Policy DM5 Flooding
All development proposals will be assessed and determined having regard to the need to manage and mitigate against flood risk from all sources. Development proposals must be supported by the relevant flood risk assessments and show that (where necessary) alternative sites of lower flood risk have been assessed, adopting a sequential approach to site selection according to the requirements of national policy and standing technical advice which supports it.
The sequential site assessment as set out in the NPPF will be expected to consider reasonable alternatives for locating the development in a zone of lower flood risk on any site elsewhere in Norwich, except in the case of:
- Proposals within the city centre regeneration areas identified on the Policies map, in which case the assessment need only take account of reasonable alternative sites within the boundary of the relevant regeneration area concerned or (where no such alternative sites exist) alternative regeneration areas elsewhere in the city centre;
- Any other proposal which is consistent with and forms part of a specific allocation for development within the Site allocations plan and other adopted development plan documents, in which case the requirement for the sequential test will not apply.
In the case of proposals in areas of higher flood risk which are within the city centre but which fall outside the regeneration areas identified on the Policies map, the search area for reasonable alternative sites should take account of:
a) the scale and function of the proposal;
b) the potential contribution of the use or uses proposed to overall regeneration of the city centre, including through the provision of new housing;
c) where the proposal is for retail, leisure or other main town centre uses, the suitability of any alternative locations in relation to policy DM18 of this plan;
d) any objectively identified need for the use proposed which justifies a location in the city centre in order to support the objectives and policies of the development plan.
For the purposes of this policy “city centre” means the area defined on the city centre Policies map insets, including both the City Centre inset and Northern City Centre Area Action Plan inset.
Sustainable drainage and surface water flooding
Mitigation measures to deal with surface water arising from development proposals should be incorporated to minimise the risk of flooding on the development site and where possible reduce the risk, otherwise at least minimise the risk, within the surrounding area.
Sustainable drainage measures appropriate to the scale and nature of the development shall be incorporated in all development proposals involving the erection of new buildings or the extension of existing buildings until such time as thresholds are established by nationally applicable standards for sustainable drainage. Such measures will be required except where this is not technically feasible or where it can be demonstrated that other factors preclude their use.
Within the critical drainage catchments as identified on the Policies map and in other areas where the best available evidence indicates that a serious and exceptional risk of surface water flooding exists, all development proposals involving new buildings, extensions and additional areas of hard surfacing should ensure that adequate and appropriate consideration has been given to mitigating surface water flood risk. Developers will be required to show that the proposed development:
a) would not increase the vulnerability of the site, or the wider catchment, to flooding from surface water run-off from existing or predicted water flows; and
b) would, wherever practicable, have a positive impact on the risk of surface water flooding in the wider area.
Development must, as appropriate, incorporate mitigation measures to reduce surface water runoff, manage surface water flood risk to the development itself and to others, maximise the use of permeable materials to increase infiltration capacity, incorporate on-site water storage and make use of green roofs and walls wherever reasonably practicable.
The use of permeable materials, on-site rainwater storage, green roofs and walls will be required unless the developer can provide justification to demonstrate that this would not be practicable or feasible within the constraints or configuration of the site, or would compromise wider regeneration objectives.
Surface treatment
Development proposals will be required to maximise the use of soft landscaping and permeable surfacing materials unless the developer can provide justification to demonstrate that this is not feasible.
Where permission is required, proposals involving the provision of new or replacement paved and other impermeable surfaced areas will only be permitted:
a) in areas of impermeable soils as identified in Appendix 1;
b) in other areas where it can be demonstrated that permeable surfaces are not practicable due to poor soil infiltration capacity, high groundwater levels or risk of subsidence; and
c) in areas with soils with average or good infiltration capacity, where it can be demonstrated that there is an exceptional and overriding justification for such surfaces.
In cases where poor soil infiltration capacity or other factors preclude the use of permeable surfacing materials, development proposals should seek to manage and minimise the impact of surface water run-off by suitable measures for water storage on-site.
Supplementary text
5.1 The purpose of this policy is to minimise flood risk to new development and to protect existing development from increased flood risk as a result of new development.
5.2 An extensive evidence base on flooding locally has informed plan making. The Level 1 Strategic flood risk assessment identifies those areas of Norwich which are at risk of flooding from the rivers Wensum and Yare (tidal and fluvial flooding) taking account of the best available evidence of predicted net sea level rise consequent on climate change. The Level 2 Strategic flood risk assessment for Norwich analysed the extent to which development in flood zone 2, with suitable flood mitigation, will be necessary in order to achieve the housing targets set out in the JCS. The Surface water management plan (SWMP) identifies areas of the city at greater risk from surface water flooding resulting from heavy rainfall events (pluvial flooding).
5.3 JCS policy 1 requires new development to be located to minimise flood risk, mitigating any such risk through design and implementing sustainable drainage. The national policy context is set out in the NPPF within Section 10: Meeting the challenge of climate change, flooding and coastal change.
5.4 Environment Agency definitions now incorporated in the technical guidance to the NPPF categorise the relative degrees of tidal and fluvial flood risk to different zones, these being Zone 1 (low probability of flooding), Zone 2 (medium probability), Zone 3a (high probability) and Zone 3b (the functional floodplain). These definitions remain in standard use for planning purposes. The applicable zones are illustrated by the flood maps produced by the Environment Agency (EA) which are available from their website.
5.5 The NPPF and its technical guidance set out the requirement to apply a sequential approach which assesses alternative potential sites for new development, to ensure that where possible, development can be brought forward in areas at little or no risk of flooding in preference to areas at higher risk. The overall aim is to steer new development to areas of lowest risk. Where there are no reasonably available alternative sites in Flood Zone 1, consideration should be given to the vulnerability of land uses and reasonably available sites in Flood Zone 2. Where there are no reasonably available alternative sites in Flood Zone 1 or 2, sites is Flood Zone 3 may be considered, taking into account the flood vulnerability of the land use proposed. The relative levels of flood vulnerability for different land uses are set out in the NPPF technical guidance.
5.6 The Level 2 Strategic flood risk assessment for Norwich shows that development in flood zone 2, with suitable flood mitigation, will be necessary in order to achieve the housing targets set out in the JCS. Such development will also help to address the social and economic needs of the city, promoting essential regeneration in the city centre and retaining its vitality as the most accessible location in the sub-region.
5.7 Other than in the city centre (including the regeneration areas detailed below), the search area for reasonable alternatives for locating proposed development in a zone of lower flood risk will extend to the whole of Norwich. However, where sites are specifically shown for development in the Site allocations plan, and in any other local plans forming part of the development plan, the principle of development is already established. The required risk assessments will already have been undertaken as part of the plan-making and sustainability appraisal process, to show that the form of development proposed for the site concerned is appropriate and justified in the context of flood risk.
5.8 Prospective developers therefore need not re-apply the sequential test for any proposal which falls within an allocated development site in a local plan and is in accordance with the applicable plan policy for that site. However, the detailed design of schemes should still follow a sequential approach to ensure that flood vulnerable uses and activities occupy areas of lowest flood risk within the site.
5.9 The Level 2 Strategic flood risk assessment also shows that the exceptions test will not be required for allocated housing sites within Norwich. Notwithstanding this, the city council considers that there may be instances where flood vulnerable uses are proposed within mixed use allocations and these would need to be subjected to the exceptions test in accordance with national guidance.
5.10 For the city centre, the JCS sets out the importance of mixed use development and regeneration to enhance its regional role, including the redevelopment of brownfield sites. Within Norwich the identified priority regeneration areas are shown on the Policies map, these being
- the northern city centre area, whose boundary is defined by the adopted NCCAAP,
- the St. Stephens masterplan area (predominantly in Flood Zone 1 and hence unlikely to be affected by significant fluvial flood risk); and
- an area in the south-east of the city centre which is intended to be the focus of mixed use regeneration and neighbourhood renewal over the course of the plan period (under the emerging South City Centre vision and investment plan). The overall strategic plan for this area may be supplemented by smaller area development briefs for opportunity sites such as Rose Lane/Mountergate.
5.11 Because of the fundamental importance of city centre regeneration in the JCS, it has been accepted by the Environment Agency that a smaller search area can be used for reasonable alternatives to new development which is in a regeneration area, the principle being that these areas have already been identified and subject to prior testing of alternative development scenarios through the plan making and sustainability appraisal process, during the preparation of the JCS and NCCAAP. Accordingly, it is not expected that the search for alternative sites for any proposal in a regeneration area would need to extend outside the boundaries of the regeneration area concerned.
5.12 For development in regeneration areas at increased risk of flooding (outside Zone 1) the flood risk assessment should focus on addressing how the flood risk will be managed and mitigated and the sequential approach should be applied when considering the location of development within the site.
5.13 Other than in the priority regeneration areas, the extent of the search area for alternative sites under the sequential test for flood risk will vary according to the scale and nature of the proposal, for example for a large development of strategic significance it will be appropriate to look across the whole of Norwich. For main town centre uses the council’s expectation will be that locations within the defined retail and leisure areas would be most preferable, in accordance with the provisions of policy DM18 and the hierarchy of centres set out in JCS policy 19. Thus a location outside the city centre or outside any other defined centre would, by definition, not be regarded as “reasonable”. The contribution of proposals to strategic objectives – for example the desirability of expanding education opportunities within the city centre – would tend to weigh in favour of a location in the city centre over one outside it. In all cases, the suitability of a location within the defined city centre retail and/or leisure areas under policy DM18 and other policies of this plan will need to be weighed against the relative risk of flooding if that location falls within a higher risk flood zone.
5.14 Where development is proposed other than in these specific locations or on sites which are specifically allocated in the Site allocations plan, the policy follows Environment Agency advice that in carrying out the sequential test, the search for reasonable alternatives should look across the whole of Norwich. In practical terms it is expected that the scope for locating development elsewhere will depend upon the nature and intended catchment area of the use proposed. For example if a proposed development in an area with some degree of flood risk was intended to serve an essential community need for residents within a particular neighbourhood of the city and would be acceptable in all other respects, it would not be reasonable to insist that it be located outside that neighbourhood if the flood risk could be adequately mitigated. In some situations it may be appropriate to consider the relevant neighbourhood area as the appropriate area of search, taking account of the advice of the Environment Agency. Flexibility will be applied in these cases to ensure that flood risk is considered alongside other needs and priorities within the locality to achieve the most appropriate development solution.
5.15 For the purposes of the sequential test, a site would not be considered to be a reasonable alternative if:
- it is developed or in the process of being developed;
- it has an extant planning permission for redevelopment or a resolution to approve;
- the owner has stated that there is no intention to develop the site within the next five years or the site is subject to a lease with an unexpired period of five years or more.
5.16 The city council’s validation checklist requires all proposals either on sites greater than one hectare or within areas at risk of flooding to provide a flood risk assessment identifying the scale of the flood risk, likely sources of flooding and flood risk mitigation and management measures.
5.17 The law relating to sustainable drainage is changing. Schedule 3 of the Flood and Water Management Act 2010 introduced standards for the design, construction, maintenance and operation of new rainwater drainage systems, and specifies that an ‘approving body’ will now be responsible for approving most types of rainwater drainage systems before any construction work with drainage implications can start. For Norwich the approving body will be Norfolk County Council as lead local flood authority. Upon full commencement of this “drainage permission” regime, sustainable drainage systems will become mandatory for most forms of development and both planning permission and SuDS approval body (SAB) approval will be required for drainage works. Notwithstanding the emergence of the new drainage approving role of the county council, the sustainable drainage section of this policy also retains a requirement for surface water drainage issues to be addressed in planning applications, both to ensure that surface water drainage issues are considered ahead of the commencement of the new regime and to ensure that the impact of drainage measures on the form and visual appearance of developments is properly taken into account in the assessment of new development, as both planning permission and SAB approval will be required for drainage works.
5.18 The SWMP highlights two particular zones within Norwich which are especially prone to surface water flooding (and which will flood in extreme rainfall events). These comprise a zone within the outer ring road between Unthank Road and Earlham Road to the west and south-west of the city centre and a zone running on a north-south axis from the outer ring road at Catton Grove Road/Oak Lane to the north end of the city centre at Magdalen Street. Both these areas coincide with the course of former streams which were tributaries of the river Wensum. Further modelling subsequent to the publication of the SWMP has defined the extent of the catchment areas which feed into these flood-vulnerable zones. These Critical Drainage Catchments are identified on the Policies map. Those parts of the city falling within the Critical Drainage Catchments will not necessarily flood in extreme rainfall events, but any development within them is likely to increase the risk of flooding in the most flood vulnerable areas without mitigation.
5.19 The modelling evidence supporting the SWMP thus provides justification for requiring new development in these areas to incorporate higher standards of flood resilience than are necessary elsewhere. Although these areas are already densely developed and may not offer many opportunities for major development, this policy requires surface water flooding issues to be fully addressed in flood risk assessments submitted with applications and flood-resistant design enhancements to be incorporated within any new development proposals which do come forward.
5.20 To prevent an increase in surface water flooding within these areas, all significant proposals involving new construction will be required to show how surface water flooding issues will be addressed and should include measures to protect against and reduce the vulnerability of the site and the wider area to the effects of surface water run-off. Appropriate measures may include the use of permeable surfaces, grey water recycling, green and brown roofs and walls, soakaways, water storage areas and water butts. Intending developers will be encouraged to consult and take account of best practice advice on this issue.
5.21 The policy allows for the emergence of more extensive technical evidence on surface water flood risk to be taken into account over the plan period, so that in areas or sites outside of the Critical Drainage Catchments where there is likely to be elevated risk of surface water flooding (e.g. as a result of specific topography) the same policy requirements would apply. The requirements of policy DM5 for proposals within the Critical Drainage Catchments are also reflected in the relevant site specific policies in the Site allocations plan.
5.22 The extensive use of permeable surfaces in all external areas can make a significant contribution towards sustainable drainage. Consequently, this is encouraged within all development which needs planning permission, including proposals for the paving of front gardens of domestic dwellings and more substantial areas of surfacing associated with commercial and other non-residential development. Amendments were made to the General Permitted Development Order in 2008 bringing the paving over of front gardens within the scope of planning control. Planning permission is now required for the paving of domestic front gardens with an area of over five square metres except in cases where permeable surfaces complying with the CLG and Environment Agency’s Guidance on the permeable surfacing of front gardens are used.
5.23 The scope for using permeable surfaces may be reduced where soils have poor infiltration capacity, where groundwater levels are high or where ground conditions present particular risks of subsidence from voids and instability in the underlying geology. Large areas of Norwich are built on chalk and some are especially prone to subsidence (see policy DM11). Where it is demonstrated that permeable surfaces are likely to be unacceptable for these reasons, hard surfaced paving may be accepted. In these cases developers will be encouraged to explore alternative means of managing surface water runoff within the development site. Where soils are well drained, impermeable surfaces will only be permitted where it is demonstrated that there is an overriding need for such a surface.
5.24 The technical data which determines soil infiltration capacity is not collected at a level of detail which enables it to be mapped at a large-scale, consequently it is not practicable to show these areas on the Policies map. An indicative infiltration capacity drainage map is instead provided in appendix 1 of this plan. As the map is indicative, it must be demonstrated on a case-by-case basis within all relevant areas of the city that permeable surfaces are not practicable. The city council will take account of any more detailed technical advice and/or mapped data that emerges over the plan period which enables a more informed judgement to be made on issues of drainage capacity in relation to this policy.
References
- NPPF CLG, 2012: Delivering sustainable development, presumption in favour of sustainable development; Section 10 – Meeting the challenge of climate change, flooding and coastal change: minimising vulnerability to climate change and managing the risk of flooding.
- National Planning Practice Guidance: CLG 2014: Climate Change; Flood Risk and Coastal Change.
- Norwich strategic flood risk assessment, Level 1 (Millard Consulting, 2008) and Level 2 (Hyder, 2010).
- Norwich Surface water management plan and associated technical studies (URS Scott Wilson, 2011).
- Provisions of the Flood and Water Management Act 2010.
- Greater Norwich integrated water cycle study, Scott Wilson, 2009.
- Guidance on the permeable surfacing of front gardens: CLG/Environment Agency, 2008.
