It could be a bumpy ride

Environmental aspects of the 51m consultation response, part 11

This is the final blog covering issues raised by the Southdowns report , in Appendix 18 to the 51m alliance response to the public consultation (available here), that I have not mentioned previously.

HS2 Ltd has provided a graph showing how the pass-by noise of HS2 (called “source level” in the caption to the figure) will vary with speed; this is figure 4 on page 45 of Appendix 5 to the AoS (available here). This graph, which is drawn for an observer at 25 metres from the track, is the basis of the noise predictions that HS2 Ltd has used in its preliminary assessment of the number of dwellings that will be affected by HS2 noise nuisance.

HS2 Ltd claims, in paragraph 5.6.1 on the previous page of Appendix 5, that the “assumed noise levels of HS2 trains are based on the noise levels of currently operating high speed trains together with noise level requirements for new trains from European specifications”.

HS2 Ltd cites the reference for the data on currently operating trains as measurements made on TGV trains and reported in a SNCF paper of 2007 (available here). The European specifications which dictate noise level requirements for new trains are the Technical Specification for Interoperability (TSI) covered by EU Commission Decision 2008/232/CE (available here).

Direct comparison between figure 4 and the data in the sources is difficult, as HS2 Ltd has insisted on using its much loved noise parameter, the equivalent continuous noise level over an eighteen-hour period (explained in my blog Taking a longer-term view, posted 22 Jun), and the two referenced documents express noise for a single train pass-by only. However, Southdowns reports that a single pass-by level, confirmed by HS2 Ltd in response to a Freedom of Information request, “is consistent with the data reported by SNCF for TGV trains”.

However Southdowns also comments that the noise levels apply only for “trains with wheels in good condition running on track which is compliant with the TSI specifications for track roughness and decay rates”.  The TSI sets out the minimum standards for the quality of the “reference track” that shall be used for noise measurements in Annex N to the specification that is covered by 2008/232/CE and the measurements of TGV noise given in the SNCF paper were made on a track that meets the “reference track” standard. However, the TSI is at pains to point out that “neither the design nor the maintenance nor the operating conditions of ‘normal’ tracks, which are not ‘reference’ tracks are covered by the reference specification.

This puts a question mark against the source noise levels that HS2 Ltd has assumed; will the operational track conditions for HS2 be as good as the TSI reference model, or will operational tolerances result in higher noise levels in practice?

A useful insight into how difficult it may be to maintain the track reference specification on a railway that is used for day-to-day operations may be found in the SNCF paper. It is reported there that some track work was carried out on the test track during the period over which measurements were being made and that “some ballast dust might have been run over by the wheels, the roughness of which significantly increased on the following days”.

This increased wheel/track roughness was reflected in the noise measurements that were taken. The paper reports that they “were then increased by 1.5 to 2.5 dB(A) after each track work episode”. The cumulative effect of these track work degradations is summarised in table 2 in the paper, which shows a 4 dB increase in noise at 300 kph and 3 dB at 330 kph.

The Southdowns report points out that the TSI for the infrastructure sub-system of the trans-European high speed rail system (2008/217/CE) is aware of this problem; this document may be downloaded here. In paragraph 4.2.19 (on page L 77/31), which covers the specification of noise and vibration, it is prescribed that “the actual track” quality should be taken into account in any assessment of pass-by noise. In order to emphasise this, the TSI document adds the footnote “It must be underlined that actual track quality is not the reference track quality defined to assess rolling stock against pass-by noise limits”. Southdowns concludes:

“It should be apparent therefore that unless assurances are provided to ensure that the track will be constructed and maintained to the TSI reference track conditions, then an acoustic track quality correction will need to be derived and should be applied to calculated noise levels in accordance with the TSI infrastructure requirements as part of the future EIA of the scheme and any subsequent noise management procedure.”

I think that we can be fairly confident that the first option offered by Southdowns, constructing and maintaining the whole track to meet the TSI reference track conditions, is an impractical way forward. This leaves making “an acoustic track quality correction”, which means adding some decibels to the assumed source level to take account of the degradation in track quality that will happen in an operating railway.

Why has HS2 Ltd not done this? A precedent already exists in the 3 dB reduction in the source level that HS2 Ltd has assumed to take account of “the anticipated noise control improvements in the next generation of high speed rolling stock”; I reported on this in A little bit of magic, which I posted on 17 May. The SNCF paper indicates that an increase of around 3 dB might be appropriate, which would negate this 3 dB reduction already employed.

So HS2 Ltd has taken account of the TSI to claim a reduction in the predicted noise nuisance levels, but has ignored a TSI requirement that will result in a compensating increase in noise; well that’s pretty much what we would expect from HS2 Ltd, isn’t it.

If you remember what I said in It used to be really quiet ‘round here (posted 12 Oct) about the sensitivity of the calculation of the number of dwellings affected by noise to small changes in the assumed noise levels, then it should be clear that upping the noise levels by around 3 dB will make a big difference to this dwelling count. Easy to see, I suppose, why HS2 Ltd wants to ignore it.


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