Publication: Pre-Submission Gloucester City Plan

Pre-Submission City Plan

Policy E6: Flooding, sustainable drainage, and wastewater

Development shall be safe from flooding and shall not lead to an increase in flood risk elsewhere. In accordance with the National Planning Policy Framework, flood risk betterment shall be sought through the development process.

Planning permission will not be granted for any development in the functional flood plain (Flood Zone 3b) except for development with 'water compatible' and 'essential infrastructure' flood risk vulnerability development classifications.

The sequential test (flood risk) and exception test will be evaluated in line with government planning guidance.

All development will be expected to incorporate Sustainable Drainage Systems (SuDS) to reduce surface water discharge rates and address water quality, unless it can be shown, to the satisfaction of the City Council, that this is not feasible.

The most up to date Environment Agency and Local Lead Flood Authority climate change guidance shall be used in the evaluation of fluvial flood risk and for the design of drainage / SuDS.

Development proposals shall facilitate watercourse restoration, exploiting opportunities to open culverts, naturalise river channels, and protect and improve the floodplain, buffer strips and adjacent terrestrial habitats and water quality, as well as the heritage value. Development proposals to impound and narrow waterways will be refused.

An 8 metre riparian buffer strip, measured from the top of bank to each side of the watercourse or the outside edge of any culverted watercourses where is necessary for the culvert to remain in situ, shall be kept free of development. As well as for flood risk reasons, this is to facilitate maintenance access and to act as a green corridor for ecological benefit.

Applicants shall demonstrate that all surface water discharge points have been selected in accordance with the principles laid out in within the SuDS/drainage hierarchy. That is, where possible, connections to the public sewerage systems, and in particular the combined sewer network, are to be avoided. Wherever possible, foul drainage from development shall connect to the mains public sewer.

Where necessary, financial contributions towards flood risk management infrastructure will be sought through the development process.


3.5.26 Proposals for new development must be in accordance with: the NPPF; Planning Policy Guidance; JCS Policy INF2; Gloucester's Strategic Flood Risk Assessment Level 1 and Level 2; Gloucestershire County Council's SuDS Design and Maintenance Guide; Gloucester City Council's Sustainable Drainage Design and Adoption Guide, or any future iterations.

Flooding

3.5.27 Gloucester has been identified as a 'Flood Risk Area' by the Environment Agency following a preliminary flood risk assessment for river, sea and reservoir flooding, carried out to meet the requirements of the European Floods Directive (2007/60/EC) (transposed into the Flood Risk Regulations (2009)). Flood Risk Areas are where the risk of flooding is likely to be significant at a national scale for people, the economy or the environment (including cultural heritage). As such, it is particularly important that Gloucester has robust policy with respect to flooding, sustainable drainage, watercourses and wastewater. All of Gloucester's watercourses are considered to lack capacity during design rainfall events; any increase in surface water discharge from development sites therefore represents an increase in flood risk.

3.5.28 Proposals should have specific regard to the design principles outlined in the National Planning Practice Guidance, including a sequential approach to site layout, ensuring safe access is available for the lifetime of the development (i.e. incorporating climate change) and that it is supported by suitable flood warning and evacuation plans.

Sequential Test & Exception Test

3.5.29 The area of search for the flood risk sequential test shall generally be the whole of the Gloucester City - unless it can be demonstrated that there is a specific need in a specific location. The City Council's aspiration to redevelop a redundant brownfield site may be considered in the evaluation of the sequential test.

3.5.30 Sleeping accommodation shall not be permitted where the floor level is below the design flood level. For the purposes of the exception test, the design flood level is that with a return period of 1% (100 year probability) with the appropriate allowance for climate change.

Sustainable Drainage Systems (SuDS)

3.5.31 Gloucester City Council actively seeks blue-green infrastructure through the development process, to mitigate against flood risk, by building with nature.

3.5.32 All development proposals will be required to manage surface water through SuDS and reduce the existing discharge rate on previously developed sites. For brownfield sites, the post-development discharge rate shall be as close to the greenfield rate as possible and, as a minimum, at least 40% lower than the pre-development discharge rate. The 40% reduction is used across all districts in Gloucestershire and is cited in the Lead Local Flood Authority (LLFA) 'SuDS Design & Maintenance Guide'. This figure reflects a consensus view amongst district drainage officers, and the consultant engaged to write the SuDS guide, about what is 'reasonably practicable'. Additionally, the LLFA and districts have been successfully applying this standard to development since November 2015, thus demonstrating that it is a viable requirement. For greenfield sites, in the absence of long-term storage, flows shall be attenuated to QBar (mean annual flood). Where parts of a brownfield site do not have an existing on-site positive drainage system, these areas shall be treated as greenfield for the purposes of the surface water discharge rate calculations.

3.5.33 Above ground SuDS (for example attenuation basins and swales), offer significant benefits over below ground systems, including water quality, biodiversity and amenity, and shall be incorporated where practicable. Larger developments will be expected to incorporate SuDS for source control (for example water butts and green roofs) and conveyance (for example swales), as well as for attenuation.

3.5.34 The design of SuDS shall be considered at the earliest possible stage. If an adequate level of SuDS cannot be provided on site, there will be a requirement for a contribution to off-site measures.

3.5.35 None of Gloucester's watercourses, currently assessed under the Water Framework Directive, have reached the targeted 'good' status. In order to achieve 'good' status by the target date of 2027, surface water discharge from developments must address water quality issues. The preferred option for addressing water quality is through the installation of SuDS, but where this is not practicable, demonstrably effective proprietary devices may be used. Development should address the water quality guidelines set out in the most up to date version of the CIRIA SuDS Manual (C753).

Climate Change

3.5.36 In calculating the attenuation volume requirements, the uplift on rainfall to allow for climate change shall be 40%, unless it can be demonstrated that the site is likely to revert to greenfield prior to 2070. It is expected that the Environment Agency climate change guidance will be updated in 2019/20 to incorporate the UK Climate Projections 2018 data (UKCP18). This is likely to increase the 40% requirement, and developments will be expected to adhere to the latest guidance, including any future upgrades to climate change guidance during the plan period.

3.5.37 For calculating the climate change uplift for river flows (i.e. to determine the design flood level for a development which influences the design of the development including floor levels, flow routes, floodplain compensation and safe access and egress arrangements), developers should refer to the latest Environment Agency climate change guidance. Developers are encouraged to assess the Upper End allowances (currently 70% for the Severn River Basin District based on 100 years lifetime of development). Major regeneration projects and infrastructure development are expected to be designed to this level.

Watercourses & culverts

3.5.38 Gloucester's waterways and watercourses are multifunctional assets. They provide transport and recreation corridors, green infrastructure, a series of diverse and important habitats, a unique backdrop for important heritage sites, landscapes, views, a backdrop for cultural and community activities, as well as drainage, flood and water management, and urban cooling functions.

3.5.39 Gloucester City Council supports the Environment Agency in terms of culvert improvement; culverts shall be improved in accordance with the following hierarchy of betterment options; (1) open the culvert (2) replace the culvert (3) leave the culvert in open space for future to open up. All the above options need to incorporate 8 metre buffer strips to allow for access. Each option will still require riparian owners to undertake their responsibilities with regards to maintenance and upkeep of the culvert. It will need to be demonstrated that options higher up the hierarchy are not practicable for them to be discounted. Applicants should contact the Environment Agency and the Lead Local Flood Authority at the earliest opportunity to understand the constraints and opportunities of culverted watercourses for their proposals.

3.5.40 Development proposals shall not remove or interrupt the continuity of existing natural or manmade drainage features, unless agreed with the city Council. Where watercourses or dry ditches are present within a development site, these should be retained and, where possible, enhanced. Enhancement measures could include removing redundant structures, improving fish passage and restoring watercourses to more natural alignments by improvingly hydromorpholoy. All measures can contribute to achieving 'good' status as required under the Water Framework Directive. Access to drainage features for maintenance should be retained and ownership of land clearly defined as part of the site maintenance plan. The removal of natural drainage features may result in an increased need to connect to the public sewerage network, and therefore be contrary to the SuDS / drainage hierarchy.

Wastewater

3.5.41 The existing sewerage network is known to have areas with capacity issues and network constraints and in some cases improvements to the network may be required for new development to connect. Applicants should contact Severn Trent at the earliest opportunity to understand if improvements to the network are required.

3.5.42 Surface water run-off discharge points should be as high up the hierarchy of drainage options as possible; (1) into the ground (infiltration) (2) to a surface water body (3) to a highway drain (4) to another drainage system and (5) to a combined sewer.

3.5.43 The creation of an overall masterplan for the development will enable strategic infrastructure serving multiple developments to be designed appropriately, providing wider benefits and efficiencies that would not otherwise be possible. The masterplan should outline key milestones that need to be achieved for critical infrastructure. This will help to align programmes between different stakeholders.

Financial contributions towards flood risk management infrastructure

3.5.44 Where appropriate, in partnership with the Environment Agency and other flood risk management bodies, the Council will seek financial contributions towards flood risk management infrastructure. The advice within paragraph 56 of the NPPF relating to planning obligations shall be key to determining appropriateness. Such instances would be rare but could include cases where the safety of a development and/or the ability to access the development safely, relies upon flood defences, the Environment Agency's Flood Warning System, or other flood risk management infrastructure.

3.5.45 The Environment Agency has experience of working with developers and Councils in Gloucestershire to secure financial contributions in such cases. Money secured through such planning obligations can be used towards maintenance and improvements of flood defences, provision and upkeep of river gauges (which support the Flood Warning Service), and other flood risk management projects.