The 2000/01 Floods — a Hydrological Appraisal

Introduction and Overview

The extensive flooding across the United Kingdom during the autumn of 2000 and the winter of 2000/01 was the most severe flood episode, on a nationwide scale, since the flooding which followed snowmelt in March 1947. Unprecedented rainfall, over periods extending beyond eight months in some regions, resulted in the redefinition of high flow regimes over wide areas and a substantial extension of the range of recorded groundwater levels. This report documents the flooding within a hydrological framework and concentrates on the period from September to December 2000, but it also addresses the remarkable groundwater conditions which characterised the winter and spring of 2000/01.

Rainfall during the latter half of September 2000 greatly reduced the capacity of soils to absorb further rainfall and, in most areas, the autumn-winter 2000/01 floods were a response to sustained frontal rainfall on near-saturated catchments. Frequent pulses of heavy rain continued through October, November and December, with the result that many new local and regional rainfall records were established across the country. England and Wales experienced its wettest autumn (September to November) on record and the September-December period established a new four-month rainfall maximum in the national rainfall series, which begins in 17661.

The heavy and widespread rainfall generated many record river flows. A significant number of individual peak flows had return periods of greater than 100 years and autumn/winter runoff totals exceeded previous maxima (often by wide margins) for the majority of gauging stations in England and Wales. Prolonged high flows sustained flooding over long periods in many catchments. A serious aggravating factor in several areas - the South-East especially — was the frequency of significant flood events; in some impermeable catchments six, or more, floodplain inundations occurred over a 13-week period.

The 2000/01 flooding may broadly be divided into four phases. The first phase, from mid September through to the end of the month, resulted in significant, but localised, urban flooding. The second phase was concentrated in southeast England as a result of the extraordinarily wet period from 9-12th October. The third phase, during which there was extensive floodplain inundation across much of the UK, was triggered by very vigorous storms on the 29/30th October; heavy rainfall continued - generating further peak flows in early November - and high flows continued in many river systems well into December. The fourth phase was the result of unprecedented rises in water-tables in the Chalk and other aquifers. During December, groundwater-fed rivers in much of southern England experienced outstanding peak flows. Exceptional spring outflows and serious flooding, associated with extremely high water-tables, continued - in many areas - well into 2001.

The Environment Agency estimated that during the autumn of 2000, 10,000 properties were flooded at over 700 locations in England and Wales2. Serious flooding also occurred in Northern Ireland and southern Scotland. There was widespread dislocation of rail and road travel, and the prolonged saturation of agricultural land greatly restricted farming activities. Significant soil erosion was also reported3. The flooding focussed public, political and media attention on the possible effects of climate change on flood magnitude and frequency, the dangers of inappropriate floodplain development and, more locally, the contribution of land management practices to flood risk.

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