Hydraulic Behavior of Spill Channel of Polavaram Dam Spillway without Left Embankment

Spill channel is a flood disposing channel excavated on the downstream of spillway which joins the main river course. The purpose of spill channel is to divert the spillway released flood safely to the downstream river course. The alignment of the spill channel may be straight of curved or partially straight and partially curved. It may be in steep gradient or in flat gradient and subjected to supercritical flows or subcritical flows. While disposing of the flood, the flow conditions in the spill channel would be straight without cross waves. Water surface profiles in the convex and concave banks of the spill channel must be confined within the top elevation of the embankments of channel to avoid spilling over the embankments. Thus, the hydraulic design of the spill channel involves keeping the velocities in the channel to allowable limits to prevent erosion of embankments and also confining the water levels to within the top elevation of the embankments. Suitable foundations are necessary to keep the embankments of spill channel stable. If a suitable foundation is not available for embankments, the embankment sometimes may have to be removed, if it would not cause problem in disposing of the desired quantum of flood. Not much literature is available relating to these kinds of studies carried out on large spill channels carrying more than 1.41 lakh m3/s discharge and without one embankment. The general procedure for selection of optimum channel parameters is of trial and error by considering number of alternatives and the hydraulic model studies play a key role in optimising the design of spill channel. Hydraulic model studies of Polavaram Irrigation Project, Andhra Pradesh were carried out in CWPRS, Pune on 1:140 scale 3-D comprehensive model to assess the subcritical flow conditions in spill channel with and without the provision of left embankment for various magnitudes of flood ranging upto 141583 m3/s. Studies enabled optimisation of layout of spill channel to pass the design flood with uniform flow conditions in the spill channel. In this paper, the various aspects of optimization of the hydraulic design of spill channel in disposing of the flood safely to downstream, based on the results of hydraulic model studies are discussed