EFFECT OF THE FLUME WIDTH ON THE SLOPE OF A “FLOOD PLAIN”

Abstract

It was noted about some rivers in Japan that a valley in the upper reach of a river has a more gentle slope than has an alluvial fan in the lower reach. A simple possible hypothesis was provided for explanation by the authors. The gentle slope in the upper reach may be due to narrow width of flood water restricted by valley walls (Fig. 1). The slope of a “flood plain” and water surface, width of water surface and velocity of flow under equilibrium condition were studied experimentally by use of a laboratory flume. Uniform and noncohensive sand, O.5_??_1.0mm in diameter was used for the experiment. The amount of sand supply and water discharge were kept constant during the operation. Six runs were carried out for different flume widths. Introduction of the water discharge and sand supply was accompanied by deposition until an equilibrium channel and a “flood plain” were established. By definition a stable equilibrium existed when the ratio of sand discharge to sand supply became to 1.0 averaged for sufficiently long time. The flume width determined the slope of running water when the flume was narrow. When the flume was sufficiently wide as compared with water discharge, the steeper “flood plain” appeared in the flume. In the latter case, the slope of the “flood plain” and running water was determined mainly by discharge, independently of flume width (Fig. 2). It is thus inferred that there is a critical value of flume width, beyond which the slope is determined only by discharge. This value is mainly a function of discharge. The result obtained by the experiment (Table 1, Fig. 2) seems to support the above hypothesis on the slope of river bed.