Local channel scour in uniformly graded sediments: the time-scale problem

The importance of the time scale during the local scour of uniformly graded sediments and the dynamics of the erosion process are discussed in the context of the scour which takes place downstream of a submerged sluice gate. In the study, the scour profiles were obtained using a video-imaging process. The dynamic nature of the process is clearly demonstrated by presenting scour profiles at various instants of time over a period of 144 h. Some of the similarity parameters identified in previous studies were found to be unsuitable for describing the scour which took place. However, as found in related studies, the nondimensional volume of scour and maximum depth of scour did follow a power-law relationship with time, with equilibrium not yet achieved by the end of the 144 h test. Two alternately occurring, distinct flow fields were recognized during the study. In relation to the type of flow field occurring, the bed material downstream of the sluice gate moved either in the upstream or the downstream direction. The downstream movement of the bed material was accompanied by the rapid digging of a trough in the sand bed relatively close to the sluice gate. The digging process was immediately followed by a gradual upstream transport of the sand particles and a refilling of the trough. Of particular significance in this regard is the fact that the maximum depth of scour observed during the short-duration digging was greater than was observed during any other phase of the scour process.