6 Selection & appraisal of measures to manage surface water flood risk
The FRM Act requires that the most sustainable measures to manage flood risk are identified and implemented. This requires effective appraisal of measures to underpin decision making. Measures should be appraised using the following Scottish Government appraisal guidance;
- Delivering Sustainable Flood Risk Management
- Sustainable Flood Risk Management - Principles of appraisal: a policy statement
- Flood Protection Schemes - Guidance for Local Authorities Chapter 5 Project Appraisal
As stated in Section 2.2 appraisals should also follow a risk based approach and can be done at different levels from strategic to detailed. The level of appraisal required will depend on the measure proposed and the data available.
The benefits and costs for all 'do something' options should be compared with those of the 'do nothing' option to provide a common baseline with which to compare options.
As stated in the Scottish Government guidance on appraisal for flood protection schemes, appraisal should start off with as wide a range of measures as possible. Cost benefit analysis can only identify the best of those options considered. A good appraisal will therefore encompass a wide range of management options, as a minimum the measures set out in section 6 should be considered.
This long list of measures should then be screened to create a short list (Figure 10 gives an overview of the appraisal process). Further appraisal on the costs and benefits of measures can then be carried out on the short list of measures.
Figure 10. Overview of measures appraisal process
6.1 Identify long-list of measures to address surface water flooding
A range of structural and non structural measures should be identified to achieve the objectives in a way that is most sustainable and follow the principles of integrated drainage set out in section 1.5 and in the Ministerial guidance on SFM.
Structural measures may include, for example, a surface water storage structure or managed overland flow pathway to protect properties from flooding. Measures can also be non structural, for example; development of new land use planning policy, consideration of surface water flooding through settlement strategies in development plans and planning decisions or an identified need for more detailed flood risk assessment. Measures should address the sources, pathways and receptors of surface water flooding.
A SWMP should identify all measures to address surface water flooding i.e. from strategic type measures (e.g. land use planning policy) to detailed structural measures to address specific problems.
A list of potential measures that should be considered to manage the risk of surface water flooding is given in Table 9.
At this stage identification of measures should not be constrained by concerns over funding or delivery mechanisms. The aim is to identify the most sustainable measures (i.e. most economically, socially and environmentally beneficial) to manage surface water flood risk in a given area. Agreements over funding and responsibilities should be made once the most sustainable measures have been identified (see Section 7).
| Table 9. Potential measures to address surface water flooding |
| Measure |
Description |
| Asset maintenance / asset management planning including SUDS ( LA and SW) |
Objectives met - Avoid / Protect Flood mechanisms addressed - Source / Pathway Geographical scale - Regional / urban area / priority drainage area Description - Ensure sewer / road drainage / SUDS and culverts are maintained for optimal performance and clearance and repair works in urban burns are carried out. Ensure LA and SW work is co-ordinated with good lines of communication. Identify where less maintenance could be carried out / where maintenance should remain the same / where more maintenance is required. This may include review of responsibilities in adopting existing (legacy) SUDS and review of new and future policy on SUDS adoption to resolve any issues. |
| Land use planning policy |
Objectives met - Avoid Flood mechanisms addressed - Receptor Geographical scale - Regional / urban area Description - Ensure new development is not at risk of surface water flooding - Ensure appropriate policies are included in strategic development plans, local development plans and supplementary planning guidance. Ensure that masterplans and development management decisions require adequate and appropriate drainage (including SUDS) and ensure developments are designed for drainage exceedance events. Additional planning requirements may be needed in areas with a high risk of surface water flooding (e.g. ensuring new development or re-developed areas improve the surface water flooding situation, policies for blue and green corridors, requirements for SUDS when discharging to sewer or coastal waters). Identification of sites constrained by surface water flood risk. References: Scottish Government 2011 Green Infrastructure: Design and Placemaking CIRIA 2006 C635 Designing for exceedance in urban drainage: good practice Glasgow and Clyde Valley Green Network web based guidance on Integrating Green Infrastructure and case studies |
| Emergency response plans |
Objectives met - Prepare Flood mechanisms addressed - Receptor Geographical scale - Regional / urban area / priority drainage area Description - Use information of surface water flood risk to improve emergency plans e.g. identify emergency routes on roads not at risk of surface water flooding. Ensure people and business are more prepared, can recover more quickly. |
| Awareness raising |
Objectives met - Prepare Flood mechanisms addressed - Receptor Geographical scale - Urban area / priority drainage area Description - Ensure people and communities at risk of surface water flooding are aware of the risk and provide information on their responsibilities and how they can protect their property. |
| Managed overland flow pathways |
Objectives met - Protect Flood mechanisms addressed - Pathway Geographical scale - Priority drainage area Description - Control of surface water flow through the urban environment to watercourses or storage areas through the creation of flow routes or using the road network as a flow pathway and avoiding property. References: CIRIA 2006 C635 Designing for exceedance in urban drainage: good practice CIRIA 2012 C713 Retrofitting to manage surface water |
| Surface water storage areas (above ground) |
Objectives met - Protect Flood mechanisms addressed - Pathway Geographical scale - Priority drainage area Description - safe above ground storage of water from surface runoff from the urban area. References: CIRIA 2006 C635 Designing for exceedance in urban drainage: good practice CIRIA 2012 C713 Retrofitting to manage surface water |
| Retrofit SUDS |
Objectives met - Protect Flood mechanisms addressed - Source / Pathway Geographical scale - urban area / priority drainage area Description - This covers all types of retrofitting SUDS that will reduce the rate and volume of surface water run-off and includes measures and strategies that will reduce and remove surface water from the sewer systems. Should include strategic / focused implementation of retrofitting e.g. in priority drainage areas and should also include policies that will ensure opportunities to retrofit are taken when they arise. References: Scottish Government 2011 Green Infrastructure: Design and Placemaking CIRIA 2012 C713 Retrofitting to manage surface water |
| Strategy for the separation of surface water run-off form the combined sewerage system |
Objectives met -Avoid / Protect Flood mechanisms addressed - Source / Pathway Geographical scale - urban area / priority drainage area Description - Separation (reduction or elimination) of surface water from the combined sewerage system and managing this water at source and above ground should be considered. This will involve a range of measures identified in this table (e.g. retrofit SUDS, above ground storage areas, flow pathways etc) however the overall impact on surface water flows to the sewerage system should be considered. This should include consideration of strategic / focused plans to separate surface water flows from the sewerage systems and to ensure opportunities to separate flows are taken when they arise. |
| Land management |
Objectives met - Avoid / Protect Flood mechanisms addressed - Source / Pathway Geographical scale - Urban area / priority drainage area Description - Runoff from more rural areas can contribute significant flows to drainage systems and watercourses that can impact roads and areas further downstream. Land management measures that reduce the rate and volume of runoff should be considered. |
| Relocation / removal of receptor |
Objectives met - Avoid / Protect Flood mechanisms addressed - Receptor Geographical scale - Priority drainage area Description - Removal and relocation of receptors e.g. properties should be considered, where it is more sustainable and feasible to do this in long term as opposed to other measures. |
| Property / Site level protection |
Objectives met - Prepare Flood mechanisms addressed - Receptor Geographical scale - Localised Description - Temporary, demountable defences can be put up at the property or street level to avoid properties getting flooded. Use of flood resilient building materials and methods of construction can reduce impacts if properties do get flooded. |
| Storage tanks (underground) |
Objectives met - Protect Flood mechanisms addressed - Pathway Geographical scale - Priority drainage area Description - flooding can be reduced by diverting surface water to storage tanks or by providing storage within the drainage network. |
| Increase size of drainage pipes (roads and sewer) |
Objectives met - Protect Flood mechanisms addressed - Pathway Geographical scale - Priority drainage area Description - flooding from drainage networks can be reduced by increasing the capacity of the under ground drainage pipes. |
| Deculverting / river restoration |
Objectives met - Protect Flood mechanisms addressed - Source / Pathway Geographical scale - Priority drainage area Description - deculverting and restoring urban watercourses can increase capacity available in these systems and reduce flooding. |
| Watercourse storage (on-line or off-line) |
Objectives met - Protect Flood mechanisms addressed - Source / Pathway Geographical scale - Priority drainage area Description - may be combined with deculverting and restoration of watercourses. Storage in urban burns may help reduce flood risk down stream, especially in areas where urban burns enter the sewer system. |
6.2 Screening
The initial 'long list' of potential measures (or groups of measures) should be screened for technical, financial and legal feasibility. The purpose of this step is to remove any potential measures that are clearly unfeasible or unrealistic at an early stage.
Table 10 describes the criteria that should be used for screening out unfeasible or unrealistic options.
| Table 10. Screening criteria |
| Feasibility |
Description |
Metric |
| 1. Technical |
Removal of any measures that are not technically feasible. e.g. Is land available for above ground storage? |
Categorical - Y/N
Expert judgement |
| 2. Legal |
Removal of any measures that represent insurmountable legal issues, including health and safety. |
Categorical - Y/N
Expert judgement |
| 3. Financial |
At this stage, is there evidence that the costs will be disproportionate compared to the benefits? Rapid assessment of cost estimates against key economic and social benefits. Please note this should not be a detailed cost benefit analysis. |
Categorical - Y/N
Estimated build and maintenance costs of measure vs benefits to economy (direct economic benefits to property) and key social impacts (risk to life/human health). |
Expert judgement is involved in making these decisions, which will invoke some element of subjectivity into the process. However the process should still be transparent and an important part of this screening process is therefore to ensure agreement with SWMP partnerships on which measures are discarded at this stage.
Where there is uncertainty about the feasibility of any measure, or where the feasibility of a measure is considered to be borderline, the measure should remain in the appraisal process because these issues will be looked at again in more detail later.
The screening process will produce a short list of potential measures for more detailed appraisal.
The more detailed appraisal of the short listed measures will consider the flood risk benefits of the measures as well as the wider impacts.
6.3 High level appraisal of short listed measures
Please note it is proposed that the Scottish Advisory and Implementation Forum for Flooding ( SAIFF) will develop a more detailed methodology on the appraisal of surface water measures to ensure a consistent approach is taken. This will also ensure consistency with the appraisal of river and coastal measures. This will include for example further information on what human health, environmental, cultural heritage and economic impacts should be assessed and how to assess these impacts.
The high level appraisal of measures for surface water flooding should follow the Scottish Government guidance Flood Protection Schemes - Guidance for Local Authorities Chapter 5 Project Appraisal. This should be a high level appraisal to agree a range of cost beneficial measures that could be put in place to address surface water flooding.
Agreement must then be made on what measures can be funded and implemented in the next FRM planning cycle and the funding arrangements for each measure. Detailed appraisal and design of measures will then be carried out at a later stage prior to being implemented.
To ensure the selection of the most sustainable measures the full range of impacts, both positive and negative (i.e. costs and benefits) on human health, the environment, cultural heritage and economic activity should be considered in an equitable manner.
Impacts that cannot be valued in monetary terms should always be described, quantified and brought into the appraisal through appraisal summary tables.
Understanding these impacts is critical to selecting sustainable actions and they should not be ignored simply because they are difficult to quantify or value in monetary terms.
