Umfolozi Road Bridge Hydraulic Model Investigation

Abstract

A movable-bed hydraulic model study was carried out of a new road bridge across the Umfolozi River in Zululand, Natal. The model was designed in accordance with the shear/settling velocity criterion, V\d〈\dr=W\dr, and crushed anthracite was used as bed material (170υ\N). The horizontal and vertical scales were 1 in 250 and 1 in 100 respectively. The velocity scale was based on Froude’s law and, with the large tractive force in this steep river, it was found necessary to deviate from the criterion Re\d〈\dr=1. Tests were run to determine the best river crossing, the required bridge length and the optimum orientation of the bridge piers, particularly with regard to minimum river bed scour. When a major flood occurred during the early stages of bridge constuction, an opportunity arose to check on the predicted scour depths. Scour depths recorded in nature were reproduced to an accuracy of about 10 percent in the model and, because of the greater confidence which could be placed on the model results, considerable savings were effected in the bridge foundations.