Enhanced river regulation following dam reoperation: Impacts on flow regime, sediment transport, and planform adjustment in a plateau river basin in the lower Ganga plains

Abstract

In the last century, more than 58,000 large dams were constructed worldwide. Meanwhile, some of these older dams were renovated/reconstructed and started reoperation with enhanced regulation capacity to meet the rising water demand. However, the impact of dam reoperation on rivers is not thoroughly understood. This study aimed to examine the impact of dam reoperation on the Kangsabati River’s flow regime, sediment transport and planform adjustments in upstream and downstream of the reconstructed anicut (like a dam/barrage) in the lower Ganga plains. The anicut structure was built by the British in 1866. It was demolished, underwent further reconstruction, and recommenced operation in 2016. While the Kangsabati reservoir (KR), with 1135 million-m3 (MCM) capacity in 80 km anicut’s upstream, has been functional since 1974. Together, these regulation structures enable irrigation across 0.4 million hectare cropland in dry regions. Meanwhile, the KR operation reduced 34 % (1974–2010) and 70 % (2010–2015) flow before the new anicut’s reoperation. Likewise, within five years, the reoperation reduced 35 % mean annual flow and two-thirds of the non-monsoon flow, despite the anicut’s small 6-MCM storage capacity that redoubled the flow regime shift. After the reoperation (16 %/year), the enhanced water impoundment reduced fluvial landform diversity by submerging upstream channel area and abandoning/exposing downstream landforms, as effectively captured with the non-water/water index (NWI). It led to significant channel planform alterations, with 85 % expansion of the upstream channel, 44 % contraction of the downstream channel, and 72 % variation in both upstream and downstream water levels, unlike before the reoperation. Reoperation disrupted sediment infilling of 1.3 MCM downstream sand mining pits. This deprived the downstream communities, relying on mining activities for their livelihoods. Thereby, the reoperation-induced regulations have ignored upstream and downstream ecological connectivity and ecosystem services, resembling the impacts of any large dam. This requires urgent attention and strategies aimed at restoring river ecology and associated benefits.