Thames Tideway Tunnel Local Impact Report: London Borough of ...

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London Borough of Hounslow, its local context and the different impacts on the following ..... 2.39 The noise emission p
Thames Tideway Tunnel Local Impact Report: London Borough of Hounslow (Unique Reference Number LBH01)

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CONTENTS 3 Introduction 6 Details

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1.0

INTRODUCTION

1.1

The Development Consent Order is a legal Order which provides consent for the project and means that a range of other consents, such as planning permission and listed building consent will not be required. A Development Consent Order can also include provisions authorising the compulsory acquisition of land or of interests in or rights over land which is the subject of an application.

1.2

The Council understands that the aim of the Local Impact Report is not to discuss the principle of a project applying for such an Order, but to state all the positive, negative and neutral local impacts that the project will have and to comment on provisions, requirement and development consent obligations, either proposed, or which the Council considers should be proposed.

1.3

The Thames Tideway Tunnel is one part of a wider sewage upgrade for the city and region, which also includes the Mogden Sewage Treatment Works in Isleworth. This aspect of the upgrade works, which opened this year, is considered to have the most impact and benefit to the overall London Borough of Hounslow. However, it is noted that sewerage overflow events from sites affected by this proposal have affected areas of the river and its banks within the Borough, with particular reference to sewage flooding (particularly in Chiswick Mall, where significant damage was cause by sewage overflow to a Grade 1 listed building in 2009) and wildlife kills. Notably the Acton Storm Relief combined sewer overflow (the CSO) spills approximately 29 times and discharges 312,000m3 of untreated sewage into the River Thames at Chiswick Eyot, a local nature reserve in the London Borough of Hounslow. On the basis that litter tonnages are proportional to discharge volumes, approximately 79 tonnes of sewage derived litter is also discharged from this CSO in an average year.

1.4

The Council will effectively be commenting on the implications of its proximity to a proposed tunnel site and its hosting of the tunnel in the east of the Borough in this report.

1.5

The Borough’s Local Impact Report principally deal with two “sites”- the Acton Storm Tanks site (a site within the London Borough of Ealing, but immediately bordering Hounslow) and the main tunnel underneath the London Borough of Hounslow, its local context and the different impacts on the following topics: Heritage Transport Noise, Vibration and Odour Land Quality Air Quality Ecology Socio-economics 3

Water Resources and Flood Risk 1.6

The application documents that address the impacts of the project and the associated mitigation measures are the Environmental Statement, The Code of Construction Practice, the Statement of Common Ground, the Design Principles and the Requirements included in the Development Consent Order. The Environmental Statement presents the findings of the assessments of the likely significant effects on the different topics addressed.

1.7

Schedule 3 of the Draft Order contains requirements (similar to Planning Conditions) both for the main tunnel and for Acton Storm Tanks. One of these requirements is the Code of Construction Practice. Other included are understood to be: Project wide: Interpretive requirement, time limits, phasing-project wide and site specific, drive strategy, design principles, air management, listed buildings and structures and their monitoring, built heritage, signage. Acton Storm Tanks: Code of Construction Practice Part B, detailed design approval for permanent above ground structures, contaminated land, archaeology, landscaping, construction management plan, travel plan, junction layout and other highway works, specification of accesses, surface water drainage and new flood protection and operational lighting.

1.8

The Code of Construction Practice contains mitigation measures to minimise the potential negative effects. They are based on a precautionary approach. The Code of Construction Practice is divided in two sections: Part A which includes the general requirements and measures and Part B which includes site specific measures. There are some site specific measures included in Part B for the Acton Storm Tanks, with general tunnelling being covered in Part A.

1.9

The Statement of Common Ground states the areas of agreement and disagreement between the Council and Thames Water. There are currently several matters still to be fully agreed, but there are no areas of great discord between Thames Water and the Council: -Other matters of the Draft DCO-generally agreed, subject to requirements -Heritage Statement-generally agreed, subject to requirements -Significant effects related to the Environmental Impact Assessment generally agreed, subject to requirements -Clarification of the Code of Construction Practice- generally agreed, subject to requirements -Air Management Plan- generally agreed, subject to requirements -Excavated Material and Waste Strategy- generally agreed, subject to requirements -Overarching Archaeological Written Scheme of Investigation - generally agreed, subject to requirements -Transport Assessment- generally agreed, subject to requirements -Certain clauses of the Property section- the Council is still considering this 4

issue and may require further time to fully respond -Final Design and COCP principles- generally agreed, subject to requirements -Impact on aquatic species- it is now agreed there could be both beneficial and negative effects, with the former outweighing the latter. 1.10

The Council is committed to working with Thames Water to address the matters not fully agreed and therefore the Statement of Common Ground is a working document and will be fully agreed by the time the Examination in Public of the project takes place.

1.11

The Council considers that there is one major omission with reference to the drafting of the requirements, which arises from the fact that most of the mitigation measures are included in the Code of Construction Practice and Design Principles. The requirements within the Development Consent Order refer to these two documents; however neither the Code of Construction Practice, nor the design principles are part of the Order. They are nonstatutory documents and therefore their implementation is not ensured unless the wording of the requirements includes the phrase “submitted and approved in accordance with the (insert supplementary document) and approved in writing by the Local Planning Authority”. All mitigation measures must be included in the Development Consent Order.

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2.0

DETAILS

2.1

Acton Storm Tanks and associated Acton to Carnwath section of tunnel

2.2

Details of the proposal.

2.3

The proposal is extensively underground, forming the sub-land main tunnel section between Acton Storm Tanks and Carnwath Road Riverside. It also intercepts the existing Acton Storm Relief combined sewer overflow (CSO). Acton Storm Tanks is an existing Thames Water pumping station and storm water tanks site.

2.4

The proposed development at Acton Storm Tanks is intended to reduce the frequency of spills to the river from the Acton Storm Relief CSO. The works would convey flows from the inlet of the Acton Storm Tanks to the main tunnel.

2.5

Proposals at Acton Storm Tanks include the construction of underground structures to connect the CSO to a shaft approximately 31 metres deep and with an internal diameter of approximately 15 metres. This shaft would be constructed towards the northern end of the site within two of the existing storm tanks, and would transfer sewage flows from the CSO to the western end of the main tunnel. While most of the operational structures would be underground, a ventilation column and other smaller structures would be permanent above ground features. The ventilation structure would be tallest as 15m (with possible lighting), but also located within the part of the site furthest from the London Borough of Hounslow.

2.6

Heritage

2.7

The scheme plays down the visual presence of the Thames Tideway Tunnel, locating it at an existing Thames Water site and improving its appearance locating the majority of the equipment below ground where it is not open to view and secure. The minimal, low-key design approach of the new infrastructure is supported. It reduces the visual impact upon the townscape and setting of the adjacent conservation area.

2.8

In fact, there is little visual impact on the Bedford Park Conservation Area other than the new vent columns. The exact position and material details of the columns have yet to be confirmed, although it is understood that Thames Water wishes to reflect local distinctiveness and make this a landmark structure, subject to consultation with local authorities at discharge of requirement stage, including potential lighting. This enables the presence of the tower when viewed from the immediately adjacent Bedford Park Conservation Area to be mitigated for.

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2.9

The non-surface section of the works passes underneath two conservations areas (Bedford Park and Old Chiswick), the edge of another (Stamford Brook) and underneath the Roman Road Archaeological Priority Area. The applicant has carried produced settlement and archaeological documents that can provide the basis for requirements to safeguard these heritage features. They have identified that 3 listed buildings would suffer some settlement as a result of the work. The resultant requirements should be phrased to require the written approval of the London Borough of Hounslow and Ealing (where relevant) and also English Heritage.

2.10

In both cases it is considered appropriate that the drafted relevant requirements are amended to include the phrase “approved by the Local Planning Authorities in writing.”

2.11

Transport

2.12

The applicant has produced highways, transport and code of construction practice documentation, as well as a travel plan. It is understood their remits will be secured by project wide and site specific requirements.

2.13

All works strictly within the London Borough of Hounslow are sub-surface and any transport impact will be related to any traffic displaced into the borough through road closure.

2.14

The Council has been assured there will be no compulsory road closures within Hounslow’s borders.

2.15

Given the potential for up to six months (out of 3.5 years) of 24 hour working practices, the potential for some Saturday working and the potential for increased traffic movements removing waste or bringing materials for the site (up to 23 HGV movements a month, over 5 months), the Council is seeking assurance that our residents will not be unduly or unreasonably disturbed. The Code of Construction Practice document is considered an adequate baseline, but its’ relevant requirement phrasing should include the request for written discharge by the Local Authorities.

2.16

This written approval is especially important, given that there will be period(s) of 24-hour working required for secondary lining of the main tunnel. During this period of continuous working, activities would be predominately below ground, where Hounslow residents could be most affected by noise an vibration from subsurface vehicles, with support activities occurring at ground level. Heavy goods vehicle (HGV) movements however, would be limited to daytime hours.

2.17

The Acton Storm Tanks site specific and project-wide Construction Plans, including specific measures on air quality and dust/mud/waste control/storage, illumination, land quality (where necessary) and all other relevant factors should, subject to Local Authority requirement discharge mitigate for this work and ensure no residents are disturbed at night

2.18

Noise, Vibration and Odour

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2.19

One of the Council’s key concerns is the potential for increases in background noise levels and vibration during demolition, construction and post development. A continuing commitment to safeguard and protect against unwanted and unreasonable noise should be secured as part of relevant requirements.

2.20

The Environmental Statement volumes 3 and 4 cover the assessed noise and vibration impacts for the work adjacent to and beneath the London Borough of Hounslow. Section 9 of both the volumes covers the assessed noise and vibration impacts. The Code of Construction Practice parts, A and B, and the Environmental Statement include mitigation measures to the generation of noise and or vibration during construction of the tunnel. These relevant requirements for the COCP should require approval in writing by the Local Planning Authorities.

2.21

Working hours are site specific and for Acton Storm Tanks and the first stretch of tunnel, they are envisaged to be standard working hours as follows: Standard working hours, including mobilisation and maintenance

2.22

08.00 to 18.00 weekdays, 08.00 to 13.00 Saturday. Plus up to one hour before and after for mobilisation, ie, 07.00 to 19.00 weekdays, 07.00 to 14.00 Saturdays. Plus maintenance periods 13.00 to 17.00 Saturdays 10.00 to 16.00 Sundays. Continuous working hours 0.00 to 24.00 Monday – Sunday. For the tunnel construction activities, the underground work and surface support activities will be undertaken on a continuous 24-hour, seven days a week basis. Underground work includes maintenance of underground machinery and plant. Surface support activities comprise works required to support tunnelling including excavated material processing and handling, shaft lifting operations, tunnel lining supply, grout and concrete batching plant operation, barge loading and movements.

2.23

The Council permits normal working hours of 8am to 6pm on Mondays to Fridays and 8am to 1pm on Saturdays, with none on Sundays and Public Holidays.

2.24

Part 6 of the Code of Construction Practice Part A sets out the s61‟Prior Consent‟ application process which all contractors must adhere to. The requirements in regard to the s61 process including noise and vibration monitoring are considered to be acceptable. Also included in part 6 are a sound insulation and temporary re-housing schemes. Should noise after the applying of mitigation and best practicable means exceed certain trigger levels, then the scheme will be available to exposed residents.

2.25

The Council is satisfied that the procedure for application for Prior Consent under section 61 of the Control of Pollution Act 1974 given in Code of 8

Construction Practice Part A will allow the Council detailed examination and approval for works at this site. 2.26

The Borough could be affected by cumulative construction noise and vibration impacts and could essentially be affected by what is also a project wide issue is the possibility of ground borne noise from the tunnel boring machine excavating the main tunnel.

2.27

The contour plot of ground borne noise from the main tunnel boring machine propagates to no more than 44 dBL . A medium and high groundborne noise impact from the operation of the TBM is predicted at residential properties adjacent to the route between Acton Storm Tanks and the River Thames and above the Frogmore long connection tunnel. This is considered to affect properties for up to 3 days. Although the residences are considered to have high sensitivity to groundborne noise, a medium/high impact is not predicted to be experienced for more than six days at any residential receptor as the TBM passes the receptor.

2.28

The applicant considers that TBM groundborne noise is predicted to exceed the likely significant effect threshold level for groundborne noise, however when the magnitude of the impact is considered in combination with the duration of the impact, the effect would be insufficient to cause sustained disturbance to occupants. Based on professional judgement the overall effect (ie, the resulting disturbance to occupants in this case) would be rated as not significant

2.29

The contractor will be required to control vibration generating activities (particularly ‘in-river’ works such as pile installation and removal, installation of jetties, cofferdams, and campsheds) with regard to protecting fish.

2.30

Section 9 of Volume 4 covers the assessment of noise and vibration in both the construction and operational phases at Acton Storm Tanks and Section 9 of Volume 4 covers main tunnelling activity.

2.31

The method used in the Environmental Statement for the establishment of significance of construction noise impact is the ABC method of BS5228:2009, which is an accepted standard.

2.32

Construction Road Traffic: Noise

2.33

The works are predicted to generate 46 additional HGVs movements on local roads; taken together with other assorted workers vehicles there would be a maximum 4.8% increase in current flows. This would result in much less than a 1dB increase in current traffic noise levels. The Council considers that this is not significant, especially given that most of these additional movements will not even enter its boundary.

2.34

This is even less likely given the Controlled Parking Zone in Bedford Park, the lack of cut-throughs large enough for construction vehicles outside the main roads.

2.35

Operational: Noise and Vibration 9

2.36

During the operation of the Thames Tideway Tunnel, triggered by combined sewage being diverted to the combined sewer overflow, noise and or vibration generated is not likely to cause any noticeable impact. The only issue is the noise generated by the effluent diverted to the combined sewer overflow falling down the drop shaft. However, this falling water will generate a vortex and this will reduce impact noise, and would not be audible within the London Borough of Hounslow.

2.37

An active ventilation system would be installed at Acton Storm Tanks. This requires plant for tunnel ventilation and odour control of air extracted from the tunnel. Preliminary noise predictions for these arrangements have been carried out and the design of the systems sized to ensure that noise levels are controlled to be within the criteria established by the British Standards and the Local Authority.

2.38

The main plant would include two 10m3/s centrifugal fans, which would push air through carbon filters, before discharging through outlet ducts at the top of the adjacent vent stack. The ventilation fans have been considered as being located within the area currently occupied by tanks 5 and 6. Small diameter ventilation columns have also been assessed on this site. The noise assessment is based on the ventilation fans being housed within acoustic enclosures and includes all aspects of noise generation such as plant noise and air flow noise through ducts and vent columns.

2.39

The noise emission predictions have been based on data for typical plant at the appropriate operating settings. FläktWood’s HCGB 080 centrifugal fans at a pressure drop of 1170Pa and a speed of 1500rpm have been applied as a typical installation for this application. A smaller ventilation system may also be included within the building to discharge exhaust air from plant inside.

2.40

It is shown from noise prediction studies of the plant and buildings, that there is sufficient potential to limit noise emissions so that receptor noise level limits are met.

2.41

The interceptor chamber and filter chamber will also help contain noise to within the drop shaft acting partially as noise reducing plenums. The ventilation stacks here are also passive with a very low efflux and influx velocity. Audible air movement noise is unlikely to be generated. The limited use throughout a year for about fifteen hours will ensure operational noise is negligible. There will be surface located plant which will need to meet the appropriate noise emission limits of Table 9.6.1 of the Environmental Statement. The plant to be installed will operate very intermittently and compliance with the noise emission limits when the plant is operational will need to be shown by way of submission of a noise report, which

2.42

Operational: Odour

2.43

During the operational phase odour the ventilation system would be mostly below ground level housed within two of the existing storm tanks with some above ground ventilation structures. The below ground air treatment chamber would have a total air treatment capacity of 20m3/s. Having been 10

treated in the underground chamber, the air would be released from the ventilation column. When air flows are greater than 20m3/s, as could occur during a heavy storm, the excess air would be released through a bypass vent in the ventilationcolumn. This would occur for about 15 hours during a typical year. 2.44

During the typical year scenario, treated air would be released from the ventilation column for 99% of the time.

2.45

An active ventilation and air treatment plant would be included as part of the design. The air treatment units would remove any odours emanating from the tunnel. Full details of the Thames Tideway Tunnel project ventilation system can be found in the Air Management Plan. Given this plan will be secured by requirement odour should not have an impact on residential occupiers adjacent to the site.

2.46

Land Quality

2.47

Council GIS systems show that there is likely to be some land contamination underground along the route of the tunnel.

2.48

Section 8 of both Volumes 2 and 4 of the Environmental Assessment covers the assessed land quality impacts. The Code of Construction Practice Part A is also part of the application and sets out the mitigation measures that will reduce the impacts of land contamination. Operational land quality effects for this site have not been assessed by the applicant. This is on the basis of the embedded measures adopted during the construction and operational phases. No significant operational effects are considered likely for Acton Storm Tanks and for this reason, only information relating to construction is presented in the assessment of effects on land quality.

2.49

In order to construct the shaft, the works at Acton Storm Tanks would involve initial extensive excavation at tanks 5 and 6 and subsequent below ground construction which may result in the exposure, subsequent movement and temporary storage of potentially contaminated soils prior to off-site removal.

2.50

The main tunnel shaft would be constructed through the upper aquifer and founded within the London Clay, the invert of which would be located at a depth of approximately 31m below ground level.

2.51

An area would also be required within the site for construction logistics, such as materials handling and storage areas, segment storage, site welfare facilities and offices.

2.52

Site-specific data for superficial soils shows levels of chemical contamination to be low in comparison with Defra/EA (2009) and Chartered Institute of Environmental Health (2009) human health risk assessment screening values. Some relatively minor biological(coliform) contamination has been recorded locally within soils which is typical of wastewater treatment areas.

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2.53

Overall and on the basis of the information available, contamination levels at the site are considered to be very low. However, it should be noted that the area immediately beneath the storm tanks has not been investigated and it is possible that some contamination may exist beneath the tanks.

2.54

Contamination would however be restricted in vertical spread by the underlying negligibly permeable London Clay (which is anticipated at a depth of approximately 4m below the base of the storm tank) reducing the amount of soils that could be impacted. The granular nature of the deposits and the principal contamination source being CSO waste water means that contamination would more likely be present in groundwater rather than soils.

2.55

The groundwater monitoring shows some bacteriological contamination and some minor volatile organic compounds (VOCs) but the quality of the groundwater does not indicate that widespread soil contamination is likely to be present in the River Terrace Deposits beneath the tanks.

2.56

The embedded measures relevant to land quality at the site are set out in Section 9 of the CoCP. The applicant considers that there are no CoCP Part B measures which are relevant. Land quality issues should be managed in close liaison with the local authority, London Borough (LB) of Ealing, and the Environment Agency (EA) prior to and during construction.

2.57

Environmental monitoring, would include the following measures: -on-site watching brief during potentially high risk activities and an on call watching brief for all other activities. Specialist watching brief may include: UXO; contaminated land; health and safety/occupational health; and ecological (for invasive species). - dust and air/vapour monitoring (see CoCP Part A Section 7 for further details). Where appropriate, this would include a combination of onsite and boundary monitoring.

2.58

There are residential receptors immediately adjacent to the site and several residential and other sensitive receptors in the near vicinity. The Environmental Statement has assessed the impact on these receptors as minor adverse once the mitigation measures in the Code of Construction Practice are implemented.

2.59

The assessment of the effects of the construction on nearby receptors is based on limited site investigation information; however the assessment assumes that the mitigation measures contained within the Code of Construction Practice are being implemented. The scope of the measures are broadly acceptable, however, their applicability will need to be further reviewed once the site is fully investigated and risk assessed.

2.60

If the investigation shows that contamination is likely to have migrated offsite and offsite receptors are not considered as part of a remediation strategy, as required by the National Planning Policy Framework, the Council would need to consider further assessment of the impacted site through Part 2A of the Environmental Protection Act 1990, if the risk was deemed to be significant. 12

2.61

The operational phase of the development is not considered as part of the Environmental Impact Assessment , as it was determined that no significant effects would be likely. It is expected that the operational phase will be considered as part of the remediation strategy once further assessment has taken place. Any removal or remediation of contamination on site is likely to have a beneficial impact on the general land quality of the area.

2.62

The draft Development Consent Order includes a requirement for the submission of a remediation strategy which will be approved by the Council before any remediation works commence. A requirement is also included for the development works to cease if unexpected contamination is encountered, a remediation strategy must then be produced and approved by the Council. These requirements will provide the Council with an opportunity to comment on any remediation proposals and ensure that the development will not pose a risk to construction workers or site users.

2.63

Within the Code of Construction Practice the requirement for site assessments and investigations is included in Chapter 9; however, there is no wording to suggest that these investigations/assessments will require approval by the Council. It states that the measures (assume mitigation measures) will be agreed with the employer and the EA, but there is nothing to suggest that the Council will have any avenue to require further works or changes if the investigation was not appropriate or if additional elements need to be addressed. A mechanism by which the Council can do this should be in place.

2.64

Although the draft Development Consent Order contains a requirement for the submission of a remediation strategy, assessing contaminated land is a phased approach and it is essential that the required steps leading up to the formation of a remediation strategy are also agreed with the Council. Without this agreement, the remediation strategy may not be considered acceptable and the earlier phases, such as site investigation, may need to be revisited. This could have a significant impact on the project delivery in terms of costs and delays. It is therefore recommended that requirements necessitating the submission and approval of a Site Investigation Scheme and Risk Assessment are also added to the consent.

2.65

Air Quality

2.66

According to the Councils Air Quality Supplementary Planning Document the area of Chiswick immediately adjacent to Acton Storm Tanks is likely to be in exceedance of the EU limit value for NO2 of 40ugm3 in 2005. Work has been ongoing to reduce this and residential development in or near the area is likely to be subject to planning conditions and S.106 agreements.

2.67

Section 4 of both volumes 2 and 4 covers the assessed air quality impacts.

2.68

The Code of Construction Practice Part A and Part B is also part of the application and outlines the mitigation measures that will reduce the impacts on air quality from the development.

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2.69

Any impact on Air Quality within the London Borough of Hounslow is considered only to relate to the aspect of the proposal at Acton Storm Tanks

2.70

Given that it has been established that construction effects on local air quality would be negligible in terms of NO2 and PM10 at Ealing receptors, this is not likely to transfer into Hounslow.

2.71

In terms of construction dust, although the receptor sensitivity (with respect to construction dust nuisance) is identified as medium for all receptors (as identified in Vol 4 Table 4.4.7), due to the duration of the works and the high PM10 background concentrations in the locality, the sensitivity of the area has been defined as ‘high’. With regard to the significance of construction dust effects, a high risk site with a high sensitivity of the area would result in an overall moderate adverse effect without control measures.

2.72

The Code of Construction Practice refers to an air quality management plan in paragraph 7.4.7, which will identify the appropriate control measures for dust. It is not made clear when this air quality management plan will be produced or if it will be submitted and approved by the local authority.

2.73

When the measures outlined in the CoCP (Section 7) are applied, the significance of the effect would be reduced to minor adverse for receptors within 20m of the site boundary (in accordance with IAQM guidance). The significance of construction dust effects at receptors greater than 20m (ie within Hounslow, from the site boundary would be negligible with the CoCP (Section 7) measures.

2.74

The proposed Air Quality requirement is considered acceptable.

2.75

Ecology- All species types

2.76

The Chiswick reach of the Thames is heavily used for competitive and recreational rowing, and Chiswick itself is home to several clubs- at least 15.

2.77

The Syon Park SSSI is also in hydrological continuity with the project. Dukes Meadow and Chiswick Eyot are both nature reserves designated by Hounslow’s Unitary Development Plan. The former, due to the increase in pH of the calcified river water provides a suitable habitat for rare molluscs (that prefer more alkaline environments for shell development), such as the Two lipped door snail (Lacinaria biplicata) and the German hairy snail (Perforatella rubiginosa), while the latter provides roosting habitats for water fowl and the common tern and bats.

2.78

The Council can only comment on the overall site impacts in relation to the overall scheme and its impact on aquatic ecology in its area, as it has no sites on the waterfront, nor is it adjacent to any no-developed sites. In addition to the species identified above, the Environmental Statement Volume 2 has identified that there were juvenile fish in Kew during 2011, but no other protected species or habitats.

2.79

Nonetheless, Catchment modelling1 indicates that in an average year, the Acton Storm Relief combined sewer overflow (the CSO) spills approximately 29 times and discharges 312,000m3 of untreated sewage into the River 14

Thames at Chiswick Eyot in the London Borough of Hounslow. On the basis that litter tonnages are proportional to discharge volumes, approximately 79 tonnes of sewage derived litter is also discharged from this CSO in an average year. 2.80

The CSO discharges have multiple impacts on water quality at the outfall location. This includes a localised effect of rapidly dropping dissolved oxygen levels, the release of pollutants and the discharge of sewage litter and effluent.

2.81

Each discharge increases the risk of exposure to harmful microscopic organisms within the untreated sewage for river users who come into contact with water. An assessment of health impacts upon recreational users of the River Thames concluded that the risk of infection can remain for two to four days following a spill as the water containing the sewage moves backward and forward with the tide.

2.82

Assuming the average 29 spills per annum from the Acton Storm Relief CSO occur on separate days, there could be up to a maximum of 116 days per year when recreational users are at risk of exposure to untreated sewage in the vicinity of the outfall as a result of the Acton Storm Relief CSO spills alone.

2.83

The CSO was identified by the Environment Agency as a CSO that needs to be controlled. The proposed solution to control the CSO is for full interception. The CSO discharges have multiple impacts on water quality in this location, including a localised effect of rapidly dropping dissolved oxygen levels, the release of pollutants and the discharge of sewage derived litter and effluent.

2.84

Catchment modelling suggests that if the project is constructed as proposed, in a typical year, the CSO would not spill into the Thames. Similarly, the tonnage of sewage derived litter from the CSO would be reduced to zero.

2.85

The number of days per year where recreational users would be at risk of exposure to harmful microscopic organisms within the untreated sewage would be reduced to zero. As a result of the improved water quality, there would be a significant beneficial impact for the recreational users of the stretch of the tidal Thames at Chiswick Eyot. There would also be significant reduction in odour at Acton Storm Tanks as a result of the proposals.

2.86

The Environment Agency have previously considered that there needs to be some link or reference to Aquatic Ecology and the impacts of noise and vibration and the Council would consider this especially important with reference to the point where the tunnel enters the riverbed. This appears to be adequately dealt with by the Code of Construction Practice Part A.

2.87

Socio-economics

2.88

The construction of the tunnel should not prejudice the future redevelopment of any of the boroughs sites. 15

2.89

Of the working age population in Hounslow, about 3% claim Job Seekers Allowance and 5.5% claim Incapacity Benefit or Employment Support Allowance. Approximately 4.4% of 16-18 year olds in Hounslow are not in education, employment or training; however, this is less than half the national rate (9.6%).

2.90

The Council would welcome that Acton Storm Tanks site would provide 40 jobs. In the absence of any requirement on economic issues, or full explanation of how any S106 obligation would work, it is unclear how Hounslow would secure the benefits of it s proportion of these jobs.

2.91

Water Resources and Flood Risk

2.92

Groundwater The Environmental Statement Volume 4, Section 13 explains that the effects of the project on groundwater are negligible both during construction and operation. As a result, no mitigation measures are proposed. It is considered that even when the predicted effects are negligible loose monitoring should be in place.

2.93

Surface water During operation, the effects on water quality will be mainly positive as there will be a considerable reduction of the combined sewer overflow spills into the river (312,000 m3 and associated litter per year). This will have a positive impact in river ecology, river users health and aesthetics. Construction impact on surface water quality is likely to be negligible if the appropriate site measures used to control pollutants in the general site run-off are in place. These measures are proposed in the Environmental Assessment and the Code of Construction Practice Part A (Section 8).

2.94

Flood Risk -The construction elements of the proposed development relevant to floodrisk would include:

The removal of two of the six storm tanks currently at the site to enable the construction of the interception chamber, main tunnel shaft and other associated hydraulic chambers that would intercept the Acton Storm Relief combined sewer overflow (CSO) and connect to the main tunnel. A change to the active management of the four remaining tanks and the Acton Storm Relief Sewer would be implemented to manage and maintain existing flood protection, as far as possible. Modification of the remaining four tanks would be undertaken as necessary, by raising wall heights, to minimise any loss in total site storage volumes due to the removal of the two northern most tanks 2.95

The site is located approximately 1.5km north of the River Thames. The Environment Agency (EA) Flood Map identifies the storm tanks to be within Flood Zone 1, 2 and 3 associated with the Tidal Thames. The location of the site in relation to the flood zones is shown in Vol 4 Figure 15.3.1 (see separate volume off figures). The area of land associated with the storm tanks is considered to be within Flood Zone 3a (as the storm tanks are situated at a lower topographic level). The periphery of the storm tanks are within Flood Zone 2. The part of the site to the southeast, north and northeast of the tanks is within Flood Zone 1. As the site is partially within Flood Zone 3a, the floodrisk from tidal sources is considered to be high. 16

2.96

The principal operational effect of the project is the improvement that it would achieve in the water quality of the tidal Thames. The Environmental Statement identifies the relationship between reductions in dissolved oxygen and hypoxia (mass fish mortalities).One of the most notable recent examples was in June 2011, described in a press release by the Environment Agency at the time as follows :“the incident…caused the release of more than 250,000 tonnes of storm sewage into the river from CSOs and at least 200,000 tonnes of storm sewage from the Mogden sewage treatment works […] more than26,000fish were killed along a 2km stretch of the river between Barnes and Chiswick”.

2.97

It would also reduce sewage flood risk. Hounslow’s Strategic Flood Risk Assessment 2007 states that: Para 23: In addition to fluvial (river) and tidal flooding, properties within the London Borough of Hounslow are also affected by a risk of flooding stemming from issues of a relatively localised nature. These include surcharging of the underground sewer system, the blockage of culverts and gullies resulting in overland flow, and surface water flooding. There is also a potential (albeit minimal) risk of groundwater flooding within the Borough. Para 97: The River Thames poses a potential risk of flooding to properties within Hounslow. Notwithstanding this however, the vast majority of property in Brentford, Chiswick and Isleworth is protected by the River Thames Tidal Defences (TTD) up to the 1 in 1000 year event. This is provided that the River Thames Barrier is operated to protect against storm surges from the North Sea and to provide a pool for the River Thames to drain into during extreme fluvial events at high tides. It is essential to recognise that defences do not fully remove the risk of flooding to properties within the Borough. There is always a residual risk of system malfunction, overtopping and/or structural failure. Whilst the risk of flooding from rivers and/or the sea may be reduced through the presence of defences, it is also important to remember that localised flooding (i.e. resulting from local catchment runoff and/or sewer system failure following heavy rainfall) may flood properties within defended areas. The overloading of the sewer system due to inflows exceeding the underground system capacity (i.e. resulting in surcharging) is a known problem in some areas. Note that surface water networks are typically designed to cater for events up to a 1 in 30 year. Surface water flooding will occur when the sewer system is overloaded. Para 208: The raising of floor levels above the anticipated maximum flood level ensures that the interior of the property is not directly affected by flooding, avoiding damage to furnishings, wiring and interior walls. It is highlighted that plumbing may still be impacted as a result of mains sewer pfailure.

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