Grade Control Sill Spillways
Grade control sill spillways are primarily suited for auxiliary and emergency spillways; however, in some cases, they can function as a service spillway. A grade control sill is a less robust, minimal spillway typically limited to a vertical reinforced concrete wall (sill) that is placed in a trench through an excavated trapezoidal channel. The grade control sill can be constructed in both rock and soil foundations. The grade control sill spillway tends to have limited reserve discharge capacity, given channel armorment could be damaged or fail due to discharges that exceed design levels. Therefore, an important consideration is to limit potential erosion during spillway operation. In summary, this type of spillway should only be used on low head situations where the hydraulic drop (vertical dimension between the reservoir water surface and the downstream river or stream) can be effectively controlled, including limiting erosion potential. Examples of grade control sill spillways include the service spillway at Reclamation’s Crane Prairie Dam (embankment) and the emergency spillway at Reclamation’s Davis Creek Dam (embankment).[1]
Best Practices Resources
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Revision ID: 7131
Revision Date: 07/11/2023