Effect of Bed Protection Length on Local Scour Downstream of Horizontal Jets

Local scour occurs due to interactions between water current, erodible bed and hydraulic structures which can endanger performance and stability of these structures. To control this phenomenon downstream of structures such as control gates, various engineering measures such as aprons and rip-rap protections have been applied so far. However, investigation on the effect of lengths of these measures requires further studies which are dealt with in the current experimental research. For this purpose, scour profile due to a horizontal jet issued from a sluice gate was initially measured the (without protection) under different hydraulic conditions. The same tests were then repeated separately for the cases of application of aprons and rip-rap protections to achieve an insight about their effects on the maximum scour depth. Results showed that increasing relative length of aprons (L̃a ≥ 1) and rip-rap protections (L̃R ≥ 2), decreased the maximum scour depth considerably so that for L̃a ≥ 27 and L̃R ≥ 7, it decreases up to 100 percent. On this basis, a properly designed and constructed rip-rap protection (L̃R ≥ 2), is more effective on decreasing maximum local scour depth than the apron measure. In this paper, suitable empirical relationships (in non-dimensional form) for estimation of maximum scour depths (considering lengths of the apron and rip-rap protections) are proposed. A comparison of the results of the proposed relationships with those of the previous researches shows that most of the data are within acceptable (±20%) error interval.