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13 reservoir so flow can enter from the entire perimeter or from one or multiple sides of the crest. The profile of a drop inlet crest can be square or round, and in some cases, can have an ogee crest-shaped lip to efficiently discharge flows. On large spillways, it is common to construct guide vanes or piers at the crest to prevent the formation of flow vortices which can lead to sub-optimal hydraulic conditions and reduce discharge. Discharge control can shift from crest to the throat, or to the conveyance tunnel for these types of spillways. Where the inlet is funnel-shaped, it is often called a “morning glory” type crest structure. Morning glory crest structures (Figure 7) are more prevalent on large dams. A more common drop inlet for smaller dams is shown on Figure 8. Figure 7: Morning Glory Figure 8: Drop Inlet Side Channel Crests Typically a side channel spillway has a control crest located parallel to, and along one side, of the spillway chute. In this case, flow enters from the crest and then turns 90-degrees into the spillway chute. In some cases, a control crest is also located at the upstream end of the spillway chute to provide additional discharge. The side channel spillway crest can be configured with most of the above-mentioned crest types. Due to its unique shape with a long crest and narrow chute, a side channel spillway can often be well-suited to a narrow and/or steep dam abutment. Where suitable, a bathtub crest structure can also be considered if a crest is located on both sides and the upstream end of the chute. Weir capacity and channel capacity must be balanced in the design of these spillways to consider the submergence effects on the weir crest. An example of a side channel spillway is shown on Figure 9. A bathtub crest structure is shown on Figure 10. Figure 9: Side Channel Spillway Figure 10: Bathtub Crest Structure (Reclamation 1987)