Surface water management planning: guidance (2018)

5 Understand surface water flood risk

Understand flood risk: considerations and example outputs
Considerations	Example outputs
Analysing and interpreting information to understand surface water flood hazard and risk. Identifying areas with greatest surface water flood risk. Consulting on this stage along with initial objectives. May need to consult Scottish Water or roads authorities before setting initial objectives if complex problems exist where the sewer network or roads drainage may influence surface water flooding. Communicating information clearly, considering different communication material for different audiences and stakeholders.	Section of SWMP report clearly communicating findings of this stage (e.g. clearly describing the sources, pathways and adverse impacts (risk) of surface water flooding in the area and identifying areas at greatest risk). Other outputs showing key information (e.g. GIS data and maps showing SWMP area, flood hazard, flood risk, areas at greatest risk).

5.1 Introduction

Understanding the causes and consequences of flooding is crucial for making well-informed decisions on how to manage flood risk. Available information should be analysed to gain an understanding of the sources, pathways, receptors and adverse impacts of flooding (flood hazard and flood risk), and the findings communicated clearly.

5.2 Analysing and communicating available information

Having analysed the available information, the findings should form part of the SWMP report. Accompanying maps, figures and key data outputs will help to convey the information more effectively. Different communication materials should be considered for different audiences (e.g. LA flooding, authorities, the public). The level of detail in the report should be proportionate to the level of surface water flood risk, the complexity of the surface water flooding mechanisms and the information available.

For example, information may include:

• Any significant surface water flood events.
• Natural drainage features (e.g. watercourses and their catchments, including small urban burns and culverted watercourses).
• Artificial drainage systems (e.g. Scottish Water sewer catchments, areas with combined sewers, areas with separate surface water and waste water sewers).
• Any interactions between natural and artificial drainage systems and pluvial/other sources of flooding (e.g. any known locations where land drainage, watercourses or the sea has affected surface water drainage or entered the combined sewer).
• Current flood risk:
  • Surface water flood hazard in the SWMP area (e.g. this may include a summary of main sources, flow pathways and depths of flooding).
  • Main adverse impacts (risk) of surface water flooding and more localised areas at greatest risk of flooding (flooding hot spots) at the neighbourhood or street scale. This should include a summary of receptors at risk at appropriate spatial scales (e.g. for the LA area, the SWMP area, flooding hotspots).
• Future flood risk, which may include information on the impacts of climate change, urban creep and population change.
• Existing actions to manage surface water flood risk.

5.3 Identifying areas with greatest flood risk

Identifying more localised areas at greatest risk of surface water flooding supports a risk-based approach by allowing efforts to be concentrated where they are most needed. So-called flooding ‘hot spots’ are likely to be found at the street or neighbourhood scale and have a single or linked cause of flooding ( Figure 5.1). At this scale objectives and actions can be more focused, allowing flood risk information to be summarised and monitored over time to determine whether objectives are being met at this more localised level.

Level of risk can be based on number of receptors at risk, such as high numbers of homes and businesses or vulnerable single receptors – e.g. a school, a hospital or a road that would cause significant disruption if flooded. Risk in different return periods should also be considered.

aThe areas at greatest risk should be defined according to where the impacts of surface water flooding occur. This boundary does not limit where actions can be implemented to manage surface water flooding, but the risk should be linked to a single cause (or linked cause) of flooding in order to understand what actions will be appropriate to manage it. When identifying single or linked sources of flooding, account should be taken of the topography and underground drainage connections that influence flood risk in the area. Flood risk across the areas should be summarised (see Table 5.1), and should include a note on confidence in the data for each area. It is important to be flexible; it is possible that the next significant surface water flood may occur outside one of the identified hotspots, thus triggering a review and update of the SWMP plan. Hence, the SWMP process should remain ‘live’.

Figure 5.1 Example showing SWMP boundary and areas at greatest risk (surface water flooding hotspots)

Table 5.1 Flood risk in SWMP area

Location	History of flooding Confidence in data	Flood risk
		Total Annual Average Damage ( AAD) (all return periods)	Businesses 1:200 yr	Homes 1:200 yr	Homes in socially vulnerable areas 1:200 yr	Community facilities 1:200 yr	Listed buildings 1:200 yr	Infrastructure 1:200 yr
Whole Town	History of surface water flooding.	£395,000	25	90	65	<10	0	<10 electricity sub- stations, 1.5km road
Neighbourhood A	History of significant surface water flooding but modelling shows low risk. Low confidence in modelled data.	£1,000	0	<10	0	0	0	100m road
Burn B	Observed flooding matches modelled data. Good confidence in modelled data.	£80,000	0	25	15	0	0	10m road
Burn C	Observed flooding in area but existing structure to manage risk not shown in modelled data.	£12,000	2	15	15	0	0	200m road
Neighbourhood D	Observed flooding matches modelled data. Good confidence in modelled data.	£90,000	<10	20	15	1 hospital	0	1 electricity sub-station, 20m road
Road E	No history of surface water flooding but area could be at risk. Medium confidence in modelled data.	£10,000	<10	<10	<10	0	0	500m road

5.4 Future flood risk

Many factors influence flood hazard and flood risk, the main ones being:

• Climate.
• Land use (urban creep can have significant impacts on surface water flooding).
• Demographics.

Knowing more about factors that will influence future flood risk is essential for managing flood risk sustainably now and in the future, because it will allow us to:

• Understand where significant flood risks may arise in the future and what areas may be more sensitive to change.
• Decide what actions can be taken now to mitigate changes in the future (see Chapter 7 Option appraisal and Appendix A7.6 Climate change mitigation).
• Make sure that any actions implemented now are resilient and adaptable to future change (see Chapter 7 Option appraisal and Appendix 6 Adaptation to future flood risk).
• Inform the choice of actions – for example, greater importance should be given to adaptability when considering options in an area that is highly sensitive to climate change.
• Identify how future risks from flooding could change due to different investment scenarios and estimate the level of investment that would maximise benefits under different circumstances (see the Environment Agency’s long-term investment scenarios for further information). ^[7]
  

At this stage, a strategic-type assessment of how these factors may affect future surface water flood risk in the SWMP area should be carried out using available information, for example identifying where significant flood risks may arise in the future and what areas may be more sensitive to change.

Impact at different spatial scales can be included where information is available (e.g. over the entire LA, SWMP areas or neighbourhoods / flooding hotspots).

Climate change
SEPA’s 2013 pluvial mapping generated two climate change scenarios: a 20% uplift in rainfall intensity for the 1:30 and 1:200 year rainfall events by 2080. Where the mapping shows good validation with observed events, it can be used to identify cities, towns and neighbourhoods that may be more sensitive to climate change or where new risks may occur.

Key flood risk indicators (e.g. number of properties at risk) can be estimated as snapshots under the climate change scenarios and compared with the current day flood risk estimate. For example, SEPA’s 2013 pluvial data climate change scenario shows that the number of homes and businesses at risk of surface water flooding in Scotland in the 1:200 year rainfall event could rise by 45% by 2080 as a result of climate change ( Figure 1.3).

SEPA’s 2013 pluvial model scenarios were based on information issued by Defra in 2006 (and subsequently updated in Environmental Agency, 2016), ^[8] which represented best understanding at that time. A new UKWIR study ^[9] suggests that a larger uplift may be appropriate for future strategic studies. UKCP18 ^[10] is also expected to improve our understanding of climate change impacts on rainfall, outputs from which are expected in 2018.

Land use - urban creep
In urban areas the trend is for permeable ground to be replaced with impermeable surfaces, e.g. gardens and other green space paved over or areas re-developed with higher density buildings. Often referred to as urban creep, this can significantly increase surface water run-off and flood risk over time.

Rates of urban creep vary. Although information may not be available for a local area, a strategic-level assessment could explore the extent to which urban creep might occur and its likely impacts on flood risk. Remodelling could be undertaken to estimate the impacts of urban creep on flood risk (further information on urban creep can be found in Appendix A6.3).

Where information on urban creep is available, it could be used to identify areas that may be more sensitive to the impacts of urban creep. Factors such as planning policy, permitted development and local housing stock will influence the rate of urban creep and the subsequent impacts on flood risk, all of which could be taken into account. This information could be used to consider what actions could be taken to mitigate or adapt to the effects of urban creep (see Chapter 7 Option appraisal and Appendix A6.3 for more on adaptation responses).

Demographics
A strategic-level assessment should describe how households and populations are likely to change in the area and show locations that have been identified for development (using information from local authority land use planners and National Records of Scotland).

Between 2014 and 2039, the number of households in Scotland is projected to rise by 14% to 2.76 million – an average annual increase of about 13,800 households. Over the same period, Scotland’s population is projected to go up by 7%. ^[11] The rise in the number of households is projected to affect almost every council area in Scotland, with the largest rises projected for Midlothian and the City of Edinburgh (of 31% and 30% respectively). In contrast, there are just three council areas where the number of households are projected to fall: Inverclyde (by 5%), Argyll and Bute (by 1%) and Na h-Eileanan Siar (by <1%).

These national changes (and regional variations) may significantly influence future flood risk. The relationship is not straightforward, as factors such as planning policy, development planning and development management will interact with demographic change to influence flood risk. ^[12] This highlights the importance of adhering to Scottish Planning Policy, which seeks to ensure that new development is not at risk of surface water flooding and does not increase surface water flood risk elsewhere.

Demographic information can be used to:

• Identify areas that are likely to experience greater growth in households, have been earmarked for development or are likely to experience significant change in use.
• Identify areas that are likely to experience a decrease in growth (which may present opportunities to plan for ‘shrinkage’ and surface water management).
• Help in selecting options (e.g. by identifying where supplementary land use planning actions could be put in place to require new development to reduce existing flood risks).
  

You can find more information on adaptation responses that may be appropriate in areas experiencing high growth, in Chapter 7 Option appraisal and Appendix 6.