Estimation of the rate of erosion of valley fill deposits in a part of the NW Lesser Himalaya

Abstract

As the discharge increases and sediment supply decreases, the rivers incise forming terraces. When considered over a long period, the sediment storage in the valleys become a source for sediment supply and may contribute a significant amount of the total sediment budget. An estimate made by mapping the area of the terraces and multiplying it with their thickness suggests ~ 4800 million tonnes of sediments is trapped between Pipalkoti and Devprayag in the Alaknanda valley (Fig. 1a). These terraces continuously get eroded by various processes such as gullying, undercutting by rivers and collapsing under the influence of gravity contributing to the adjacent rivers. Therefore, apart from the rate of erosion of the bedrock, it is essential to calculate the rates of erosion of such valley fill deposits and quantify the amount of sediment supplied by them to the river systems. In this study, an attempt has been made to determine the rate of erosion by calculating the volume of sediment removed from ã 17 ka terrace located in NW Lesser Himalaya. River terraces (referred to as terraces from here onwards) form when a river becomes unstable and incises its bed (e.g. Schumm et al., 1987; Bridgland, 2000; Wegmann and Pazzaglia, 2009). Thus, terraces record the time of instability of the channel, making it an excellent geomorphic marker. Accordingly, terraces are commonly studied to identify tectonic and climatic events causing changes in the river dynamics in the geological past (e.g. Wegmann and Pazzaglia, 2009). Study area A requirement of this study was a terrace of known age with least interference from external processes such as anthropogenic activity or sediment supply on terrace surface by local stream, so that sediment volume of the terrace is only altered due to natural erosional processes. Thus, a terrace located in the Srinagar (Garhwal) region along the left bank of the Alaknanda River was chosen for the study (Figs. 1a,b) which fulfilled the above requirement. A village named Swit is located on top of the terrace under investigation, hence from here onwards this terrace is referred to as the Swit Terrace. It is located 5 km ENE from the main Srinagar city (Figs. 1a,b). The elevation of the Swit terrace is ~ 655 m asl (above sea level) and covers an area of 0.6 km 2 and the elevation of its top from the river bed is ~ 100 m. A small stream originating from the adjacent valley walls, and locally called Swit nala, flows into the Alaknanda river. The local streams are seasonal and are fed mainly by the monsoon. The study area receives an average annual rainfall between 1000 mm to 1500 mm (Sati et al., 2007) with maximum rainfall occurring between mid-June to mid-September. Method A geomorphic map of the study area prepared by previous worker Depositional river terraces in tectonically active regions are used to determine the relative roles of tectonic and climatic changes in the landscape evolution of an area. Apart from providing evidences of tectonic and climatic shifts, these terraces over a long period act as a s...