Spillway Discharge Coefficients for Rock Island Dam

A method for selecting prototype spillway discharge coefficients on the basis of model data is presented. The effects of adjacent gates opened, partially opened gates, number of gates opened, and tailwater conditions are included. The average discrepancy between coefficients selected by the recommended procedure and coefficients computed from the model data is 0.85%. The average discrepancy between prototype flow rates computed with these selected coefficients and the measured prototype flow rates is 1.65%. The usual adjustments for adjacent gate effects were not applicable for this spillway. The spillway studied was unusual in that the flow approached part of the spillway at a very oblique angle because of the shallow topography upstream of the spillway. The discharge coefficients were affected by this oblique angle of approach when coupled with numerous gates open.