Experimental investigation on the parameters influencing local scour types around scaled triangular nose bridge pier in nonuniform mobile bed

The local scour at the triangular nose pier of the Al Kufa bridge, Kufa, Iraq was experimentally investigated using a pier model with a scale of 1:155. Special arrangements were followed to fix the pier model in the middle ofthe mobile bed with nonuniform sediments of median diameter, d₅₀=0.30 mm and geometric standard deviation, σg=2.78. The mobile bed had a depth of 100 mm and a length of2m and was located 6 m from the inlet ofa glasssided titling flume. For five different discharges, the effect offlow parameters (velocity, u, water depth, y and Froude number, Fr), pier dimensions (pier width, b and pier length, l), bed sediment characteristics (d₅₀, σg and threshold velocity for sediments, ut) and time, t on local scour depth were studied. Throughout the experiments, clear water scour and lived bed scour were achieved. For clear water conditions, the relationship between normalized scour depth (ds/b) and velocity ratio was found almost linear up to ua/ut=l while for live bed scour conditions, a nonlinear relationship with a decreasing trend was found. For the thin pier, the laboratory data obtainedfrom the present study demonstrated that the depth ofthe local scour (ds) was independent ofthe water depth. When Fr=0.061 in both model and prototype, the measured scour depth at the pier model was overestimated by 0.4 m at thepier ofAl Kufa Bridge. In addition, a relationship between scour depth ratio (ds/dseq) and time ratio (t/tseq) waspreparedfrom laboratory data. The relationship is useful in determining the equilibrium scour depth at the Al Kufa Bridge. Furthermore, the angle of the scour holes (0) resulted from the equilibrium scour depth was measured andfound to be ranges between 14' and 18.4'.