Going over the top

Environmental aspects of the 51m consultation response, part 15

So we have come to our final task in reviewing what Southdowns Environmental Consultants Ltd has said in Appendix 18 to the 51m alliance response to the public consultation (available here), which is to review anything new that Southdowns has to say about criticisms that I have already made in earlier blogs. I plan to devote this and the next seven blogs to this task.

In A little bit of magic (posted 17 May) I expressed my doubts about whether the three metre high noise absorbent barriers proposed by HS2 Ltd would provide adequate noise screening for a train travelling at over 300 kph and cited a US authority that was recommending a minimum height of at least half as much again.

This potential problem has been acknowledged in the AoS (in paragraph 6.3.3 on page 50 of Appendix 5, available here), which admits that barriers “are not likely to be as effective or feasible, due to the required increase in barrier height, to provide shielding for the entire train”. The alternative solution that is proposed by HS2 Ltd is “to control aerodynamic noise through advanced rolling stock design”.

The Southdowns report summarises this situation as follows:

“Whilst it is acknowledged in the AoS that aerodynamic noise needs to be controlled through advanced rolling stock design, the sources of aerodynamic noise are many e.g. pantograph, pantograph recess, gaps between coaches, driver door handles, bogies and other surface irregularities. If substantial reductions in aerodynamic noise are not achieved at source, then higher barriers will be required and/or reductions in train speed will be required in the event that there are practicable reasons to the installation of noise barriers.”

Southdowns appears to be (rightly) sceptical about the proposal to overcome the high barrier problem by “advanced rolling stock design”:

“The identification of train noise mitigation requirements on the proposed route alignment requires detailed consideration of the numbers and magnitudes of noise impacts or changes at both residential and non-residential receptors. This in turn requires the application of robust calculation procedures and development of input assumptions, as well as the development of relevant noise appraisal criteria. The contribution of aerodynamic noise to overall train noise levels at higher speeds requires further and more detailed consideration, as does the contribution of mechanical noise from train motors, fans and ancillary equipment at low speeds if the acoustic benefits of barriers and other forms of screening are not to be overestimated because of inadequate assumptions about source heights and other acoustic characteristics.”

The Southdowns report also cautions:

“The noise criteria and assumptions incorporated into the HS2 noise model represent a significant project risk in the event that significant noise effects have been underestimated and engineering alignment options are constrained to the published alignment. Options for additional mitigation will then be limited to the installation of higher noise barriers at 4-5m height or more above ground and/or long term speed restrictions.”

Further evidence that substantial noise contributions are generated high up on a train travelling at aerodynamic speeds may be found in the SNCF paper of 2007 that HS2 Ltd has used to obtain data on pass-by noise levels (available here). This paper describes some measurements that were carried out using acoustic arrays, which allow some preliminary conclusions to be drawn about where the primary sources of noise are located on a train. Although it is admitted in the paper that “source identification is still an open point to some extent”, a table is provided that identifies noise levels from particular sources on the train. The data in this table indicates that, at 350 kph, the highest levels of noise are generated by the pantograph and the front window and roof; these sources are likely to be largely unattenuated by three-metre noise barriers.

Some insight into the effectiveness of measures that might be incorporated into the “advanced rolling stock design” that HS2 Ltd is relying on to keep noise barriers low may be gained from a paper reporting on research carried out on aerodynamic noise reduction in pantographs at the Japanese Railway Technical Research Institute  (available here). This research describes a number of design modifications that achieve around a 4 dB reduction in aerodynamic noise from the pantograph, compared with the “current low-noise design”.

So the work in Japan indicates that it is possible “to control aerodynamic noise through advanced rolling stock design”, but the 4 dB reduction in noise achieved by the Japanese is insignificant compared by the around 20 dB that HS2 Ltd assumed absorbent noise barriers would achieve for the noise simulations provided at the consultation roadshows. I think that HS2 Ltd has quite a task on to convince us that advanced aerodynamics can reduce the source noise from high level generators on the train by anything remotely approaching 20 dB.

A review of the calculation methods used in Germany (available here) indicates that barrier performance, even at relatively low speeds, may not be nearly as good as HS2 Ltd is assuming, and that the beneficial effect is reduced with increasing distance from the track (see Figure 5). The reason given is “reduced effect on high positioned noise sources”.

As it says in the Southdowns report, HS2 Ltd has a great deal more work to do on the sources of noise from high speed trains and ways of abating that noise.

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