Abstract

Abstract : Groynes are man - made hydraulic structures constructed for a variety of pu rposes. They extend outside the bank in the depth of streams. Local scour usually occurs around groynes resulting in problems of interest to hydraulic engineers. In this study, laboratory experiments were conducted to evaluate the local scour around differ ent number of groynes distributed on different spacing with different shapes. Physical models have been developed for groynes installed in a straight hydraulic channel grounded with uniform cohesionless (soil) as bed material wit h medium grain size (d50 = 0.69 mm). The experiments were conducted under subcritical flow and Clear - water conditions. Three different numbers of groynes (single, double and triple) were used to indicate the impact of the numbers on the local scour; such impact have been observed t o be as reverse relationship, specially, for the intermediate groynes. Different spacing between groynes as (1, 1.5, and 2) times the length of groynes were used in this study. For the range tested, it has been observed that for each spacing decrease by (0 .5 Lg) there was a decrease in scour depth about (20%). Also, several lengths of groynes (13, 10, and 7) cm were used, and it has been observed that scour depth is decreased about (20 - 53)% by decreasing the ratio of groyne length to main flow width (Lg/B ) by (7.5%). Two different shapes of curved groynes, namely as, quadrant (quarter of a cylinder) and semi - parabolic shaped groynes were used to indicate the effect of groyne shape on the local scour. Generally, scour is decreased about (75) % in quadrant s hape groynes as compared with that of semi - parabolic shape groynes