Assessing PPG25 flood risks for brown-field developments

Flood risk assessments based on the government's PPG25 guidelines are now an integral part of brown-field development planning applications. Bob Williams of consultants Fenland Hydrotech looks at the implications of PPG25.
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Flooding is now regarded as a serious problem in this country. Apart from risks to life, financial losses are regularly measured in millions of pounds. Insurance companies are demanding greatly increased premiums on properties liable to flooding, and are sometimes even refusing to offer cover.

Apart from coastal areas, most flooding occurs because rivers cannot convey or store the rate and volume of run-off water directed into them. When it rains, some water is retained in the soil; some will evaporate; a little is absorbed by vegetation, and the remainder runs into streams and rivers. In a rural environment, something less than 30 per cent of run-off drains directly to rivers; the remainder soaks into the ground, reaching rivers eventually, or evaporates.

Urban environments are quite different. With far less absorbent surface areas, almost all the rainwater which collects on roofs, roads and paved areas must be disposed of by man-made drainage systems. In practice, this has usually meant piping it into the nearest river.

Apart from small movements due to climate change, today's increased flood risks largely result from rapid expansion in urban development. The government's plans to build millions more homes, mainly on 'brown-field' sites, offers valuable opportunities to incorporate new surface water management techniques into developments and minimise the overall impact.

The current and projected future incidence of flooding has risen to such an extent that the government has issued a 47-page Planning Policy Guidance document, PPG25, about flooding and flood risk assessments.

PPG25 states that susceptibility of land to flooding must be treated as a material consideration in all development planning applications, whether on brown-field or green-field sites. Indeed, some planning applications have already been rejected because a proper flood risk assessment has not been carried out. And it is not only the immediate development area that must be considered; the applicant must also show that run-off will not add to the risk of flooding elsewhere - downstream of it, for example.

With the government's present emphasis on urban regeneration and the redevelopment of brown-field sites, paragraph 35 of PPG25 provides general guidance. It cannot be assumed that, if a site has already played host to housing or industrial development in the past, a developer will not have to worry about flood risks in his planning application; a risk assessment will still be required.

The extent of such assessments will depend partly on whether the developer intends to maintain the status quo - in other words, replace like with like - or is applying for a change of use from, say, industrial to residential. For example, PPG25 points out that much past industrial development took place on flat land alongside rivers - possibly an inviting target for a high-quality residential or holiday development. However, with rising river levels some of this land may now be - or could be in future - subject to flooding, and PPG25 says that for any proposals for redevelopment, '…local authorities should take into account the risks of flooding, the standards of existing flood defences and the ability to improve them… such redevelopment should avoid interference with flood plain flows or compromising future shoreline or river management operations.'

The levels of flood risk may also determine the type of development that might be allowable for individual sectors within the overall development area. For instance, the risk of flooding in one sector may restrict it to non-residential usage such as parkland or sports facilities. Certainly, possible risks to life as well as property in the event of flooding must be at the forefront of any assessment. If part of the site is subject to flash flooding at short notice, it is highly unlikely to be suitable for occupation by people with restricted mobility or other disabilities such as blindness - a retirement home, for example. However, PPG25 says that an acknowledged risk of flooding on a brown-field site '… might be mitigated by confirmed good levels of protection, including protected access, prudent design of development and effective public warning systems.'

It also needs to be remembered that if a flood risk assessment indicates the need for precautions against flooding to be included in the development project, the costs of these will have to be met by the developer. This will apply not only to the risk on the development site, but also to any risk of increased flooding elsewhere which may be caused by the development.

The government's approach to redevelopment projects is summed up in PPG25 as follows: 'A balanced, flexible approach is required which addresses the risks of flooding whilst recognising the benefits of recycling previously developed land and the damage to urban regeneration caused by under-investment and urban blight.' The general rule of thumb is that a brown-field development must not create a greater flood risk than previously. And while PPG25 itself does not call for reductions in previous flood risk levels, some local authorities and/or Environmental Agencies in high-risk areas - the Nene Valley area in Northamptonshire is one example - may expect any proposal to include measures to actually lessen existing risks of flooding in the area where possible.

Carrying out a full flood risk assessment on a brown-field site, particularly if there is to be a change of use, will involve discussions with the local Environmental Agency, planning authority and water utility, and in today's climate the success of a planning application may hinge upon the assessment. It will also be advisable to bring the insurance industry into the consultative procedure.

Under PPG25 the developer has to assess (a) whether the development will be susceptible to flooding, and (b) whether it will increase flood risks elsewhere. He must carry out a Sequential Test of flood risk, with categorisation into one of three Flood Zones, and local planners are requested to give priority to permitting sites within the lower risk zones. What at first sight may appear a fairly simple task can quite quickly become very complex, and making a proper assessment calls for a considerable range of expertise and technical resources including experience in soil types, drainage systems, hydrology, computer software such as Hydroworks for drainage and Mike 11 for river flow modelling, the Flood Estimation Handbook (FEH) to predict rainfall and runoff, and climatology prediction data. If the developer does not have these in-house, he would be well advised to seek the services of a suitably qualified engineering consultancy such as my own company, Fenland Hydrotech.

Another benefit of using professionals is the advice they can give about possible ways to minimise flood risk problems by the creation of flood defences and SUDS (Sustainable Urban Drainage Systems). If costs are not excessive in comparison with the value of the development, these may well tip the scales in favour of project. Because a professional consultant must act impartially there is a possibility that his Assessment may show that the flood risk is too high for the project to proceed. However, a good consultant will usually spot this possibility well before the expense of a formal PPG25 assessment is entered into.

The volume and nature of flood risk regulations are constantly changing, and engineers must keep abreast of these changes. However, take heart; the quality and quantity of data on matters such as rainfall, climate change, urbanisation runoff characteristics and river flows is improving steadily, and computer modelling is becoming more accurate. One day, perhaps, flood risk assessments may become a simple formality - but not yet!