annex 3 wsp surface water drainage strategy · 2.1 wsp development and transportation have been...

Proof of Evidence - Flooding APPENDICES

Annex 3 WSP Surface Water Drainage Strategy

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LAA (London Ashford Airport) March 2009

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WSP Development & Transportation has prepared this report for LAA, the conditions of which were set out in its appointment for consulting engineering services by the Client. WSP Development & Transportation shall not be responsible for the use of the report or its contents for any purposes other than those for which it was prepared and provided. Should the Client require to pass copies of the report to other parties for information, the whole of the report should be so copied, but no liability shall be incurred or warranty extended by WSP Development & Transportation to any other parties in connection with this report without explicit written agreement thereto by WSP Development & Transportation. WSP Development & Transportation is part of WSP UK Limited. Report Number: 2009/14

Issue/revision Issue 1 Revision 1 Revision 2 Revision 3

Remarks

Date March 2009

Prepared by J Daly

Signature ��

Checked by E. Isnenghi

Signature � ��

Authorised by E. Isnenghi

Signature � ��

Project number 11111559

File reference Q:\London - Ashford Airport, Lydd, Shepway BC, 1559\TEXT\REPORTS\LAA_

Drainage.DOC

WSP Development and Transportation Kent House Romney Place Maidstone Kent ME15 6JB Tel: +44 (0)1622 760 800 Fax: +44 (0)1622 760 801 http://www.wspgroup.com WSP UK Limited | Registered Address WSP House, 70 Chancery Lane, London, WC2A 1AF, UK | Reg No. 01383511 England | WSP Group plc | Offices worldwide

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1 Executive Summary

1.1 This Surface Water Drainage Strategy supports the planning application for the runway extension at London Ashford Airport and has been prepared in consultation with the Environment Agency ("EA") and the Romney Marshes Area Internal Drainage Board ("RMAIDB").

1.2 In order to extend the runway it will be necessary to relocate part of the existing ditches which cross the area.

1.3 The surface water runoff generated by the additional impermeable area of the proposed runway extension will be collected into a traditional piped system which contains the appropriate level of pollution control measures.

1.4 This system would then discharge into the realigned land drainage system which includes the new ditches and also serves the local area.

1.5 The ditches will be realigned around the perimeter of the airport controlled land so as to maximise the offset distance from the runway in line with the Civil Aviation Authority ("CAA") guidelines.

1.6 Storage capacity for the water generated by the proposed runway extension has been shown to exist within the proposed ditch system..

1.7 Ecological issues have been accounted for and mitigation proposed for the interim timeframe before the new ditch system matures.

1.8 Access to the ditches will be provided to RMAIDB as required for their routine maintenance.


2 Introduction

2.1 WSP Development and Transportation have been commissioned by London Ashford Airport Limited ("LAA") to provide a series of drainage strategy principles for the proposed runway extension at London Ashford Airport (Lydd).

2.2 The purpose of this design exercise is to inform the relevant stakeholders of the proposed ditch layout for the runway extension and the effects to the surrounding surface water drainage network.

2.3 These details are submitted in support of a planning application for the runway extension (ref: Y06/1648/SH).

2.4 This Surface Water Drainage Strategy has been prepared in consultation with the EA and the RMAIDB and will be subject to further discussions with them.

2.5 The strategy will be finalised pursuant to a planning condition on the grant of permission for the runway extension (ref: Y06/1648/SH).

2.6 This Strategy should be read in conjunction with the Flood Risk Assessment, 2006 and supplements and refines the following documents:

• Chapter 10 of the 2006 Environmental Statement for the Runway Extension; • Appendix 5.1 of Volume 3A to the 2007 Supplementary Information; • Statement to Inform on the Predicted Impacts from the Proposed Runway

Extension on the Dungeness SAC contained in Volume 4 of the 2007 Supplementary Information;

• Appendix 3 of Volume 6 to the 2008 Supplementary Information.


3 Site Location

3.1 London Ashford Airport is located on the Dungeness peninsular on the south-east coast of Kent, less than 2km east of Lydd. The site sits within an area of marsh land known as the Denge Marsh, an area of reclaimed marsh land, which runs between the coast and old sea cliffs to the north and west. The eastern boundaries overlap with the shingle formations of Denge Beach. The nature of the land upon which the site is located is low lying flat land and the site is approximately 3.5m above sea level.

3.2 Broadly speaking the airport is connected to Rye in the west and New Romney to the north-east via the B2075 with onward connections to Folkestone and Ashford via A259(T) and A2070 respectively with the M20 motorway in close proximity.

3.3 The postcode for the airport is TN29 9QL and the site location is shown in Figure 1.


4 The Site

Site Constraints 4.1 The proposed runway extension footprint (known in this strategy as the "development site") is presently laid to a combination of rough grassland and worked arable farmland. The development site is criss-crossed by a network of land drainage ditches which serve to drain the marshes and through control structures help maintain appropriate water levels throughout the year. These ditches are a mixture of open sewers under the control of the RMAIDB and private drains.

4.2 A recent topographic survey of the ditch system was undertaken by SLC Associates and the plans are included in Appendix A to this report for information. Sections through the ditches were also taken at regular intervals and the relevant sheets have also been included.

4.3 Although there are no main rivers in close proximity to the development site, the EA does control the Denge Main Sewer. This open ditch sits some 500m to the west of the northern end of the runway and discharge from the proposed extension into the RMAIDB controlled Mockmill Sewer will eventually discharge into this watercourse.

4.4 Given the nature of surrounding land and the adjacent airport use, levels are relatively consistent over the area with most locations around 3.5mAOD although ground level highs and lows of 4.09mAOD and 2.94mAOD respectively are shown on the survey.

4.5 Access to the development site is currently along private farm tracks which connect to the local road network or directly from the airport circulation routes (runway, taxiway and service roads).

4.6 The development site is abutted by a National Nature Reserve and a Special Protection Area (SPA), all of which lie along the south eastern boundary of the development site. The tip of the development site lies within a Special Area of Conservation (SAC). The airport is also surrounded to the north and south west by a site of Special Scientific Interest (SSSI). The ecological impact of the proposal is considered at Appendix H.

Geotechnical Data 4.7 It is understood from site visits carried out by WSP personnel on 25th February and 4th March 2009 and from other evidence that the ground broadly consists of fine clayey sands over the majority of the development site which blend into the shingle and gravel bed of Denge Beach towards the eastern boundary of the development site. The area is then generally overlain by a thin covering of topsoil.


5 The Proposed Works

5.1 The proposed runway extension will comprise an extension to the existing north-south runway (Runway 03/21) of 294 metres of additional pavement to its northern end, taking its length from 1,505m to 1,799 metres. A 150 metre ‘starter extension’ beyond the threshold of Runway 21 is required in line with current CAA guidelines.

5.2 A clear and graded area of 105m (from the runway centre line) either side of the extension is also required, along with a Runway End Safety Area ("RESA") in order to comply with CAA recommendations. Only the runway extension and the starter extension consist of hard paving. The clear and graded area and RESA are areas of semi-improved grassland. The total runway and starter extension amounts to some 444 linear metres of additional impermeable pavement.

5.3 A consequence of the extended runway is disruption of existing drainage ditches that form part of the wider drainage network and will need to be relocated and realigned as part of the scheme. The affected sections of existing ditch are highlighted in WSP drawing number 1559-GA-01 rev A enclosed in Appendix B.

5.4 The surface water drainage strategy contained herein looks at the affected ditches and their realignment as well as run-off from the proposed works.

5.5 The proposed ditch realignment works are shown in WSP drawing number 1559-GA-02 rev A ‘Drainage Strategy – Proposed Ditch System’ and the runway drainage scheme is shown in WSP drawing number 1559-GA-03 rev A ‘ Drainage Strategy – Proposed Runway Drainage’. Both drawings are included within Appendix C and D respectively.

5.6 These drawings supersede the ditch and drainage proposals shown on Figures 1A, 1B and 1C within Appendix 3 of Volume 6 to the 2008 Supplementary Information. The designation of the ecological areas as shown on Figures 1A, 1B and 1C, however, remain and Figures 1A, 1B and 1C should only be referred to in this regard.


6 Surface Water Design

Existing System 6.1 There are no existing hard drainage features in the area of the proposed works that serve the airport directly and the system described below forms part of the wider land drainage network that runs throughout the low lying Denge marsh area.

6.2 A system of interconnected, open drainage ditches criss-cross the area with flow generally to the west. A couple of private drains sit on the periphery of the development site but the main affected watercourses, Mockmill Sewer, Paines Field Petty Sewer and School Petty Sewer, fall under the control of the RMAIDB. The drainage board have a responsibility for the maintenance of these watercourses and one of their roles is to control the water levels in the ditches and thereby the marshes throughout the year.

6.3 This level of control is facilitated through simple wooden stop boards inserted upstream of selected culverted control structures - the boards holding water back, holding more water back if increased in height and dropping water levels if boards are removed. The process of adjustment is ongoing, seasonal to some degree but also has to be adjusted in line with the prevailing weather conditions. WSP has been informed by RMAIDB that their summer retention level in the local ditch system is 2.53mAOD and they anticipate no imminent changes to this benchmark level.

6.4 In the area of the development site there are two control structures, one to the west known as the Northlade stops which will be unaffected by the proposed works. The other structure, Mockmill stops, sits directly under the path of the proposed runway extension and will need to be relocated as discussed below.

6.5 All the watercourses mentioned above collect in the Mockmill Sewer, flow westwards and connects into the EA controlled Denge Main Sewer 500m to the west of the development site. From here the watercourse winds a route across the countryside, eventually finding its way towards an outfall to sea approximately 8km to the south.

6.6 A copy of the IDB’s asset plan for the area has been included Appendix E for information.

Design Criteria 6.7 A Flood Risk Assessment has previously been undertaken by BSF Consulting Engineers Ltd and issued as version 2.0 in October 2006 in support of the runway application (ref: Y06/1648/SH). It is understood that this report has been approved by the EA and the hydraulic basis and methodology within the FRA document will be followed or improved upon where possible.


6.8 To enable the surface water design, a number of criteria have been established. These are from a number of sources and are summarised below:

Item Source Information

Greenfield run-off rate BSF Flood Risk

Assessment

7 l/s/ha (litres per second

per hectare)

Summer Retention Level in

the IDB ditch system

RMAIDB 2.53m AOD

Design storm - allowing for

the year 2115 design

horizon

PPS25 1 in 100 year storm +30%

Drainage Strategy : Ditches 6.9 The principles used in designing the new ditch system are listed below and the proposal is illustrated in WSP drawing 1559-GA02 (see Appendix C);

• All ditches to be outside of the clear and graded area of 105m from the runway centre line (CAA requirement).

• Ditches sited as near to the existing or proposed airport boundary so as to still allow construction from within airport controlled land. This will typically mean a 2m distance from airport boundary to the edge of ditch.

• Ditches to be set as close to 150m from the runway centre line as recommended by CAA guidelines for a clear area. This will also marginally improve maintenance access for the RMAIDB as ditches will generally be further from the runway than at present.

• All existing control structures will be replaced and improved where possible. • The prevailing direction of water flow will be maintained. • Smooth transitions will be employed for any changes in direction within an open

ditch (100m radius curve proposed). • More abrupt changes in direction will be facilitated through the use of a piped

culvert and headwall structure, minimising erosion potential and providing regular crossing points to aid access requirements.

• All existing ditches that are to be truncated or remain will be connected into the new system as described above.

• The Mockmill Stops will be relocated some 210m south east at the point where the existing Mockmill Sewer is proposed to change direction. This will keep the control structure on broadly the same part of the network.Additional control structures (with stop boards) will be included where appropriate to provide attenuation within the system to allow for the proposed runway runoff. In this way, all new discharge points from the runway drainage system will be into waters that are locally controlled by stop boards, thereby attenuating the water and restricting its onward transmission into the wider network.

• The new ditches will be larger in section than the majority of existing network they will replace thereby improving flow capacity and available storage within the system (Figure 2).


• Bed levels of the new ditches will generally be inline with those in Mockmill

Sewer beside the existing Northlade stops. This is deeper than the majority are currently and will also aid the hydraulic gradient from the east and provide additional storage capacity within the system compared with the existing.

• Since the sewers in question are generally under the control of RMAIDB it is envisaged that this situation would remain after the works. Since the scheme involves an increase in the length of the ditch system.

6.10 Below is a table which provides a comparison between the main statistics for the existing and proposed ditch systems: Further details have been provided in Appendix F.

Existing Proposed Difference

Length of ditch affected 801m 1300m +501m

Volume below Summer Retention

Level (SRL) 3284m3 5178m3 +1894m3

Surface Area at SRL 4480m2 6487m2 +2007m2

Average width at SRL 5.52m 4.99m -0.53m

Approximate Volume available for

Storage above SRL (assumes GL

of 3.5mAOD)

4346m3 6292m3 +1946m3

6.11 It should be noted that the volume of storage above the Summer Retention Level has been obtained with a crude surface area by depth calculation and that given the sloping sides of the ditches, the actual volume of storage available would be greater still.

Drainage Strategy : Runway Runoff 6.12 As previously mentioned, the land under the proposed runway extension is currently agricultural land or rough grassland. As such, run-off from the new runway extension has been designed assuming that discharge up to an equivalent Greenfield run-off rate is allowed for and any additional discharge will be temporarily stored within the new ditch system and released in a controlled manner by the proposed and existing control structures and stop boards. From the Flood Risk Assessment a Greenfield run-off rate of 7l/s/ha has been used.


New Impermeable Surface Area 17,455 m2

17.5ha

Greenfield run-off rate 7 l/s/ha

Allowable discharge 12.25 l/s 6.13 Using WinDES Microdrainage suite of hydraulic modelling software and the Quick Storage Estimate function we find that given the above discharge rates, the runway extension would require the following additional storage volumes for the surface water runoff (copies of the results are included in Appendix G)

Event 1:100 year + 30%

Minimum Volume Required 913m3

Maximum Volume Required 1245m3

Volume provided above SRL

(See previous section)

6292m3

6.14 Taking a precautionary approach the worst case event has been designed for and storage provided above the Summer Retention Level (SRL) of 2.53mAOD.

6.15 Given the ditch characteristics provided in the last section, the worst case storage condition would result in an increase in water level above the SRL of less than 190mm.

6.16 Since freeboard within the local ditch system would be around 0.97m during the summer when levels are kept higher (therefore the worst case) it is believed the increase is acceptable and will cause no detrimental impacts to the system, adjacent landowners or the wider network either upstream or downstream.

6.17 Further modelling of a basic pipe network suggests that during a 1:100 year storm event +30% for climate change, runoff from the proposed runway extension would generate;

• A peak discharge into the ditch system of 360l/s • This would occur during the 30 minute winter storm.

Pollution Prevention & Control Measures 6.18 There are broadly two types of potential pollutants which are used, or could be used at the airport. They fall into the following categories;

• Hydrocarbons – Fuels, oils and some de-icing chemicals • Water soluble substances – Some de-icing chemicals.


6.19 It should be noted at this point that both refuelling of aircraft and any chemical de-icing of aircraft (a seldom used procedure at Lydd given the prevailing weather conditions at the airport) take place away from the runway and the proposed runway extension on the aprons or within hangars whilst the aircraft are on stand. Surface water runoff from these areas is already dealt with by an existing and completely separate drainage system. This system will remain in place after the runway extension is in operation. LAA has confirmed that no changes are proposed to this system or the current procedures for refuelling of aircraft, or their maintenance. 6.20 As far as the runway is concerned, no chemicals are used to de-ice the runway pavement. Should snow and ice become an issue, then only mechanical means are employed to remove them. This process has been in place for over 20 years at the airport. LAA has confirmed there is no intention to change de-icing procedures for the runways, taxiway and apron areas etc.

6.21 In summary, exposure to chemicals and pollutants from runway runoff to the new drainage system is considered to be extremely low.

6.22 However, for best practice and as incidents and accidents may still occur, the proposed drainage system has been designed to collect and remove pollutants from the system through tried, tested and industry standard practice before being discharged into the local watercourse. It is proposed to achieve this through the use of numerous control measures throughout the drainage system;

• Channel drains at the edge of the runway pavement. • Catchpits at every point of connection from the sections of channel drainage

into the piped carrier system. • A terminal catchpit before the petrol interceptor. • A petrol interceptor ("PI") is to be located just before the outfall location on any

and all new drainage connection into the local watercourses. 6.23 The PIs are sized according to the catchments they serve and will be in accordance with EA Pollution Prevention Guidelines, PPG3 ‘Use and design of oil separators in surface water drainage systems’.

Ecological Issues 6.24 From this strategy, it is calculated that 801m of the existing ditch network will need to be realigned (a reduction in length of 212m from previous calculations as set out in Appendix 3 of Volume 6 to the 2008 Supplementary Information thus emphasising the improvements in the design). Of this 801m, approximately 250m of ditches are within the Dungeness SAC, which is again a reduction in the length of the ditch network that is to be realigned within the SAC of 33m.

6.25 The ecological impacts of the updated drainage proposals for the proposed new runway extension has been considered and the findings are set out at Appendix H.

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Future Access & Maintenance 6.26 Access to the new drainage ditches will be maintained in line with current procedures allowing the RMAIDB full access to the ditch system for routine maintenance, weeding cutting and de-silting as required.


6.27 The system of work will not alter greatly from that currently in place but LAA has confirmed in writing to RMAIDB the procedures necessary which are no more onerous than current conditions. A copy of the LAA letter, dated 6th February 2009, which outlines the detail has been included in Appendix I for information.


7 Summary

7.1 The surface water runoff generated by the additional impermeable area of the proposed runway extension will be collected initially into a traditional piped system.

7.2 The piped network will contain several levels of pollution control measures in the form of numerous catchpits and petrol interceptors.

7.3 Discharge from this system will be directed into a realigned land drainage system that also serves the local area. This system will be realigned around the perimeter of the airport controlled land so as to maximise the offset distance from the runway in line with CAA guidelines.

7.4 Access to the ditches will be provided to RMAIDB as required for their routine maintenance as confirmed by LAA.

7.5 Storage capacity for the water generated by the scheme has been shown to exist within the proposed ditch system.

7.6 Ecological issues have been accounted for and mitigation proposed for the interim timeframe before the new ditch system matures.

Appendices, Figures & Tables

Appendix A SLC Associates Topographic Survey and Ditch Cross Sections

S-09-459-01 Sheet 1

S-09-459-01 Sheet 2

S-09-459-02 Dyke Sections 1 to 15 S-09-459-03 Dyke Sections 16 to 30 S-09-459-05 Dyke Sections 46 to 60 S-09-459-07 Dyke Sections 76 to 90

Appendix B WSP Drawing 1559-GA01 Ditch Removal/Realignment

Appendix C WSP Drawing 1559-GA02 Proposed Ditch System

Appendix D WSP Drawing 1559-GA03 Proposed Runway Drainage

Appendix E RMAIDB Asset Plan

Appendix F Ditch Calculations – New & Proposed

Appendix G WinDES QSE Results

Appendix H GHD Summary of Ecological Impacts of the Updated Drainage Proposals for the Proposed New Runway Extension

Appendix I Letter From LAA To RMAIDB Dated 6th February 2009

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