Catchment runoff totals for 1998 were broadly consistent with the corresponding rainfall totals in much of western and northern Britain. In the east however, where many rivers are sustained principally from groundwater, above average catchment rainfall corresponded with runoff totals which were well below average. For the River Mimram, to the north of London, 1998 rainfall was around 110% of average but river flows remained depressed throughout most of the year and the annual runoff total ranks seventh lowest in a series from 1952; a direct reflection of the low groundwater levels throughout 1998. Runoff totals in impermeable catchments were much healthier throughout the UK and new maximum annual totals were established for a significant minority of rivers including the Whiteadder, Cree and Cynon (see map for their locations). Notably high annual runoff totals characterised much of western Scotland contributing to an exaggeration in the north-west/south-east runoff gradient across the country - a recurring feature of the last 20 years.
The pattern of monthly river flows during 1998, and over the preceding four years, is shown on Figure 1. February flows were generally very modest but, contrary to the normal seasonal pattern, flows increased through March and April. With catchments saturated, inflows to reservoirs were heavy and overall stocks for England Wales were close to capacity through into the late spring. From mid-March many catchments were particularly vulnerable to further significant rainfall. The notable storm on the 9th April (see Rainfall) produced rainfall intensities which on impermeable, sodden catchments meant that severe flooding was inevitable. The resulting flooding was the most severe ever recorded in a number of rivers with headwaters in the south and east Midlands (e.g. the Leam, Warwickshire Avon, Nene and Cherwell); return periods of 50-150 years were ascribed to the most extreme peak flows. Overall, it was the most damaging flood episode in the UK since the summer floods of 1968 in southern England. Five fatalities were attributed to the event and the total cost was assessed at around £350 million. Around 4500 people were made temporarily homeless (some homes were lost permanently) and the associated disruption was exacerbated by the fact that most flood peaks occurred at night (and during the Easter break). The unprecedented flooding prompted a wide-ranging review of existing flood warning procedures, flood alleviation strategies and planning constraints on floodplain development (the growth in the number of river-side caravan sites being a particular concern).
Flows in most rivers remained above average through most of the summer and, following a short low-flow interlude in August and early September, began a strong seasonal recovery early in the autumn - this gathered momentum in October and over the period of the 20-25th a second major flooding episode - less extreme but more extensive than in April - produced notable peaks over wide areas. Damage was especially severe in South Wales where in some catchments it was the worst for 20 years; issues highlighted included the effect of increasing urbanisation (e.g. in the Merthyr Tydfil area) on runoff rates and, locally, the exacerbating influence of the depositing of tip material (following the 1966 Aberfan disaster) on the floodplain of the River Taff. In larger catchments it was a complex event with multiple peaks and the flood risks extending over periods of a week or more.
Runoff rates during the last two months of the year were generally closer to the seasonal average but November flows in many permeable catchments reflected a lagged response to the very heavy late-October rainfall. The November runoff total was the highest for 24 years for the Thames despite the catchment rainfall being less than 90% of the monthly average. December began with most rivers in recession but widespread spates occurred in the week following Christmas - in eastern Scotland, the Earn recorded its second highest December flow in 32 years and bankfull flows were very widespread. Flow rates on the Mimram exceeded the December average at year-end after a very lengthy sequence of below average monthly runoff totals - establishing a new 36-month runoff minimum for the catchment.
The flow duration curves featured on Figure 2 confirm that in most flow ranges, flows exceeded the average in 1998. Exceptions included many rivers in northern Britain where the flows exceeded 10% of the time were close to the average - and, more notably - flows in spring-fed lowland rivers which, though remaining above drought minima, were especially depressed - relative to the seasonal average - during the first quarter of the year.