Hydrologic Assessment of Environmental Flows for Downstream Ecosystem of Indus River at Ghazi Barrage

Abstract

Water resources are developed across the world mainly for the cheap production of power to fuel the economic development of countries. To make these development of water resources sustainable, minimum quantity of water should be released downstream of the hydraulic structures for sustainable growth of riverine ecosystem. This minimum amount of water is termed as environmental flows. This study has been carried out for the hydrologic assessment of environmental flows for the downstream ecosystem of the Indus River at Ghazi Barrage. Ghazi-Barotha Hydropower Project was designed to meet the acute power shortage in Pakistan. The project has diverted almost the entire outflows of Tarbela Dam into the power channel, thus, leaving no flow for the downstream ecosystem. The diversion of river water and mixing of industrial waste of Gadoon Amazai and domestic sewage of riparian villages into low flows has caused devastating impacts on the downstream ecosystem of the Indus River at Ghazi Barrage. To make Ghazi-Barotha Hydropower Project sustainable, environmental flows are assessed by applying the most widely used hydrological techniques. These hydrological techniques are Tennant’s Method, Tessman’s Method, Monthly Modified Tennant Method, Low Flow Indices and Exceedance Percentile method, and single technique from hydraulic methods. For the assessment of environmental flows, hydrologic data of 40-years (1974-2013), in the form of daily mean inflows and outflow of Tarbela Dam, is used in the present study. The assessed values of environmental flows are then compared with the outflows released downstream of Ghazi Barrage. For comparison, hydrologic data of 8-years (2004-2012) in the form of daily outflows is used. A small survey was also conducted in the month of October, 2014 in 9 riparian villages of the affected reach of Indus River to evaluate the adverse impacts of reduced flows during winter seasons. The results of survey and comparisons of flows has indicated that environmental flows should be maintained in order to make the Ghazi-Barotha Hydropower Project sustainable. It is recommended that environmental flows should be maintained during winter season as suggested by different methods. Also, hydraulic methods, which are usually site specific, should be used for the assessment of environmental flows.