Flood Risk Assessment, Fairmead, Church Road, High Beach, Loughton, IG10 4AJ, for the Discharge of Planning Condition 10 on EPF/2903/15
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Flood Risk Assessment, Fairmead, Church Road, High Beach, Loughton, IG10 4AJ, for the Discharge of Planning Condition 10 on EPF/2903/15 1 Background ‘Fairmead’ is a single plot of some 1.47 ha situated in Epping Forest – see Figures 1 and 2. The site is currently occupied by a single detached house and several outbuildings that have previously been used for riding stables and a cattery. The proposed development is for the removal of many of the outbuildings, replacement of the existing house by a larger detached dwelling on Plot 3. The total area of Plot 3 is 7,650m2 (0.765ha). The Planning Application EPF/2903/15 has been granted permission by Epping Forest District Council (EFDC) in their letter of 16 January 2016), subject to a number of Conditions needing to be met before construction may begin. This report sets out to address Condition 10, which requires an assessment of any flood risk that might result from increased surface water runoff. The assessment will follow the guidance set out in the National Planning Policy Framework (Refs. 1 & 2) and policy U2B of the adopted Local Plan and Alterations (Ref. 3). Policy U2B requires a Flood Risk Assessment for any development in excess of 235m2, irrespective of whether the development is located within a flood risk zone or not. The incremental area of the new house is estimated to be approximately 270 m2. Location of site Figure 1. Location of proposed development 1 Figure 2. Plan for new house on Plot 3. 2 The site lies on the south-east side of the high ridge that runs between Lippits Hill and High Beach. A small ditch runs along the northern boundary of the site, crossing a culvert under Church Road, forming the headwater of a minor stream. This stream turns southwards and flows through the forest into Connaught Water, some 2km away to the south. The outflow of Connaught Water is the origin of the River Ching, which eventually joins the River Lee. As there is a large flood risk area associated with the main Lee valley, as part of their Strategic Flood Plan (Ref. 4), Epping Forest District Council requires a Flood Risk Assessment to consider the impact of additional flood runoff from any new development sites greater than 50 m2, to avoid creating any additional flood risk downstream, and propose any runoff control measures that may be required. In terms of geology – see Figure 3, the site is underlain by London Clay, but is located just south of its junction with overlying deposits of the Claygate member, which is formed of sands and gravel. The higher parts of the ridge are formed of Bagshot Sands, which are overlain by superficial (Pleistocene) deposits of the Woodford Gravel Formation of sands and gravel. All the strata of the ridge are permeable to a greater or lesser extent, and being underlain by the impermeable London Clay, this will result in groundwater from the ridge appearing at the surface as springs and minor streams, the ditch to the east of the site being an example of these conditions. Woodford Gravel Bagshot Sands Claygate mem. Development site London Clay Figure 3. Superficial and bedrock geology (from BGS Onshore Viewer) 3 2 Existing Flood Risk The site lies entirely within Flood Risk Zone 1, as defined by the Environment Agency (EA), and so has a risk of flooding of 0.1% or less, i.e. 1 in 1,000-years or less frequently from rivers (fluvial flooding). Figure 4, obtained from the EA website, shows that the closest flood risk area, which is defined as Food Risk Zone 2, occurs downstream of Connaught Water, 2km away. This is a narrow strip associated with the course of the stream: as defined by the EA, Flood Risk Zone 2 has a probability of flooding between 1% and 0.1%, i.e. an average return period between 100 and 1,000 years Development site Connaught Water Flood Zone 2 Figure 4. Flood Risk Zones in the Fairmead area. (Source: Environment Agency Flood Map for Planning) The EA also provides maps of flood risk due to surface water flooding, sometimes referred to as ‘Pluvial’ flooding, i.e. direct runoff from heavy rainfall. By definition, these types of events occur in relation to intense, localised and short duration rainfall. Areas susceptible to pluvial flooding in the vicinity of the proposed development are shown in Figure 5. Pluvial flooding is closely associated with the minor watercourses in the headwater network that feeds the small stream flowing into Connaught Water. These areas are entirely within the forest area, and do not affect any dwellings. The shading used in Figure 5 grades from the dark blue along the line of the streams, indicating a ‘high’ flood risk, (1 in 30 years), to the light blue areas which are defined as ‘low’ flood risk (1 in 100 years or longer). The small area associated with the culvert passing under Church Road in front of the development is defined as having a ‘medium’ level of flood risk, i.e. between 1 in 30 and 100 years. The scale of the map makes it difficult to ascertain whether or not surface water flooding would extend onto the road adjacent to the culvert, but would not encroach upon the sites of the new houses. The culvert, which is in good condition and maintained by Corporation of London, was inspected on 3 February at the end of a long wet period, and was functioning well. The owner of the site has experienced some surface water flooding from ridge to the west of the existing house, and already has plans in hand for the construction of a system of field drains to direct 4 flow downslope towards the ditch. Development site Connaught Water Figure 5. Areas susceptible to surface water (pluvial) flooding. (Source: Environment Agency Flood Map for Planning) 3 Nature of the Development The proposed development is for a single detached house with a detached garage and a stable block, along with a driveway and parking and hard standing areas, as shown in Figures 2. The existing house on the Plot 3 site will be entirely replaced. The outlines of the existing house and various outbuildings are also shown in Figure 2. The breakdown of the footprint for the house, stable and internal access is as follows. House, garage and stable structures 450 m2 Driveways (3No) & hard-standing. (‘Forecourts’ on map) 320 m2 Thus the total footprint of the development will be 770 m2. Scaling from the architect’s drawings, the footprint of the existing buildings on the site is approximately 182 m2, and the site of the former house will be mostly taken up as the forecourt of the new house and garage. Thus the incremental footprint will be in the order of 500 m2, but the net effect of the rebuilding on runoff generation will be reduced by the nature of the building and other hard areas. The calculation of runoff impacts is dealt with in the following section. 4 Estimation of runoff generated by the development Initial estimates for the appropriate design rainfall and runoff rates have been obtained using the UK Sustainable Drainage – Guidance and Tools website (Ref. 5), which is a spreadsheet version of the long-established Institute of Hydrology Report IH 124 (Ref. 6) for estimating runoff. The results obtained by entering the location, size, rainfall and soil type into the spreadsheet are shown in Appendix 1. The calculation method gives a total site (0.77ha) Greenfield runoff estimate of 0.19 l/s for Qbar1, and a 100-year estimate of 0.60 l/s, a runoff growth factor of 3.19*. 1 In IH-124, the Greenfield rate for lowland SE England is given as 5.6 l/s/ha, based on the M5-1-hour rainfall 5 *Note that the growth factor for rainfall is different from that of runoff, primarily because runoff takes into account changes in catchment saturation with probability, which cannot be assigned to hard surfaces. The estimate for the required surface water storage is based on the new hard surface cover of 776m2 (0.078 ha), using the same catchment descriptors and rainfall inputs as for the Greenfield calculation. Qbar is increased to 0.38 l/s, and a total storage requirement of 28.88m3 has been estimated. The results of the calculation sheet are reproduced in Appendix 1, and a summary is given in Table 1. Frequency Runoff (l/s) Storage type Storage volume (m3) Qbar 0.38 Interception 3.20 1 in 1 year 5.00 Attenuation 25.68 1 in 30 year 5.00 Treatment 9.60 1 in 100 year 5.00 Total 28.88 Table 1. Surface water storage requirements (m3) and runoff (l/s) for the proposed development at Plot 3, Fairmead, High Beach However, the Guidance to the SuDS calculator recommends that a minimum discharge rate of 5 l/s be used as a minimum flow rate for any site and this rate is also recommended in the Code for Sustainable Homes (Ref. 7). ‘Hard’ areas do not produce 100% runoff from rainfall, but involve some losses from detention and evaporation, so runoff rates from the current built on area will nominally be taken as 80%. In the case of developments, the EFDC Planning policy follows the current recommendation of the Environment Agency to add a factor to design rainfall estimates to allow for assumed increased rainfall depths and intensities in the future. An additional 40% is recommended in the latest guidance issued by the EA in February 2016 (Ref.