Studies on the Effect of Unsymmetri-cal Operation of Radial Gates on the Flow Conditions in the Stilling Basin of Bhakra Dam Spillway
Of the four radial gates provided for the passage of flood waters over the Bhakra Bam Spillway, one or more gates might get jammed at any time due to mechanical defects or damage and thus cause un-symmetrical flow in the two halves of the spillway during the flood period. With the uneven flow over the spillway crest the conditions of flow in the stilling basin were also likely to be very much different than under normal conditions when all the four gates were worked uniformly. It was essential to investigate the effect of such an unbalanced flow caused by unsymmetrical operation of the spillway gates on the conditions of flow in the stilling basin and in ike river downstream. Model studies were, therefore, made at the Hydraulic Research Station, Malakpur to examine the effect of different combinations of radial gates working at a time on the position of the hydraulic jump, the turbulence, the water surface profile, the wave lap and erosion of the river bed downstream of the stilling basin.
A detailed study has been made of the effect of opening of one, two and three gates of the Bhakra Dam Spillway. All the necessary studies, such as position of hydraulic jump, the flow conditions in the stilling basin, wave action along the training walls and scour in the river bed, etc., etc., were carried out. The conditions of flow have been illustrated with the help of photographs taken in each case. The amplitudes of wave lap against the training walls for each case are given and the bed scours for each condition plotted.
It was observed that when only one gate in one half of the spillway was open, the jet spread over the spillway and there was no action on the river bed downstream of the stilling basin. The wave lap against the training walls was also very small Opening of one gate, therefore, did not cause any adverse effect on the training walls or the river bed. When two gates in the same half of the spillway were operated,, a strong back flow in the other half took place which went back again to the other half from over the central training wall resulting in the shifting of the hydraulic jump and extending the turbulence into the river beyond the stilling basin. This strong back flow in one half brought a lot of bed material from the river into the stilling basin. This was a very undesirable effect and as far as possible, opening of the two gates only in one half of the spillway when the other half was closed was to be avoided. Pulsations against the right training wall were also higher.
When three gates were opened, two in one half and one in the other half, the conditions were much better than with two gates in the same half working.
These studies are dealt with in this paper in certain details. The results of these studies are of general application as they provide a very good guide for regulation of escapage at the dams.