Managing the dam breach modelling and mapping of reservoirs in Wales

Abstract

The Welsh Government enacted an amendment to the Reservoirs Act 1975 reducing the qualifying capacity of large raised reservoirs to 10,000m3 from 25,000m3.  This led to an increase in the number of reservoirs falling within the Reservoirs Act in Wales. In 2019, Natural Resources Wales (NRW) appointed Arcadis to undertake the production of a suite of reservoir inundation maps for over 180 reservoirs across Wales using the Environment Agency's Reservoir Flood Mapping specification. Following production of these maps an opinion on the risk designation was provided. Throughout the project several Arcadis teams collaborated to produce project specific documentation, develop tools for automation, run hydraulic models and produce the mapping, in line with the specification requirements. Upon request by NRW, modelling of the cascades was modified, allowing cascade reservoirs to breach due to a fluvial scenario. Following this modification, it became apparent some cascade reservoirs were being incorrectly triggered. Therefore, further changes to how cascade reservoirs were modelled were made to ensure they were triggered correctly.