Why do Planning Authorities ask for flood risk assessments?

The Easter floods of 1998 in England resulted in five deaths and estimated damages to property of £400M.  In autumn and winter of 2000, 10,000 properties were flooded with estimated damages amounting to £700M. Approximately 10% of the population in England and approximately 8% of the land area are at risk from flooding.

In recognition of these problems, the Government published its guidance on Development and Flood Risk (PPG 25) in 2001, which places a duty on local planning authorities to implement the Government’s policy of reducing the risks to people and property by ensuring that flood risk is properly taken into account in the planning of developments. 

The susceptibility of land to flooding is therefore a material planning consideration and developers of land in the flood plane, or in areas where there are known drainage problems, can expect to be asked to provide a risk assessment with their planning application.  As a statutory consultee, the Environment Agency will generally object to planning applications for developments in land that could be affected by flooding if a satisfactory flood risk assessment is not provided with the application.

Assessment of Flood Risk

Assessing flood risk is complex and requires specialist expertise in hydrology, open-channel and pipe hydraulics and land drainage practise.  The assessment should consider risks to the development in question and risks caused by the development.

Flood events are usually classified according to their estimated frequency of occurrence.  This is expressed as a “return period”. A flood having a return period of 100 years is the flood event that will be exceeded, on average, once in 100 years.  In other words, the probability that a 1 in 100 years flood will be exceeded in any one year is 1%.  This defines the flood risk

Run-off assessments

In locations where there are known drainage problems, a run-off assessment is likely to be required.  “Run-off” is the surface water resulting from heavy rainfall which flows over the ground surface to a drainage channel such as a river or stream, or to a drainage gulley leading to a sewer.  Surface flooding can occur when the volume of run-off exceeds the capacity of the receiving drainage system.  Where this occurs some form of mitigation, such as flood storage, will be required.

Sustainable Urban Drainage Systems (SUDS)

Until comparatively recently, drainage practise was based on collecting and discharging rainwater as quickly as possible.  This ensured that developed land was kept free of ponding water but it had the undesirable effect of causing rapid changes in the flow rates of the main drainage channels such as streams and rivers.  This in turn resulted in sudden changes in river levels as high volumes of run-off were added to the channel.  The consequence of this is “flash flooding”.  A flood wave travels down the river, overtopping the channel wherever it can and causing sudden flooding.  High flow velocities in the stream and river channels can also be damaging to the aquatic ecosystem since erosion of river channels causes loss of habitat.

Modern drainage practise recognises this problem and aims to reduce the speed and volume of run-off by increasing the opportunity for rainwater to infiltrate into the ground, and also by providing storage for excess water.

SUDS seek initially to prevent problems by good site design which minimises directly-drained areas and contains potentially polluting areas.  Any residual problems that are not addressed by prevention techniques are then addressed using source control methods such as infiltration devices.  Excess run-off is accommodated in suitably designed conveyance devices such as filter strips, swales or oversized pipes. Ponds can be incorporated into landscape designs and can be effective, provided there is sufficient land space. Buried tanks, large diameter pipes, or modular storage units may also be used.

The distinguishing feature of SUDS is that, unlike previous drainage practise, they address the problems of quantity and quality of run-off.  The devices used in SUDS help to filter and cleanse run-off before it eventually reaches the arterial drainage system of streams and rivers.  This has a very positive environmental impact on the quality of rivers and their ecosystems.