Surface Water (Pluvial) Flood Map
Pluvial flooding is usually related to short duration high intensity rainfall. It occurs when the ground is unable to absorb the rainwater, causing it to flow across the surface and filling depressions and low spots in the landscape where local natural and engineered drainage systems are overwhelmed. The Surface Water Map layer illustrates the low lying areas and hollows that are estimated to be prone to flooding to surface from an extreme 1 in 200yr rainfall event. The Surface Water Map does not specifically illustrate areas that are at risk from flooding due to local deficiencies in sewerage or drainage systems which are at best designed to cope with a 1 in 30yr rainfall event.
Only areas which are estimated to be inundated from surface water flooding to a depth of 100mm are highlighted in the map. The main reason for this threshold is that the likelihood of flooding to property at depths up to 100mm is reduced by the step up in height from street level to ground floor level (which typically exceeds 150mm).
The surface water flood outlines have been generated using a relatively new and largely untested strategic surface water modelling technique. Although the surface water model used is currently the best available for a broad scale strategic assessment and similar to the models used by the other flood authorities within the UK and Ireland, the results should be treated with a degree of caution due to uncertainties in the methodology. For example, there is some difficulty in establishing an appropriate extreme rainfall event storm profile for specific return periods; there are deficiencies in the topographical data which can markedly affect the flood routing within the model and broad assumptions have been made about the capacity of existing drainage systems which affect the estimates of net surface water flows, particularly in urban areas. As a consequence the flooded area maps derived from the model should be treated with caution and viewed as indicative rather than accurate or precise.
