SURFACE FLOW HYDRAULICS OF TAUNSA BARRAGE: BEFORE AND AFTER REHABILITATION

Abstract

Taunsa barrage is one of the important diversion structures, built across Indus River. The barrage experienced ripping of downstream floor and damages to impact/friction blocks in 1959, just after one year of its completion. Similar damages were reported and subsequently got repaired in years 1962, 1986, 1994, 1998 and 2003. Excessive retrogression and consequently the sweeping of hydraulic jump were noted as the main reasons for the damages. Feasibility study on “Rehabilitation and Modernization of Taunsa Barrage” completed in Year 2005, proposed subsidiary weir with crest at EL424, to be constructed downstream of the barrage. The subsidiary weir location and crest level was finally fixed on the basis of physical model study carried by Irrigation Research Institute (IRI), Lahore, under the technical supervision of Punjab Barrages Consultants (Joint Venture of NDC-NESPAK in association with ATKINS Consulting Engineers, UK). Arguments have emerged regarding technical rationality of the provision of subsidiary weir downstream of Taunsa Barrage as rehabilitation structure. The surface flow analyses were carried out using computer software HEC-RAS to establish the location of hydraulic jump for various tail water level scenarios. This paper discusses the calibration of HEC-RAS model and the surface flow hydraulics of Taunsa barrage before and after its rehabilitation. The RAS model results indicate that under the existing condition (without subsidiary) the tail water depth was sufficient to develop hydraulic jump over the glacis.