Experimental and Numerical Investigations of Design Tractive Force in the Compound Open Channel

Abstract

Tractive forces in the compound open channels are distributed differently according to section shapes. In river design works, design tractive forces are evaluated usually using cross sectional averaged values or results by two- dimensional numerical models, and applied to revetment design as external forces. However, cross sectional distributions of tractive forces in compound open channels are directly affected by sectional shapes and show three dimensional characteristics. Design tractive forces evaluated by cross sectional averaged values are underestimated or overestimated, and it causes bank erosion and an excess design. Although there are many experimental and numerical studies of compound open channels, results of the studies are not applied to river design works. As most previous studies have focused mainly on the narrow width channel or some selected shapes, results of the studies could not be applied to river design works. The objective of this study is to develop the design factors for tractive force evaluation in compound open channel through the investigation of the relationship between averaged tractive forces and local tractive forces along the cross sectional shapes. In this study tractive forces in representative compound open channels are measured by experiments, and three dimensional CFD model is calibrated by the experimental results. Using the calibrated numerical model, characteristics of tractive force distribution with floodplain width and height are analyzed. Characteristics of tractive force distribution compared with average tractive forcers are investigated on bank, floodplain and levee. In the experiment tractive force distributions are measured using Preston tube. In the numerical simulation, OpenFoam model is applied. OpenFoam model is verified by the experimental results for LES. Numerical simulation are conducted for various width ratio and depth ratio on the compound open channel. The nomograph for determining tractive force is suggested by the numerical experiments. The nomograph developed in this study can be applied for the given geometric ratio and help to calculate the design tractive forces in the simple procedure.