Surface water management planning: guidance (2018)

Guidance to assist the responsible authorities in preparation of Surface Water Management Plans (SWMPs) to help with the management of surface water flooding.

Appendix 6 Adaptation to future flood risk

A6.1 Introduction

This appendix provides further guidance on considering adaptation to future flood risk in an options appraisal. It covers three main influences on future flood risk:

  • Climate change
  • Urban creep
  • Demographic change.

The information available on these different factors will vary and should be summarised at the ‘Understand flood risk’ stage. Authorities should consider all influences in an integrated way where possible.

Further guidance on considering adaptation in option appraisals can be found in:

Two climate change adaptation approaches are described in the Defra 2009 policy statement and in Chapter 7 Option appraisal: managed adaptive and precautionary. Both can also be applied to ensure that options are adaptable to changes in land use (urban creep) or population growth and new development.

A6.2 Information on the impacts of climate change on rainfall

When designing options, allowance should be made for climate change. What allowance to include will vary, depending, for example, on the type or location of the option and whether a managed adaptive or precautionary approach is being taken.

SEPA’s 2013 pluvial model climate change scenarios assumed a 20% uplift in rainfall intensity for 2080. This assumption was based on Defra 2006 guidance and subsequently updated EA 2016 guidance, which represented the best understanding at that time. UKWIR (2015) rainfall intensity for sewer design has estimated uplifts for different regions of the UK, showing that for the east in particular larger uplifts may be more appropriate. UKCP18 should further refine our understanding of climate change impacts on rainfall, outputs from which are expected in 2018.

Industry design manuals (e.g. CIRIA 753 (2015) SUDS manual and Scottish Water Sewers for Scotland) may also contain guidance on climate change allowance when designing infrastructure.

Information on the impacts of climate change on rainfall include:

A6.3 Information on rate and impact of urban creep

When designing options, some allowance for urban creep can be included. What allowance to include may vary, depending, for example, on the type and location of the option and whether a managed adaptive or precautionary approach is being taken.

Information on rates of urban creep, which will vary, may not be available for a local area. Instead modelling can be carried out to estimate the impacts of different urban creep scenarios on flood risk for various options. Whatever approach is used is likely to call for an increase in impermeable area in the model. This could be a fixed percentage increase, as per the 10% allowance in hydraulic design for urban creep recommended in the CIRIA C753 (2015) SUDS Manual and Scottish Water’s Sewers for Scotland, for example.

Kelly (2016) [21] showed that:

  • Modelled increases in run-off are directly proportional to growth in impermeable cover – the contribution of existing paved front gardens to the overall urban drainage burden is already substantial.
  • The effects of climate change are likely to exacerbate the problem further – higher rainfall in the future is likely to increase run-off from paved front gardens, putting additional strain on already struggling drainage systems.

Soil type is likely to influence the extent of future flood risk. Published data on rates of urban creep includes:

  • UKWIR [22] data on average rates of urban creep in sample English cities showed between 0.4 and 1.1m2 per house per year.
  • Wright et al (2011) [23] found a near quadrupling of the area of impermeable hardstanding in three typical residential areas of Edinburgh.

A6.4 Information on demographic change

It is important to adhere to land use planning policies to ensure that new development is not at risk of surface water flooding and does not increase elsewhere.

Further information, including projections for new homes, is available from local authority land use planners and National Records of Scotland (see Section 5.4 for further information).

Growth can be taken into account if deemed necessary (e.g. in areas of high growth) along with urban creep and loss of green space, and a managed adaptive or precautionary approach taken.


Gordon Robertson:

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