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

摘要

Although the sediment concentration to water discharge ratio (￡) 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 index. These rivers can be classified as four channel patterns, i. e., braided, low concentration meandering, hypercon-centration meandering and island. It was found that, when mean annual water discharge (Q) is given, the 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 low-est. A discriminating model has been proposed for the above four channel patterns, based on the relationship between and Q. Three threshold lines between the four channel patterns arc well identified. The limits between channel patterns have close relation with the phenomenon of "double-thresholds for fill-scour behaviors of rivers with wide-range sediment concentra-tions", 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 index is related to specific stream power (Ω) and channel sinuosity, and the result indicates that the complicated variation of with is one of the causes for the complicated variation of river channel patterns with the index. This complicated variation may be regarded as a complex response of river channel pattern to the varying index. This study also shows that the clay content in river bank material is controlled by the index to some degree. Thus, the index also contains some information of erosion-resistance of channel boundary materials. Through the effect of on channel boundary material, channel pattern is affected by The proposed discriminating model may be used to predict the trend of channel pattern changes. Based on the data of Q and the location of a given river for the pre- and post-impact periods can be plot in the diagram. If the river crosses the threshold line between two channel patterns, for instance the line between braided pattern and low concentration mean-dering pattern, or vice versa, channel pattern transformation may be expected to occur.