OCSC were appointed to conduct a Flood Risk Assessment for a new development comprising retail and student accommodation in the centre of London. The site is located in the natural floodplain of the River Thames and is defended by flood defence walls. The Flood Risk Assessment dealt with all possible sources of flooding including fluvial, tidal, off-site infrastructure, groundwater, development runoff and failure of the existing flood defence walls.
The existing flood defence walls are maintained and controlled by the UK’s Environment Agency and provide the required level of protection from tidal and fluvial flooding. However, the Flood Risk Assessment identified a residual flood risk arising from partial failure of the flood defence walls during a flood event in the River Thames. To evaluate this residual risk, OCSC constructed a 2-dimensional computer model of central London using ISIS software.
The study area comprised 2km² of densely populated urban area in the centre of London. The inflow hydrograph was defined using data for an extreme tidal flood event in the River Thames. The breach was selected at a location critical to the development site and the duration of the breach was based on the Environment Agency’s expected response time for repair works. A high-resolution 2m-grid was used to effectively model the flow paths along streets and laneways in the study area.
The analysis produced flood mapping of the study area showing extent of flooding, flood depth and flood flow velocity at selected time intervals. The resulting data was used to determine the flood hazard rating at critical points within the study area. The Flood Risk Assessment was prepared in accordance with PPS25 and was approved by the UK’s Environment Agency.