DM25 — Sustainable Drainage Systems and reducing flood risk.

8.29 The National Planning Policy Framework states that development should be directed away from areas at a highest risk of flooding and that Local Plans should apply a sequential, risk-based approach to the location of development. Where development is required in areas at risk of flooding, it should be safe for the lifetime of development without increasing flood risk elsewhere. DM25.1 The Council will ensure that development in the borough reduces flood risk and minimises the impact of flooding by: 8.30 The National Planning Policy Framework and Planning Practice Guidance state that sustainable drainage systems should be given priority in major developments unless demonstrated to be inappropriate. However, the use of sustainable drainage systems in all developments provides the opportunity to manage surface water as close to the source as possible and provide wider amenity and biodiversity benefits. d. Applying the sequential approach to site layout by locating the most vulnerable uses in parts of the site at the lowest risk of flooding. a. Steering development to the areas with a lower risk of flooding; b. Applying the Sequential Test and Exception Test in accord with Table 8.1; c. Taking account of all sources of flooding from fluvial, surface water, groundwater, sewers, reservoirs and ordinary watercourses; and DM25.2 In areas at risk of flooding development should be safe for the lifetime of development and should incorporate flood resilience and resistant measures into the design, layout and form of buildings to reduce the level of flood risk both on site and elsewhere. DM25.3 Sustainable drainage systems are required in all development and should: a. Ensure surface run-off is managed as close to the source as possible; b. Accord with the London Plan Sustainable Drainage Hierarchy; c. Achieve better than greenfield runoff rates, where feasible; d. Be designed to be multifunctional and incorporate sustainable drainage into landscaping and public realm to provide opportunities to improve amenity and biodiversity; e. Achieve improvements in water quality through a sustainable drainage system management train; and f. Be designed with consideration of future maintenance. Croydon Local Plan 2024 A Place with a Sustainable Future: Environment and Climate Change Croydon Local Plan 2024 How the policy works Table 8.1 Application of Sequential test, Exception test and Flood Risk Assessment to applications in Croydon Flood Zones Land uses Sequential Test Exception Test Flood Risk Assessment Flood Zone 1 All uses are permitted Required if identified at risk from other sources of flooding. Not applicable for minor developments. Not applicable All major developments and all developments in areas identified at risk from other sources of flooding Flood Zone 2 Highly vulnerable uses will only be permitted if the Exception Test is passed Required for all development unless allocated in the Croydon Local Plan Required for highly vulnerable uses All development Required for all development unless allocated in the Croydon Local Plan Required for essential infrastructure and more vulnerable uses All development More vulnerable and Highly vulnerable uses should set Finished Floor Levels a minimum of 300mm above the known or modelled 1% annual probability flood level (1 in 100 year) including climate change Flood Zone 3a Highly vulnerable uses will not be permitted More vulnerable uses should set Finished Floor Levels a minimum of 300mm above the known or modelled 1% annual probability flood level (1 in 100 year) including climate change Basements dwellings will not be permitted Flood Zone 3b Water compatible uses will be permitted Highly vulnerable, more vulnerable and less vulnerable uses will not be permitted Basements dwellings will not be permitted Required for all development unless allocated in the Croydon Local Plan Required for essential infrastructure All development 8.31 When preparing Flood Risk Assessments regard should be had to the Council’s Strategic Flood Risk Assessment, Surface Water Management Plan, Local Flood Risk Management Strategy, other local flood history, relevant flood defence asset information, Section 19 reports and Environment Agency flood maps. Flood Risk Assessments should assess the risk from all sources of flooding and should be informed by the latest evidence on climate change allowances. 8.32 The Exception Test is not required for sites within Flood Zone 1. For development in Flood Zones 2 and 3 the Sequential Test and Exception Test should form part of the Flood Risk Assessment as given in Table 8.1, having regard to the guidance in the Council’s Strategic Flood Risk Assessment. A Sequential Test is required for development (not minor) in Flood Zone 1, if the area has been identified at risk from other sources of flooding. 8.33 For residential development, a Sequential Test may be made against the Council’s published five year supply of housing land and should demonstrate that the five year supply of housing land cannot be met on sites with a lower risk of flooding. For all other uses a Sequential Test should be based on the catchment of the proposed use. 8.34 Where a site is at risk of groundwater flooding, the Council will request a Basement Impact Assessment as part of the Flood Risk Assessment for any application with basement or lower ground floor development. These assessments should be informed by ground investigations to help assess the flood risks to basement development and the surrounding area. In addition, any new basement development connected to the sewerage network shall be fitted with a positive pumped device to protect the basement from the risk of sewer flooding. 8.35 Due to the risk posed by surface water flooding in Croydon, development should utilise sustainable drainage systems to achieve better than greenfield runoff rates from the site where feasible. Greenfield runoff rates are defined as the runoff rates from a site, in its natural state, prior to any redevelopment. If better than greenfield runoff rates cannot be achieved, this should be justified to the Local Planning Authority and Lead Local Flood Authority as part of a drainage strategy. In these instances greenfield runoff rates should be achieved as a minimum in line with the London Plan. 8.36 Sustainable drainage systems should always be considered as early in the design process in synergy with the green infrastructure objectives to inform the design of the development. Proposals should demonstrate an understanding of how surface water will flow across the site, taking account of topography and locating drainage features accordingly. A drainage strategy should demonstrate that the site will achieve better than greenfield runoff rates and that sustainable drainage systems have been designed in line with the London Plan drainage hierarchy. Drainage design should follow the principles of water sensitive urban design and demonstrate a sustainable drainage management train. A sustainable drainage management train identifies the different stages of movement of water through and across a site, identifying suitable sustainable drainage techniques and opportunities for ecological improvements at each stage. For example, a management train could consist of a green roof, a soakaway and permeable paving used in different parts of a development. The drainage strategy should also demonstrate how the drainage system will be managed and maintained for the lifetime of the development.

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