Surface water management planning: guidance (2013)

Guidance to assist the responsible authorities in preparation of Surface Water Management Plans (SWMPs) to help with the management of surface water flooding. This guidance has now been superseded by the 2018 edition.

4 Assessment of flood risk

4.1 Introduction

Flood risk is a combination of the probability of a flood occurring (flood hazard) and the potential adverse consequences of that flooding on receptors.

Flood risk assessment is used to account for the consequences of flooding and is a necessary precursor to the process of options appraisal where appropriate.

This section includes information on:

  • How to determine if further modelling and mapping may be necessary to supplement that provided by SEPA or Scottish Water (i.e. if a greater level of detail is required)
  • How to undertake a flood risk assessment as part of a SWMP using SEPA and / or Scottish Water provided mapping outputs or new modelling and mapping generated for the purpose.
  • The outputs from a risk assessment that will be needed to support cost benefit appraisal of measures (see section 7 for guidance on appraisal).

The approach is 'risk-based' and flexible, meaning that the level of detail can be varied to suit the understood risk and the complexity of the flooding mechanisms as they are appreciated locally. The approach is not prescriptive but provides a framework around which different partnership organisations can collaborate under the leadership of a local authority.

The ultimate purpose is to provide sufficient information to support the development of a locally relevant and agreed strategy for the reduction of surface water flood risk. It is not intended to provide the type of information to support detailed appraisal or engineering design. These more detailed stages can occur once a locally agreed strategy is in place through the SWMP.

Guidance on the principles of modelling surface water flooding is provided in Appendix 3.

4.2 Approach to SWMP flood risk assessment

A process is illustrated below ( Figure 8) to support local authorities undertaking a SWMP risk assessment. The process is described in 3 steps and further key references are identified as required.

The process is best followed with the involvement of all key partners in the SWMP so that outcomes have full agreement. The process is indicative only; local authorities are encouraged to be innovative and apply equivalent methodologies to suit local circumstances.

The purpose of the SWMP risk assessment is to provide a clear explanation around the probability and adverse impacts of surface water flooding across a whole SWMP area or focussed around one or more flooding hotspots or drainage areas.

Figure 8. Overview of SWMP risk assessment process

Figure 8. Overview of SWMP risk assessment process

SEPA, Scottish Water and local authorities have duties under the FRM Act to provide information on flood risk (flood hazard and adverse impacts of flooding) and other information that is required for the management of flood risk. This information will be provided to the local authorities leading on the production of SWMPs and is the starting point for the production of SWMPs including the identification of measures to address surface water flooding.

A summary of the information that is being produced as a requirement of the FRM Act can be seen in Appendices 4 and 5.

Two key datasets are being provided under the FRM Act that will contribute to the production of SWMPs:

  • Regional pluvial flood hazard mapping
  • Regional pluvial impacts of flooding (baseline impacts)

Regional pluvial flood hazard mapping

SEPA have undertaken regional pluvial modelling to produce pluvial flood extents, depths, velocities and hazard ratings. The regional areas modelled were those considered most at risk from pluvial flooding based on the national pluvial modelling that was carried out as part of the NFRA and the availability of LiDAR (light detection and ranging) data. The regional pluvial hazard models have been developed based on a ground model consisting of combined LiDAR and NextMap datasets, applying a 300mm uplift of the ground model to represent buildings and a blanket 1 in 5yr drainage loss allowance for a range of return periods (including climate change scenarios). More information on the regional pluvial hazard modelling can be found in Appendix 5.

Regional pluvial impacts of flooding (baseline impacts)

SEPA will also be determining the adverse impacts of the regional pluvial flood hazards on human health, the environment, cultural heritage and economic activity under the range of return periods. For example this will include information on monetary damages to homes and businesses, the number of people at risk of flooding, community facilities effected etc. With the data provide the local authorities will be able to summarise the adverse impacts within any area they require. More information on how the adverse impacts of flooding are assessed can be found in Appendix 4.

In addition, the Scottish Water assessment of flood risk from sewerage systems (Section 16 of the FRM Act) when used in conjunction with the regional pluvial flood hazard and risk mapping and local knowledge can provide insight into likely flood mechanisms and solutions.

In many cases this information can be used in SWMPs without the need for further modelling. In other cases, further modelling and new hazard and risk mapping may be required or warranted. In these cases, SEPA can provide data to make this further hazard modelling and risk mapping relatively straightforward (and consistent with national flood risk assessments) using a variety of different modelling platforms (software) and methods.

Areas where there is no regional pluvial hazard mapping are covered by the national pluvial hazard mapping. This was undertaken using a coarser type of pluvial modelling and can provide some supporting information for local authorities looking to undertake SWMPs outside of PVAs. However the types of measures that can be identified from this data will be limited as there is more uncertainty associated with this information.

The responsible authorities may have other information that can be used to inform the SWMP, and this should be used where available.

4.3 STEP 1 - Verify flood hazard information and determine if further modelling is required

The purpose of Step 1 is to determine whether the SEPA regional pluvial hazard map and associated impacts of flooding is an appropriate basis for the SWMP, including the strategic identification of measures to address surface water flooding or whether further modelling or assessment is required prior to identification of measures. It is generally expected that the regional pluvial hazard maps and associated pluvial baseline impacts should be appropriate for the following tasks:

  • Understanding flood hazard and flood risk
  • Understanding flood mechanisms
  • Establishing objectives for surface water flooding
  • Identification of measures (long list)
  • Strategic cost benefit appraisal of measures (structural and non structural)

4.3.1 Comparison of modelled flooding with historical observations

In order to verify the regional pluvial hazard maps, they should be compared to observed flooding events. The regional pluvial hazard maps show a range of modelled scenarios. Initially the 1 in 50 year regional pluvial flood extent should be compared to local records of flooding and other anecdotal information around the location and frequency of previous flooding. The 1 in 50 year pluvial flood extent is used as a starting point as it should represent rainfall events that have been experienced, as opposed to using a more rarely occurring and larger magnitude flood (e.g. 1 in 200 year) that may not have been experienced.

There is no reliable, scientific way to compare historical and modelled flooding; the judgement of all SWMP partners should be applied. The model should be verified against know flooding locations because alignment (or failure of alignment) between modelled flooding locations and flooding observations is likely to be a combination of:

  • locations where flooding is predicted but has never occurred. In this case the model may be accurate but there has been no flood event to verify the model in the given location. Just because a location has not experienced flooding in the past does not mean that it is at risk of flooding in the future.
  • locations where flooding has occurred but is not predicted. In this case the model is failing to predict the observed flooding and further information is likely to be required.

Good alignment around known flooding locations is an ideal even if predicted flooding is not matched by observations elsewhere. Where there is good alignment between modelled and observed flooding locations, and the flooding mechanisms are understood, then Step 2 should be carried out.

Complete failure to replicate known flooding locations indicates that undertaking further modelling may be necessary to correctly represent flood mechanisms. If this is the case, then the reasons why this might be the case should be identified in order to help make a decision on what type of further modelling is required.

Verification of modelling in Dundee

4.3.2 Reasons why modelled flooding might not be predicting observed flooding

The regional pluvial hazard mapping was carried out using standard inputs for a range of modelling parameters such as drainage capacity, runoff coefficient and topography. These will not be appropriate in all situations and can be adjusted in further modelling to improve verification and model certainty. The development of further modelling can be used to address modelling uncertainties of the following type:

Drainage capacity

Section 16 sewer flooding mapping (where available) is useful to determine local drainage capacity. This knowledge can help inform whether the default 1 in 5 year return period drainage capacity (used in the regional pluvial hazard modelling) is appropriate.

Similarly, Section 16 results can be used to determine the critical duration for drainage exceedance; this will vary with gradient and other factors. This knowledge will guide the selection of mapping scenarios with either a 1 hour or 3 hour storm event duration. If, in consultation with Scottish Water and other partners, it is clear that none of the default drainage capacity or event duration scenarios are suitable then further pluvial modelling should be considered using data supplied by SEPA (Option A in Table 6).

Runoff coefficient

Similarly, if default run-off coefficients for land use types (used in the Regional Pluvial model) are considered inappropriate then the regional pluvial models can be re-run with locally appropriate runoff coefficients. See Appendix 4 for further information on re-running the regional pluvial hazard models (Option A in Table 6).

Inaccuracies in the Digital Terrain Model ( DTM)

The regional pluvial mapping may not be representing observed flooding because of inaccuracies or simplifications in the DTM used. The DTM used by SEPA in the regional pluvial hazard maps is based on NextMap and LiDAR that has been processed to remove false blockages and introduce building footprints as indicated on Ordnance Survey data. No ground truthing of the DTM was undertaken. Where it is thought that local topography is not represented in the DTM and it is having an impact on surface water flow routes and flooding locations then topographical surveys can be carried out and this surveyed detail can be added to the DTM e.g. where roads are known to convey significant flows, these can be modelled as 150mm depressions in the DTM, other features can also be added including kerb-lines / heights, low walls, additional buildings and known flow routes (Option A in Table 6).

Further information on this can be found in CIWEM Urban Drainage Group's guide to Integrated Urban Drainage Modelling (2009).

Interactions with other sources of flooding

The regional pluvial modelling may be inaccurate because it's not representing the dynamic interaction of above and below ground flows correctly; this can occur when large sewer pipes transfer flooding from one location to another or where the catchment of the sewer system does not follow above-ground topography. Section 16 sewer flooding data and other sewer asset information can be used to infer the importance of sewer and surface interactions. There can be similar interactions with watercourses (which may be culverted) and coastal waters which can impede discharge from surface water drainage outfalls resulting in a locally reduced drainage capacity. Such dynamic interactions require a more detailed type of model that can represent above and below ground interactions. This is often called an 'integrated urban drainage model' which should be developed in high risk areas or where the appraisal of solutions requires a more detailed understanding of these interactions (Option B or C in table 6).

Whether SEPA regional pluvial mapping is used directly, or amended, or another modelling approach adopted, the end of Step 1 results in a set of surface water flood maps which then form the basis of subsequent analysis and assist in the appraisal of solutions. Mapping outputs on their own can be used to powerfully communicate the location, depth and pathways of surface water flooding. This is useful information for land use planners and emergency planners.

4.4 Options for further modelling

This section describes the modelling options in more detail. As described in section 4.3.2 further modelling will be required if modelled outputs do not reflect observed flooding and the type of further modelling required will depend on the reasons that have been identified.

A risk based approach should be adopted to select the modelling method. The simplest approach that will support robust strategic decision making should be applied at this time. Complexity can be added in small areas (e.g. for priority drainage area(s)) or be recommended as a future investment after a high level assessment has determined high risk areas within the SWMP area.

It is anticipated that most SWMP partnerships should be able to work very effectively with the supplied regional pluvial and Section 16 sewer flooding without having to undertake further modelling at this stage.

Table 6. Options for further modelling
A) Re-run regional pluvial hazard model The regional pluvial hazard maps can be re-run with improved information on data inputs for:
  • drainage capacity
  • run-off coefficients
  • DTM

SEPA can provide the original input data (to local authorities or consultants acting on behalf of local authorities) to help make this straightforward using a range of software platforms.

The parameters in section 4.3.2 (e.g. drainage capacity, DTM accuracy) could be considered for adjustment to better reflect the real conditions with the intention of improving the representation of pluvial flooding. Local knowledge held by local authorities which is considered more representative of individual catchments can be adopted, following appropriate advice by SEPA, and models adjusted accordingly to reflect this information. This can either be via an adjustment to the original model, an alternative modelling package utilising the available data, or via other supporting information pertinent to the area in question.

SEPA agrees with the principle of sharing models and model data; however this will be dependent upon licensing conditions. These licensing conditions will apply to both the model themselves, the model outputs developed by the contractor and the datasets supplied for input to the models. This may limit what information SEPA can share until licensing conditions are agreed with the licensors.
B) Sewer and pluvial modelling Coupled 1D (underground sewer network) and 2D (above ground) model. This model allows water to flow across the modelled urban surface and re-enter the sewer network where there is an inlet and underground capacity.
C) Integrated Catchment modelling This usually involves combining existing sewerage models with watercourse models and a 2D representation of the urban surface and can also model the influence of other sources of flooding including river and coastal flooding on surface water flooding. This approach is costly, time-consuming and requires a high degree of collaboration between partners. It is already being applied in areas of very high risk (e.g. Glasgow).

4.5 STEP 2 - Calculating / summarising the consequences of flooding

Step 2 can proceed once a locally agreed version of the regional pluvial mapping has been adopted by the SWMP partners, or further modelling has been considered necessary and completed.

SEPA will determine the consequences of flooding based on the initial outputs of the regional pluvial hazard maps in the regional pluvial impact assessment. This information will be provided to local authorities, and it can be summarised at any scale.

The SEPA regional pluvial impact assessment will show the adverse impacts of pluvial flooding on economic activity, human health, the environment and cultural heritage. This will include direct and indirect impacts and monetised and non monetised information.

If further flood hazard modelling is undertaken through the SWMP, the SWMP partnership will need to re-run the impact assessment based on the updated hazard modelling. The SWMP partnerships should re-run the baseline impact methodology that can be provided by SEPA. SEPA may be able to re-run the base line impact assessment if new hazard outputs are provided but the ability of SEPA to do this will be based on the resources available at the time.

The SWMP should summarise the impact information for each drainage area. This is a necessary first step in understanding the relationship between positive and negative impacts of improvements and to ensure compliance with the Scottish Government's flood appraisal guidance [8] .

The scale at which flood consequences are described and summarised within a SWMP is entirely flexible to suit local needs. It may be sensible to first describe / summarise consequences across a whole town but then break this down further by drainage area.

4.6 STEP 3 - Identify and prioritise drainage areas

The purpose of this step is to identify and prioritise drainage areas within the SWMP to give focus to the subsequent options appraisal stage. While some measures will apply across the whole SWMP area (e.g. land use planning measures) some measures will need to be focussed on one or more drainage areas (e.g. structural measures, awareness raising). The number of drainage areas that should be considered will vary depending on the level of surface water flood risk (the number of high risk areas) and available resources to consider solutions at this time. A risk based approach is flexible, allowing the SWMP to focus on a small number of priority areas in line with available resources.

Drainage areas can be prioritised using a number of criteria. There is no single recommended method for prioritisation and this should be a joint decision by the SWMP partners. Factors for prioritisation could include:

  • Surface water flood risk (information from impacts of flooding summarised at the drainage area scale)
  • Surface water flood risk to priority receptor groups (information from impacts of flooding summarised at the drainage area scale)
  • Locations with a history of flooding
  • Locations of internal / external sewer flooding (information from Scottish Water)
  • River Basin Management Plan - potential to improve water quality (information from SEPA)
  • Climate change vulnerability
  • Redevelopment / regeneration opportunities (information from local authority)
  • Areas which are predicted to flood but where there is no history of flooding, should be treated with caution, particularly where more detailed models aren't available. It is sensible to balance predicted and actual flooding information when prioritising drainage areas. It is advisable to follow a process for the prioritisation of drainage areas that is transparent, defensible and sensitive to local requirements.

Finally, it is important to be flexible. It is highly probable that the next significant surface water flood will occur outside one of the selected hotspots. SWMP partners should be looking to keep the process 'live' and ongoing.


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