Hydrological impact of an extreme precipitation event the Carlisle January 2005 flood

The flooding of Carlisle in January 2005 was part of a much wider event that resulted from very heavy rainfalls across the whole of Cumbria. The rainfall was caused by a flow of unusually warm air, forced northwards ahead of an Atlantic cold front. The rainfall was initially enhanced by strong orographic effects over the mountains of the Lake District and later by strong frontal uplift and convection, as a depression centre passed further north. The maximum observed rainfall during the 7/8 January was 213 mm at Honister in the centre of the Lake District with over 200 mm recorded further east within the River Eden catchment that drains through Carlisle to the Solway Firth (see Fig. 9.4 and Table 9.4). Peak rainfall intensities recorded over 15-min periods were over 20 mm h-1 but high intensities were observed over long periods. In fact the initial flooding at Carlisle was pluvial flooding resulting from the drainage system being overwhelmed by the sustained rainfall intensities. Later, fluvial flooding resulted from the River Eden and its tributaries in Carlisle, the Petteril and Caldew, going out of bank. More than 1800 properties were affected and two elderly residents were drowned. The police, fire and rescue services properties in the city centre were flooded and had to be temporarily relocated.

This event also occurred on an already wet catchment area. The calculated runoff coefficient for this event was exceptionally high at 73 per cent. The estimated peak flow of 1520 m3 s-1 at the Sheepmount gauging station in Carlisle is the highest recorded in the catchment (and the highest recorded in the Environment Agency's archive of river flows in England and Wales). At the time of the event, plans for new flood defences for Carlisle were displayed for a final public consultation. They were designed to protect against an event that might occur once in 100 years. Even if they had been built, however, the water levels achieved in this event were higher than the level of protection in the design. The plans have now been revised and new defences built to a higher level of protection.

Fig. 9.4 The January 2005 Carlisle flood event: (a) rainfall totals for 6-8 January 2005; (b) 15-min rainfalls recorded at Wet Sleddale and Brothers Water in the period 5-10 January 2005 (redrawn from Environment Agency, 2006).

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