Flood Risk Assessment, Fairmead, Church Road, High Beach, , 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 – 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 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 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. 8) though this may seem excessive for a small development such as in this case.

In accordance with the latest recommendations for rainfall and runoff management produced by the Environment Agency (Ref. 9), the drainage design will incorporate provision for longer duration storms. The design of various components of the runoff management system will use the latest estimates for rainfall depth-duration-frequency available on the FEH Web Service (www.FEHweb.ceh.ac.uk). This data source uses the original FEH CD-ROM method and data, updated to 2013 (Ref. 10). The output from the web service provides comprehensive rainfall depth-duration- frequency estimates for the 1 km grid square enclosing the site. The detailed drainage design plan will need to meet the requirements to accommodate the 1-hour duration events and the 6-hour event. Results for key return periods (frequencies) are given in Table 2. The up-rating of the 100-year rainfall by 40% in Table 2 is as recommended by the latest EA/Defra guidance (Ref. 8).

Frequency (Return Period) Duration 1 hour 6 hour 5 years 19.0 37.0 30 years 33.3 56.7 100 years 44.2 76.4 100 years + 40% (CC) 61.9 107.0

Table 2. Design storm rainfalls (mm) for a range of durations and frequencies Note. These statistics are somewhat different from those generated by the UK-SuDS calculator, as they are using an updated, expanded and re-worked dataset. A separate detailed drainage design plan will be submitted, but at this stage it is envisaged that estimate, which at the time of publication was 21mm-22mm. In Table 2 the runoff rate for the 5-year events use the rainfall multiplied by area, rather than Qbar, which has a return period of 2.33 years.

6 additional runoff from the new development can be accommodate by an on-site piped drainage system which discharges into an existing pond on the site. It is not considered that soakaways will provide an effective solution due to the clay subsoil. It will be necessary to arrange for an overflow, especially in the conditions of a 100-year storm in the future. The pond within the site would provide an ideal facility to receive excess runoff. It has a surface area of 183m2, and thus a good capacity for storage. As an example, the total runoff volume from roof areas in the 100-year + climate change storm is 48m3: its entry into the pond would cause a level rise of 0.26m. Any outflow from the pond to the stream in longer duration severe events can be through a formal overflow structure.

Given appropriate drainage and storage arrangement, the resulting runoff rates from the site would be trivial at the 1 in 5year level. Even at the 100-year + climate change level, the estimated runoff rate is at the de minimus rate of 5 l/s, and will have no significant effect on downstream flow conditions. There are no river gauges on the River Ching with which to compare these flow rates. The nearest similar catchment is Cobbin’s Brook which drains the north side of Epping Forest and includes a larger lowland catchment. The river has been measured at Sewardstone Bridge since 1971. Key data extracted from the National River Flow Archive (NWFA, http://nrfa.ceh.ac.uk) are as follows:

Catchment area 38.4km2 Qmed 7.57m3/s Peak recorded flow 35m3/s

Converting these flow rates to specific flows (Q/A) gives the following; Qmed 19.7 l/s/ha Peak flow 91.1 l/s/ha

Thus the increased flow rates from the redevelopment are insignificant when compared with catchment- scale discharges. The requirements of the National Planning Guidance and its associated Technical Guidance (Refs. 1 and 2) require that flood management should endeavour to control flood runoff to at least the 100-year level. This remains essentially the same as in the requirements given in PPS25 (Ref. 11). Runoff control on the site is aimed at limiting discharge and delay entry of the additional runoff into the watercourse leading from the site into the stream that eventually reaches Connaught Water.

5 Concluding remarks The approved Planning Application entails the redevelopment and removal of the existing buildings on the site. The site lies within the EA defined Flood Risk Zone 1, thus having a low probability of flooding. The EA information suggests that some localised risk from surface water (pluvial) flooding exists, but it is confined to the line of existing watercourses, and does not affect the planned position of the proposed new house. However, in order to discharge Planning Condition ? as required by EFDC, the possible impact of increased runoff downstream has to be considered. Comparison of plans of existing and proposed hard surfaces has shown that a net increase of approximately 500m2 in hard surfaces will result.

Runoff rates and volumes for the 1 in 5 year and 1 in 100 year (+ climate change adjustment) design storms have been estimated, and additional runoff rates and volumes produced would be very small, and have minimal impact to downstream areas. As a preliminary to the development of a detailed drainage design and runoff management plan, estimates of current Greenfield runoff rates and runoff and storage requirements has been provided, using the UK-SuDS calculator. The capacity of the SuDS system and the use of an existing pond on the plot should effectively contain all runoff, except in the most extreme conditions. In addition, there are no buildings or dwellings immediately downstream of the site that might be affected by any additional runoff, and the eventual receptor of any additional flows will be Connaught Water, which has a capacity for storage far in excess of the volumes generated form the site.

It is therefore my conclusion that successful runoff management facilities for the proposed site will be fully effective and the relevant Conditions can be discharged.

7 James E Dent, B.Sc., FRMetS, C.Met., C.Env., Inst. of Water (Corporate Member) September 2016

8 References

1. National Planning Policy Framework. Department for Communities and Local Government, March 2012.

2. Technical Guidance to the National Planning Policy Framework. Department for Communities and Local Government, March 2012.

3. Epping Forest District Local Plan. Local Plan Alterations, Adopted July 2006.

4. Strategic Flood Risk Assessment – Map 4 Flood Risk Zones. Epping Forest District Council 2011.

5. UK Sustainable Drainage – Guidelines and Tools. HR Waliingford. www.uksuds.com/greenfieldrunoff_js.htm

6. Code for Sustainable Homes – Technical Guide. Communities and Local Government, November 2010.

7. Flood Estimation for small catchments. Report No. 124, Institute of Hydrology, 1994.

8. Flood risk assessments: climate change allowances. Environment Agency, February 2016. (Update to: “Adapting to climate change – advice for flood and coastal erosion risk management authorities”. EA/Defra Aug 2013.)

9. Rainfall runoff management for developments. Report SC030219, Defra/Environment Agency, October 2013.

10. Flood Estimation Handbook (FEH), Institute of Hydrology, Wallingford, 1999.

11. Planning Policy Statement 25 (PPS25): Development and Flood Risk. Dept. of Communities and Local Government, December 2006.

9 APPENDIX 1

UK SuDS Spreadsheet Calculations

1. Greenfield Runoff Estimation

10 2. Surface Water Storage requirement

11