Hydrological impact of an extreme precipitation event the UK Summer 2007 floods

The early summer of 2007 in the UK was, in many places, the wettest on record (Marsh, 2008) though there is some dispute as to whether it was really the wettest May to July period in England and Wales since records began in 1776 as widely reported at the time (Eden, 2009). Table 9.3 shows the 20 most extreme 3-month rainfall totals recorded since records began. There is some uncertainty about what the total average over England and Wales should be, but 2007 ranks no higher than 26th in this list. In June, two spells of heavy rainfall, on the 15th and 25th, resulted in 'pluvial' flooding of properties in Hull in Yorkshire as drainage systems were overwhelmed. Fluvial flooding occurred in Sheffield and Doncaster as the River Don overtopped its banks. Totals of over 110 mm were recorded in the Sheffield and Hull areas. July was particularly unsettled and, on the 20th, an active frontal system resulted in daily totals of over 120 mm falling on already wet ground (Fig. 9.5a). The highest recorded rainfall total over 24 h was 142.6 mm at Pershore College, Worcestershire (Fig. 9.5b). This resulted in widespread flooding in several major river catchments in England, such as the Severn in Gloucestershire and Thames in Oxfordshire. In Gloucester, water treatment plants were affected and 400 000 people were left without drinking water. Power was also cut when one electricity substation was flooded, while another was kept in operation by the deployment of temporary flood defences. The total damages in this period in 2007 were of the order of £2bn.

What made this event particularly unusual was the occurrence of such widespread heavy rainfalls and precipitation in summer. In the past, most notably in 1947 but also more recently in 2000 and 2002, such widespread flooding has resulted from synoptic

Table 9.3 The 20 most extreme 3-month periods of rainfalls in the entire England and Wales rainfall record (1776-2007; data from Eden, 2006)

Rank

Period

Year

Rainfall (mm)

1

Oct-Dec

2000

512

2

Sep-Nov

2000

503

3

Nov-Jan

1929/1930

500

4

Oct-Dec

1929

499

5

Aug-Oct

1799

491

6

Jul-Sep

1799

487

7

Oct-Dec

2002

468

8

Sep-Nov

1852

456

9

Oct-Dec

1852

450

10

Nov-Jan

1876-77

449

11

Aug-Oct

1903

444

12

Sep-Nov

I960

439

13

Oct-Dec

I960

438

14

Oct-Dec

1770

433

15

Jul-Sep

1775

431

16

Oct-Dec

1872

429

17

Sep-Nov

I935

424

18

Dec-Feb

1914/1915

423

19

Nov-Jan

1852/1953

423

20

Dec-Feb

1989/1990

4I9

Fig. 9.5 (a) Pattern oftotal recorded rainfall in southern England, 19-20 July 2007; (b) hourly rainfalls at Pershore College, Worcester, 19-21 July 2007 (redrawn from Marsh and Hannaford, 2007).

scale heavy rainfalls in winter. The particular conditions in 2007 have been linked to a weakened El Niño effect and an extreme southerly position of the jet stream over the North Atlantic, leading to strong depressions tracking further south than normal. The high summer rainfalls also resulted in high groundwater levels in many aquifers, although the second driest August to October period in the record also allowed levels in both rivers and ground waters to fall again.

These extreme rainfalls and consequent flooding resulted in a number of official enquiries. The most significant of these, commissioned by the Government, resulted in the Pitt Report,2 which made 97 recommendations to improve the management of flood risk (see Chapter 16 for further discussion).

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