FLOOD CONSEQUENCE ASSESSMENT FOR A PROPOSED RESIDENTIAL DEVELOPMENT SITE ADJACENT TO GLANDULAS FARM, NEWTOWN, POWYS

Transcription

1 FOR A PROPOSED RESIDENTIAL DEVELOPMENT SITE ADJACENT TO GLANDULAS FARM, NEWTOWN, POWYS Report Reference: 218/FCA Draft: Version D1 July 2016 Report prepared for: Berry Brothers Willow House East Shrewsbury Business Park SHREWSBURY Shropshire SY2 6LG B a r k e r s C h a m b e r s B a r k e r S t r e e t S h r e w s b u r y U n i t e d K i n g d o m S Y 1 1 S B T : F : E : i n f h a f r e n w a t e r. c o m

2 GENERAL NOTES Title of report: Flood Consequence Assessment for a proposed residential development Site: Site adjacent to Glandulas Farm, Report ref: 218/FCA Date: July 2016 Version Date Issued to Draft version D1 July 2016 Berry Brothers Author: Reviewer: Rebecca John BSc FGS Chris Leake BSc MSc FGS This report has been prepared by Hafren Water Ltd for the named Client, with reasonable skill, care and diligence within the agreed scope and terms of contract. Hafren Water Ltd disclaims any responsibility to the client and others in respect of any matters outside the agreed scope of work. This report has been prepared for use by the client and others acting on their behalf. The report may be passed to regulators. This report does not constitute legal advice or opinion. This report does not represent advice to third parties and no reliance is offered to third parties. No liability is accepted with regard to third parties. Reliance required by any specific Third Party must be agreed in writing with Hafren Water Ltd. P:\Projects\ (218)\Reports\FCA\Draft\218-FCA vn D1 (Jul 16).docx July 16 Page i

3 CONTENTS 1 INTRODUCTION Background Data sources Technical Advice Note 1 (TAN1) Development and Flood Risk Local policy Regulatory requirement for this assessment BASELINE CONDITIONS Location and setting Topography Hydrology Watercourses Waterbodies Culverts Ground conditions Proposed development APPROACH TO THE FLOOD RISK ASSESSMENT Flood risk Climate change Flood receptors Receptors internal to the site Receptors external to the site Design events and flooding pathways MODELLING CARRIED OUT BY THE LEAD LOCAL FLOOD AUTHORITY... FLOOD RISK TO SITE Background Fluvial flooding Surface water flooding Groundwater flooding Flooding from sewers and water mains FLOOD RISK FROM THE SITE TO THE SURROUNDING AREA Fluvial flooding Surface water flooding MITIGATION MEASURES Background Methodology for calculation of run-off and storage volume Greenfield run-off rates Run-off rates for the developed site Mitigation measures Surface water run-off: external receptors Adherence to Local Policy Objectives Natural Resources Wales flood warning service July 16 Page i

4 8 SUMMARY AND CONCLUSIONS TABLES 218/FCA/T1: Recommended increases in rainfall parameters due to climate change /FCA/T2: Rational method run-off co-efficient values /FCA/T3: Site surface areas /FCA/T4: Run-off volumes and attenuation storage DRAWINGS 218/FCA/01 218/FCA/02 218/FCA/03 218/FCA/04 Site location plan Water features TAN1 Development Advice Map: Flood Map Natural Resources Wales surface water flood map APPENDICES 218/FCA/A1 218/FCA/A2 Indicative development layout Run-off calculations July 16 Page ii

5 1 INTRODUCTION 1.1 Background A Planning Application has been prepared for a proposed residential development at a site adjacent to Glandulas Farm in. The site comprises 2 areas; one to the north of the A489 and one to the south. The location of the site is shown on Drawing 218/FCA/01. The Application Area has a total area of approximately 6.2 hectares (ha); the northern area being 1.34 ha and the southern 4.89 ha. The Technical Advice Note 1 (TAN1) Development Advice Map (DAM) shows that the Application Area is located mostly within Flood Zone A, where land is considered to be at little or no risk of fluvial or tidal/coastal flooding. The eastern boundary of both parts of the site (north and south) is adjacent to the Mochdre Brook and there is an area adjacent to this classified as Flood Zone C2; this being defined as floodplain without significant flood defence infrastructure. However, it should be noted that all of the proposed development is located within Flood Zone A. Hafren Water has been commissioned to undertake this Flood Consequence Assessment (FCA) as part of the Planning Application. 1.2 Data sources The following data sources were used in this assessment: Berry Brothers Indicative site layout Ordnance Survey (OS) (emapsite for digital maps) 1:2,000 and 1:0,000 series mapping British Geological Survey (BGS) Geological information (BGS website) Natural Resources Wales (NRW)/Environment Agency (EA) Flood risk maps (NRW website) for flooding from rivers and surface water Development Advice Maps (DAM) Cranfield Soil and Agrifood Institute (ukso.org/maps website) Soils map July 16 Page 1

6 Powys County Council (PCC) Strategic Flood Consequence Assessment Stage 1 (March 2012) Strategic Flood Consequence Assessment Stage 2 (September 2013) 1.3 Technical Advice Note 1 (TAN1) Development and Flood Risk This FCA has been undertaken with due regard to the statutory requirements of TAN1 in relation to development and flood risk, to ensure that flood risk is taken into account at all stages of the planning process and to avoid inappropriate development in areas potentially at risk of flooding. TAN1 classifies the flood risk vulnerability of sites used for residential developments as highly vulnerable development. 1.4 Local policy Powys County Council (PCC) is the Lead Local Flood Authority (LLFA) and local Planning Authority for the site. The PCC Stage 2 Strategic Flood Consequence Assessment (SFCA) states in Section 6.2: The following policy objectives are recommended for all sites that may come forward for development within Powys: Ensure no increase in flood risk elsewhere Development may increase run-off and/or reduce the existing storage capacity afforded by pre-development topography. The aim should therefore be to ensure that a development does not increase flood risk elsewhere. This would ordinarily be assessed via a site-specific flood consequence assessment. Identify opportunities to reduce existing flood risk In areas identified at risk of flooding (fluvial, surface water) or where a watercourse has insufficient channel capacity opportunities to improve existing flood risk by using SuDS, wetlands or other agreed and appropriate measures as well as reducing run-off rates should be investigated and implemented wherever possible. A sequential approach and use of the Justification Test - Use the principles of TAN1, taking a sequential approach to steer development to Flood Zone A. Where this is not always possible, the Justification Test will need to be applied. July 16 Page 2

7 Protect the functional floodplain (in greenfield and previously developed areas) Avoid development in the greenfield functional floodplain in the first instance. Identify opportunities for making space for water on previously developed areas by reinstating the functional floodplain. Site layout - Apply a sequential approach within the development site by locating the most vulnerable elements of a development in the lowest flood risk / hazard areas in the first instance. The use of Flood Zones C2 areas should be primarily retained for recreation, amenity and environmental purposes, which can also provide an effective means of flood risk management as well as providing connected green spaces with consequent social and environmental benefits. Set development back from watercourses - Any developments located adjacent to a watercourse should leave an appropriate undeveloped buffer strip, maintaining the watercourse and the immediate riparian zone as an enhancement feature and allowing for routine maintenance. The width of any buffer strip should be agreed with the relevant authorities on a site by site basis. Reduce surface water run-off from new developments Any development must ensure that post development run-off volumes and peak flow rates are maintained at either the Greenfield rate (for greenfield sites) or deliver a 0% reduction to surface water run-off rates for brownfield sites (up to and including the 1 in 100-year event inclusive of an appropriate allowance for climate change for both development scenarios). SuDS should also be a requirement for all new development and space should be specifically set aside for SuDS and used to inform the overall site layout. Hardstanding areas should be kept to a minimum and infiltration techniques and the re-use of water should be considered before attenuation devices in accordance with the SuDS hierarchy. SuDS will need to have a maintenance strategy to ensure they are maintained and working efficiently. Subject to commencement of the relevant Schedule of the Flood and Water Management Act 2010, the Lead Local Flood Authority as SuDS approval body, should be consulted to confirm the appropriate requirements and specification of SuDS components that are to be adopted. The policies outlined above have been reviewed and the FCA and recommendations therein have been compiled in accordance with the relevant objectives. July 16 Page 3

8 1. Regulatory requirement for this assessment The site is mostly classified as Zone A as defined in TAN1 and shown on Welsh Government s DAM, where the probability of fluvial flooding in any one year is less than 1 in 1,000 (Annual Exceedance Probability, AEP <0.1%). A smaller area of the site along the eastern edge is bounded by Mochdre Brook, and is designated as Flood Zone C2. This is an area where the probability of fluvial flooding in any one year is 1:100 (AEP 1%). An FCA is required in accordance with TAN1. July 16 Page 4

9 2 BASELINE CONDITIONS 2.1 Location and setting The Application Area ( the site ) is situated approximately 1.6 km southwest of the centre of. It has a total area of approximately 6.2 ha and is bisected by the A489 trunk road. The centre of the site is located at National Grid Reference (NGR) SO (Drawing 218/FCA/01). The site is currently used to graze livestock, with surrounding land used for agriculture, a caravan park and an industrial estate. 2.2 Topography The site lies within the valley of the River Severn and is generally low-lying, with an elevation of between 120 and 2 metres Above Ordnance Datum (maod). Land to the north declines towards the River Severn, which is situated at an elevation of approximately 0 maod. The south area of the site is bisected by a field boundary; comprising a 2 m wide hedgerow. Ground to the west of the hedgerow, where the proposed development will be sited, is higher than that to the east. To the east of the hedgerow the ground is undulating, but overall gently decreases in elevation to the Mochdre Brook. 2.3 Hydrology The hydrological characteristics of the site and its environs have been derived from a site visit (8 th July 2016), Ordnance Survey maps, the Natural Resources Wales (NRW) website and GoogleEarth aerial imagery. Water features are shown on Drawing 218/FCA/ Watercourses The Mochdre Brook forms the eastern boundary of the north and south areas of the site (see Drawing 218/FCA/02). It is an incised stream that flows northwards to its confluence with the River Severn. The River Severn flows in a broadly eastern direction approximately 16 m north of the northern area. The Llyfnant Brook flows eastwards to join the Mochdre Brook 900 m to the southwest and upstream of the site. Further to the southwest the Cwm-y-rhiwdre Brook flows northwards to join the Mochdre Brook upstream of the site, some 1.4 km distant. July 16 Page

10 2.3.2 Waterbodies Waterbodies are not present within the site boundaries. The nearest waterbody to the site is located 00 m to its north. This is a small pond adjacent to houses at Bryn-hyfryd. A second small pond is located approximately 720 m to the south of the site. It has an outflow that joins Mochdre Brook to its east Culverts A concrete culvert (Culvert 1) is located on the east side of Mochdre Brook, adjacent to Mochdre Industrial Estate. Due to its location it is assumed that this culvert discharges surface water run-off from the adjacent Mochdre Industrial Estate to the Mochdre Brook. 2.4 Ground conditions The solid geology beneath the site comprises the Nantglyn Flags Formation consisting of mudstone, siltstone and sandstone of Silurian age. Information from a borehole within the vicinity of the site (information from online BGS Geology Viewer) describes the local geology as medium black/grey shale. The Nantglyn Formation is understood to be designated as a Secondary aquifer; these are predominately lower permeability layers which may store and yield limited amounts of groundwater due to localised features such as fissures, thin permeable horizons and weathering. The superficial geology on-site comprises Alluvial/Fan deposits (Quaternary) which consist of sand, gravel and clays adjacent to the Mochdre Brook. These deposits are understood to be designated as a Secondary aquifer; these are permeable layers capable of supporting water supplies at a local rather than strategic (regional) scale, and in some cases forming an important source of base flow to rivers. According to the classifications given by the Cranfield National Soils Resources Institute, the predominant soil on-site is free-draining, slightly acid, loamy floodplain soils. July 16 Page 6

11 2. Proposed development The proposed development comprises a residential development of 89 houses, with associated landscaped public open space and areas of tree planting. Within the northern parcel of land it is proposed to build 30 houses and within the southern area it is proposed to build 9 houses. Further details of the proposed development are included elsewhere within the Planning Application. July 16 Page 7

12 3 APPROACH TO THE FLOOD RISK ASSESSMENT 3.1 Flood risk The FCA considers the likelihood of flooding, the associated hazards and the vulnerability of the flood receptor using a mixture of quantitative and qualitative methods. These factors are combined to produce the single measure, flood risk. Requirements for assessment of flood risk are addressed by due consideration of TAN1, DAM maps and components of flood Management Strategy (see Section 1.4 of this report) as required by Powys County Council (the LLFA). 3.2 Climate change Within the UK, projections of future climate change indicate that there will be more frequent, short duration, high intensity rainfall events and periods of long duration rainfall. TAN1 recommends that the effects of climate change are incorporated into Flood Consequence Assessments. Recommended precautionary sensitivity ranges for peak rainfall intensities and peak river flows are outlined in the Environment Agency document, Adapting to climate change: Advice for flood and coastal erosion risk management authorities (April 2016) and are summarised in Table 218/FCA/T1, below. 218/FCA/T1: Recommended increases in rainfall parameters due to climate change Peak rainfall intensity - applies across all of England 2020s (201 to 2039) Years 200s (2040 to 2069) 2080s (2070 to 2) Upper estimate +10% +20% +40% Central estimate +% +10% +20% Appendix 2 of TAN1 suggests a 20% increase in the 100-year return period flood peak discharge. As such, a precautionary sensitivity range for peak rainfall intensities of 20% for climate change has been used to calculate run-off rates and volumes for the site, given the lifetime of the development (see calculation sheets in Appendix 218/FCA/A2). 3.3 Flood receptors Receptors internal to the site At present the Application Area is used for agricultural purposes to graze livestock. The flood vulnerability class of these areas, as defined in the TAN1, is less vulnerable. July 16 Page 8

13 The proposed site s use will be for residential development, and therefore post-development the site would be classified as highly vulnerable Receptors external to the site Glandulas Farm is located immediately to the south of the site. Glandulas Caravan Park borders the southern area to the west. Glan Hafren Hall is located approximately 21 m to the northwest of the northern area. These properties form the highly vulnerable receptors within the locality of the site. This level of vulnerability increases the potential severity of the consequences of flooding for these receptors. The overall degree of flood risk may thus be higher for such receptors for a given severity of flood event. To the east of the site, on the opposite side of Mochdre Brook is Mochdre Industrial Estate comprising light industrial use. This area forms a less vulnerable receptor. Other receptors outside the site, comprising roads and agricultural fields, are all classed as less vulnerable. 3.4 Design events and flooding pathways As required by TAN1, the return period to be considered for fluvial and rainfall events is 100 years and the effects of climate change will be factored in as described in Section 3.2 of this report. This design event is referred to as the climate-changed, 1% AEP event where AEP means Annual Exceedance Probability. This FCA considers the following hydrological components: Fluvial flow Surface water run-off Groundwater flow Sewer and/or water mains leakage Pathways for flooding may involve two or more such components in sequence, such that the type of flow at the source or site of perturbation is not the same as that delivering flooding to the receptor. Within this FCA, flood risk to both internal and external receptors is assessed with reference to interactions between the development site and the hydrological components itemised July 16 Page 9

14 above. For internal receptors this gives information on the degree of flood hazard and hence the degree of flood risk. In the case of external receptors, flood hazard and hence flood risk during the design events is not evaluated. Instead the potential for the development to qualitatively increase or decrease flood risk at external receptors is assessed so that targeted measures to ensure a qualitative reduction can subsequently be taken, if necessary. July 16 Page 10

15 4 MODELLING CARRIED OUT BY THE LEAD LOCAL FLOOD AUTHORITY Powys County Council has carried out a Stage 2 Strategic Flood Consequences Assessment (SFCA) that builds on work carried out in their Stage 1 SFCA. The Stage 2 SFCA provides an overview of the level of flood risk for a number of Candidate Sites considered for development and inclusion within the Powys County Council (PCC) Local Development Plan (LDP). The Level 2 SFCA provides evidence to inform PCC (the Local Planning Authority (LPA)) of the potential consequences of flooding to development, and allow a sequential and risk based approach to site development. Of the 30 Candidate Sites (CS) that were considered, CS 92 is adjacent to the northern part of the site being considered as part of this FCA. As such, it is pertinent to outline findings of the Stage 2 SFCA flood modelling for CS 92 in relation to this Planning Application. Both the Application Area and CS 92 area are located mostly in Flood Zone A, with a smaller margin of Flood Zone C2. Flood modelling for CS 92 was carried out for the following flood return periods: 1 in 20, 1 in 100, 1 in 100 (+ climate change) and the 1000-year event. Using these flood return events allows modelling results to be correlated with DAM and TAN1 information. The hydraulic (fluvial) model comprised a new 2D TUFLOW model and enabled flood hazard mapping of the floodplain to be completed. Results from modelling indicate that the area of Zone C2 flood risk along the eastern boundary of the Application Area is affected by the 1 in 20, 1 in 100, 1 in climate change and 1 in 1000-year return period. The recommendations for CS 92 are: Acceptable management of flood consequences to be demonstrated in a site-specific FCA. Access to the site - even for areas outside of the modelled flood extents needs to be carefully considered. The LPA will need to be satisfied that access/egress to the site can be achieved in line with guidance set out in TAN1. Recommend consultation with Emergency Services/ Emergency Planners re: access/egress. July 16 Page

16 FLOOD RISK TO SITE.1 Background The risk of flooding at the site has been assessed by examining the likelihood of flooding, the hazard caused if it were to flood and the site s vulnerability. This has been undertaken for a range of likely mechanisms using both quantitative and qualitative methods. In terms of TAN1 Flood Risk Vulnerability Classification, the proposed site use of residential development comprises highly vulnerable flood classification..2 Fluvial flooding The Application Area (north and south area) is mostly situated within Flood Zone A, as shown on the Welsh Government s DAM indicative flood map. Flood Zone A is classified as very low flood risk with a 1 in 1,000-year or less (<0.1%) annual probability of fluvial flooding (Drawing 218/FCA/03). The eastern boundary of both north and south areas is adjacent to Mochdre Brook. In the northern area, the eastern boundary borders Flood Zone C2; classified as floodplain without significant flood defence infrastructure. Proposed development within the northern area is therefore limited to the low risk Flood Zone A area, and is not sited within the C2 Zone. The southern area is divided by a hedgerow (as previously described). To the west of the hedgerow the site is entirely contained within Flood Zone A, and as such the fluvial flood risk is very low. The area east of the hedge is partially within Flood Zone C2, however no development is proposed for this area. The overall risk of fluvial flooding for proposed development within the northern and southern areas of the site is considered to be low and mitigation measures are therefore not required. Within Figure 2 of the TAN1, this style of development is appropriate for this flood classification..3 Surface water flooding Surface water flooding may occur when rainwater does not drain away through the normal drainage systems or percolate to ground, but lies on or flows over the ground surface. Natural Resources Wales Risk of Flooding from Surface Water map indicates that the northern area, and the majority of the southern area are considered to be at low risk. This means that in a year, the area has a chance of flooding of less than 1 in 1000 (<0.1%). July 16 Page 12

17 The only areas potentially affected by surface water flooding are located immediately adjacent to Mochdre Brook and a low-lying channel which crosses the southeastern part of the site, east of the proposed development (see Drawing 218/FCA/04). Based on the above, current surface water flood risk for development at the site is considered to be low..4 Groundwater flooding Powys County Council s Preliminary Flood Risk Assessment (SFRA) contains no records of groundwater flooding within the area. The proposed development will be sited on Alluvial and fan deposits as confirmed during the site visit. It is highly likely that good hydraulic connectivity exists between the Mochdre Brook and the shallow sands and gravels hence groundwater levels will be closely linked with river water levels. Modelling carried out by Natural Resources Wales has been used to inform local flood risk mapping. Peak water levels for model node points in the vicinity of the site may be used to further inform peak water flood levels. The nearest node, RS21A, is 170 m downstream of the site and has a peak water level for the 1 in 100-year event + 20% climate change of 2 maod. If this water level is assumed to be constant upstream of node RS21A, ie within Mochdre Brook, the peak flood water adjacent to the site would be 2 maod. The elevations of the areas for development are in excess of 3 maod in the north area and in excess of 6 maod in the south area. It is therefore considered that the risk posed by groundwater flooding (due to hydraulic conductivity with the Mochdre Brook) is low, and mitigation measures are not necessary.. Flooding from sewers and water mains Information from a previous report carried out by Hafren Water for Glandulas Caravan Site (Drainage Report, April 2010) indicates that pipework associated with foul drainage for part of the adjacent caravan site may be present beneath the southern area of the site. Although manhole covers and a pipe outfall on Mochdre Brook were observed during the site visit (see Drawing 218/FCA/02), further investigation would be required during detailed design stage to identify the location and nature of this infrastructure. July 16 Page 13

18 However, due to the relatively small size of the caravan site, it is assumed that discharge volumes would be small. As such, flood risk posed by site interaction with sewers and water mains is not considered significant for any internal receptors. July 16 Page 14

19 6 FLOOD RISK FROM THE SITE TO THE SURROUNDING AREA 6.1 Fluvial flooding The area to the west of the site is located within Flood Zone A on the Welsh Government s DAM indicative flood map. This is classified as very low flood risk (less than 1 in 1,000-year (<0.1%) annual probability of fluvial flooding). The area to the east of the site is adjacent to Mochdre Brook and is classified as Flood Zone C2, which is classified as high flood risk (1 in 100-year or greater (10%) annual probability of fluvial flooding). It is not proposed to discharge surface water off-site. Drainage features will be designed for infiltration and attenuation; hence the flood risk to neighbouring land due to the proposed development is considered to be very low. Given the proposed surface water management system (SuDs-based), the likelihood of fluvial flooding to the external receptors is considered to be low, hence the overall risk is low. 6.2 Surface water flooding The proposed development will result in changes to run-off characteristics of the site due to the proposed increase in impermeable surfaces. Surface water from the proposed site will run-off to the proposed sustainable drainage system that will be designed as part of the development works. In order to prevent impact on external receptors it will be necessary to ensure that these features are sized to accommodate the required volume generated in a significant rainfall event. Greenfield run-off rates have been calculated together with runoff rates for the existing site and the proposed development and are provided in Appendix 218/FRA/A2. Location and sizing of proposed attenuation features is discussed under mitigation measures in Section 7 below. There is currently an overall low risk of surface water flooding in the areas of proposed development (see Section.3) and this is not expected to change as a result of the development. It is considered therefore that with an appropriately sized sustainable drainage system the development will not increase the flood risk to external receptors. July 16 Page 1

20 7 MITIGATION MEASURES 7.1 Background The proposed development will result in some changes to the run-off characteristics of the existing site. Greenfield run-off rates have been calculated, together with run-off rates for the proposed development and are provided in Appendix 218/FCA/A Methodology for calculation of run-off and storage volume Greenfield run-off rates The peak run-off rate from the greenfield site (ie pre-development) has been estimated using the IH124 method (Equation 7.1, Institute for Hydrology Report N o 124, 1994). The method to estimate mean annual peak flow (QBAR) is of the form: QBAR(rural) = AREA 0.89 SAAR 1.17 SOIL 2.17 Where: QBAR(rural) AREA mean annual flood, with a return period of 2.3 years (m³/s) catchment area (km²) SAAR(4170) Standard Average Annual Rainfall (1941 to 1970) (mm) SOIL soil index This method does not account for slope or storm duration and provides a conservative estimate of the peak greenfield run-off rate. QBAR(rural) can be multiplied using the UK Flood Studies Report regional growth curves to produce peak flood flows for any return period. The greenfield run-off rate for the Application Area is provided in Table 218/FCA/T Run-off rates for the developed site Peak run-off rates for the completed residential development have been estimated. The peak run-off rate for the 1 in 100-year event and storm duration of 6-hours has been estimated using the Rational Method. The Rational Method to give peak flows (Qp) is of the form: Qp = 2.78 CiA Where: C = run-off co-efficient (dimensionless) i = rainfall intensity (mm/hr) A = catchment area (ha) July 16 Page 16

21 The run-off co-efficient, C, varies for different surfaces. The C values used for this assessment are consistent with the Wallingford Procedure for design and analysis of urban storm drainage (HR Wallingford, 1981) and other published research and can be summarised as follows: 218/FCA/T2: Rational method run-off co-efficient values Description of area Run-off coefficient Impermeable (roof) C = 0.9 Impermeable (concrete or hard surface) C = 0.80 Permeable (grass, garden, open space) C = 0.40 Rainfall intensities at the site were obtained from the Flood Estimation Handbook (FEH) CD- ROM, Version 3. Rainfall intensity is dependent on storm duration, representing the likely critical duration for the receiving drainage system. For this assessment, a 6-hour storm duration has been used. The effect of climate change on rainfall intensities has been taken into account in accordance with TAN1 and Planning Policy Guidance (PPG), assuming that the development lifetime will be 7 to 100 years; the maximum period under current provisions. Areas of the differing surface types are tabulated below. It is assumed that run-off from external source areas will not enter the site. 218/FCA/T3: Site surface areas Zone Proposed development Area (m²) Northern area: Impermeable (roof) 1,712 Impermeable (concrete or hard surface) 3,81 Permeable (grass, garden, open space) 7,846 Total 13,409 Southern area: Impermeable (roof) 2,737 Impermeable (concrete or hard surface) 7,863 Permeable (grass, garden, open space) 38,238 Total 48,838 July 16 Page 17

22 Calculations in Appendix 218/FCA/A2 provide estimates of the greenfield and postdevelopment run-off values from the site, using the IH124 and Rational Method. Key information from these calculations is summarised in Table 218/FCA/T4. 218/FCA/T4: Run-off volumes and attenuation storage Phase of development Method Return period Run-off rate (l/s) Run-off volume (m 3 ) Northern area of site Greenfield Post-development IH124 Rational 1 in 2 years in 100 years in 100 years in 100 years (+CC 20%) Southern area of site Greenfield Post-development CC = Climate Change IH124 Rational 1 in 2 years in 100 years in 100 years 71 1, 1 in 100 years (+CC 20%) 86 1,866 Total run-off volume for site 2,470 1 For a critical storm duration of 6-hours As summarised above, run-off for a 1 in 100-year return period (excluding climate change) for the north area of the proposed development increases from 207 m³ to approximately 04 m³ post-development. Including the allowance of 20% for climate change, run-off from the post-development site increases to approximately 60 m 3. For the south area, run-off for a 1 in 100-year return period (excluding climate change) increases from 76 m³ to approximately 1, m³ post-development. With the allowance for 20% climate change, run-off from the post-development site will increase to approximately 1,866 m 3. Mitigation measures are outlined below. The increased run-off will be accommodated through the implementation of a Sustainable Urban Drainage Scheme (SuDS); the details of which are outlined in Section Mitigation measures Surface water run-off: external receptors Run-off characteristics for the site will be altered as a result of the proposed development. In accordance with Powys County Council s policies, SuDS will be used in order to reduce flood risk. July 16 Page 18

23 Run-off calculations have been completed for the 1 in 100-year event, for a storm duration of 6-hours, in accordance with TAN1 and CIRIA SuDS Manual (C697). The total postdevelopment attenuation storage required for the 1 in 100-year event + climate change 20%, with critical storm duration of 6-hours, is approximately 2,470 m 3. To reduce the risk of surface water flooding, the proposed SuDS system will include ephemeral retention basins and infiltration areas. The attenuation features will be of sufficient capacity to provide adequate storage to retain water generated by the design storm event. It is planned to construct the SuDS features on land to the east of the hedgerow in the southern area, adjacent to the C2 Flood Zone. This places it within the proposed area of Public Open Space of the development. Attenuation feature dimensions will be confirmed before construction. If swales are located across topographic contours check dams will be installed to prevent erosion. All SuDS features will be located outside areas of flood risk. The site occupier will be responsible for future maintenance of the SuDS system. In the pre-development site, run-off discharged directly onto surrounding land and into Mochdre Brook. However, all run-off post-development will be directed into the SuDS system, reducing the risk of flooding to external receptors and increasing the capacity of Mochdre Brook. 7.4 Adherence to Local Policy Objectives With regard to the objectives set out by Powys County Council in the Stage 2 SFCA the following section describes how the proposed development meets these criteria (policy objectives in bold). Ensure no increase in flood risk elsewhere Evidence from run-off calculations and Sustainable Urban Drainage Design (SuDS) indicates that the proposed development does not increase flood risk for external receptors. Identify opportunities to reduce existing flood risk It is likely that existing flood risk will be reduced through the use of SuDS as part of the proposed development. A sequential approach and use of the Justification Test The principles of TAN1 have been used to steer development away from high risk flood areas and in to Flood Zone A. July 16 Page 19

24 Protect the functional floodplain (in greenfield and previously developed areas) Development has been moved away from the greenfield functional floodplain within the southern area of the site and in to Flood Zone A. Site layout - A sequential approach has been taken within the development site by locating the most vulnerable elements of the development (ie the houses) in the lowest flood risk/hazard areas. This has been achieved through ensuring that all housing is within Flood Zone A. Land use in Flood Zone C2 has been limited to recreation and environmental purposes, as well as to facilitate flood risk management. Set development back from watercourses Development at the site has been designed to allow a suitably wide undeveloped buffer strip between the watercourse and the proposed housing. Reduce surface water run-off from new developments Calculations for the proposed development show that run-off volumes and peak flow rates are maintained at either the greenfield rate (up to and including the 1 in 100-year event +20% climate change. Development space has been set aside for SuDS features. Flood risk designations have informed the overall site layout. With regard to modelling carried out by Powys County Council for Candidate Sites within the county, the recommendations for CS 92 have been addressed: The LPA will need to be satisfied that access/egress to the site can be achieved in line with guidance set out in TAN1 Access and egress from the area of proposed development will be from higher ground to the west. This area is designated as Flood Zone A and is considered to provide safe access and egress from the site. 7. Natural Resources Wales flood warning service The Application Area is located within a Natural Resources Wales Flood Alert Area (Upper Severn in Powys). The NRW flood warning service in Powys issues warnings by telephone, mobile, , text message and by fax. Registration can be completed on the NRW website or by contacting the Floodline on It is recommended that the site be registered with the NRW flood warning service in order to allow preparatory action to be taken and reduce residual flood risk. July 16 Page 20

25 8 SUMMARY and CONCLUSIONS An assessment has been undertaken of flood risk in and around the proposed development of 89 residential properties in. The Application Area covers approximately 6.2 hectares (ha) and is centred on National Grid Reference (NGR) SO The proposed development is located within Flood Zone A as defined in TAN1 and shown on Welsh Government s DAM, where the probability of fluvial flooding in any one year is less than 1 in 1,000 (AEP <0.1%). A smaller area along the eastern boundary of the site is designated as Flood Zone C2 where the probability of fluvial flooding in any one year is 1:100 (AEP 1%). An FCA is required in accordance with TAN1. Risks of flooding have been considered for fluvial, surface water, groundwater and sewers/mains. Risks from fluvial, groundwater and sewers/mains flooding are low for the site and surrounding area. There is a risk of surface water flooding to the surrounding area as a result of the development. Mitigation measures in terms of surface water storage (attenuation features) have been proposed. The total post-development attenuation storage required for the 1 in 100-year event (plus climate change) is 2,470 m 3. The proposed SuDS scheme will reduce surface water flood risk to external receptors. Drainage features will be designed for infiltration and attenuation, hence the flood risk to external receptors will be significantly reduced. As mitigation of the associated residual risk, it is recommended that the site should be registered with Natural Resource Wales flood warning service and to make contact with the local flood warden. In light of the above, the Application Area is considered to satisfy the flood risk requirements of TAN1 and associated technical guidance. July 16 Page 21

26 DRAWINGS July 16

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31 APPENDIX 218/FCA/A1 Indicative site layout July 16

32 ED Bd OS Mapping obtained from Promap Licence No Reproduced by Berrys under OS Licence No y EP 0.96 SP N T191 river bank T190 approx size only T T T ST T T370 SR10A SP9 P/W mh EP ge D id Br hedge 1mw MS T water markers 4.6 WSV WO WSV 3.9 T T hedge 2mw MS T SP IC cl IC cl IC cl 6.32 hd hedge 2mw T3 mh 1.0 P/R R P/ Pl 6.9 G T MS T346 MS T N MS no tag 14 P/ R traffic calming T T SP N 7.32 T199 WO MS 0 trees and shrubs G W P/ G 7.48 MS T h 0m STW h 2m hedge mw T344 MS no.77 tag 6.09 SP Roa d Adop table N g traffic calming G tin an Amendments: 4.9 hedge 6mw water lev post 7. 0m h shrubs G IC cl trees and shrubs h T N 8.84 Project: TP Project Glandulas Drawing: Drawing piers not shown gravel SN piers not shown DPC level FL m MS approx size unable to tag 8.19 mh P/W 1.0 trees and shrubs LP concrete P/W 9.43 Client rid 8.27 h 2m 1. Client: 9.84 water trough piers not shown bitmac notice board 8.74 R P/ approx CL only 9.92 ST SR IC cl IC cl IC cl G Caravan Park Tel: Fax: e stone ret wall wider at base SN Lin pe Willow House East Shrewsbury Business Park Shrewsbury SY2 6LG Pi SN ca ble G N 9.98 ge erh ea d hedge foliage water lev ov enu ned av MS T192 Tree li P/ 7 typical tree size T s landula G g in t e fron h 0m 7.6 G edge of foliage P/ W mh post T typical tree size W BERRYS mh post 29022N 9.34 P/R SN T MS no tag 8 trees and shrubs traffic calming MS T19 G T W P/ DK 8.27 SP trees and shrubs post DK T T30 T T32 T ee.73 0mh W P/ water lev Tr h m BT exposed electrical cable ST T33 Landscaped Public Open Space stay wire line of marker flags h.0m P/W 29032N EP borehole MS T overhead cable SP T stone ret wall notice boards 6.61 NO TAG edge of sm trees e ag stone ret wall foli ST T to N e du all T utm 6.07 Sta k m oo Br post 300 square post 300 square MS T MS T BT RS mh water marker IC cl 6.48 hedge 2mw IC cl P/R e Tre ting n Pla dense overgrowth JOI re N El Sub line of marker flags ST h 2.2m S/S 2 6.m 2.92 oc water lev T361 river approx il MS headwall soffit IC cl 6.38 meter M.0.48 hedge 3.mw 6.32 CS N MS T ST IC cl 3.23 WSV WO WSV 3.3 WSV EP 3.81 dense overgrowth N P/W 1.0mh Wilmar 4 is a l u hedge foliage N 6.9 hedge 1mw IC cl A/B footpath sign 4.3 FM Depot G IC cl post /B A IC cl h 30 8 A hedge.mw DK 1 29 P/W G IC cl m G 4.10 il 3. 10mm pipe 4.68 stile edge of concrete 1.70 ducts top edge of bridge deck water marker P/ R edge of concrete 2m BT h G.49 B A/ BT 0 il 3.67 edge of concrete mm pipe water marker ST4.8 IC cl hedge 2mw EP dense overgrowth.3.36 all.17 m 1.0 hedge foliage h water marker 2m R P/ WSVs edge of bridge deck sh me hedge foliage edge of concrete water lev.1.70 B MS T368 A/ hedge 2.mw hedge 4mw SP sparse 4.4 footpath post water marker 1.77 MS T P/W 1.0mh o Ad pta ble hedge 2mw WSV N 4.46 DK ad Ro 4.4 edge of foliage 4.4.2mh P/R WO tree foliage water marker 3.6 h m 1.2 R P/ hedge 2mw and trees SR N water lev hedge 1mw N N rid g window head e1 26 window head window head window head Drawing Number: Drawing Rev. Scale 1:70 Paper Drawn By: AB A1 Date:

33 APPENDIX 218/FCA/A2 Run-off calculations July 16

34 Greenfield Runoff Estimate for Application Area (north) Parameters Results Area km 2 QBAR(rural) 3.7 l/s SAAR 1007 Q (1in1yr)* 3.2 l/s SOIL 0.30 FSR region 4 QBAR 2.8 l/s/ha Return period 2 Q (1in1yr) 2.4 l/s/ha Growth curve factor 0.89 Q (1in100yr) 7.2 l/s/ha NB: calculation based on 0. km2 and then scaled down to actual catchment size. The IH124 methodology is designed for sites > 0. km2 but can be linearly interpolated to represent smaller catchments. Q (1in1yr)*: approximate calculation using a ratio of 0.8 (R&D Technical Report W-074/A Preliminary Rainfall Runoff Management For Developments. Revision D - January 2012) Return period (yr) Q (l/s/ha) Q (l/s) Barkers Chambers Barker Street Shrewsbury, Shropshire SY1 1SB UK Tel: Client: Berry Brothers Willow House East Shrewsbury Business Park Shrewsbury SY2 6LG Title: Greenfield run-off rates from Application Area_North area, using IH124 formula Project: Calc Sheet: 1 of 6 Date: Jul-16 P:\Projects\ (218)\Working\Runoff Newtown_North area v2/greenfield_field C_IH124

35 Storage Volumes vs Storm Duration (1-in-100-year storm) for North area Roofs Impermeable (road, diveway) Permeable (garden, tree planting, publoc open space) Contribution Coefficient Area Ha Climate change (% rainfall increase) 0 % IH124 Estimate of 0% AEP Greenfield Discharge 0.0 l/s Groundwater Inflow Rate (-ve for Outflow) 0.0 l/s Duration Rainfall* 2 Rainfall intensity 100 year event Accretion Rate from Roofs * 3 Accretion Rate from Impermeable surfaces * 3 Accretion Rate from Permeable surfaces * 3 Accretion Rate Accretion Rate from from Net Accretion Groundwater * 3 Watercourse * 3 Rate in Storage Net Accretion Volume in Storage hours mm mm/hr l/s l/s l/s l/s l/s l/s m Barkers Chambers Barker Street Shrewsbury, Shropshire SY1 1SB UK Tel: Client: Title: Runoff rates and retention volumes for north area, Post development Berry Brothers Willow House East Shrewsbury Business Park Shrewsbury SY2 6LG Project: Calc Sheet: 2 of 6 Date: Jul-16 * 2 Obtained from FEH CD-ROM v3 * 3 Climate change factored into rainfall intensity at this stage P:\Projects\ (218)\Working\Runoff Newtown_North area v2/post Dev RUNOFF

36 Storage Volumes vs Storm Duration (1-in-100-year storm) for North area Roofs Impermeable (road, diveway) Permeable (garden, tree planting, publoc open space) Contribution Coefficient Area Ha Climate change (% rainfall increase) 20 % IH124 Estimate of 0% AEP Greenfield Discharge 0.0 l/s Groundwater Inflow Rate (-ve for Outflow) 0.0 l/s Duration Rainfall* 2 Rainfall intensity 100 year event Accretion Rate from Roofs * 3 Accretion Rate from Impermeable surfaces * 3 Accretion Rate from Permeable surfaces * 3 Accretion Rate Accretion Rate from from Net Accretion Groundwater * 3 Watercourse * 3 Rate in Storage Net Accretion Volume in Storage hours mm mm/hr l/s l/s l/s l/s l/s l/s m Barkers Chambers Barker Street Shrewsbury, Shropshire SY1 1SB UK Tel: Client: Berry Brothers Willow House East Shrewsbury Business Park Shrewsbury SY2 6LG Title: Runoff rates and retention volumes for north area + 20% Climate Change, Post development Project: Calc Sheet: 3 of 6 Date: Jul-16 * 2 Obtained from FEH CD-ROM v3 * 3 Climate change factored into rainfall intensity at this stage P:\Projects\ (218)\Working\Runoff Newtown_North area v2/post Dev RUNOFF +20%CC