Surface Water Flows and Flooding

The recent ASC report on flooding and water scarcity makes some interesting points about surface water flows, particularly those associated with flood hazard in urban areas. The report states that ‘Every millimetre of rainfall deposits a litre of water on a square metre of land’. Water falling onto a paved, impermeable will not infiltrate into the ground and so the volume has to move somewhere. The amount of paved surface is increasing as the report noted that green spaces in urban areas have been paved over and so surface water flows in urban areas are increasing even before the more intense rainfall associated with climate change is considered. The figures cited are that the proportion of paved gardens has increased from 20%in 2001 to 48% in 2011 of the total garden area of 340,000 hectares.

To combat this increase in paved area contributing to runoff the report suggest that urban creep should be minimized, sustainable urban drainage (SUDS) (SUDS) (SUDS-EA) should be improve to slow down water flows and store water above ground, and that conventional sewers should be maintained or upgraded: all good ideas. Recent floods in urban areas have highlighted the importance of such measures. Paved surfaces permit no storage of water, runoff is almost immediate and, with intense rainfall, the volumes of runoff involved can be huge within a short period of time. Overwhelmed urban drainage systems mean that the water moves rapidly across impermeable surfaces and flows through streets and roadways using them like predefined river channels. Similarly, when a river bursts its banks the water tends to use the paved, impermeable surface as a routeway for movement. The urban road network provides a convenient substitute for natural channels providing water with a rapid means of moving across an urban area.
A great deal of the potential damage from a flood and even flash floods could be mapped using a detailed digital elevation model (DEM) and a knowledge of past events in an urban area. This will help map out previous routeways that surface flows have used. Future events may be harder to predict, as the urban infrastructure changes and precautions are taken by planners to block or re-route surface flow, then the microtopogaphy of the urban area may be a guide to patterns of surface flow but other factors will also affect the detailed routes the water takes. The local detail is a bugger for modelling flow patterns. It will be interesting to see what, if any, use is made of the information about flood damage from the recent floods. There is a great deal of information online from Twitter, as well as local blogs and newspapers accounts that could provide a great deal of information about how surface water moved through urban areas. The potential for ‘citizen science’, for ordinary people (a horrible term that seems to imply scientists and planners are extraordinary) to contribute to the scientific investigation of flooding is immense. Co-ordination of this type of information, the mere exercise of collecting and collating information, or judging its quality and usefulness fro modelling and understanding urban surface flow is immense. Time, expertise and, potential funds, are needed for these activities but by who is unclear. Once the aftermath of the floods disappears from public view, the chances of funding such work drops dramatically. The need for people, the public (rather than the ordinary – anyone got a better term that isn’t condescending?) to be involved is important, however, if some of the recommendations of the ASC report are put into practice. In particular, the emphasis on households undertaking property-level flood protection measures might be enhanced if they were also actively involved in monitoring and in the feedback loop from modelling studies of their local areas. This would not only mean they were better informed about the risks of flooding but also more likely to act in the manner hoped for by planners if they felt they were an active part of preventing flood damage rather than passive victims in urban flooding.

Flooding and Development in the UK: Some thoughts on ASC Report

The recent ASC Report provides some interesting insights into current planning practices in the UK in relation to flooding. The ASC report (chapter 2) suggests that climate change is not the only factor increasing flood risk (page 27). It points out that risk changes if the probability of an event occurring changes OR if the consequences of an event alter. The first aspect is meant to cover the purely physical aspects of a hazard or rather of climate change in affecting flooding, whilst the second aspect is meant to relate to the socio-economic aspects of a hazard. It could be argued that a hazard isn’t really a hazard unless people are involved so an ‘event’ isn’t really a hazardous event unless there is a vulnerable population, so the two aspects may be more tricky to separate than appears. The two may go hand in hand.


Leaving this aside, the report does highlight the importance of the need for planning that bears these aspects in mind. The tables on pages 28 and 29 also provides some estimates of the amount of stock at risk from different types of flooding with 1.2 million properties at risk from river flooding, 230,000 of these at significant risk (a greater than 1 in 75 chance in any given year). The report points out that most floodplain development is within built-up areas that already have flood defences. Continuing development behind these existing defences increases the total value of assets that are being protected. In any cost-benefit analysis this increased value will make any future investment decisions easy – keep investing in defences as the value of assets keeps increasing. This means that current flood defences lock-in long-term investment, meaning that higher and stronger defences are continually required. The report recognises that this has been known for a while as the ‘escalator effect’.

The report also identifies that only a small number of planning applications have been approved when there was a sustained objection from the Environment Agency (EA). This implies that most planning applications meet the requirements of the EA for taking into account flood risk – a comforting point. However, the report also notes that, in general, local authorities are implementing national planning policy by continuing to build with protection in floodplains (page 36 of the report). In addition most flood risk management policies in plans focus on making development safe once the strategic decision to build in floodplains has been taken (page 37 of the report).

Combined this implies that floodplain development will continue and will produce an investment strategy for flood defences that encourages further development in already protected areas, forcing further and more extensive protection of these areas. The report states that the EA as taken a strategic approach to funding structural flood defences ‘targeting investment towards communities at greater flood risk and with the highest social vulnerability’ (page 40). The justification for this approach, however, is then expressed in terms of average cost-benefit ratio of 8:1, i.e. for every £1 spent on flood defences there is an expected reduction in long-term cost of flood damage of £8. This implies that social vulnerability is defined in terms of money as are any other benefits. This would suggest that assets that are relatively easy to attach a monetary value to will weigh heavily in these calculations. Again this would encourage development in flood protected areas as any additional value from the buildings an infrastructure will increase the cost-benefit ratio and so ensure the continuation of more, stronger and higher flood defences. Whilst here is nothing inherently wrong with this it does mean that if or when the defences are breached the cost of flood damage will be huge. How is this potentially high magnitude cost worked into the cost-benefit equations? Is the potential loss or cost discounted and over what time scale? How does the probability of such a high magnitude loss change as both aspects of flood risk mentioned above, the physical and the socio-economic, change into the future? Is the potential cost so huge that the defences must alwasy be enhanced into the future no matter what the rate of cliamte change or the cost? Is the current strategy commiting the future to locational inertia of housing, business and infrastructual investment?

Future Flood Risk in England: Committee on Climate Change Report

The recently published report ‘Climate change – is the UK preparing for flooding and water scarcity?’ an Adaptation Sub-Committee (ASC) Progress Report of the Committee on Climate Change, could not have asked for better timing really. The ongoing unseasonal rain (and by rain I mean torrential downpours rather than light summer showers) has brought the issue of flooding to the forefront of many people’s minds, not least those inundated with dirty brown flood waters. The headline grabbing pieces of the report are:


• floodplains have been developed faster than anywhere else in England over the past decade; one in five properties in floodplains were in areas of significant flood risk

• the current ‘build and protect’ policy will leave a legacy of rising protection costs; current levels of investment in flood defences will not keep pace with increasing risks with the number of properties at significant risk of flooding double by 2035

• increasing investment in flood defences and property protection measures could halve the number of properties at risk by 2035 (relative to current levels)

The recommendations of the report in relation to flooding (in the executive summary and helpfully laid out in a box on page 17) sound reasonable but they need to also be looked at in the light of the government’s recent modernization of planning policy (National Planning Policy Framework) which highlights the needs to undertake major and sustainable development in the UK in the near future. The ASC report advises that for flooding robust and transparent implementation of planning policy in flood risk areas is required and that local authorities should be consistent and explicitly takes into account long-term risk of flooding when deciding the location of new developments (page 17 of the report). In addition, there should be support for sustained and increased investment in flood defences by both private and public sources as current spending will not keep pace with the increasing risk. Failing increased expenditure, ways to manage the social and economic consequences of increased flood frequency should be identified. Lastly, there should be increased enabling of the uptake of property-level measures to protect against floods and encouragement of the increased use of sustainable drainage systems to manage surface water.

The National Planning Policy Framework highlights the importance of sustainable development particularly of housing and the importance of community involvement. The ASC report even recognises this on page 12 when it states that ‘Development in the floodplain may be rational decision in cases where the wider social and economic benefits outweigh the flood risk, even when accounting for climate change’ but then adds that in a review of 42 recent local development plans there was mixed evidence of transparency from local authorities in terms of locating development on areas other than floodplains and in including the long-term costs of flooding associated with climate change. So no cosnsitent approach to the issue yet.

So how do you decide (and who decides) if development on the floodplain is worth it or not? The decision seems to be located at the local authority level where contradictory messages are being delivered – develop sustainably (whatever that means) and don’t develop there unless you absolutely have to. There is also an assumption that the decision can be rationalised, presumably using cost-benefit analysis. This puts the valuation of property, the environment, business and infrastructure at the centre of any argument. How does this square with the ‘sustainability’ issues raised in the National Planning Policy Framework? Even worse it is not necessarily the current valuation that will be important by the future valuation of both the land and its use as well as the costs of clearing up flood damage. How do you work this out in a consistent and mutually agreed manner for the whole country for every land-use or potential land-use? Even if the local authorities made their valuations explicit would every stakeholder agree? What of localism as well? How much input will communities have into the decision-making process if valuation is the central pillar of decision-making for housing? Can they, the communities, compete in any discussion with the complicated economic modelling available to local authorities?

An interesting aspect of the report is the focus on ‘property-level protection measures’ by which they means things such as door guards and airbrick covers which the report points out require a take-up rate increase of 25-30 times by 2037 to reach all 200,000 to 330,000 properties that could benefit from their use. Although the report discusses these measures in combination with government investment as being a means of reducing flood risk I do wonder if this is more evidence of the ‘moral hazard’ argument coming into the discussion. In economic theory this refers to the tendency to take undue risks when the costs of those risks are not borne by the individual (or entity) taking those risks (a nice discussion of this idea can be found on Wikipedia – (I am not adverse to using this source if it is well done but wouldn’t advice it for any of my students reading this!) Property-level protection measures seem to throw responsible for tackling the risk (and increased risk of flooding) onto the property owner. Although not put into these words, does that mean it is their fault? They brought the house there, so the risks are theirs to bear? They have to implement property-level protection measures or else why should anyone else help them, such as insurers or local authorities? Odd when developers are allowed to develop on floodplains and then sell houses on – do home buyers have choice in where to buy if housing development is concentrated in floodplains or if those are the only locations they can find houses cheap enough (or in the right price range) for them to buy? Where exactly does responsibility for living in a floodplain lie?