Reservoir Sedimentation and Potential Impact of the Proposed Smithfield Dam on the uMkhomazi River on the Coastal Sediment Budget and Shoreline Stability

This conference paper investigates reservoir sedimentation and downstream coastal impacts associated with the proposed Smithfield Dam on the uMkhomazi River, KwaZulu‑Natal, South Africa. A review of regional sediment yields was conducted and combined with 2‑dimensional hydrodynamic modelling to simulate reservoir sedimentation over a 100‑year period, indicating potential storage capacity losses of 36–66% depending on future sediment yield scenarios. A 1‑dimensional hydrodynamic and sediment transport model was applied to assess pre‑ and post‑dam sediment routing from the dam site to the river mouth. Results show that the dam would trap nearly all coarse sediment, leading to a long‑term reduction of approximately 74 000 t/a (≈21%) in sand delivery to the coast. This reduction corresponds to a 10% decrease in total longshore sediment supply at Durban, with implications for coastal sediment budgets and shoreline stability north of the uMkhomazi River mouth.

The study further evaluates cumulative impacts of existing sand mining, showing that dam sediment trapping and sand mining would together reduce fluvial sand supply by up to 37%. Potential mitigation measures include sediment bypass tunnels, improved sand‑mining regulation, catchment management, and targeted beach nourishment. The findings highlight the importance of integrated river–coast sediment management in future dam planning.