The 2000/01 Floods — a Hydrological Appraisal
In common with most protracted flood episodes, the 2000/01 floods was characterised by large spatial and temporal variations in severity, but four reasonably distinct phases could be identified:
Intense rainfall was reported from many localities in across southern Britain at the end of the second week of September — causing local, mostly urban, flooding in (amongst other areas) Portsmouth, parts of west London, Penzance and West Sussex. On the 19th, large areas of the Midlands and East of England reported more than 20 mm of rainfall. The 28/29th September period was also very wet, notably in northern parts of the UK, and flooding was reported in Bangor (Northern Ireland).
The worst hit areas were southwest Kent and East Sussex, where a near-stationary front (with embedded thunder calls) was responsible for exceptional rainfall. Barcombe recorded 175 mm in less than 72 hours (return period approx. 400 years). Rivers draining impermeable catchments in Kent and Sussex (e.g. the Uck, Ouse and Teise) registered very high flows, with particularly severe flooding across parts of the catchments of the Medway and Sussex Ouse. In Lewes, the Environment Agency estimated that on 12th and 13th October 817 properties and business were flooded and needed to be evacuated4. Flooding also occurred across southwest England, in parts of Northern Ireland and in Dumfries and Galloway in Scotland.
There was persistent rainfall across many catchments on the 29/30th October, with many areas reporting falls of greater than 40 mm. Persistent rainfall continued through the first week of November, generating notably high flows (over the 5-8th November) in catchments across much of the country. The areas most severely affected were the South-East, Yorkshire, western catchments (especially the Severn Basin), parts of northern Britain and Northern Ireland. On 9th November, the Environment Agency reported that the area of land under floodwater around Selby on the lower Yorkshire Ouse was “bigger than Lake Windermere”5. Return periods in the 15-30 year range were ascribed to peak flows for many rivers in England and Wales, and maximum recorded flows were exceeded in some catchments — for example, the Whiteadder (a tributary of the Tweed) and the Annacloy in Northern Ireland. High flows continued in many river systems well into December but declined steeply in most impermeable catchments towards year-end.
Caused by the remarkable rises in water tables levels in the chalk and other permeable aquifers. Records from the Environment Agency confirmed that the recharge in some eastern outcrops was the highest for the autumn in a series since 19206. This resulted in exceptional outflows from high level springs and extensive groundwater flooding. In December significant peak flows were recorded on the Avon and Itchen in Hampshire and the Lavant in West Sussex. Localised ‘clear-water’ flooding from springs, resulting from high water-tables, continued in many areas (e.g. the Berkshire Downs and Chilterns) well into spring 2001. Although drier than the preceding four months the January-April 2001 period was the wettest start to the year for England and Wales since 1951. Many catchments therefore remained saturated - and sensitive to further flooding — until the late spring of 2001. Many rivers and streams supported principally by groundwater remained above previous maximum flows for lengthy periods.