The Environment Agency estimates that 200,000 homes in the North West are at risk from flooding, Sefton Council takes this risk seriously, having suffered severe surface water flooding a number of times since 1990 with significant flooding events occurring in 2001, 2004, 2008, 2010 and 2012. In addition, parts of Sefton are influenced by shallow groundwater and a fluvial flood risk that is currently managed by pumping. These factors make Sefton sensitive to the effects of climate change. In order to address some gaps in understanding from previous Merseyside Flood Management Plans, Sefton Council, in partnership with the Environment Agency, Capita Symonds and United Utilities developed a Surface Water Management Plan (2011). Alongside the SWMP a Strategic Risk Assessment was produced, this applied a risk based approach to spatially identify urban and highly developed areas at most risk from flooding. Hydraulic modelling was undertaken of both sewer flooding and surface water flooding for storm events with a 1 in 30 (3.3%), 1 in 100 (1%) and 1 in 100+cc (1%+cc) chance of occurring in any given year, cc represents a climate change factor. This data is the 1 in 100+cc (3.3%) model. The impacts of climate change were also considered by increasing rainfall intensity by 30%. The SWMP models were updated to include new information in 2013.
The models have been constructed for the purpose of assessing surface water flood risk in Sefton. This modelling was undertaken by Capita Symonds using ESTRY-TUFLOW software version 2012-05-AE. Each model has been built upon a 5m cell size and simulated using the software. The drainage network has not been explicitly modelled however it has been represented by applying a continuing loss to impermeable land uses.
The modelled outputs were based on flood velocity, depth and hazard, a debris factor was used in calculating hazard rating e.g. debris factor of 0.5 for depths,=0.25 and 1 for depths>0.25, based on DEFRA’s guidance methodology, for further details please reference DEFRA’s Flood Risks to People Phase 2 – FD2321/TR1.
The Environment Agency estimates that 95% of all flood damage occurs during the first 23 cm of external floodwater. In order to assess the potential damage to property in Sefton, the models used an impact based on there being a depth of flooding at each property of at least 0.08m to produce the flood outlines. However, because of the way in which buildings are represented, this equates to a flood depth around the building of 0.23m. (i.e. the mean flood depth is simulated to exceed 80mm inside the property, which is equivalent to 230mm externally).
The GIS data has been checked for geographic errors and is accurate.