Select Committee on Environment, Food and Rural Affairs Minutes of Evidence


Memorandum submitted by Professor Howard Wheater (FL 119)

SOME OBSERVATIONS ON UK FLOODING

EXECUTIVE SUMMARY

Is UK flood hazard increasing?

    —  urbanisation increases flood risk, but the problem is well known and mitigation measures are normally put in place

    —  agricultural intensification increases flood risk at local scale—effects at larger catchment scales are not yet known

    —  land use effects have greatest impact for frequent floods

    —  there is potential to mitigate flood risk by changing land management and the return of floodplains to washland; the magnitude of effects is uncertain, but research to quantify effects is ongoing

    —  climate change is expected to increase the intensity of storms and to increase flood risk, but quantitative estimates are highly uncertain

    —  there is currently no guidance concerning climate change impacts on extreme events such as the Boscastle storm.

Does the UK have the policy, resources and technical tools in place to manage flood risk effectively?

    —  DEFRA's "Making Space for Water" provides a far-sighted basis for flood management policy, but tools for implementation are not yet available

    —  Responsibilities for flood management in urban areas are unclear. Technical advances will shortly provide new tools for urban flood design

    —  There is legacy of inappropriate floodplain development that requires policy attention

    —  Technical developments are needed for groundwater flooding, but research is in hand. Other technical needs were discussed above and include modelling tools for land use management. There is a particular need to represent and reduce uncertainty in estimates of climate change, and to provide some guidance on climate change effects for high return period extreme floods

    —  Human resources are limited by a lack of numerate graduates

    —  If improved standards of flood protection are to be implemented in a reasonable period of time, substantial additional financial resources will be required.

MEMORANDUM

  1.  Floods will always occur and are part of the natural functioning of river systems. Floods cannot be prevented, but people and infrastructure can be protected. Key strategic questions are:

    a.  What level of flood hazard is acceptable?

    b.  Is flood hazard increasing?

    c.  Does the UK have the policy, resources and technical tools in place to manage flood risk effectively?

  2.  This note attempts to provide an overview of the UK situation and a personal perspective. I would be happy to provide amplification and further information as appropriate.

3.  What level of flood hazard is acceptable?

  4.  The term "return period" is commonly used to describe flood frequency, and is easily misinterpreted. A return period of 10 years means that such a flood will occur on average once every 10 years, or more usefully, that there is a 1 in 10 chance of such a flood occurring in any given year. Thus a 100 year flood has a 1 in 100 chance each year of occurrence. The cumulative risk over a period of time can easily be calculated; for example, over a 70 year period (a human lifetime, or a design lifetime of a structure or facility, perhaps), there is a roughly 50:50 chance of a 100 year flood occurring.

  5.  Floods occur across the full range of frequencies. The Boscastle flood (August 2004) was associated with an extreme rainfall (181 mm in 5 hours), estimated to have a return period of the order of 1000 years or more. It is inconceivable that protection could be provided nation-wide for such an extreme event—the costs would be huge, and the landscape altered unacceptably. The Carlisle flood (January 2005) is a different story. Damage was estimated at £450 million, and 2 lives lost. The flood had an estimated return period of 150 years, whereas the defences were designed for levels of protection from 20 years to 70 years. This raises the question of what is an acceptable level of risk for an urban area, which is essentially a political question. Conventional practice in the funding of flood protection schemes weighs the costs of protection against the economic benefits from preventing flooding for a range of frequencies. This ignores wider issues of social equity (protecting the rich is economically more justified than protecting the poor), as has recently been recognised by DEFRA, and a target level of the 1 in 100 year flood has been identified. However, very few UK towns and cities achieve this level of protection. For example the Jubilee River, recently built to protect Maidenhead from Thames floods, was designed for an approximately 1 in 60 year event (similar to the 1947 Thames flood). Clearly where major assets are at risk, higher levels will be appropriate, but is 1 in 100 years (with a roughly 50/50 lifetime chance) an appropriate target level of risk as a baseline for our towns and cities?

  6.  There are two important footnotes to the above discussion. One is that any level of flood protection has a finite risk of being exceeded, and there are important issues of (lack of) public awareness of that risk. The second is that, for certain facilities, lifetime risk is important. In a recent case, consultants unthinkingly suggested a 1 in 100 year level of flood protection for an important facility with a lifetime of 70 years. As noted earlier, that gives a 50/50 chance of flooding—which in that case was completely unacceptable. Where lifetime risks are important, much rarer floods must be considered.

  7.  A final point concerns extreme flood hazard. Where facilities such as reservoirs have significant risks associated with potential dam failure, such as substantial loss of life, under UK practice rare extreme events are considered, for example the 1 in 10,000 year flood, and the Probable Maximum Flood (the largest flood that is considered possible). I return to this point, below.

8.  Is UK flood hazard increasing?

  9.  Flood risk is generally considered to be a product of a) flood hazard (the chance of flooding) and b) the consequences of flooding. Nationally (and globally) flood risk is increasing, as more properties are built in vulnerable areas, and as the value of properties and infrastructure at risk increases with economic growth. Here I consider just flood hazard. A brief discussion follows—more detail can be found in Wheater (2006).

10.  Land use change

  11.  Man's activities continuously change the environment, and hence flood risk. New build replaces vegetated sites with impermeable surfaces—roofs, roads, car parks, etc. This changes the runoff—more surface runoff, less soil storage, with runoff usually collected in storm drains and channelled rapidly to watercourses. This increases river flood peaks and may reduce low flows. It may also change flood seasonality—rural streams mainly flood in winter, urban areas generate high runoff from intense summer storms. These effects are well known and hence urban development usually comes with a requirement to provide temporary storage (for example in detention reservoirs) to mitigate these effects. There is also interest in SUDS—sustainable urban drainage systems—where for example soakaways, permeable pavements and in-pipe storage can be used to reduce the rate of runoff. Confused responsibilities for flood management and maintenance in urban areas have inhibited uptake of these methods in England and Wales. Nevertheless, effects of urbanisation are well known and design solutions are commonly provided to mitigate the effects. (Flooding within urban areas is of increasing concern, not least to the water utilities responsible for sewerage in England and Wales and the insurance industry. Flooding may occur due to intense rainfall within an urban area overloading the drainage systems (sometimes called pluvial flooding), due to a failure in the sewer system, or due to interactions with river or coastal flooding. I return to this issue below.)

  12.  While urban development is a clear and dramatic example of changes to the environment, other changes are more subtle. There has in recent years been much concern about the effects of agricultural intensification. Changing arable land management practices include changing cropping patterns (with increased working of bare soils in Autumn and Winter) and the use of contractors (with increased size and weight of machinery, and constraints on time available for land access). There is evidence of associated degradation of soil structure and mainly anecdotal evidence of associated "muddy floods". In the uplands, there has also been intensification. In Wales, for example, sheep numbers increased by a factor of 6 from the 1970s to the 1990s, and changing breeds led in some cases to a doubling of weight per animal. The DEFRA/EA Flood and Coastal Defence R&D programme recently funded a definitive review (project FD2114) which concluded that there has been a lack of hard evidence on the local scale effects, and a lack of methods to predict with reasonable confidence the effects at the scale of river systems. Under the Flood Risk Management Research Consortium (FRMRC), led by the Engineering and Physical Sciences Research Council, but co-funded by DEFRA, the EA, the Scottish Executive and the Northern Ireland Rivers Authority, a research programme was established in 2004 to develop the science base in this area. Current results suggest that agricultural intensification has led to locally-increased flood risk and demonstrate that changes to upland land management practices can reduce flood runoff, at least at local scale (individual fields up to 20km2 river catchments). Effects at larger scale are the subject of continuing studies, funded variously by DEFRA/EA, the FRMRC and NERC'S FREE (Flood Risk from Extreme Events) programme.

  13.  For both urban development and agricultural intensification, impacts are greatest for frequent flood events. As the severity of the rainfall increases, so the relative effects of the land use change decrease. For urban areas, it is speculated in UK practice that impacts are minimal for the 500 year event. Research to quantify rural effects is yet to report.

  14.  The previous discussion in this section has focussed on runoff generation. River flows are routed downstream in river channels and, under flood conditions, in the associated flood plain. Flood protection works often have the effect of protecting flood plain areas from flooding—either to protect properties built on the floodplain, or to allow agricultural development in the floodplain. This involves disconnecting the river channel from the natural floodplain storage, so that the natural storage and attenuation is lost. This has the effect of transmitting increased flood peaks downstream. This issue is of major concern in Europe, for rivers such as the Rhine. Flood risk for downstream towns and cities on the Rhine has significantly increased as a result of river engineering and floodplain disconnection and efforts are being made to re-establish some floodplain storage. In the UK, creation of washland storage may be considered an option in the design of flood relief schemes, and there is considerable interest in the potential use of floodplain agricultural land to provide flood storage, at the expense of a loss in agricultural flood protection.

15.  Climate Change

  16.  A detailed discussion of climate change and flooding issues can be found in the 2002 Proceedings of the Royal Society (Phil. Trans. R. Soc. Lond. A, Vol 360, 2002).

  17.  Scenarios of climate change for the UK, developed from Global Climate Models and embedded Regional Climate Models, suggest a change in the South to warmer, wetter winters and hotter drier summers, and in the North, to wetter summers and winters. It is important to note that Global Climate Models are impressive in explaining global temperature change, but are poor at representing rainfall—for example they fail to capture the correct daily cycle of rainfall in the tropics. And as the scale of interpretation is reduced, the uncertainty in rainfall estimates increases. It is recognised that there are large differences in quantitative estimates of response to emissions scenarios between models, and even between the same model, with different initial conditions. Hence while there is reasonable consensus between models concerning the direction of climate change for the UK, quantitative estimates are highly uncertain. Recent work by NERC's Centre for Hydrology and Ecology, Wallingford, has shown that when Hadley Centre climate model scenarios are combined with hydrological models, results can be mixed. In many catchments flood risk is increased, but in some it is reduced, as a result of the effects of the drier summers. The main message is that estimates of future rainfall and consequent flooding are extremely uncertain. Recent work funded by DEFRA and the EA (e.g. project FD2113) has sought to develop new methods to improve the confidence in estimates based on Global and Regional Climate Models, and this task remains an important research challenge.

  18.  Climate models provide a basis for estimating extreme rainfall with fairly frequent occurrence (return periods of say a few decades). However, there is no guidance currently available to quantify climate change impacts on extreme events, such as the Boscastle storm, for example, or the rare events considered for dam safety. This is a strategically important gap in knowledge.

  19.  An alternative to modelling to evaluate climate change is to look at the historical record to detect change. However, since extreme events are by definition unusual, there are major technical difficulties in determining whether they indicate changing response. My understanding of the literature is that, although short records may indicate change, there is as yet no evidence for the UK from long term data that floods have increased. However it seems likely that within the long term natural variability, flood occurrence is currently greater than 30 years ago, which was a relatively dry period with respect to flood occurrence.

20.  Does the UK have the policy, resources and technical tools in place to manage flood risk effectively?

21.  Making Space for Water

  22.  DEFRA has recently developed a vision for water management "Making Space for Water" (MSW) which is a visionary and far-sighted document. MSW embodies a radical change in perspective for flood risk management from earlier approaches that focussed on local assessment of hard defences. MSW emphasizes the need for integrated management of flood risk at the spatial scale of the whole river catchment or the whole shoreline. This requires consideration of both structural and non-structural measures, including rural land use solutions, and a more integrated approach to specific issues such as urban drainage, coastal flooding and erosion. MSW also emphasizes the need to "deliver the greatest environmental, social and economic benefits consistent with the Government's sustainable development principles," which requires broadly-based multi-criterion assessment. Implicit in this new perspective therefore is the need for new and broader approaches to decision support systems and modelling. MSW must also be seen in the context of European developments, in particular the Water Framework Directive, which has wide-ranging implications for water management and the protection of ecological quality, and the forthcoming Floods Directive.

  23.  There are, however, major technical challenges in taking forward such broad-based assessment. The DEFRA/EA R&D programme has recently funded an expert group, which I led, that has mapped out a technical vision to achieve this (project FD2118) over a 5 to 10 year timescale. In the meantime, the EA has been developing Catchment Flood Management Plans (CFMPs), with the aim of providing more integrated assessment with the more limited tools currently available (these tools are unable adequately to represent features such as land use change, or changes in river geomorphology). It is however unfortunate in this context that the recent reorganisation of the EA has removed the river catchment as a basic management unit—a structure that had been in place for 30 years or more and viewed internationally as a flagship example of how to organise river management.

  24.  MSW recognises that land use management and flood management are intimately related, and the current structure of DEFRA combines these responsibilities. However, a truly holistic approach has yet to be achieved. For example, changes to rural land use to mitigate flood risk are likely to have associated benefits for diffuse pollution, but a framework to account for such multiple benefits has yet to be achieved.

  25.  Another issue of coordination within DEFRA was that until recently, responsibility for reservoir safety (and the associated issue of design for very extreme floods, discussed in para 7 above) was divorced from fluvial flood protection. This led to a situation where a new code of flood design practice was introduced for fluvial flooding in 1999, which when extrapolated beyond the return periods for which it was intended, gave very large flood estimates, in some cases bigger than the previously considered Probable Maximum Flood values. This left the dam owner community in a difficult position, with potentially large costs to be incurred to meet new standards of protection, which might be incorrect. Work was eventually funded by DEFRA to reconcile this issue, but in 2007 it has yet to report. Recently these responsibilities have been integrated, however.

26.  Urban flood management

  27.  In the urban situation, as noted above, there has, in England and Wales, been divided responsibility for flooding between the EA (responsible for main rivers), the water utilities (responsible for sewer flooding) and Local Authorities (responsible for lesser watercourses). DEFRA has been aware of these problems, which inhibit the development of integrated solutions to flooding problems and implementation of methods such as SUDS, where responsibilities for maintenance have been unclear.

  28.  For urban flooding there has also been an important technical gap. Storm sewer design has been based on models to simulate flow within the piped system, and criteria for the acceptable frequency of pipe-full flow (return periods of just a few years). The relationship between sewers flowing full or under pressure (surcharge), and surface emergence of flows is site-specific, and the tools have not been available to represent surface flooding for the sort of return periods commonly considered for river flooding (eg 20-100 years). New technology is changing that, and within the FRMRC, colleagues are using detailed remote sensing topographic data (from airborne LiDAR) to identify surface flow paths in urban areas. These can then be used with models that can represent the interaction of storm drain and surface runoff in urban areas. This provides the potential for a completely new approach to the design and management of urban flood infrastructure in the future.

29.  Groundwater flooding

  30.  Current fluvial flood estimation methods focus on surface water flooding. Extensive and prolonged groundwater flooding occurred in the wet Autumn and Winter of 2000-01, affecting areas such as the Chalk landscape of South-East England where streams are predominantly fed by groundwater. In these areas, normal river flows are low (typically just 2% of rainfall appears as rapid runoff), but under exceptional periods of prolonged wet weather, dry valleys begin to flow, springs break out in new areas, and much larger volumes of runoff occur. A notable example is the flooding of Chichester in 1994. New technical methods are needed to asses and manage groundwater flooding. NERC's FREE research programme has commissioned some of the first studies in this area, led by me, which began in 2007.

31.  Floodplain management

  32.  There is a major legacy of inappropriate floodplain development, approved by local authorities against the advice of the EA (and its predecessors). Recent strengthening of the EA's voice in floodplain planning issues is welcome, but as the floods of this Summer have demonstrated, there is a major issue of legacy development and inappropriate siting of buildings and other infrastructure in floodplains. Apart from the recent example of Severn-Trent's water works, it is not uncommon to find hospitals, old peoples' homes and the bases for emergency services located on sites at risk of flooding. This is clearly an unsatisfactory situation.

33.  Human resources

  34.  As Director of the UK's oldest MSc programme in Hydrology (which started in 1955), I should report that there is a severe shortage of technically competent and experienced hydrological specialists in industry and the EA, which is reflected in demand for the engineering-based students which we produce. However, in particular over the last 5 years, we have struggled to recruit suitably qualified students to both MSc and PhD programmes, despite the availability of funding from UK Research Councils for UK students. There are large numbers of students wishing to follow careers in this area, but lacking the necessary background in Mathematics to enable them to develop the appropriate technical skills. One consequence of this is that many overseas graduates from our MSc course take up UK posts, another is that funded training opportunities remain unfilled, a third is that employers use underqualified staff. It would in my opinion be most helpful to the provision of skilled manpower if Research Council grants could be made available in full to EU residents.

35.  Financial resources

  36.  In my opinion, much of the UK's flood infrastructure provides levels of protection that are incompatible with society's expectations of risk. This has implicitly been recognised by DEFRA, in establishing guidelines of 1 in 100 risk as appropriate for urban areas. Raising defences nationally to this level (or better) within a reasonable time frame will require very significant expenditure.

REFERENCE:

  Wheater, H.S. (2006) Flood hazard and management: a UK perspective. Phil Trans R Soc A, 364, 2135-2145.

Professor Howard Wheater FREng

Department of Civil & Environmental Engineering

Imperial College London

September 2007





 
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