MEDMERRY PARK, EARNLEY, CHICHESTER, WEST SUSSEX PO20 7JP

PROPOSED HOLIDAY VILLAGE DEVELOPMENT

Surface Water Drainage Strategy

Prepared on Behalf of

Manhire Surveyors LLP

2019/D1663/SW/DS/1.1

November 2019

RGP Design Limited – Transport Planning and Infrastructure Design Consultants www.rgpdesign.co.uk South Coast 2, West Barn, Norton Lane, Chichester, PO20 3AF T: 01243 210418 RGP Design Limited VAT Registration No: 219 6736 80 Registered in England No: 09674169 Registered Office: Unit 2 West Barn, Norton Lane, Chichester, West Sussex PO20 3AF

DOCUMENT CONTROL

Project: Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP Proposed Holiday Village Development

Document: Surface Water Drainage Strategy

Client: Manhire Surveyors LLP

Reference: 2019/D1663/SW/DS/1.1

Document Checking:

Author: SPB Date: 11/11/2019

Checked by: MJA Date: 11/11/2019

Approved by: MJA Date: 11/11/2019

Status:

Issue Date Status Issued By

1. 29/10/2019 DRAFT SPB 2. 11/11/2019 FINAL SPB

© Copyright RGP Design Limited 2019 No part of this publication may be reproduced by any means without the prior permission of RGP Design Limited

Manhire Surveyors LLP 2 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

CONTENTS

1 Introduction ...... 5 1.1 Background ...... 5 1.2 Site Location and Description ...... 5 1.3 Flood Risk ...... 6 2 Existing Drainage ...... 11 2.2 Highway Drainage ...... 11 2.3 Ground Conditions ...... 11 3 Proposed Development ...... 16 3.2 Climate Change ...... 20 3.3 Urban Creep ...... 20 3.4 Treatment ...... 21 4 Offsite Impacts ...... 22 5 Summary ...... 23

Manhire Surveyors LLP 3 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

DRAWINGS

PL200 Existing Impermeable Areas PL201 Proposed Impermeable Areas PL300 Surface Water Drainage Strategy

APPENDICIES

Appendix A Architectural Design Proposals Appendix B Southern Water Public Sewer Records Appendix C Enzygo Environmental Consultants Correspondence Appendix D West Sussex County Council Correspondence Appendix E Environment Agency Correspondence Appendix F MicroDrainage Calculations

Manhire Surveyors LLP 4 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

1 INTRODUCTION

1.1 Background

1.1.1 RGP Design Ltd is instructed by Manhire Surveyors LLP to assist in preparing a Surface Water Drainage Strategy in support of the proposed re-development at Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP.

1.1.2 The proposed re-development seeks to provide 518 No. new holiday lodges and caravans, replacing the existing 308 No. units with development being phased from 2020 (Phase 1) up to 2030 (Phase 5).

1.1.3 In preparing this report, RGP Design has referred to the following documents and information:

• Environment Agency Flood Maps for Planning

• Long Term Flood Risk Assessment Surface Water Flood Risk Maps

• Long Term Flood Risk Assessment Reservoir Flood Risk Maps

• Environment Agency Product 4 Data

• Southern Water Drainage Records

• West Sussex County Council and Chichester District Council Drainage Records

• West Sussex LLFA Policy for the Management of Surface Water

• Enzygo Environmental Consultants Soakaway Letter Report

• British Geological Survey and Records

• Site-specific Flood Risk Assessment (Herrington Consulting Ltd.)

• Flood Defence Strategy for Medmerry Park Development Report – Ian S Heijne

1.1.4 This Drainage Strategy report should be read in conjunction with the site-specific Flood Risk Assessment produced by Herrington Consulting Ltd, as well as the Flood Defence Strategy prepared by Ian S Heijne.

1.1.5 This report has been prepared for the benefit of the parties listed in the Terms of Appointment.

1.2 Site Location and Description

1.2.1 The site totals 33.09Ha in size and is located off of Stoney Lane, approximately 1.20km east of Bracklesham Bay. A site location plan has been included within this report (Appendix A).

Manhire Surveyors LLP 5 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

1.2.2 The site is located approximately 0.26km inland from the English Channel and is confirmed as falling within Flood Zone 3, as stated by the Environment Agency and Flood Maps for Planning (Rivers and Sea), included within Appendix E.

1.2.3 The site falls under the jurisdiction of Chichester District Council and West Sussex County Council, and is currently occupied by Medmerry Park Holiday Village, which consists of 308 No. holiday lodges.

1.2.4 It is proposed that the site will be re-developed, to provide 518 No. holiday caravans and lodges – an increase of 210 No. lodges when compared with the existing development. A phased approach to developing the site has been proposed, as detailed within the ‘Proposed Development’ section of this report.

1.2.5 A topographical survey has been undertaken at the site, conducted by Sumo Surveys Ltd, dated March 2014 which indicates variable falls across the site, with levels ranging from in the region of 1.60m AOD to 4.20m, broadly falling from north-west to south-east.

1.2.6 Park Rife (designated Ordinary Watercourse) is identified as crossing through the site, from the north to the south.

1.2.7 Earnley Rife/Broad Rife (designated Main River) is located to the west/south-west/south of the site.

1.3 Flood Risk

1.3.1 Herrington Consulting Ltd. has been appointed by the client to prepare a site-specific Flood Risk Assessment in support of the proposed re-development and the associated planning application. In view of this, this Drainage Strategy has been prepared in view of the findings of the Flood Risk Assessment undertaken.

1.3.2 As a result of 1.3.1, this report will not detail Flood Risk, although a brief summary is outlined, below.

1.3.3 The Environment Agency’s on-line Flood Maps for Planning show that the site is located within Flood Zone 3, as demonstrated by Figure 1. The definitions of Flood Zones are confirmed within the table, below (Figure 2).

1.3.4 Herrington Consulting Ltd. has advised that the Flood Risk Assessment which they have undertaken, inclusive of numerical flood modelling (NFM) appraises current day conditions, to provide an accurate baseline scenario for which comparisons can be drawn with future development proposals. Herrington Consulting Ltd. has confirmed that – in view of the work undertaken, the Environment Agency’s Product 4 data is not reflective of the true baseline, hence why NFM was undertaken.

1.3.5 Herrington Consulting Ltd. has confirmed that, following NFM, parts of the site are located within the functional floodplain (Flood Zone 3b), including some of the areas where development is proposed – such as in Field E, where Phase 2 is located.

Manhire Surveyors LLP 6 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

Figure 1. Flood Maps for Planning (Gov.uk)

Flood Zone Definition Zone 1 Low Probability Land having a less than 1 in 1,000 annual probability of river or sea flooding. (Shown as ‘clear’ on the Flood Map – all land outside Zones 2 and 3) Zone 2 Medium Probability Land having between a 1 in 100 and 1 in 1,000 annual probability of river flooding; or land having between a 1 in 200 and 1 in 1,000 annual probability of sea flooding. (Land shown in light blue on the Flood Map) Zone 3a High Probability Land having a 1 in 100 or greater annual probability of river flooding; or Land having a 1 in 200 or greater annual probability of sea flooding. (Land shown in dark blue on the Flood Map) Zone 3b The Functional Floodplain This zone comprises land where water has to flow or be stored in times of flood. Local planning authorities should identify in their Strategic Flood Risk Assessments areas of functional floodplain and its boundaries accordingly, in agreement with the Environment Agency. (Not separately distinguished from Zone 3a on the Flood Map) Figure 2. Flood Zone Definitions

1.3.6 The Long-Term Flood Map (Rivers or the Sea) confirms that the site is as varying levels of Flood Risk from Rivers or the Sea, ranging from ‘Low’ to ‘High’.

Manhire Surveyors LLP 7 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

Figure 3. Long-Term Flood Map – Flood Risk from River or the Sea

1.3.7 The Long-Term Flood Map (Surface Water) confirms that the site is also at varying levels of susceptibility whereby potential surface water flooding is concerned – ranging from ‘Very Low’ to ‘High’. The areas identified as being at risk appear to follow the course of the existing watercourse network which passes through the site.

Manhire Surveyors LLP 8 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

Figure 4. Long-Term Flood Map – Flood Risk from Surface Water

1.3.8 The Long-Term Flood Map (Reservoirs) confirms that the site is not at risk of flooding from such sources.

Manhire Surveyors LLP 9 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

Figure 5. Long-Term Flood Map – Flood Risk from Reservoirs

Manhire Surveyors LLP 10 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

2 EXISTING DRAINAGE

2.1.1 The existing drainage systems have been identified by reviewing the site-specific topographical survey undertaken by Sumo Surveys Ltd, dated March 2014. In addition to this, statutory records obtained from Southern Water (Appendix B) have been reviewed, whilst the client has also confirmed existing drainage infrastructure arrangements understood to currently be serving the site and the existing development.

2.1.2 The public sewer records provided by Southern Water (Appendix B) do not show any public surface water sewers in the immediate vicinity of the site. The nearest public surface water sewers are located approximately 200m west of the western most extent of the western boundary of the site, located in East Bracklesham Drive.

2.1.3 The topographical survey undertaken at the site confirms that the site is served by private surface water drains and sewers, which are understood to outfall to the numerous watercourses and open water features located within the bounds of the site.

2.1.4 It is likely that the site benefits from minimal levels of infiltration, owing to the factors detailed within 2.3 – ‘Ground Conditions’, supported by the on-site investigation undertaken by Enzygo Environmental Consultants.

2.1.5 Although the site is currently developed – housing 308 No. existing holiday caravans and lodges, an assessment has been undertaken to establish likely greenfield run-off rates for a development site of this size.

2.1.6 The greenfield run-off rate (Qbar) has been calculated using MicroDrainage. Qbar has been determined to be 112.6l/s; Q1 = 95.8l/s, Q30 = 255.1L/S, Q100 = 359.1l/s.

2.1.7 The greenfield run-off volume has been calculated to be 7,517.171m3 (Based on FEH, SPR of 37.00).

2.1.8 As established within 2.1.5, the site is not a greenfield site, and houses significant existing development, incorporating areas of hardstanding which would generate an increased rate and volume of run-off.

2.1.9 A high-level assessment has been undertaken to establish likely increased levels of run-off generated by the site in view of the existing development and areas of hardstanding. In view of the existing impermeable area at the site, it is anticipated actual run-off rates may be up to 525.24l/s (15.87l/s per hectare).

2.1.10 Supporting MicroDrainage calculations are included within this report, within Appendix F.

2.2 Highway Drainage

2.2.1 The estate roads on site supporting the existing development are not understood to be served by any positive drainage infrastructure – it is believed that the roads drain ‘over the edge’.

2.3 Ground Conditions

2.3.1 Reference has been made to the British Geological Survey (BGS) website in order to gain an understanding of the likely geological composition of the soil(s) underlying the site.

2.3.2 Superficial deposits are recorded as being variable, consisting of ‘Raised Marine Deposits – Clay, Silt, Sand and Gravel’ as well as ‘Raised Beach Deposits – Sand and Gravel’ as demonstrated by Figure 6, below.

Manhire Surveyors LLP 11 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

Figure 6. BGS Superficial Deposits Geology Map

2.3.3 Bedrock geology is recorded as ‘Earnley Sand Formation – Sand, Silt and Clay’, as demonstrated in Figure 7, below.

Figure 7. BGS Bedrock Geology Map

Manhire Surveyors LLP 12 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

2.3.4 Further to a review of the BGS geology maps, reference has also been made to a site-specific investigation undertaken by Enzygo Environmental Consultants, conducted on site between 25th to 27th February 2019. The purpose of the investigation was to undertake trial pitting and soakage testing in accordance with BRE365, along with groundwater monitoring works.

2.3.5 A copy of Enzygo Environmental Consultants’ letter report can be found within Appendix C of this report. The ground conditions encountered during on-site investigation are summarised in the table, below;

Strata Summary Description Depth (m) Topsoil 0.00m - 0.30m Made Ground Orange brown clayey sandy 0.00m - 0.80m gravel Firm consistency pale brown 0.30m - 0.80m sandy gravelly clay Very soft to stiff consistency grey 0.20m - 3.50m green/grey brown/orange brown sandy gravelly clay Superficial Yellow brown gravelly sand 0.30m - 1.20m Firm dark grey sandy silty clay 0.80m - 1.00m Green grey/orange brown silty 1.20m - 4.45m clayey fine to medium sand Soft consistency blue grey clay 1.40m - 2.00m Orange brown sandy gravel 1.40m - 2.10m Firm dark grey silty clay 3.50m - 4.45m Figure 8. Enzygo Environmental Consultants’ Geological Summary

2.3.6 Made ground was encountered across the site, typically up to depths of 0.30m below ground level, consisting of topsoil underlain by reworked deposits of sandy clayey gravel from 0.15m to 0.80m below ground level.

2.3.7 Superficial deposits were confirmed as being ‘generally consistent with the published BGS information and varied in thickness across site and comprised of interbedded cohesive and granular strata to a maximum depth of 4.45m below ground level.’

2.3.8 Bedrock is stated as not having been encountered during on-site investigation.

2.3.9 Groundwater was confirmed as being encountered during the on-site investigation works. Enzygo Environmental Consultants’ letter report states that ‘Groundwater seepage was encountered at depths ranging from 1.90m begl (SA05) to 3.30m begl (WS01). Likely due to the proximity of surface water features.’ Groundwater monitoring is currently ongoing at the site.

2.3.10 The on-site soakage testing confirms that the underlying soils at the site are highly unlikely to be conducive in terms of permitting infiltration methods and techniques as a means of supporting the proposed development. The report produced by Enzygo Environmental Consultants confirms that ‘None of the soakaway tests were successful due to insufficient infiltration rates being recorded’. The report continues to state that ‘This is likely related to the encountered shallow groundwater, most likely due to the presence of surface water features on site and the nearby watercourse.’

2.3.11 Mapping managed by MAGIC (magic.defra.gov.uk) provides authoritive geographic information concerning the natural environment from across government. This mapping tool has been referred to in order to confirm whether the site is affected by any underlying aquifers, and also whether located within either any groundwater source protection zones or groundwater vulnerability zones.

Manhire Surveyors LLP 13 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

2.3.12 The Aquifer Designation Map (Bedrock) confirms that the site is classified as falling within a ‘Secondary A’ aquifer.

Figure 9. Aquifer Designation Map - Bedrock (magic.defra.gov.uk)

2.3.13 The Aquifer Designation Map (Superficial Drift) confirms that the site is classified as falling within a ‘Secondary A’ aquifer.

Figure 10. Aquifer Designation Map – Superficial Drift (magic.defra.gov.uk)

2.3.14 Environment Agency Groundwater Mapping indicates that the site is not located within or nearby to any Groundwater Source Protection Zones.

Manhire Surveyors LLP 14 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

Figure 11. Groundwater Source Protection Zone Map (magic.defra.gov.uk)

2.3.15 The same groundwater mapping tool demonstrates that the site is, however, located within a Groundwater Vulnerability Zone – classified as ‘Minor Aquifer High’.

Figure 12. Groundwater Vulnerability Map (Magic.gov.uk)

Manhire Surveyors LLP 15 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

3 PROPOSED DEVELOPMENT

3.1.1 The SUDS Manual (C753) and Building Regulations, Part H (Drainage and Waste Disposal) set out a hierarchy of drainage methods to ensure that developments maximise the use of sustainable drainage techniques. The hierarchy favours infiltration methods of disposal over other methods such as watercourse and sewers, as detailed below;

i. Utilise infiltration techniques

ii. Attenuate rainwater in ponds or open water features for gradual release

iii. Attenuate rainwater by storing in tanks or sealed water features for gradual release

iv. Discharge rainwater direct to a watercourse

v. Discharge rainwater to a surface water sewer/drain

vi. Discharge rainwater to a combined sewer

SUDS Technique Suitable Comments

Living Roof No Alternative means and measures are deemed more suitable given the development proposals at the site

Basins and Ponds Yes No initial constraints identified – existing features located on site - Constructed Wetlands including the existing Rife, - Balancing Ponds watercourse network and pond(s) should be considered before - Detention Basins introducing additional ‘new’ features. - Retention Ponds

Filter strips and swales Yes No initial constraints identified

Infiltration Devices No On-site investigation undertaken by Enzygo Environmental Consultants - Soakaways confirmed that infiltration rates are - Infiltration trenches and basins poor, and therefore infiltration devices are not deemed to be viable as a means of dealing with surface water run-off generated by the proposed development.

Permeable surfaces and filter Yes As infiltration methods and techniques are not viable, as drains confirmed by on-site testing, such - Gravelled areas methods and techniques are unlikely to offer much benefit.

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- Solid paving blocks Features such as permeable paving could be utilised – if tanked – in - Porous pavement order to provide attenuated storage.

Tanked systems Yes No initial constraints identified, although more sustainable ‘above Oversized pipes ground’ storage should be Cellular tanks considered, particularly in view of existing features located on site.

3.1.2 As established in 1.1.2, the proposed re-development of the holiday park will see an increase in the total number of caravans and lodges at the site from 308 No. units, to 518 No units.

3.1.3 The proposed re-development of the site will be phased over a number of years, to coincide with the gradual expiration and release of existing leasehold agreements on existing plots. The current anticipated phasing of the re-development is detailed, below;

Phase Location (Field) Date No. of Units

Phase 1 Field C 2020-2023 107

Phase 2 Field E 2023-2028 135

Phase 3 Field D 2028-2029 38

Phase 4 Field B 2029-2030 43

Phase 5 Field A 2030 195

Total; 518

3.1.4 In order to allow the above phased development to be fulfilled, various Flood Risk defence works will be required alongside each phase to ensure that the development remains safe throughout its design life.

3.1.5 The necessary defence works have been identified further to the findings detailed within Herrington Consultancy Ltd’s Flood Risk Assessment, following Fluvial and Pluvial Flood Modelling of the Rife(s) and existing watercourse network, with consideration also given to the impacts of potential Coastal flooding. The engineering solution proposed in order to provide the required level of protection as determined by Herrington Consulting Ltd, has been prepared by Ian S Heijne, as detailed within the Flood Defence Strategy Report.

Manhire Surveyors LLP 17 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

3.1.6 In view of the fact that the site drains poorly via infiltration as confirmed by on-site investigation and testing conducted by Enzygo Environmental Consultants – where the infiltration tests were abandoned due to poor infiltration rates, as well as in view of the fact that Groundwater is present at shallow depths underlying the site, it has been determined that infiltration methods and techniques are not viable as a means of dealing with run-off which would be generated by the proposed development.

3.1.7 As a result of the findings established in 3.1.6, it is proposed that post-development run-off will discharge to the existing watercourse network which currently conveys flows through the site.

3.1.8 ‘Significant improvements’ are being proposed for the Park Rife, which passes through the site. In particular, this includes re-instating sections of the watercourse as ‘open ditch’, which are currently culverted (up to 140m), whilst the ‘refurbished’ watercourse will also incorporate two larger areas (ponds) which will serve as detention areas for more extreme storm events.

3.1.9 Herrington Consulting Ltd’s Flood Risk Assessment acknowledges that the ultimate outfall of Earnley Rife and Park Rife is tidally influenced, whilst Product 4 modelling data provided by the Environment Agency confirms that Earnley Rife and Park Rife are tidally influenced. Correspondence from the Environment Agency can be found within Appendix E of this report.

3.1.10 Given that the ultimate outfall which serves the catchment associated with the existing development at the site has been confirmed to be tidally influenced, and the associated waterbodies are identified as being tidally influenced too, it is proposed that run-off generated by the proposed re-development of the site can discharge at an uncontrolled rate to the existing watercourse network – no attenuation is required. In such circumstances, the impacts of a surcharged outfall should be considered however, to ensure that the site is not at increased risk of flooding if flows cannot leave the site owing to the surcharged outfall.

3.1.11 Following completion of all phases of development, the total impermeable area at the site has been determined to be 64,156m2 (6.42Ha).

3.1.12 The impermeable areas associated exclusively with Phase 1 of the development has been calculated as totalling 12,792m2 (1.28Ha). The impermeable area associated with each separate phase of development is detailed, below;

Phase Location Extents of Phase Associated Impermeable Associated (Field) (Area) Area (Proposed) Impermeable Area (Proposed) – Inclusive of Urban Creep

Phase 1 Field C 43,554m2 (4.36Ha) 12,792m2 (1.28Ha) 14,071m2 (1.41Ha)

Phase 2 Field E 62,598m2 (6.26Ha) 15,538m2 (1.55Ha) 17,092m2 (1.709Ha)

Phase 3 Field D 20,763m2 (2.08Ha) 3,283m2 (0.33Ha) 3,611m2 (0.36Ha)

Phase 4 Field B 21,690m2 (2.17Ha) 5,905m2 (0.59Ha) 6,496m2 (0.65Ha)

Phase 5 Field A 99,124m2 (9.91Ha) 26,496m2 (2.65Ha) 29,146m2 (2.91Ha)

Manhire Surveyors LLP 18 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

3.1.13 Despite the findings as identified within 3.1.9 and 3.1.10, an assessment has been undertaken to ascertain the storage requirements in view of greenfield run-off rates – in theory identifying the ‘worst case scenario’ whereby attenuated storage is concerned.

3.1.14 Further to the proposed impermeable area(s) as identified in 3.1.11 and 3.1.12, the table below identifies high-level attenuated storage requirements should run-off be restricted to greenfield rate;

Phase Location (Field) Estimated Storage Provision

Phase 1 Field C 735 – 1,064m3

Phase 2 Field E 848 – 1,232m3

Phase 3 Field D 152 – 227m3

Phase 4 Field B 332 – 480m3

Phase 5 Field A 1,482 – 2,145m3

Full Site N/A 3,226 – 4,725m3

*Figures include an allowance of 40% for Climate Change, and 10% Urban Creep

3.1.15 As the proposed development is to be implemented in phases in order to honour leasehold agreements on existing plots, the overall impermeable area will be greater throughout construction, owing to the retention of existing areas of hardstanding which will not (or cannot) be removed from consideration until works associated with Phase 5 (which focuses on the areas of existing development) commence. Phases 1-4 will be implemented around the existing development. In view of this, a cumulative impermeable area – incorporating each new phase of development in addition to the retained existing development, has been determined.

3.1.16 In view of 3.1.15, the existing impermeable area at the site (which will be retained until Phase 5 is under construction) totals 37,817m2 (3.78Ha). In view of greenfield run-off rates, in addition to the estimated storage provision required to support each phase – as detailed above, it has been determined that a further 2,091 – 3,005m3 may be required in order to support the run-off associated with the existing impermeable area at the site.

3.1.17 Herrington Consulting Ltd and Ian S Heijne have confirmed that, following the Fluvial and Pluvial modelling undertaken in relation to Park Rife, that sufficient capacity exists within the rife network to accommodate post-development run-off, inclusive of an allowance of 10% of the total post- development impermeable area, and an allowance of 40% where climate change is concerned.

3.1.18 The scheme will utilise above ground, sustainable means of draining the site – with the use of swales and ponds, as well as french drains – mimicking historic greenfield, as well as existing, conditions.

3.1.19 As opposed to hard engineered solutions (such as permeable paving, cellular storage and so forth), the development will instead focus on managing overland flows and the conveyance of flows within the existing and proposed features, including ponds, swales and watercourses.

Manhire Surveyors LLP 19 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

3.1.20 The proposed features, where to be implemented, will provide additional storage whilst calculations indicates that – for the entire development, storage to the order of 3,226 – 4,725m3 will be required (based on greenfield run-off rates) – which has been confirmed can be provided within the re-worked Park Rife network. New storage feature should be provided behind the proposed flood defence bund, which is to be prepared by others.

3.1.21 Herrington Consulting Ltd. has indicated areas of the site which are likely to fall within Flood Zone 3b (functional floodplain), following NFM. Potential flooding has been identified in Field E (Phase 2) which is currently connected to a ‘blind ditch’. At present, run-off from the site flows into this ditch, which backs up and partially floods the east of Field E. Mitigation is to be provided in view of this by introducing a new ‘holding pond’ in Field E, during the first phase of works, which would reduce the extent of the functional floodplain. As there is no opportunity to connect to the Park Rife, a temporary link to Earnley Rife will need to be provided.

3.1.22 In developing this Drainage Strategy in conjunction with the Flood Risk Assessment undertaken by Herrington Consulting Ltd, inclusive of flood modelling, as well as the proposals as detailed within the Flood Defence Strategy prepared by Ian S Heijne, the Drainage Strategy will be able to support a 1-in- 100-year critical storm event, inclusive of an allowance of 40% for the anticipated effects of climate change. A further allowance has also been made for Urban Creep (10%).

3.1.23 A high-level surface water drainage strategy has been prepared, as demonstrated on drawing PL300, covering the full extents of the site.

3.2 Climate Change

3.2.1 NPPF sets out percentage increases to be applied to the peak rainfall intensity values used in the design of a surface water drainage system to allow for the effects of climate change.

3.2.2 In accordance with NPPF ‘climate change allowances’ updated 15 February 2019, an increase of 20% should be applied for this development and a review should be undertaken to determine the impact of an increase of 40%.

3.2.3 A 40% level of climate change has been accounted for within the preliminary design and supporting calculations produced at this feasibility stage.

3.2.4 Herrington Consulting Ltd has confirmed that climate change allowances have been made whereby the flood modelling they have undertaken is concerned, in accordance with the requirements as details above in 3.2.1 – 3.2.3.

3.3 Urban Creep

3.3.1 Based upon the assumption that there will be an urban creep on new developments through their lifetime, an allowance for potential urban creep should be considered.

3.3.2 West Sussex County Council’s ‘Policy for the Management of Surface Water’, dated 27th November 2018 confirms that an allowance of 10% in view of urban creep is required for residential development density of less than 25 No. dwellings per hectare, as stated within Table 5.2.

3.3.3 In view of 3.3.2, an allowance of 10% should be allowed for where urban creep is concerned.

Manhire Surveyors LLP 20 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

3.4 Treatment

3.4.1 The CIRIA SuDS Manual C753 outlines three categories of pollution hazard identification for the various types of land use as follows:

• Total Suspended Solids (TSS).

• Metals (M).

• Hydrocarbons (H).

3.4.2 In accordance with C753 table 26.2 the proposed land uses at the site are categorised as follows:

• Residential Roofs – Very low/TSS-0.2/M-0.2/H-0.05.

• Car parking and other impermeable surfaces - low/TSS-0.5/M-0.4/H-0.4.

3.4.3 In accordance with C753 table 26.3 to establish whether a proposed surface water management technique can provide sufficient treatment for runoff, the values are compared. If the treatment score for each pollutant category is higher than the pollution score, then the chosen technique is sufficient. If a single technique is not judged to provide enough treatment, then other techniques can be added to the system.

3.4.4 The treatment afforded by the proposed outline surface water management strategy has been assessed based on the following components:

• Filter Drain – TSS-0.4/M-0.4/H-0.4

• Pond – TSS-0.7/M-0.7/H-0.5

• Swale – TSS-0.7/M-0.6/H-0.7

3.4.5 In conclusion, the proposed SuDS will provide sufficient treatment of run-off.

Manhire Surveyors LLP 21 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

4 OFFSITE IMPACTS

4.1.1 The proposed development will see an increase in the impermeable area at the site from 3.55Ha to 6.42Ha. Surface water run-off will be managed within the bounds of the site as confirmed by the proposed drainage strategy to minimise the risk of flooding both on and off-site.

4.1.2 The effects of climate change and urban creep are considered in the design of the proposed surface water drainage system, to ensure there is no off-site impact, supported by the findings of third-party reports (Flood Risk Assessment and Flood Defence Strategy).

4.1.3 The ultimate outfall of the existing watercourse network has been confirmed to be tidally influenced – consideration has been given to the potential for the outfall to become surcharged, as detailed within the modelling undertaken by Herrington Consulting Ltd.

Manhire Surveyors LLP 22 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019

5 SUMMARY

5.2.1 The proposed development seeks to replace the existing 308 No. holiday lodges with 518 No. new caravans and lodges, producing an increase of 210 No. units in total at the site, with development to be phased.

5.2.2 The total developable area totals 30.09Ha. Levels range in the region of 1.60m AOD to 4.20m, broadly falling from north-west to south-east.

5.2.3 Further to on-site investigation and testing, it has been confirmed that infiltration is not viable – owing to poor infiltration rates and Groundwater being present at shallow depths.

5.2.4 The existing run-off from the site is believed to follow existing overland flow routes with run-off discharging to the existing watercourse network within the vicinity of the site, as well as features located within the bounds of the site (such as existing ponds).

5.2.5 The proposed development results in an increase in the impermeable area on the site to 64,156m2 (6.42Ha).

5.2.6 It is proposed that surface water run-off generated as a result of the development will discharge to the existing watercourse network and associated ditches within the vicinity of the site. Improvements will be made to the Park Rife, further to the findings of the Flood Risk Assessment produced by Herrington Consulting Ltd, as well as the Flood Defence Strategy, prepared by Ian S Heijne.

5.2.7 The proposed drainage strategy is designed to cater for a 1 in 100-year critical storm event and include an allowance of 40% to cater for the effects of climate change. Urban creep has also been considered, with an allowance of 10% being made. Flood modelling undertaken by Herrington Consulting Ltd. confirms that sufficient capacity is available within the existing watercourse network in order to support the impermeable area associated with the re-development of the site.

5.2.8 In conclusion, the proposed drainage strategy will ensure that following completion of the development the flood risk both on and off the site will remain unchanged.

Manhire Surveyors LLP 23 Medmerry Park, Earnley, Chichester, West Sussex PO20 7JP November 2019