Abstract:
Presented herein is a comprehensive experimental investigation of the effects of various important parameters on the depth of local scour at abutments. The parameters are approach flow depth and flow velocity, approach channel (crosssectional) geometry, abutment length, shape and orientation, and sediment size and gradation. Generally, scour depth ds increases with both increasing flow depth Yo and abutment length L. Abutments are classified as short, medium or long, in terms of the ratio L/Yo. For short abutments, ds scales with L, the maximum scour depth 2L and this occurs at Yo/L=3 for vertical wall abutments. For long abutments, ds scales with Yo, the maximum scour depth is 10Yo and this occurs at L/Yo=100. These values are affected by sediment size and gradation, velocity conditions and abutment shape and alignment. The effect of flow velocity on scour depth is expressed in terms of the ratio of approach flow mean velocity U to that at the threshold condition Uc for uniform sediments. For nonuniform sediments, Uc is replaced by Ua, the armour peak velocity. A method to determine Ua is given. Scour depth increases with U/Uc or U/Ua towards a maximum value at the threshold or armour peak condition. At higher velocities, the scour depth changes consequent upon the size and shape of the bed features present. Scour depths in nonuniform sediments are always less than those in similar-sized uniform sediments due to armouring effects. The scour depth variations and the underlying processes are described herein. Sediment size effects are examined in terms of the ratio, L/d50, where d50 is the median particle size. ds increases with increasing L/d50 towards a maximum value at L/d50=40. For different abutment shapes, the sediment size ratio affects the flow depth ratio at which the maximum scour depth occurs. Maximum scour depths occur at vertical wall abutment shapes, with streamlining of the abutment shape acting to reduce the scour depth. The minimum scour depths were measured at the most streamlined spill through abutments. For each abutment shape, the shape effect diminishes with increasing L/Yo. The effects of compound channel cross-sectional shape are investigated. ds decreases with an increase in the width of the flood channel relative to the abutment length, and a decrease in the flow depth in the flood channel relative to that in the main channel. Finally, a design method is presented in which the effect of each parameter is expressed in terms of a scour depth multiplying factor.