Drop Spillways
Drop inlet spillways (including morning glory / shaft and other drop inlet control structures): These types of control structures are suited for service and auxiliary spillways.[1]
Morning Glory Control Structures
This structure is a type of drop inlet spillway. This type of control structure should be considered when there is very limited space and there is adequate rock foundation. The morning glory spillway (sometimes referred to as a “shaft” spillway) has the potential of small to moderate discharge capacity and is used with conduit and tunnel conveyance features. This type of spillway is applicable to concrete, embankment, and composite dams, and it can be gated or ungated. As hydraulic head on the crest increases, the flow transitions from crest control to throat (orifice) control and, in some cases, to pipe (pressure) control, resulting in reduced discharge efficiency. Morning glory spillways are typically designed to only operate in the crest control range. Larger discharges could result in adverse hydraulics in the downstream conduit or tunnel (slug and/or pressure flow). Also, this spillway type may be vulnerable to debris plugging. If heavy debris loads are anticipated during flood events, consider defensive measures to protect the control structure (such as debris booms) or consider other types of spillway control structures. Examples of morning glory spillways include the service spillways at Reclamation’s Hungry Horse Dam (concrete), Ridgway Dam (embankment), Trinity Dam (embankment), and Owyhee Dam (concrete).[1]
Other Drop Inlet Spillways
This type of spillway is most applicable where there is a small amount of space to locate the control structure, there is adequate rock foundation, and the conveyance feature will be a conduit or tunnel. This type of spillway is applicable to concrete, embankment, and composite dams, and it can be gated or ungated. This spillway is very similar to an outlet works intake tower and may be in combination with an outlet works. This spillway has the potential of small discharge capacity. Larger discharges than design levels can result in hydraulic control shifts (crest to orifice and/or orifice to pipe control) that could result in adverse hydraulics in the downstream conduit or tunnel (slug and/or pressure flow). Also, this spillway type may be vulnerable to debris plugging. If heavy debris loads are anticipated during flood events, consider defensive measures to protect the control structure (such as debris booms) or consider other types of spillway control structures. Examples of other drop inlet spillways include the service spillways at Reclamation’s B.F. Sisk Dam (embankment) and Lake Sherburne Dam (embankment).[1]
Best Practices Resources
Citations:
Revision ID: 7129
Revision Date: 07/11/2023