Analysis of the Rainfall-Runoff-Inundation Characteristics with and without Considering the Sediment Transport: A Watershed Scale Modeling in an Ephemeral Hilly Catchment of Nepal.

Highly variable topography coupled with concentrated monsoon renders almost the entire parts of Nepal prone to flash floods and landslides. In this context, Siwalik Hills of Nepal which are geo-morphologically considered as the most fragile regions of Nepal faces the issues of soil erosion and land degradation. Majority of the rivers originating from these hills are ephemeral in nature where the flow only occurs as a direct response to the rainfall events. Flash floods are common in these rivers which tend to affect the channel morphology with every flood events. Furthermore, human-induced land-use change and haphazard construction work combined with extreme rainfall events have resulted in a significant increase in the sediment yield from these catchments. High sediment influx alters the dynamic equilibrium of the channel causing higher deposition due to the reduced sediment transport capacity in the lower reach. Such processes have led to the continuous aggradation of the riverbed to such extent that the adjacent communities in the floodplain are now well below the river bed level at many places. Under such condition, the risk of flooding and the consequent damage to the adjacent areas naturally increases. Majority of the inundation studies have not incorporated sediment transport in their analysis. In the current study, we are making an attempt to characterize the inundation scenario with and without considering the sediment in the simulation. In this regard, Bakra river basin originating in the Siwalik region is chosen for the study where the problem of soil erosion, riverbed aggradation and inundation are critical. The basin elevation varies between 200m to above 2380m above sea level in a short stretch of about 12kilometers. A catchment scale landscape evolution model (CAESAR-LISFLOOD) is applied to the watershed to investigate the inundation scenario under unsteady non-uniform rainfall input with and without considering the sediment. Preliminary simulation has demonstrated a reasonable match with the real field cases in terms of the critical location and the magnitude of inundation. Differences with and without sediment in the inundation extent, depth, timing, etc. will be analyzed.