The program automatically accounts for submergence on the weir when the tailwater is high enough to slow down the flow. Submergence is defined as the depth of water above the weir on the downstream side divided by the headwater energy depth of water above the weir on the upstream side. As the degree of submergence increases, the program reduces the weir flow coefficient. Submergence corrections are based on the shape of the spillway crest (broad crested, ogee shaped weir, or sharp crested). If the spillway is a broad crested shape, then the same submergence curve that is used for flow over a roadway at a bridge (the figure below) is used. If the spillway crest is ogee shaped, a submergence curve from the USACE EM 1110-2-1603 (Plate 3-5, A-A) is used. If the spillway is sharp crested, then the Villemonte equation (Villemonte, 1947) is used to compute the flow reduction coefficient.
