Doing things by the book

Environmental aspects of the 51m consultation response, part 22

This is my final blog on what Southdowns Environmental Consultants Ltd has said about HS2 noise nuisance in Appendix 18 to the 51m alliance response to the public consultation (available here).

In Trust me, I’m an acoustic engineer (posted 4 Jul) I expressed concern that the model used to predict the noise impacts of HS2 in the AoS, dubbed the Calculation of Railway Noise 1995 (CRN), had not been designed for predicting the effects of predominantly aerodynamic noise sources. This is admitted in paragraph 5.6.9 on page 46 of Appendix 5 to the AoS (available here):

“For very high speed rail, i.e. above 300km/h it is likely that CRN would need to be adapted to have sources at two or more heights above rail: for example rolling noise and the second for aerodynamic noise, however the research basis for this change in calculation methodology is not currently available.”

The way that HS2 Ltd has “overcome” this problem is described in paragraphs 5.6.10 and 5.6.11 on page 46 of Appendix 5 to the AoS:

“It was decided that some modification to the base CRN calculation should be included to account for aerodynamic noise. The best option at this stage was to retain a single noise source but alter the source height.

“A source located 1.0m above the head of the near rail was used as a series of comparative calculations indicated that this gave the most consistent results when compared with SNCF data for speeds in excess of 300km/h. For train speeds less than 300km/h the rolling noise source location of CRN was used (rail head height).”

In Trust me, I’m an acoustic engineer I described this as a “bodge”. Although characteristically the criticism in the Southdowns report is somewhat less direct, it does seem to be broadly in sympathy with the sentiment of my accusation:

“A primary concern with regards to HS2 adaptation of CRN for trains travelling at high speed around or above the transition speed between rolling and aerodynamic noise lies with HS2’s assumed source position, albeit 1m above rail head height rather than the rail head assumed in CRN. Whilst train noise source height may not be of major concern with respect to free-field noise propagation for receptors at 4m above ground, in the absence of any interference to the propagation path between source and receiver, the distribution of individual noise sources and their respective contribution to overall train pass-by noise levels is critical to the calculation of acoustic screening associated with any structures or ground topography which interfere with the propagation path.”

So what Southdowns is saying is that, if HS2 Ltd does not make a proper modification to the CRN to adapt it for aerodynamic noise, then the noise contour maps that it (eventually) publishes may inaccurately assess the impact of man-made or natural obstacles (including hills and the like) in the noise propagation path.

In Going over the top (posted 9 Nov) I referred to a paper reviewing the noise calculation methods used in Germany (available here). The German calculation tool, known as Schall 03 2006, assumes three equivalent noise source heights to account for the various noisy elements on the train, such as roughness of wheels and rails, pantograph noise and engine noise; these heights are 0, 4 and 5 metres above the railhead. This adds force to the argument that the single noise source height of the CRN, as adapted for the AoS, is a somewhat inadequate (and probably inaccurate) model.

This leaves one final matter to report briefly. Southdowns introduces us to a publication by the Highways Agency, Design Manual for Roads and Bridges, and specifically Volume 11, Section 3, Part 7 (available here), which covers environmental assessment techniques for noise and vibration. The Southdowns report comments that:

“Whilst prescriptive methodologies for the calculation and presentation of road traffic noise impacts and their numbers are documented in the Design Manual for Roads and Bridges, there are no equivalent guidelines for the assessment of train noise impacts nor indeed are there any fixed national limits for the operation of new railway lines. Furthermore, currently available railway noise dose response relationships are based on historical data obtained from epidemiological studies of communities to established railways, whilst the response to high speed trains is more uncertain.”

Since it appears very likely that trains travelling at speeds over 300 kph are going to become a fact of life in the UK within an expanding high speed rail network, it is essential that this omission in the planning and design guidelines is addressed as a matter of urgency. We have seen what happens if a company charged with the promotion and design of a particular scheme is left to write its own rulebook; the errors and omissions by HS2 Ltd have proved fertile material for a whole tranche of my blogs.

The UK needs a properly researched manual to guide the designers of high speed railway projects, written by those not directly involved in any one project, satisfying the latest recommendations of the WHO and other international bodies and subjected to rigorous peer review. It is time that HS2 Ltd was stopped from writing its own rules.


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