The stability of stone on a submerged spur crest

Abstract

A spur is a low height earth or rockfill projection extending from the bank of a stream into the channel. It may be used to redirect low flow currents, to stabilize the spur-side bank, to maintain a deeper channel opposite the spur, or to create a backwater upstream. A typical spur or groin is usually overtopped during high flood stages. During the time the spur is submerged, erosive forces will be acting on the crest. The purpose of this work is to define the rock size required on the crest for stability.

This problem is complex and required physical modelling. A simple relationship for the required stone size was developed as a function only of the difference in water depths upstream and downstream of the structure. The difference in water levels, or backwater caused by the spur, was found to be a function of the contraction ratio, the downstream Froude number, and the ratio of the upstream flow depth to spur height.

Given a stage-discharge relationship for the stream and the geometry of the river and spur, the backwater effects may be predicted and the required stone size determined.