Climate change and land-use change impact the sediment flux and grain size delivered to rivers. This in turn influences channel morphologies and so modifies flood risk. High mountain catchments are particularly sensitive to morphological changes in channels forced by changing sediment supply due to the close connection between hillslope sediment supply and channels in such environments. In this study, we explored the Nakkhu River, the largest southern tributary within the Kathmandu basin, Nepal. The mobility of the channel is well documented and is caused by bank erosion, downcutting, and accumulation of bar forms; these processes are particularly important during extreme flood events, as evidenced by analysis of repeat satellite images. Bank erosion and down-cutting increase a channel’s discharge capacity leading to a reduction in flood inundation and upstream water storage. In contrast, the aggradation of river bars reduces a channel’s conveyance capacity. These vertical and lateral geomorphological changes impact flood risk downstream. Here, we investigate how changes in sediment grain size affect river morphology and flood inundation in the Nakkhu River. This is motivated by an awareness of gravel extraction in the upper reaches of the channel. We use the landscape evolution model CAESAR-Lisflood, combined with a newly generated (2019) 10 m digital elevation model, field-derived grain size data, and daily discharge data, to simulate erosion and deposition along a 14 km reach of the river. In a series of numerical experiments, we compare riverbed cross-sections, flood extent, and water depths for three model scenarios where we vary sediment grain size from fine sand to coarse gravel dominated distributions representing the natural variability of measured sediment data.

These simulations suggest that channel morphologies are sensitive to changes in sediment grain size distribution, and therefore that consideration of grain size as well as sediment supply is important for accurate flood hazard mapping in sediment-rich catchments. In addition, this study highlights some of the challenges regarding the grain size parameter and uncertainty to the landscape evolution model that need to be addressed.