The UK’s Natural Environment Research Council (NERC) has ear-marked a £250K pot of money for new research applications investigating the causes and consequences of pluvial flooding. For those prospective bidders who do not know the meaning of “pluvial”, a definition is reassuringly offered (which is useful, as any such ignorance might potentially prejudice grant application outcomes): “Pluvial flooding is defined as flooding that results from rainfall-generated overland flow, before the runoff enters any watercourse or sewer. It is usually associated with high intensity rainfall events (typically >30mm/h) but can also occur with lower intensity rainfall or melting snow where the ground is saturated, frozen, developed or otherwise has low permeability resulting in overland flow and ponding in depressions in the topography. Urban pluvial flooding arises from high intensity ‘extreme’ rainfall events. In such situations urban underground sewerage/drainage systems and surface watercourses may be completely overwhelmed.”
The pluvial brouhaha is a direct result of unprecedented insurance losses brought by the June and July 2007 floods to England. Total losses of £3bn have meant that many of the UK’s major insurers will struggle to post any profit for the financial year. Fingers are pointing at pluvial flooding, which is not currently included in any systematic risk assessment and warning policy. The existing UK flood protection paradigm (in non-coastal areas) has evolved out of concerns with fluvial flooding - those instances when rivers exceed bankfull discharge and inundation of the floodplain follows. This monolithic approach has now been found wanting. It is widely being assumed that the incidence and severity of pluvial flooding is likely to increase with climate change. Additional factors exacerbating pluvial problems in a local context may include the “urban heat island” effect and the presence of too many sheep (see the previous mid-Wales post http://waterworlds.wordpress.com/2007/11/12/5-when-sheep-cause-floods/).