Discrimination of channel patterns for alluvial rivers based on the sediment concentration to water discharge ratio

Abstract

Although the sediment concentration to water discharge ratio (zeta) is an empirical coefficient, it contains manifold physical meanings. Based on data from 107 Chinese alluvial rivers, a study has been made to relate river channel patterns to the zeta index. These rivers can be classified as four channel patterns, i. e., braided, low concentration meandering, hyperconcentration meandering and island. It was found that, when mean annual water discharge (Q) is given, the zeta index for hyperconcentration meandering rivers is the highest, for braided rivers the second, for low-concentration meandering rivers the third, and for island rivers the lowest. A discriminating model has been proposed for the above four channel patterns, based on the relationship between zeta and Q. Three threshold lines between the four channel patterns are well identified. The limits between channel patterns have close relation with the phenomenon of for fill-scour behaviors of rivers with wide-range sediment concentrations, which was previously described by the author. The limit between hyperconcentration-meandering rivers and braided rivers reflects the upper threshold of the double-thresholds phenomenon, and the limit between braided rivers and low-concentration meandering rivers reflects the lower threshold of the double-thresholds phenomenon. Of the 107 Chinese rivers studied, 84.7% are correctly predicted using the proposed model. Based on some Chinese rivers, the zeta index is related to specific stream power (Omega) and channel sinuosity, and the result indicates that the complicated variation of Omega with zeta is one of the causes for the complicated variation of river channel patterns with the zeta index. This complicated variation may be regarded as a complex response of river channel pattern to the varying zeta index. This study also shows that the clay content in river bank material is controlled by the zeta index to some degree. Thus, the zeta index also contains some information of erosion-resistance of channel boundary materials. Through the effect of zeta on channel boundary material, channel pattern is affected by zeta. The proposed discriminating model may be used to predict the trend of channel pattern changes. Based on the data of Q and zeta, the location of a given river for the pre- and post-impact periods can be plot in the zeta-Q diagram. If the river crosses the threshold line between two channel patterns, for instance the line between braided pattern and low concentration meandering pattern, or vice versa, channel pattern transformation may be expected to occur.