planning stage drainage strategy vaughan road aparthotel
TRANSCRIPT
Dandi Nine Ltd Vaughan Road Aparthotel
Planning Stage Drainage Strategy Vaughan Road Aparthotel
Vaughan Road Aparthotel Planning Stage Drainage Strategy copy MLM Consulting Engineers Limited
mlmipscomworkTier2MLM66010006601343MLMREPDrainage Report
Notice
This document and its contents have been prepared and intended solely for Dandi Nine Ltd information and use in relation to the Vaughan Road Aparthotel scheme
MLM Consulting Engineers Limited assumes no responsibility to any other party in respect of or arising out of or in connection with this document andor its contents
Document History
Client Dandi Nine Ltd Project Vaughan Road Aparthotel Document Title Planning Stage Drainage Strategy Document Reference 6601343-MLM-ZZ-XX-RP-C-0001 MLM Reference JR 6601343 JJB
Revision Status Description Author CheckedApproved Date
P01 S2 Issued for Comment James Noble Martin Cozens 28082020
P02 S2 Amendments throughout James Noble John Barrell 22092020
Vaughan Road Aparthotel Planning Stage Drainage Strategy copy MLM Consulting Engineers Limited
mlmipscomworkTier2MLM66010006601343MLMREPDrainage Report
Contents
1 Introduction 1
2 Thames Water Asset Map 1
3 Foul Drainage 2
31 Existing Foul Water Drainage 2 32 Proposed Foul Water Drainage 2
4 Flood Risk 3
5 Exceedance Routes 3
6 Surface Water Drainage 4
61 Existing Surface Water Drainage 4 62 Proposed Surface Water Drainage 4
7 Treatment 5
8 Future Maintenance 6
9 Conclusion 6
91 Foul Water 6 92 Surface Water 7
Appendix A - Thames Water Correspondence
Appendix B - Thames Water Pre-planning Enquiry
Appendix C - Flood Exceedance Routes
Appendix D - Drainage Calculations
Appendix E - Drainage Strategy
Appendix F - SuDS Maintenance Plan
Appendix G - Foul Water Pump Specification
Appendix H - Surface Water Pump Specification
Vaughan Road Aparthotel Planning Stage Drainage Strategy copy MLM Consulting Engineers Limited
mlmipscomworkTier2MLM66010006601343MLMREPDrainage Report 1
1 Introduction
This report has been prepared for the sole use of Dandi Nine Limited and the contents should not be relied upon by others without the express written authority of MLM Consulting Engineers Limited If any unauthorised third party makes use of this report they do so at their own risk and MLM owes them no duty of care
MLM have been appointed for the Civil and Structural design of a new aparthotel in Harrow
The proposed 129 room aparthotel with associated fifth-floor restaurant gym ground floor commercial space external hard and soft landscaping small car parking area to the rear of the site and external amenity space located on an existing (disused) carpark on Vaughan Road Harrow The current site is approximately 012ha and mostly impermeable with a small area of vegetation verge running along the northern and eastern boundaries
MLM were also appointed in 2017 for Civil and Structural design for a previous residential scheme on the same site which gained various approvals in relation to the sites surface amp foul water connections and flood exceedance routes
This report has been produced in support of the proposed developments planning application and relates to all onsite private drainage proposals within the sites red line boundary only All offsite works beyond the sites boundary is not included within this report
Whilst this report will discuss elements of flood risk a flood risk assessment (FRA) is not required under NPPF guidelines due to the size of the site being below 10ha and its location within flood zone 1
2 Thames Water Asset Map
Figure 1 below is an extract of Thames Water assets located in surrounding the site The map has been extracted from correspondence with Thames Water within Appendix A
Figure 1
Vaughan Road Aparthotel Planning Stage Drainage Strategy copy MLM Consulting Engineers Limited
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3 Foul Drainage
31 Existing Foul Water Drainage
There are no known existing foul sewers located on the existing car park site With reference to Thames Water Asset plans all potential offsite existing foul sewer connection points have been assessed as follows
There are a number of other existing Thames Water Foul manholes located approximately 45m to the East adjacent to the BP garage Any proposed connection to these sewers would require crossing the busy Bessborough Road therefore connection into this sewer has been discounted
The nearest existing Thames Water Foul sewer is located 25m to the West along Vaughan Road indicated as Manhole EXFW A on the drainage strategy drawing 6601343-MLM-ZZ-XX-DR-C-0001 within Appendix E A number of smaller adoptable sewers connect from the recently built houses (4No) to the West of the development which connect into the head of the existing Thames Water main foul sewer
No manhole reference cover or invert level information has been identified on the Thames Water records and all existing site surveys do not extend to this location It is proposed to connect the developments onsite foul network to a new length of gravity sewer within Vaughan Road discharging into manhole EXFW A
In advance of detail design further survey works will need to be carried out to confirm the assumed invert level of existing Thames water manhole EXFW A
32 Proposed Foul Water Drainage
The Northern lower half of the development is at a lower level that Vaughan Road with an approximate proposed floor level of 649m The proposed private onsite foul network therefore will have an approximate outfall invert level of 6374m
The developments foul outfall connection point at Thames Water manhole EXFW A is assumed to have the water authority minimum standard depth to invert of 12m resulting in an invert level of approximately 6395m
The invert level of existing manhole EXFW A is therefore higher than the onsite invert level which means a full gravity connection will not be feasible
It is therefore proposed to use an onsite private package pump with associated 24hr storage (storage in manhole or tank) and pump flows at an agreed rate with Thames Water to a final onsite gravity manhole Details of the foul pump can be found within Appendix G
From the boundary of the development a new foul gravity sewer will be constructed below Vaughan Road to existing manhole EXFW A The new foul sewer will be constructed in accordance with Thames Water requirements and will be offered for adoption under a Section 104 agreement
A pre-planning enquiry was submitted to Thames Water for the proposed connection into the existing manhole EXFW A at a peak discharge rate of 82ls Thames Water have confirmed in principle that this connection point and discharge rate are acceptable For further detail refer to Thames Water Pre-planning enquiry Appendix B
All proposals are subject to the additional survey work described above a Thames Water Section 106 and Section 104 application (and Thames Water approval) and full coordination with the design team at detailed design
Vaughan Road Aparthotel Planning Stage Drainage Strategy copy MLM Consulting Engineers Limited
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Details of the proposed foul drainage strategy can be found on drawing 6601343-MLM-ZZ-ZZ-DR-C-0001 within Appendix E
4 Flood Risk
The site is located within flood Zone 1 and is smaller than 10ha therefore the site is considered Low Risk and a scheme specific flood risk assessment (FRA) is not required
The below extract taken from the GOVUK Flood Risk from Surface Water indicates a very low risk with no surface water flooding onsite
There is however Low Risk surface water flooding on the adjacent highway (Vaughan road) A new drop crossing is required to access the site from Vaughan Road This crossing and all works along the existing highway site boundary to the south will retain the existing levels Therefore risk of flooding from offsite will not be effected with the onsite risk remaining very low
5 Exceedance Routes
In accordance with good drainage practice consideration must be given to the effects of drainage system failure or for storm events greater than those used in the design analysis
lsquoSurface Water Flood Flow and Exceedance Routesrsquo drawing 6601343-MLM-ZZ-XX-DR-C-0002 shows proposed finished floor and external levels above ground exceedance flow routes (blue arrows) across the site which routes run-off away from the new building and prevents water leaving site In extreme cases the flood waters will overtop into a high level overflow pipe as indicated by the red arrow preventing surface water from entering the building The lsquoSurface Water Flood Flow and Exceedance Routesrsquo drawing is included as Appendix C
Vaughan Road Aparthotel Planning Stage Drainage Strategy copy MLM Consulting Engineers Limited
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6 Surface Water Drainage
61 Existing Surface Water Drainage
An existing 25m diameter 135m deep concrete Thames Water trunk sewer crosses the site in the South East corner
During the previous residential scheme Thames Water requested a build over or near agreement be entered into to ensure the proposed development does not impact the trunk sewer The new Aparthotel scheme has where appropriate ensure that Thames Waterrsquos build over or near requirements have been met when positioning the building and developing the preliminary foundation design A full detailed application will be made to Thames Water at the detail design stage
The scheme site survey has confirmed that the previous car park drained via five existing gullies located on the lower Northern boundary of the car park A CCTV survey confirmed that these gullies discharge to a lsquoyrsquo connection at point 901B as shown on the Thames Water asset map From point 901B the sitersquos surface water discharges into the existing Thames Water network within the adjacent underpass manhole ref 9009
The existing underpass network then outfalls to an existing 28m diameter 812m deep Thames Water surface water catchpit ref 901A located in the South Eastern corner of the site The catchpit then discharges via a 300dia sewer to the trunk sewer below
62 Proposed Surface Water Drainage
The developments proposed surface water run-off from the building will be collected via a combination of green blue roof and rainwater pipes with the external hard paved areas by porous paving
The inclusion of a large roof garden and smaller planted roof terraces (green roofs) not only increases the bio-diversity water treatment and improves air quality of the existing site which currently is predominantly hard paved but also aids with the capture control and delay of surface water runoff at source A portion of the surface water will also be harvested for irrigation purposes The inclusion of porous paving for the external hard paved areas will also contribute to water treatment control and delay of surface water runoff at source
Due to the non-porous nature and restricted external space infiltration is not feasible The surface water run-off will therefore be conveyed to a below ground attenuation tank located below the external car park The attenuation tank and associated network will be installed approximately 2-25m deep due to the low lying level of the adjacent underpass and eastern courtyard
The developments will re-use the existing carparks surface water outfall A new adoptable onsite demarcation chamber will be required at the boundary of the site and a new adoptable manhole (ref 901B) will be built on the line of the existing 150dia lateral sewer The existing offsite 150dia sewer downstream of the new manhole is to be offered for adoption by Thames Water
The surface water offsite discharge rate will be restricted to 50 ls for up to and including the 1 in 100 year storm event plus a 40 allowance for climate change Due to the depth of the proposed drainage network and attenuation it will be necessary to pump surface water flows up into the demarcation chamber From this point surface water will flow by gravity into manhole 901B Details of the surface water pump can be found within Appendix H
A high level overflow from the pump chamber to the demarcation chamber will also be installed to ensure that in the unlikely event of pump failure surface water flows will overtop into the network and not flood the building or leave site
Vaughan Road Aparthotel Planning Stage Drainage Strategy copy MLM Consulting Engineers Limited
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The proposed discharge rate of 50ls connection point and adoption of the existing 150dia lateral sewer have been approved in principle by Thames Water Correspondence can be found within Appendix A The proposed connection will be subject to full technical approval and a Thames Water Section 106 agreement during the detailed design
The design team have liaised with Harrow InfrastructureHighways team throughout the pre-planning stage and have received confirmation that in principle the strategy is acceptable
It is proposed to attenuate in below ground storage tanks and where achievable within the porous paving sub-base Based on the restricted 50ls an onsite attenuation volume approximately 50m3 ndash 72m3 (with the 40 climate change) will be required for the development For the surface water calculations please refer to Appendix D
Details of the proposed surface water drainage strategy can be found on drawing 6601343-MLM-ZZ-ZZ-DR-C-0001 within Appendix E
7 Treatment
With reference to the SuDS manualrsquos pollution hazard land use classification table 262 the Aparthotel roof and external pedestrian areas parking spaces are classified as having a very low and low pollution hazard levels respectively
The required hazard indices for a site of this type should achieve or better the hazard indices identified for Total suspended Solids Metals and Hydrocarbons Refer to Figure 1 below indicating the SuDS manual
Table 262 Figure 1 ndash Pollution hazard indices for different land use classification Table 262
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In order to provide adequate treatment the proposed drainage network components should have a total pollution migration index that equals or exceeds the pollution hazard index In order to comply with these requirements a series of individual components within the proposed network is required In line with the SuDS manual factor of 05 is to be used to account for the reduced performance of the secondary or tertiary components associated with already inflow concentrates
Please refer to table 2 for scheme specific pollution mitigation indices
Surface Water Runoff
Treatment -Individual devicesequence
Total suspended solids mitigation index
Total metals mitigation index
Hydrocarbons3
Residential Roof Debris leaf guard within roof gutters
Sealed downpipes
Catchpit Siltrap Manholes
Blue Green Roof
Reference to Chapter 24 of the SuDS manual table 43 Minimum water quality requirements for discharge to receiving surface waters and groundwater
States for very low pollution hazard level the requirement for discharging to groundwater is the lsquoRemoval of gross
solids and sediment onlyrsquo
Private porous external pedestrian areas and parking spaces
Porous Paving with type 3 sub-base
07 06 07
Table 2 ndash Scheme Pollution Mitigation Indices
The total proposed treatment for the private drainage networks will provide appropriate levels of water quality treatment prior to discharge to Thames Water network
8 Future Maintenance
Maintenance of SuDS features shall be carried out in accordance with the MLM document entitled lsquoSurface Water Drainage Maintenance Planrsquo This is included as Appendix F
9 Conclusion
The schemes proposed foul and surface water drainage strategy detailed within the report identifies the intended means of connection and discharge of estimated peak flows The design team have liaised with Harrow Highways Infrastructure team and Thames Water throughout the pre-planning stage and have received confirmation in principle that both the surface water and foul water strategies are acceptable
Summary of foul and surface water drainage strategy
91 Foul Water
911 The developments foul water connection is based on a peak flow rate of 82 ls discharging into the existing Thames Water manhole EXFW A to the West of the development in Vaughan Road
912 Thames Water have confirmed that the proposed peak flow rate and connection into their foul sewer in Vaughan Road is acceptable in principle and will not have a detrimental effect on their network
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913 Connection details of the proposed Section 104 gravity sewer within Vaughan Road into the existing Thames Water foul manhole will be subject to confirmation of the invert level
92 Surface Water
921 The developments surface water will discharge at a maximum of 5ls for all storms up to and including the 1 in 100 year storm event plus 40 allowance for climate change The development will discharge via the existing car parks outfall lateral outfall
922 Thames Water have confirmed that the proposed surface water discharge rate adoption of the 150dia lateral and ultimate connection into their trunk sewer below site is acceptable in principle and will not have a detrimental effect on their network
An approved Thames Water pre-planning enquiry has been received for the developments foul and surface water discharge Refer to Appendix B for details
Both connections will be subject to Thames Water full technical approval at detail design
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Appendix A - Thames Water Correspondence
From Siva RajaratnamTo John BarrellCc James NobleSubject RE 6601343 - RE RE DS6075526 - Pre-planning enquiry Car Park Vaughan Road Harrow HA1 3XDDate 14 August 2020 102423Attachments image001jpg
image003jpgimage004jpg
Hi John This will be acceptable to Thames Water RegardsSiva RajaratnamDeveloper Services ndash Adoptions Engineer Mobile 07747 640477 Landline 0203 577 9811 sivarajaratnamthameswatercouk
Clearwater Court Vastern Road Reading RG1 8DB Find us online at developersthameswatercouk
From John Barrell ltJohnBarrellmlmgroupcomgt Sent 13 August 2020 1827To Siva Rajaratnam ltSivaRajaratnamthameswatercoukgtCc James Noble ltJamesNoblemlmgroupcomgtSubject RE 6601343 - RE RE DS6075526 - Pre-planning enquiry Car Park Vaughan Road Harrow HA13XDImportance High
This email contains a reference to Coronavirus or COVID-19 Please be aware ofcoronavirus-themed active phishing campaigns and use extra vigilance whenresponding or clicking
Many thanks for the swift reply Siva Yes we would pump up to a manhole within the site boundary then gravity into a new manhole atlocation 901A We do not believe there is currently a manhole at location 901A we believe there is a lsquoyrsquo connectiononly On this basis we would propose a new manhole (to adoptable standards) at location 901A Wewould then put new manhole 901A and the existing downstream 150mm dia lateral to manhole 9009forward for Thames Water adoption This would of course be subject to a formal adoption process toThames Waters requirements at detail design We have updated the design to reflect the above and also updated manholes pipes that are nowconfirmed to be in Thames Waters ownership Details have been clouded on the attached drawing6601343-MLML-ZZ-XX-DR-C-0001 P03 Please note that the development boundary is indicated by the red dashed line and so the area to theeast is currently highway owned It should also be noted that this existing sewer does pass parallel with the new building frontagehowever all necessary protection measure will be in place and CCTV survey carried out (if required)to ensure all existing sewers are not affected by the works If you could confirm the above is acceptable in principle (subject to Thames Waters adoption S106