Understanding Sustainable Drainage

Sustainable flood drainage systems are processes which regulate water in a way that imitates natural drainage systems.

Designed to control water flow and the distribution of polluted water in developed, urban areas, SuDS work by enabling water to soak into the ground (infiltration systems), slowing down the flow of water into watercourses (retention systems), storing and transferring water from vegetation into the atmosphere (evapotranspiration) and removing pollutants from water (filtration).

Faced with a future of climate uncertainty, SuDS are a far more competent water drainer, container and manager than their traditional counterparts like underground pipe systems, borehole soakaways and drainage bricks. 

Sustainable drainage solutions come in several forms, each of which have different purposes and suit a specific type of landscape.

Variations of surface water drainage solutions examples include:

• Linear Wetlands Gullies direct water into downstream wetlands, where vegetation stores water and breaks down pollutants.
• Detention Basins Dry or waterlogged concave areas temporarily hold rainwater during storms to slow runoff (also called attenuation basins or SuDS ponds).
• Green Roofs Specially designed roofs with plants and drainage layers manage rainwater and stormwater flow.
• SuDS Paving Gravel and porous paving allow rainwater to soak into the ground, reducing surface flooding.
• Swales Shallow, vegetated channels collect runoff and move it to ponds or wetlands.
• Wetland and Retention Ponds Water-saturated areas that support biodiversity and collect runoff from swales.
• Vegetative Filtration Plants absorb water and release it into the atmosphere through evapotranspiration, helping to filter excess water.

Join our next CPD webinar on SuDS

This session introduces what is meant by contaminated land, the legislative background and how this can impact property development and site acquisition. It also examines planning conditions and the help that is available in response to them.

SuDS provide many advantages to people, society, wildlife and the environment alike due to their variety. 

Through implementing SuDS developers can:

• Create and protect natural habitats such as wetlands and ponds.
• Support wildlife by providing spaces for breeding, shelter, and feeding.
• Reduce flood risk by capturing and managing surface water.
• Enhance drainage for homes and developments with smarter designs.
• Improve water quality by limiting pollutants entering rivers and wetlands.
• Boost urban sewage capacity and improve drain flow.
• Filter pollutants from urban areas and the wider environment.
• Enhance town and city landscapes with green spaces and planting.
• Ease pressure on traditional drainage systems during heavy storms.
• Support natural groundwater recharge and aquifer replenishment.
• Protect river flow patterns and maintain natural regimes.
• Help communities adapt to heavier rainfall caused by climate change.

Whilst the positives of sustainable drainage systems far outweigh the negatives, there are a couple of downsides to SuDS:

• Space for development may be limited, as green areas like wetlands and marshlands are protected for their biodiversity and water management roles.
• SuDS relying on infiltration need permeable ground conditions, such as sands or gravels.
  In areas with impermeable soils, surface water may not drain properly, increasing flood risk or pollution of local watercourses.

Firstly, from a financial perspective, the construction and maintenance of SuDS has been proven to be a cheaper alternative to traditional drainage systems.

In addition, SuDS can improve the financial value of land and property with the integration of green, natural spaces which combat flooding and pollution in a more attractive way.

Developments constructed in an area with sustainable drainage systems will also improve their aesthetic value and offer wellbeing benefits to potential occupants.

Furthermore, the presence of SuDS will reduce the time taken for planning permission to be granted, especially in instances where SuDS schemes are key to drainage strategies of local planning authorities.

Building on a SuDS strategy, Detailed Drainage Design provides calculations and drawings that shows what to build where to satisfy the agreed drainage strategy. This can cover both surface water and foul drainage.

Every project is different and therefore the drainage design is unique. In some cases, planners may require a significant level of detail to ensure the feasibility of the design and drawings are used by the regulators to ensure the approved scheme is installed correctly.

If you intend to build a house, or any kind of development, you will need a detailed drainage design in order for your builders (or perhaps yourself) to know what drainage to install. The design has to consider all relevant guidance, from the Local Authority, DEFRA and best practice from the SuDS manual (CIRIA, 2015) and demonstrate a reduction in discharge rate where required.

England

Most local authorities in England will require a SUDS strategy to be undertaken before planning is approved, even in areas with a relatively low flood risk. You may also need to undertake Detailed Drainage Design (following the strategy being produced) before construction can begin.

In January 2023, the Department for Environment, Food and Rural Affairs (DEFRA) and Environment Minister, Rebecca Pow, announced the government’s decision to implement Schedule 3 of the Flood and Water Management Act 2010 in England to better control flooding and wastewater discharges. One of the most notable proposals in the schedule is for the introduction of SuDS approval bodies (SAB), whose duty it will be to adopt new drainage systems on the basis that they meet certain conditions. 

Once Schedule 3 is implemented, developers and other professionals involved in developments must ensure drainage approval is sought from a SuDS approval body before construction begins as a legal requirement.

Schedule 3 was expected to be implemented in 2024, but has been delayed.

Learn more about Schedule 3 of the Flood and Water Management Act

Wales

In Wales, Schedule 3 was commenced into law in 2019. In cases where new developments of more than one dwelling are planned for construction, or in instances where the area that will be covered by the development equals or exceeds 100 square metres, approval is required from the SuDS Approval Body (SAB) prior to any construction.

The SAB approval will also require developers to demonstrate:

• Arrangements for the adoption and management of SuDS.
• How maintenance of the SuDS and its drainage elements will be funded.

Moreover, in cases where infiltration SuDS are not suitable, the policy states that “consultation with drainage bodies and NRW should be undertaken and relevant evidence and information drawn from Area Statements taken into account.”

Scotland

The Scottish Planning Policy states that the planning system should promote avoidance of increased surface water flooding through requirements for Sustainable Drainage Systems (SuDS) and minimising the area of impermeable surface.

Proposed arrangements for SuDS should be adequate for the development and appropriate long-term maintenance arrangements should be put in place.

According to the policy, planning should support developments which showcase six qualities, one of which integrates SuDS “to conserve and enhance natural features whilst reducing the risk of flooding.”