Effect of Erodible Bed on Backwater Rise Due to Bridge Piers Only in Case of Subcritical Flow

Abstract

This experimental research aims to study the effect of movable bed scour on the backwater rise upstream bridge piers. The physical models were adjusted for all runs to generate subcritical flow between bridge piers using four different values of downstream Froude number equal 0.0958, 0.107, 0.277 and 0.3527. Different three nose shapes of bridge piers were used, (rectangular, triangular and circular nose), under four different values of contraction ratio (Cr) of; 0.625, 0.70, 0.75 and 0.875 using four discharge values (10 l/s., 11 l/s., 13 l/s. and 15 l/s.). The experimental work was carried out using fine sand sample with geometric mean size (d50) equals 0.167 (classified according to American code). Generally, this research illustrated that the backwater rise upstream bridge piers affected directly with scour propagation around piers. It was appeared that the values of backwater rise using triangular and rectangular noses were greater than the corresponding ones using circular nose by an average percentage of 15.50% and 31.50% respectively. This means that the circular nose gave the least values of backwater rise than other nose shapes, so it is recommend to use it as the best nose shape. It was noticed that the backwater rise decreased gradually with the increasing of run time and the scour hole around piers. It was also showed that the change of Froude number, shape of pier nose and contraction ratio had a noticeable effect on the backwater rise upstream bridge.