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

Document Type : Research Article



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 riprap
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 (L=a > 1) and
rip-rap protections (L=R > 2), decreased the maximum scour depth considerably so that for L=a > 27 and
L=R > 7, it decreases up to 100 percent. On this basis, a properly designed and constructed rip-rap
protection (L=R > 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.