WATER-SURFACE PROFILES IN COMPOUND CHANNEL WITH MULTIPLE CRITICAL DEPTHS

Water surface profiles measured in a laboratory compound channel that has rough floodplains and multiple critical depths are compared with c omputed water surface profiles obtained by numerical solution of the o ne-dimensional equation of gradually varied flow in a form that includ es a compound channel Froude number. Manning's n is found to vary with depth in the floodplain, and the interaction between the main channel and floodplain is represented as a correction to the main channel res istance coefficient. Outside the discharge range of multiple critical depths, good agreement is demonstrated between measured and computed w ater surface profiles. Within the discharge range of multiple critical depths, the measured water surface profile near a free overfall canno t be computed as a one-dimensional flow because it separates into dist inctly different drawdown profiles in the main channel and floodplain. A method is proposed for identifying the discharge range within which multiple critical depths exist in order to identify potential problem s with interpretation of water surface profiles computed by one-dimens ional methods in this range.