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Planning Advice Note PAN 69
Planning and Building Standards Advice on Flooding

Dear Colleague,

PLANNING ADVICE NOTE 69: Planning and Building Standards Advice on Flooding

I am pleased to enclose a copy of PAN 69: Planning and Building Standards Advice on Flooding which provides good practice advice on planning and building standards in areas where there is a risk of flooding.

The PAN fulfils a commitment made in Scottish Planning Policy (SPP) 7: Planning and Flooding, published earlier this year, and supports the policy set out in the SPP. It also supports the Technical Handbooks published by the Scottish Building Standards Agency which provide guidance for the Building (Scotland) Regulations 2004. We are grateful to members of the SPP 7 Advisory Group for their input into the advice.

The Scottish Executive has also established the National Technical Advisory Group on Flood Risk to address many of the wider technical issues. This includes sustainable flood management, and in due course the Scottish Executive Environment and Rural Affairs Department will issue guidance with further explanation of responsibilities. The PAN also refers to the Water Framework Directive and Water Environment and Water Services (Scotland) Act 2003, and the forthcoming system of River Basin Management Plans.

Further copies of the PAN are available from the Scottish Executive Development Department, Planning Division Area 2-H, Victoria Quay, Edinburgh EH6 6QQ (0131 244 7066) and on the Scottish Executive web site at www.scotland.gov.uk/planning .

Yours faithfully

ALAN DENHAM
Assistant Chief Planner

Planning Advice Note PAN 69
Planning and Building Standards Advice on Flooding

INTRODUCTION

1. The planning and building standards systems both play important roles in safeguarding land and development from the consequences of flooding. This advice note provides background information and best practice advice in support of Scottish Planning Policy (SPP) 7: Planning and Flooding, and the Technical Handbooks published by the Scottish Building Standards Agency which provide guidance for the Building (Scotland) Regulations 2004. SPP 7 aims to prevent future development which would have a significant probability of being affected by flooding or which would increase the probability of flooding elsewhere. The Technical Handbooks provide improved guidance on building in areas where there is a risk of flooding.

2. This Advice takes as a starting point the responsibilities of local authorities and developers in ensuring that future built development is not located in areas with a significant risk of flooding, including functional flood plains. However, there are circumstances where development would benefit from selecting designs, forms of construction and materials which may help to minimise the effects of a flood event on the property.

3. The first part of this Advice sets out background information on the water environment and the factors which contribute to flooding. This includes watercourses, coasts, sewer surcharging, groundwater, and the influence of climate change. The document also contains advice on addressing flood risk in development plans and in dealing with planning applications, and promoting a flood prevention scheme. It provides background information on the impact of floodwater on buildings and materials, and advice on flood resistant materials and forms of construction that may be necessary to obtain a building warrant. Many of the measures described in the PAN will have a role to play as part of a sustainable approach to flood management. It also covers the role and remit of Flood Liaison and Advice Groups (FLAGs).

4. The Scottish Executive encourages joint working between local authorities, Scottish Water and SEPA to improve flood risk management, and has established a National Technical Advisory Group on flooding. A summary of the responsibilities of the different bodies and individuals in relation to flooding is set out at Appendix A.

BACKGROUND AND CONTEXT

5. Flooding can be created by a combination of human activity and natural physical conditions. Paragraphs 6 to 28 provide an explanation of the main causes of flooding in Scotland and the way in which different factors may interact.

WATERCOURSE AND COASTAL FLOODING

6. The principal cause of watercourse flooding is excessive rainfall or snow melt within a limited period, which overwhelms the natural drainage capacity, particularly when the ground is already saturated. Inundation by the sea is largely due to combinations of high tide, storm surge and wave activity raising the level of the sea above adjoining land. Floods can also occur during lesser events e.g. when river channels become blocked with debris, watercourses which are culverted or pass under bridges being the most vulnerable, and in the event of a structural failure of defences. Some areas are subject to combinations of tidal and watercourse impacts.

7. The impacts of flooding vary at different locations. For example, flooding of agricultural land can be costly to the individual farmer, but is unlikely to involve a serious threat to human life. However, the potential overtopping and possible failure of a high flood bank defending a densely populated area presents a greater threat to life and property. Rapid flows due to flash flooding following failure of defences pose a greater risk to life than a steady rise in water level.

8. The impacts of watercourse flooding can be aggravated by:

• the growth of built development in catchments and other changes in land use, which increase the rate and volume of run-off;
• sediment deposition that has changed river cross-sections and affected channel capacity particularly in culverted watercourses;
• lack of maintenance of flood defence systems, watercourses and culverts particularly where this leads to channel blockage;
• canalisation, modification and diversion of rivers, which increase the rate of flow and decrease the time taken for water to travel within a catchment; and
• building of structures (e.g. embankments) which restrict flows over historical flood plains and thereby create additional flood risks both upstream and downstream.

9. For the coast, tide-tables ¹ give predictions of astronomical tides and take into account seasonal average weather conditions for the locality. However meteorological conditions can cause storm surges and if in phase with the normal tidal cycle, can result in levels considerably higher than those predicted by reference to tide-tables. Storm surges are associated with intense depressions and in most years, several surges of 1 to 1.5 metres are experienced and 2 metre surges are not uncommon. An additional hazard in coastal flooding is the height of waves which over a long fetch can add considerably to the level of the water. Fortunately, the extremes of tide, surge and wave rarely coincide. The damage in the Firth of Clyde in January 1991 resulted from a surge of 1.38 metres above predicted high tide. The disastrous flooding on the east coast of England in 1953 was caused by a massive storm surge in phase with a spring tide and resulted in significant loss of life and millions of pounds worth of damage.

FLOOD PLAINS

10. The flood plain is a geographical term for the generally flat areas of land adjacent to watercourses or on the coast. They are typically created by the natural processes of erosion and deposition, particularly through changes in river meanders and the gradual build up of sediment as it is deposited by successive floods over a very long timespan.

11. Flood water may flow onto those parts of the flood plain closest to the watercourse fairly frequently, perhaps even annually, but at the furthest margins of the flood plain this may happen only during the most extreme flood events, e.g. those with an annual probability of about 0.1% (1:1000). So while flood water may flow across any part of the flood plain, whether it does so depends on the severity of the flood and local differences in ground level.

12. Flood plains convey and store water when river flows exceed their channel capacity. Flood plain storage reduces the peak flow in the river which has the effect of reducing flood levels and the risk of flooding downstream. Maintaining this function of the flood plain is important to the wider management of flooding. See Case Study 1: Insh Marshes for an example of a functional flood plain.

13. SPP 7 identifies the area of the flood plain where this function should be safeguarded, i.e. the 'functional flood plain', as areas which will generally have a greater than 0.5% (1:200) probability of flooding in any year. It also recognises their role in conveying flood water back to the watercourse or sea as a flood subsides.

14. Agricultural flood embankments play an important role in relation to farming areas and in reducing the duration of peak flows downstream. They are not built or maintained to the same standards as those forming part of a flood prevention scheme and are more easily overtopped or breached. Increasing the height and standard of existing agricultural flood embankments could affect flood flows downstream (see The River Tay Catchment Study, Ove Arup). It is unlikely that areas behind agricultural flood embankments will be outwith the functional flood plain as defined in SPP 7.

SEWER FLOODING

15. Sewer flooding occurs when the sewerage infrastructure has to deal with loads beyond its design capacity. This occurs most often as a result of high intensity rainfall events such as were experienced in various parts of Scotland during 2002. For example, in Glasgow the amount of rainfall expected for the whole of July fell in only 10 hours. This storm was assessed as having a 1% probability of occurring in any year, referred to as the 1:100 year rainfall event.

16. The sewerage infrastructure in many parts of Scotland is an old combined system, taking both foul sewage and surface water. In many places it is of limited capacity. This means that when there is heavy rainfall, the system may be unable to deal with the volume of water, which can back up causing spills of foul sewage through WCs and manhole covers. Additional development may increase the volume of surface water, adding to the risk of flooding. This has also resulted in situations where combined sewer overflows, designed to relieve pressure from excessive surface water, may discharge into watercourses and have the potential to affect flooded areas. Since the 1960s however, developers have been encouraged to separate surface and foul water discharges to reduce the volume of water entering the public sewer system. Other factors can contribute to sewer flooding, such as blockages, illegal connections to the public sewer system, or infiltration of surface water. The latter two can contribute flows beyond the capacity of the sewer.

17. One of the difficulties in addressing this issue has been the lack of satisfactory data on the condition of existing infrastructure, which limits the ability to predict the possibility of flooding in particular areas. However, Scottish Water has been undertaking a significant amount of work on its underground systems, and expects to have drainage area studies completed for the systems serving the majority of the population by the end of 2005.

18. Dealing with the issue of sewer flooding is a very high priority within Scottish Water. Significant investment continues to be made in water and sewerage infrastructure and the number of houses suffering foul flooding as a result of sewer discharges during heavy rainfall events has been falling steadily in recent years. Scottish Water's investment programme for the period beyond 2006 is currently being formulated and local authorities and development interests are closely involved in the process.

19. Sustainable urban drainage systems (SuDS) are designed to manage the flow of surface water which could otherwise find its way into the public sewer network or receiving watercourse. By reducing the volume of storm water in a sewer, SuDS will reduce the likelihood of sewer or watercourse flooding. To date the principles of SuDS have predominantly featured in greenfield development. To adequately manage flooding in the urban area, where drainage is predominantly on a combined system and there is a significant interaction between watercourses and the public sewerage system there may be potential to 'retro-fit' SuDS. This will reduce levels of surface water entering sewers and watercourses at times of peak flow and thereby reduce the potential for flooding. Further information on SuDS design is contained in PAN 61 and the CIRIA SuDS Design Manual for Scotland and Northern Ireland. The Water Environment and Water Services (Scotland) Act 2003 gives Scottish Water responsibility for the future maintenance of public SuDS which have been constructed to an appropriate standard. See also paragraphs 120 to 125 on SuDS.

GROUND WATER

20. Groundwater is an important natural resource, providing domestic and industrial water supplies, and is a source for the wider aquatic environment when it filters into lochs, watercourses and wetlands. Groundwater comes from rain that has collected in permeable rocks underground. These bodies of groundwater are known as aquifers. During its slow percolation through the ground, it is filtered, generally ensuring good quality water in its natural state. Where the pores or cracks of underground rock are completely filled with water is known as the saturated zone - the upper level where this occurs is called the water table. The rock above the water table is known as the unsaturated zone.

21. In Scotland the volume of groundwater is greater than the volume of water in lochs and rivers. However, most of our needs for water are met from lochs and rivers, and relatively little use is made of groundwater in comparison with other European countries. Groundwater flooding occurs when the water table rises above ground level. Water tables are not rising generally in Scotland, but there can be localised problems in some areas.

LAND DRAINAGE

22. Land is naturally drained by watercourses and percolation into the ground although in many parts of Scotland the soils are impermeable and percolation is limited. Development is likely to affect this natural system. Scottish Water has a duty to effectually drain its area and to convey to a suitable point of discharge both foul effluent and surface water entering their sewers, but does not have a duty to take the water draining from gardens or open spaces into their public sewers, drains or SuDS, and they will not usually do so. If land drainage is required, it has to be provided as part of the development and a separate private means of discharge identified.

23. Intense rainfall storms can also produce flash flooding, particularly if the ground is hard and dry, or already saturated, promoting run off and local flooding. For example, flooding occurred in Perth on 6 August 2002 when around 30 mm of rain fell in one hour on the Craigie Burn catchment (Cargill et al, 2004).

CLIMATE CHANGE

24. The Intergovernmental Panel on Climate Change (IPCC) records of the average surface air temperature of the planet show a rise of about 0.6°C since the start of the 20th century, with 0.4°C of this warming occurring since the 1970s. In the UK, air temperatures and coastal water temperatures have risen. Other observed changes include: an increase in the frequency of summer heatwaves; fewer frosts and cold spells in winter; drier summers and wetter winters; a larger proportion of winter precipitation falling on heavy rainfall days than was the case 50 years ago; and a 10 cm rise in the average sea level around the UK compared to 100 years ago.

25. In future years, climate change is expected to continue. The UK Climate Impacts Programme (UKCIP ²) helps organisations assess how they might be affected by climate change so they can prepare for its impact and coordinates research on the effects of climate change at regional and national level. UKCIP has also developed a set of future climate change scenarios, usually known as the UKCIP02 climate change scenarios for the UK. ³ They are based on global greenhouse gas emissions scenarios published by the IPCC and describe four equally possible future climates, based on low, medium-low, medium-high and high emissions. The Scottish Executive ⁴ has published Climate Change: Review of Levels of Protection Offered by Flood Prevention Schemes UKCIP02 ⁵ update (2003).

26. In winter, average seasonal rainfall and the likelihood of intense rainfall is expected to increase in the east, south and centre. The scenarios suggest that the east of Scotland, and to a lesser extent the south and central areas will experience worsening flooding conditions. Conditions in the north west are likely to experience less change. By the 2080s, the 100-year events in eastern Scotland may increase in magnitude by between 20% and 30%. In the north and north-west, peak flows are suggested to increase by between 0% and 10% over this period. There is a relatively high level of uncertainty with these predictions. They do however indicate that there may be marked regional differences around Scotland.

27. According to UKCIP02 Scotland will experience net sea-level rise of around 1 cm to 60 cm by the 2080s, depending on the emissions scenario. These figures include the effect of vertical land movement due to isostatic adjustment, which is particularly important for Scotland. Initial work on modelling future surge conditions suggests peak levels will increase by a similar amount, or possibly more than mean sea level rise. Extreme winds and storminess, influencing tidal surges and waves, may be modestly affected; very severe winter gales are suggested to become a little more frequent. However there are significant prediction uncertainties on these three factors, especially the future surge and wave conditions. Research on future coastal flood risk based on the UKCIP02 scenarios suggests that the 1990s 100-year level will become on average a 20 to 40-year event by the 2050s, and 10 to 30-year event by the 2080s.

28. SPP 7 says that for planning purposes it is not national policy with regard to watercourse and coastal flooding to add an additional allowance for climate change above the 0.5% probability but planning authorities may do so if it can be justified. Flood risk assessments and SEPA's advice will take account of the latest climate change predictions.

RISK AND PROBABILITY

29. The calculation, analysis and understanding of risk lies at the heart of planning for flooding. Many everyday activities involve some risk which people are prepared to accept (e.g. crossing a busy road) because they can take precautions which reduce the risk to a negligible level and the benefits outweigh the slight chance of an accident. When considering any activity where risk is an issue the important question is: 'what level of risk is involved?' It will not usually be relevant to ask 'is there a risk or not?'

30. Where flood risk is significant, applicants and the public should be able to rely on the advice of experts and the decisions taken by responsible authorities. The overwhelming principle is that the probability and the consequences are understood and communicated to the people who will bear them. This will enable them to make informed decisions. In the case of new development there should be a chain of trust from the occupier through to the developer, their consultants, the planning authority and their advisors, including SEPA.

Risk is taken to mean:

The statistical probability of flooding x the consequences

where consequences relate to people and the environment

Source: Office of Science and Technology, Foresight Future Flooding Executive Summary, 2004

Probability

31. The probability of flooding is calculated from historic data and sometimes computer modelling and is expressed in terms of the likelihood of a flood of a given magnitude in any year e.g. the 1% (1:100), sometimes described as an average statistical return period of once in 100 years. Return periods are often misunderstood to mean that another flood will not occur until the end of the return period with the implication that a development is "safe" for the foreseeable future. This is not the case and therefore the probability should be described as a percentage or ratio rather than a return period.

Longer Term Probability of Flooding

32. While the probability of flooding is usually expressed on an annual basis (e.g. 0.5%), where development is concerned it is worth considering the probability of flooding over a much longer period, in keeping with the development's expected life. For example, for a development with a 0.5% annual probability of flooding there is a 39% probability that the development will flood at least once in a 100 year period (and a 26% probability that it will flood at least twice. The expected lifespan of a proposed development and the prospect of the site being subsequently redeveloped have to be considered so that we can avoid adding to the flood risk problem for the future.

Table 1: Lifetime Probabilities of Flooding

33. The flow of water in a watercourse is related to the rainfall which feeds it but the severity of the two may differ, so that a rainfall event with an annual probability of 1% (1:100) will not necessarily result in a river flow event of 1% (1:100). This is because the relationship between the amount of rainfall and the watercourse flow will depend on catchment characteristics such as size and ground porosity, antecedent catchment conditions such as soil moisture, and any available storage before the rainfall reaches the watercourse. Additionally a rainfall event may not affect the whole of a catchment. In general, a rainfall event of a given probability, say 2% (1:50), will lead to a lower magnitude flood event, say 4% (1:25).

IMPLICATIONS FOR INSURANCE COVER

34. Where flood risk is an issue, SPP 7 highlights the need for developers to consider the availability of insurance at an early stage of their evaluation of a site. The Association of British Insurers (ABI) has set out the level of protection for new development required by the industry to enable insurance cover under normal terms. The ABI have set this out as supplementary advice on implementing the ODPM ⁶ policy on planning and flooding, PPG 25. It is available at www.abi.org.uk/Display/File/78/PPG25guidance2.pdf

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