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

Abstract

River regulation by dams and embankments drastically reduce/alter flow, which affects the natural channel pattern. Existing braiding indices have seldom incorporated the effects of diurnal flow variations caused by hydropeaking, leading to over/underestimation of the braiding intensity. These indices consider only the visible wet channels, ignoring the existence of dry channels that are activated only episodically during phases of water release from hydropower dams. We have extracted the dry channels (those that are periodically wet) coursing across the channel belt of the highly regulated River Tista from Landsat images between 1977 and 2014, using Normalized Difference Wetness Index values. These were combined with existing wet channel widths and numbers to formulate the Regulated Braiding Index (RBi) for characterizing channel braiding in the Tista's course over the Himalayan foothill plains. Overall, the widths and numbers of wet channels decreased by 63% and 25%, respectively, during the regulated years (2003 and 2014) as compared to pre‐dam years (1977 and 1995) due to the collective operation of 14 upstream dams/barrages (having cumulative ~89 million m3 reservoir capacity), whose operations reduced the braiding intensity (eliciting lower RBi values). Further, the number of high braiding reaches decreased by half while low braiding stretches rose by 10% in comparison to the pre‐dam period due to reduced/regulated flow. Comparative analysis of derived RBi values with three existing braiding indices revealed that RBi values consistently decreased near barrages, bridges, and within embankment‐confined reaches, unlike the other indices, thus providing a better framework for assessing expected river regulation effects.