Formerly Addl. Director
Central Water and Power Research Station
While discharge characteristics of submerged ogee spillways have been investigated widely, very little has been done to study the effect of submergence of crest of low head barrages by downstream water levels. The results in respect of submerged ogee spillways can not be applied to broad crested barrages. The only reference available in this regard is the outcome of a study made by the Punjab Irrigation Research Institute, Malikpur, India, in the form of a drawing showing variation of the coefficient of discharge C, in the equation Q= C L H^3/2 with the Drowning ratio defined as (downstream water depth over crest/upstream water depth over crest), expressed in percentage. It is seen that the value of C starts decreasing as the drowning ratio increases beyond about 70% or so. This result has also been adopted in Indian Standard IS 6966 (Part 1):1989 titled Hydraulic Design of Barrages and Weirs- Guidelines, Part 1 Alluvial Reaches.
Recently, an attempt was made to assess the discharging capacity of a low height barrage by applying the above results. A 2-D flume model was also available for verification. The barrage had its crest near the river bed and had 45 spans each 15.5m wide separated by 4.5m thick piers. A typical section of the barrage is shown in Figure 1.It was found that the calculated values of discharges varied widely form those obtained from model study. This is shown in Figure 1. An analysis revealed that the Malikpur curve related the coefficient of discharge C only with the drowning ratio, while other parameters likely to influence C, such as length of the crest and height of the crest above the river bed in terms of head over the crest were not considered.
Since a low height barrage with high piers can be treated as a bridge also, it was decided to explore the possibility of applying the concept of bridge pier losses to arrive at the discharge passing under the bridge (i.e. over the barrage) with help of standard charts prepared by US WES in their Hydraulic Design Criteria. In this regard, charts under Open Channel Flow, Bridge Pier Losses, No. 010-6/1 and 010-6/2 were relevant. Using these charts, it is quite easy to calculate upstream water level corresponding to a set of total discharge and downstream water level. The upstream water levels for different discharges, as calculated above, are plotted in the same figure. It would be seen that the agreement between the observed and calculated water levels was excellent.
Figure 1: Comparison of observed and calculated values of discharges and upstream water levels.
The above case study reveals that a barrage with its crest level at or near the river bed can be treated as a bridge and the bridge pier losses can be added to the downstream water levels to obtain corresponding upstream water levels with good accuracy. On the other hand, application of Malikpur curve in such a situation will not result in reliable assessment of discharges since this is not akin to an overflow weir condition.
- Indian Standard IS 6966 (Part 1):1989 titled Hydraulic Design of Barrages and Weirs- Guidelines, Part 1 Alluvial Reaches.
- US WES : Hydraulic Design Criteria. Open Channel Flow, Bridge Pier Losses, Chart No. 010-6/1 and 010-6/2