Earth-Cut/Vegetated Spillways
“Vegetated auxiliary spillways typically have trapezoidal cross-sections and a grass cover to protect them from damaging erosion. They adapt well to sites capable of sustaining vigorous grass growth by normal maintenance without irrigation." [1]
"Designs may use earth auxiliary spillways at sites incapable of maintaining vegetative growth. While like vegetated spillways, NRCS typically designs earth spillways for lower velocities, lower stresses, and less frequent use. They typically require more maintenance than vegetated spillways after a flow occurs." [1]
"No damage should occur to vegetated or earth spillways during passage of all flows up to the auxiliary spillway hydrograph. Although vegetated and earth auxiliary spillways may experience minor surface erosion during passage of larger infrequent storms, the spillway must not breach during passage of the freeboard storm." [1]
"Designers can use hydraulic data in TR 210-39, “Hydraulics of Broad Crested Spillways,” in the design of vegetated and earth auxiliary spillways. Use a minimum vegetal retardance curve index of 5.6, as defined in Agricultural Research Service (ARS) Agriculture Handbook 667 (AH–667), “Stability Design of Grass-Lined Open Channels,” to determine hydraulic capacity and vegetal stress in vegetated spillways. Use a minimum Manning’s roughness coefficient, n, of 0.02 for earth spillway design. Base the actual hydraulic capacity of the spillway on an appraisal of the expected roughness condition at the site."[1]
"Maintenance for auxiliary spillways increases as the frequency and duration of flow increase. Good designs balance maintenance cost against the cost of modifying the other elements of the dam to reduce the flow frequency. Strategies to economize the design of auxiliary spillways include raising the crest elevation, increasing the capacity of the principal spillway, adding a structural primary auxiliary spillway, providing erosion protection, or any combination thereof." [1]
"Maintaining the effectiveness of vegetated and earth spillways requires a systematic and regular maintenance program. If damage to a spillway occurs during a flow event, complete a repair of the spillway as soon as possible following the flow event. The O&M plan for the dam should describe the expected maintenance and repair for conditions such as vehicle ruts created during wet conditions, degradation from burning surface vegetation, and impact of mower blades on components above the soil surface." [1]
Best Practices Resources
Technical Release 210-60: Earth Dams and Reservoirs, NRCS
National Engineering Handbook: Chapter 50 - Earth Spillway Design, NRCS
National Engineering Handbook: Chapter 51 - Earth Spillway Erosion Model, NRCS
Technical Release 210-39: Hydraulics of Broad-Crested Spillways, NRCS
Citations:
Revision ID: 7110
Revision Date: 07/11/2023