Planning Policy on development and flood risk is defined in Planning Policy Statement (PPS) 25 which planning authorities are required to implement. 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, and if a planning authority disregards such an objection, the Environment Agency may refer the matter to the Secretary of State.

A key feature of PPS 25 is the Sequential Test.  This is a device by which planning authorities can steer development towards areas of lowest flood risk, in accordance with the policy. A planning application can fail if the applicant has not demonstrated to the satisfaction of the planning authority, that there are no suitable alternative sites for the development in a lower risk flood zone.  The Sequential Test can sometimes prove to be an unexpected obstacle to planning applicants and our advice is always to examine this issue at an early stage, before substantial costs are incurred in the preparation of the planning 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. A flood risk assessment must consider all potential risks of flooding including:

• Fluvial flooding (flooding from rivers and other open watercourses);
• Tidal flooding;
• Surface water flooding;
• Groundwater flooding;
• Flooding from sewerage systems.

Fluvial and tidal 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.

Although it is possible to assess the annual probability of rainfall events, this does not translate conveniently into an equivalent probability for surface water flooding due to the many interacting factors which influence this type of flooding.  The approach usually required by the Environment Agency is precautionary: there should be no surface water flooding of a proposed development from rainfall events having return periods up to 30 years and only “tolerable” surface water flooding from a rainfall event having a return period of 100 years.  Tolerable flooding usually means no flooding of buildings and of a depth that would not impede safe access and egress to and from the site of the development. 

Sustainable Urban Drainage Systems (SUDS)

Until comparatively recently, drainage practice 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.

 

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