Issued 26th July 2007: 1500GMT
The weather conditions experienced across much of the UK throughout the summer of 2007 have been truly exceptional. The jet stream (which influences the paths taken by weather systems in the North Atlantic) has followed an abnormally southerly track and the extension of the Azores high pressure cell across the UK – which brings settled weather conditions in most summers – has failed to become established. Correspondingly, a sustained sequence of rain-bearing low pressure systems has produced outstanding 12-week rainfall totals, and a series of flood events culminating in widespread severe flooding in late July.
The combined May and June rainfall total is the highest on record for the UK (in a series from 1914) by a considerable margin and the exceptional weather conditions continued into July. Provisional data indicate that the May-July period will be the wettest for England and Wales in a series from 1766 with many areas registering more than twice the long term average – remarkable for a three-month period..
As warm and very moist air moved north from France, the volatile July weather patterns culminated in an extremely wet episode on the 20th. Outstanding storm rainfall totals were reported across much of southern Britain. These included 145 mm in Pershore (Hereford and Worcestershire), 111 mm at Chieveley (Berkshire) & c120 mm at Brize Norton (Oxfordshire).
Statistical analyses confirm the extreme nature of such storms – on the basis of historical data they would be expected to occur, on average, only once in several hundred years (longer in the case of the Pershore event). Thundery interludes contributed to substantial spatial and temporal variations in rainfall intensity but catchment rainfall totals were exceptional over wide areas. Localised storms of tropical intensity are a feature of many English summers but a distinguishing characteristic of the July 2007 storms was the spatial extent of the extreme rainfall totals.
Characteristics of the flooding
Normally, flood risk during the summer is substantially diminished by dry soil conditions. Following the record late spring and early summer rainfall, accompanied by widespread flooding in June, soils were close to their wettest on record (for mid-summer) in early July across much of England. This rare circumstance left many catchments vulnerable to further significant rainfall.
The exceptional rainfall on the 20th July triggered a sequence of relatively distinctive flood episodes: localised (mostly urban) flash floods, extremely high flows in small responsive (impermeable) catchments and, subsequently, extensive floodplain inundations as the runoff concentrated in the major rivers of southern Britain (including the Severn, Warwickshire Avon, Bedford Ouse, Trent and Thames).
Initially, the intense rainfall overwhelmed many urban drainage systems producing localised but severe flash floods. The emergency services were widely deployed to rescue stranded individuals and organise evacuations from the most severely affected localities. These contributed to massive and extensive transport disruption across southern Britain, exacerbated by the volume of holiday traffic (the 20th being the end of the school term in many areas). Subsequently, floodplain inundations caused extensive crop damage and the need to move livestock to higher ground. The sustained high levels in the major rivers also hampered the drainage of flood waters away from the urban areas inundated on the 20th.
Preliminary data suggest that a significant minority of rivers across southern Britain exceeded their previous maximum recorded flow and many eclipsed previous maxima for the summer half-year (April-September) – often by very wide margins.
In the worst affected areas (e.g in the lower parts of the Severn and Warwickshire Avon basins and some upper reaches of the Thames catchment), flood flows may have exceeded those of March 1947 - the most severe flood in southern Britain in over 100 years (note however that the 1947 event was primarily the result of rapid snowmelt over still-frozen ground and its overall impact was substantially more severe than the current flooding).
An historical perspective on the July floods
An indication of the rarity of the hydrological conditions experienced this summer is provided by the recent increases in groundwater levels in some parts of eastern and southern England. Generally, groundwater levels decline over the May to September period, due to an absence of natural replenishment (recharge). This summer, groundwater levels in the Cotswolds rose rapidly in mid-July and by the 24th stood above normal winter levels; this is reflected in the exceptionally high recent flows reported for many spring-fed streams. In the 19th century, significant summer recharge was recorded in a number of years (e.g. 1860 and 1879) but examples of significant and widespread summer recharge in the 20th century are very rare.
Episodes of extensive summer flooding may be found in the historical record (e.g. in 1875) – particularly in the nineteenth century when summer half-year (May-Oct) rainfall often exceeded that for the winter half-year. There are, however, no close modern parallels to the scale of the summer flooding experienced this year. It has served to underline our continuing vulnerability to very exceptional summer rainfall and to, as yet poorly understood, changes in the position of the Jet Stream.
In the medium term (2-4 months) it is likely that soil conditions will remain wetter than the seasonal norm. This will encourage an early onset of the seasonal recovery in river flows and groundwater levels in the autumn. Correspondingly, an extended flood season throughout the autumn and winter of 2007/08 may be expected.
How influential is climate change?
By their nature, individual extreme flood events cannot be linked directly to climate change. If they form part of a developing pattern or emerging trend, then a causative association becomes more plausible. In England and Wales, evidence for long term increases in fluvial flood magnitude is elusive – one factor here is that in a warming world snowmelt (a primary cause of the 1947 flood) is very likely to decline as an exacerbating factor as temperatures increase. Warmer, drier summers would also tend to produce very dry soil conditions which, unlike this year, should help moderate fluvial flood risk. On the other hand, more intense summer rainfall would increase the risk of localised flash flooding – with associated drainage problems, particularly in urban areas.
Further information concerning the impact of climate change on flood risk in the UK is given here.
A more comprehensive analysis of the floods will appear in the July edition of the Hydrological Summary for the United Kingdom [NEED NEW INTERNAL LINK] which will be released in mid-August by the Centre for Ecology & Hydrology and the British Geological Survey.
Media enquiries on the current flooding situation should be directed to the CEH Press Officer, Barnaby Smith (mobile 07920 295384). Our experts are available to provide explanation and analysis of historic flooding patterns, possible future scenarios under climate change and scientific understanding of the current flooding situation.
General enquiries should be made via our web feedback form.