Evaluating methods for discharge prediction of straight asymmetric compound channels

Accurate prediction of flow discharge in a compound channel is increasingly important in river flood risk management. This paper evaluates four most recently developed 1-D methods for discharge prediction. The four methods, which have considered the impact of momentum exchange, are Interacting Divided Channel Method (IDCM), Momentum-Transfer Divided Channel Method (MTDCM), Modified Divided Channel Method (MDCM) and Apparent Shear Stress Method (ASSM). The four methods are compared with 20 experimental datasets from the author and the literature. These datasets include both homogeneous (8 datasets) and heterogeneous (12 datasets) asymmetric compound channels, which have various width ratios (B/b) of 1.5 ~ 5 [channel total width B at bankfull / main channel bottom b] and bed slopes of 2.65×10-4 to 1.3×10-2. This study shows that the four methods performed reasonably well (in averaged errors < 6.5%) against all the datasets except in a very steep channel with high width ratio (e.g. B/b ≥ 5 in So = 0.013), particularly with improved discharge predictions of main channels compared with conventional divided channel method (DCM). It appears that the MDCM shows the best overall performance for homogeneous channels whereas all four methods perform similarly for heterogeneous compound channels. Close examination reveals that the error percentage by all four methods increases as increasing width ratio (B/b) for roughened floodplain channels, but it seems in reverse for homogeneous channels. Finally, all four methods have shown improved flow predictions of main channels compared with the DCM.