The challenge of flooding

Despite advances in understanding of the underlying science, and the extent of experience flood risk management is not widely available, writes David Balmforth:

There is nothing like a major flood to capture the spotlight, and this year has brought flooding to the UK on an almost unprecedented scale as an exceptional combination of prolonged rainfall followed by very intense storms (over 100 mm of rainfall in 24 hours) was repeated over several weeks.

The result was flash floods first in the North of England, the Midlands, in the South West and finally in the Thames Valley. Few parts of England and Wales were spared. In Hull alone over 10,000 properties were flooded and over 4000 school children could not attend school. There were also several deaths as a direct or indirect result of the floods.

The questions now are: Why did this happen? Are we seeing the effects of climate change?, Why was more not done to defend land and property from flooding? Who is responsible? The fact that these points are being raised is in itself an indication of the complexities of flooding and the uncertainty over understanding potential climate change impacts.

Flooding is one of the greatest global hazards, forming 65% of all natural disasters and affecting over 6 billion people worldwide. In the UK over 2 million homes are at risk of flooding with the average annual damage cost exceeding £1.4m. Over £800m is spent annually on flood risk management. Areas prone to flooding cannot support agriculture and cannot contribute effectively to wealth creation.

Despite advances in understanding of the underlying science, and the extent of experience in flood risk management, comprehensive flood risk management is not available to most of the world’s population. Even in developed countries significant parts of the population are at risk from flooding, and flood-warning systems are often rudimentary.

The cause of flooding is often complex. Flooding occurs when drainage channels have insufficient capacity to drain surface run-off. In natural systems, streams and river channels will overflow creating a flood plain which has the dual function of providing additional conveyance capacity and temporary flood storage.

Flooding may be caused by the local lack of channel capacity, by flow backing up from downstream overloaded channels, and/or by the passage of flood flow across the flood plain. It can be localised, due say to the run-off from a single field, or widespread. Regions near the coast are susceptible to flooding from the sea which can be caused by particularly high tides and/or storm surges.

For historic and legal reasons the responsibility for managing flood risk is also complex. Many different bodies are engaged in managing flood risk, ranging from local authorities, highway authorities, water companies, land drainage boards, the Environment Agency and individual property owners.

Responsibility for the flooding ultimately fell across five different bodies. Identifying the causes of flooding and developing a holistic solution is often hampered by the different structures and statutory duties of these bodies. It makes engagement with the public, the ultimate customer in flood defence, particularly difficult.

Real space for water

If all this was not bad enough, current research on climate change, as set out in the OST Foresight Report, shows that the infrequent extreme events of recent weeks could become much more the norm in the future. By 2080 flood damage costs could rise to 8 times the present value with the cost of flood defence rising by an even greater amount. Even in the unlikely event that resources became available for flood defences, such extensive construction would badly damage our ability to meet carbon reduction targets. Future flooding can only be tackled through a radically different approach.

Important conclusions can be drawn from examining actual extreme flood events, such as that which occurred in Glasgow in 2002. Here approximately one third of the flood was due to overloaded watercourses, one third due to overloaded sewers and a further third as the result of direct land drainage.

Apart from the usual conclusion that water flows downhill and accumulates in low spots, it was also shown that around 80% of the flood water was conveyed on the surface, leading to indiscriminate flooding of property, often as a result of relative minor detail such as kerb height. Managing the flood water on the surface offers an opportunity for much more effective control of flood risk in extreme events in the future.

The foundations for that approach have been laid in Defra’s Making Space for Water and in the Planning Policy Guidance PPS25. Both these raise the need to embed flood risk management into urban design. However current interpretation of PPS25 is unlikely to deliver the radical change that is needed without better engagement with planners. This is essential in the context of planning for 3 million new homes.

The Construction Industry Research and Information Association have produced some valuable guidance on how to design urban areas to safely and effectively manage flood flow on the surface. It includes advice on how to design roads to also act as flood channels, to use car parks as temporary flood storage areas: even to designate parts of our urban areas to be “sacrificial” flood areas during extreme flood events.

There is little evidence that such guidance is being actively taken up. If we are exceptionally (and it must be exceptionally) to build on flood plains then we must allow space for flood waters to pass through. “Compensating” storage and sustainable drainage systems have little effect during extreme events. Conveyance capacity is the answer and we need to lay out our new urban areas to achieve this. The CIRIA Guidance should be followed. In addition, housing must be made to be much more flood resilient, raising living areas above street level, and using materials more resistive to flood damage.

Fresh leadership

For such a radical new approach to flood risk, the public will need to be convinced these measures are workable, equitable and affordable. This requires much better engagement with the public, both strategically and locally, than has been evident in the past. There needs to be a much more coordinated approach between the different stakeholders responsible for flooding in our urban areas.

The Defra Integrated Urban Drainage Pilots are an important step in the right direction. An early appraisal of PPS25 and the value of Strategic Flood Risk Assessments would be a further step forward. Overall there is a strong argument for one lead industry body with an overarching responsibility for managing urban flood risk. This could provide a welcome focal point for the public and also be the important catalyst for change.

David Balmforth is a technical director with the Engineering Company MWH, a visiting professor at Imperial College and Editor in Chief of the CIWEM Journal on Flood Risk Management.