Seismic survey for detecting scour depths downstream of the Srisailam dam, Andhra Pradesh, India

During the construction of the Srisailam dam, deep scour was observed near the dam toe. Various protection measures were taken between 1977 and 1987, but in 1987 soundings revealed a scour hole at a distance of 206 m downstream of the reference line. In 1995 this scour hole was about 61 m upstream. It was thus feared that scouring may affect the stability of the downstream protection works. To define this problem more clearly, a seismic reflection survey was conducted along 24 traverses covering an area of 67,000 m2 and a seismic refraction survey was carried out along eight profiles with shots at either end. The refraction profiling, using an array of vertical hydrophones, was carried out in the deepest part of the downstream pool of water. This survey revealed an overburden with a velocity varying from 2000 to 2600 m/s that was underlain by quartzite with a velocity of 4000 to 5500 m/s. The depth to rock varied between 2.0 and 14.0 m from the river bed. Seismic reflection results determined the areal configuration of the river bed and the sediment thickness. The river bed level varied from 137.2 to 155.4 m, and slopes steeply from both the banks. The rock level is 152.4 m at a distance of 50 m from the reference line of the dam towards the power house, and is 134.1 m at a distance of 130 m. The rock levels towards the left bank are deeper than the 1968/1969 borehole rock levels, indicating that scouring has taken place, and the depth of scouring was found to vary from 2 to 4 m.