Determining the depth of local scour near intermediate piers and shore structures in the presence of a markedly unsteady flow

Abstract

In rivers the flow is, as a rule, unsteady, with different degrees of variability. Floods can apparently be considered as slowly varying flows. Flows in the form of breakthrough waves, which can arise upon failure of dams and as a result of natural phenomena, such as earthquakes, collapse of large masses of earth into a reservoir, bursting of natural dams, heavy storms, etc. are very unsteady. Just in the s,~mer of 1976 three cases of the formation of breakthrough waves were reported: upon failure of the Teton Dam in the USA, bursting of a wall of the canal connecting the Ruhr region with Hamburg in West Germany, and collapse of a levee during flooding in the Ashkhabad region. The experiments were conducted in a long rectangular flume. The models of the intermediate piers were arranged along the axis of the flume, which corresponds to the scheme of the flow of an infinitely wide stream past piers, which was used also in the earlier investigation of local scour under steady flow conditions.The shore structures were located at the edge of the flume, that is, embankments of the bridge approaches did not constrict the flow.The head of the flume was used as a storage reservoir. Upon rapid lifting of the vertical gate installed at the end of the pool a traveling breakthrough wave was produced in the flume. During passage of the wave (about 20 sec) we measured the depth of the flow with a string wave meter and velocity by four current meters installed on the same vertical. The the wave shape and velocity were recorded on an N-700 oscillograph. readings of u, h. cm/sec cn~