A Spatial Analysis of Sediment Connectivity of Ranikhola River System in Sikkim Himalaya, India

Abstract

The concept of sediment connectivity refers to the degree of linkage between sediment sources and downstream areas. It is mainly affected by the longitudinal (dis)continuity of channels associated with depressions and confluences along the channel, and it represents variations in the balance between the sediment load and the transport capacity of the channel along a line. This study analyzed the stream channels as pathways and sinks of runoff carried with sediment along a mountainous catchment. The aim of this study was to investigate the effect of streams on connectivity and the linkage between sediment source area, stream network and outlet, and their contribution to sediment connectivity patterns in the mountainous catchment. In addition, to validate the output through field investigation at specific locations considering the degree of (de)connectivity. The index of sediment connectivity (IC) is used to analyze the sediment connectivity in the study area. The shape of the glacier cirque significantly influences the connectivity in the upper hanging valleys (Messenzehl et al., 2014). IC decreases when channels are considered conduits and sinks for sediment-loaded runoff, particularly on upper hill slopes. The map shows that the closer the distance to the channels, the more significant impact on the IC. The IC channels map showed the areas closer to the main channel have high IC values (1.160 to - 0.608), while sub-catchments and steep hill slopes have low values (- 5.912 to -7.680). On the other hand, it is evident from the IC Outlet that the upper Ranikhola region has a low IC value (i.e. -9.21), while the lower reach of the main channel receives a high value (i.e. 2.14). So, there was a striking difference between the IC channel and the IC outlet value for the Ranikhola basin. The formation of the flat regions on steep slopes caused by terrace farming typically reduced connectivity, although, at some locations, topographic convergence brought about by terraces enhanced connectivity. In the river bed, the presence of an excessive range of loads, ranging from coarser particulate matters to erratics with a mean load diameter size of 6 to 7 feet (6 ft., which indicates medium to larger monolithic deposits), and boulders that are commonly found to be coarse to very coarse in size (ranging from approximately 6.56 metres to 13.12 metres in length), makes it easier for sand to become trapped. This study also revealed that this model is helpful when understanding the sediment movement in a catchment situated at the foothills of the mountain. However, in this context, systematic field investigation was complex to uncover the links between hill slopes and river networks.