A Study on the Roughness Factor and the Stability of a Composite Channel.

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The roughness factor of a composite channel increses when there is a strong lateral mixing between the low water channel and the flood plain. Therefore, We carried out an experiment on the lateral momentum mixing in a shallow open channel where the primary shear is in vertical. The following three types of flow were examined. (1) A couple of flows parallel to each other with a velocity difference. (2) A wake behind a piar. (3) A boundary layer on the side bank. Results showed that the lateral mixing in a shallow open channel is similar to the mixing of free turbulent flow and that the lateral eddy viscosity is proportional to the product of a lateral velocity difference and a lateral shear layer thickness. A shallow water equation accompanied by the above expression of the eddy viscosity led to a method of estimating the roughness factor of a composite channel.The flow resistance of bushes which often glow between the low water channel and the flood plain was studied by a field observation and a flume experiment. It is noteworthy that the coupling of the motion of bushes and the lateral seiche makes an regular emition of voltexes and enlarges the resistance.A numerical calculation method was developed to estimate a secondary circulation intensity which influences a large effect on the stability of the low water channel bank. Experiments verified that this method is useful especially in a natural river channel which is often bending in ancomprecated manner.The above results are served for the planning and designing a composite channel.