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«LONDONWEST MIDLANDS ENVIRONMENTAL STATEMENT Volume 2 | Community Forum Area report CFA7 | Colne Valley November 2013 ES VOL LondonWest Midlands ...»

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2.6.2 Since April 2012, as part of the design development process, a series of local alternatives have been reviewed within workshops attended by engineering, planning and environmental specialists. During these workshops, the likely significant environmental effects of each design option have been reviewed.

The purpose of these reviews has been to ensure that the Proposed Scheme draws the right balance between engineering requirements, cost and potential environmental impacts.

2.6.3 For alternatives considered for the sustainable placement area please refer to the CFA6 report.

Position of the Colne Valley viaduct 2.6.4 The Proposed Scheme will pass on a viaduct over the Colne Valley Regional Park, including parts of the Mid Colne Valley SSSI, the A412 Denham Way/North Orbital Road and adjacent to local properties including listed buildings, notably Savay Farm and the locally listed Dew’s Farm which will be demolished under the Proposed Scheme.

2.6.5 The viaduct will curve away from the Chiltern Main Line, passing over the HOAC, the Grand Union Canal, Moorhall Road, the southern part of the Mid Colne Valley SSSI, the River Colne and between the settlements of South Harefield and Denham Village.

CFA Report – Colne Valley/No 7 | Overview of the area and description of the Proposed Scheme 2.6.6 The viaduct was part of the scheme announced in January 2012, however, the alignment has been moved approximately 60m at its centre point to the northeast in the Proposed Scheme compared with the January 2012 announced route to reduce its environmental impact.

2.6.7 The viaduct crosses the River Colne and alternative options were considered in order to try and reduce the potential impacts of this crossing. Three horizontal

alignments were considered for the position of the viaduct. These include:

 Option A: the January 2012 announced route;

 Option B: the Proposed Scheme, a moderate straightening of the viaduct, moving it approximately 60m north-east at its centre point; and  Option C: a minor straightening of the viaduct, moving it approximately 25m north-east at its centre point.

2.6.8 All three options would result in construction and operational impacts on ecology including impacts to parts of the SSSI and the River Colne. There would also be visual impacts from all the options. However, Option B provided environmental benefits compared with Option A and C because it reduced the number of piers in the channel of the River Colne to one, rather than six and three respectively for Options A and C. Therefore, Option B will have reduced impacts for both ecology and water. All options would alter the position of the piers placed within the SSSI however there was not considered to be a significant difference in terms of the impact because all the alternative alignments have a similar construction and operational footprint within the SSSI.

2.6.9 The various alignment options would also affect the property impacts at the southern end of the viaduct. Option B will have a reduced direct impact on the buildings affected at the HOAC site compared with Options A and C. However, it will also cause an additional demolition at Dew’s Farm which would not have been required under Option A or C. This property would still have been affected under Option A and C because it would have been close to the viaduct under these options.

2.6.10 Option B has been taken forward in the Proposed Scheme because it will have less impact on the River Colne and overall was considered a better environmental outcome. The options for the river crossing and associated river diversion are described below.

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CFA Report – Colne Valley/No 7 | Overview of the area and description of the Proposed Scheme existing river channel. When combined with the pile cap beneath the pier structure this will affect the entire width of the watercourse.

2.6.12 The alignment of the viaduct as presented in the January 2012 announced scheme has been altered as described previously; this reduced the numbers of piers in the river from six to one. The aim of this change in alignment is to reduce impacts on the River Colne. Whilst this has been achieved the remaining pier in the River required consideration of further mitigation options.

2.6.13 Six options were considered to address the effects arising from the pier in the

channel. These were:

 Option A: the January 2012 announced route with the changed horizontal alignment as described previously and presented in the Draft ES; 40m pier spacing and a pier located in the channel of the River Colne;

 Option B: a single 90m viaduct span over the River Colne allowing the watercourse to remain in its original location;

 Option C: a 450m river diversion to the west of its current alignment, whilst retaining the original section of river as backwater channel;

 Option D: a 250m river diversion to the west of its current alignment, whilst retaining the original section of river as backwater channel;

 Option E: a 420m river diversion to the west of its current alignment, whilst retaining the original section of river as backwater channel; and  Option F: the Proposed Scheme, comprising a 170m long localised diversion of the existing channel. This would alter and narrow a short section of the River Colne to allow it to pass between two piers.

2.6.14 Option A would generate increased flow rates as the water would have to pass between the pier and the banks resulting in significantly higher flood risk to in the immediate area and surroundings. This would be likely to result in erosion of the existing river banks and adverse impacts on the habitats and species in the vicinity, many of which would be associated with the Mid-Colne SSSI. Without further mitigation this was not considered a feasible option on environmental grounds.

2.6.15 In addition, as the pile cap for the viaduct pier would be as wide as the existing channel Option A would still require a temporary diversion of the river during construction.

2.6.16 Option B would allow the original channel to be retained, maintaining the existing conditions within the river and associated biodiversity. However the design of the viaduct would need to be changed as the longer span would need a much deeper vertical section, approximately 7m deep. This would be necessary to provide structural support for the longer span. This would have CFA Report – Colne Valley/No 7 | Overview of the area and description of the Proposed Scheme increased visual impacts on the surrounding area including the SSSI and recreational lakes. In particular, adopting a different design for this section of the viaduct would create a non-uniform appearance and increased visual intrusion. There would be ecological benefits from this option; however, these benefits would be reduced as a result of the construction method that would be required. This is because in order to build the longer span the proposed construct method would have to change and temporary supports would be needed in the river. This would impact on the river channel. The larger structure and the change in construction method would also increase the costs of the project. For these reasons this option was not adopted.

2.6.17 Of the longer river diversion Options C and E, construction impacts on the land identified for diversion, including parts of the SSSI, would be greater than Option A due to the increased length of river diversion. Over a number of years this habitat would regenerate and the retention of the backwater channel would allow a new wetland habitat to develop. Option C would require slightly more vegetation to be cleared from the adjacent ancient woodland during construction of the diverted channel, although during operation for both options there would be no increased risk to flooding as the flow rates within the river are maintainable.

2.6.18 Option D would be the shortest of the diversion routes and is broadly similar in terms of construction and operational impacts to that of Options C and E although the vegetation to be removed would be considerably less during construction and the rivers meander would follow more closely to its original condition, thereby maintaining more of the existing character of the watercourse.

2.6.19 For Options C, D and E all of the river diversions would pass through areas of higher ground. The result of this is that the river channel would be lower than existing ground level, by up to 8m. In this situation to meet principles of good river restoration and design and WFD objectives extensive earthworks and vegetation removal would be required to create a naturalised channel. This in itself would increase the amount of land required for these options and consequently have additional environmental impacts on the SSSI. Therefore, given these constraints it was unlikely that the longer channel diversions would be as ecological valuable as the existing channel. Consequently trying to maximise the retention of the existing channel under Option F was preferred.

2.6.20 Option F involves a realignment of the existing river channel to enable it to pass between two piers under the viaduct. This presented the smallest construction footprint in relation to the diversion options C, D and E and so will reduce the impact on the SSSI and associated species as well as avoiding the adjacent ancient woodland.

2.6.21 As the channel is narrowed slightly in Option F, flow rates are likely to increase through this section of the River potentially affecting flood risk. Therefore an CFA Report – Colne Valley/No 7 | Overview of the area and description of the Proposed Scheme area of adjacent land has been included to mitigate this effect as appropriate.

With this in place effects associated with this option during operation will not be significant.

2.6.22 Option F was included in the Proposed Scheme because it will have fewer impacts on the River Colne itself and fewer impacts on the surrounding SSSI and ancient woodland. Detailed design of the river diversion will be completed with the Environment Agency to ensure that they address the Environment Agency requirements with respect to the hydraulic capacity, flood risk, ecology and hydromorphology.

Height of the Colne Valley viaduct 2.6.23 The Proposed Scheme includes a viaduct over the Colne Valley. The viaduct height will vary from approximately 11m to 15m above ground/water level through this section of the route.

2.6.24 The viaduct was part of the scheme announced in January 2012, however, the alignment has been moved approximately 60m to the north-east in the Proposed Scheme compared with the January 2012 announced route. The operators of Denham Aerodrome raised concerns about the possibility of aircraft from the aerodrome colliding with the viaduct structure and proposed that the height of the viaduct be lowered.

2.6.25 HS2 Ltd has undertaken a safety assessment of the proposed alignment in response to the concerns raised by the Aerodrome and do not consider that there is a risk. Therefore, lowering the height of the viaduct will not be necessary to allow the safe operation of the Aerodrome and altering the vertical alignment has not been investigated further. This decision was predominately based on health and safety requirements rather than environmental considerations. Environmental constraints were considered in the initial decision to include a viaduct over the Colne Valley as part of the work to develop the scheme consulted upon in 2011 and contained within the route announced in January 2012.

Tunnel under the Colne Valley 2.6.26 The Colne Valley Community Forum requested that a tunnel be constructed to pass beneath the lakes between Ruislip and the M25. This would be considered preferable by the community because it would avoid above ground disturbance from construction and would remove visual and noise impacts during construction and operation.

2.6.27 HS2 Ltd acknowledges that there would be environmental benefits if a tunnel was proposed however, the use of the viaduct to cross the Colne Valley was based on a combination of practical, financial and safety considerations. The lakes are large former gravel pits and the ground beneath falls well below the water level. This means that tunnelling would likely be more difficult and expensive than elsewhere on the route.

CFA Report – Colne Valley/No 7 | Overview of the area and description of the Proposed Scheme 2.6.28 Consequently it was determined early in the project that tunnelling was not appropriate and an option for tunnelling has not been re-visited in detail as part of the work since the announcement of the scheme in January 2012.

2.6.29 HS2 Ltd maintains that in order to pass beneath the lakes at sufficient depth it would not be feasible for the Proposed Scheme to meet the surface again before reaching the southern portal of the Chiltern tunnel. The consequence of this is that a tunnel under the Colne Valley would need to be an extension of the Chiltern tunnel. This longer tunnel would require more extensive provision for fire safety and emergency public evacuation in the event of a train failure or fire, such as a third tunnel bore and/or an emergency intervention station in the middle of the extended tunnel length. This would in itself introduce additional potentially significant adverse effects and additional cost.

2.6.30 A long tunnel option from the edge of London and heading north under the Colne Valley and into the Chiltern tunnel would also mean that all excavated tunnelling material would have to be extracted and transported from either the north end of the Chiltern tunnel in the Chilterns AONB or at the south end of the tunnel in central London.

2.6.31 For these reasons, tunnelling beneath the Colne Valley has not been considered in detail since January 2012 and has not been incorporated into the Proposed Scheme.

Location of the Chiltern tunnel portal 2.6.32 The Proposed Scheme includes a tunnel portal east of the M25 for the Chiltern bored tunnel.

2.6.33 In the scheme announced in January 2012 the location of the southern portal of the Chiltern tunnel would have impacted on the piled bridge foundations of the Chalfont Lane overbridge to the M25. Four alternatives were assessed as a

result of identifying this impact:

 Option A: the January 2012 announced route;

 Option B: increase the separation between the two bores within the tunnel, i.e. move one bore to the south of the underground structure and the other north of it;

 Option C: the Proposed Scheme, move both bores to the west; and  Option D: move both bores to the east.

2.6.34 Option A was not considered feasible because of the impact on the bridge foundations and the additional work this would involve. All three of the other options (B, C and D) would be similar in terms of environmental impacts. Option C was chosen as it will avoid the need for a highways diversion to Chalfont Lane and avoid potential impacts to the overhead power lines on the eastern side of the M25.

CFA Report – Colne Valley/No 7 | Overview of the area and description of the Proposed Scheme 2.6.35 For these reasons Option C was included in the Proposed Scheme.

Porous tunnel portal 2.6.36 The Proposed Scheme includes a porous tunnel section at the southern end of the bored Chiltern tunnel, located within the section of cutting adjacent to the M25. The scheme in January 2012 did not identify the need for a porous portal and this has been identified as part of the design development of the scheme.

The purpose of the porous portal is to maintain passenger comfort and to reduce the pressure transient created by a train when it enters/exits the tunnel

and the associated impact on local receptors. Two options were considered:

 Option A: the Proposed Scheme, with the porous portal at the southern end of the tunnel, external to it; and  Option B: that the porous portal is contained within the tunnel length.

2.6.37 To achieve the reduction in air pressure the porous portal needs to allow air to pass through it. It therefore needs to be exposed to the surface in some way.

Placing the portal within the tunnel structure in this location would make it impossible to achieve this, as it would be located directly under the M25.

Consequently Option B was not considered feasible in this situation and was not assessed in detail.

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