ENVIRONMENTAL ASSESSMENT OF THE AUXILIARY KANDAR DAM AND ITS INFLUENCE ON THE LIFESPAN EXTENSION OF THE MAIN RESERVOIR: EVIDENCE FROM KOHAT, PAKISTAN
ENVIRONMENTAL ASSESSMENT OF THE AUXILIARY KANDAR DAM AND ITS INFLUENCE ON THE LIFESPAN EXTENSION OF THE MAIN RESERVOIR: EVIDENCE FROM KOHAT, PAKISTAN

Summary/Abstract: Reservoir sedimentation poses a significant challenge to water resource sustainability, as it reduces storage capacity, compromises hydraulic performance, and shortens the operational lifespan of dams. In Pakistan, many reservoirs face accelerated sediment accumulation due to steep catchment slopes and intensive land use, yet limited research has focused on the role of auxiliary dams in mitigating these effects. Objectives: This study aimed to assess the environmental impact of the Auxiliary Kandar Dam on extending the lifespan of the Main Reservoir in District Kohat, Khyber Pakhtunkhwa. It sought to quantify sediment deposition, evaluate trapping efficiencies, and model future storage scena rios under different auxiliary dam construction timelines. The research addressed a key knowledge gap regarding how auxiliary dam construction can optimize sediment retention and improve reservoir sustainability in semi-arid regions. Methods: A detailed grid survey was conducted to determine the storage capacities of both reservoirs, with volumetric and spatial analyses performed using SURFER software. Sediment yield estimations were calculated through the HR Wallingford Sediment Yield Prediction Model (WSYPM). Field observations, hydrological data, and chronological sediment records were integrated to validate trapping efficiencies and predict future capacity loss scenarios. Results: The initial capacities of the Main and Auxiliary Dams were 1,000,365 m3 and 1,994,974 m3 , respectively. Over nine years, the Auxiliary Dam accumulated 196,654 m3 of sediment, resulting in a 9% loss in storage capacity. The Main Dam exhibited a trapping efficiency of 90.48%, which declined with a dec reasing inflow ratio from 0.508 to 0.194. Within six years, approximately 175,501 m3 of sediment were deposited in the Main Dam, causing a 61.78% reduction in total storage capacity over 41 years. The observed sedimentation patterns closely matched WSYPM predictions. Importantly, the construction of the Auxiliary Dam in 2014 extended the Main Dam's effective lifespan by 34 years (2022 – 2056). Modelling suggested that additional auxiliary structures in 2016 or 2022 could further prolong the lifespan by 80 years (2022 – 2102) or 68 years (2022 – 2090), respectively, while a proposed left Auxiliary Dam (2025) would extend it by 66 years (2022 – 2088), yielding an overall operational duration of 107 years (1972 – 2079). Conclusion: The findings confirm that the construction of the Auxiliary Kandar Dam significantly reduced sediment inflow into the Main Reservoir, effectively extending its operational lifespan and ensuring sustainable water storage for the region. The study successfully demonstrates, for the first time in this geographic context, the quantitative relationship between auxiliary dam placement and sediment reduction efficiency.

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