It used to be really quiet ’round here

Environmental aspects of the 51m consultation response, part 8

In this blog I will begin my review of the issues on noise nuisance that Southdowns Environmental Consultants Ltd has raised in Appendix 18 to the 51m alliance response to the public consultation (available here) that have not been mentioned previously in my blogs.

The Southdowns report points out the importance of existing ambient noise levels in any consideration of the impact that noise from HS2 will have. Using the tranquillity maps that have been published by the Campaign to Protect Rural England (CPRE) as a data source, it comments that the proposed route of HS2 “passes through extensive stretches of tranquil areas” and that it “represents a new source of transportation noise into these areas that are unlikely to be currently affected by significant sources of transportation noise”.

I touched on this feature of the proposed HS2 route when I blogged about the impact of HS2 on the Priors Hardwick/Wormleighton area of Warwickshire in This blessèd plot (posted 29 Aug); I said then that the “beauty, peace and sensitivity of this area mean that HS2 will have a much more severe impact here than at other places”.

The AoS appears to acknowledge this problem in paragraph 8.5.2 on page 53 of Appendix 5 of the AoS (available here), which says:

“A mitigation strategy that takes into account the relative importance of different factors affecting relative tranquillity … could help to reduce the potential impacts.”

This gives little assurance that HS2 Ltd has a solution to this problem.

Based upon its extensive experience with HS1 in Kent, Southdowns makes a comparison in the number of properties that will experience a “noticeable noise increase” forecast for HS2 in the AoS, and between what was achieved by HS1. The AoS predicts that up to 4,700 dwellings will experience a noticeable noise increase (3 dB or more above the existing noise levels) due to HS2. Southdowns comments that “this represents a significant increase in the numbers of noise impacts that were identified during the EIA” for HS1 and that the final HS1 reference design, including options for further mitigation “resulted in less than 250 impacts the majority of which fell within the SLIGHT category of noise impact”.

Even the figure of 4,700 properties affected could be considered optimistic as it is calculated after the application of “additional indicative mitigation” and excludes the increase in traffic that will result when the Manchester and Leeds links come into operation. Removing the mitigation increases the figure to 24,300 and adding in the “Y” traffic further increases it to 33,600; refer to Table 3 and Table 4 on pages 51 and 52 of Appendix 5 of the AoS.

According to Southdowns the principal reason for this difference between the noise impacts of HS2 and HS1 is “attributable to the design of the 108km alignment [of HS1] which largely followed established road and railway transportation corridors”. Southdowns also points out that the design speed of HS1 was lower than HS2, which of course results in a lower source noise level.

So next time you hear someone say that noise was not a problem with HS1 and, therefore, there is no reason to make a fuss about HS2 noise, you will know what to say to them.

The Southdowns report also picks up on an interesting point that arises from the calculations in the AoS of the numbers of residential properties affected by noise. According to Table 1 on page 45 of Appendix 5 of the AoS  the number of trains per day on the base of the “Y” rises from 432 to 576 once the links to Manchester and Leeds are operational; an increase of 33%. Southdowns comments that this 33% difference “corresponds to a difference in average 18-hour daytime noise levels of 1.25 dB”.

Comparing the “noticeable noise increase” figures in Table 3 and Table 4 on pages 51 and 52 of Appendix 5 of the AoS reveals that the number of properties impacted rises by about 40% for this 1.25 dB increase in noise. Southdowns comments that “relatively small changes of this magnitude can result in a significant increase in the numbers of properties affected, which provides evidence of the sensitivity of noise impacts to small changes in noise input and calculation assumptions”.

In other words the assumptions made about noise levels only have to be a couple of dB or more out for the estimate of the number of properties affected to change dramatically. Or, put another way, you can’t place much credence on the numbers in the AoS.

In the next blog I will consider two further matters associated with the calculation of impacts on properties that are mentioned in the Southdowns report.

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