A novel approach to quantify the changing channel pattern in a regulated Himalayan river

The multi-channel braided pattern is very common across the alluvial plains of the major Himalayan rivers. The braided pattern indicates the relationship between streamflow and sediment that carries down from the Himalayan hinterlands to the low gradient alluvial plains. Hence, previous studies have used braiding indices to quantify the channel pattern to understand such fluvial processes. The optical satellite images are the most reliable source to quantify the spatiotemporal dynamics of braiding but essentially provide information on visible wet channels. Whereas, many hydropower dams across the Himalayan river basins are usually impounding the diurnal water into the reservoirs and releasing water during the night to generate hydroelectricity in the non-monsoon months. Consequently, existing braiding indices are sensitive to a few visible wet channels during daytime. However, the diurnal dry channels in the optical imageries are in fact the active channels during the night, which can be extracted using image-based indices substituting the visible channel pattern. Based on this principle, the present study compared the existing braiding indices with the proposed regulated braiding index (RBI) for 30 years of spatiotemporal analysis for the regulated braided river Tista in the Eastern Himalaya, India. Again, dry channels are rarely used as parameter of braiding in previous studies.

The study has divided the ~100 km river stretches on the alluvial plain into 5 km reaches and further 1 km sub-reaches for comparison. More than 13 hydropower reservoirs in the mountain basin and a diversion barrage in the plain have been altering the streamflow. Eventually, the RBI values decreased during the post-regulation period, compared with the pre-regulation natural condition with relatively high streamflow. Thus, reducing wet channels represents the degree of regulation and remain a significant parameter of braiding. However, integration of diurnal dry channels has improved the RBI value during the post-regulation period representing the actual braiding condition. Reaches regulating water by the barrage and channel constriction at the railroad bridge observed distinct RBI values compared to other indices. Therefore, the new index has shown better results than the existing indices for the regulated river condition.