Retrofitting overflow weirs to safely pass extreme floods at UK reservoirs

Many UK reservoirs were originally designed for historical flood standards that are now considered insufficient, especially with updated climate change guidance and revised probable maximum flood (PMF) estimates. Reservoir overflows are critical safety elements of the reservoir and retrofitting them to safely pass extreme floods is a key priority under post-Toddbrook safety reviews. The aim of this paper is to demonstrate a structured methodology for selecting and justifying reservoir overflow retrofit solutions to safely pass extreme floods (PMF, 10,000-year flood). The approach focuses on increasing the discharge capacity of the overflow as identified through flood study results to meet wave freeboard requirements, or on implementing additional changes to increase the reservoir storage capacity, in compliance with current UK dam safety requirements. Using four different case studies of reservoirs in UK, the paper demonstrates how different retrofit options such as piano-key weirs, labyrinth weirs, broad-crested weir lowering, and other modified broad-crested weir configurations, were evaluated and selected. The preferred solution for each reservoir is selected based on hydraulic performance, site constraints, cost-effectiveness, carbon footprint, and constructability.