The sinuous bedrock channel of the Tapi River, Central India: Its form and processes

The Tapi Gorge lies in the monsoon-dominated region of the Indian subcontinent. Because of the seasonality of rainfall and flows all the fluvial activity in the bedrock gorge is confined to the monsoon season, in general, and during a few high-magnitude monsoon floods in particular. Field investigations along a 30-km reach of the sinuous bedrock gorge indicate that the river displays all the morphologic properties of a meandering alluvial channel albeit with a much higher level of energy expenditure. Considering the perimeter lithology and channel morphology two types of reaches are evident in the field: a predominantly rocky and relatively straight reach close to the gorge-head, and a longer, sinuous reach of gravel deposition downstream. Hydraulic modeling of a rainfall-induced dam-failure flood indicates that large-magnitude events that exceed the threshold of bedrock resistance for a sustained length of time are capable of erosion. It appears that the overall channel and gorge morphology is adjusted to two types of thresholds. A threshold of boulder-transport, which is associated with large floods that are competent to entrain boulders but are incapable of bedrock erosion; and another higher threshold that is exceeded by truly high-energy processes that generate large total energy and exceed the threshold of bedrock resistance. The later threshold is exceeded only episodically, with fairly long periods of little or no bedrock erosion in between. stingly, meso-scale erosional features such as inner channels and well-developed potholes are nearly absent or inconspicuous within the gorge section. Whilst this could be partly attributed to the bedrock resistance, it appears that under the present hydro-geomorphic conditions the dominant fluvial activity is not directed towards the channel bed, but towards the banks. This is evident from the concentration of erosion on the outer banks and deposition of coarse gravel on the inner banks, and armoring of the channel bed. The main conclusion of the study is that the bedrock channel is increasing the flow resistance and energy losses by developing and enhancing the meandering pattern.