170331 32113 fra addendum - aylesbury vale€¦ · subject: addendum to march 2016 flood risk...

TECHNICAL NOTE

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Job Name: Aylesbury Woodlands

Job No: 32113/4001

Note No: TN001

Date: 24/03/17

Prepared By: S Bari

Subject: Addendum to March 2016 Flood Risk Assessment

Peter Brett Associates LLP (PBA) undertook a Flood Risk Assessment (FRA) for the Aylesbury Woodlands site in March 2016, in accordance with national and local flood risk policy, which formed part of the Outline Planning Application submission (Ref:16/01040/AOP).

Since the submission of the outline planning application, liaison with stakeholders has continued and the first phase of development has been defined. This Addendum to the March 2016 FRA therefore incorporates the following:

Confirmation of the proposed Phase 1 of development; Details of correspondence and discussions with the Environment Agency (EA) regarding the

hydraulic modelling which supports the outline application; An updated Baseline Modelling Report, incorporating EA comments; Updated fluvial climate change allowances to respond to the February 2016 Planning

Practice Guidance update ‘Flood risk assessments: climate change allowances’; A Sequential Test for Water Compatible Development within Flood Zone 3.

The proposed Phase 1 of the development, up to 2022, is shown in the plan titled Parameter Six: Phasing in Appendix A (Figure EDP2524/98e). This Phase 1 will consist of the following elements.

The Eastern Link Road (South) (ELR(S)); ELR(S) Flood mitigation works; 3,070 sq m B1 floorspace; 39,850 sq m B2 floorspace; 32,800 sq m B8 floorspace; and Associated landscape works.

Planning Use classes B1, B2 and B8 are all ‘less vulnerable’ uses with regard to flood risk within the Planning Practice Guide. The ELR (S) is considered to be ‘essential infrastructure’. The flood mitigation works and landscape works are both classified as ‘water compatible development’.

The FRA (March 2016) set out recommendations with regard to flood risk for both the ELR(S) in Section 5.1, and the commercial element of the Phase 1 indicated on the Parameter plan in Section 5.3. Section 5.3 sets out clear recommendations in the event of the Phase 1 commercial development progressing in advance of the ELR(S) and the flood mitigation works. As such, no changes are necessary to the FRA as a result of the confirmation of Phase 1. It should be noted that the Phase 1 area within the March 2016 FRA excluded the ELR(S); however, the recommendations within Sections 5.1 and 5.3 of the FRA are still valid and describe appropriate flood risk mitigation measures for what is now defined as Phase 1 in the plan Parameter Six: Phasing.

Appendix A also includes the following additional updated drawings: Illustrative Masterplan (edp2524/45v)

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• Parameter One: Land use and amount (edp2524/52l) • Parameter Two: Access and movement (edp2524/54k)

Since March 2016, there have been ongoing discussions between the EA and PBA, particularly regarding the hydraulic modelling that supports the outline FRA. As part of the outline planning submission (March 2016), hydraulic modelling was undertaken to design the mitigation scheme for the ELR (S), which was then submitted to the EA for review in advance of the March 2016 planning submission.

Since the March 2016 submission, the following information and documents have been exchanged between the EA and PBA:

Technical Review Certificate reference 2016s3840, EA, 29th April 2016 EA Model Review Response, PBA, 18th July 2016 Technical Review Certificate reference 2016s4490, EA, August 2016 Hydraulic Model – Grid Size Sensitivity, PBA, 3rd November 2016 Figure 32113/4001/002 rev B Indicative Areas requiring Flood Compensation Storage, PBA

– drawing updated to include a 70% allowance for climate change Summary of discussions with the Environment Agency Regarding Flood Risk, PBA, 16th

December 2016 Email discussion between Amy Hensler (PBA) and Nick Read (EA) dated 16th December

2016 Letter from EA to AVDC (reference WA/2016/122289/01-L01), dated 11th January 2017

All the documents listed above can be found in Appendix C of this Addendum.

Following the submission of the additional information listed above, the EA has agreed that the hydraulic modelling undertaken to date is suitable to inform strategic development decisions at the outline application stage, as stated in a letter (reference WA/2016/122289/01-L01), dated 11th January 2017 (presented in Appendix C) . It is further agreed that this model will require refinement post outline planning consent, including a reduced 2d domain grid size, prior to informing detailed design. Draft conditions are proposed that will secure this.

Details of the correspondence and additional technical information provided to the EA are included in Appendix B. The PBA Baseline Hydraulic Modelling Report, which formed Appendix E of the March 2016 FRA, has been updated to incorporate EA comments. The updated Baseline Hydraulic Modelling Report (March 2017) is provided in Appendix C of this Addendum. Specifically, the changes relate to the hydrological methodology, as detailed below:

Clarification on the scope for the hydrological and hydraulic modelling exercise; Addition justification regarding the selection of the donor catchment used in the FEH

Statistical analysis; Clarification on how the urban adjustment factor UAF has been derived; Additional information regarding the selection of the pooling group used in the FEH Statistical

analysis; Confirmation of the statistical distribution used; Validation of the estimated 1 in 100yr growth factor used in the Statistical analysis; Further commentary on the 2008 and 2015 hydrological approaches.

It should be noted that both the hydrological and hydraulic modelling within the Baseline Hydraulic Modelling Report are unchanged and only additional clarification and commentary has been provided within the update to the report.

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The March 2016 FRA that accompanied the March 2016 outline planning submission for Aylesbury Woodlands utilised a 20% climate change allowance as per the guidance at the time that the hydraulic modelling work was undertaken. The national guidance for climate change within the Planning Practice Guide was updated in February 2016 (‘Flood risk assessments: climate change allowances’). Post-submission, PBA has undertaken supplementary hydraulic modelling to assess the impacts of the updated climate change allowances. This analysis is provided in Appendix D of this Addendum (32113/4001/TN001 New Climate Change Allowances dated August 2016).

The 2016 climate change guidance provides several bands of climate change, with guidance on the appropriate band for use with a development being dependent on the geographical location, proposed design life and vulnerability use of any proposed development.

The ELR(S) constitutes an ‘Essential Infrastructure’ use, and as the site is located within Flood Zone 3 of the River Thames Basin, the recommended climate change allowance was the Upper End estimate of 70%.

The figures in Appendix D within the technical note 32113/4001/TN001 show the impact of the proposals when the updated climate change scenarios are considered across the site and surrounding areas. These figures showed that the proposed flood mitigation scheme for the Eastern Link Road (South) , as described in Section 5.1 of the March 2016 FRA performs as intended without increasing flood risk offsite or within the proposed built development shown within the Illustrative Masterplan

A Sequential Test for the ELR(S), essential infrastructure proposed within Flood Zones 3a and 3b, was completed and is detailed within Appendix H of the March 2016 FRA.

As requested by Aylesbury Vale District Council (AVDC), a further Sequential Test has been undertaken for Water-Compatible Development within Flood Zone 3 and 3b. This Flood Risk Sequential Assessment – Water Compatible Development (March 2017) is provided in Appendix E of this Addendum.

This Sequential Assessment seeks to demonstrate that there are no reasonably available sites in areas with a lower probability of flooding that would be appropriate for the proposed provision of formal play space and open space. Adopted and emerging planning policy documents, with evidence base documents, have been reviewed to identify potential development sites across the district suitable to accommodate formal play space and open space, including: Aylesbury Vale District Local Plan (2004); Vale of Aylesbury Local Plan (Draft – Summer 2016); Aylesbury Vale Housing and Economic Land Availability Assessment (January 2017); Aylesbury Vale District Council – Five year housing land supply interim position statement, October 2016. A desk-based assessment has been undertaken of shortlisted sites, taking into account key considerations, including availability, ownership, planning history and development constraints.

The Flood Risk Sequential Assessment – Water Compatible Development has demonstrated that there are no sequentially preferable, available sites that could accommodate the amount and type of formal play space or open space proposed within areas of flood risk as part of the Aylesbury Woodlands development. Collectively, when read together with the Sequential Assessment relating to the ELR(S), we conclude that the proposed development as a whole satisfies the Sequential Test requirements set out in the NPPF and the PPG.

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This FRA Addendum incorporates further details to support the Aylesbury Woodlands outline planning application (March 2016).

PBA has had continued, post March 2016 submission, discussions with the EA and provided updated information to address their comments arising.

This FRA Addendum demonstrates that the findings of the Flood Risk Assessment (March 2016) remain valid including consideration of the updated climate change allowances.

The additional technical information within this Addendum has been shared with the EA during ongoing discussions and the EA have advised that they have no objection to the outline application, subject to conditions (letter reference WA/2016/122289/01-L01 dated 11th January 2017, reproduced in Appendix C).

The Flood Risk Sequential Assessment – Water Compatible Development has demonstrated that there are no sequentially preferable, available sites that could accommodate the amount and type of formal play space or open space proposed within areas of flood risk as part of the Aylesbury Woodlands development. Together with the Sequential Assessment relating to the ELR(S) (March 2016), the proposed development as a whole satisfies the Sequential Test requirements set out in the NPPF and the PPG.

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A.1 EDP drawing EDP2524/45/v Illustrative Masterplan

A.2 EDP drawing EDP2524/52l Parameter one: Land Use and Amount

A.3 EDP drawing EDP2524/54k Parameter two: Access and Movement

A.4 EDP drawing EDP2524/98e Parameter six: Phasing

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B.1 Aylesbury Woodlands Baseline Hydraulic Modelling Report Revision B (March 2017)

On behalf of

Project Ref: 32113/4003 | Rev: B | Date: November 2016

Office Address: Caversham Bridge House, Waterman Place, Reading, Berkshire RG1 8DN T: +44 (0)118 950 0761 F: +44 (0)118 959 7498 E: [email protected]

Aylesbury Woodlands Baseline Hydraulic Modelling Report

Baseline Hydraulic Modelling Report Aylesbury Woodlands

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Document Control Sheet Project Name: Aylesbury Woodlands

Project Ref: 32113

Report Title: Baseline Hydraulic Modelling Report

Doc Ref: 32113 Aylesbury Baseline Modelling report

Date: 02/10/15

Name Position Signature Date

Prepared by: S Bari S Kirby

Assistant Modeller Engineer

02/10/15

Reviewed by: A Hensler Associate 02/10/15

Approved by: P Jenkin Partner 02/10/15

For and on behalf of Peter Brett Associates LLP

Revision Date Description Prepared Reviewed Approved

A 06/01/16 . DRAFT Revised Baseline modelling

S Bari/ S Kirby A Hensler A Hensler

B 22/03/17 Updated following EA review and comments

S Kirby / S Bari A Hensler A Hensler

Peter Brett Associates LLP disclaims any responsibility to the Client and others in respect of any matters outside the scope of this report. This report has been prepared with reasonable skill, care and diligence within the terms of the Contract with the Client and generally in accordance with the appropriate ACE Agreement and taking account of the manpower, resources, investigations and testing devoted to it by agreement with the Client. This report is confidential to the Client and Peter Brett Associates LLP accepts no responsibility of whatsoever nature to third parties to whom this report or any part thereof is made known. Any such party relies upon the report at their own risk.

© Peter Brett Associates LLP 2016



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Contents

1 Introduction ................................................................................................................................. 11.1 Brief ............................................................................................................................... 11.2 Location ......................................................................................................................... 11.3 Existing Modelling.......................................................................................................... 11.4 Review of existing modelling ......................................................................................... 2

2 Hydrology ..................................................................................................................................... 52.2 Definition of catchments and flow estimation points ..................................................... 52.3 Flow Estimation: FEH Statistical Method ...................................................................... 62.4 Flow Estimation: ReFH Method ..................................................................................... 92.5 Summary ..................................................................................................................... 102.6 Amendments to sub-catchment conceptualisation ...................................................... 122.7 Rescaling of the model inflows .................................................................................... 152.8 Scaling Factors ............................................................................................................ 15

3 Hydraulic Model ......................................................................................................................... 183.1 Introduction .................................................................................................................. 183.2 Software versions ........................................................................................................ 183.3 Data available .............................................................................................................. 193.4 New modelled watercourses/channels ........................................................................ 193.5 Culverts under the A41 and GUC ................................................................................ 203.6 ISIS model ................................................................................................................... 213.7 ESTRY model .............................................................................................................. 213.8 TUFLOW model ........................................................................................................... 223.9 Critical Duration ........................................................................................................... 26

4 Modelled results ........................................................................................................................ 274.1 ISIS model results ....................................................................................................... 274.2 TUFLOW modelled flood extents ................................................................................ 30

5 Summary and conclusions ...................................................................................................... 335.1 Model refinements ....................................................................................................... 335.2 Model results ............................................................................................................... 335.3 Conclusion ................................................................................................................... 33

Figures

Figure 1.1: Aylesbury Woodlands site location (as of Dec 2015) ........................................................... 1Figure 1.2: Bear Brook and Upper Thame model extent (site in solid light blue modelled watercourses in blue, model boundary in red line) ........................................................................................................ 2Figure 2.1: FEH CD-ROM (Version 3): Total Catchment at the downstream extent of the modelled reach 6Figure 2.2: Log graph comparing 2008 and 2015 estimated flows ....................................................... 11



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Figure 2.3: Halton Catchment with location of Photos in Figures 2.4, 2.5 and 2.6 ............................... 13Figure 2.4: Photo 1 - Confluence of Halton channel (from left) and Bear Brook (flowing in up direction) 14Figure 2.5: Photo 2 - Plastic pipe/culvert along direction Bear Brook channel (flowing right to left)..... 14Figure 2.6: Photo 3 - View looking south (upstream) along Halton channel at confluence with Bear Brook (flows to the right) ........................................................................................................................ 15Figure 2.7: Table 8.1 from Bear Brook and Upper Thame Flood Risk Mapping Study, Final Modelling Report March 2008. ............................................................................................................................... 16Figure 2.8: Downstream flow location arrangement (ISIS nodes pink, PO lines red) ........................... 17Figure 3.1: Identified significant ditches from Greenhatch Survey to be included in PBA modelling.... 20Figure 3.2: ISIS (Blue) and ESTRY (Red) channels and Estry culverts (Green) in updated model ..... 22Figure 3.3: HX lines with 0.3 coefficient applied for stability (dashed blue lines) .................................. 23Figure 3.4: Multiple Domain TUFLOW model (blue is 20m domain, red is 10m Domain, pink dotted lines are 2d-2d boundaries) ................................................................................................................... 24Figure 3.5: 5% AEP ISIS convergence plot ........................................................................................... 25Figure 3.6: 1% AEP ISIS convergence plot ........................................................................................... 25Figure 3.7: 0.1% AEP ISIS convergence plot........................................................................................ 26Figure 4.1: ISIS model node locations .................................................................................................. 27Figure 4.2: Modelled 5%, 1% and 0.1% AEP flood extents on the topographic survey DEM. ............. 30Figure 4.3: Modelled Baseline (blue hatch) and EA Baseline 5% AEP extents .................................... 31Figure 4.4: Modelled Baseline (red hatch) and EA baseline 1% AEP flood extents ............................. 31Figure 4.5: Modelled TUFLOW flood levels for 1% AEP event (EA levels in Red, PBA baseline levels in Blue) 32

Tables

Table 1.1 Limitations of EA modelling and updates applied for PBA model ....................................... 2Table 2.1 Catchment descriptors at downstream extent of modelled reach ....................................... 5Table 2.2 Growth Curve Fittings and Flood Frequency curve ............................................................ 9Table 2.3 ReFH flows for total catchment ........................................................................................... 9Table 2.4 Comparison of 2008 and 2015 FEH and ReFH flows for total catchment ........................ 10Table 2.5 Comparison of scaling factor flows to downstream target flows (All flows in m3/s) .......... 17Table 4.1 ISIS node modelled water levels (m AOD) along the Bear Brook past the site ................ 27. 27Table 4.2 ISIS node modelled water levels (m AOD) along the Burcott Brook and tributaries through the site. 29

Appendices

Appendix A HydrologyAppendix B Sub-catchment delineationAppendix C ISIS flood level comparisonAppendix D Topographic dataAppendix E Baseline Results



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1 Introduction 1.1 Brief

1.1.1 Peter Brett Associates LLP (PBA) was commissioned by Buckinghamshire Advantage to provide flood risk advice and hydraulic modelling for the proposed Aylesbury Woodlands development, to the east of Aylesbury in Buckinghamshire.

1.1.2 This note details the hydraulic modelling undertaken to determine the baseline in terms of fluvial flood risk and is based on the existing Environment Agency (EA) modelling for the region.

1.2 Location

1.2.1 The Aylesbury Woodlands site is shown in Figure 1.1 and is located to the east of Aylesbury, Buckinghamshire. The site is south of the Grand Union Canal (GUC) and north of the A41, the Bear Brook forms most of eastern boundary and College Road North lies along the western boundary of the site towards the ‘Arla Diary Site’.

1.3 Existing Modelling

1.3.1 PBA were provided with the current EA modelling of the Bear Brook and Upper Thame in Aylesbury. This was the Bear Brook and Upper Thame Flood Risk Mapping Study from March 2008.

1.3.2 The model is predominantly an ISIS-TUFLOW model with several ESTRY elements. The model covers the reach of the Bear Brook (including tributaries) from Aston Clinton to the south of the site, through to its confluence with the River Thame to the east of Aylesbury. The downstream



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limit of the model is the Eythrope Bridge on the River Thame, approximately 2.9km from the confluence with the Bear Brook.

1.3.3 The EA model is a 20m TUFLOW model linked to an ISIS model of the watercourses. There were several ESTRY culverts modelled in the 2d model domain (Figure 1.2).

1.3.4 A FEH hybrid methodology was adopted to derive the hydrological inputs. A FEH rainfall-runoff model was calibrated in conjunction with the hydraulic model using gauged level data provided by the EA. The FEH Rainfall Runoff design flows were then scaled to fit the FEH Statistical Method estimates at the downstream boundary.

1.4 Review of existing modelling

1.4.1 The Bear Brook and Upper Thame model was developed in 2008 for the purposes of flood mapping and was subsequently taken forward to inform the Strategic Flood Risk Assessment (SFRA) prepared in 2009 on behalf of Aylesbury Vale District Council. The SFRA model is currently not available for use in this study. The EA Final Modelling Report (March 2008) which accompanied the Mapping Study made note of limitations in the modelling and made recommendations regarding further use or development of the model (column ‘2008 Report Comments’, Table 1.1). PBA’s response to the recommendations are summarised in column ‘PBA updates applied’ in Table 1.1 below:

2008 Report Comments PBA updates appliedEA model report (2008) Section 10.2 Limitations



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Recommended that further flow and level gauging be undertaken in the Upper Thame and Bear Brook catchments.

Outside of the scope of the Aylesbury Woodlands study.

Recommended that any future analysis should use the FEH CD-ROM (version 2) and The WinFAP-FEH (version 2) software, though noted that it is unlikely that these two updates will have any significant effect on the peak flow estimates. Any future hydrological analysis should use the latest HiFlows-UK dataset, Investigations should be made into the use of the Revitalised Rainfall-Runoff method (ReFH) instead of the Rainfall-Runoff method.

Latest versions of FEH methodologies will be used to compare to previous estimates.

Investigations could be made into running the Bear Brook and River Thame ISIS-TUFLOW model using the ISIS-TUFLOW Link as the runs as part of this study have been made using the January 2006 Beta version.

Latest ISIS-TUFLOW executable will be used to simulate the model and compare model results to previous results.

Composite flood extents be “smoothed” by projecting the flood levels out onto the LiDAR

This will be done using topographic information for the site and surroundings.

We would always recommend that individual developers satisfy themselves that the modelling is sufficiently detailed for their purposes

A multiple domain model will be created to reduce the 2d model cell size from 20m to 10m over the site.

EA model report (2008) Appendix GThe West End Ditch, Bedgrove Brook and Southcourt Brook channel survey was carried out in 1986 while the upper reach of the Bear Brook from the main river limit at Aston Clinton to Vatches Farm Track was carried out in 1989.

All located upstream and downstream of the site. Access not available to resurvey, beyond scope of study and unlikely to impact on results at the site. Following desk-based review and site walkover, there is, no evidence that channel geometry has changed significantly on site and no records of any works having been completed

The 2D results north of the canal (north of Western Mead Farm – Burcott Brook) and downstream of Eythrope Bridge have also been removed as they are outside of the study area and were only included in the model to provide the necessary flow routes.

Same approach will be adopted north of the canal. Access is restricted and model cannot be refined. Detailed representation of these areas is highly unlikely to impact upon results at the site of interest.

Location 1 Aston Clinton Bypass Figure G1 shows the area around the new Aston Clinton Bypass road. Nine culverts were counted under the bypass, eight of which no information was available. These culverts were assumed to have a 1m diameter. The ninth

Topographic survey of the Aston Clinton Bypass culverts and as built drawings from the Highways Agency (via Canal and Rivers Trust) of the structures has been obtained and incorporated into the model to refine and improve the model along the southern boundary of the site.



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culvert is for the Bear Brook and channel survey data was available. Location 2:Canal siphons A culvert was inserted underneath the canal at this location to provide the flow route northwards. Limited information was available for this culvert, therefore a 0.5m diameter culvert was assumed.

Topographic survey of the routes under the canal adjacent to the site has been obtained and incorporated into the model to refine and improve the model. Asset/structure information was also obtained from the Canal and River Trust, but was superseded by the topographic survey data.

Locations 3-7 Not relevant to the Aylesbury Woodlands site because the locations are far enough from the site that the hydraulic schematisation in these locations would have negligible impact on water levels on the Aylesbury Woodlands site.



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2 Hydrology 2.1.1 The EA have provided the Bear Brook and Upper Thame model (2008) as the appropriate model

for use in this study and consider the hydrology used in this model as being appropriate.

2.1.2 The EA model is a rainfall-runoff model, using standard FEH (Flood Estimation Handbook) methodologies, connected to the hydraulic model and calibrated concurrently. The lack of recorded flow data precluded the construction of a hydrological model calibrated independently to the hydraulic model. FEH statistical analysis has been undertaken and design flows derived using the FEH hybrid methodology. FEH CD Rom version 1 and WinFAP 1.1.002 were used in deriving the hydrological estimates.

2.1.3 Following consultation with the EA, the principal areas of work to refine/update the hydrological assessment were identified as:

Refine the existing hydrological sub-catchments in the vicinity of the site based on detailed topographic survey and site observations. Regenerate the hydrological inflows using the same methodology as the existing model. These changes will be small and local and it is therefore assumed that it will not be necessary to recalibrate.

Estimate flows to the downstream boundary (Eythrope) using more recent hydrological methods such as ReFH and the latest FEH Statistical methodology and compare the flows to the 2008 flows.

Derive 1 in 1,000 year hydrology to use in defining Flood Zone 2

Use only one of the three previous critical durations, as this is the only one relevant for the site. Assume that critical duration is appropriate for all the minor ditches on site.

2.2 Definition of catchments and flow estimation points

2.2.1 ReFH and FEH Statistical analysis using up to date techniques was undertaken at the downstream boundary (SP 768135) at Eythrope Park Bridge).

2.2.2 FEH catchment descriptors were extracted from the FEH CD-ROM version 3 (relating to soil, permeability, rainfall, urban extent) for the total catchment to the flow estimation point. These are presented in Table 2.1

Descriptor Value Descriptor Value

AREA 219.92 URBCONC1990 0.671

ALTBAR 107 URBEXT1990 0.0344

ASPBAR 313 URBLOC1990 0.821

ASPVAR 0.1 URBCONC2000 0.855

BFIHOST 0.463 URBEXT2000 0.0607

DPLBAR 15.79 URBLOC2000 0.836

DPSBAR 33.6 C -0.0259

FARL 0.976 D1 0.31422

FPEXT 0.1666 D2 0.28689

170331 32113 fra addendum - aylesbury vale€¦ · subject: addendum to march 2016 flood risk...

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