Prediction of scour profile in cohesive beds downstream of vertical ‎sluice gates

Scour phenomena can impair infrastructures and result in non-reparable damages. Few researches conducted so far show that application of non-cohesive criteria for situations where the bed is formed of cohesive materials، overestimates dimensions of the scour hole and will impose large costs to the project. According to the previous researches، type and content of clay also significantly affects scour in the cohesive materials. In this paper، the local scour in the cohesive bed due to a horizontal submerged jet issued from a vertical sluice gate is experimentally studied. Initially، the effective factors were identified and Bentonite clay was selected as cohesive material. A total of 48 tests، considering four different clay contents، three amounts of gate openings and four different tailwater depths، were conducted in a rectangular flume of Soil Conservation and Watershed Management Research Institute laboratory. The results show that by adding Bentonite clay to the erodible bed، effect of cohesion appears in the 10% ratio and with further increase of the clay content the scour depth decreases considerably. So that in a bed containing 20% Bentonite clay the maximum scour depth reduces by 76% on average in relation with a non-cohesive bed. Based on the experimental results، a general equation for predicting the maximum scour depth valid in both conditions of presence and absence of the cohesive materials is presented. A sensitivity analysis on the existing parameters that effect on scour depth was conducted and compared with the results of previous researches. Finally، two types of scour hole profiles were defined which showed a relatively good agreement with previous empirical relationships.