November 20, 2009

experts react to flooding in Cumbria

Heavy rain in the north-west of England has caused devastating flooding, driving hundreds from their homes and causing extensive damage to roads and bridges. The Environment Agency described the level of rainfall as ‘unprecedented’.

Jamie Hannaford, Centre for Ecology & Hydrology (CEH), said:

“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.

“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 the 24h period, for Seathwaite (Cumbria), if confirmed, would set a new daily 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.”

NOTES – 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 new totals would 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 rain gauges) rainfall record, available from 1788 – 2000, the average November rainfall is 230 mm. The daily total 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.

Bob Sargent, Director of Water Environment, Hyder Consulting (UK) Limited, in response the following questions, said:

“Q: If this is a once on a 1000 year occurrence can we design flood defences for an event that is this rate or would it be too expensive?
A: “Flood defences for a 1000 year event would be too expensive to build everywhere and unsustainable (and impractical in many cases). We need to reduce our vulnerability by increasing the resilience of existing buildings in floodplains and reducing the runoff from high rainfall events through better land management. Return periods are a poor guide – we can expect events like this to occur more frequently in future.”

Q: Would any flood defences – no matter how expensive – withstand an event like this?
A: “The flood plain is part of the river we should not divorce the two. Yes, technically it could be done but it is not practical. Nor is it desirable – imaging every river barricaded behind high flood walls, each one requiring constant maintenance. And a higher event could always happen and create a worse disaster.”

Q: Have we implemented the recommendations of the Pitt review yet?
A: “Not all of them. The Flood and Water Management Bill announced in the Queen’s speech on Wednesday is intended to implement some of them – to improve management of flood risk in England and Wales by giving local authorities a flood risk management role, overseen by the Environment Agency. This could help reduce flooding in some circumstances, particularly in large urban areas, but will not address the causes of the flooding just experienced. Flooding on this scale could be reduced by land use change in the rural catchment, which could reduce the amount of runoff created and the speed of the flood response, but this is outside the scope of the Bill, or the ability of local authorities to deliver.”

Additional information from Bob Sargent:
“The flooding in NW England and SW Scotland is the result of persistent heavy rainfall over the past two days, caused by a slow moving weather front. This weather situation is normal for the time of year, although the rainfall totals are particularly high.

“Rainfall like that experienced is bound to create flooding in rivers, particularly in steep areas, and affected rivers will break their banks and flow across their floodplains. This is natural, but creates a problem where property and roads have built in the flood plain.

“The Flood and Water Management Bill announced in the Queen’s speech on Wednesday is intended to improve management of flood risk in England and Wales by giving local authorities a flood risk management role, overseen by the Environment Agency. This could help reduce flooding in some circumstances, particularly in large urban areas, but will not address the causes of the flooding just experienced. Flooding on this scale could be reduced by land use change in the rural catchment, which could reduce the amount of runoff created and the speed of the flood response, but this is outside the scope of the Bill, or the ability of local authorities to deliver.

“The Bill also aims to ensure new development is not in areas subject to flooding. This is sensible, but again, will not address the problem of existing developments built if floodplains. Defending all these from flooding is very expensive and cannot be achieved. The Environment Agency already spends over £½ billion in building and maintaining flood defences. In some areas we will have to increase the resilience of buildings and structures to flooding. We may also have to consider abandoning some areas where flood defence is not viable, and some compensation scheme may prove cost effective.”

The following is information from Dr Harvey J E Rodda, Director of Hydrology, Hydro-GIS Ltd & Dr Max Little, University of Oxford: “Reports of the current flooding in Cumbria have listed record 24 hour rainfall totals for the UK, for example, the BBC quoted the Environment Minister as saying “We have seen extraordinary amounts of rainfall. The rain gauge at Seathwaite registered 314mm [12.4in] in 24 hours which could make it the wettest day ever recorded …” . According to the Environment Agency this total was for the 24 hours up to 00:45 on 20th November 2009, although proper quality controlled data to confirm this observation may not be available for some time. “The standard UK 24 hour rainfall period is however from 9:00 to 9:00, and a 24 hour total not within this period cannot be listed as a daily record. The 9:00 to 9:00 period was a convention started by the British Rainfall Organisation for their network of observers in 1860 who would manually empty the gauges at this time. It was not until the 1970s that automated rain gauges provided a readily available continuous rainfall depth observation. “A digital archive of UK 24 hour extreme rainfalls, compiled over the past 3 years by Hydro-GIS Ltd, a specialist hydrological consultancy company, and researchers at the Department of Physics, Oxford University, as part of a NERC research grant (under the FREE programme), includes all extreme UK rainfall observations back to the year 1866. The current standard 24 hour record is 279.4mm measured at Martinstown, Dorset on 18th July 1955. A total of 64 extreme rainfall observations for the standard 24 hour period are listed in the archive for Seathwaite, with the largest being 203.9mm on 12th November 1897. “Assuming no climate change, historical data such as that recorded at Seathwaite would suggest that this recent record rainfall is a 1-in-5000 year event (Figure 1, attached PDF) however other research undertaken in the above study has shown that that this region has an increased risk of extreme rainfall, a possible effect of climate change (Figure 2, attached PDF). Dr Max Little, at Oxford University says “current statistical models used to estimate the frequency of these kinds of events probably need updating in the light of climate change. Many UK rainfall records show similar, unprecedented recent extremes that do not fit the pattern of the historical data. Therefore, although these events are extraordinary and devastating to those affected, better models would downgrade these sorts of frequency figures.”

David Balmforth, flooding expert, Institution of Civil Engineers, said:

“The events of the last few days yet again illustrates that flooding remains a very real risk across the UK. Climate change means that is only going to get worse. We cannot hope to defend ourselves from flooding on this scale. Instead we need to make our communities much more resilient to flooding and this must be placed at the heart of the way we plan, design and build our towns and villages. This preventative approach, combined with continued investment, will help us to avoid the damage, loss of life and disruption to society that have been the all too familiar consequences of flooding in recent years.”

Prof Stuart Lane, Executive Director of the Institute of Hazard and Risk Research, Durham University, said:

“These are unusual events in many senses, but it is particularly important that we don’t see them as exceptional or that we simply assume that climate change is responsible. We are a naturally wet country. Historically, evidence identifies years to decades when we have many major flood events and then years to decades when we don’t. Many of these most extreme flood events can’t be stopped. We can only learn to live with them, and that is hard to do when we haven’t experienced them before.”

Nick Reeves, Executive Director, Chartered Institution of Water and Environmental Management (CIWEM), said:

“These latest floods will test the lessons learned from previous flooding events. But until the Floods and Water Management Bill becomes law, when the Environment Agency is given strategic oversight of flood risk and when local councils become responsible for mitigating and managing flood risk, more communities will be at risk of flooding and those already at risk will continue to be vulnerable. We know that climate change will bring warmer and wetter winters so we need to plan, now, for the possibility of more frequent flooding with measures in place to protect critical infrastructure such as water supply and treatment plants, energy supply networks, hospitals and community services. Politicians must not backslide on the recommendations of the Pitt Review and local councils must be given the extra resources they will need to meet their obligations in the Bill.

“The impact of flooding, both on individuals and the communities affected, is considerable. With increased incidence of extreme weather events due to climate change, coupled with ambitious Government housing targets, flood risk will increase and flooding will affect many thousands of households for the first time. We need to adapt to these increases in risk and embrace integrated management of all sources of flooding, including helping communities adapt to future increases in flood risk.

“The Government must recognise that emergency management measures and follow-on community support will become increasingly important. The role of the insurance industry will also be increasingly crucial to community resilience and recovery. The Government must ensure that the insurance industry provides a service that meets individual needs at a fair and equitable cost.

“The Floods and Water Management Bill is a key piece of legislation produced largely in response to the 2007 events and the subsequent Pitt Review[i]. It puts in place a range of significant changes that should deliver positive benefits and more integrated management of flood risk which works far better with natural processes. The next Government must ensure that this legislation is delivered upon and the responsible authorities are properly resourced to do so.”

[i] CABINET OFFICE. 2008. The Pitt Review: Lessons learned from the 2007 floods.

Laura Grant, Policy Officer, CIWEM, said:

“The Floods and Water Management Bill is a key piece of legislation that should put in place a range of significant changes to deliver positive benefits and more integrated management of flood risk which works far better with natural processes. The Government must ensure that this legislation is delivered upon and the responsible authorities are properly resourced to do so.

“CIWEM has called for adequate funding by central Government, secured over at least a three-year rolling programme, to allow effective management of flood and coastal erosion risk. This should underpin proper and adequate resourcing of local authorities to discharge their wider responsibilities for flood risk management fully. There must be a national programme to support local authorities and others in capacity building as well as clear guidance, champions and training for integrated approaches to flood management and the development and strengthening of partnership working.”