Impact of human intervention structures on the rivers: An investigation of the spatiotemporal variation of grain size in the Tista River, eastern Himalayas

Abstract

In order to investigate the effect of intervention structures like dams and barrage on the sediment grain size distribution, we examined a $75 km stretch of the Tista River in the foothill alluvial plain of the eastern Himalayas. However, studies on the impact of dams on streamflow and sediment in the Himalayan rivers are constrained due to inadequate historical gauge data. Therefore, we employed a simple method to track the initial formation of some of the existing river landforms that formed before the dam/barrage operations from yearly satellite images for in situ sediment sample collection. Hence, the collected sediment grain sizes analysis is used here as a proxy to comprehend the pre-, syn-and post-dam river sediment-transport pattern. We have grouped 23 landforms into three temporal clusters: (i) those formed before 1997, earlier than the dams/barrage interventions (TOL); (ii) formed between 1998 and 2010 during the upstream dam construction (TML) and (iii) formed after 2010 (TNL) when upstream dams and the barrage were fully operational. A total of 116 sediment samples from these landforms indicate the overall grain size reduction for TML and TNL sediment due to reservoir sequestration of coarser grains. In comparison, the coarsest TOL sediment with downstream fining trend represents the pristine river condition. The coarsening trend with a poor correlation coefficient of TML is primarily assigned to partial trapping of the coarse bedload and massive sediment generation by the upstream under-construction dams, resulting in the downstream erosion of the riverbed and transport of the coarser bedload further down.Similarly, post-dam sediment trapping and controlled flow were responsible for the weaker trend with a distinctly finer size of TNL. The historical satellite images and data on streamflow and suspended sediment reveal that contemporary river behaviour is consistent with the earlier interpretation.

A new braiding index to assess river regulation effects in multi‐thread channels: Insights from a highly regulated Himalayan river

“…Rainfall and seasonal streamflow continually erode the eastern Himalayas, resulting in very high denudation rates of 500–1000 t km −2 year −1 (Froehlich & Walling, 2007; Ghosh, 2021; Goswami, 1985; Walling & Webb, 1983). Anthropogenic activities resulting in land cover changes, allied with dam construction, have accelerated hillslope erosion in the Tista basin (Ghosh & Chakraborty, 2022). The river's steep gradient due to its elevation drop from ~7000 m near its source to ~50 m in the plains, enables it to transport coarse gravels to the foothills from the mountain hinterland and fine to coarser sands over its downstream alluvial plains (Ghosh, 2021).…”

“…For the present study, a ~100 km stretch of the Tista, from its mountain front at Sevoke to the extended alluvial plains till the India‐Bangladesh border, is considered (Figure 1). Within this riparian tract are multiple geomorphic features like gravel bars near the foothills to sandy bars further downstream (longitudinal, transverse, bank‐attached lateral bars and mid‐channel bars), islands, elevated riverbanks, floodplains with fine silt, sand and coarser sand, pebble, and gravel deposits dissected by multiple sinuous channels (Ghosh & Chakraborty, 2022). In the alluvial plains, several notable tributaries meet the studied stretch, such as the Lish, Gish, Dharala, and Karala.…”

“…It has channeled 2.3 billion m 3 /year of water and a 9.4-million-ton (mt) sediment load between 1998 and 2016, through two major canals, for irrigating 0.92 million hectares of croplands (Ghosh, 2021). However, more than 6000 m 3 /s of the peak monsoon flow is hardly retained in the reservoirs and only 5% of this is diverted by the barrage, compared to half the average 200 m 3 /s non-monsoon flow diversion and reservoir regulation (Ghosh & Chakraborty, 2022). The cumulative $89 mm 3 (million meters cubic) capacity reservoirs of these dams and the barrage (Figure 1) are responsible for the daily hydropeaking and episodic pulse-flow release phenomenon, which occurs primarily in the non-monsoon period to produce hydropower and augment irrigation (Ghosh & Munoz-Arriola, 2023).…”

“…Collectively, streamflow modifications (Figure 5a) and sediment alterations (Ghosh & Chakraborty, 2022;Ghosh & Munoz-Arriola, 2023) indicate that the role of regulation has potentially altered the character of channel braiding in the Tista. However, the changing braided patterns are complex, varying in space and time and depend upon multiple controls which are highlighted in the following subsections.…”

“…It Therefore, the RBi values avoid underestimation of the braiding condition (as noticed in Figure 9), confirming that the River Tista continues to exhibit a braided channel pattern throughout. However, due to the frequent and marked flow regulations in this river, modifications in channel braiding are numerous and over shorter terms (spatiotemporal) due to periodic dam releases and hydropeaking effects (Ghosh, 2021;Ghosh & Chakraborty, 2022). Despite such evident scenarios, only wet channels are popularly used to typify the braiding nature, as these strands are prominently marked in topographical maps or can be easily extracted from multi-spectral images using parameters like the NDWI (Kleinhans & van den Berg, 2011;Kuo et al, 2017).…”