Hydrological Evaluation of an Artificial Lake

Abstract

The Irrigation and Flood Control (I & FC) Department of J & K State planned to construct a barrage across the Tawi River at Jammu for creating a lake for recreation purpose. The aim of the present study is to evaluate the hydrology of the lake and ascertain its sustainability based on the trend of flows in the river. Various ancillary methods have been used to develop the minimum database required for the study. The elevation-area-capacity table of the likely pondage has been obtained by observation of spot levels in the submergence area of lake by GPS and then by processing the data in GIS. Daily inflow data of 30 years of record at Jammu on Tawi River has been obtained from the CWC. Pan-evaporation depth data of 11 years has been obtained from the observatory of SKUAST, Ponichak, Jammu. In/around the lake, tests using double ring infiltrometer and Guelph permeameter have been carried out at 8 locations to estimate the infiltration capacity and hydraulic conductivity of lake bed. The infiltration capacity is found to range from 2 to 8 m/day. A preliminary groundwater flow model (Visual MODFLOW) has been set up and the water loss through seepage from the lake bed has been estimated. The analysis shows negligible seepage losses from the lake under the conditions of impervious left and right embankments, impervious barrage resting on relatively impervious stratum and high water table in the lake bed. Long-term simulation analysis of the lake has been carried out using daily inflow series from June 1977 to May 2007. A computer program has been developed that computes the reliability of maintaining the lake levels above specified elevations. A number of simulation runs have been taken with different scenarios of seepage loss rate (1%, 10%, 20%, and 30% of the storage capacity at an elevation) from the lake. Detailed tables have been prepared specifying the reliability of maintaining the lake above specified elevations assuming different loss rates. In addition to cumulative results for entire period, reliability analysis for individual months has also been carried out. Average annual values of hydrological variables (inflow, evaporation loss, seepage loss, and spill) under different scenarios have also been computed. Trend analysis has been performed on the inflow data considering annual and monthly series. Annual flows in the River Tawi show decreasing trend with a average reduction of 1.26% per year with respect to the mean. Monthly trends show decreasing trend for months of January, February, March, April, July, and August while increasing trend for other months. To account for the trend of inflows in the river Tawi at Jammu, the simulation analysis for the lake has also been carried out with the modified inflow series using annual trend analysis and monthly trend analysis. The daily inflow values have been modified as per the trend analysis assuming that the present trend continues for another 30 years. Reliability analysis has been carried out with the modified inflow series and tables showing reliability of maintaining the lake above specified levels under different scenarios of seepage losses have been prepared. In general, it is observed that if seepage losses from the lake can be controlled (which is quite likely as per the findings from the preliminary groundwater model study), then lake can be maintained at higher levels (with depth exceeding 4 m) with high reliability exceeding 80%. Further, the results show decreased reliability in winter months only when the recreation activities in the lake generally tend to minimize. Reliability tables for different scenarios of inflows and seepage losses have been prepared which may help the concerned department in arriving at various decisions related to the lake. The developed model can also be used to further analyze any one or a combination of developmental scenarios.