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UK Flooding – Briefing from the Centre for Ecology & Hydrology

20 November 2009

Context

The Centre for Ecology & Hydrology jointly operates the National Hydrological Monitoring Programme in conjunction with with the British Geological Survey. NHMP scientists produce the UK Monthly Hydrological Summary which assesses rainfall, river flows, groundwater and reservoir levels. They also operate the National River Flow archive. The NHMP also has a remit to analyse major flood and drought events in the UK and analyse long term trends in UK hydrological data.

Background Situation

In the last few days widespread flooding has been observed across County Galway in the Republic of Ireland, Gwynedd in North Wales Cumbria and the Scottish Borders. Extreme rainfall totals have been reported for across a wide area including sites at Eskdalemuir (Dumfries and Galloway), Seathwaite (Cumbria).

The flooding reflects a combination of circumstances which have led to elevated flood risk.   Whilst October rainfall was below average in most of the affected areas, there has recently been some some very wet weather in the north and west of the UK.   River catchments were therefore already saturated, and levels in many rivers were already high.   A persistent sub-tropical south westerly airstream has occurred, over oceans which are still very warm, meaning the air mass has been holding exceptional amounts of moisture.  As this air has encountered mountainous areas in the west of the British isles, orographic enhancement has occurred (i.e. the air has risen and become even wetter).  This combination of factors has led to the exceptional rainfall totals which have been observed in recent days.

Rarity of this event and historical data

Flooding during the autumn/winter season is relatively common in the UK.  However, the magnitude of the provisional rainfall totals suggests this is a monumental event, and perhaps a ‘record breaker’. 

Provisional data of 314mm for a 24h period, for Seathwaite (Cumbria), if confirmed, could set a new 24-hour rainfall record for the UK.  The previous maximum rainfall 24h total for England was 279 mm (at Martinstown, Dorset, in July 1955).     Provisional analyses suggest that the rainfall at Seathwaite has a return period of in excess of 300 years, but it may have been even more extreme if it fell over shorter periods.  Seathwaite has some rainfall records from the 1800s and is regularly mentioned in the British Hydrological Society “chronology of hydrological events”, which has anecdotal evidence of many historical floods.   There is a record of 8.52 inches (204 mm) on 12 Nov 1897 and  7.52 inches (191 mm) 26 Nov 1861;  6 inch totals are not uncommon in the record.   This is one of the wettest parts of the country, and clearly has seen some very notable rainfall totals in the past.  However, the recent rainfall seems to eclipse anything previously seen, by some margin.

The average November rainfall for the Derwent catchment at Camerton is 194 mm.  In the Central Lake District (CLD, an amalgamation of several raingauges) rainfall record, available from 1788 – 2000, the average November rainfall is 230 mm.   The 24-hour total provisionally registered at Seathwaite is therefore considerably higher than the rainfall that would be expected in a typical month.  The CLD series, from 1788 – 2000, lists the maximum monthly rainfall (for any month of the year) over this period as 658mm, in 1852.  The recent Seathwaite total is around half the value of the previous monthly maximum, in a record from over 200 years long – this further underlines the extreme nature of the daily rainfall.   More analyses will be carried out to examine rainfall totals over a range of durations.

We do not yet have access to any flow data.  However, we have historical flow data for Camerton (an Environment Agency index site between Cockermouth & Workington) from 1960.  The maximum monthly rainfall in this catchment was in October 1967, with 468 mm.  The highest daily flow in this catchment was 282 cumecs in January 2005.    The highest flood peak was for October 2008, at 302 cumecs.  Major flooding occurred in north-west England in January 2005, with the river Eden registering the highest river flow ever recorded for England.   Provisional data suggests the rainfall associated with the current event is likely to be higher than the January 2005 event.

Climate Change

In the areas currently affected by flooding, there have been major flood events in the recent past. These areas were not seriously affected by the 2007 summer flooding, but damaging flooding occurred in the north-west of England in January 2005.  

There are inevitably concerns that these events have occurred as a result of climate change. In general, climate change is expected to intensify the hydrological cycle, as a warmer atmosphere can hold more moisture.  Climate change is expected to lead to increased rainfall in some seasons in some areas of the globe, and, in particular, is expected to lead to increased rainfall extremes which may cause flooding.  However, while the evidence for a warming climate is unequivocal, it is still uncertain as to what this means for rainfall patterns; furthermore, there are still many uncertainties associated with future climate projections.

Firstly, it is important to remember that any single flood event cannot be attributed to climate change. We can only establish whether observed events are part of a long-term trend or pattern, and we must examine the historical record to do this.  This flooding is occurring in a part of the country which has become wetter in the recent past.  The latest research from the UK Climate Projections team (UKCP09) indicates that winter rainfall has increased in the north-west of the UK since the early 1960s.   Other research suggests that extreme rainfalls have also increased in frequency in northern and western areas of the UK, and one recent study which involved CEH scientists found that, over the last 50 years, the daily maximum rainfall has increased by 25% in northern and western areas relative to the previous 50 years.   UKCP09 projects increases in winter rainfall for the future in these areas, and there are concerns that extreme rainfall will become more common in future.   In the context of these studies, it can be said that extreme rainfalls of the last few days are consistent with a tendency towards the north west becoming wetter and more extreme, but that it is not possible to attribute these with certainty to climate change.

CEH has recently published a paper in the International Journal of Climatology, which examines trends in river flow and flood records around the country.  This indicates that there have been increases in river flows, leading to longer periods of high river flow and increases in flood frequency, in the last 30 - 40 years in upland areas of the north and west of the UK.  Whether this is due to climate change is an open question; these records are fairly short, and increases in rainfall and flow over this period may reflect variability associated with changes in atmospheric circulation over the North Atlantic.  The issue is very complex, however, as climate change may still be the underlying cause of this variability.  This published research suggests that trends over a longer period (>50 years) are generally much less compelling, and in general, there is limited evidence for long-term trends in flood frequency or magnitude anywhere in the UK.

Jamie Hannaford

Centre for Ecology & Hydrology

20 November 2009

 

Additional information

Latest monthly hydrological summary for the UK - October 2009 [PDF]

National River Flow Archive

National Hydrological Monitoring Programme

Recent scientific paper by Hannaford and Marsh: High-flow and flood trends in a network of undisturbed catchments in the UK (subscription required)