THE STABLE CHANNEL AS SHAPED TO FLOW AND SEDIMENT

Abstract

Stable channel design is a very important element in many water resources projects. Both bed and bank stability are necessary design criteria. The channel is designed for some critical flow rate and sediment load, where the bank erodibility, sediment size distribution, and channel resistance to flow are imposed conditions. For these conditions the stable channel width, depth, and slope are predicted. Earlier studies by Lacey, Blench, and others related the channel dimensions to the flow rate as a power function. In Blench's study the coefficient of the function was dependent on the nature and charge of the bed material, and the erodibility of the sides, while the exponent was a constant. This study extends the power function equation relationship. The width, depth, and width/depth ratio were considered functions of the flow rate, and the coefficients and exponents were both found to be dependent on the sediment concentration and the bank erodibility. The tractive force method was used in this analysis. A set of design graphs were determined from simultaneous solutions of the Manning and Laursen equations. From the graphs design equations were formulated. Some simple example problems were solved using this method. In the analysis the bank erodibility (maximum permissible bank shear) needed to be quantified. Experiments were performed with a Preston tube to determine the shear distributions in channels with various roughness patterns. From the results the maximum bank shear could be determined as a coefficient times the maximum bed shear. When the smooth channel and rough channel were tested, the results compared well with the values used by Lane (coefficient approximately 0.76). When the banks were smooth and the bed was rough, or vice versa, the coefficient was found to be different than 0.76. More testing is considered necessary to determine if the difference is significant.