Delivering sustainable flood risk management: guidance (2019)

4. Surface water management

Introduction

Urbanisation has altered the natural drainage process. Rain falls everywhere, so all features of our urban landscape, by design or otherwise, influence surface water run-off and flooding. Surface water flooding is often a complex interaction of many sources of flooding, including flooding from piped systems when their capacity is exceeded, small urban watercourses and direct inundation from surface water run-off.

It is widely recognised that sustainable surface water management ensures that above and below ground parts of the drainage system can work in concert to deliver benefits for flood risk management, people, the water environment and biodiversity, while also making our urban areas more adaptable to future changes and more resilient to climate change.

This section provides guidance on:

• delivering a sustainable approach to surface water management with a focus on urban areas in particular;
• inclusion of surface water management within the preparation of flood risk management strategies and local plans;
• interactions with other processes to deliver multiple benefits.

Surface water drainage and flooding

Surface water management involves the interaction of many different components of above-ground and below-ground drainage systems. Following a rainfall event, surface water runoff will normally flow above-ground until it reaches a receiving body (storage pond, stream or low point in the catchment), or it enters the below-ground piped drainage system, typically through a series of gullies, eventually making its way to a receiving body of water or a wastewater treatment works.

These systems are not designed to deal with severe storms and can never be built large enough to accommodate the most extreme rainfall events. This means heavy rainfall events can cause flooding which is often a complex interaction of many sources including:

• direct inundation from surface water run-off;
• the capacity of part or all of the drainage system is exceeded, which can include:
  • capacity of the below-ground system is overwhelmed by the rate of flow;
  • surface runoff cannot enter the below-ground drainage system due to limited capacity of drainage inlets (by design or through poor maintenance);
  • systems cannot drain effectively because they cannot discharge at their downstream outfall, possibly due to high levels in receiving waters;
  • flooding from small urban watercourses.

Where run-off is conveyed through combined sewers, as is the case in older developments, a mixture of surface water and untreated sewerage discharges can spill out from the system if it becomes overwhelmed.

Potentially hazardous contaminants can also enter the system at several points and lead to pollution of land and receiving watercourses. Under the Controlled Activities Regulations, all new developments must drain surface water through Sustainable Urban Drainage systems (SUDs) before it enters receiving watercourses.

A number of factors can, if uncontrolled, place additional pressure on urban drainage, potentially resulting in increased flood risk and pollution. For instance increases in the proportion of impermeable surfaces in existing developments as new roads and car parks are constructed and people pave over gardens. Likewise climate change is likely to place increasing pressure on existing drainage systems and increase the risk of surface water flooding. It is also important that land use planning policies are adhered to and new development is not at risk of flooding and does not increase flood risk elsewhere.

Sustainable surface water management

The long term answer to urban surface water drainage is not the perpetual upgrading of sewerage infrastructure, for instance by creating ever larger pipes and subsurface storage, as this is impractical and prohibitively expensive.

Instead, a sustainable approach to surface water management 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 during any event the flows created can be managed in a way that will cause minimum harm to people, buildings, cultural heritage the environment and businesses.

A key component of this approach is to mimic natural drainage systems by managing rainfall and surface water above ground, maximising the use of permeable surfaces and planted landscapes. Surface water should be conveyed to watercourses using the natural topography. Increasing surface water in the sewers should be avoided as this can lead to flooding elsewhere. Surface water in the sewers should be reduced where possible.

This approach not only helps reduce surface water flooding. With coordination between authorities using multidisciplinary teams (including landscape architects), it will also help to realise multiple benefits including, integrating with and enhancing the urban landscape to provide better places for people, reducing surface water in the sewers (reducing sewer flooding and spills from sewer overflows), improving the quality and physical habitat of watercourses, increasing biodiversity and making the urban environment more adaptable to future change.

The governance of surface water requires coordination between authorities to achieve these multiple benefits. In addition to flood risk management planning, this should include the interaction of:

• development planning and development management (local authority);
• sewer network management (Scottish Water);
• road management (roads authorities including local authorities);
• enforcement of Building Regulations (local authority);
• river basin management plans (SEPA);
• Local Biodiversity Actions Plans (local authority).

Sustainable surface water management will involve increased use of SUDS and creation of surface water flow routes that divert ‘safe’ floods to areas where impacts will be minimised. The best solutions will be achieved when the full drainage system, from source to receiving water, is designed from the outset. This allows the optimum balance between source, site and regional controls to be achieved.

To deliver these changes sustainable surface water management must be considered in existing urban areas and in new developments. In new developments consideration in the land use planning system is essential so that sustainable surface water management is embedded into the fabric of our urban and rural landscapes.

The principles set out in BOX 2 should be adopted by SEPA and the responsible authorities to support the delivery of sustainable surface water management.

Local flood risk management plans and surface water

Local authorities will be expected to lead on the preparation of surface water management plans that will identify and implement the most sustainable actions to reduce the risk of surface water flooding. These actions should be co-ordinated in the local flood risk management plans and flood risk management strategies. Separate guidance for delivering surface water management plans is available^[7]. This work should be taken forward within the context of wider flood risk management strategies prepared by SEPA in order to ensure that surface water management decisions are undertaken in consideration of other flood management actions and interactions with the wider catchment.

In taking this work forward, careful consideration will need to be given to responsibilities for delivering and maintaining all parts of the drainage system, with particular attention given to responsibilities where the system, or parts of the system, performs more than one function. The aim should be to find a fair and practical way to share costs and responsibilities for the whole drainage system.

SEPA and Scottish Water will need to engage proactively in this work, offering support, expertise, data and models to responsible authorities. This should include drainage studies and, wherever possible, contributions to overland flow modelling and mapping.

SEPA’s modelling guidance^[8] should be considered when developing models.

Prioritising effort

The level of effort invested in understanding and tackling surface water and drainage flooding problems must be proportionate to the risks they present. In complex urban settings where the risk of surface water flooding is significant, detailed urban studies and planning is likely to be required. In determining the level on the effort needed to investigate and manage surface water flooding, consideration should be given to:

• future urbanisation/redevelopment –urban expansion or regeneration presents a challenge to existing drainage systems but can also become an opportunity to address long-standing problems;
• opportunities to retrofit sustainable drainage and surface water management systems;
• evidence of surface water and sewer flooding– past flooding is a reliable indicator of future flooding;
• asset knowledge- where there are complex drainage systems, solutions are more likely to require detailed studies and a partnership approach.

SEPA and the responsible authorities will identify areas where surface water management plans will be necessary to tackle surface water flooding. These areas should be reviewed for each flood risk management cycle to take account of new information. The national flood risk assessment prepared by SEPA should provide information to help target effort. Longer term needs should be coordinated through the flood risk management strategies and local plans, for instance by identifying where detailed drainage studies are required.

Principles of sustainable surface water management

Manage rain and surface water in a way that mimics natural systems and protects and enhances both the built and natural environment.

Manage rainfall and surface water safely above ground, avoiding harm to people, homes, businesses and other adverse impacts of flooding. Maximise the use of permeable surfaces and planted landscapes and convey water to watercourses using the natural topography. Avoid increasing surface water in the sewers as this can lead to flooding elsewhere. Reduce surface water in the sewers where possible.

Manage all rainfall events:

• Everyday rain – manage rain locally at source, maximise infiltration and evapotranspiration by maximising use of permeable surfaces and plants. Water can be collected for use. There should be little or no surface water run-off in these frequent events.
• More rain – collect, delay and convey safely above ground to watercourses following the natural topography. Do not increase surface water in the combined sewers as this can cause flooding elsewhere. Water can be collected for use.
• Extreme rain – delay, store and convey safely above ground to watercourses following the natural topography.

Multifunctional – maximise all benefits:

• People – integrate with, protect and enhance the urban landscape, provide attractive places for people to live, work and visit.
• Drainage and flood management – manage all rainfall events, avoid flooding to people and buildings, avoid increasing flows to receiving watercourses and combined sewers.
• Water quality – protect and enhance the quality and physical habitat of receiving watercourses. Collecting water for use can reduce the need to abstract water elsewhere.
• Biodiversity – protect and enhance biodiversity, maximising permeable surfaces and plants to attract wildlife.
• Adaptability to future change – help the urban environment adapt to future challenges of climate change (increasing rainfall, rising temperatures) and mitigate the loss of green space.

Co-ordinate with other authorities and projects to help to maximise benefits (e.g. using foot paths and cycle paths as routes for infiltration and conveying water, contributing to ‘green and blue networks’).

Think of different spatial scales required to manage surface water (e.g. what can be done locally at the building and street level; what regional and more strategic management is required; and what connections between these different scales are required?).