West Herts College Deculverting Proposals

Environment Agency Response

133834-EAR-(0)

DECEMBER 2020

RSK GENERAL NOTES

Project No.: 133834

Title: West Herts College Deculverting Proposals

Client: Bellway Homes Ltd. (North London)

Date: 7th December 2020

Office: Hemel Hempstead

Status: Final (For Planning Purposes Only)

Author G Turner Technical reviewer C Ariyaratne Date: 4th December 2020 Date: 7th December 2020

Project manager C Ariyaratne Date: 7th December 2020

Revision Comments Author Technical Reviewer 0 1st issue GT CA

RSK Land and Development Engineering Ltd (RSK) has prepared this report for the sole use of the client, showing reasonable skill and care, for the intended purposes as stated in the agreement under which this work was completed. The report may not be relied upon by any other party without the express agreement of the client and RSK. No other warranty, expressed or implied, is made as to the professional advice included in this report. Where any data supplied by the client or from other sources have been used, it has been assumed that the information is correct. No responsibility can be accepted by RSK for inaccuracies in the data supplied by any other party. The conclusions and recommendations in this report are based on the assumption that all relevant information has been supplied by those bodies from whom it was requested. No part of this report may be copied or duplicated without the express permission of RSK and the party for whom it was prepared. Where field investigations have been carried out, these have been restricted to a level of detail required to achieve the stated objectives of the work. This work has been undertaken in accordance with the quality management system of RSK Land and Development Engineering Ltd.

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0)

CONTENTS

Contents 1 INTRODUCTION ...... 1 1.1 Purpose of this report ...... 1 1.2 Development Proposals ...... 1 1.2.1 Main Site ...... 1 1.2.2 Deculverting and Formation of Naturalized Channel ...... 1 2 PLANNING HISTORY ...... 2 2.1 General ...... 2 2.2 Planning Application ...... 2 2.3 EA Consultation ...... 3 3 CONSTRAINTS AFFECTING THE DECULVERTING PROPOSALS ...... 4 3.1 General ...... 4 3.2 Existing Site Conditions ...... 4 3.2.1 Boundary Conditions ...... 4 3.2.2 Existing Culvert ...... 6 3.2.3 Site Levels ...... 7 3.2.4 Flows to be Catered For ...... 8 3.3 Planning Requirements ...... 8 3.3.1 Amenity Space ...... 9 3.3.2 Parking ...... 9 3.3.3 Access requirements ...... 10 3.3.4 Maintenance Access ...... 10 3.4 EA Objections ...... 10 4 CHANNEL GEOMETRY ...... 12 4.1 EA Objection ...... 12 4.2 Designer’s Response ...... 12 4.2.1 General ...... 12 4.2.2 Constraints Specific to the Open Channel Geometry ...... 12 4.2.3 Edge Treatment ...... 14 4.2.4 Bed Level ...... 17 4.2.5 Gabions ...... 19 5 CHANNEL LINING ...... 21 5.1 EA Objection ...... 21 5.2 Designer’s Response ...... 21 5.2.1 Channel Liner ...... 21 5.2.2 Gravels to Base of Open Channel ...... 21 6 ENERGY DISSIPATION STRUCTURE ...... 23 6.1 EA Objection ...... 23 6.2 Designer’s Response ...... 23 6.2.1 Requirement for This Structure ...... 23 6.2.2 Fish Passage ...... 24 7 HEADWALL ...... 25

Bellway Homes (North London) Drainage Strategy Statement 133834-DSS-(02)

7.1 EA Objection ...... 25 7.2 Designer’s Response ...... 25 8 FLOOD RISK ...... 27 8.1 EA Comments Following the Meeting on 26th November 2020 ...... 27 8.2 Designer’s Response ...... 27 9 DESIGN CONCLUSION ...... 29 9.1 Original Open Channel Proposal Objected to by EA ...... 29 9.2 Revised Open Channel Proposal Amended in Line with EA Comments ...... 29 APPENDIX A – EA CORRESPONDENCE ...... 30 APPENDIX B – SITE CONSTRAINTS ...... 31 APPENDIX C - SCHEME PROPOSALS ...... 32 APPENDIX D – HYDRAULIC CALCULATIONS ...... 33 APPENDIX E – FLOOD RISK TECHNICAL NOTE ...... 34

Bellway Homes (North London) Drainage Strategy Statement 133834-DSS-(02)

1 INTRODUCTION

1.1 Purpose of this report

This report has been prepared to provide a formal response to the objection raised by Environment Agency (EA) in relation to the deculverting proposals associated with the former West Herts College, Home Park Mill Link Road, Kings Langley. For the purposes of this report this development will simply be referred to as the, “Site”.

1.2 Development Proposals

1.2.1 Main Site Bellway Homes Ltd (North London) have submitted a Reserved Matters (RM) application (LPA ref. 20/1858/AOD) on land at the above Site. The application provides details of the appearance, scale, layout, and landscaping of the development pursuant to an outline planning permission for the demolition of the existing building and its redevelopment for up to 65 flats (LPA ref. 18/1034/OUT). The reserved matters scheme originally proposed to retain the existing culvert in-situ, however following detailed site investigation and inspection of the culvert structure by the Applicant it was determined that its long term viability was compromised due to significant structural issues. The application was therefore amended in October 2020 to include an in-principle proposal for deculverting. The Environment Agency had initially objected to the proposal to retain the culvert in situ and the updated consultation response (attached under Appendix A) supported the principle of deculverting but contained several specific objections. A meeting was held with Officers of the EA on 26th November 2020 to discuss their concerns and this report provides a detailed response on behalf of the Applicant. It should be noted that Condition 12 of the outline application requires the submission of a landscape management plan, to include details of the proposed new section of watercourse such as cross sections, bank gradients, materials, a method statement for the deculverting works, details of management regimes, etc.

1.2.2 Deculverting and Formation of Naturalized Channel Following comments from the EA on the reserved matters application, proposals have been developed to replace the existing culvert with a naturalized open channel. Please see Section 2 of this report on the planning history associated with this. Full details respective to the naturalized open channel proposals are provided in the relevant sections and the appendices of this report below.

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 1

2 PLANNING HISTORY

2.1 General

This section provides a summary of the planning history associated with the current planning application for the Site.

2.2 Planning Application

The following table provides a timeline of the planning application and relevant conditions discharge submissions.

Table 2.1 - Planning Application History

Date Ref Description Comments

15/08/2019 18/1034/OUT Outline planning permission. Established the principle of residential development at the site. The Illustrative layout showed de-culverting of Mill Stream, but detailed investigations had not been undertaken and no details were provided as landscaping was defined as a reserved matter. A requirement to provide details of the landscaping and deculverting proposals were secured under Condition 12

17/07/2020 20/1193/NMA Non-material amendment to amend Refused as the amendment the wording of Condition 12 of the was material and therefore the

outline planning permission. S.73 application below was submitted.

05/10/2020 20/1512/FUL S.73 application to vary Condition 12 Application proposed to vary of the outline planning permission. the wording of Condition 12 to allow for retaining the culvert over the Mill Stream. Withdrawn when decision was taken to proceed with deculverting.

Submitted 20/1858/AOD Reserved Matters application for Decision by Three Rivers DC 08/09/20 (yet details of scale, layout, appearance awaited, pending outcome of to be and landscaping pursuant to the ongoing discussions between determined) outline planning application. the Applicant and the EA.

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2.3 EA Consultation

The following table provides a timeline of the EA consolation process undertaken as a part of the application process. Table 2.2 - EA Correspondence History

Date Description Summary

12/06/2018 Consultation response to outline planning application No objection subject to conditions. (EA ref. NE/2018/128647/01-L01).

14/08/2020 Consultation response to S.73 application to vary Objection to the proposals to amend the Condition 12 (EA ref. NE/2020/132088/01-L01). wondering of Condition 12 which removed reference to deculverting.

05/10/2020 Consultation response to Reserved Matters Objected to as the application proposed application ref. 20/1858/AOD (EA ref. to retain the existing culvert in situ. NE/2020/132302/01-L01).

17/11/2020 Consultation response to Reserved Matters Supported the principle of deculverting application ref. 20/1858/AOD - amended scheme but objections maintained in respect of proposals (EA ref NE/2020/132302/02-L01) the specific proposals for the naturalized open channel.

Letter enclosed under Appendix A.

26/11/2020 Meeting with EA – Attended by Meeting to discuss the proposals submitted for deculverting. Subsequent EA, Bellway (Client), RSK (Civil Engineer) and information provided on the requirements Savills (Planners) for flood risk review.

Meeting record enclosed under Appendix A.

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3 CONSTRAINTS AFFECTING THE DECULVERTING PROPOSALS

3.1 General

This section of the report provides a detailed summary of the existing conditions, constraints and other factors that have a material effect on the deculverting proposal, and which limit the options respective to the provision of a naturalized open channel.

3.2 Existing Site Conditions

3.2.1 Boundary Conditions The existing site boundaries have associated constraints which have influenced the design of the Site. These boundaries are clearly indicated on Figure 3.1 below.

Figure 3.1 – Site location plan

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The constraints imposed by the boundaries to the site are scheduled in Table 3.1 below Table 3.1 – Boundary conditions Boundary Feature Comments North Home Park An existing open-ended culvert enters the site on the northern Mill Link boundary, this emerging from a under a short section of third party Road and owned land that lies between the Site and Home Park Mill Link Road. adjacent third-party The invert level to this open-ended culvert is only marginally below land. the existing surround ground level, with the crown of this culvert being approx. 1m above existing ground level. The culvert is separated off from the rest of the site by a retaining wall which forms the edge of an existing spillway. This boundary feature imposes significant constraint on design options and has had to be dealt accommodated within the deculverting proposal.

The photographic image below highlights this constraint.

The retaining wall to the left of the open channel, which lies directly on the eastern boundary of the site, is also significant and this is discussed below under the constraints imposed by the eastern boundary. South Station A sustainable objective of the consented scheme as defined within footpath. the planning approved Design Access Statement is to create:

“Permeable street pattern of indicative scheme that gives priority to pedestrians and cyclists linking outwards to as many local facilities, public transport and natural green spaces as possible”.

This is discussed in more detail under Section 3.3.3 of this report.

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Boundary Feature Comments East Commercial The adjacent commercial premises to the east of the Site are properties elevated at a much higher level, in the region of 2.2m. A combination of earthworks batter slopes and retaining walls are situated both on and immediately adjacent to the eastern boundary and these acts to accommodate the aforementioned level differential. The deculverting works proposals have been designed to consider this constraint, which affects the whole of the eastern boundary to the Site. The photographic image below demonstrated the constraints of the eastern boundary to the site.

West Pinnacle Whilst there are no constraints on this boundary which have a direct House impact on the deculverting proposal, Pinnacle House, which lies on the western boundary, is service by an existing 300mm diameter surface water drain which passes through the narrow corridor in which the deculverting works are to be undertaken. This surface water drain is to remain in place and be left fully operational and therefore the proposed naturalized open channel has had to be designed around this buried obstruction.

The constraints imposed by the above tabulated boundary conditions were highlighted to the EA via a short presentation undertaken by RSK at the meeting of the 26th November 2020. It was generally accepted that without intimate knowledge of the site these constraints would not be obvious from a desk review of the deculverting proposals.

3.2.2 Existing Culvert The existing culvert required to be decommissioned as part of the deculverting works runs on an angle below the Site heading in a northeast to southwest direction. Part of the upstream end to the culvert lies below the site boundary and it is suspected that some portions of the existing boundary retaining walls between the Site and the commercial premises to the east may sit over the culvert structure, refer to Figure 3.2 below which shows the proximity of the existing culvert to the Site boundary.

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Mill Steam flows under the road

Existing culvert location

Proposed building outline Existing stream

Figure 3.2 – Proposed building outline with the current location of the culvert The removal of the culvert in the north east corner of the Site is not viable whilst maintaining the stability of the boundary. The deculverting proposal must therefore leaving a section of culvert insitu and stabilizing the culvert structure by back filling with foamed concrete. This in turn drives both the requirement for the energy dissipation structure and the offsetting of the proposed open channel away from the eastern boundary, refer to drawing no. 133938-RSK-ZZ-00-DR-P-00031 under Appendix B for further details.

3.2.3 Site Levels The site levels for all the boundaries need to be maintained as close to existing ground level as possible and as such the proposed on-site levels have been developed to accommodate this. The depth of the existing culvert structure relative to existing ground level is significant and has been taken into account in determining the hard bed level for the naturalized channel that will replace the culvert. The depth of the existing spillway relative to both the site boundary to the east and the existing levels to the west has also placed constraints on the deculverting proposals and the options available for redirecting the flows in this spillway over to the location of the proposed open channel.

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See photographic image below illustrating this constraint

Figure 3.3 – Retaining wall on boundary of site adjacent to the spillway, demonstrating the level differential between the site and the adjacent land

3.2.4 Flows to be Catered For The flow of water through the site is effectively controlled by an inlet weir structure located upstream of the Site, however ultimately it is suspected that the hydraulic limitations of the twin piped culverted passing under Home Park Mill Link Road is probably the ultimate physical constraint on the volume of flow able to passing into the culvert below the Site. Both the upstream weir and the tin piped culvert under Home Park Mill Link Road are outside of the control of the developer, Bellway Homes. As the hydraulic capacity of the existing box culvert below the Site is known to exceed that of the twin piped section passing below Home Park Mill Link Road then it can be argued that, in its current form, that the culverted watercourse below the site never acted to constrain the flows it received from upstream. The offsite features which have influence over the flows able to pass through the site are detailed on the drawings contained under Appendix B, however in designing the naturalized open channel cross section it has been assumed that the flows predicted to pass over the upstream weir located at the head of the Home Park canal arm are able to enter the Site unconstrained. Predicted flow and the hydraulic design of the open channel are discussed in detail under the FRA Technical Note contained under Appendix D.

3.3 Planning Requirements

The redevelopment proposals for the site are governed by the principles set out in the outline planning permission dated August 2019. These include that redevelopment of the site for residential use is acceptable in principle and that 65 flats would be an appropriate quantum of development for the site.

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Further, the reserved matters (proposals for scale, appearance, layout and landscaping of the development) are required to demonstrate conformity with adopted policies of the Development Plan, in order to create a high quality development that provides appropriate standards of living to future occupiers and would not adversely affect existing neighbours. Some of these, where they constrain potential options for de-culverting the Mill Stream, are explained in more detail below.

3.3.1 Amenity Space There is a need to provide for amenity open space in association with the new residential development. Policy DM1 of the adopted ‘Development Management Policies’ Local Development Plan Document requires that all applications for residential development should satisfy the detailed design standards set out in Appendix 2 of the document. These include standards for the provision of amenity space in new development which amount to 21m2 per 1 bed unit and 31m2 per 2 bed unit. Policy DM11 of the document also requires that for developments of more than 25 dwellings, 10% of the site area should be set aside for open space. The scheme proposals provide for public and private amenity space in the form of communal landscaped areas (including around the Mill Stream), and balconies, respectively. The total amount of open space proposed accords with the 10% requirement set out in Policy DM11. However, the scheme proposals fall short of the more detailed standards set out in Policy DM1; proposing an average of 21m2 per 1 bed unit and an average of 22.8m2 per 2 bed unit. The total amount of amenity space provided is considered to be the maximum possible given the need to balance competing demands for open space and car parking on the Site (see below).

3.3.2 Parking There is a need to provide car parking facilities for the new residents of the development. Policy DM13 of the adopted ‘Development Management Policies’ Local Development Plan Document states that developments should make provision for parking in accordance with the standards set out in Appendix 5 of the document. This sets vehicular parking standards of 1.75 spaces per 1 bed unit and 2 spaces per 2 bed unit. On this basis, the 65 units (split between 20 1 bed units and 45 2 bed units) would require a total of 125 parking spaces. At the time of considering the outline planning application it was acknowledged that a lower standard of car parking may be appropriate given the site is sustainably located and well connected. A total of 72 spaces were shown at the outline stage, and the same number have been shown in the detailed proposals put forward by the Reserved Matters application. Again, whilst the total parking provision falls short of the standards set out in the Policy, this is considered to be the maximum possible given the need to balance competing demands for car parking and open space.

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3.3.3 Access requirements As noted above, the site is sustainably located having good pedestrian links to the wider retail/employment opportunities in Kings Langley and in particular to the railway station via the public footpath to the south of the site. The presence and availability of use of the footpath by future residents has been an important consideration in the principle of redeveloping the site for residential use, as well as informing the decision as to what is an appropriate level of parking provision on the site, As such both the applicant and the planning authority are keen to retain access to the footpath for future residents of the development.

3.3.4 Maintenance Access The deculverting works lie within a main river corridor which has severe limitation on its width and therefore, as far as practicable, ease of access to the proposed open channel has been taken into consideration. The side slopes to all banks have been limited at 1 in 2 max. Where it has not been possible to deal with level differentials without the introduction of retaining walls, these walls have been designed to be maintenance free, having a minimum life span equal to that of the apartment building to be constructed on the site.

3.4 EA Objections

A proposal for deculverting was presented as a part of the reserve matters submission. Two images demonstrating these original proposals are presented in Figure 3.4 and Figure 3.5 below.

Figure 3.4 – Proposed layout for reserved matters

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Figure 3.5 – CGI of the Deculverting Proposal

Following the submission of Reserved Matters information relative to deculverting, the EA objected based on the points of concern listed below. 1. The cross-sectional shape of the open channel and its edge treatment. 2. The channel lining. 3. The energy dissipation structure. 4. New headwall proposal. The following sections of this report provide responses to each of the points raised in the aforementioned EA objection and the responses below also take in to account the principles for dealing with these points as agreed during the meeting held with the EA on 26th November 2020.

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4 CHANNEL GEOMETRY

4.1 EA Objection

The channel geometry: the proposed plans will create a box-shaped channel, canalized by gabion walls either side.  We would like to see the channel sides be made softer and less vertical. What ‘greener’ options have been/ can be considered?  We would like to see the channel cross-sectional shape be made more natural (e.g. riverbeds are not naturally flat).  Gabions generally have a short life span, and you should seek to explore erosion protection that works with natural processes for a long-term solution. For example, one option is bank re-grading. If further bank stability is required could the banks be capped with clay (then covered with topsoil and re-vegetated)

4.2 Designer’s Response

4.2.1 General As discussed at the meeting of the 26th November, in removing the culvert we are effectively creating a transitional section of open channel that ties a fast flowing canalized body of water, with significant potential energy, into a slow moving man made body of water that behaves more like a canal than a regular watercourse. In this regard we are creating a short section of open channel that has been designed to deal with: i) The speed and volume of the incoming body of water which is dictated by the upstream weir located at the end of the branch canal, known as the, “Home Park Arm”. ii) Dissipating the potential energy of this flowing body of water created by the significant drop in the hydraulic grade line between the upper section of the branch canal (Home Park Arm) to the north and the lower section of manmade naturalized channel to the south (Mill Stream), with an overall vertical drop approximating to 3.5m.

4.2.2 Constraints Specific to the Open Channel Geometry Hydraulic Constraints Refer to drawing no. 133938-RSK-ZZ-00-DR-P-00030 contained under Appendix B for details of the hydraulic features and constraints which have significant influence over the flows that the open channel is required to cater for.

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The vertical drop discussed under section 4.2.1 above is comprised as follows: i. Canal branch arm water level behind the control weir down to the upstream headwall of the twin pipes passing under Home Park Mill Link Road = 1.5m approx. ii. Fall in the piped / culverted watercourse from the upstream headwall to the outfall on to site = 0.7m. iii. Fall in the open spillway linking the offsite culvert outfall with the on-site culvert = 1.2m. iv. Fall over the length of the on-site culvert = 0.1m Total vertical drop between the upstream and downstream water level = 3.5m. Corridor & Level Constraints The constraints of the site in terms of corridor width and the depth of the existing culvert’s invert level relative to the surround ground level significantly limit the scope of what is achievable The proximity of the existing culvert to the site boundary and the necessity to, in part, leave this culvert insitu thereby avoiding the destabilization of the site boundary also limits what is achievable and both these factors are demonstrated in the Typical Section incorporated under Appendix B Corridor Width and Depth of Open Channel The corridor available in which the open channel is to be formed ranges in width from 15.3m at the widest, to 11.9m minimum at the narrowest point. This width of corridor leaves very little space for batter slopes or engineered reinforced earthworks once the designed open channel width of 4m is taken into account. To assist in overcoming this problem the invert level of the proposed open channel has been raised significantly respective to that of the culvert it replaces. The existing culvert is almost level falling only 100mm over its full length and has an average invert level of 65.15m. The proposed open channel has a bed gradient of 1% (1 in 100) with an invert level of 66.00m at its head, therefore the open channel has been raised 0.85m to mitigate the problem of depth respective to corridor width. 66.00 – 65.15 = 0.85m The limitation on how much the channel could be raised has been dictated by the hard bed tie-in level with Mill Steam and also the need to ensure that the open channel gradient does not become significant enough to introduce excessive turbulent flow within it. As will be discussed in more detail under Section 4.2.3 below, the speed of flow through the open channel acts to limit available options for edge treatment and therefore the lower the speed of flow that can be achieved, the less engineered the edge treatment needs to be.

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The width of the designed open channel has been dictated by the following:  Requirement to introduce an irregular shape into the channel base  Necessity to ensure a suitable flow velocity is attained for all circumstances ranging from the dry weather flow condition up to the most extreme event catered for (1 in 100 + 70%).  Requirement to introduce berms into the open channel to facilitated edge planting for the purposes of softening the design and for the introduction of wildlife habitat.  Ensuring that, as with the original culvert, flow depths are contained entirely within the channel for all storm scenarios up to and including the 1 in 100yr + climate change event, thereby ensuring that the open channel poses no risk of flooding both on site and upstream of the site. Based on the issues highlighted it is obvious that the wider the open channel is made the greater the necessity will be to create vertical retaining features to accommodate the level differential between the open channel bed and the adjacent ground level. Following discussions with the EA on 26th November 2020, the sectional profile of the open channel has been amended, whilst still limiting its overall width to 4m. Any further widening of the channel would require the complete removal of the footbridge to the south and significant works on third party land to both replace this bridge and to accommodate the transition of a wider open channel into the downstream watercourse (Mill Stream).

4.2.3 Edge Treatment In terms of edge treatment to the proposed open channel, the anticipated speed of flow though the channel limits the options available when considering long term durability and scour. The design requires a relatively fast flowing body of water to be directed through a meandering channel of limited width and therefore the associated edge restraint will need to be sufficiently robustly to accommodate this issue. The open channel to be created at only 35m in length is too short to consider more than one type of edge restraint feature per side of whilst ensuring there is a sufficiently meaningful length of feature for it to become fully established and integrated into the surrounding landscaping. Knowing the predicted speed of flows through the open channel a suitable green measure has been adopted for replacement of the low level gabions along the western edge of the open channel, these now being replaced with staked coir roll, pre-planted to assist in the early establishment of edge side vegetation for this side of the channel. With respect to the replacement of the gabion retaining wall on the eastern edge of the site the following options have been considered. Green Measures  Fascines / brushwood  Willow spiling  Woody material

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Green – Grey Measures  Geocell system  Vegetated concrete blocks  Vegetated reinforced earthworks  Vegetated riprap

Green Measures Fascines / Brushwood

As shown opposite fascines / brushwood are not suitable for creating a retaining feature capable of support up to 1.5m of retained earth. In addition, this type of edge feature does not have the longevity of design life to be viable for the eastern bank of the open channel.

Willow Spiling As shown opposite whilst willow spiling can provide earth retention it requires too much land take from the very restricted corridor to the east. Space is at a premium in this corridor as it forms part of the useable public realm necessary to meet planning requirements. The necessity to also have a maintenance free life span equivalent to that of the development building cannot be guaranteed with this retaining feature.

Woody Materials As shown opposite whilst woody materials can be used to provide an earth retention solution this is very limited in nature and will not have the longevity of design life required nor retention capability for the eastern bank.

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Grey – Green Systems Geocell system Geo cell systems whilst able to provide the retaining properties required do not have the longevity in term of design life and require significant space behind to anchor back the earth filled cells. The presence of the existing culvert will impeded the installation of this type of retaining feature.

Vegetated concrete blocks Vegetated concrete blocks are a hard- engineered solution and therefore provide no significant benefit over the gabions as they are less easily screened and require more working space for installation.

Vegetated reinforced earthworks Vegetated reinforced earthwork whilst able to provide the retaining properties required do not have the longevity in term of design life and require significant space behind to anchor back the earthworks. The presence of the existing culvert will impeded the installation of this type of retaining feature.

Vegetated riprap Vegetated riprap whilst able to provide the retaining properties required and the longevity required cannot be installed as it required too much space not being able to install this with a vertical face.

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Without reverting to a fully engineered solution (Grey measure) a gabion retaining wall provides the most viable long term edge treatment to the east side of the open channel, however the effect of this on the channel itself can be mitigated as described under Section 4.2.5 below. It should be noted that with the introduction of a dry berm in front of the gabion wall it is the berm itself which actually forms the edge treatment to the open channel, with the gabion wall being seen to be set back as an adjacent retaining feature.

4.2.4 Bed Level It can be stated that the proposed open channel is not transitioning into a natural watercourse but into a man-made open channel cut into the ground which is more akin to a canal. Mill Stream has a relatively flat base as determined by the results of the survey works undertaken to establish its hard bed level at the proposed point of tie-in. Following the discussions with the EA on 26th November 2020 a revised open channel profile has been developed which has an irregular shape rather than the previous flat base across its full width. This has been achieve by the introduction of two berms into the open channel, one each side, thereby creating a lower trapezoidal shaped channel between them, see typical channel profile below.

4000

Dry Berm Partially Submerged Berm

Normalized Flow Water Level

Trapezoidal Channel

Figure 4.1 – Revised Geometry of Open Channel Profile

Details of how the above open channel profile will operate during varying flow conditions has been detailed within the calculations contained under Appendix D, where flow volumes and velocities have been calculated for varying storm event scenarios.

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Refer to the Typical Section extracts below to see how this profile has been integrated within the overall naturalized open channel design. Full sized versions of these Typical Sections below are also incorporated under Appendix C of this report.

Figure 4.1 – Updated section following EA meeting, Chainage 5m

Figure 4.2 – Updated section following EA meeting, Chainage 20m

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4.2.5 Gabions General In consideration of the comments made by the EA respective to the use of gabions the site constraints must be taken into account as they impose severe limitations on the options available, especially for eastern bank to the open channel. Site constraints As previously highlighted under Section 3 above a public realm area is required to be formed on the eastern side of the proposed open channel. This is necessary to meet minimum planning requirements for public realm space and also to facilitate a pleasant route corridor in which a footpath can be constructed that will link the development with the strategic public footpath to the south of the Site. The narrowness of the corridor in which the open channel is to be constructed has previously been discussed and its relative depth respective to the existing adjacent ground levels requires a vertical faced retaining feature to be installed to ensure adequate space can be maintained east of the open channel for the purposes mentioned above, whilst also ensuring that this feature does not clash with the portion of existing culvert required to be left insitu supporting the site boundary. The consideration of various greener options to a gabion retaining wall have been reviewed under Section 4.2.3 above, however in order to have the required retaining capability coupled with a maintenance free serviceable life exceeding 100yrs leaves gabions as the only viable solution for the space available. Mitigation Measures To mitigate the effect of the gabions on the open channel they will be set back behind an earth berm which will form a soft edge to the open channel. The landscaping proposals for this berm will act in tandem with planting within the gabions baskets themselves to screen this retaining wall. Refer to the image below as an example of proposed planting within the gabions.

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Lifespan of Gabions In answer to the EA’s concerns over the life span of a gabion retaining feature, this is dependent upon the quality of the mesh cages / baskets utilized. Gabions can have a design life exceeding 120yrs if durable mesh cages are employed with full BBA certification. Refer to an example of a gabion mesh cage which is BBA compliant under Appendix C and for which the design life is predicted to be more than 120yrs, as defined within the BBA certification. The gabion retaining wall proposed for the eastern side of the open channel would utilize BBA certified gabion mesh boxes thereby ensuring its design life is more than fit for purpose.

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5 CHANNEL LINING

5.1 EA Objection

Channel lining (bentonite geosynthetic clay liner) underneath uniformly sized gravels.  Please explain why channel lining is required. If it is necessary, this should be a clay layer (without introducing synthetic materials). Synthetic matting tends to come away overtime and cause further problems.  Gravels should be of mixed size to create more habitat niches (we recommend 10-50mm mixed gravels, locally derived and clean).

5.2 Designer’s Response

5.2.1 Channel Liner A Geoenvironmental appraisal of the development site has been undertaken by RSK. The full findings of this report are too extensive to discuss here but a summary statement is provided below. “The deculverting proposals have the potential to impact the hydrogeology of the site and water quality within the watercourse downgradient of the site. Whilst robust site investigation works have assessed the ground conditions in the south-eastern corner of the site, and identified areas of contamination that require targeted removal in order to be protective of controlled waters, removal of the culvert would increase infiltration through the unsaturated zone and made ground soils. Therefore, to mitigate the potential for the leaching and mobilisation of any previously unidentified contaminants, and also the possibility of a significantly increased sediment load entering the channel, it will be necessary to place a clay liner at the base of the newly formed open channel to create a separation layer”. Based on the above summary statement and adopting a precautionary approach, the intention is to provide a clay liner to the open channel. Following objection from the EA to incorporating a Geosynthetic Clay Liner (GCL) it is now intend to utilize an inert, “Puddle Clay”, material placed in a layer thickness of 200mm minimum as shown on the typical cross section details located under Appendix C. The specification for the puddle clay will be to the River and Canal Trust (Former British Waterways) standard, a copy of which is also contained under Appendix C.

5.2.2 Gravels to Base of Open Channel The hydraulic modelling of the proposed open channel is covered in detail under Appendix D and from this modelling the anticipated flow volumes and associated velocities of flow have been derived based on the predicted flows that the open channel will be required to convey. A summary of these flows and their associated velocity through the open channel are presented in the Table 5.1 on the following page.

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 21

Table 5.1: Predicted Flows and Flow Velocities in the Designed Channel

Return Period Normalized 1 in 2yr 1 in 50yr 1 in 100yr

(Dry Weather) + 70%

* Modelled Flows (Cumecs) #0.45 1.48 2.55 3.43

Designed Open Channel 0.51 1.48 2.60 3.45

Flow Capacity (Cumecs)

Velocity of flow (m/s) 0.89 1.21 1.37 1.51

Depth of flow (m) 0.35 0.57 0.75 0.85

* Modelled flows given in the table above have been taken directly from the following source: “Gade and Bulbourne Modelling Study (JBA, 2019)”.

# Flow calculated by RSK based on depth of flow passing over the control weir at the end of the Home Park branch canal, as this is not available within the abovementioned JBA report. Refer to Appendix D for further details on how this flow was determined. The velocity of the anticipated flows defined in the Table 5.1 above have been used to determine the size of random aggregate / cobbles that will be utilized as the finish to the open channel. Table 5.2: Class of Stone for the Lining of Watercourses Relative to Water Velocity

Source: HR Wallingford, “Green Approached to River Engineering – Chapter 3 Decision Support Framework”. Based on Table 5.2 above it is clear that natural gravel aggregate alone will not be sufficient and cobbles in the size range of 64 to 128 mm will be needed in order to resist the shear forces that will be imposed by the velocity of the body of water which will flow over them. Considering the narrow nature of the open channel it is therefore proposed to utilize cobble sizes tending to the bottom end of the aforementioned range with the larger voids between the cobbles being filled with a 10-50mm mixed of clean gravels, locally sourced.

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 22

6 ENERGY DISSIPATION STRUCTURE

6.1 EA Objection

The energy dissipation structure  We have concerns over this and would like to see alternatives considered. For example, a rockramp.  How has fish passage been considered as part of this design?

6.2 Designer’s Response

6.2.1 Requirement for This Structure The requirement for an energy dissipation structure was presented by RSK in the meeting with the EA held on the 26th November 2020. The general reasoning for this structure was accepted by the EA subject to some comments, however for the completeness of this response report this reasoning has been formalized below. The energy dissipation structure is essential to perform the following functions:  Diverting incoming flow entering the Site on the north eastern edge away from this boundary as quickly as possible in a safe and controlled manner that avoids conflict with the existing culvert structure.  To perform the above task whilst also mitigating its impact on land take, thereby ensuring that the planning requirements for parking space provision can be attained, by allowing development spaces over this structure.  To maximize the length of open channel which can be accommodated on Site, thereby maximizing its ecological and wildlife habitat benefit.  To accommodate, in a short length, the level differential between the fixed incoming culvert invert level and the design bed level to the open channel. This ensures that a suitably shallow gradient can be attained for the open channel downstream of this structure.  To contain the hazard generated when a significant volume of fast flowing water falls a vertical height of 750mm. The energy dissipation structure provides a controlled environment in which this hazard is kept separated from future residents and the public.  To convert turbulent incoming flow into controlled constant laminar flow passing out through the lower slot of outfall portion of this structure. Note that the upper slot would only be triggered in the most extreme storm events, or should the lower slot become temporarily clogged as it is effectively an emergency overflow provision.  To provide a robust flow control structure that will have a design life at least equivalent to the apartment building to be constructed on the Site. The above functional requirements have effectively dictated that a hard-engineered solution is the only feasible option, with a cast insitu reinforced concrete structure being

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 23

ideal for this purpose. The mitigating factor to this design is that only a very limited portion of this structure will be seen when the deculverting works have been completed. In direct response to the EA’s comments on the structure itself it is confirmed that it has been designed for a serviceable life equivalent to that of the apartment building to be built on the Site and to be fully accessible. It will have both man access entry points along with inspection viewing slots that will help minimize the necessity for actual man entry into this structure. This structure will have no mechanical or moving parts within it and will therefore effectively be maintenance free, only requiring periodic inspections to assess debris and silt build up within it. The hydraulic performance of this structure exceeds the predicted volume of flow that will ever pass through it based on the JBA, “Gade and Bulbourne Modelling Study (2019)”. Modelling Study for the 1 in 1000yr event predicts flow through the site to be 3.67 Cumecs The hydraulic capacity of the 2m wide channel (Narrowest internal portion) within the energy dissipation structure is calculated to be 3.78 Cumecs at a depth of 240mm. 3.78 > 3.67 therefore OK. Details in respect of the derivation of the hydraulic capacity of the energy dissipation structure are provided under Appendix D.

6.2.2 Fish Passage As previously noted, an energy dissipation structure is essential to accommodate the deculverting works due to the existing overall level difference between the upper section of the branch canal (Home Park Arm) and the lower section of Mill Stream south of the site. The design of this structure has been heavily affected by the existing site constraints and planning requirements for the main development. The details of the energy dissipation structure are provide on RSK drawing 133938-RSK-ZZ-00-DR-P-00041 and the lower slot in the outfall portion of the structure has been given a depth of 250mm, which is considered adequate to allow the passage of fish through it. The potential flow speeds considered as a part of the design are presented in Appendix D. As stated above the lower slot is sufficiently wide and deep for the fish to pass through, however as highlighted in a previous sections of this report the flow and associated speed of flow through the energy dissipation structure will be dictated by flow control features that are located off the Site and are not under the control of the developer, Bellway Homes. In this regard it is not clear if the velocity of water passing through the structure will significant enough to prevent fish movement.

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 24

7 HEADWALL

7.1 EA Objection

New headwall  The installation of outfalls on a watercourse will contribute cumulatively to the degradation of this waterbody. If it is necessary to install an outfall into the watercourse, it is advised that soft options are proposed, such as:  Outfalls cut flush with the gradient of the bank (or at a shallower gradient if the bank has been unnaturally steepened).  No headwalls or wingwalls if these are not necessary.  If a headwall/wingwall is required, bespoke brickwork is preferred as this will allow the outfall to be as small necessary, to reduce the break in riparian continuity.

7.2 Designer’s Response

The comments of the EA are noted and are agreed with. We believe the headwall in question is the existing headwall on the end of the 300mm diameter surface drain servicing Pinnacle House. This existing headwall is located immediately adjacent to the end of the existing culvert and has a concrete headwall which is tied in with the culvert structure. As the downstream section of the culvert is required to be removed, then the headwall associated with the existing 450mm diameter outfall pipe will also be removed. The revised outfall will for obvious reasons have to be accommodated but as its location is very close to the existing footbridge then it will pass through the gabion retaining wall that is necessary at this location. As such the proposed gabion retaining wall itself will become the headwall for this outfall and will present a much neater and more aesthetically pleasing appearance than a harsh concrete headwall. The outfall pipe will be finished just proud of the face of the gabion wall and in this regard, it will therefore not require its own individual headwall and / or associated wing walls. This type of details present s neat and tidy finish and as the pipe outfall invert level is below the existing level of Mill Stream it will be partially submerged as is currently the case. Based on the fact that runoff from the former college site is to be removed from this existing outfall, it will be possible to reduce its size down from the existing 450mm diameter to 300mm diameter without the potential of causing upstream flooding. This will be of benefit to ensuring that a non-intrusive outfall pipe can be built into the gabion retaining wall feature to be built in this location. Refer to the following page for a photographic image of what the new headwall would look like, bearing in mind that the example shown is 450mm diameter and the proposal would incorporate a 300mm diameter pipe. Also shown on the following page is a photographic image of the existing headwall and outfall pipe that is currently tied into the culvert structure.

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 25

Photograph 7.1 – Existing Outfall with Concrete Headwall

Photograph 7.2 – Example of Proposal for the New Headwall

The situation as shown above is very similar in that the outfall site is very close to a footbridge and therefore must pass through a gabion retaining wall that runs into an adjoining bridge abutment.

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 26

8 FLOOD RISK

8.1 EA Comments Following the Meeting on 26th November 2020

Usually for deculverting, we would request detailed hydraulic modelling, however rather than having to include full detailed modelling in the FRA, you could do some initial assessments to demonstrate flood risk will not increase as a result of the proposed development: The updated FRA could include, but not be limited to, the following:  an initial assessment using topo and site surveys to demonstrate no increase in flood risk.  rough assessment on channel capacity  demonstrate the proposal is not changing the cross-sectional area of the river channel  demonstrate no inspection chambers will covered and access is not impeded by the proposal, in case we require access for emergency works  written justification for need of the energy dissipating structure, demonstrate it is built to allow sufficient capacity/flows for the channel  demonstrate there will be no adverse impact on the stability of the bank by including an adequate undeveloped buffer zone from the top of the main riverbank to the edge of the built footprint If the FRA shows that there could be a breach point/increase in flooding, we would require more detailed information to demonstrate flood risk is not increased.

8.2 Designer’s Response

The naturalized open channel that will replace the existing culvert has been designed to ensure that predicted flows for the 1 in 100yr + 70% event will remain within this channel and therefore the channel presents no increase in flood risk over the original culvert. Testing the channel against predicted flows for the 1 in 1000yr only results in a minor breach of the channel cause the extent of flooding to slightly submerge the coir roll edge restraint to the western side of the channel. The extents of flooding for the Site along with predicted flow depths in the open channel are detailed on RSK drawing number 133938-RSK-ZZ-00-DR-P-00045 contained under Appendix D. The hydraulic calculations undertaken to determine the channel capacity are also contained under Appendix D. The proposed naturalized open channel is narrower and shallower than the existing Mill Steam and will therefore taper out to the existing Mill Stream profile and hard bed level at the tie-in point. In this regard the open channel proposals will not change the cross- sectional area of the existing river channel. Access to the naturalized open channel will not be impeded by the proposals, however being in a confined corridor this access will be limited and the features making up the channel have been design to have longevity with minimum maintenance requirement.

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 27

Written justification for the energy dissipation structure has been provided under Section 6 of this report. The open channel design has been developed to provide as much buffer space as can be attained within the severe constraint of the corridor in which it is to be implemented. More specific details relative to the issue of Flood Risk are presented in the FRA Technical Note contained under Appendix E.

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 28

9 DESIGN CONCLUSION

9.1 Original Open Channel Proposal Objected to by EA

9.2 Revised Open Channel Proposal Amended in Line with EA Comments

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 29

APPENDIX A – EA CORRESPONDENCE

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 30

Adam Ralton Our ref: NE/2018/128647/01-L01 Three Rivers District Council Your ref: 18/1034/OUT Development Control Three Rivers House Northway Date: 12 June 2018 Rickmansworth Hertfordshire WD3 1RL

Dear Adam

West Herts College, Home Park Mill Link Road, Station Road, Kings Langley, Hertfordshire, WD4 8LZ

Outline application: demolition of existing college building and redevelopment for a residential development of 65 flats [appearance, landscaping, layout and scale Reserved]

Thank you for consulting us on the above planning application. The proposed development will be acceptable subject to the following planning conditions. We ask to be consulted on the details submitted for approval to your Authority to discharge these conditions and on any subsequent amendments/alterations

Condition 1 – Remediation Strategy No development approved by this planning permission shall commence until a remediation strategy to deal with the risks associated with contamination of the site has been submitted to, and approved in writing by, the Local Planning Authority. This strategy will include the following components:

1. An updated preliminary risk assessment which has identified:  all previous uses;  all proposed activities  potential contaminants associated with those uses;  a conceptual model of the site indicating sources, pathways and receptors; and  potentially unacceptable risks arising from contamination at the site.

2. A site investigation scheme, based on (1) to provide information for a detailed assessment of the risk to all receptors that may be affected, including those off site.

3. The results of the site investigation and the detailed risk assessment referred to in (2) and, based on these, an options appraisal and remediation strategy giving full details of the remediation measures required and how they are to be undertaken.

Cont/d..

4. A verification plan providing details of the data that will be collected in order to demonstrate that the works set out in the remediation strategy in (3) are complete and identifying any requirements for longer-term monitoring of pollutant linkages, maintenance and arrangements for contingency action.

Any changes to these components require the written consent of the local planning authority. The scheme shall be implemented as approved.

Reason To ensure that the development is not put at unacceptable risk from, or adversely affected by, unacceptable levels water pollution in line with paragraph 109 of the National Planning Policy Framework (NPPF) and your local plan policy DM9 (Contamination and Pollution Control). The previous use of the proposed development site as a papermill and industrial estate presents a high risk of contamination that could be mobilised during construction to pollute controlled waters. Controlled waters including groundwater are particularly sensitive in this location because the proposed development site:

 is within Source Protection Zone (SPZ) 1 for public water supply  is located upon a Secondary aquifer A in hydraulic continuity with a Principal aquifer.  is located in close proximity of the River Gade.

In addition, the Thames river basin management plan requires the restoration and enhancement of water bodies to prevent deterioration and promote recovery of water bodies. Without this condition, the impact of contamination from the site may cause further deterioration and prevent the recovery of a drinking water protected area in the Mid-Chilterns Chalk GB40601G601200.

The documents submitted in support of this planning application provides us with confidence that it will be possible to suitably manage the risk posed to controlled waters by this development. Further detailed information will however be required before built development is undertaken. It is our opinion that it would place an unreasonable burden on the developer to ask for more detailed information prior to the granting of planning permission but respect that this is a decision for the Local Planning Authority.

In light of the above, the proposed development will be acceptable if a planning condition is included requiring the submission of a remediation strategy, carried out by a competent person in line with paragraph 121 NPPF.

The Planning Practice Guidance defines a "Competent Person (to prepare site investigation information): A person with a recognised relevant qualification, sufficient experience in dealing with the type(s) of pollution or land instability, and membership of a relevant professional organisation."(http://planningguidance.planningportal.gov.uk/blog/policy/achieving- sustainable-development/annex-2-glossary/)”

Condition 2 – Verification report Prior to any part of the permitted development being occupied, a verification report demonstrating the completion of works set out in the approved remediation strategy and the effectiveness of the remediation shall be submitted to, and approved in writing, by the local planning authority. The report shall include results of sampling and monitoring carried out in accordance with the approved verification plan to demonstrate that the

Cont/d.. 2 site remediation criteria have been met.

Reasons To ensure that the site does not pose any further risk to human health or the water environment by demonstrating that the requirements of the approved verification plan have been met and that remediation of the site is complete. This is in line with paragraph 109 of the NPPF and your local plan policy DM9 (Contamination and Pollution Control). This condition will also prevent further deterioration of a water quality element to a lower status class and prevent the recovery of a drinking water protected area in the Mid-Chilterns Chalk GB40601G601200.

Condition 3 – Maintenance and Monitoring plan The development hereby permitted may not commence until a monitoring and maintenance plan in respect of contamination, including a timetable of monitoring and submission of reports to the Local Planning Authority, has been submitted to, and approved in writing by, the Local Planning Authority. Reports as specified in the approved plan, including details of any necessary contingency action arising from the monitoring, shall be submitted to, and approved in writing by, the Local Planning Authority.

Reasons To ensure that the site does not pose any further risk to human health or the water environment by demonstrating that the requirements of the approved verification plan have been met and that remediation of the site is complete. This is in line with paragraph 109 of the NPPF and your local plan policy DM9 (Contamination and Pollution Control). This condition will also prevent further deterioration of a water quality element to a lower status class and prevent the recovery of a drinking water protected area in the Mid-Chilterns Chalk GB40601G601200.

Condition 4 – Unsuspected Contamination If, during development, contamination not previously identified is found to be present at the site then no further development (unless otherwise agreed in writing with the Local Planning Authority) shall be carried out until a remediation strategy detailing how this contamination will be dealt with has been submitted to and approved in writing by the Local Planning Authority. The remediation strategy shall be implemented as approved.

Reasons To ensure that the development is not put at unacceptable risk from, or adversely affected by, unacceptable levels water pollution from previously unidentified contamination sources at the development site in line with paragraph 109 of the NPPF and your local plan policy DM9 (Contamination and Pollution Control). No investigation can completely characterise a site. The condition may be appropriate where some parts of the site are less well characterised than others, or in areas where contamination was not expected and therefore not included in the original remediation proposals.

Condition 5 – Use of Infiltration Surface Water Sustainable Drainage Systems (SuDS) No infiltration of surface water drainage into the ground at this site is permitted other than with the written consent of the Local Planning Authority. The development shall be carried out in accordance with the approved details.

Reasons To ensure that the development is not put at unacceptable risk from, or adversely affected by, unacceptable levels water pollution caused by mobilised contaminants in

Cont/d.. 3 line with paragraph 109 of the NPPF and your local plan policy DM9 (Contamination and Pollution Control).

In light of the previous use of the site, we do not believe that the use of infiltration SuDS is appropriate in this location. Without this condition we would object to the proposal in line with paragraph 109 of the NPPF because it cannot be guaranteed that the development will not be put at unacceptable risk from, or be adversely affected by, unacceptable levels of water pollution.

Condition 6 - Use of Piling, Boreholes, Tunnel Shafts, Ground Source heating And Cooling Systems Piling and other foundation designs, deep excavations, investigation boreholes and ground source heating and cooling systems using penetrative methods shall not be carried out other than with the written consent of the local planning authority. The development shall be carried out in accordance with the approved details.

Reasons To ensure that the proposed piling and foundations, installation of groundwater boreholes do not harm groundwater resources in line with paragraph 109 of the NPPF, your local plan policy DM9 (Contamination and Pollution Control) and the Environment Agency’s approach to groundwater protection, February 2018 Version 1.2. https://www.gov.uk/government/publications/groundwater-protection-position- statements

Some piling techniques can cause preferential pathways for contaminants to migrate to groundwater and cause pollution. A piling risk assessment and appropriate mitigation measures should be submitted with consideration of the EA guidance. During piling works (especially if the piles extend to the Chalk within SPZ1 saturated zone) due to the proximity of nearby potable abstractions the weekly groundwater monitoring for insitu parameters and turbidity should be considered. EA guidance can be found here: http://webarchive.nationalarchives.gov.uk/20140328084622/http://cdn.environment- agency.gov.uk/scho0202bisw-e-e.pdf

Condition 7 – Borehole Management Scheme A scheme for managing any borehole installed for the investigation of soils, groundwater or geotechnical purposes shall be submitted to and approved in writing by the local planning authority. The scheme shall provide details of how redundant boreholes are to be decommissioned and how any boreholes that need to be retained, post-development, for monitoring purposes will be secured, protected and inspected. The scheme as approved shall be implemented prior to the occupation of any part of the permitted development.

Reasons To ensure that redundant boreholes are safe and secure, and do not cause groundwater pollution or loss of water supplies in line with paragraph 109 of the NPPF, your local plan policy DM9 (Contamination and Pollution Control) and The Environment Agency’s approach to groundwater protection March 2017 Version 1.0 https://www.gov.uk/government/publications/groundwater-protection-position- statements

The submitted planning application indicates that boreholes will need to be installed at the development site to investigate groundwater. If these boreholes are not decommissioned correctly they can provide preferential pathways for contaminant movement which poses a risk to groundwater quality. Groundwater is particularly

Cont/d.. 4 sensitive in this location because the proposed development site is within SPZ1.

Condition 8 – De-culverting and Landscape Management Plans No development shall take place until a landscape management plan, including long- term design objectives, management responsibilities and maintenance schedules for all landscaped areas (except privately owned domestic gardens), shall be submitted to and approved in writing by the local planning authority. The landscape management plan shall be carried out as approved and any subsequent variations shall be agreed in writing by the local planning authority.

The scheme shall include the following elements:  Provide detailed cross sections for the new proposed section of river. The upstream culvert and downstream bridge width on site will constrain flows, meaning the width of the new section of river should take this in to account.  Provide information on the bank gradients, material and planting regime  Provide information regarding the substrate of the river  Provide a method statement for the river works  Detail extent and type of new planting (NB planting to be of native species)  Details of maintenance regimes  Provide information on whether a new bridge is proposed for this development. If a new bridge is proposed, we would require the abutments to be set back.

Reasons This condition is necessary to ensure the protection of wildlife and supporting habitat and secure opportunities for the enhancement of the nature conservation value of the site in line with NPPF paragraph 109 which recognises that the planning system should aim to conserve and enhance the natural and local environment by minimising impacts on biodiversity and providing net gains in biodiversity where possible, contributing to the Government’s commitment to halt the overall decline in biodiversity, including by establishing coherent ecological networks that are more resilient to current and future pressures. Paragraph 118 of the NPPF also states that opportunities to incorporate biodiversity in and around developments should be encouraged. This condition also supports your local plan policy DM6 (Biodiversity, Trees, Woodlands, Watercourses and Landscaping)

Advice to applicant - Flood Risk Activity Permit Under the terms of the Environmental Permitting Regulations a Flood Risk Activity Permit is required from the Environment Agency for any proposed works or structures, in, under, over or within 8 metres of the top of the bank of the Mill Stream/River Gade, designated a ‘main river’. Details of lower risk activities that may be Excluded or Exempt from the Permitting Regulations can be found on the gov.uk website. Please contact us at [email protected].

We support the proposal to deculvert a section of the Mill Stream (River Gade), but the car parking spaces shown in drawing '114822-IBI-XX-XX-XX-PL-A-200-03' should be set back from the top of bank of the deculverted section of the Mill Stream.

Further Advice to Applicant From the design and access statement, the proposed development includes 1 no. 5- storey building (65 residential units made up of 1-2 bedroom properties). Table 12: Preliminary CSM should be updated to include the construction activities and additional details for the drainage and foundation designs for the proposed development.

Due to the age of the former papermill (1873-1988) it may have had on-site power

Cont/d.. 5 source and the contaminants of concern associated with this activity should also be considered in the site investigation.

We recommend that developers should: 1. Follow the risk management framework provided in CLR11, Model Procedures for the Management of Land Contamination, when dealing with land affected by contamination. 2. Refer to the Environment Agency Guiding principles for land contamination for the type of information that we required in order to assess risks to controlled waters from the site. The Local Authority can advise on risk to other receptors, such as human health. 3. Consider using the National Quality Mark Scheme for Land Contamination Management which involves the use of competent persons to ensure that land contamination risks are appropriately managed. 4. Refer to the contaminated land pages on GOV.UK for more information. 5. We expect the site investigations to be carried out in accordance with best practice guidance for site investigations on land affected by land contamination. E.g. British Standards when investigating potentially contaminated sites and groundwater, and references with these documents:  BS5930:2015 Code of practice for site investigations;  BS 10175:2011 A1:2013 Code of practice for investigation of potentially contaminated sites;  BS ISO 5667-22:2010 Water quality. Sampling. Guidance on the design and installation of groundwater monitoring points;  BS ISO 5667-11:2009 Water quality. Sampling. Guidance on sampling of groundwaters (A minimum of 3 groundwater monitoring boreholes are required to establish the groundwater levels, flow patterns but more may be required to establish the conceptual site model and groundwater quality. See RTM 2006 and MNA guidance for further details).  Use MCERTS accredited methods for testing contaminated soils at the site. A Detailed Quantitative Risk Assessment (DQRA) for controlled waters using the results of the site investigations with consideration of the hydrogeology of the site and the degree of any existing groundwater and surface water pollution should be carried out. This increased provision of information by the applicant reflects the potentially greater risk to the water environment. The DQRA report should be prepared by a “Competent person” E.g. a suitably qualified hydrogeologist. In the absence of any applicable on-site data, a range of values should be used to calculate the sensitivity of the input parameter on the outcome of the risk assessment.  GP3 version 1.1 August 2013 provided further guidance on setting compliance points in DQRAs. This is now available as online guidance: https://www.gov.uk/guidance/land-contamination-groundwater-compliance- points-quantitative-risk-assessments  Where groundwater has been impacted by contamination on site, the default compliance point for both Principal and Secondary aquifers is 50m.

Where leaching tests are used it is strongly recommended that BS ISO 18772:2008 is followed as a logical process to aid the selection and justification of appropriate tests based on a conceptual understanding of soil and contaminant properties, likely and worst-case exposure conditions, leaching mechanisms, and study objectives. During risk assessment one should characterise the leaching behaviour of contaminated soils using an appropriate suite of tests. As a minimum these tests should be:  upflow percolation column test, run to LS 2 – to derive kappa values;  pH dependence test if pH shifts are realistically predicted with regard to soil properties and exposure scenario; and

Cont/d.. 6  LS 2 batch test – to benchmark results of a simple compliance test against the final step of the column test.

Following the DQRA, a Remediation Options Appraisal to determine the Remediation Strategy in accordance with CRL11. The verification plan should include proposals for a groundwater-monitoring programme to encompass regular monitoring for a period before, during and after ground works. E.g. monthly monitoring before, during and for at least the first quarter after completion of ground works, and then quarterly for the remaining 9-month period.) The verification report should be undertaken in accordance with in our guidance Verification of Remediation of Land Contamination http://publications.environment- agency.gov.uk/pdf/SCHO0210BRXF-e-e.pdf

1. Where SUDs are proposed; infiltration SUDs should not be located in unsuitable and unstable ground conditions such as land affected by contamination or solution features. Where infiltration SuDS are to be used for surface run-off from roads, car parking and public or amenity areas, they should have a suitable series of treatment steps to prevent the pollution of groundwater. For the immediate drainage catchment areas used for handling and storage of chemicals and fuel, handling and storage of waste and lorry, bus and coach parking or turning areas, infiltration SuDS are not permitted without an environmental permit. Further advice is available in the updated CIRIA SUDs manual http://www.ciria.org/Resources/Free_publications/SuDS_manual_C753.aspx USTs

2. The Environment Agency recommends the removal of all underground storage tanks (USTs) that are unlikely to be reused. Once the tanks and associated pipelines have been removed, samples of soil and groundwater should be taken to check for subsurface contamination. If soil or groundwater contamination is found, additional investigations (possibly including a risk assessment) should be carried out to determine the need for remediation. Refer to ‘Pollution Prevention Advice and Guidance on Storing and handling materials and products’ https://www.gov.uk/government/publications/underground-storage-tanks-ppg27- prevent-pollution and ‘Defra - The Groundwater Protection Code: Petrol stations and other fuel dispensing facilities involving underground storage tanks - for England and Wales’ http://archive.defra.gov.uk/environment/quality/water/waterquality/ground/docum ents/groundwater-petrol.pdf specifically those sections relating to decommissioning redundant underground fuel storage tanks and infrastructure.

Should you have any queries regarding this response, please contact me.

Yours sincerely

Chris Padley Sustainable Places Planning Advisor

Direct dial 0208 4749329 E-mail [email protected]

End 7

Scott Volker Our ref: NE/2020/132088/01-L01 Three Rivers District Council Your ref: 20/1512/FUL Development Control Three Rivers House Northway Date: 14 August 2020 Rickmansworth Hertfordshire WD3 1RL

Dear Scott

West Herts College, Langley Waters, Home Park Mill Link Road, Kings Langley, WD4 8LZ

Variation of condition 12 (landscape management) of application 18/1034/OUT: (outline application: demolition of existing college building and redevelopment for a residential development of up to 65 flats [appearance, landscaping, layout and scale reserved]) to allow changes to landscaping scheme

Thank you for consulting us on this application. We have reviewed the information submitted and do not agree that condition 12 should be amended to remove the reference to the deculverting of the Mill Stream.

This deculverting was in fact clearly shown in illustrative site plan 114822-IBI-XX-XX- XX-PL-A-200-03 submitted as part of application 18/1034/OUT and would have been the reason we did not object to the application at the time. The deculverting of this section of the Gade is a specific Water Framework Directive action in the current Thames River Basin Management Plan and this development is a once in a generation opportunity to open up this section of the river.

As such we feel there are still opportunities for the river here. We have addressed some of the arguments raised in the technical note below;

Opening of the culvert would provide limited amenity and ecological benefit. No prospect that two open channel sections will ever be linked via a naturalised channel.

We don’t agree with this, the opening up the culvert would create further space for nature with opportunities to enhance the channel to benefit wildlife. Under the Water Framework Directive, the Gade is currently at moderate status with ecological potential assessed as moderate and we should be looking to naturalise and enhance watercourses through deculverting. This has multiple benefits and can:  provide habitat for wildlife and improving its connectivity;  provide additional flood storage capacity and slowing flows;  ameliorate the urban heat island effect;  provide areas for recreational use;  Reduce maintenance costs and improve safety.

Cont/d.. We do not accept the view that greater biodiversity gain could be delivered through other landscape features. The design of the deculverted channel should be such that biodiversity net gain will be achievable, with planting and wetland features forming a part of this.

Given the nature of the made ground deposits, and also the underlying natural sands and gravels, consideration needs to be given to scouring of the channel and associated impact to the existing naturalised channel down-gradient as potential removal of the weir would create a high energy flow through the realigned watercourse and increase the sediment load within the watercourse. Scouring of the channel may also have an impact upon the adjacent site boundary which is locally elevated above site levels and supported by a retaining wall.

The new channel design should accommodate the expected flows within the channel. Methods to minimise adverse scour can be included within the design, with strategically placed planting and soft engineering. Silt mitigation would be required at build stage to minimise the risk of additional sediment loading within the watercourse. The flows through the channel would be expected to settle at low energy levels as is found on chalk streams and the deculverted channel will adjust to flow levels over time and any arising issues can be addressed as would be the case with any river channel.

Removing the culvert will ensure that any heritage features that may be present will be preserved.

Has it been confirmed that any heritage features exist? And is there the option to incorporate any of these features into the channel design?

Engineering constraints- culvert is acting as a propped cantilever retaining wall. What would happen when this culvert comes to the end of its design life? How would these constraints be addressed when the culvert needs replacing? We would be asking for deculverting at this stage too.

When deculverting or daylighting a channel, the river does not have to follow the course of the current culvert. In fact, realigning the channel to a more natural character could include features such as meanders. Will moving the channel away from the eastern boundary provide an opportunity to include more natural banks? Have there been calculations to prove that the culvert is acting as expected?

Not possible to link the two sections of Mill Stream due to the presence of the Home Park Mill Link Road.

There is always possibility to enhance the crossing of home park mill link road when this culvert is due for replacement. We cannot predict the future, and as such we would argue that this is not a valid argument for failing to deculverting at this site.

Yours sincerely

Mr Kai Mitchell Sustainable Places Planning Specialist

E-mail [email protected]

End 2

THREE RIVERS DISTRICT COUNCIL

REFUSAL OF NON-MATERIAL AMENDMENT

TOWN AND COUNTRY PLANNING ACT 1990: SECTION 96A (AS INSERTED BY SECTION 190 OF PLANNING ACT 2008)

To : Mrs Jo Unsworth On behalf of: Bellway Homes Limited Savills (North London) C/O Agent Hawker House 5-6 Napier Court Napier Road Reading RG1 8BW

Site : West Herts College Home Park Mill Link Kings Langley Non Material 20/1193/NMA Amendment Ref: Proposed Amendment : Non-material amendment to outline planning permission 18/1034/OUT: amended wording of condition 12 (Landscape Management) Relating to Planning 18/1034/OUT Application No : Approved Development: Outline Application: Demolition of existing college building and redevelopment for a residential development of up to 65 flats [Appearance, Landscaping, Layout and Scale reserved]

I refer to your application validated in this office on 19th June 2020 for a non-material amendment to planning permission 18/1034/OUT at the above address.

This non-material amendment seeks to amend the wording of condition 12 (Landscape Management) of outline planning permission 18/1034/OUT. The wording of the condition reads as follows:

‘No development shall take place until a landscape management plan, including long-term design objectives, management responsibilities and maintenance schedules for all landscaped areas (except privately owned domestic gardens), shall be submitted to and approved in writing by the Local Planning Authority. The landscape management plan shall be implemented as approved.

The scheme shall include the following elements:  Provide detailed cross sections for the new proposed section of river. The upstream culvert and downstream bridge width on site will constrain flows, meaning the width of the new section of river should take this in to account.  Provide information on the bank gradients, material and planting regime  Provide information regarding the substrate of the river  Provide a method statement for the river works  Provide details of timescales/timetabling for the works to take place  Detail extent and type of new planting (NB planting to be of native species)  Details of maintenance regimes  Provide information on whether a new bridge is proposed for this development. If a new bridge is proposed, we would require the abutments to be set back.

Reasons: This is a pre commencement condition to ensure the protection of wildlife and supporting habitat and secure opportunities for the enhancement of the nature conservation value of the site in accordance with paragraph 170 and 175 of the National Planning Policy Framework and Policies DM6 and DM9 of the Development Management Policies LDD (July 2013).’

The application seeks to amend the wording of the condition to omit reference to the culverting of Mill Stream on the eastern boundary of the site on the basis that the opportunity for de-culverting part of Mill Stream is no longer considered practical or viable due to further technical work undertaken on the scheme. As such it is proposed to amend condition to be read as follows: renmaz

‘No development, with the exception of demolition, shall take place until a landscape management plan, including long-term design objectives, management responsibilities and maintenance schedules for all landscaped areas (except privately owned domestic gardens), has been submitted to and approved in writing by the Local Planning Authority. The landscape management plan shall be implemented as approved.

The scheme shall include the following elements:  Provide detailed cross sections through the landscaped wetland features  Provide information on wetland features adjacent to the culvert in terms of the bank gradients, material and planting regime  Provide details of timescales/timetabling for the works to take place  Detail extent and type of new planting (NB planting to be of native species)  Details of maintenance regimes  Provide information on whether a new bridge is proposed for this development. If a new bridge is proposed, we would require the abutments to be set back.’

A covering letter has been submitted providing further detail of the justification for the amendment stating that whilst the outline application noted the potential opportunity to de-culvert part of the Mill Stream, this was not supported by any technical work to support the proposal. Crucially, no assessment of the structural implications for nearby buildings had been undertaken; nor any consideration of the implications of de-culverting on issues such as landscaping, archaeology and ecology.

In line with paragraph 005 Reference ID: 17a-005-20140306 of the NPPG the LPA considered it reasonable to consult the Environment Agency on this NMA given that they were supportive of works to de-culvert the Mill Stream in their comments at outline stage. Whilst no formal comments have been received, informal correspondence has raised concerns with the absence of de-culverting works and they considered this to be a material change to the original permission.

Furthermore, Policy DM6 section g) Watercourses states that any development adjacent to, over or in a watercourse needs to take into account consideration of the ‘Water Framework Directive’ requirements and opportunities outlined in the ‘Thames River Basin Management Plan’ (“TRBMP”). All developments should seek to improve the biodiversity of the site and contribute towards the riparian corridor’s ability to be used by migrating species. In this case, the Mill Stream is mentioned in the TRBMP and the EA do have de-culverting this site as a current Water Framework Directive action.

The proposed amendment to the condition would result in the potential for the scheme to not comply in full with the requirements of Development Plan Policy DM6, and such non-compliance would represent a material change which requires full and formal assessment and consideration.

As such, the Council is not satisfied that the proposed amendment described above is non-material. In reaching this decision regard has been paid to the effect of the change on the outline planning permission as originally granted.

I must therefore inform you that your application for a non-material amendment to outline planning permission 18/1034/OUT dated 15th August 2019 is hereby REFUSED.

Date: 17th July 2020

I trust that this information is of assistance to you.

Yours sincerely

Kimberley Rowley Head of Regulatory Services On behalf of Director of Community & Environmental Services, Three Rivers District Council, Three Rivers House, Northway, Rickmansworth, Herts WD3 1RL

renmaz

Our ref: NE/2020/132302/02-L01 Scott Volker Your ref: 20/1858/AOD Three Rivers District Council Development Control Date: 17 November 2020 Three Rivers House Northway Rickmansworth Hertfordshire WD3 1RL

Dear Scott

Approval of Reserved Matters: demolition of existing college building and redevelopment for a residential development of up to 65 flats in a five storey building (submission of Reserved Matters of appearance, landscaping, layout and scale for consideration pursuant to condition 1 of outline planning permission 18/1034/OUT)

West Herts College, Kings Langley, Hertfordshire WD4 8LZ

Thank you for consulting us on the above reserved matters application.

We are pleased to see that deculverting the river has been included within this new landscape design for the development and are therefore able to remove our objection due to the lack of deculverting.

However, in its current form, the designs for the new river channel are not acceptable from a biodiversity and flood risk perspective and so we MAINTAIN an OBJECTION. Should these designs have been submitted in respect of the related discharge of condition 12 of planning permission ref. 18/1034/OUT, we would not have found the design acceptable and therefore wouldn’t be recommending discharge. We would not expect to see a poorer quality design in terms of biodiversity and nature conservation than was secured via conditions at the outline stage.

Overcoming our objection To address our outstanding concerns, the following should be addressed.

1. The channel geometry: the proposed plans will create a box-shaped channel, canalised by gabion walls either side.  We would like to see the channel sides be made softer and less vertical. What ‘greener’ options have been/ can be considered?  We would like to see the channel cross-sectional shape be made more natural (e.g. river beds are not naturally flat).  Gabions generally have a short life span and you should seek to explore erosion protection that works with natural processes for a long term solution. For example, one option is bank re-grading. If further bank stability is required could the banks be capped with clay (then covered with topsoil and re-vegetated) Environment Agency, Alchemy, Bessemer Road, Welwyn Garden City, Hertfordshire, AL7 1HE

2. Channel lining (bentonite geosynthetic clay liner) underneath uniformly sized gravels.  Please explain why channel lining is required. If it is necessary, this should be a clay layer (without introducing synthetic materials). Synthetic matting tends to come away overtime and cause further problems.  Gravels should be of mixed size to create more habitat niches (we recommend 10-50mm mixed gravels, locally derived and clean).

3. The energy dissipation structure  We have concerns over this and would like to see alternatives considered. For example, a rockramp.  How has fish passage been considered as part of this design?

4. New headwall  The installation of outfalls on a watercourse will contribute cumulatively to the degradation of this waterbody. If it is necessary to install an outfall into the watercourse, it is advised that soft options are proposed, such as: o Outfalls cut flush with the gradient of the bank (or at a shallower gradient if the bank has been unnaturally steepened). o No headwalls or wingwalls if these are not necessary. o If a headwall/wingwall is required, bespoke brickwork is preferred as this will allow the outfall to be as small necessary, to reduce the break in riparian continuity.

We understand that this reach of river does not have all the characteristics of a typical chalk stream and there will need to be a balance between the engineered aspects and a natural channel.

The above concerns could be addressed through a detailed Water Framework Directive Assessment that considers how the design may impact on Waterbody Status under the Water Framework Directive.

Once the final design is agreed an update to the Flood Risk Assessment will be required to show that the changes will not increase flood risk.

We would be willing to meet to discuss the details listed above. In addition it would be useful to receive at this stage the details of the method statement, timings of works and maintenance requirements as described in planning condition 12.

Please let me know if you have any questions.

Yours sincerely

Deborah Simons Planning Advisor Direct e-mail [email protected]

Grant Turner

From: Simons, Deborah Sent: 30 November 2020 12:31 To: Jo Unsworth; Scott Volker Subject: Planning App Consultation for Environment Agency on 20/1858/AOD at West Herts College Home Park Mill Link Kings Langley Hertfordshire WD4 8LZ Attachments: RE: 20/1858/AOD: West Herts College, Home Park Mill Link Road, Kings Lan... (87.6 KB); NERC Green Engineering.pdf

Dear Scott, Jo

Thank you for the useful meeting we attended last week to discuss our outstanding objection. Following this, we agreed to forward on some points of clarification and further explanation.

1. Flood Risk Assessment Update I understand Nina has already sent over some points regarding the FRA. I’ve attached a copy of her email.

2. Fish Passage (WFD actions) I can confirm that there is a Water Framework Directive action to address fish passage on the weir above the development site Action ID: 33,621, Home Park Industrial Estate construct fish easement at weir. It is therefore crucial that fish passage is considered as part of this proposal.

Following the meeting, our Fisheries Officer (who wasn’t able to be there) had the following questions. How does the applicant see fish and eels being able to utilise and successfully migrate past this structure ? What is the gradient of this structure ? What will be the water depths in the channel in this structure? What is the purpose of the slots on either end of the structure? How wide are the slots? What will flow velocities be through the slots? Will fish be able to navigate through the slots when the water levels are low in periods of prolonged dry weather?

3. Fish Pass Manual Our Fisheries Officer notes that, while every structure is unique with regards to velocities and channel depths necessary for fish passage, this manual may hold some relevant information. https://ifm.org.uk/wp-content/uploads/2020/09/Fish-Pass-Manual.-GoodVersion-pdf.pdf.

4. Water Framework Directive assessment We would want to see a Water Framework Directive assessment of alternative options to gabions for both banks of the channel (our preference is no gabions). I have attached a guide that I think could be useful. The use of gabions must be proved to be the only practicable, reasonable option if you are to proceed with them.

5. Channel lining & channel cross sections We would need an assessment of the need for lining the channel and alternative options to a geotextile (our preference is no lining). Changes need to be made to the shape of the channel cross section, to include more diversity in bed levels as is appropriate to a natural river channel, using features such as berms. Features such as berms could help to protect the steep banks of the channel and will also form valuable habitat in the channel.

6. Outfall headwall The outfall headwall should fit neatly into the bank, angled downstream and should not be oversized.

Jo - Please can you forward any reports directly to me at the [email protected] email address and copy in Scott? That way we’ll be able to start looking at these as soon as possible.

Please let me know if you have any questions.

Kind regards Deborah

Deborah Simons Planning Advisor, Hertfordshire and North London Sustainable Places 1 Environment Agency | Alchemy, Bessemer Road, Welwyn Garden City, Hertfordshire, AL7 1HE  0203 025 9020 [email protected]

Does Your Proposal Have Environmental Issues or Opportunities? Speak To Us Early!

If you’re planning a new development, we want to work with you to make the process as smooth as possible. We offer a bespoke advice service where you will be assigned a project manager who will be a single point of contact for you at the EA, giving you detailed specialist advice. This early engagement can significantly reduce uncertainty and delays to your project. More information can be found on our website here.

Information in this message may be confidential and may be legally privileged. If you have received this message by mistake, please notify the sender immediately, delete it and do not copy it to anyone else. We have checked this email and its attachments for viruses. But you should still check any attachment before opening it. We may have to make this message and any reply to it public if asked to under the Freedom of Information Act, Data Protection Act or for litigation. Email messages and attachments sent to or from any Environment Agency address may also be accessed by someone other than the sender or recipient, for business purposes.

2 Cham Ariyaratne

From: Garner, Nina Sent: 26 November 2020 14:20 To: Jo Unsworth Cc: Grant Turner; Owen Mesley; Cham Ariyaratne; HNL Sustainable Places Subject: RE: 20/1858/AOD: West Herts College, Home Park Mill Link Road, Kings Langley

Afternoon Jo,

Yes of course. As per our response letter, we requested an updated Flood Risk Assessment for the application including the proposal to deculvert a section of the river.

Usually for deculverting, we would request detailed hydraulic modelling, however rather than having to include full detailed modelling in the FRA, you could do some initial assessments to demonstrate flood risk will not increase as a result of the proposed development:

The updated FRA could include, but not be limited to, the following: - an intial assessment using topo and site surveys to demonstrate no increase in flood risk. - rough assessment on channel capacity - demonstrate the proposal is not changing the cross sectional area of the river channel - demonstrate no inspection chambers will covered and access is not impeded by the proposal, in case we require access for emergency works - written justification for need of the energy dissipating structure, demonstrate it is built to allow sufficient capacity/flows for the channel - demonstrate there will be no adverse impact on the stability of the bank by including an adequate undeveloped buffer zone from the top of the main river bank to the edge of the built footprint

If the FRA shows that there could be a breach point/increase in flooding, we would require more detailed information to demonstrate flood risk is not increased.

Just a note that the works will require a Flood Risk Activity Permit. You can find more information on the gov.uk website here, or email [email protected] on how to apply (we offer a free pre-application service).

If you have any questions on the above information, please do ask.

Kind regards,

Nina

Nina Garner Flood Risk Management Officer PSO Luton, Hertfordshire and Essex

Environment Agency | Hertfordshire and North London Alchemy, Bessemer Road, Welwyn Garden City, Hertfordshire, AL7 1HE [email protected] Telephone: 07769367251

1

APPENDIX B – SITE CONSTRAINTS

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 31 Notes:

1. This drawing presents the significant hydraulic features both on and off the site which dictate the volume of flow that can pass through the culvert below the proposed Millworks Development Site (Former West Herts College).

2. The hydraulic design of the open channel that will replace the culvert LOCATION 1 has been developed taking into account the features shown on this HOME PARK MILL LOCK (N0. 70) drawing.

The operation of this lock influences the 3. For more specific information respective to the existing culvert that water level within the Grand Union Canal is required to be decommissioned and replaced, refer to the Beaver and ultimately controls the highest level to Bridges report no. BB1219-19,"Kings Langley Culvert". which the water within the canal can rise.

GRAND UNION CANAL

HOME PARK MILL LINK ROAD

PINNACLE HOUSE LOCATION 8 MILL STREAM CONCEPT HOUSE Existing open watercourse to the south of the Development Site.

The culvert below the site discharges into this watercourse and therefore the naturalized open channel required to replace the culvert will be engineered to tie into this watercourse.

MILL WORKS HOME PARK ARM DEVELOPMENT SITE

PUBLIC FOOTPATH P01 01/10/20 First Issue MFP GT GT

LOCATION 2 Rev. Date Amendment Drawn Chkd. Appd. HOME PARK ARM (BRANCH CANAL) INNOVATION CENTRE The Home Park Arm is in direct continuity with the main canal and its water level is relatively consistent with that of the Grand Union Canal.

LAND & DEVELOPMENT ENGINEERING LTD

MILL 18 Frogmore Road Tel: +44 (0) 1442 437500 STREAM Hemel Hempstead Fax: +44 (0) 1442 437550 Hertfordshire Email: [email protected] HP3 9RT Web: www.rsk.co.uk United Kingdom LOCATION 7 SOUTH HEADWALL (CULVERT OUTFALL) Client

This is the end of the culvert where the flow enters Mill Stream and this is effectively the point at which the formation of a naturalized channel to replace the culvert will commence.

Project Title SMA HOUSE MILLWORKS HOME PARK MILL LINK ROAD NOTE KINGS LANGLEY This detail below is opposite handed to the aerial image of the site. LOCATION 3 STATION ROAD BROAD CREST WEIR Status ISSUED FOR PLANNING This weir controls the volume of water that Drawing Title is able to leave the Home Park Arm of the Grand Union Canal and pass under Home HYDRAULIC FEATURES Park Mill Link Road before entering the spillway located on the Development Site. AND CONSTRAINTS

LOCATION 6 LOCATION 5 LOCATION 4 Drawn Date Checked Date Approved Date CULVERT BELOW SITE OPEN SPILLWAY (OUTFALL) OPEN SPILLWAY (INLET) MFP 29.0.20 GT 30.09.20 GT 01.10.20 The adjacent detail has been extracted from The spillway appears to have a This spillway directs the flows entering the the Beaver Bridges report no. BB1219-19 sinusoidal profile to its invert and Development Site into the downstream Scale Orig Size Dimensions has a vertical drop of approx.1.5m culvert lying below. The culvert section N.T.S. A1 Metres This culvert passes below the development over a length of 9.5m. shown entering the spillway lies under site and is required to be decommissioned adjacent private land and is outside the Project No. Drawing File and replaced with a naturalized open Flows through this spillway are control of Bellway Homes 133938 channel. relatively consistent as would be expected due to the control Drawing No. Rev. imposed by the broad crest weir. 133938-RSK-ZZ-00-DR-P-00030 P01 Notes:

1. This drawing presents the enabling works required to allow the

x2 0.45d x2 construction of an open channel. EATH ROD

66.85 66.91

0.90d 2. To ensure the stability of the existing site boundary remains intact CL67.57 requires a portion of this culvert to remain insitu and be backfilled

67.57

IC with foam concrete. 0.65d

3. The extents of culvert removal shown on this drawing are based on 0.80d

67.63

Ri the necessity to have a 2m wide working zone for the construction of

Bin

100Ø

67.01

66.94 the east bank gabion wall. Some temporary works may also be

67.53 67.58

67.09 required to support the excavation whilst this wall is installed.

67.55

67.55 G

67.44 67.53 67.54 150Øfws

67.11 67.16

(possible water main) water (possible 67.55

67.45 67.61

67.65

CABLES 67.69

EOT 100Ø 67.61

x2 0.50d x2

0.65d Ri

CONCRETE SLABS CONCRETE 67.71 66.78 Legend:

67.44

67.54

67.67 67.71 67.45 67.69

CL66.92

IC

IL 66.12 IL UTT

CL 67.72 CL

water riser water

GROUND BT

2"pe 67.65

67.55 0.90d

67.45 GABION WALL CONSTRCUTION ZONE (2m wide) 66.98

ARAR 67.62

67.46 Metal Grid Fence Ht=2.20m 67.69

66.90 67.45

66.85 Outline of existing college building 67.53

Ht/Z=67.606

N/Y=202001.143 PORTION OF CULVERT TO BE LEFT INSITU

66.83 E/X=507859.977 67.69

EOT JAG03

ER 150Ø 67.54

67.15

x2 0.50d CL67.54 IC

IL 67.01 IL

IL67.10

67.74 CL 67.60 CL

CL67.58

ER MH

MH

CL67.59

IC 67.53 CL67.56 67.49

Concrete PORTION OF CULVERT TO BE DEMOLISHED 66.94 67.53 IC

100Ø

67.07 100Ø Ri Ri 67.66

IL 66.95 IL

67.61 67.57 CL

67.17 Blocked 67.60 67.59

67.58 MH

67.68 67.62

67.56 67.60

67.61 TAP

67.66

67.66

Concrete Vents

Ri

CO SLABS CO

UTR 67.58

CO SLABS CO

67.64

67.58

67.57

W/M Ht=0.95m 67.60

67.56

CO/SLABS ER 67.63 150Ø

67.06 Ht=1.75m 67.58

150Ø

67.59 150Ø 67.59

67.59 Vents 67.40

Concrete 67.44

66.99 x2 0.45d 66.75 67.06 CL67.58

67.39 67.43 0.70d 67.57 67.42

4 STEPS 4 IC

67.18 67.44 67.49

67.41

G Ht=0.60m

67.34 67.38 Blocked G 67.41 67.50

UTR

Ht=1.14m

G Ht=1.53m

G Cycle Racks Cycle 67.14

67.19 G CL66.80

67.30 IC

67.38 67.52 CO RAMP CO

4STEPS 67.44

66.95 67.27

Ht=1.05m

66.82 67.40

66.91 Blocked

67.02 67.41

1.00d G

0.65d 67.06 67.43 67.54

66.81 66.81 67.43

67.44

67.07 67.47

67.41

67.03 Ht=1.12m BW

66.91

IL65.78

CL66.78

MH

67.02 67.55

67.42 67.69

66.84

66.83 67.50 67.09 67.44

66.99

67.20

67.15

67.26

67.32

67.03

67.51 67.53 67.55

66.94 67.45 0.60d 67.50 67.46

67.48

x2 0.30d x2

67.37 67.57

Ht=1.10m

67.40 67.51 67.43 67.51

BinS

67.25

66.84

0.35d

67.24 Energy dissipation and diversion

CL67.04

IC structure, buried below car park

CL67.56 IC

67.52

67.50 Existing culvert area and not visible at the surface.

67.64 67.62

structure outline 67.72

Ht=0.45m On top Wall 67.60

x2 0.15d Asphalt This structure re-directs the

x2 6"ci pipes visible on the wall

incoming flow away from the site BinS

Metal Grid Fence Ht=2.50m

0.50d boundary. Refer to drawing no. 133934-RSK-ZZ-00-PL-00041 for

details of this structure

BRIDGE

67.47 67.52 67.55

MILL STREAM 67.54

66.91 67.29

67.64

67.61

67.67 Ht=0.50m

67.19 MC

67.40 MC

67.35

67.24 Ht=0.50m

67.29 67.14 67.55

MC MC 67.91

66.99 67.08 MC

67.46 67.04

66.95 67.57

67.71 67.82 67.76 MC

67.50 67.59

Ht=0.65m 67.66

Ht=0.50m

67.48

66.89 67.65

67.49

67.90 67.66

67.62 67.55

Ht=2.46m

67.53 Ht=2.46m

67.53

67.55 67.59 BW Ht=0.65m 66.94

66.89 67.56

67.56

67.47 67.57

OSA Ht=2.46m OSA 67.30

67.30 67.62

Smoking Smoking

66.97 Shelter

67.56 67.23

67.04 67.45

67.14

67.09 67.00

67.29 67.39

67.34 67.18 P01 2-10-2020 First Issue JO'N GT GT

66.96

67.24

66.89

66.97

67.17 67.48

67.39 Rev. Date Amendment Drawn Chkd. Appd.

67.46

67.15

Ht=2.46m 67.36 OSA Ht=2.46m 67.22

Ht=2.46m 67.43 67.54

BT 67.42

Existing portion of buried culvert This portion of the spillway is to remain OSA Existing culvert

CL67.57 operational but will be slabbed over as part

IC to be broken out to facilitate the structure outline earthworks required for the of the construction works for the energy formation of an open channel Portion of existing spillway to be dissipation and diversion structure. This is required to make this area safe. Access into 67.98 Existing portion of buried culvert made redundant. This area to be this portion of the spillway will be provided by LAND & DEVELOPMENT ENGINEERING LTD 67.98 to remain insitu and be backfilled backfilled with earth and Existing site a manhole cover built into the cover slab. with foam concrete to maintain landscaped over 18 Frogmore Road Tel: +44 (0) 1442 437500 boundary the stability of the site boundary Existing open spillway Hemel Hempstead Fax: +44 (0) 1442 437550 68.07 New energy dissipation and Hertfordshire Email: [email protected]

68.07 diversion structure to be HP3 9RT Web: www.rsk.co.uk built over this portion of United Kingdom existing spillway. Client

Project Title MILLWORKS HOME PARK, MILL LINK ROAD KINGS LANGLEY

Status ISSUED FOR PLANNING Drawing Title ENABLING WORKS DECOMMISIONING OF EXISTING CULVERT

Drawn Date Checked Date Approved Date JO'N Oct'20 GT Oct'20 GT Oct'20

Scale Orig Size Dimensions 1:100 A1 Metres

Project No. Drawing File 133938

Drawing No. Rev.

C:\USERS\YAMAH\DROPBOX\RSK PROJECTS\133834 KINGS LANGLEY\DECULVERTING\DWGS\133938-RSK-ZZ-ZZ-DR-P-00031 - ENABLING WORKS DECOMISSIONING OF CULVERT.DWG DECOMISSIONING WORKS - ENABLING LANGLEY\DECULVERTING\DWGS\133938-RSK-ZZ-ZZ-DR-P-00031 KINGS PROJECTS\133834 C:\USERS\YAMAH\DROPBOX\RSK 133938-RSK-ZZ-00-DR-P-00031 P01

Scale 1:200 0 2 4 6 8 10m Notes:

1. This drawing presents a typical section through the proposed open channel at the head of the channel, chainage 0.00m. Facade to existing building required to be demolished. 2. The purpose of this section is to demonstrate how the existing culvert and the adjacent boundary retaining wall constrain the space available in which a retaining feature is required to be built, thereby limiting the options available for this feature.

Facade to new building

Existing retaining wall located on the boundary to the site

Gabion wall with vertical face to maximize the useable public realm space behind

Existing Ground Level

Public Realm Amenity Space Open Channel Public Realm Amenity Space Terrace To Ground Floor Apartment

Removal of culvert structure FFL 68.10m would undermine the existing retaining wall. Existing Ground Level

IL 66.00m

EXISTING CULVERT

IL 65.15m

Invert level to open channel Working space required to elevated above the invert level install gabion retaining wall of the culvert it replaces. Portion of existing culvert This portion of the culvert to be removed to facilitate to be left insitu and be the installation of the backfilled with foam concrete. gabion retaining wall

P01 07-12-2020 First Issue MFP GT GT

Rev. Date Amendment Drawn Chkd. Appd.

LAND & DEVELOPMENT ENGINEERING LTD

18 Frogmore Road Tel: +44 (0) 1442 437500 Hemel Hempstead Fax: +44 (0) 1442 437550 Hertfordshire Email: [email protected] HP3 9RT Web: www.rsk.co.uk United Kingdom

Client

Project Title MILLWORKS HOME PARK, MILL LINK ROAD KINGS LANGLEY

Status FOR INFORMATION Drawing Title

TYPICAL CROSS SECTIONS AT HEAD OF OPEN CHANNEL CH0

Drawn Date Checked Date Approved Date MFP Dec'20 GT Dec'20 GT Dec'20

Scale Orig Size Dimensions 1:50 A1 Millimetres

Project No. Drawing File 133834 133834-RSK-EA-SK01

Drawing No. Rev. 133834-RSK-EA-SK01 P01

Scale 1:50 0 500 1000 1500 2000 2500mm

APPENDIX C - SCHEME PROPOSALS

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 32 Notes:

1. This drawing presents typical sections through the proposed open channel.

2. For the cross-section details through the open channel, refer to Building facade drawing nos.

133938-RSK-ZZ-00-DR-P-00033 133938-RSK-ZZ-00-DR-P-00034 133938-RSK-ZZ-00-DR-P-00035 Fence to site 133938-RSK-ZZ-00-DR-P-00036 boundary

Terrace railing 200mm Topsoil to verge, overlying sub-soil Existing ground profile Parapet rail

Bituminous construction 200mm Topsoil to with resin bonded gravel batter slope, finish Highest predicted water overlying sub-soil level (1 in 100yr +70%) Existing ground profile 500 x 500 Gabion basket with integral planting

1000 x 1000 Coir Rolls Gabion basket Chestnut Planted berm stakes

Well compacted type1

sub-base as backfill to 500 retaining wall 350

Concrete foundation Normalized overlying type 1 sub-base water level Random rock Formation fill to bed Clay subsoil 15025

Verge Footpath Verge Open Channel Batter Slope / Soft Landscape Verge Terrace

Varies 1200 1000 4000 Varies Varies

1 in 40 1 in 40 1 in 20 Varies 1 in 20

TYPICAL SECTION CHAINAGE 5M

P02 01-12-20 Adjusted in line with EA comments MFP GT GT

T01 24-11-2020 Tender Issue MFP GT GT

P01 2-10-2020 First Issue JO'N GT GT

Rev. Date Amendment Drawn Chkd. Appd.

Building facade

Fence to site LAND & DEVELOPMENT ENGINEERING LTD boundary 18 Frogmore Road Tel: +44 (0) 1442 437500 Hemel Hempstead Fax: +44 (0) 1442 437550 Terrace railing Hertfordshire Email: [email protected] Existing ground profile HP3 9RT 200mm Topsoil to Web: www.rsk.co.uk verge, overlying United Kingdom sub-soil Parapet rail 200mm Topsoil to Client Bituminous construction batter slope, with resin bonded gravel overlying sub-soil finish 500 x 500 Highest predicted water Gabion basket 500 x 500 level (1 in 100yr +70%) Gabion basket 200mm Topsoil to with integral planting batter slope, Existing ground profile Project Title overlying sub-soil MILLWORKS 1000 x 1000 Coir Rolls Gabion basket Chestnut Planted berm HOME PARK, MILL LINK ROAD stakes Concrete foundation overlying type 1 sub-base KINGS LANGLEY Well compacted type1 500 sub-base as backfill to

retaining wall 350 Status FOR INFORMATION Drawing Title Normalized water level Random rock Formation Concrete foundation fill to bed overlying type 1 sub-base Clay subsoil TYPICAL CROSS SECTIONS

11748 CHAINAGE 5m TO 20m

Drawn Date Checked Date Approved Date JO'N Oct'20 GT Oct'20 GT Oct'20

Scale Orig Size Dimensions Landscape Area Footpath Verge Open Channel Batter Slope / Soft Landscape Verge Terrace 1:50 A1 Millimetres Varies 1200 1000 4000 Varies Varies Project No. Drawing File 1 in 40 1 in 40 1 in 20 Varies 1 in 20 133938 133938-RSK-ZZ-00-DR-C-00037

Drawing No. Rev. 133938-RSK-ZZ-00-DR-C-00037 P02

TYPICAL SECTION CHAINAGE 20M Scale 1:50 0 500 1000 1500 2000 2500mm Notes:

1. This drawing presents the Engineering Layout for the decuverting works proposal.

2. Refer to drawing no, 133938-RSK-ZZ-00-DR-C-00037 for typical

section details at the chaniage locations shown on this drawing, PUBLIC FOOTPATH PUBLIC

MILLWORKS

New boundary fence (PROPOSED BUILDING) CH5

Coir rolls to edge TERRACES CH20 of channel

Graded earthworks batter slope. Head wall at start of open channel, with vehicle Coir roll to abut the Maximum gradient 1 in 2 restraint barrier. This headwall forms an integrated higher gabion wall at Minimum gradient 1 in 3 Side slope at 1 in 2 to part of the energy dissipation and diversion structure; this point TERRACES create a non level base and is visible at the surface to the open channel

Energy dissipation and diversion structure, buried Small submerged below car park area and not visible at the surface. Parapet railing berm in front of coir for fall protection roll edge treatment This structure re-directs the incoming flow away from the site boundary. Refer to drawing no. 133934-RSK-ZZ-00-PL-00041 for details of this structure Gabion wall 0.5m high Channel

at top of batter slope EXISTING FOOTBRIDGE EXISTING

MILL STREAM Channel

P02 01-12-20 Adjusted in line with EA comments MFP GT GT

Parapet railing to top of T01 24-11-2020 Tender Issue MFP GT GT wall for fall protection

P01 17.04.20 First Issue MFP GT GT Proposed footpath

Rev. Date Amendment Drawn Chkd. Appd.

This portion of the spillway is to remain Gabion wall height here operational but will be slabbed over as part matches the level of the New fence on Gabion retaining wall approx. of the construction works for the energy existing footbridge site boundary 1.5m high to retain earthworks and site boundary. CH5 dissipation and diversion structure. This is required to make this area safe. Access into Spillway enters the culvert at this point. Exposed planted this portion of the spillway will be provided by LAND & DEVELOPMENT ENGINEERING LTD For details of the culvert decommissioning berm in front of a manhole cover built into the cover slab. works, refer to drawing no. 18 Frogmore Road Tel: +44 (0) 1442 437500 gabion wall 133934-RSK-ZZ-00-DR-P-00031 Existing open spillway Hemel Hempstead Fax: +44 (0) 1442 437550 Hertfordshire Email: [email protected] HP3 9RT Web: www.rsk.co.uk New energy dissipation and United Kingdom

CH20 Portion of existing spillway to be diversion structure to be made redundant. This area to be built over this portion of Client backfilled with earth and existing spillway. landscaped over

Project Title MILLWORKS HOMEPARK MILL LINK ROAD

KINS LANGLEY PUBLIC FOOTPATH PUBLIC Status FOR INFORMATION Drawing Title ENGINEERING LAYOUT

Drawn Date Checked Date Approved Date SMA HOUSE MFP 01.10.20 GT 02.10.20 GT 02.10.20

Scale Orig Size Dimensions 1:100 A1 Metres

Project No. Drawing File 133938

Drawing No. Rev. 133938-RSK-ZZ-00-DR-P-00038 P02

Scale 1:100 0 1 2 3 4 5m PLANTING PROPOSALS TO BE INCOPORATED AS PART OF THE EDGE TREATMENT FOR THE OPEN CHANNEL

Example Of Pre-planted Coir Rolls To Be Utilized As Edge Treatment To The Western Side Of The Open Channel

Example of Verge Planting To Be Utilized On The Berm In Front Of The Gabion Retaining Wall Along The Eastern Edge Of The Open Channel

MILLWORKS KINGS LANGLEY SPECIFICATION FOR CLAY LINER TO OPEN CHANNEL WATERCOURSE

PUDDLE CLAY SPECIFICATION

1) General

Material to be used as puddle clay shall be naturally occurring homogeneous plastic material. It shall be free from deleterious matter such as sand, stones and organic material. The use of lime-stabilised clays shall not be allowed.

2) Properties

a) More than 65% of the natural material shall be finer than 0.06mm and more than 40% shall be finer than 0.002mm

b) The natural material shall be defined as firm clay in accordance with BS5930:1981 Table 8 (Cu 40-75 kPa).

c) The natural material shall be defined as clay of intermediate to extremely high plasticity in accordance with BS5930:1981, figure 31 and the liquid limit shall not be less than 35%.

d) The coefficient of permeability (k) of the remoulded material shall not be greater than 10 –9 m/s.

e) The remoulded material shall be defined as Non-dispersive (ND1) in accordance with BS1377:Part 5:1990, Table 2.

3) Identification

An indication of a material’s suitability may be obtained from the following empirical tests, at the moisture content agreed for placement.

a) Tenacity Test

A 300mm long, 25mm diameter cylinder of clay is held vertically for 15 seconds so that at least 200mm is unsupported and in tension under its weight. If the cylinder breaks the clay will be rejected as unsuitable.

b) Pinch Test

A 75mm diameter ball of remoulded clay is squeezed into a 25mm thick flat disc. If any cracks appear the clay may be rejected as unsuitable.

c) Slaking Test

A 50mm diameter ball of remoulded clay is placed in a 600ml beaker and covered with water. If the ball disintegrates within 24 hours the clay may be rejected as unsuitable.

d) Permeability Test

A sample of remoulded clay shall be formed into a tray to hold 20 litres of water and the loss measured after 24 hours. This shall be compared with the water loss from a metal tray of the same surface area holding the same quantity of water. If the difference is greater than 1% the clay may be rejected. 4) Acceptance

a) A representative sample of the proposed clay material, not less than 10kg in weight together with appropriate tests results shall be supplied to the Engineering Manager for his acceptance not more than two weeks after acceptance of the Tender, and at least four weeks in advance of any proposed change in source or quality of the material.

b) Test results to BS1377 are required as follows:

i) Grading ii) Liquid and Plastic Limits iii) Natural Moisture Content iv) Coefficient of Permeability of remoulded clay v) Pinhole Dispersion of remoulded Clay vi) Compaction (2.5kg rammer)

5) Emplacement

a) The clay should be reworked in a stockpile on site and water added as necessary to destroy the original structure of the clay and produce a smooth plastic homogeneous puddle clay with a moisture content of a minimum of 1.3 times the plastic limit. Reworking of the clay should be carried out in such a manner as to prevent contamination.

b) The method of placing the clay shall be agreed by the Engineering Manager before work commences. Whatever means are adopted they shall produce a continuous homogeneous plastic mass of puddle clay effectively free from voids, laminations or imperfections which could affect its water retaining properties.

c) The clay shall be placed in horizontal layers not exceeding 150mm consolidated thickness and compacted by an approved method to an air void content not exceeding 5%.

d) Unless agreed otherwise with the Engineering Manager, the type of compaction plant and number of passes shall conform with the requirements of Clause 608 and Tables 6/1 and 6/4 for material Class 7C (selected wet cohesive material) of the DoT specification for Highway Works Part 2.

e) Before placing a further layer of puddle, the surface of the previous layer shall be cleansed of all slurry and surplus water and the surface prepared to ensure that the clay to be placed shall be integrated with that already placed. Preparation of surfaces between successive layers shall be formed by frequent non-continuous spade cuts into the upper surface of the clay to depth of 75mm.

f) Where clay puddle is to be joined with existing clay puddle, the existing clay shall be cut back and stepped to form a good key between the existing and new clay puddle over a distance to be agreed by the Engineering Manager, but not less than 1000mm. All trace of junction marks shall be wholly eliminated.

g) Precautions shall be taken to ensure any puddle clay awaiting placing, puddle clay which has been placed and any puddle clay in dry areas shall be kept continuously wet to prevent it drying out and covered by waterproof sheets to protect it from rain damage. Precautions shall be taken to prevent the material freezing. Maccaferri Ltd Building 168 Harwell Campus Didcot Oxfordshire OX11 0QT HAPAS Certificate Tel: 01865 770555 Fax: 01865 774550 16/H247 website: www.maccaferri.com/uk Product Sheet 1 MACCAFERRI CIVIL ENGINEERING PRODUCTS MACCAFERRI GABIONS

This HAPAS Certificate Product Sheet(1) is issued by the British Board of Agrément (BBA), supported by Highways England (HE) (acting on behalf of the Overseeing Organisations of the Department for Transport; Transport Scotland; the Welsh Government and the Department for Infrastructure, Northern Ireland), the Association of Directors of Environment, Economy, Planning and Transport (ADEPT), the Local Government Technical Advisers Group and industry bodies. HAPAS Certificates are normally each subject to a review every three years.

This Certificate relates to Maccaferri Gabions, cages formed from hexagonal double-twisted mesh of polymer coated galvanized wire, used with selected fill material for retaining wall structures, river training works and erosion control, in both temporary and long-term applications. CERTIFICATION INCLUDES: HAPAS requirements compliance with Regulations where applicable

chnical investigations

-yearly review. KEY FACTORS ASSESSED Structural performance the products, when used correctly in a fully designed application, will have adequate strength to resist the anticipated loads and the designed long-term performance (see section 6). Resistance to weathering the products will have adequate resistance to weathering (see section 7). Durability when used in accordance with the requirements of this Certificate, the polymer-coated gabions may be considered to have a life expectancy in excess of 120(1) years (see section 9). (1) Excludes the effects of adverse abrasion. The BBA has awarded this Certificate to the company named above for the products described herein. These products have been assessed by the BBA as being fit for their intended use provided they are installed, used and maintained as set out in this Certificate. On behalf of the British Board of Agrément

Date of Second issue: 11 March 2019 Paul Valentine Claire Curtis-Thomas Originally certificated on 22 June 2017 Technical Excellence Director Chief Executive The BBA is a UKAS accredited certification body Number 113. The schedule of the current scope of accreditation for product certification is available in pdf format via the UKAS link on the BBA website at www.bbacerts.co.uk Readers are advised to check the validity and latest issue number of this Agrément Certificate by either referring to the BBA website or contacting the BBA direct. Any photographs are for illustrative purposes only, do not constitute advice and should not be relied upon. British Board of Agrément Bucknalls Lane tel: 01923 665300 Watford [email protected] Herts WD25 9BA ©2019 www.bbacerts.co.uk Page 1 of 13 Requirements

The Highways Technical Advisory Committee has agreed with the BBA the aspects of performance to be used by the BBA in assessing Maccaferri Gabions.

In the opinion of the BBA, Maccaferri Gabions, when manufactured and installed in accordance with the provisions of this Certificate and the Department of the Environment, Transport and the Regions, Highways England (DETR, HE) Standard HD22 Ground Investigations and Earthworks Procedure for Geotechnical Certification, are suitable for use in short- and long-term applications including retaining wall structures, river training and erosion control.

The design, materials specification and construction methods adopted must be in accordance with the Manual of Contract Documents for Highway Works (MCHW)(1) Volume 1 Specification for Highway Works (SHW), BS 8002 : 2015, and BS EN 1997-1 : 2004 and its UK National Annex.

Additional site requirements may be included on particular projects.

(1) The MCHW is operated by the Overseeing Organisations: Highways England (HE), Transport Scotland, the Welsh Government and the Department for Infrastructure (Northern Ireland).

Regulations

Construction (Design and Management) Regulations 2015 Construction (Design and Management) Regulations (Northern Ireland) 2016

Information in this Certificate may assist the client, designer (including Principal Designer) and contractor (including Principal Contractor) to address their obligations under these Regulations.

See section: 3 Delivery and site handling (3.1, 3.3 and 3.4) of this Certificate.

Additional Information

CE marking

The Certificate holder has taken the responsibility of CE marking the products and their applications in accordance with

Declaration of Performance.

Technical Specification

1 Description

General

1.1 Maccaferri Gabions are cages formed from hexagonal double-twisted wire mesh panels made of galvanized (zinc- aluminium alloy) steel wire protected with PVC(1), PA6(2) or PoliMac(3)(4) polymer coating, fabricated into cage structures to be filled with selected granular material (gabion fill).

(1) PVC coating in accordance with BS EN 10245-2 : 2011. (2) Maccaferri PA6 is an environmentally safe, extruded polyamide coating applied to galvanized mild steel wire to provide increased resistance against mechanical damage, low friction coefficients, long term strength and elasticity, improved adhesion to wire, low liquid penetration for corrosion process, and improved performance in cold temperature. (3) Maccaferri PoliMac is an environmentally safe an extruded polymer specifically developed with an additional masterbatch, to provide increased resistance to abrasion and to mechanical damage, an improved performance in cold and hot temperatures, and a better performance to UV radiation. The properties of the Polimac coating are in accordance with BS EN 10245-1 : 2011. (4) Polimac is a registered trademark.

1.2 The products are available in a wide range of standard sizes, as defined in Table 1. Non-standard sizes can be manufactured for specific design requirements.

Page 2 of 13 Table 1 Standard sizes of Maccaferri Gabions fabricated from mesh with a core diameter of 2.7 mm

Length(1) (mm) Width(1) (mm) Height(1) (mm) 1 1 0.5 or 1 1.5 1 0.5 or 1 2 1 0.5 or 1 3 1 or 2 or 3 0.5 or 1 4 1 or 2 or 3 0.5 or 1 5 2 or 3 0.5 6 2 or 3 0.5 (1) Values are given in ETA 15/0219. Note: Non-standard sizes are available for special order with specific design requirements to be checked with the Certificate holder.

Wire mesh

1.3 The hexagonal double-twisted wire mesh is manufactured from steel wire in compliance with BS EN 10218-1 : 2012 and BS EN 10218-2 : 2012. The steel wire has a minimum tensile strength in the range of 350 to 550 N·mm2, with a minimum elongation at rupture of 8%, and is drawn from rods complying with BS EN ISO 16120-1 : 2017 and BS EN ISO 16120-2 : 2017.

1.4 The metallic coating is classified Class A(1) for PVC and PoliMac coated wire with nominal thickness of 0.5 mm, and Class E(2) for PA6 coated wire with nominal thickness of 0.4 mm. Wire specifications are given in Table 2.

(1) Class A coating, as defined in BS EN 10244-2 : 2009, Table 2, relates to the highest coating thickness. (2) Class E coating, as defined in BS EN 10244-2 : 2009, Table 2, for which corrosion resistance must be at least equivalent to that of zinc coating to Class B in BS EN 10244-2 : 2011, Table 1.

Table 2 Standard mesh wire specification

Galvanized wire Minimum Zn or Minimum Zn-Al Galvanized wire without polymer coating(1) with PVC, PA6 or Zn-Al coating coating for PA6 PoliMac coating(2) Class A coated wire Class E

Nominal diameter Tolerance Nominal diameter (g·m2) (g·m2) (mm) (mm) (external) (mm) 2.0 ± 0.05 3.0 215 60 2.2* ± 0.06* 3.2* 230* 60 2.4* ± 0.06* 3.4 230* 60 2.7* ± 0.06* 3.7* 245* 60 3.0* ± 0.07* 4.0 255* 60 3.4* ± 0.07* 4.4* 265* 60 3.9* ± 0.07* 4.9 275* 60

(1) Manufacturing reference: 8 x 10 mesh type. (2) Values are given in ETA 15/0219.

1.5 The wire is woven into a hexagonal pattern mesh (see Figure 1) with double-twist joints. Larger diameter wire is introduced along the edges to create the selvedge at cut ends.

Page 3 of 13 Figure 1 Details of hexagonal mesh

1.6 A gabion unit is fabricated from a base panel, two end panels and diaphragm panels (as appropriate, see Figure 2). To form these panels, the mesh is cut, during manufacture, to the required dimensions and selvedge wire positioned and mechanically fixed to the base panel. The gabion end panels are fixed to the base panel. Diaphragm panels, where provided, are connected to the base panel. The fabricated unit is conveniently folded, then packed into bundles for delivery.

1.7 Items used with the products, but outside the scope of this Certificate, include:

gabion filling material backfill material.

1.8 An ancillary item for use with the products is lacing wire.

1.9 The Maccaferri Pneumatic Fixing Tool is a mechanical device for crimping stainless steel or galvanized steel Flex-C rings to the polymer-coated mesh or the galvanized steel mesh, respectively. Stainless steel rings are manufactured from 3 mm diameter stainless steel wire with a minimum tensile strength of 1550 MPa. Galvanized steel rings are manufactured from 3 mm diameter stainless steel wire with a minimum tensile strength of 1720 MPa, and coated with 255 g·m-2 of Zn or Zn/5%Al. The pull-apart strength of the lacing rings is minimum of 2.0 kN.

2 Manufacture

2.1 The products are manufactured from galvanized steel wire, with or without polymer-coating, which is either and/or mill certificates are supplied with each batch of bought-in wire.

2.2 Maccaferri Gabion end panels and diaphragms have a selvedge wire along their upper edges. The cut ends are mechanically selvedged with a wire of greater diameter than that used for the mesh (see Table 2 and Figure 1).

2.3 As part of the assessment and ongoing surveillance of product quality, the BBA has:

agreed with the manufacturer the quality control procedures and product testing to be undertaken assessed and agreed the quality control operated over batches of incoming materials monitored the production process and verified that it is in accordance with the documented process evaluated the process for management of nonconformities checked that equipment has been properly tested and calibrated undertaken to carry out the above measures on a regular basis through a surveillance process, to verify that the specifications and quality control being operated by the manufacturer are being maintained.

Page 4 of 13 2.4 The management system of Maccaferri Ltd has been assessed and registered as meeting the requirements of BS EN ISO 9001 : 2015 by CERTICOM/Slovak Republic (Certificate CM5620718/1). The management system of Maccaferri Ltd has also been certified by SGS UK for geotechnical design and supplying of retaining structures, erosion protection works, highway and bio-engineering systems (Certificate GB01/52789).

3 Delivery and site handling

3.1 The gabions are delivered to site in bundles weighing from 600 to 900 kg, depending on unit size and mesh specification. A label bearing the , and the BBA logo incorporating the number of this Certificate, is attached to each bundle.

3.2 The products may be stored in the open, but away from site traffic to avoid the risk of accidental damage, and should remain packaged until required.

3.3 All bundles must be handled with due care to avoid damage to the polymer coating. Individual units can be manhandled. Individual units vary in weight between approximately 10 and 75 kg, and the appropriate number of people required to lift the units must therefore be assessed to satisfy manual lifting limits.

3.4 Lacing wire is supplied in 25 kg coils.

Assessment and Technical Investigations

The following is a summary of the assessment and technical investigations carried out on Maccaferri Gabions.

Design Considerations

4 Use

4.1 Maccaferri Gabions, used with selected fill materials, are satisfactory for use as retaining wall structures, river training and erosion control, in both short- and long-term applications. Where required by the overseeing authority, the gabions must be protected by the use of environmental barriers, as recommended in HE 85/01.

4.2 The design of structures using Maccaferri Gabions should be carried out by a suitably qualified engineer in accordance with BS EN 1997-1 : 2004, BS 8002 : 2015 and BS 6031 : 2009.

4.3 The design of the gabion structures must be based on the principle of mass gravity earth-retaining walls. Additional allowances may be made to account for the effect of the wire mesh.

4.4 The magnitude and distribution of the earth pressures and earth resistance should be calculated in accordance with current design practices and available guidelines.

4.5 As with other earth-retaining structures, it is necessary to ensure by design that a suitable factor of safety is achieved against the following potential modes of failure:

overall stability overturning shearing pressure sliding internal stability.

4.6 The designer should specify the relevant properties of both the gabion stone fill and backfill material (placed behind the cages) as deemed acceptable for the purpose of the design. Acceptable materials must meet the requirements of the MCHW, Volume 1 SHW.

4.7 The density of filled gabions depends on both the granulometric curve of the filling stones and the installation procedures: generally it should be taken as 60 to 70% of the density of the solid material. A higher value may be

Page 5 of 13 appropriate in certain circumstances, but this will be the responsibility of the consulting engineer who must ensure that the design value is achieved on site.

4.8 The stone infill of the gabions is normally sized between 100 and 200 mm and will depend on the mesh size, the lower limiting size being controlled by the dimensions of the mesh. The fill should satisfy the requirements of the MCHW, Volume 1 SHW, Class 6G(1). The fill must be a hard, durable material, eg stone as quarried or naturally occurring rounded stone (in accordance with BS EN 13383-1 : 2002).

(1) The MCHW, Volume 1, Series NG 600 Earthworks.

4.9 Gabion walls can be constructed with a minimum radius of curvature of 25 m on plan without modification of the gabion structure.

4.10 Watercourse linings, weirs and/or other hydraulic or erosion protection structures may require special consideration in regard to scour, uplift, wave action, seepage, etc.

4.11 The specification of the mesh for the gabions should be chosen to achieve the required design life (see section 9).

5 Practicability of installation

The products are designed to be installed by a competent general builder, or contractor, experienced with these types of products, and are easily installed under normal site conditions.

6 Structural performance

installation guidelines and this Certificate, the gabions have adequate strength to resist all loads associated with handling, positioning and filling.

6.2 Gabion walls are permeable and, in general, will not allow hydrostatic pressure to build up behind the wall. Gabion earth-retaining structures are not normally designed to withstand hydrostatic pressure.

6.3 Where cohesive material, such as clay, is retained, water movement may cause it to exude into the gabion structure and block the passage of water. To reduce the risk of a build-up of hydrostatic pressure in these conditions, it may be necessary to provide additional granular layers behind the gabion structure to allow water to drain away.

6.4 Maccaferri Gabion cages have adequate strength to permit pre-filling and placing by crane, carried out in accordance with the Certifi

7 Resistance to weathering

7.1 Some localised damage may occur to the polymer coating during installation and in exposed areas this may lead to local corrosion of the mesh. Site evidence and test data indicate that such damage will not spread and will not cause sequential corrosion underneath the intact adjacent area of mesh. The design of the wire mesh, and the fact that the strength of the external facing mesh is not used in the design of the structure as a whole, indicate that this local damage will not affect the integrity of the structure.

7.2 A gabion wall is a mass earth-retaining structure and settlement may occur under the action of its self-weight. The movement associated with this will gradually increase the density of the structure and reduce the volume of voids. Additional settlement may occur when gabions are founded on weak soils.

7.3 A gabion wall will permit the growth of vegetation, which will contribute to the integrity of the structure and to maintaining a natural appearance.

8 Maintenance and repair

Routine maintenance is not normally required; however, should the exposed gabion be damaged, additional or replacement panels can readily be fixed to the structure.

Page 6 of 13 9 Durability

9.1 For a particular installation environment, the most appropriate Maccaferri mesh wire coating specification may be selected in accordance with BS EN 10223-3 : 2013, Annex A, to satisfy the required project design life.

9.2 Evidence from tests show that the polymer-coated, Zn-Al alloy galvanized wire, has good resistance to chemical corrosion, bio-degradation, temperature effects and ultraviolet exposure and will not be affected by the chemicals normally encountered in earth-retaining structures.

9.3 The life of a gabion structure is dependent on the quality of the mesh, the durability of the stone fill and, in the longer term, the stability of the consolidated mass of the infill material and the conditions of exposure encountered esign life.

9.4 In the opinion of the BBA, when used and installed in accordance with this Certificate, the polymer coated gabions may be considered to have a design life in excess of 120 years.

9.5 The Certificate holder can advise on the life expectancy of products used in river erosion and coastal protection schemes and the design of such installation if required to optimise the performance of the system.

10 Reuse and recyclability

The products are made from polymer-coated (PVC, PA6 or PoliMac), galvanized (zinc-aluminium alloy) steel wire, which can be recycled.

Installation

11 General

11.1 Installation of Maccaferri installation instructions.

11.2 Prior to and during installation, particular care must be taken to ensure that:

site preparation and gabion foundation are in accordance with the contract drawings gabion construction is as detailed in section 12 fill properties (gabion stone and backfill) satisfy the design specification drainage is adequate at all stages of construction, as required by the contract documents the system is protected against damage from site traffic and installation equipment the stability of existing structures is not affected.

Page 7 of 13 12 Procedure

In-situ filled gabions units

12.1 The flat-packed gabion cages are opened and folded on a hard surface, pressing out any unwanted creases (see Figure 2). Figure 2 Preparation of gabion boxes

12.2 The front and rear sides, ends and diaphragm are lifted into position to form a box shape (see Figure 3).

Figure 3 Forming a gabion box

12.3 Top corners are secured with the projecting lengths of thick selvedge wire. Vertical edges are then joined together, starting from the top, using either the appropriate lacing wire, in a continuous operation with alternate single and double twists, or with rings applied using the Maccaferri Pneumatic Fixing Tool (see Figure 4).

Page 8 of 13 Figure 4 Typical Wiring pattern

12.4 A number of empty gabions are placed in position on a flat surface and secured together as described in section 12.3.

12.5 The gabions are filled progressively, in one third height layers, with suitable stone. To ensure a good finished appearance, all visible faces should be carefully hand-packed. Gabions should be overfilled to allow for natural settlement of the rock fill, see Figure 5.

Figure 5 Filling Sequence

Page 9 of 13 12.6 Gabions forming the exposed face of a structure should be filled to one-third height, braced from front to rear with bracing wires (lacing wire) located as shown in Figure 6, then filled to two-thirds height and again braced (see Figure 6). Four bracing wires should be provided for each square metre of visible face. Filling may then be completed.

Figure 6 Bracing Filling Sequence

12.7 When considered necessary for aesthetic or other considerations, the gabion may be filled whilst under tension. Gabions are tensioned by applying a load, distributed over the full end panel of a row of gabions, to the first cell which has been anchored in position. An external frame may also be used.

12.8 On completion of the gabion cage filling, the mesh lid is folded down, stretched into position and laced to the top of the front and side panels/adjacent gabion, and the top of any diaphragms.

Figure 7 Closing the lid on gabions

12.9 The remaining row of gabions is then filled sequentially in the same manner.

Page 10 of 13 12.10 It is essential that each gabion is properly secured to adjacent gabions above, below and on each side, using the lacing wire as described in section 12.3.

Pre-filled gabions units

12.11 The Certificate holder should be contacted for specific advice on the pre-filling procedure.

12.12 Prefilled Gabion units [traded under the name Readymac(1) or Cubiroc(1)] are constructed as described in sections 12.1 to 12.9, but with double loop lacing throughout. It is advantageous to construct a slightly oversized frame within which the empty unit can be held, stretched taut during filling. For example, Cubiroc units are filled using a special frame combined with a shaking table to ensure that the stone are compacted by vibration.

(1) Readymac and Cubiroc are registered trademarks.

12.13 During the filling operations, reinforcing steel bars or lifting straps are included to maintain the shape of the gabion during lifting.

12.14 Purpose-made lifting frames and slings must be used for lifting the filled units, which weigh up to 1.8 tonnes per cubic metre.

Technical Investigations

13 Investigations

13.1 The manufacturing process was evaluated, including the methods adopted for quality control, and details were obtained of the quality and composition of the materials used.

13.2 An assessment of data was made to determine:

dimensional accuracy* tensile strength* quantity of galvanized coating* the effect of tolerances strength of wire, mesh and filled gabions quality of materials quality of plastic coating ease of assembly compliance with the MCHW, Volume 1, Clause 262 specification (see sections 1.1 and 5.1) design procedures strength of the lifting frame durability effect of site damage equivalent performance of zinc-aluminium alloy steel and galvanized coated wire.

13.3 Site visits were carried out to assess the practicability, ease of handling and installation under various site conditions.

Page 11 of 13 Bibliography

BS 6031 : 2009 Code of practice for earthworks

BS 8002 : 2015 Code of practice for earth retaining structures

BS EN 1997-1 : 2004 + A1 : 2013 Eurocode 7 : Geotechnical design General rules NA to BS EN 1997-1 : 2004 + A1 : 2013 UK National Annex to Eurocode 7 : Geotechnical design General rules

BS EN 10218-1 : 2012 Steel wire and wire products General Test methods BS EN 10218-2 : 2012 Steel wire and wire products General Wire dimensions and tolerances

BS EN 10223-3 : 2013 Steel wire and wire products for fencing and netting Hexagonal steel wire mesh products for civil engineering purposes

BS EN 10244-2 : 2009 Steel wire and wire products. Non-ferrous metallic coatings on steel wire. Zinc or zinc alloy coatings

BS EN 10245-1 : 2011 Steel wire and wire products Organic coatings on steel wire General rules BS EN 10245-2 : 2011 Steel wire and wire products Organic coatings on steel wire PVC finished wire

BS EN 13383-1 : 2002 Armourstone Specification

BS EN ISO 9001 : 2015 Quality management systems Requirements

BS EN ISO 16120-1 : 2017 Non-alloy steel wire rod for conversion to wire General requirements BS EN ISO 16120-2 : 2017 Non-alloy steel wire rod for conversion to wire Specific requirements for general-purpose wire rod

Manual of Contract Documents for Highway Works, Volume 1 Specification for Highway Works, Series 600 Earthworks Manual of Contract Documents for Highways Works, Volume 2- Series NG 600 Earthworks

ETA 15/0219 European Technical Assessment- Hexagona Woven Mesh Gabion Boxes and Mattresses, version 01 of 07/04/201 ETA 16/0758 Officine Maccaferri S.p.A. Hexagonal double twisted wire meshes, and hexagonal double twisted wire meshes with inserted wire ropes

Page 12 of 13 Conditions of Certification

14 Conditions

14.1 This Certificate:

relates only to the product/system that is named and described on the front page is issued only to the company, firm, organisation or person named on the front page no other company, firm, organisation or person may hold claim that this Certificate has been issued to them is valid only within the UK has to be read, considered and used as a whole document it may be misleading and will be incomplete to be selective is copyright of the BBA is subject to English Law.

14.2 Publications, documents, specifications, legislation, regulations, standards and the like referenced in this Certificate are those that were current and/or deemed relevant by the BBA at the date of issue or reissue of this Certificate.

14.3 This Certificate will remain valid for an unlimited period provided that the product/system and its manufacture and/or fabrication, including all related and relevant parts and processes thereof:

are maintained at or above the levels which have been assessed and found to be satisfactory by the BBA continue to be checked as and when deemed appropriate by the BBA under arrangements that it will determine are reviewed by the BBA as and when it considers appropriate.

14.4 The BBA has used due skill, care and diligence in preparing this Certificate, but no warranty is provided.

14.5 In issuing this Certificate the BBA is not responsible and is excluded from any liability to any company, firm, organisation or person, for any matters arising directly or indirectly from:

the presence or absence of any patent, intellectual property or similar rights subsisting in the product/system or any other product/system the right of the Certificate holder to manufacture, supply, install, maintain or market the product/system actual installations of the product/system, including their nature, design, methods, performance, workmanship and maintenance any works and constructions in which the product/system is installed, including their nature, design, methods, performance, workmanship and maintenance any loss or damage, including personal injury, howsoever caused by the product/system, including its manufacture, supply, installation, use, maintenance and removal any claims by the manufacturer relating to CE marking.

14.6 Any information relating to the manufacture, supply, installation, use, maintenance and removal of this product/system which is contained or referred to in this Certificate is the minimum required to be met when the product/system is manufactured, supplied, installed, used, maintained and removed. It does not purport in any way to restate the requirements of the Health and Safety at Work etc. Act 1974, or of any other statutory, common law or other duty which may exist at the date of issue or reissue of this Certificate; nor is conformity with such information to be taken as satisfying the requirements of the 1974 Act or of any statutory, common law or other duty of care.

British Board of Agrément Bucknalls Lane tel: 01923 665300 Watford [email protected] Herts WD25 9BA ©2019 www.bbacerts.co.uk

Page 13 of 13

APPENDIX D – HYDRAULIC CALCULATIONS

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 33 Project Project No. Former West Herts College 133834 Kings Langley Section CONTENTS Office HEMEL HEMPSTEAD Department Prep by Date Check by Date Land & Development Engineering GT 07/12/2020 CA 07/12/2020

Contents

Section Description Sheet

1. Flow Control Weir Calculation to determine the normalized flow passing over the upstream weir offsite 1 which control flow leaving the Home Park Arm canal branch.

2. Hydraulic Check Manning's calculations to determine steady state constant flow within the proposed 2,3,4, 5 & 6 On Capacity of open channel, for varying storm events. Open Channel

3. Hydraulic check Manning's calculations to confirm that the flow capacity of this structure, at its narrowest, 7 On Capacity of Energy will take the the predict flow associated with a 1 in 1000yr event. Dissipation Structure

Supplementary Cowan Procedure 8 & 9 EA Level 4 Data - Predicted Flows 9

NOTES 1. It should be noted that the Mannings Equation has been adopted to provide a simple check on the capacity of the proposed open channel. The Mannings Equation assumes that all flows are constant and in a steady state conditon. This is unlikely to be the case with the open channel as there will be a degree of turbulance as flow passes down this channel. 2. The Mannings number used in the equation for the open channel hydraulic assessment has been derived utilzing the Cowan procedure to more accurately define this co-efficent. 3. As no measured flow data is available for the existing culvert entering the site at the boundary it has been assumed that the flows as schedule under the Level 4 product data supplied by the Environment Agency (Ref: HNL 188265 BC) are correct and therefore this data has been included within this calculation pack. 5. The aforementioned EA data does not provide a constant flow value for the, "Normalized Flow", dry weather conditon. This flow has therefore been calculated based on the observed depth of flow passing over the upsteam control weir on site (EA node reference HMPS1056). the measued depth of flow passing over the weir during dry weather conditons on two separate occasions has been recorded at 60mm. 6. In the enclosed Mannings Sheets, "Pipe Area", is the actual open cahnnel crossectional area and "Pipe Perimeter", is the wetted perimeter of the channel based on the flow depth.

Calculations cover sheet Millworks, Kings Langley Flow Volume Over Existing Weir Based On The Observe Depth Normalized Condition (Dry Weather Constant Flow)

Weir WEIR CALCULATOR

I want to find the flow discharge of my rectangular sharp crested weir

Cross Section Input

Weir Width (b) 18 m

Channel Width (B) 18 m

Upstream depth to crest (P1) 1.5 m

Downstream depth to crest (P2) 2.5 m Downstream depth of water above weir (h ) 2 m (leave blank for free discharge)

Other Inputs

Upstream depth of water above weir (h1) 0.06 m Observed depth of flow over the Weir

Coefficient of discharge (optional) 0.62

CALCULATE

Outputs

Coefficient of Discharge 0.62

Flow Discharge 0.48434 m³/s

SHEET 1 Registered User: Single User License © 2019 CivilWeb Project: Millworks Kings Langley By: GT Calculation: Manning Formula - Channel Flow Analysis (Normalized Constant Flow Condition) Approved: Sheet Number: 2 Date: 04.12.20 CA

Channel Info Roughness Coefficient (n) 0.04 Hydraulic Gradient (S) 1 % = 0.01 m/m Pipe Area 0.58 m2 1 in 100or Pipe Perimeter 2,728 mm

 3/2   1  A  2/1  Q       AS  n  P  

Results When Flowing Full Velocity (v) 0.89 m/s Flow (Q) 510.60 l/s = 0.51 m³/s Depth of Flow (d) 0.35m 10000 Manning Chart (Non-Standard Pipe)

1000

511

100 (l/s) Discharge

10

1 0.01 0.1 Hydraulic Gradient (%) 1 10 Registered User: Single User License © 2019 CivilWeb Project: Millworks Kings Langley By: GT Calculation: Manning Formula - Channel Flow Analysis (1 in 2 yr Constant Flow Condition) Approved: Sheet Number: 3 Date: 04.12.20 CA

Channel Info Roughness Coefficient (n) 0.04 Hydraulic Gradient (S) 1 % = 0.01 m/m Pipe Area 1.23 m2 1 in 100or Pipe Perimeter 3,668 mm

 3/2   1  A  2/1  Q       AS  n  P  

Results When Flowing Full Velocity (v) 1.21 m/s Flow (Q) 1,484.21 l/s = 1.48 m³/s Depth of Flow (d) 0.35m0,57 m 10000 Manning Chart (Non-Standard Pipe)

1,484

1000

100 (l/s) Discharge

10

1 0.01 0.1 Hydraulic Gradient (%) 1 10 Registered User: Single User License © 2019 CivilWeb Project: Millworks Kings Langley By: GT Calculation: Manning Formula - Channel Flow Analysis (1 in 50 yr Constant Flow Condition) Approved: Sheet Number: 4 Date: 04.12.20 CA

Channel Info Roughness Coefficient (n) 0.04 Hydraulic Gradient (S) 1 % = 0.01 m/m Pipe Area 1.89 m2 1 in 100or Pipe Perimeter 4,634 mm

 3/2   1  A  2/1  Q       AS  n  P  

Results When Flowing Full Velocity (v) 1.37 m/s Flow (Q) 2,598.60 l/s = 2.60 m³/s Depth of Flow (d) 0.75m m 10000 Manning Chart (Non-Standard Pipe) 2,599

1000

100 (l/s) Discharge

10

1 0.01 0.1 Hydraulic Gradient (%) 1 10 Registered User: Single User License © 2019 CivilWeb Project: Millworks Kings Langley By: GT Calculation: Manning Formula - Channel Flow Analysis (1 in 100 yr + 70% Constant Flow Condition) Approved: Sheet Number: 5 Date: 04.12.20 CA

Channel Info Roughness Coefficient (n) 0.04 Hydraulic Gradient (S) 1 % = 0.01 m/m Pipe Area 2.28 m2 1 in 100or Pipe Perimeter 4,834 mm

 3/2   1  A  2/1  Q       AS  n  P  

Results When Flowing Full Velocity (v) 1.51 m/s Flow (Q) 3,453.77 l/s = 3.45 m³/s Depth of Flow (d) 0.85m m 100000 Manning Chart (Non-Standard Pipe)

10000

3,454

1000 (l/s)

100 Discharge

10

1 0.01 0.1 Hydraulic Gradient (%) 1 10 Registered User: Single User License © 2019 CivilWeb Project: Millworks Kings Langley By: GT Calculation: Manning Formula - Channel Flow Analysis (1 in 1000 yr Flow Through Energy Dissipation Structure) Approved: Sheet Number: 7 Date: 04.12.20 CA

Channel Info Roughness Coefficient (n) 0.012 Hydraulic Gradient (S) 8 % = 0.08 m/m Pipe Area 0.48 m2 or 1 in 12.5 Pipe Perimeter 2,480 mm

 3/2   1  A  2/1  Q       AS  n  P  

Results When Flowing Full Velocity (v) 7.89 m/s Flow (Q) 3,785.57 l/s = 3.79 m³/s Depth of Flow (d) 0.24m m 10000 Manning Chart 3,786(Non-Standard Pipe)

1000

100 (l/s) Discharge

10

1 0.01 0.1 Hydraulic Gradient (%) 1 10 MANNING ROUGHNESS (n)

Introduction

The Manning’s n is a coefficient which represents the roughness or friction applied to the flow by the channel. Manning’s n-values are often selected from tables, but can be back calculated from field measurements. In many flow conditions the selection of a Manning’s roughness coefficient can greatly affect computational results when applying the Manning’s formula.

As the proposed open channel has not been built we therefore have no field measurement for flow in the channel, therefore a first principles derivation of a suitable n value has been undertaken using the Cowan Methodology.

Cowan (1956) developed a methodology to more accurately estimate the value of the Manning roughness n, for irregular naturalized open channels by using the associated geometry and hydraulic parameters of the channel under consideration.

Cowan Methodology Application

Factors to be incorporated in determining the Manning value n.

N1 = Base roughness

N2 = Irregularity modifier

N3 = Cross sectional modifier

N4 = Obstruction modifier

N5 = Vegetation modifier

N6 = Meandering modifier

Manning number (n) = N1 + N2 + N3 + N4 + N5 + N6

N1 - Base Roughness

The open channel will have a random rock / aggregate finish to its base and side slopes, therefore N1 can be considered to be coarse with a value of 0.028.

N2 – Irregularity modifier

The value of the irregularity modifier is based on the degree of irregularity. For the channel in question irregularity would be considered, “Minor”, based on it having a regular finish to the base and side slopes. In this regard this modifier would be 0.005

N3 -- Cross sectional modifier

The cross-sectional modifier is based on the character and variation in the size and shape of cross section. The cross-sectional area of the proposed open channel is substantially regular over its entire length, therefore in this regard this modifier would be 0.00 (Change in size or shape only occurs very gradually).

N4 – Obstruction Modifier

There will be no significant obstructions located with the proposed open channel therefore the relative effect of obstructions would be negligible and in this regard the modifier value would be 0.00. MANNING ROUGHNESS (n)

N5 – Vegetation Modifier

Vegetation will only be present on the berm to the east side of the open channel and in normalized flow conditions this planting will be elevated above the anticipated flow level. Only in more extreme events would this vegetation impose any influence on flows, therefore this modifier has been classified as, “Low”, at 0.005.

N6 – Meandering Modifier

Whilst a slight meander has been built into the alignment of the open channel the degree of curvature relative to the short length of channel under consideration is not significant and therefore it has been classified as being, “Minor”. In this regard the modifier has been determined as 0.00.

The Manning number to be applied in determining flow through the proposed naturalized open channel is determined as shown below n = N1 + N2 + N3 + N4 + N5 + N6 n = 0.028 + 0.005 + 0.00 + 0.00 + 0.005 + 0.00 n = 0.038

Assuming a precautionary approach, a value of 0.040 will be adopted for all the open channel flow calculations.

Detailed FRA centred on: West Herts College Kings Langley Campus, WD4 8DH - 12/10/2020 - HNL 188265 BC

GUC1506 GUC1478In

GUC1451In

GUC1424In Environment Agency GUC1397In Alchemy, Bessemer Road, GUC1370In Welwyn Garden City, Hertfordshire, GUC1344 AL7 1HE HPMS1155 GUC1344d 0 50 100 200 HPMS1131In HPMS1107In

GUC1291In HPMS1084In Metres GUC1264 Legend HHPMS1056PMS1056 GUC1239u GUC1239 Main Rivers GUC1219 Site location HPMS993 GUC1179 HPMS990 GUC1161 HPMS997 1D Node Results

GUC1162 HHPMS960PMS960 1D Nodes GUC1143In GUC1159 GUC1125 HPMS931In

GUC1095In HPMS903In

HPMS874In GUC1065In The data in this map has been extracted the Gade HPMS846In and Bulbourne Modelling Study (JBA, 2019). GUC1035In This study is a catchment scale mapping study and so may need local updates for site specific decisions. It HPMS817In should be noted that it was not created to produce flood GUC1006In levels for specific development sites within the HPMS789In catchment. Modelled outlines take into account catchment wide GUC976In defences. HPMS760In

GUC946In HPMS732In GUC916In HPMS704 HPMS698 GUC887In HPMS668In

GUC857In https://www.gov.uk/guidance/flood-risk-assessments- HPMS639In climate-change-allowances Produced by: Partnerships & Strategic Overview, This map is based upon Ordnance Survey Material with the permission of Ordnance Survey on behalf of the controller of Her Majesty's Stationery Office Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. Environment Agency 100024198, 2020 Hertfordshire & North London UNCLASSIFIED

MODELLED FLOWS Return Period Node Label Easting Northing 2 yr 5 yr 10 yr 20 yr 50yr 75yr 100yr 100yr+25%cc 100yr+35%cc 100yr+70%cc 200yr 1000yr GUC1506 507715 202343 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.17 3.53 3.02 3.84 GUC1478In 507726 202314 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.17 3.53 3.02 3.84 GUC1159 507730 202014 0.76 0.98 1.22 1.45 1.73 1.95 2.10 2.81 3.11 4.10 2.66 5.23 GUC1162 507731 202024 0.76 0.98 1.22 1.45 1.73 1.95 2.10 2.81 3.11 4.10 2.66 5.23 GUC1143In 507731 201995 0.76 0.98 1.22 1.45 1.73 1.95 2.10 2.81 3.11 4.10 2.66 5.23 GUC1161 507732 202027 0.76 0.98 1.22 1.45 1.73 1.95 2.10 2.81 3.11 4.10 2.66 5.23 GUC1125 507734 201978 0.76 0.98 1.22 1.45 1.73 1.95 2.10 2.81 3.11 4.10 2.66 5.23 GUC1451In 507736 202289 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.17 3.52 3.02 3.84 GUC1179 507737 202037 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.02 0.04 0.15 0.02 0.32 GUC1095In 507741 201949 0.77 0.99 1.23 1.46 1.74 1.97 2.12 2.83 3.13 4.12 2.67 5.26 GUC1424In 507746 202263 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.17 3.52 3.02 3.84 GUC1065In 507749 201921 0.77 0.99 1.23 1.46 1.74 1.97 2.12 2.83 3.13 4.12 2.67 5.23 GUC1219d 507752 202066 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.07 0.00 0.16 GUC1219 507754 202068 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.07 0.00 0.16 GUC1397In 507755 202238 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.17 3.51 3.02 3.84 GUC1035In 507757 201892 0.77 0.99 1.23 1.46 1.74 1.97 2.12 2.83 3.13 4.12 2.67 5.23 GUC1239 507760 202083 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.07 0.00 0.16 GUC1239u 507762 202087 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.07 0.00 0.16 GUC1006In 507764 201863 0.77 0.99 1.23 1.46 1.74 1.97 2.12 2.83 3.13 4.07 2.67 5.11 GUC1370In 507765 202213 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.17 3.51 3.02 3.84 GUC1264 507768 202109 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.09 0.01 0.16 GUC1344d 507772 202167 0.01 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.13 0.02 0.16 GUC976In 507772 201835 0.77 0.99 1.23 1.46 1.74 1.97 2.12 2.83 3.13 4.05 2.67 5.07 GUC1291In 507772 202126 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.11 0.01 0.16 GUC1344 507775 202188 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.17 3.50 3.02 3.84 GUC946In 507780 201806 0.77 0.97 1.14 1.32 1.53 1.71 1.94 2.66 2.94 3.80 2.48 4.76 GUC916In 507788 201777 0.77 0.96 1.11 1.25 1.42 1.56 1.73 2.37 2.63 3.45 2.21 4.33 HPMS1155 507792 202175 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.02 3.67 GUC887In 507795 201748 0.77 0.96 1.11 1.25 1.40 1.53 1.70 2.31 2.55 3.35 2.15 4.21 GUC857In 507803 201720 0.77 0.96 1.11 1.25 1.40 1.53 1.70 2.30 2.54 3.31 2.15 4.18 HPMS1131In 507814 202167 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.02 3.67 HPMS1107In 507825 202148 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.02 3.67 HPMS1084In 507836 202120 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.02 3.67 HPMS1056 507847 2020901.48 1.842.11 2.32 2.552.69 2.77 3.07 3.16 3.43 3.02 3.67 HPMS1002 507861 202050 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.02 3.67 HPMS997 507862 202046 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.02 3.67 HPMS993 507863 202042 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.02 3.67 HPMS990 507865 202037 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.50 3.02 3.67 HPMS960 507870 202007 1.481.84 2.11 2.32 2.552.69 2.77 3.07 3.16 3.43 3.02 3.67 HPMS931In 507879 201980 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.02 3.67 HPMS903In 507892 201953 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.02 3.67 HPMS874In 507905 201928 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.01 3.67 HPMS846In 507918 201903 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.01 3.67 HPMS817In 507928 201877 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.01 3.82 HPMS789In 507941 201852 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.01 3.82 HPMS760In 507953 201827 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.01 3.82 HPMS639In 507963 201712 1.48 1.83 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.01 3.82 HPMS732In 507963 201796 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.01 3.82 HPMS704 507968 201770 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.01 3.82 HPMS698 507968 201762 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.01 3.82 HPMS668In 507969 201740 1.48 1.84 2.11 2.32 2.55 2.69 2.77 3.07 3.16 3.43 3.01 3.82

APPENDIX E – FLOOD RISK TECHNICAL NOTE

Bellway Homes (North London) Environment Agency Response 133834-EAR-(0) 34

Our Ref: 133834-FRS-(0) 18 Frogmore Road Hemel Hempstead 7th December 2020 Hertfordshire HP3 9RT UK Nina Garner Telephone: +44 (0)1442 437500 Flood Risk Management Officer- PSO Luton, Hertfordshire and Essex Fax: +44 (0)1442 437552 Environment Agency www.rsk.co.uk Alchemy, Bessemer Road, Welwyn Garden City, Hertfordshire, AL7 1HE

By email only

Dear Nina,

RE: FLOOD RISK STATEMENT FOR CULVERT OPENING UP WORKS WEST HERTS COLLEGE, HOME PARK MILL LINK ROAD, KINGS LANGLEY 1. INTRODUCTION RSK Land and Development Engineering Ltd were commissioned by Bellway Homes (North London) to provide a review of the flood risk associated with the proposed culvert opening up works in relation to the development at West Herts College, Home Park Mill Link Road, Kings Langley. Bellway Homes Ltd (North London) have submitted a Reserved Matters (RM) application - LPA ref. 20/1858/AOD - on land at the above site. The application provides details of the appearance, scale, layout and landscaping of the development pursuant to an outline planning permission (LPA ref. 18/1034/OUT) for the demolition of the existing building and its redevelopment for up to 65 flats. A detailed flood risk assessment (FRA) produced by GHD, has been submitted as a part of the outline application. The proposals associated with the main site have not changed in principle from the outline planning stage. Therefore, the conclusions provided in the GHD FRA are still valid for the main development. The purpose of this statement is to demonstrate that the works associated with culvert opening up will not increase the flood risk elsewhere. An updated assessment has been provided of the flood risk to the site from a range of sources, where applicable to the proposed culvert opening up works. The comments given in this report and opinions expressed are subject to RSK Group Service Constraints provided in Appendix A.

2. ENVIRONMENT ANGENCY REQUIREMENTS This statement specifically addresses the following items requested by Environment Agency (EA) following a meeting held between EA, Bellway Homes and the design team on 26th November 2020. a) An initial assessment using topographical and site surveys to demonstrate no increase in flood risk b) Rough assessment on channel capacity c) Demonstrate the proposal is not changing the cross-sectional area of the river channel

RSK Land & Development Engineering Ltd Registered office Spring Lodge • 172 Chester Road • Helsby • Cheshire • WA6 0AR • UK Registered in England No. 4723837 www.rsk.co.uk

West Herts College, Kings Langley – Flood Risk Statement Ref: 133834-FRS-(0) – Page 2 of 7

d) Demonstrate no inspection chambers will be covered and access is not impeded by the proposal, in case we require access for emergency works e) Written justification for need of the energy dissipating structure, demonstrate it is built to allow sufficient capacity/flows for the channel f) Demonstrate there will be no adverse impact on the stability of the bank by including an adequate undeveloped buffer zone from the top of the main riverbank to the edge of the built footprint All of these items have already been addressed as a part of the design development undertaken following the meeting. Hence, where relevant, this report makes references to other supporting documents produced by the design team during the planning process.

3. SITE DETAILS AND PROPOSALS The development site is located off Home Park Mill Link Road, bounded by this road to the north, station footpath to the south, industrial buildings to the east and Pinnacle House to the west; grid reference Easting 507831, Northing 202026. As demonstrated in Figure 1, the culvert is located between the proposed development and the existing commercial development.

Mill Steam flows under the road

Existing culvert location

Proposed building outline Existing stream

Figure 1: Proposed building outline with the current location of the culvert

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4. BASELINE CONDITIONS

Topography The site general falls from north to south with a level of 68.47mAOD on the north-western corner falling to 66.98mAOD on the south-western corner. The culvert is located along the eastern boundary of site with the commercial properties to the east located approximately 2.2m above the levels of the development site.

Hydrology A culverted watercourse known as Mill Stream passes through the West Herts College site. This is connected to the Grand Union canal north and south of the site, outside the site boundaries. Connection to the north of Grand Union Canal is approximately 120m north of the site.

Geology Based on published geological records for the area (British Geological Survey (BGS) online mapping) the site exhibits the following geology: • Superficial Geology: Alluvium - Clay, Silt, Sand and Gravel. • Bedrock Geology: Lewes Nodular Chalk Formation and Seaford Chalk Formation (undifferentiated) - Chalk.

Hydrogeology Hydrogeological information was obtained from the online Magic Maps service. These maps indicate that the site is underlain by a Principal bedrock and a Secondary superficial aquifer.

Fluvial Flood Risk A Product 4 flood risk dataset has been obtained from the EA, based on a modelling study undertaken by JBA in 2019. The dataset indicates that the flows within the River Gade remain in-channel where it passes through the site for all return period events modelled including the 1 in 100 year plus 70% climate change and 1 in 1000 year events. The River Gade is shown to overtop the channel downstream of the site but this overtopping does not result in any out-of-channel flooding on the subject site. This confirms the site’s location within Flood Zone 1 prior to the proposed channel opening up works. The maximum modelled 1 in 100 year plus 70% climate change flood level within the channel as it passes through the subject site is 67.4m AOD. The maximum 1 in 1000 year flood level is 67.44m AOD at the top of the culvert and 65.86mAOD at the bottom. There is no risk to the subject site from the Grand Union Canal along the site’s western boundary for the 1 in 100 year plus 70% climate change or 1 in 1000 year modelled events. These findings correspond with those in the GHD FRA.

Flood Risk from Other Sources The EA’s flood risk from surface water mapping indicates that the site is primarily at a ‘low’ risk from surface water flooding, with small isolated pockets of ‘medium’ risk indicated in the west of the site (away from the culvert location). EA mapping also indicates the site is not at risk from reservoir flooding. In view of the underlying geology, there is the potential for shallow groundwater to exist beneath the site. These findings correspond with those in the GHD FRA.

5. FLOOD RISK ASSESSMENT - CULVERT OPENING UP WORKS This section of this statement addresses the EA requirements as listed under Section 2. Figure 2 shows the updated cross section of the proposed channel following discussions with the EA.

West Herts College, Kings Langley – Flood Risk Statement Ref: 133834-FRS-(0) – Page 4 of 7

Figure 2 – Updated section following EA meeting, Chainage 20m a) Topography

The levels of the existing culvert has been interpreted from information presented in Culvert Condition Report Ref: BB1219-19, produced by Beaver Bridges. An extract from the northern headwall elevation is reproduced below for reference. Similar information is available for the southern headwall and our interpretation of the existing levels are presented in Table 1 below.

Figure 3 – North headwall information from Culvert Condition report

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Table 1 – Existing topography of the culvert

Location Existing ground level (mAOD) Estimated Culvert level (mAOD) North headwall 67.5 64.99 South headwall 67.5 64.95

The culverted section inside the site boundary has an average invert level of 65.15mAOD. As the opened channel required battered slopes at least on one side of the arrangment, as agreed with EA, the proposed channel bed level has been raised from the exising level. The new propsoed levles are summaries below.

Table 2- Proposed Channel levels

Location Proposed ground level Estimated Culvert level (mAOD) (mAOD) Head of the proposed open channel 67.49 66.0 Bottom of the proposed open channel 67.3 65.5

As demonstrated the proposed site levels are relatively comparable to the existing site level in the proposals. The long section of the culvert is presented below, showing the arrangements in relation to the existing ground level. Further details on how existing levels relate to the new proposals are provided under Appendix C of RSK EA Response Report 133834-EAR-(0).

Figure 4 – Estimated depth of the channel available for flow

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b) and c) Channel capacity and cross section The existing culvert varies in width and depth along its length. The cross sectional area of the existing culvert has been estimated from the information presented in Culvert Condition Report by Beaver Bridges. The southern end of the site has a reduced cross section compared to the northern head wall area.

Figure 5 – South headwall information from Culvert Condition Report Ref: BB1219-19, produced by Beaver Bridges

Based on the information above, the existing cross section area has been estimated as below. Approximate existing cross-sectional area = (2.25 x 1) + (2.22 x 0.9) = 4.25m2 However, the flow is currently unable to utilize this full cross-sectional area due to the flow within the channel been controlled by the twin pipe system located to the north of the site. Proposals for the new opened up channel has a consistent channel bed width of 4m through the site. The southern end of the site where both sides of the opening are formed of gabion baskets can be considered as the narrowest point of this new arrangement.

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Figure 6 – Proposed channel width

For a simplified version of the calculation if it is assumed that channel is rectangular at chainage 26.655 (see Figure 4), the total depth between channel bed and proposed site level is approximately 1.16m. This is equivalent to 4.64m2 of cross-sectional area. In reality, the cross-sectional area is higher than this value due to it being a trapezoidal profile. A cross section for the channel further downstream is presented below. This is the location at the existing foot bridge to the south of the site.

Figure 7 – Proposed cross section underneath foot bridge

Further to the information provided under sections 4 and 5 of this report, we have interpreted the flood depth for 1 in 1000-year event based on this data for the new open channel as shown on figure 8. As demonstrated above, there is sufficient capacity to accommodate the 1 in 1000-year flood levels within the channel. It should also be noted that the maximum flow volume within the channel even in an extreme rainfall event throttled by the twin-pipe arrangement to the north of the site. d) Inspection chambers No inspection chambers are proposed as a part of this proposal to open up the culvert. The channel section is open and the energy dissipation structure is located belowground. With regards to the access there is access available to both areas. There is a 1m corridor between the development and the channel at the pinch point at the head of the open channel. Please refer to Section 3.3.4 of the RSK EA Response Report 133834-EAR-(0) regarding maintenance access and section 6 of the same report regarding access to the energy dissipation structure. Gabion basket with same design life as the development (100 years) has been proposed to minimize maintenance requirements. e) Energy dissipating structure As noted under the topography section of this report, the existing site constraints means that an energy dissipation structure is required to achieve the open channel feature as required by the EA. The structure has been designed to accommodate the required flow volumes. A full justification of the use of the energy dissipation structure is provided under section 6 of the RSK EA Response Report 133834-EAR-(0). Manning calculations for flows have been undertaken and these are presented in Appendix D of the abovementioned report.

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f) Stability of slopes Due to the site constraints there is only limited space to accommodate the slopes and there is a 1m width available at its narrowest point. This has been discussed with EA during the meeting and the reasoning behind the arrangement presented with drawings. This corridor widens as you travels south along the channel. Further details of this are provided under section 4.2.3 of the RSK EA Response Report 133834- EAR-(0).

6. CONCLUSIONS As noted in the FRA produced by GHD, the main development is at low risk of flooding. Although there is some localized surface water flooding noted, this will be dealt with as a part of the proposed surface water drainage strategy. Within the application site, the proposed open channel can contain the 1 in 1000 year flow with no overtopping of the banks. Therefore, the proposals for opening up the culvert will not alter the flood risk on- site (with the site remaining within Flood Zone 1) or downstream of the site. The flow in the Mill Stream is restricted upstream of the site due to the presence of culverts underneath the Home Park Mill Link Road. There are no alterations to these as a part of works undertaken on site. Therefore, overall there is no increased flood risk as a result of this proposal. We trust the above is useful, but should you require any additional information, please do not hesitate to contact the undersigned.

For RSK LDE Limited, Yours faithfully,

Cham Ariyaratne Grant Turner Civil Engineer Civil Engineer Author Reviewer

APPENDICES Appendix A: Service Constraints

APPENDIX A Service Constraints 1. This report and the Drainage design carried out in connection with the report (together the "Services") were compiled and carried out by RSK LDE Ltd (RSK) for Bellway Homes (North London) (the "client") in accordance with the terms of a contract between RSK and the "client" dated December 2020. The Services were performed by RSK with the skill and care ordinarily exercised by a reasonable Civil Engineer at the time the Services were performed. Further, and in particular, the Services were performed by RSK taking into account the limits of the scope of works required by the client, the time scale involved and the resources, including financial and manpower resources, agreed between RSK and the client. 2. Other than that expressly contained in paragraph 1 above, RSK provides no other representation or warranty whether express or implied, in relation to the Services. 3. Unless otherwise agreed the Services were performed by RSK exclusively for the purposes of the client. RSK is not aware of any interest of or reliance by any party other than the client in or on the Services. Unless expressly provided in writing, RSK does not authorise, consent or condone any party other than the client relying upon the Services. Should this report or any part of this report, or otherwise details of the Services or any part of the Services be made known to any such party, and such party relies thereon that party does so wholly at its own and sole risk and RSK disclaims any liability to such parties. Any such party would be well advised to seek independent advice from a competent environmental consultant and/or lawyer. 4. It is RSK’s understanding that this report is to be used for the purpose described in the introduction to the report. That purpose was a significant factor in determining the scope and level of the Services. Should the purpose for which the report is used, or the proposed use of the site change, this report may no longer be valid and any further use of or reliance upon the report in those circumstances by the client without RSK's review and advice shall be at the client's sole and own risk. Should RSK be requested to review the report after the date hereof, RSK shall be entitled to additional payment at the then existing rates or such other terms as agreed between RSK and the client. 5. The passage of time may result in changes in site conditions, regulatory or other legal provisions, technology or economic conditions which could render the report inaccurate or unreliable. The information and conclusions contained in this report should not be relied upon in the future without the written advice of RSK. In the absence of such written advice of RSK, reliance on the report in the future shall be at the client's own and sole risk. Should RSK be requested to review the report in the future, RSK shall be entitled to additional payment at the then existing rate or such other terms as may be agreed between RSK and the client. 6. The observations and conclusions described in this report are based solely upon the Services, which were provided pursuant to the agreement between the client and RSK. RSK has not performed any observations, investigations, studies or testing not specifically set out or required by the contract between the client and RSK. RSK is not liable for the existence of any condition, the discovery of which would require performance of services not otherwise contained in the Services. For the avoidance of doubt, unless otherwise expressly referred to in the introduction to this report, RSK did not seek to evaluate the presence on or off the site of asbestos, electromagnetic fields, lead paint, heavy metals, radon gas or other radioactive or hazardous materials. 7. The Services are based upon RSK's observations of existing physical conditions at the site gained from a walk-over survey of the site together with RSK's interpretation of information including documentation, obtained from third parties and from the client on the history and usage of the site. The Services are also based on information and/or analysis provided by independent testing and information services or laboratories upon which RSK was reasonably entitled to rely. The Services clearly are limited by the accuracy of the information, including documentation, reviewed by RSK and the observations possible at the time of the walk-over survey. Further RSK was not authorised and did not attempt to independently verify the accuracy or completeness of information, documentation or materials received from the client or third parties, including laboratories and information services, during the performance of the Services. RSK is not liable for any inaccurate information or conclusions, the discovery of which inaccuracies required the doing of any act including the gathering of any information which was not reasonably available to RSK and including the doing of any independent investigation of the information provided to RSK save as otherwise provided in the terms of the contract between the client and RSK. 8. The phase II or intrusive environmental site investigation aspects of the Services is a limited sampling of the site at pre- determined borehole and soil vapour locations based on the operational configuration of the site. The conclusions given in this report are based on information gathered at the specific test locations and can only be extrapolated to an undefined limited area around those locations. The extent of the limited area depends on the soil and groundwater conditions, together with the position of any current structures and underground facilities and natural and other activities on site. In addition chemical analysis was carried out for a limited number of parameters [as stipulated in the contract between the client and RSK] [based on an understanding of the available operational and historical information,] and it should not be inferred that other chemical species are not present. 9. Any site drawing(s) provided in this report is (are) not meant to be an accurate base plan, but is (are) used to present the general relative locations of features on, and surrounding, the site.