Environmental Impact :: Flood Risk Assessments
Flood Risk and Planning Policy
3.1 Policy
The Government's approach to flood risk in the context of the planning regime is detailed in Planning Policy Guidance Note 25 "Development and Flood Risk" (PPG 25) which was published by DTLR in July 2001, and details how flood risk should be considered at all stages of the planning and development process.
PPG 25 is currently under review and is due to be replaced by Planning Policy Statement 25 “Development and Flood Risk” (PPS 25) in Autumn 2006. Whilst this present report complies with PPG 25, account has also been taken of proposals contained in the draft version of PPS 25 (published December 2005), and also take account of further proposals in respect of climate change and lifetime of the development anticipated in the final version.
3.2 Responsibility for Protecting Land and Property against Flooding
The primary responsibility for safeguarding land and other property against flooding remains with the owner / developer. Within this context, those proposing particular development are responsible for providing "an assessment of whether any proposed development is likely to be affected by flooding and whether it will increase flood risk elsewhere and of the measures proposed to deal with these effects and risks". This is the purpose of the Flood Risk Assessment (FRA).
3.3 Flood Risk Assessment
Appendix F of PPG 25 gives guidance on carrying out a Flood Risk Assessment. This report conforms to the guidance provided by PPG 25.
3.4 Flood Risk Zone and Appropriate Planning Response
As set out in PPG 25 Table 1, the site lies within an area classified as Flood Zone 3(a) - developed area within an area of High Flood Risk - based on current indicative mapping.
The "appropriate planning response" for Zone 3(a) - developed areas - is that "these areas may be suitable for residential, commercial and industrial development provided the minimum standard of flood defence (including suitable warning and evacuation procedures) can be maintained for the lifetime of the development…"
3.5 Previously Developed Land
Paragraphs 35 and 36 of PPG 25 deal with Previously Developed Land and the need for "urban regeneration and the redevelopment of previously developed land to minimise the need for development of green-field land". In particular section 36 states that "Planning guidance on housing (PPG 3) already advises local planning authorities to take account of physical and environmental constraints on the development of land" and that "nothing in PPG 25 should be taken as departing from this guidance".
Recognising that land use sustainability and regeneration objectives may conflict with flood risk exposure for certain sites, paragraph 35 states that “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 acknowledged risks of flooding might be mitigated by confirmed good levels of protection, including protected access, prudent design of development and effective public warning mechanisms”.
3.6 “Sequential Test”
Paragraph 31 of PPG 25 requires a "sequential test" to be applied by the local planning authority when considering an application for development in a Flood Zone 3(a) area. This test should take into account -
* the Environment Agency's advice on the distribution of flood risk and the availability of flood defences….
* the resulting level of actual risk in…considering proposals and applications for development.
Essentially, the sequential test aims at generally limiting development / redevelopment in a high risk zone unless there is reasonable justification for such development in such a location, and there is no suitable location in a lower risk zone.
3.7 Sustainable Drainage Systems (SUDS)
Both PPG 25 and Draft PPS 25 recommend the use of Sustainable Drainage Systems (SUDS) where practicable.
Paragraphs 40-42 of PPG 25 deal with Sustainable Drainage Systems.
Paragraph 40 states that “consideration of flood issues should not be confined to river and coastal flood plains. Development throughout a river catchment can have a significant impact on flooding simply by increasing run-off resulting in increased flows downstream and this increasing the risk of flooding.
Paragraph 40 acknowledges that a change in land uses can result in an increase in vegetated areas (in gardens etc) and thus reduce run-off.
Paragraph 41 continues, “the restriction and reduction of surface water run-off from new developments can be encouraged by the provision of surface water storage areas, flow limiting devices in conjunction with surface and subsurface storage or, where ground condition permits, the use of infiltration areas or soakaways”.
Paragraph 42 states that “local planning authorities should, therefore, work closely with the Environment Agency and prospective developers to enable surface water run-off to be controlled as near to the source as possible by the encouragement of sustainable drainage systems”.
The present site lies in a “coastal flood” area where sea wave overtopping can contribute to surface water run-off.
3.8 Minimum Standards of Flood Defence
Paragraph 31 of PPG25 states that the minimum standard for flood defence for commercial and industrial development should aim at achieving the same minimum standards as new housing, namely protection against flooding with an annual probability of 1% for river flooding and 0.5% for coastal flooding for a period of 50 years, taking into account the appropriate allowances for climate change.
3.9 Scheme Lifetime and Climate Change
Appendix A, Table A1 of PPG 25 provides recommended allowances for regional rates of relative sea level rise in the interval AD 2000 – AD 2050. For the South West this allowance is currently 5mm per year. (This table is repeated in PPS 25 as Table B.1.).
However, urban residential development is likely to be required to last typically in excess of 70 years (based on recent historical trends), and paragraph 31 of PPG 25 also recommends that where development is to take place on “…sparsely developed flood plain, the threat of flooding should be managed to ensure that the development is and remains safe throughout its lifetime ….”
In this case, therefore, a scheme life of 75 years is proposed for the residential elements of the development. (A lower scheme life of 50 years would be acceptable for commercial and retail elements, since these are subject major refurbishment and re-building at typically much more frequent intervals than housing).
Paragraph A7 also indicates that annual rainfall rates are also expected to rise by up to 10% by the 2050s with winter rainfall increased by up to 20% in some scenarios. In assessing flood risk due to surface water run-off, therefore an additional allowance of +20% has been considered above the figures derived from the Wallingford Procedure (1982) and the Flood Estimation Handbook (1999).
Draft PPS 25 goes further and recommends a precautionary response allowing for up to 20% increase in peak river flows by AD 2050, increasing to 30% by AD 2110, and allowing for up to 10% increase in rainfall intensities by AD 2050 and up to 15% by AD 2110.
Draft PPS 25 refers to the climate change projections published in the UK CIP02 Scientific Report “Climate Change Scenarios for the United Kingdom”, April 2002. This indicates, for the South West, net sea level changes of between +160mm and +760mm (depending on low-emission and high-emission climate impact scenarios) by the 2080s. (Because of its proximity to the “South Coast”, UK CIP data for the south coast has also been considered alongside the “South West” data, and the more onerous of the two used in the following report, averaging at +6mm per year for the next 75 years, assuming a medium-emissions climate impact scenario).
Draft PPS 25 also suggests that an allowance of +10% be added to offshore wind speeds and wave heights to account for changes by the 2080s. (No further increase is currently indicated for offshore wind speeds by the 2110s.)
