A.M. Negm - Professor of Hydraulics, Civil Engg. Dept., College of Engg., Garyounis University, Benghazi, Libya

Scour downstream (DS) of hydraulic structures (HS) may endanger the safety of the structures if the necessary precautions are not considered during the design stage. Normally, different measures produce different effects on reducing the maximum scour depth DS of HS. One important HSs is the abruptly enlarged (or suddenly expanding) stilling basin. In this paper, the effects of different arrangements of sills inside an abruptly enlarged stilling basin will be treated. An experimental program was conducted to investigate the effects of a continuous end sill, one asymmetric side sill, double staggered asymmetric side sills, symmetric side sills, a central sill and a continuous central sill. The flow patterns were observed and the maximum scour depths were recorded. The results revealed that in most of the cases the flow patterns are asymmetric resulting in asymmetric scours. The reduction in the maximum scour depth depended on the sill arrangement and on the flow conditions represented by the Froude number. The central sill with limited width and the continuous central sill improved flow symmetry and the latter yielded the least depth of scour