Methods for predicting discharge of straight asymmetric compound channels

Accurate prediction of flow discharge in a compound channel is increasingly important in river engineering and management. Due to velocity difference between the main channel and its floodplains, the momentum exchange plays a significant role in the prediction of flow, and also there are some different characteristics between in a symmetric compound channel and an asymmetric compound channel that has relatively less been investigated. This paper evaluates four most recently developed 1 -D methods for flow calculation, which have considered the momentum exchange in their methods, namely Interacting Divided Channel Method (IDCM), Momentum-Transfer Divided Channel Method (MTDCM) and Modified Divided Channel Method (MDCM) and Apparent Shear Stress Method (ASSM). The four methods are compared against our data and other experimental data available in the literature. The 20 datasets used include both homogeneous compound channels (8 datasets) and heterogeneously roughened channels (12 datasets), which have various aspect ratios [channel total width {B) at bankfull / main channel bottom (b) =1.5 ∼ 5] and bed slopes (So = 2.65x10^-4∼ 1.3x10^-2). Our studies show that the four methods performed reasonably well (in average errors less than 6.5%) against all the datasets except in a very steep channel with high aspect ratio (e.g. B/b ≥ 5 in S_o = 0.013). Among the four methods, the MDCM has shown the best overall performance for homogeneous channels whereas all four methods perform similarly for heterogeneous compound channels. Close examination shows that the error percentage by all four methods increases as increasing aspect ratio (B/b) for roughened channels, but it seems in reverse for homogeneous channels. Finally, all four methods have shown improved flow predictions of main channels compared with the conventional divided channel method (DCM).