Surface water management planning: guidance (2013)

Introduction

1.1 Purpose

The purpose of this guidance is to assist responsible authorities in preparation of Surface Water Management Plans ( SWMPs) to help with the management of surface water flooding as required under the Flood Risk Management (Scotland) Act 2009 (the FRM Act).

The guidance has been developed by the Scottish Advisory and Implementation Forum for Flooding ( SAIFF) which has representatives from Scottish Government, local authorities, Scottish Water and SEPA.

1.2 Background

The Flood Risk Management (Scotland) Act 2009 (the FRM Act) establishes a flood risk management planning process for the assessment and sustainable management of flood risks with the aim of reducing the adverse consequences of flooding from all sources, including surface water flooding.

The FRM Act requires two sets of complementary plans, Flood Risk Management ( FRM) Strategies produced by SEPA, and Local Flood Risk Management Plans ( LFRMPs) produced by lead local authorities ^[1] .

The FRM Act requires SEPA and the responsible authorities to co-operate with each other and to co-ordinate the exercise of their functions ^[2] with a view to reducing overall flood risk and to achieve the objectives of the FRM Strategies and LFRMPs.

The FRM Strategies and LFRMPs must set objectives for the management of flood risk (including surface water flood risk) in Potentially Vulnerable Areas ( PVAs) and identify the most sustainable measures to achieve those objectives. The objectives and measures in the FRM Strategies and LFRMPs must take account of all sources of flooding including surface water management and urban drainage.

The surface water management planning process described in this guidance is currently considered best practice by which to identify the most sustainable measures to manage urban drainage and the risk of surface water flooding. It is expected that the FRM Strategies and LFRMPs will identify the production of Surface Water Management Plans ( SWMP) as a measure to manage the risk of surface water flooding.

The Ministerial Guidance ^[3] on Delivering Sustainable Flood Risk Management (the SFM guidance) states that surface water flooding will be addressed through SWMPs and local authorities will lead on the preparation of SWMPs which will be co-ordinated within the flood risk management planning process set out in the FRM Act.

The surface water management planning process will help to deliver the Scottish Government outcomes for sustainable flood risk management:

1. A reduction in the number of people, homes and property at risk of flooding as a result of public funds being invested in actions that protect the most vulnerable and those areas at greatest risk of flooding
2. Rural and urban landscapes with space to store water and slow down the progress of floods
3. Integrated drainage that decreases burdens on our sewer systems while also delivering reduced flood risk and an improved water environment
4. A well informed public who understand flood risk and adopt actions to protect themselves, their property or their businesses
5. Flood management actions undertaken that will stand the test of time and be adaptable to future changes in the climate

1.3 Surface water flooding in Scotland

Surface water flooding is a significant problem in Scotland. The National Flood Risk Assessment ( NFRA) published by SEPA in December 2011 estimated that around 125,000 properties are at risk of flooding from all sources. This represents 1 in 22 homes and 1 in 13 businesses with the average annual cost of damages estimated to be between £720 million and £850 million. The NFRA estimated that surface water accounts for approximately 38% of these predicted impacts in Scotland.

Figure 1. Main sources of flood risk in Scotland

1.4 What is surface water flooding?

In natural (undeveloped) catchments, when rain falls onto a surface, some will evaporate directly back into the atmosphere (evaporation) and the remainder will infiltrate into the ground (groundwater). Some of this will then be taken up by vegetation and evaporates back into the atmosphere (transpiration). Any excess surface water runoff will drain via a network of small and large watercourses and lochs to the sea. During higher rainfall watercourses can reach their bank full capacity and overflow onto floodplains.

Development and urbanisation has fundamentally altered this natural drainage process. Removing vegetation and building over green space reduces infiltration and evapotranspiration (evaporation and transpiration). This has the dual effect of increasing both the volume and rate of surface water runoff in urban areas. This increased runoff combined with the replacement of some watercourses (and other natural drainage features) with drains and culverts (that have a finite capacity) constrains the ability of the drainage network to cope with the surface water, causing flooding when surface water can't reach the drainage network or when the drainage capacity is exceeded. When this increased surface water runoff reaches watercourses it also exacerbates river flooding ( Figure 4).

The term surface water flooding is often used to describe flooding from high intensity rainfall events that cause flooding from rainfall runoff flowing and ponding on the ground and also flooding from sewers and other artificial drainage systems such as road drainage when the capacity of drainage systems is exceeded. It is distinct from flooding that occurs from larger rivers and the sea. In reality the general term of surface water flooding is often a complex interaction of many sources of flooding, including flooding from the natural (e.g. smaller watercourses) and artificial (e.g. sewers) drainage systems and direct inundation of areas from surface water runoff. Other sources of flooding can exacerbate surface water flooding for example where high sea levels or river levels prevent drainage systems from discharging freely. The term surface water flooding for the purpose of this guidance includes flooding from the following sources:

• Pluvial flooding - flooding as a result of rainfall runoff flowing or ponding over the ground before it enters a natural (e.g. watercourse) or artificial (e.g. sewer) drainage system or when it cannot enter a drainage system (e.g. because the system is already full to capacity or the drainage inlets have a limited capacity).
• Sewer flooding and other artificial drainage system flooding - flooding as a result of the sewer or other artificial drainage system (e.g. road drainage) capacity being exceeded by rainfall runoff or the drainage system cannot discharge water at the outfall due to high water levels (river and sea levels) in receiving waters.
• Groundwater flooding - flooding as a result of the water table rising to the surface.
• Flooding from small urban watercourses (including culverted watercourses) - flooding which occurs from small watercourses (including culverted watercourses) that receive most of their flow from inside the urban area and perform an urban drainage function. It should be noted for consideration that SEPA will not be assessing flood risk from watercourses with a catchment area less than 3km ².

Figure 4. Overview of surface water flows in a natural catchment and in a developed catchment, illustrating the impact of urbanisation showing increased surface water flows and increased river flows. Illustration courtesy of CIRIA and www.susdrain.org.

1.5 Principles to support the sustainable management of surface water flood risk and urban drainage

The risk of surface water flooding has the potential to increase in the future due to climate change, population growth and urban creep (the gradual loss of permeable surfaces from urban areas e.g. paving over gardens to create driveways) ^[4] .

To manage surface water flooding and urban drainage in the long-term, it is not sustainable to rely on continual upgrading of 'traditional' sewerage and surface water infrastructure. Creating ever larger pipes and subsurface storage is impractical, prohibitively expensive and not adaptable to climate change.

Instead, an integrated approach to drainage that takes account of all aspects of the urban drainage systems and produces long-term and sustainable actions must be deployed.

This requires examination of the sources, pathways and receptors of flood waters to ensure that a full range of measures can be applied across the urban area and during any event the flows created can be managed in a way that will cause minimum harm to people, buildings, the environment and businesses and be adaptable to climate change.

A key component of this approach is to manage surface water before it enters the sewer system or receiving watercourse by allowing for the increased capture and reuse of water; increased absorption through the ground; and more above-ground storage and routing of surface water separate from the sewerage system.

This approach will not only help reduce surface water flooding, it will also help to reduce pollutant inputs to watercourses and reduce the reliance on infrastructure, e.g. culverts that can damage the water environment. It can also create other recreational, amenity and economic benefits through the creation of green spaces and opportunities for urban regeneration and become more resilient to climate change.

The SFM guidance states that principles of integrated drainage should be adopted by SEPA and the responsible authorities (Table 1).

Table 1. Principles of integrated drainage
Increase the percentage of new surfaces that are permeable
Aim to deal with storm water runoff from impermeable surfaces as close to source as possible
Replace existing impermeable surfaces with permeable surfaces where practicable
Minimise the amount of drainage going underground as this is often an inflexible solution that cannot deliver wider benefits or be easily adapted to future conditions
Maximise opportunities to manage surface water before it enters the sewer system
Design for exceedance by ensuring that existing and new developments have flood plains and safe flow paths.

1.6 What is surface water management planning?

Surface water management planning is the process by which the most sustainable measures to manage the risk of surface water flooding are identified in order that they can be described as required in the FRM Strategies and LFRMPs. Further detail on the flood risk management planning process is given in Section 2.

The purpose of a Surface Water Management Plan ( SWMP) is to provide sufficient information to support the development of an agreed strategic approach to the management of surface water flood risk within a given geographical area by ensuring the most sustainable measures are identified (i.e. the most economically, socially and environmentally beneficial measures). SWMPs can be implemented at any scale, and should follow a risk based approach, where most effort should be focused in areas of highest risk and where the most complex problems exist. SWMPs can therefore vary in detail to suit local requirements and the amount of detail that a SWMP contains should be proportionate to the surface water flood risk and the complexity of the problem. The principles in this guidance can therefore be followed to address surface water flooding in any area, however the FRM Strategies and LFRMPs will identify where the risk of surface water flooding is greatest and where priorities for surface water management planning should be focused.

SWMPs within the flood risk management planning process should be considered a long-term plan for managing surface water flooding. The SWM planning process should assess the risk ^[5] of surface water flooding, set objectives and identify measures for the management of surface water flood risk in an area. This should include a description of measures currently in place, measures that can be implemented in the short term and longer term aspirational aims (e.g. where redevelopment provides a cost-effective opportunity to improve surface water management). The SWMP should therefore include a range of structural (e.g. surface water storage structures) and non structural (e.g. emergency response) measures including:

• Policy recommendations to influence land use planning
• Policy recommendations to influence emergency planning
• Ensuring better co-ordination between different authorities
• Identification of where improved maintenance / asset management by all partners will help to reduce surface water flood risk
• Structural measures where informed principally by cost benefit appraisal
• Aspirational options to reduce surface water flooding, which may not be deliverable in the short-term, but nonetheless could become feasible in the longer term

Once an agreed strategy, including a set of agreed measures, has been reached between SWMP partners, detailed appraisal and design of any structural measures identified can then be taken forward under agreed timescales ( see section 2 for more information on the flood risk management planning process).