The 2000/01 Floods — a Hydrological Appraisal
A particular feature of the weather during autumn 2000 was the frequency of low-pressure systems — some notably intense - crossing the UK from the Atlantic. Most followed tracks further south than usual, a reflection of the unusual southerly position of the Polar Front. Exceptional sea surface temperature gradients over the North Atlantic (as occurred in late 2000) have been shown to be associated with the passage of vigorous frontal systems7. During the autumn, the paths of the most intense low pressure systems largely determined which catchments were most severely affected by the associated flooding.
Extensive rainfall generated by particularly vigorous frontal systems during October 2000 resulted in the second wettest October (after 1903) on record for England and Wales, and the highest monthly rainfall total since November 1970. Thunder cells embedded in a semi-stationary front caused especially heavy rainfall across parts of Kent and Sussex in mid October. Low pressure systems, commonly with notably low central pressures, continued to track across central and southern Britain, producing outstanding rainfall accumulations. However, the speed of some frontal systems was a moderating factor; their rapid passage across the country helped to reduce storm rainfall totals. There was no respite from frontal rainfall until well into December when, despite the final two weeks being relatively dry, regional monthly rainfall totals again comfortably exceeded the December average across most of the UK.
During January 2001, many parts of the UK recorded below average rainfall, but the weather across the South-East continued wet. Vigorous frontal systems tracked across the UK early in February and, with only modest evaporation demands typical of the late winter, most catchments remained close to saturation. Southern England was again very wet during March. However, regional variations were large - northern England, Scotland and Northern Ireland were comparatively dry during March. During April, low-pressure systems continued to track across central and southern Britain. For England and Wales, April 2001 ranked amongst the wettest half dozen since 1935. May 2001 was generally dry, ending the remarkable period of eight successive wet months across much of the English lowlands. The warmer, drier weather continued into June and finally brought to a close an extraordinarily prolonged wet episode — although the risk of localised summer flooding from intense convective storms remained.
A distinctive feature of the weather during the autumn and winter of 2000 was that, in addition to intense storms which triggered almost immediate flooding, there were prolonged periods of rainfall across many catchments. The frequency of these wet interludes is reflected in the accumulated monthly rainfall totals for England and Wales over the September 2000 to April 2001 period. Figure 1 compares the progressive monthly rainfall accumulations from September 2000 to April 2001 with the corresponding maximum accumulated rainfall totals for the full England and Wales rainfall series (1766-1999). For each of the periods from November onwards (i.e. the September to November period, then September to December, etc.), the 2000/01 accumulated rainfall totals exceeded the previous maxima. The outstanding nature of the rainfall is emphasised in Table 1, which ranks the highest rainfall accumulations over four- and eight- month periods (starting in any month of the year) using the homogenised England and Wales rainfall series1. The 2000/01 totals rank as the highest over both the four- and eight-month periods. Largely as a result of the extraordinary autumn and early winter rainfall, record England and Wales rainfall totals were established for accumulations extending up to 14 months.
The regional distribution of accumulated rainfall across the UK during the period September to December 2000 is shown in Figure 2 and Table 2. Across all regions of England rainfall totals over that period exceeded 150% of the 1961-90 average. Values equal to or greater than 180% of the 1961-90 average indicate clearly the very significant rainfall recorded from Kent to Dorset, and northwards through the Thames, Severn and Trent basins to Yorkshire. It was the wettest four-month period across the Thames catchment since records began in 1883. Rainfall over the September-December period also exceeded existing four-month maxima across most gauged catchments in England and Wales; testimony to the singular nature of the late-2000 rainfall (but note, relatively few gauged catchments have areal rainfall records extending back more than 40 years).
The rainfall gradient across Great Britain, evident from Figure 2, is of particular interest. Regions along a southeast-northwest line received the following percentages of the 1961-90 average rainfall for September to December 2000 (actual totals in brackets):
Southern 201% (634 mm)
Midlands (Severn-Trent) 180% (496 mm)
Solway 156% (531 mm)
Highland 102% (787 mm)
This shows a significant moderation in the normally much more pronounced ‘drier south-east to wetter north-west’ rainfall gradient. It reflects the tendency of rain-bearing weather systems from the Atlantic to take more southerly routes than usual during the autumn and early winter of 2000.
There were comparatively few dry spells across many parts of the UK during the period from September to December 2000 but the frequency of thunderstorms (often associated with frontal systems) resulted in large spatial variations in rainfall amounts. Many areas recorded notable rainfalls across a range of timespans during the autumn-winter 2000/01 period. From the 9-12th October Barcombe (East Sussex) recorded 175 mm in less than 72 hours (return period: approx. 400 years) and West Freugh (near Stranraer in Dumfries and Galloway) received 125 mm in 48 hours. Over the 29/30th October Andover (Hampshire) recorded 62.2 mm in 16 hours. Linton-on-Ouse (North Yorkshire) reported 154.8 mm over the 10 days beginning 29th October. In Northern Ireland, 167 mm was recorded in a 48-hour period during 5-7th November at Silent Valley.
Figure 3 provides a guide to the pattern of rainfall over the September-December period; it shows daily totals for Barcombe, beside the Sussex Ouse, and Scar House in the Pennines. The former experienced a number of very significant short duration rainfall events as well as notable n-day accumulations. Daily totals of around 26 mm or more may be expected in this area, on average, once every 7-10 months. However, no less than eight daily totals reached this threshold over the September to December period; the most outstanding were 115 mm on October 11th (>450 year return period) and 46 mm on October 29th (> 60 year RP). The four-month rainfall total of 970 mm for September to December 2000 for the Barcombe raingauge was the equivalent of 120% of the annual average rainfall (832 mm).
At Scar House Reservoir (in the upper Nidd catchment) average annual rainfall is around 70% greater than at Barcombe. Some similarities to the rainfall distribution in the South-East can be recognised, but a substantially higher proportion of the autumn rainfall was concentrated into a remarkably wet episode across the Pennines, beginning in late October. Over the six days up to and including November 2nd, rainfall totalled 196 mm (RP approx. 60 years) and the 11-day total reached 300 mm (RP approx. 250 years). These exceptional accumulations resulted from sustained rainfall associated with the passage of low pressure systems rather than short, intense storm events.