Down in the valley, part 3

(… continued from Down in the valley, part 2, posted on 16 Feb 2014).

In the second part of this three-part blog I speculated on the extent to which the two large earthworks proposed for the Leam valley would mitigate the visual impact of running HS2 across it. In this final part I will look at mitigating the airborne noise impacts of the trains, termed “operational noise” by HS2 Ltd.

The noise footprint in the northern part of the valley is depicted in drawing SV-05-046, which is in Volume 2 Map Book CFA17 of the Environmental Statement (ES), from which the section below has been taken.

HS2 noise footprint in the Leam valley (Source: HS2 Ltd)

HS2 noise footprint in the Leam valley (Source: HS2 Ltd)

As was the case with visual mitigation, this plan view is really all we have to go on to assess the efficiency of the mitigation that is on offer; the text of the ES offers little enlightenment, except that paragraph 11.4.6 of ES Volume 2 Report CFA17 informs us that:

“Generally, the assessment has been based on noise barriers having a noise reduction performance equivalent to a noise fence barrier with a top level 3m above the top of the rail, which is acoustically absorbent on the railway side, and which is located 5m to the side of the outer rail. In practice, barriers may differ from this description, but will provide the same acoustic performance. For example, where noise barriers are in the form of landscape earthworks they will need to be higher above rail level to achieve similar noise attenuation to a 3m barrier because the crest of the earthwork will be further than 5m from the outer rail.”

What is very striking is the estimated “noise footprint” (see footnote) arising from HS2 operations is very much larger in the valley than for the adjoining section of track running northwards, which is in cutting; the valley footprint is up to 700 metres across, whereas the retained cutting through South Cubbington Wood has an associated sound footprint that is only 150 metres across. We need to exercise some caution in making this comparison, because the “throw” of the noise will be affected by the local terrain, as well as the conditions in the immediate vicinity of the passing train. So, for example, the “bite” taken out of the lower left of the main valley footprint, as shown on the above noise map, is almost certainly the result of the shielding effect of a bluff that protrudes from the valley side just north of Lower Grange farm. However, I think that it is safe to assume, from the much broader noise footprint in the valley, that a “false” cutting is nowhere near as good at mitigating noise as the real McCoy.

There are also two obvious breaches in the noise defences in the valley, indicated by the bulges in the pink shading that depicts a “significant” noise impact. The right-hand bulge is associated with the viaduct, and reflects the generally poor acoustic screening that is proposed for such structures. Paragraph 5.9.2 of ES Volume 1 advises that viaducts will “typically include a solid 1.4m (above rail level) safety barrier located close to the rail” and that this “will also act as a noise barrier and is termed a ‘low level’ barrier”. Paragraph 5.9.3 adds:

“Where further noise mitigation is required to avoid or reduce significant noise effects, three further levels of mitigation have been incorporated where necessary. The first is to provide acoustic absorption in the railway side of the 1.4m low level barrier. The second is to provide a 3m (above rail level) acoustically absorptive parapet noise barrier. The third is to increase the height of the parapet noise barrier to 4m above rail.”

Report CFA17 (paragraph 11.4.7) appears to indicate that a low-level absorptive barrier is proposed for the viaduct over the Leam, but this is not completely clear.

The bulge in the centre of the map appears to be the result of there being no raised earthwork proposed for the eastern (upper) side of that track section; again no explanation for this omission has been provided.

It is not possible to say how much influence these two noise hot spots have had on the placement of the outline of the noise contour, and hence the size of the noise footprint in the valley, and the ES is silent on this matter. However, the shape of the eastern (upper) contour, in particular, appears to mirror the two bulges in the pink shading, and so it does not seem unreasonable to suspect that the hot spots could be the dominant cause of the noise footpath being so large.

Accordingly, the question for HS2 Ltd to answer is not just whether the proposed earthworks would provide appropriate noise mitigation, but also whether the mitigation they afford would be adequate. One obvious improvement that could be made simply is to increase the height of the barriers on the viaduct section, as suggested in paragraph 5.9.3 quoted above. Another would be to employ noise fence barriers along the section of track where no raised earthwork is proposed. If these measures are insufficient to reduce the noise footprint to an acceptable size, then further deployment of noise fence barriers should be considered to “beef up” the screening offered by the raised earthworks.

Indeed, HS2 Ltd is proposing to supplement the noise screening of the raised earthworks in just this way on a small section of the southern false cutting, as shown in the following section of drawing SV-05-045 from Map Book CFA17.

Proposed deployment of fence noise barriers on southern Leam valley earthwork (Source: HS2 Ltd)

Proposed deployment of fence noise barriers on southern Leam valley earthwork (Source: HS2 Ltd)

The noise barriers are the royal blue lines on the western (lower) side of the false cutting. The map legend identifies royal blue lines as indicating “landscaping and/or fence barriers”, but assurance that fence barriers are proposed is provided by paragraph 2.2.8 of Report CFA17. Also according to the legend, the labels “7-9m”, “5-6m” and “3m” indicate the “total barrier height above rail level”.

Yet again, the ES provides absolutely no explanation of why fence noise barriers have been proposed for this location, although an educated guess on my part would be to prevent the western (lower) edge of the noise footprint drifting into the village of Offchurch, which lies perilously close. Neither can we tell how successful the ploy has been, although the noise contour on the western side does look a little pinched in at this point indicating that the fence noise barriers would provide some benefit.

If there is a consistent theme in this posting it must be a plea to HS2 Ltd to “sell the benefits” of what they are proposing for noise mitigation better, including explaining what is proposed, what alternative mitigation might be employed, what additional measures could be used to supplement the mitigation, and why any such measures have been rejected. When considering such alternatives it would be helpful to see comparative noise footprint plots.

I realise that this posting is getting quite long, but I want to mention one other thing before I close bringing me back to the theme of the disposal of surplus material. In the Dunsmore, Wendover and Halton Community Forum Area (CFA 10) it would appear that, even with the inventiveness of the design engineers of HS2 Ltd in applying the integrated earthworks design approach, it proved impossible to find sufficient locations for environmental mitigation earthworks to balance the books. Left with a surplus of about one million cubic metres of excavated spoil to find a home for HS2 Ltd has dropped the pretence of mitigation and has, quite literally, resorted to dumping. The chosen site, just north of the proposed Leather Lane overbridge near Great Missenden in Buckinghamshire, is marked in the map section reproduced below, which is taken from drawing CT-06-035 in ES Volume 2 Map Book CFA10.

Leather Lane sustainable placement area (Source: HS2 Ltd)

Leather Lane sustainable placement area (Source: HS2 Ltd)

Paragraph 2.2.6 in ES Volume 2 Report CFA10 describes the large green area as a “sustainable placement area” and states that “the area will be approximately 1.3km long, up to 450m wide, and up to 5m in height” – and this, would you believe, is within the Chilterns Area of Outstanding Natural Beauty.

Footnote: By “noise footprint” I mean all of the area contained by the 40dB night contour. According to paragraph 8.9.28 of ES Volume 1, “The extent of the 40dB night-time sound level contour is equivalent to, or slightly larger than, the 50dB daytime contour” and, accordingly, the ES employs the same footprint for the 40dB (night) and 50dB (day) thresholds. It is HS2 Ltd’s position that “generally no adverse effect [from airborne operational noise is] expected” outside of this area. My view is that this an optimistic assertion in the light of World Health Organisation guidance and noise levels quoted elsewhere in the ES.

Acknowledgement: The Ordinance Survey mapping upon which the noise contours and route design are overlaid has been reproduced in accordance with the principles of fair dealing as set out in the Copyright, Designs and Patents Act 1988.  On this basis, this mapping is:

Reproduced by permission of Ordnance Survey on behalf of HMSO.

© Crown Copyright. All rights reserved.


One response to this post.

  1. Posted by chriseaglen on February 20, 2014 at 11:10 am

    Educated guesses and inexperienced guesses are each possible due to the lack of detailed verification. There are still the issues of what the viaduct will be made from and also the distance and volumes from overbridges also which are noticeable at 1Km.
    There are more dumping zones in other CFAs you and others have not studied in depth. Better to think also what your area can put to the Environmental Audit Select Committee within the terms and copy to National Audit Office also.


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