Sediment Transport and Flood Risk: Impact of Newly Constructed Embankments on River Morphology and Flood Dynamics in Kathmandu, Nepal

Floodplain encroachment by embankments heightens flood risk. This is exacerbated by climate change and land-use modifications. This paper assesses the impact of embankments on sediment transport, channel geometry, conveyance capacity, and flood inundation of a reach of the Nakkhu River, Nepal. Using the CAESAR-Lisflood landscape evolution model based on a 2-m digital elevation model, we simulate four flood scenarios with and without embankments and sediment transport: a historical 25-year return period flood event used to design the embankments, 50-year, 100-year, and 1000-year return period flood events forecast using the Generalized Logistic Model (using data from 1992 to 2017). Our results indicate that flow confinement by embankments reduces inundation by 99% (from 22.5 to 0.3 ha) for the historical 25-year flood discharge of 42.23 m3/s ${\mathrm{m}}<^>{3}/\mathrm{s}$ and by 15% (from 28.8 to 24.4 ha) for the 1000-year return period flood discharge of 95 m3/s ${\mathrm{m}}<^>{3}\mathrm{/}\mathrm{s}$ (similar to a 25-year maximum mid-future). The presence of embankments increases downstream sediment transport by more than 32% for all flood scenarios considered. Inclusion of sediment transport leads to a fivefold increase in predicted inundation area for a 25-year maximum mid-future flood compared to the no-sediment case in the embanked channel. Changes in channel geometry due to sedimentation significantly reduce conveyance capacity increasing overtopping flood risk, particularly where the channel is sinuous or located on flat terrain. Our results indicate that sediment erosion in outer meanders may threaten embankment stability by promoting undercuts. It is recommended that sediment transport effects be factored into embankment design and floodplain planning. Our research explores the impact of flood protection embankments being constructed along the Nakkhu River in the Kathmandu Valley, Nepal, in a region that is experiencing rapid urban growth. Using advanced computer simulations, we study how these embankments influence the erosion and deposition of sediment in the river, and hence impact flood risk. Our findings indicate that the construction of embankments increases sediment transport, and alters the geometry of the river increasing downstream flood risk during extreme flood events. This is particularly the case for embankments designed to follow natural, meandering river courses. We recommend incorporating sediment transport analysis into the planning and design of embankments and developments in floodplain areas to reduce the risk of flooding. Our study indicates that embankment construction by itself may not always be a sustainable long-term flood-protection measure for rivers carrying high sediment loads. Inclusion of sediment processes is very important in predicting the effect of embankments on river flood risk For the embanked Nakkhu, predicted inundation is fivefold larger for 25-year maximum mid-future event when sediment transport is included Sedimentation reduces channel capacity for flat terrain and large meanders; erosion at outer meanders threatens Nakkhu embankment stability