A Hydraulic Model for the Prediction of Wash Load in Mountainous Drainage Basins

Abstract

This study has been carried out to develop a hydraulic method for the prediction of wash load concentration during a runoff due to a rain fall. In order to develop such a method, efforts should be made on the following two problems: Where and how is wash load produced? How can the distribution of flow discharge be evaluated in a drainage basin? After one clarifies these problems, one should study the model for wash load prediction by combining the knowledge of these together. From such a point of view, the production regions associated with wash load are discussed first on the basis of field data, and it is found out that the main production regions are both bare slopes and erodible banks. Secondly, mechanisms of the yield and transport of wash load are considered with the theoretical backgrounds and a new idea introduced here. According to the results, the yield and transport of wash load are predominantly subjected to gully erosion at bare slopes and bank erosion and exchange velocity between flowing water and the pore water in exchange layer. The theoretical results are supported by the data obtained from experiments. Thirdly, the authors show how the flow discharge can be evaluated in bare slopes and stream channels, and then propose a model to predict wash load concentration. If some geophysical and hydrologic parameters were given, the concentration of wash load could be calculated from the model at any time and stream section. The model has been applied to Kawarabi River Basin to test its applicability. It was found that the concentration curves predicted by the method supported the data well.