Strategic Urban Drainage Systems (SuDs)

Chaper 5 – Topic Policies

Section D – The Natural and Built Environment

ENV9 Strategic Urban Drainage Systems (SuDs) (2016 Draft / 2017 Main Changes)

Water supply, flood risk and water quality have all been identified as critical constraints to the sustainable growth of Ashford. Ashford’s water environment needs to be managed carefully and the multi-benefits of implementing SuDS within local developments can not be overstated.

SuDS can make a real different to flood risk by managing the quantity of surface water run-off from development, they can also moderate flow rates and prevent sudden water level rises following heavy rain. SuDS can significantly reduce harm to valuable water resources by retaining water within the local hydrological system as well as protecting water resources from pollution by filtering run-off.

Sustainable drainage systems are designed to control surface water run off close to where it falls and mimic natural drainage as closely as possible. Sustainable drainage systems also provide opportunities (in line with NPPF) to:

  • reduce the causes and impacts of flooding;
  • remove pollutants from urban run-off at source;
  • combine water management with green space with benefits for amenity, recreation and wildlife.

The findings of the Discharge Rates Assessment demonstrated that the current Ashford Borough Council SuDS SPD policy (CS20) standard of 4 l/s/ha is difficult to achieve when applying to small site developments. The infrastructure required to store the quantity of water needed to achieve this discharge rate would not be feasible within smaller sites, as long term storage requires large areas of land.

The Assessment also recommended that discharge requirements should be based on site-specific conditions and monitoring (if available). By undertaking site-specific studies, a detailed analysis of what SuDS could be implemented into the site could also be achieved.

Finally, it was also recommended that the Council should consider a higher discharge rate than 4 l/s/ha to apply borough wide if the site is brownfield. This study has outlined that the majority of the allocated sites would be capable of accommodating drainage infrastructure that would be able to discharge to half-capacity within 24 hours, based on a discharge rate of 5 l/s/ha.

In light of the recommendations of this report, the SPD is to be updated to reflect recent changes in local and national policies with respect to the requirements for discharge runoff rates throughout the borough and recent government changes on SuDS.

The updated Sustainable Drainage SPD is anticipated to recommend the following discharge rates based upon the Discharge Rates Assessment:

Greenfield –Discharge rates for undeveloped sites should discharge at a maximum of 5l/s/ha, or 10% below current greenfield rates for the existing 1:100 storm event, whichever is lower. There must be no increase in discharge rate from less severe rainfall events, with evidence submitted to demonstrate this principle.
Previously Developed – Discharge rates for previously developed sites must meet at a minimum a reduction of 10% of existing runoff rates where this existing discharge rate can be established or 10.26l/s/ha where this cannot be established; but must endeavour to achieve 5 l/s/ha or seek to achieve 50% reduction from existing runoff rates for the site (where this can be established).

The above proposals set out the continuation of reducing runoff rates within the Borough through development.

Within the Ashford Borough the requirement for the inclusion of SuDS within major development has been extended beyond that set out within the NPPF, and also includes minor developments. Permitted developments are also encouraged to integrate SuDS into development.

Policy ENV9 – Sustainable Drainage

All development should include appropriate sustainable drainage systems (SuDS) for the disposal of surface water, in order to avoid any increase in flood risk or adverse impact on water quality, and to mimic the drainage from the pre-developed site.

Any SuDS scheme should be compliant with the adopted Sustainable Drainage SPD and any subsequent revisions.

SuDS features should always be the preferred option and provided onsite wherever practicable. In the Ashford urban area if this cannot be achieved, then more strategic forms of SuDS may be appropriate. In such circumstances, developers will need to contribute towards the costs of provision via Section 106 Agreements.

All development proposals will be required to:

  • Ensure all new developments are designed to reduce the risk of flooding, and maximise environmental gain, such as: water quality, water resources, biodiversity, landscape and recreational open space.
  • Ensure that all new developments are designed to mitigate and adapt to the effects of climate change.
  • Lower runoff flow rates, reducing the impact of urbanisation on flooding.
  • Protect or enhance water quality. Incorporating appropriate pollution control measures, to ensure there are no adverse impacts on the water quality of receiving waters, both during construction and in operation;
  • Be sympathetic to the environmental setting and the needs of the local community.
  • Incorporate a SuDS scheme that is coherent with the surrounding landscape and/or townscape;
  • Provide a habitat for wildlife in urban watercourses; and encourage natural groundwater recharge (where appropriate).
  • Demonstrate that opportunities have been taken to integrate sustainable drainage with biodiversity enhancements through appropriately designed surface water systems, as well as contribute to amenity and open spaces;
  • Demonstrate that the first 5mm of any rainfall event can be accommodated and disposed of on-site;
  • Demonstrate that clear arrangements have been established for the operation and maintenance of the SuDS component for the lifetime of the development.

Developers must notify the Council to discharge any relevant conditions before commencement of works on site to ensure a compliant design has been submitted.

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