Abstract
Water reservoirs planned or constructed to meet the burgeoning energy and irrigation demands in Pakistan face a significant loss of storage capacity due to heavy sediment load from the upper Indus basin (UIB). Given their importance and the huge investment, assessments of current UIB sediment load and possible future changes are crucial for informed decisions on planning of optimal dams’ operation and ensuring their prolonged lifespan. In this regard, the daily suspended sediment loads (SSLs) and their changes are analyzed for the meltwater-dominated zone up to the Partab Bridge and the whole UIB up to Besham Qila, which is additionally influenced by monsoonal rainfall. The gaps between intermittent suspended sediment concentration (SSC) samples are filled by wavelet neural networks (WA-ANNs) using discharges for each site. The temporal dynamics of SSLs and discharges are analyzed using a suite of three non-parametric trend tests while the slope is identified using Sen’s slope estimator. We found disproportional spatio-temporal trends between SSLs and discharges caused primarily by intra-annual shifts in flows, which can lead to increased trap efficiency in planned reservoirs, especially upstream of Besham Qila. Moreover, a discernible increase in SSLs recorded at Partab Bridge during summer is being deposited downstream in the river channel. This is due to a decrease in river transport capacity in the monsoonal zone. These findings will not only help to identify these morphological problems, but also accurately anticipate the spatio-temporal changes in the sediment budget of the upper Indus River. Our results will help improve reservoir operational rules and sediment management strategies for existing and 30,000-MW planned dams in the UIB.
Highlights
Estimation of the suspended sediment loads (SSLs) is important in the design and operation of water structures and in the planning of sediment management to preserve their live storage capacities [1,2,3,4,5]
The root mean square error (RMSE) and mean absolute error (MAE) for Partab Bridge were approximately two times more than Besham Qila; likewise, the standard deviation (SD) and mean in the actual suspended sediment concentration (SSC) samples (Table 2). This difference shows the complexity in the transport process in the glacier influence zone of the upper Indus River at the Partab Bridge gauge
Our findings show that the variation in flow patterns have been causing a significantly increasing trend in suspended sediment loads (SSLs) in May and a significantly decreasing trend in August at both Besham Qila and Partab Bridge gauges in the upper Indus River (Table 7)
Summary
Estimation of the suspended sediment loads (SSLs) is important in the design and operation of water structures and in the planning of sediment management (yield reduction, routing and removal) to preserve their live storage capacities [1,2,3,4,5]. The Warsak dam constructed on Kabul River has filled with 60 Mt of SSL annually in the 30 years after its construction, and no structural or non-structural remedies can reverse its depleting storage. The second largest Pakistani water storing facility, had an initial storage of 7.1 billion m3 (BCM), which was reduced to 5.6 BCM in 2005 due to sedimentation. In 2009, an additional 9 m rise of the dam increased the storage to 9.1 BCM, which cost one billion USD over five years. In view of the transboundary nature of the source of water, such a decrease in water storage capacity in Pakistan exacerbates the instability and geopolitical tensions of the region [12]
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