As in the preceding two years, many new runoff records - mostly over lengthy timespans - were established in 2002. Although notable localised - mostly urban - flood events were common, the frequency of widespread floodplain inundations was relatively modest given the preponderance of wet months and the length of time most catchments were close to saturation. However, spates were common in most river basins and high flows were maintained over substantial periods. These episodes are reflected in Figure 1 which illustrates 2002 runoff totals - expressed as a percentage of the period of record average - for a network of index gauging stations throughout the UK. Annual runoff totals were depressed in north- west Scotland, where the River Carron reported its second lowest annual mean flow in a 24-year record. This region was however very atypical of the UK as a whole. A significant proportion of rivers in eastern Scotland and in Northern Ireland (where flow records generally begin in the 1970s) established new maximum annual runoff totals; in the case of the River Annacloy the previous maxima was exceeded by >25%. Mean flows were also notably high in many catchments in southern England; some spring-fed rivers registered significantly above average monthly flows throughout the year.
Figure 2 shows daily flow hydrographs for 20 index gauging stations throughout the UK; the 2002 hydrographs are illustrated by the solid trace and the shaded envelopes illustrate the maximum and minimum daily flows over the preceding record. The year began with seasonally depressed flows across much of the UK but, thereafter, only brief approaches were made to the low flow envelope (e.g in April and early October). The unusually high frequency of, mostly minor, spates in the late spring and early summer of 2002 resulted in the normal seasonal flow contrasts being less evident than in a typical year. High flows, with significant flooding, were common in February and over the last three months of the year; an exceptionally severe event afflicted parts of eastern Scotland in November. Localised flooding was frequent in the summer and autumn, particularly in urban areas where drainage systems were overwhelmed during intense storms. Minimum flows during 2002 were generally experienced in the early autumn and, with the exception of rivers in north-west Scotland, were typically very healthy - substantially greater than those which characterised the 1989-1997 period. For the third successive year, river flow patterns across the UK departed significantly from those experienced in a typical year. The unusual nature of the 2002 flow patterns is evident from the flow duration curves featured in Figure 3. These curves allow the proportion of time that river flows are above, or below, any given threshold to be identified - they also provide a means of comparing the regime for a particular year with that for the previous record. The 2002 duration curves confirm the notable regional contrasts in runoff patterns. Flows in rivers throughout north-west Scotland were generally depressed throughout the full flow range whereas in eastern Scotland flows were atypically high; this was true of many rivers in Northern Ireland and some draining impermeable catchments in the English Lowlands also. The 2002 duration curves for rivers in northern England, Wales and the South-West were more representative of the normal regime but many spring-fed rivers in eastern and southern England, which in any case are characterised by stable regimes, registered an exceptionally modest range of flows. Away from rivers in northern Scotland, a feature of most flow duration curves is the degree by which the 95% exceedance flow (a commonly used index of low flows) was greater than the long term average, confirming the transient nature of depressed runoff rates in the summer and autumn of 2002.
Following lengthy recessions in December 2001, freezing weather conditions in early January further reduced runoff rates and resulted in flows approaching monthly minima in many rivers across the country (from the Tay to the Lymington). The influx of a mild westerly airflow during the second week was accompanied by a dramatic increase in runoff rates in impermeable catchments. Spates were common towards month end and storms on the 26-28th resulted in widespread flood alerts (>100 across England and Wales). However, January runoff totals were well below average in most impermeable lowland catchments. In Northern Ireland, the River Annacloy registered a ninth successive month with below average flows, establishing a new May-January runoff minimum - the wide margin by which the previous minimum was eclipsed provides a guide to the limited replenishment of the Silent Valley reservoirs over the period beginning in the spring of 2001.
February began with many fluvial and tidal flood warnings in operation, and bankfull - or higher - flows were common throughout the month. Fortunately, the rapid passage of most frontal systems helped to moderate the risk of severe flooding. The ability of the natural drainage network to cope with sustained high runoff was also well demonstrated. Nonetheless, significant flooding was experienced in some locations (e.g. Monmouth) and floodplain inundations were particularly common in mid-month. In a broad zone from South Wales to the Southern Uplands many rivers registered flows around the mean annual flood level on four or more occasions in the month. Correspondingly, gauging stations establishing new maximum February runoff totals showed a very wide distribution; runoff for the River Dee (north Wales) eclipsed the November 2000 total to establish a new single-month record in a series from 1937.
Most rivers were in brisk recession in early March, with an associated reduction in flood risk. In Scotland, flows recovered smartly in the second week and short-lived spates were common in mid-month across southern England before recessions became re-established. Runoff totals were within the normal range in almost all index catchments but mostly lower in rivers draining impermeable catchments than those where groundwater makes a major contribution to river flow. Winter half-year (October 2001-March 2002) runoff totals were generally also well within the normal range but substantially above average in many Scottish catchments - for the sixth successive year in some cases.
The early spring recessions continued into April and although interrupted by spates in mid-month (e.g. in Northern Ireland), monthly minimum flows were approached over wide areas as flow patterns showed similarities to the sustained recessions experienced in the spring of 1997. Heavy late-April rainfall produced a very brisk recovery in many catchments - triggering modest floodplain inundations (e.g. in South Wales). Notwithstanding this upturn, April runoff totals were mostly well below average, notably so on the River Spey (at the Boat of Garten gauging station) which reported its lowest April flow since 1981.
Aside from those rivers primarily reliant on groundwater, most rivers registered a very wide range of flows in May, but spatial variations in runoff totals were rather more marked than in April. Notably high flows were reported at the beginning and during the third week of the month for many western rivers. The Dart, Teifi and Yscir were amongst a significant number establishing new maximum flows for May. The associated modest floodplain inundations were complemented by significant urban flooding caused by intense storm events (e.g. in Belfast and Swansea). May runoff totals were unprecedented for a few rivers in south-western Britain but depressed in a zone from the Humber to the Forth estuary - the River Leven (Cleveland) reported its lowest May runoff since 1960.
June witnessed outstanding runoff totals in many catchments in Scotland and Northern Ireland where especially high flows - for the early summer - were recorded in mid-month. The River Camowen established a new maximum June flow on the 14th; the following day the Clyde recorded its highest June flow since 1966. Flooding was generally very modest but thunderstorms resulted in some significant local - mostly urban - flooding (e.g. Belfast on the 22nd). Many rivers, including the Camowen, Nith, Tay and Mole, established new maximum June runoff totals but in a zone from Cleveland to the East Midlands mean flows were depressed - only around 50% of the June average for the River Torne.
Most July runoff totals were within the normal range but moderate floodplain inundations were again common (significant flooding occurred on the River Etherow at Glossop) and the River Tay and River Clyde - both with flow records of >40 years - reported their highest July flows (on the 30th). Of greater significance in terms of their impact were a series of intense rainfall events - often with heavy surface runoff and, in Scotland especially, landslides - transport disruption was severe and extensive. Flash floods afflicted Belfast again on the 12th, Nottingham on the 20th, and the Pennines and Scotland on the 30th. Evacuation of residents from stricken areas was necessary (e.g. Lincoln, the Trossachs, and Glasgow where a major event caused 500 residential properties were inundated and required an estimated 200 people to be evacuated).
Other than at the very local scale, it is unusual for flood risk to extend into August but in 2002 rivers draining the Pennines and North York Moors were very vulnerable to further rainfall. The Wharfe reported its highest August flow on the 2nd and, in Scarborough, intense storm runoff caused a caravan park to be evacuated. Localised flooding also occurred in Perth and Pitlochry on the 6th. To the south, thunderstorms generated severe localised flooding and substantial transport disruption - on the 7th the rail network in London was badly affected. In many eastern catchments August runoff totals were amongst the highest three or four on record. By contrast, runoff was depressed in much of Wales and western Scotland - where the River Carron reported its second lowest August total. Away from such areas, summer (June-August) runoff totals were well above average - for the fourth or fifth year in succession in many lowland catchments.
The late summer recessions resumed in September after significant spates during the second week - on the 8th, the Mimram recorded its highest September flow since 1975. River flows generally remained in-bank but some locally significant flooding did occur as a consequence of intense storms overwhelming urban drainage capacities e.g. at Swanage, Inverness, Burntisland (Fife); landslides were triggered and transport disruption was again substantial. Of wider hydrological significance were the depressed flows reported at month-end - across much of southern Britain flows were at their lowest since 1996 or 1997 and new September minima were established in a number of western rivers, including the Cynon and Ewe.
In many catchments flows continued to decline in early October. On the 10th, the River Tawe (South Wales) recorded its lowest October flow since the 1959 drought. But thereafter - again echoing the 1959 drought - river flow recoveries were dramatic with moderate flooding in south-western Britain in the following week. In parts of western Scotland, the recovery was insufficient to prevent a new October runoff minimum being established on the River Carron. To the east however, the recovery was dramatic culminating around the 22nd when all three gauging stations on the Don eclipsed previous maxima, as did the River Tweed at Sprouston; flooding affected Eyemouth. Inundation of agricultural land was very widespread and many rivers in the English Lowlands - most of which respond much more slowly than rivers draining the western hills - were close to bankfull.
The continuing passage of active frontal systems further increased the risk of flooding in November which was characterised by sustained high flows in most river basins away from north-west Scotland. Severe flooding affected the South-West peninsular around the 14th (e.g. at St Ives and Newquay) by which time flood warnings applied across a broad swathe of England and parts of Scotland. On the 15/16th many rivers draining to the Moray Firth overtopped their banks - the Lossie exceeded its previous maximum in a 44-year record, resulting in severe flooding; evacuations were required in Elgin and Moray. The following week the maximum flows established on the River Don in October were eclipsed - further flooding occurred in Brechin and other parts of Aberdeenshire. Rivers establishing new November runoff maxima displayed a wide distribution - including the Annacloy, Kenwyn and Kennet (which, like other chalk rivers, benefited substantially from increasing baseflows as groundwater levels rose).
December was a month of stark regional contrasts in runoff patterns. New December runoff minima were established in many catchments in northern Scotland where mean flows were typically 10%-30% of the monthly average. Flows fell below previous December minima in mid-month on the River Ewe where the accumulated deficiency for the August-December period was, remarkably, more than 20% below the previous minimum in a 32-year record. By contrast, in much of southern Britain, the English Lowlands especially, flows were well above average and, as further sustained rainfall fell on saturated catchments, flood warnings were increasingly frequent from mid-month; by early January 2003 around 300 were in operation. On the 28/29th December flooding occurred at many flood-prone localities (from Elvington in North Yorks to Yalding in Kent) and further rainfall at year-end heralded the most extensive flooding since 1947 in a few areas - e.g. the middle reaches of the Thames.