Sustainable drainage is a concept that includes long term environmental and social factors in decisions about drainage. It takes account of the quantity and quality of runoff, and the amenity value of surface water in the urban environment. Some existing urban drainage systems can be the source of flooding, pollution or damage to the environment and are not to be sustainable.
New development can create a potential impact on existing watercourses and sewer systems by increasing surface water run-off. New Development is therefore one area where the strategies of sustainable drainage are required be put into practice.
Planning Guidance - (April 2015)
Planning guidance which came into effect on the 6 April 2015 dictated that the planning system will lead the drive for the delivery of Sustainable Drainage Systems.
Key changes were that:
- SuDS are to be provided in new developments of 10 dwellings or more, or the corresponding nonresidential or mixed use development;
- Lead Local Flood Authorities are made Statutory Consultees on planning applications for surface water management; and
- Planning Authorities are required to ensure that adequate arrangements are in place for the future maintenance of SuDS for the lifetime of the development either by condition or obligations.
In March 2015, the Department for Environment, Food and Rural Affairs introduced the Non-statutory technical standards for sustainable drainage system document which is to be used in conjunction with the National Planning Policy Framework and the Planning Practice Guidance documents and contains a number of standards. New standards which may significantly change how a site is developed and should be considered carefully are as follows:
- New discharge rates for Greenfield developments should not exceed the 1 in 100 year rainfall events and wherever practicable the volume of runoff for the 100 year 6 hour event should not exceed the equivalent Greenfield runoff volume; and
- New discharge rates for previously developed sites should be close to the greenfield runoff rate for the 1 in 100 year event but never exceed the previous runoff rate for the existing site. The volume of discharge for the 100 year 6 hour event should be limited to as close to the equivalent Greenfield runoff volume as possible but should never exceed the volume for the site prior to redevelopment.
Built-up areas need to be drained to remove surface water and outfall to a suitable watercourse. Traditionally this has been done using underground pipe systems designed for quantity, to prevent flooding locally by conveying the water away as quickly as possible.
The alteration of natural flow patterns can lead to problems elsewhere in the catchment and increase the risk of flooding to other areas.
The traditional way of drainage from developments has been superseded by the use of SuDS for new development to control the rate of surface water discharge from new developments restricting to the existing rate of run off or less.
SuDS also affords major benefits in terms of amenity aspects, such as water resources, water quality and provision of varied wildlife habitats. Conventional drainage systems are not designed with these wider considerations in mind.
Surface water drainage methods that take account of quantity, quality and amenity issues are referred to as Sustainable Drainage Systems (SuDS). These systems are more sustainable than conventional drainage methods because they:
- Manage surface water runoff flow rates, reducing the impact of new development on flooding by dealing with runoff close to where the rain falls
- Protect or enhance water quality by managing potential pollution at its source.
- Provide a habitat for wildlife in urban watercourses.
- Encourage natural groundwater recharge (where appropriate).
- Allow new development in areas where existing sewerage systems are close to full capacity, thereby enabling development within existing urban areas.
SuDS are made up of one or more structures built to manage the rate of surface water runoff. They are used in conjunction with good management of the site, to prevent flooding and pollution. There are four general methods of control:
- Filter strips and swales
- Permeable surfaces and filter drains
- Infiltration devices
- Basins and ponds
These controls should be located as close as possible to the source (i.e. on the site), providing attenuation for the runoff. They also provide varying degrees of treatment for surface water, using natural processes.
SuDS can be designed to function in most urban settings and the type of SuDS can be dictated by the receiving environment, from hard-surfaced areas to soft landscaped features. The variety of design options available allows designers to consider land use, land take and future management.
Sanderson Associates are able to design a bespoke sustainable design system to meet your requirements at a competitive price and provide you with a first class service.
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