Flood, RET and tiers: application of a Tier 3 risk assessment for reservoir safety

This paper documents the approach taken to carry out a Risk Assessment for Reservoir Safety Management Tier 3 risk assessment in response to Measures in the Interest of Safety recommended at Weston Turville Reservoir, Aylesbury, England. A risk estimation team (RET) was convened to identify, define and agree the relevant failure modes and to develop detailed event trees describing the stages leading from flood loading through to breach. The Tier 3 assessment included obtaining and reviewing topographic survey and ground investigation data, detailed one-dimensional (1D) hydraulic modelling of the bywash channel, erodibility and wave overtopping analyses, and 2D breach modelling to assess downstream consequences due to a breach of the subsidiary embankment. The paper summarises how the RET identified and defined the relevant failure modes, how the failure modes under consideration were used to determine the scope of further investigations and analysis and how these were used to carry out a quantitative risk assessment, including considerations of tolerability and proportionality. The study demonstrated that the identified failure risk was already as low as reasonably practicable and informed the certification of the Measure without the need for physical improvement works.