Delivering sustainable flood risk management: guidance

4. Integrated drainage

Introduction

Drainage is a complex interaction of piped systems, watercourses, and other features of the landscape, like roads and paths, that by design or otherwise perform a drainage function.

It is widely recognised that an integrated approach to drainage, ensures that above and below ground parts of the drainage system can work in concert to deliver benefits for flood risk management, the environment and water treatment.

This section provides guidance on:

• delivering an integrated approach to drainage with a focus on urban areas in particular;
• inclusion of surface water management within the preparation of flood risk management plans;
• interactions with River Basin Management.

Drainage and flooding

Drainage involves the interaction of many different components of above-ground and below-ground drainage. 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 when the capacity of part or all of the drainage system is exceeded, which can include when:

• 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 of they cannot discharge at their downstream outfall, possibly due to high levels in receiving waters.

Where run-off is conveyed through combined sewers, as is the case in older developments, a mixture of surface water and untreated 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.

Where possible opportunities to retrofit sustainable urban drainage and surface water management systems in existing developments should be identified and encouraged to keep water from entering the sewers in the first place.

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.

An integrated approach to urban drainage

The long term answer to urban water management cannot be continual upgrading of sewerage infrastructure, for instance by creating ever larger pipes and subsurface storage, as this would be impractical and prohibitively expensive.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 during any event the flows created can be managed in a way that will causes minimum harm to people, buildings, the environment and businesses.

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 foul sewer 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.

Good surface water management will involve increased use of SUDS and creation of surface water flow routes that divert 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, integrated drainage must be a key consideration in planning decisions, so that sustainable drainage 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 integrated drainage.

BOX 2 Principles to support 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 possible; 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 exceedence by ensuring that existing and new developments have flood plains and safe flow paths.

Local flood risk management plans and integrated drainage

Local authorities will be expected to lead on the preparation of surface water management plans for inclusion in the local flood risk management plans, and to do so in a way that respects the principles of integrated drainage (BOX 2). 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 local authorities. This should include drainage studies and, wherever possible, contributions to overland flow modelling and mapping. SEPA should also provide guidance to support the development of integrated modelling, both above and below ground and with the wider catchment. This could be delivered through the provision of advice or the establishment of guidance standards for modelling.

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 need to decide early in the first planning cycle where surface water management plans will be necessary to tackle surface water flooding. The national flood risk assessment prepared by SEPA should provide information to help target effort in the first planning cycle. Longer term needs should then be coordinated through the strategic flood risk management plans, for instance by identifying where detailed drainage studies are required.

Interactions with River Basin Management

Water quality problems can occur where surface water and sewage are transported in the same pipes. The overflows that are designed to help protect properties during periods of heavy rainfall can significantly increase pollution to receiving watercourses. Contaminants can also enter the system at several points and lead to pollution of watercourses.

Man made changes to the morphology of watercourses can also cause widespread environmental damage. River straightening, culverting and alterations to bank side vegetation can damage habitats. These actions can also lower the natural resilience of watercourses to erosion which can cause sediments to accumulate leading to increased flood risk.

SEPA must work closely with Scottish Water and local authorities to ensure that opportunities to improve the quality of Scotland's water environment are exploited. This will require close coordination with River Basin Management Plans.