A hydraulic and morphologic criterion for upstram slopes in local-scour holes

Scour is a natural phenomenon caused by the flow of water in rivers and streams. Scour occurs naturally as part of the morphological changes of rivers and as result of structures man-made. Several types of scour can be distinghuished. Experience has shown that due to sand or flow slides or micro instabilities at the end of the bed protection, the scour process can progressively damage the bed protection, leading eventually to the failure of the hydraulic structure for which the bed protection was meant. The length of the bed protection depends on the permissible amount of scour (maximum scour depth and the upstream scour slope) and the geotechnical structure of the soil involved (Pilarczyk, 1984). In the scope of the Dutch Delta works, a systematical investigation of time scale for two and three-dimensional local scour in loose sediments was conducted by Delft Hydraulics and the Department of Public Works (Rijkswaterstaat). From model experiments on different scale and bed materials, relations were derived in order to predict the steepness of the upstream scour slope (De Graauw and Pilarczyk, 1981 and De Graauw, 1983). Since the predictability of these relations is poor, especially for prototype conditions, a theoretical study concerning upstream scour slopes is carried out. In the present study a stability criterion is deduced which is based on the mass-balance equation and a stochastical bed-load predictor (Van Rijn, 1985) and fitted using approximately 250 clear-water scour experiments. In addition a empirical relation for undermining is discussed. The stability criterion for upstream scour slopes and the model relation for undermining are verified applying some prototype experiments (Delft Hydraulics, 1979 and De Graauw & Pilarczyk, 1981)