Sustainable Drainage Systems (SuDS)

SuDS are designed to mimic natural drainage and will contribute to lowering the city’s risk of flooding.

How do SuDS work?

They do this by slowing down the speed at which water moves and reduce the volume of water entering our sewer networks. They achieve this by retaining water within development sites until periods of reduced pressure or by encouraging the loss of water through natural processes such as; infiltration (discharge into the ground), evaporation (discharge into the air), interception (creating vegetation barriers) and absorption (being used by vegetation through their roots).

The city’s topography, soil composition, industrial heritage and historic development mean Leicester is at particular risk of surface water (pluvial) flooding. In addition, urbanisation and most forms of development increases the risk of surface water flooding. This is because it increases the proportion of impermeable surfaces (which do not allow water to pass through). So all development needs to integrate SuDS to manage surface water in a responsible manner, reduce pressure on sewer systems and watercourses, and to prevent their surface water runoff affecting others.

SuDS are key components of any drainage strategy for a development, as they aid in reducing the surface water discharge rate for the site, whilst replicating natural conditions. This is particularly important during extreme weather events, which are increasing in frequency due to climate change.

As a Lead Local Flood Authority (LLFA) for Leicester, Leicester City Council is a statutory consultee to the planning process and is responsible for reviewing SuDS in developments in Leicester; thereby reducing flood risk for the city and the wider catchment.

It is expected that all development schemes will aim to match the Greenfield runoff rate of five litres per second per hectare (l/s/ha).

With Brownfield sites, it is expected that development schemes achieve 50% betterment on the existing surface water discharge rate. SuDS help achieve this.

In order to achieve Planning Validation, a Sustainable Drainage Strategy should accompany all planning applications for new built development that involves one or more new dwellings or over 200 square metres of building footprint for all other types of development (with exception to householder developments). See: local validation list requirements (PDF).

Benefits of SuDS

• Reduce both the rate and volume of surface water runoff, and so reduce the likelihood of flooding
• Maintain and improve water quality
• Improve biodiversity and ecology
• Create attractive places for people to live, work and play by enhancing the natural environment and improving visual amenity
• Allows evapotranspiration from plants, which cools down the area and reduces the urban heat island effect in urban areas
• Allows groundwater recharge where ground conditions allow
• Promote the efficient use of land; often through multiple uses and functions
• Well-designed SuDS can be less expensive to construct and maintain than traditional piped drainage and can be easier to access and inspect

SuDS hierarchy

SuDS are designed to manage rainwater close to where it falls. The SuDS hierarchy of techniques that should be considered to select the most appropriate SuDS for a particular area are given below.

The SuDS hierarchy has four elements:

1. Prevention: SuDS measures are used to prevent pollution and runoff from leaving the site. Examples include rainwater harvesting and raingardens.
2. Source control: These are used to control runoff from near the source (such as soakaways, other infiltration methods, green rooves and permeable paving). Mainly used in minor developments or individual houses.
3. Site control: is used to manage runoff from a site using multiple SuDS methods (such as conveyance systems, managing runoff from car parks, buildings with green rooves, detention and retention basins, infiltration methods). Used in both minor and major developments.
4. Regional control: is used to manage surface water runoff from a large site or several sites (such as balancing ponds, detention and retention basins and wetlands). Mainly used in major developments. In Leicester, developments like North Hamilton and Ashton Green fall into the regional control category.

Further information on the SuDS Hierarchy can be found in CIRIA (C753F) The SuDS manual, and guidance on the construction of SuDS is presented in CIRIA (C768F) Guidance on the Construction of SuDS. Both are available for free download.

Types of SuDS

There are two groups of SuDS:

1. Infiltration SuDS
2. Non-infiltration SuDS

The most appropriate SuDS are determined by examining the ground conditions. Any sites where water can soak into the ground is called an infiltration site. Infiltration helps to recharge the ground water table. If the underlying soil is not suitable for infiltration, then a non-infiltration SuDS should be considered.

In Leicester, the main soil type is clay which is unsuitable for infiltration SuDS. Therefore, an infiltration test is required to confirm the suitability of individual sites before selecting an infiltration SuDS method. All infiltration testing must be completed in accordance with latest version of BRE Digest 365.

For a detailed description of infiltration and non-infiltration SuDS - see CIRIA (C753F) The SuDS Manual.

Retrofitting SuDS

SuDS can also be installed into existing developed areas or areas where brownfield development is to take place. This is also referred to as retrofitting SuDS and by doing this as part of development, it provides multiple benefits. For example, reducing the speed and volume of surface water runoff entering nearby sewer systems or watercourses, which reduces the likelihood of flooding. They also improve water quality, enhance biodiversity, provide visual amenity and can be an efficient use of land with multiple use classifications and functions. Examples include; rainwater harvesting, raingardens, permeable pavements and tree pits.

SuDS technical guidance

We are at the forefront of delivering innovative SuDS and have developed the SuDS Technical Guidance (2021), which builds on the principles set out in SuDS Drainage Guide (2015). This technical guidance is primarily for use by developers and outlines how to achieve the adoption of sustainable drainage in developments. With the overall aim of the guide being to help developers and applicants deliver high quality, easily maintained SuDS that are safe and contribute to the public realm.

This guidance:

1. Defines the principles of SuDS, their benefits and their importance as part of development.
2. Explains how to achieve planning and other approvals for your development in respect of the provision of SuDS.
3. Provides step by step information, which should ensure a greater likelihood of an easier and faster route through the planning approvals process regarding sustainable drainage.
4. Provides clarification on what types of SuDS we will or will not adopt and outlines the processes and procedures for adoption.

Both the SuDS Technical Guidance (2021) and the SuDS Drainage Guide (2015) documents can be downloaded at the bottom of this webpage.