Technical Note Strategic Flood Risk Assessment Strategic Flood Risk Assessment in Respect of the Midlothian Local Development Plan Main Issues Report 2013
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Midlothian Local Development Plan Main Issues Report 2013: Technical Note Strategic Flood Risk Assessment Strategic Flood Risk Assessment in respect of the Midlothian Local Development Plan Main Issues Report 2013 Note: Appendix 4 explains abbreviations 1 Background 1.1 The Scottish Environment Protection Agency (SEPA) encourages planning authorities to make use of Strategic Flood Risk Assessment (SFRA) to consider flood risk matters early on in the development plan process. Under the Flood Risk Management (Scotland) Act 2009 there is a general duty upon local authorities when carrying out their functions, to act with a view to reducing flood risk. 1.2 An SFRA is designed for the purposes of specifically informing the development planning process, and to assist flood risk reduction by avoiding areas at significant risk of flooding. Development should not take place on land that could otherwise contribute to managing flood risk. An SFRA involves the collection, analysis, and presentation, of all existing and readily available flood risk information for the area of interest. It constitutes a strategic overview of flood risk, without necessarily meeting the reporting requirements of a detailed Flood Risk Assessment (FRA). SEPA expect SFRAs to be principally desk-based studies, making use of existing sources of information. However, in some instances, greater detail may be required to inform the Development Plan. Part of an SFRA could be the identification of priority areas for more detailed analysis. 1.3 The SFRA should be used to apply the risk based approach to the identification of land for development (in Midlothian the sites assessment process has anticipated this task: see Development Sites Assessment Technical Note for details) and for the development of policies for flood risk management, including promotion of surface water management. The SFRA should be prepared in consultation with SEPA and other interested parties. 1.4 Midlothian Council expects to update this SFRA as the Midlothian Local Development Plan (MLDP) proceeds to adoption, taking into account any new information about the nature of flooding and flood defences, and reflecting changing proposals for the use of the land. Interested parties will be invited to respond to this SFRA for the MLDP Main Issues Report (MIR), which indicates where Midlothian Council’s knowledge is incomplete. Future SFRAs may benefit from work being undertaken in connection with the Local Flood Risk Management Plan for the area. 1.5 Key elements of an SFRA are as follows: • Information on all potential sources of flooding • Information on climate change impacts 2 • Information on existing flood defences • Identification of the functional flood plain (built-up areas and sparsely developed) • Identification of relevant drainage issues • Identification of sites or areas constrained by flood risk 2 Information on all potential sources of flooding 2.1 The SFRA should take account of all sources of flooding. SEPA guidance (SFRA - SEPA technical guidance to support Development Planning) on sustainable flood risk management identifies the following primary sources of flooding: • River (fluvial) flooding - this occurs when the water draining from the surrounding land exceeds the capacity of the watercourse. • Coastal flooding - this is not applicable in Midlothian. • Surface water (pluvial) flooding - this is caused when rainfall water ponds or flows over the ground before it enters a natural or man- made drainage system or watercourse, or when it cannot enter the drainage system because the system is already full to capacity. • Sewer flooding - this occurs when combined sewers are overwhelmed by heavy rainfall. • Groundwater flooding - this occurs when water levels below ground (i.e. in soils, sands and gravels or rock formations) rise above surface levels. • Reservoir flooding and flooding from other infrastructure - Although unlikely, failure of infrastructure such as dams or canals could result in a large volume of water being released quickly. Taking these potential flood sources in turn: 2.2 River Flooding. The SEPA Indicative River and Coastal Flood Map (Scotland) published in 2006, broadly defines the area at varying degrees of annual probability of flooding: 1:100, 1:200 and 1:1000. The notation refers to the chance of flooding happening over a time period of one year – another way of expressing it as a percentage probability. For the probabilities set out above the risk of flooding over a period of 1 year are 1%, 0.5% and 0.1% respectively. These parameters allow decisions to be made with reference to the risk framework set out in Scottish Planning Policy (SPP) – set out in Appendix 2. 2.3 The Indicative River and Coastal Flood Map (Scotland) (or IRCFM) is just that – indicative. The IRCFM has been produced following a consistent, nationally applied methodology for catchment areas equal to or greater than 3km2 using a Digital Terrain Model (DTM) to define river cross sections and low lying coastal land. The outlines do not account for flooding arising from sources such as surface water runoff, surcharged culverts or drainage systems. The methodology was not designed to quantify the impacts of factors such as flood alleviation measures, buildings and transport infrastructure on flood conveyance and storage. The IRCFM is not a substitute for a Flood Risk 3 Assessment, but it is useful for indicating where flood risk may be a concern, and where further study may be required. The IRCFM used climate inputs based on the observed position when it was produced: it does not include an allowance for future climate change. This matter is considered further in the section on climate change below. 2.4 The IRCFM is used to indicate where flooding may pose a problem, and is often a trigger for further investigation, including a full Flood Risk Assessment (FRA). The SEPA –Planning Authority Protocol (Policy 41) contains principles to be followed by SEPA and planning authorities regarding advice and consultation on flood risk issues. The Town and Country Planning (Development Management Procedure) (Scotland) Regulations 2008 require that planning authorities must, before determining an application for planning permission for development, consult with SEPA where the development is likely to result in a material increase in the number of buildings at risk of being damaged by flooding (Schedule 5.1(1)). The SEPA paper, Technical Flood Risk Guidance for Stakeholders, considers different types of FRA methodology and what information SEPA requires to be submitted as part of an FRA. The extent of the indicative flood risk zones may increase over time if, as predicted, climate change induces more intensive rainfall, so there is a potential gap arising from use of the current IRCFM. Future SFRAs may benefit from any revision to the IRCFM which takes future climate change impacts into account (set out in Table 1, below). 2.5 SEPA has prepared an assessment of candidate development sites being considered for inclusion in the preferred development strategy of the Main Issues Report (MIR): this represents their best available knowledge, and is most developed with regard to river flooding. Section 7 of this SFRA, considers site specific matters. 2.6 Midlothian Council has commissioned no flood risk mapping for rivers. It logs flood incidents in its area as part of its responsibilities under flood prevention legislation. A summary of all events recorded in the Council’s biennial flooding reports is set out in Appendix 1. Most of these incidents relate to surface water flooding, but have also arisen from temporary obstructions in small watercourses, particularly where they run through culverts. The Council’s flood prevention work has been focussed on repair and maintenance, de-sedimentation, and fitment of screens to assist in avoiding future incidents. The existing local plan (Midlothian Local Plan 2008) contains detailed development policies relating to wayleaves around watercourses to aid maintenance, and the Council will review these, to ensure they meet current best practice. The Council has replaced culverts with new pipes and enhanced localised drainage capacity to remedy identified flood problems. The work to produce the Forth Estuary Local Flood Risk Management Plan (LFRMP) may generate additional information on river flooding, which will inform future SFRAs (more background on LFRMPs is set out in Section 8). 2.7 Where development raises flood risk concerns, a Flood Risk Assessment (FRA) is required. The coverage of these is haphazard, reflecting the distribution of previous development proposals. These can be of use for the SFRA, where one has been carried out at or near a 4 location that is the subject of continuing interest in the MLDP process. Where the Council has additional information in respect of a site, this is indicated in the site assessment section (7). Appendix 3 contains a summary of FRA activity. 2.8 Surface water (pluvial) flooding. Surface water flooding can occur where rainfall overwhelms local drain systems: this form of flooding can be particularly exacerbated by the topography, where rainfall runs off steeply sloping ground before it can be absorbed. Most of the incidents logged in Appendix 1 are remote from watercourses, and are the result of surface water flooding. Surface water flooding is often caused by temporary and localised obstructions in the drainage system and can be resolved by maintenance, although redesign to include greater resilience is an alternative solution pursued by the Council (para 2.6 refers). 2.9 SEPA has developed a national pluvial flood map which identifies areas susceptible to surface water flooding, although at present this is not available to local authorities. The national pluvial flood map may be used during the next review period. The impact of pluvial flooding, where known or inferred to be problematic, is considered in the site assessment section. New developments are likely to be accompanied by changes in the landform and the installation of a new drainage system, based on the principles of Sustainable Urban Drainage Systems (SUDS).