Not a precise science, part 6

(… continued from Not a precise science, part 5, posted on 21 Oct 2015).

In this final part of this series on noise propagation modelling I want to add some additional thoughts that supplement what I have already discussed.

The first of these was something that was brought to the attention of the HS2 Select Committee by petitioner David Ball (see footnote 1) and concerns the scatter diagram that I reproduced in part 1 (see footnote 2). Mr Ball used this scatter diagram as one of his exhibits (see footnote 3), but advised the Committee that he had “introduced a [blue] line that goes from the top right-hand corner all the way to the bottom left-hand corner”. This line is the locus of all points where the predicted and measured values are the same, i.e. where the scatter points lie on this line the model has been demonstrated to be spot-on accurate. I reproduce the top right-hand quarter of Mr Ball’s exhibit below.

Source: HS2 Select Committee exhibit A1275(14)

Source: HS2 Select Committee exhibit A1275(14)

This section of the diagram covers predicted and measured noise levels above about 80dB LAmax and illustrates how the blue line is diverging from the best-fit line for the scatter points with increasing level. Up at the top end, albeit at the very highest levels that will be encountered from HS2 (very close to the track), this divergence is around 3dB(A).

Whilst I am perfectly happy that the practical impacts of this divergence are not likely to be of any great significance, it does illustrate that there is a shortcoming in the model; it is certainly something that should have been noted in the HS2 Phase 1 Environmental Specification (ES).

Something that I should mention whilst I am referring to Mr Ball’s petition hearing is that it included what has been, in my opinion, one of the most effective demonstrations of the impacts of HS2 noise that the Committee has witnessed to date. This was the simple ploy of playing a recording of a train pass-by every ninety seconds whilst Mr Ball was making his pitch. The volume of the playback had been set by the petitioners to be at the level that, according to the Promoter, HS2 would be heard in Mr Ball’s garden (60dB), and this was confirmed by a sound level meter on the desk in front of Mr Ball.

This simple device appeared to impress even the normal noise-sceptical Sir Peter Bottomley (see footnote 5).

My second postscript is to note that the review of the prediction methodology in Annex D2 of Appendix SV-001-000 makes no mention whether the presence of water, either as precipitation or as atmospheric vapour, has any influence on sound propagation.

My researches indicate that water in the air, in droplet form (rain) or as vapour/condensing vapour (mist or fog) or in solid (snow and hail), has little impact on sound propagation, but that snow and ice on the ground may be more of an influence. It is a disappointment that the ES provides no enlightenment.

I can’t let this examination of how comfortable we should feel with the noise level predictions in the ES pass without emphasising again that, although HS2 is being professedly designed to accommodate a maximum 400km/hr operating speed, this is not the case for the noise mitigation. This is an old moan of mine; I first raised it as an issue in my blog “We don’t believe you” (posted 25 May 2011). In that blog I complained that the pass-by speed being used to calculate source noise was 360km/hr, not 400km/hr, so HS2 Ltd was “stealing” around 2dB. This remains the case for the calculation of LAmax. However, we now know from the ES that even more has been whittled off when it comes to the LAeq calculation (see footnote 6):

“Passenger services have been assumed to operate at up to 360kph (225mph) unless constrained by the design of the route as described the relevant Volume 2: CFA report. Where the maximum operational speed of 360 kph applies, services will be timetabled based on a speed of 330 kph. It is assumed that around 10% of services will operate at the full speed of 360 kph.”

Finally, I should comment on the train source height model that HS2 Ltd has assumed. The HS1 propagation model provided for a single noise source, on the basis that at 300km/hr the noise is dominated by rolling noise. The HS2 model has enhanced the HS1 version, by allowing up to five heights of noise source to be specified (see footnote 7). This development is essential for the accurate modelling of trains travelling in excess of 300km/hr; as the ES points out “source height is an important factor especially when considering noise barriers” (see footnote 8).

The problem as I see it is that HS2 Ltd has set the power of the higher-up noise sources – those associated with aerodynamic noise – so low that their influence is significantly diminished, and this is particularly the case with noise associated with the pantograph. The evidence that this is a prudent assumption is, in my view, not really in the ES. I well remember the view expressed in 2011 of the consultants to the 51m consortium regarding the approach that HS2 Ltd was taking to pantograph noise that it represented “a significant project risk” (see footnote 9).

Footnotes:

  1. Mr Ball presented his petition, together with those of some of his neighbours in the north of Wendover, jointly with Bob Lewis during the evening session of the HS2 Select Committee held on Monday 7thSeptember 2015. Their session is recorded from paragraph 367 in the transcript and from 21:30 hrs in the video.
  2. The source of the diagram is Figure 7 in Annex D2 to Appendix SV-001-000 in Volume 5 of the ES.
  3. It is exhibit A1275(14) on page 199 of the pack for 7thSeptember 2015.
  4. See paragraphs 388 and 389 in the transcript of the evening session of the HS2 Select Committee that was held on Monday 7thSeptember 2015.
  5. See paragraphs 392 to 404 in the transcript of the evening session of the HS2 Select Committee that was held on Monday 7th September 2015. Sir Peter’s reaction is probably better appreciate from the video (at around 21:43 hrs).
  6. The extract from the ES can be found in paragraph 1.2.10 of Annex D2 to Appendix SV-001-000 in Volume 5 of the ES.
  7. See paragraphs 1.1.26 and 1.1.27 of Annex D2 to Appendix SV-001-000 in Volume 5 of the ES.
  8. See paragraph 1.1.28 of Annex D2 to Appendix SV-001-000 in Volume 5 of the ES.
  9. I quoted this in my blog Going over the top (posted 9 Nov 2011).

Important Note: The record of the proceedings of the HS2 Select Committee from which the quotes reproduced in this blog have been taken is an uncorrected transcript of evidence, which is not yet an approved formal record. Neither witnesses nor Members have had the opportunity to correct the record in such instances, and it may therefore be subject to changes being made in the light of any such corrections being requested.

 

 

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