Local Plan to 2030 - Main Modifications September 2018

Main Modifications to Ashford Local Plan

Policy ENV9 – Sustainable Drainage (MM90)

Amend supporting text as follows:

5.346 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 cannot be overstated.

5.347 SuDS can make a real different to flood risk by managing the quantity of surface water runoff 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. SuDS can form an integral part of both soft or hard landscaping and can contribute to the quality of green space for the benefit of amenity, recreation and wildlife. SuDS may also allow new development in areas where existing drainage systems are close to capacity, thereby enabling development within existing urban areas.

5.348 The Flood and Water Management Act 2010 introduced the concept of flood risk management into law and sets out the intention for SuDS in all new development. The NPPF requires LPAs to minimise vulnerability and provide resilience to the impacts of climate change, and requires all new developments in areas at risk of flooding to give ‘priority to the use of sustainable drainage systems’. The Government have recently made changes to the NPPF making it clear that they expect SuDS to be provided in all new major development wherever it is appropriate. DEFRA have published ‘non-statutory technical standards for sustainable drainage systems’ which provides guidance on minimum standards of design, maintenance and operation of SuDS systems and sits alongside the Planning Practice Guidance. These documents together with the Ashford Stage 1 Surface Water Management Plan provide information and guidance in formulating planning proposals.

5.349 The Ashford Integrated Water Management Study identified that SuDS with restricted discharges would be integral to managing flood risk as Ashford grows.

5.350 SuDS are designed to control surface water run off close to where it falls and mimic natural drainage as closely as possible. The four pillars of a successful integrated SuDS system include the provision of amenity, biodiversity, landscaping, and water quality control. SuDS 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.

5.351 In April 2015 KCC, as Lead Local Flood Authority for Kent, become a statutory consultee as per national requirements following a parliamentary statement in December 2014. Kent County Council have an adopted “Drainage and Planning Policy Statement” which should inform development of drainage schemes. There are also situations where consultation with the Environment Agency will be necessary in relation fluvial flood risk, water quality, biodiversity and groundwater protection, which may impact measures proposed for surface water management.

5.352 A recent discharge rates assessment based on the latest flood modelling for Ashford confirms the potential to reduce flood risk in Ashford through development appropriately managing and ultimately reducing site runoff rates through new development will assist in ensuring that the floodplain extents within Ashford do not increase even in light of expected changes in rainfall intensity as a result of predicted climate change.

5.353 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. Therefore, on sites below 0.25ha it is advised that a maximum discharge limit of 2l/s is sought through the incorporation of flow-control devices.

5.354 The Discharge Rates 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.

5.355 Finally, it was also recommended noted that the Council should could consider a higher discharge rate than 4 l/s/ha to apply for some brownfield sites borough wide if the site is brownfield. This study has outlined that the majority of the allocated sites in this plan 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.

5.356 In light of the recommendations of this report, the SPD is to be updated a more focused approach will be taken 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. These changes will seek to continue the reduction in flood risk through development across the borough, targeting larger sites where multifaceted benefits can be obtained by the introduction of appropriate SuDS and discharge rate reduction. The discharge rates will also be simplified to be more aligned with current guidance / established best practice wherever possible to ensure a robust reasoning behind the policy document. Moreover, discharge rates will be more site specific and should Drainage should seek to mimic the current drainage regime for a site, whilst also reducing the peak discharges and volumes from the critical storms, paying keen attention to drainage destination. The ultimate aim of the policy is to improve flood risk management in the Borough through future development.

5.357 The existing Sustainable Drainage SPD (2010) rates remain in place until such time that an updated Sustainable Drainage SPD is released, unless alternative discharge rates are agreed by the Council in consultation with KCC as Lead Local Flood Authority.

5.358 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).

5.359 The above proposals set out the continuation of discharge rates outlined below will enable continued success in reducing runoff rates within the Borough through development.

5.360 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 schemes.

5.361 Developers will normally be expected to make provision for SuDS on-site where it is practical to do so. As an In the unlikely case of an exception, where SuDS cannot be achieved on developments in the Ashford urban area, developers will be required to make suitable in-lieu financial contributions through Section 106 Agreements. Consideration should also be given to ‘strategic SuDS’ where a limited number of attenuation and treatment areas are needed downstream in areas of significant planned development.

5.362 The Council expects SuDS to form an integral part of the development design process. This is because successful SuDS require a range of discharge or infiltration techniques that need to be designed in a sequential order. Whilst primarily used to attenuate runoff, early consideration of SuDS provides the opportunity to design-in other benefits which will deliver more sustainable developments. Whilst it is acknowledged that some sites can be more challenging than others, SuDS can be applied to any site. For the reasons set out above, the Council advocates the use of masterplanning in SuDS. Useful guidance on how to successfully integrate SuDS through the masterplanning process has been development by KCC in “Water.People.Place.” Kent County Council as Lead Local Flood Authority and statutory consultee has also produced a Drainage and Planning Policy Statement which should be referred to in the consideration of planning applications.

 

Amend policy wording as follows:

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.

On greenfield sites, development should discharge at a maximum of 4l/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.

On Previously Developed Land, development must endeavour to achieve 4 l/s/ha runoff or seek to achieve 50% reduction of existing peak runoff rates for the site where existing discharge rates can be established.

On smaller sites (less than 0.25ha), development should achieve a maximum discharge of 2l/s.

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

SuDS features should always be the preferred option and provided on site 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.