the controlling limits for weir, orifice, and pipe flows.

7.1.j. Risers and Drop Inlets: Risers shall be protected with a designed trash rack and anti-vortex device. The drop inlet shall be sized to provide a rapid transition from partial to full pipe flow conditions.

7.1.k. Stage Discharge: When a closed conduit system is used as a principal system, a stage discharge rating shall be developed using standard engineering practices for weir, orifice, and pipe flow conditions.

7.1.l. An adequate foundation and bedding shall be designed for all pipes and risers.

7.1.m. All pipe spillways shall be designed to provide seepage control along the conduit.

7.1.n. Use of Corrugated Metal Pipes -Corrugated metal pipes, whether coated or uncoated, shall not be used in new or unconstructed refuse impoundments or slurry cells. If an existing corrugated metal pipe has developed leaks or otherwise deteriorated so as to cause the pipe to not function properly and such deterioration constitutes a hazard to the proper operation of the impoundment, the Secretary will require the corrugated metal pipe to be either repaired or replaced. Provided, however, sediment control or other water retention structures used for the treatment of effluent and designated as Class A Dams under 3.4.b of this rule are exempt from this prohibition.

7.1.o. The pipe spillway shall be of sufficient strength to withstand the maximum load of the fill above it.

7.1.p. The pipe spillway shall be of suitable material to resist deterioration for the design life of the facility.

7.1.q. The pipe spillway must be designed to avoid formation of alternating partial and full pipe flow conditions through proper selection of pipe slope and headwater or tail water conditions.

7.1.r. The outlet of all conduits where blockage by animals can occur must be protected by an animal guide.

7.1.s. Landslide Potential: When locating all hydraulic structures the potential for landslides or slope failures as determined in the initial site investigation shall be evaluated according to Section 10.6.

Notes

W. Va. Code R. § 38-4-7