Critical Flow in Rivers with Flood Plains

Problems with the two existing critical flow computation procedures are examined, and a simplified method is proposed for composite section channels. Existing methods, which are based on the minimum in the specific energy curve, can result in two major problems. The first one is that excessively high flow depths may be defined as critical. The second problem is that more than one minimum in the specific energy curve can occur; i.e., more than one flow depth may be defined as critical for a given discharge. The proposed method defines critical depth as the level where the discharge weighted Froude number is equal to 1.00. Example computations are carried out for a river and flood plain case. They illustrate the proposed computation procedure and compare the results obtained by the three methods of analysis.