For much of the work we do there are clear sets of noise criteria and assessment methodologies. If someone wants to install air conditioning plant this is assessed to British Standard 4142:2014, if housing is to be built close to a road then noise levels within the houses is assessed to British Standard 8233:2014. But how do you assess noise from a foot bridge?
This is the question that arose from a planning condition for a new foot bridge spanning the floating harbour in Bristol as part of the Finzels Reach development. The curved bridge is currently under construction and will form a public right of way from the main shopping area of Bristol towards Temple Meads Railway Station through the converted brewery site along the edge of the harbour.
Within Bristol and other cities there are many beautiful foot bridges spanning rivers. We learnt that some of these, due to their construction, create a lot of noise when people walk over them or bikes are ridden on them. This seems to be because of the texture and material of the bridge deck and the way that this generated vibration can interact with the building structure. So the first thing we needed to know is how is the bridge built and how loud will it be?
Discussions with the project architect and bridge contractor identified the construction and a number of locations where this had been constructed and could be measured. We then needed to find a site where we could physically place a microphone to undertake measurements of bridge noise without noise from other nearby sources. This ruled out any bridge over a road and any bridges at significant height.
Luckily for us, we discovered that a section of tow path along the floating harbour was constructed from timber fixed to supporting metal beams, it was bordered by office buildings on both sides of the river which minimised noise from nearby roads and there was a mooring alongside where we could undertake our measurements. All we had to do was wait for the lunchtime rush for people to walk and cycle past us.
Once we had the accurate measurement data we then had to determine how to assess this type of noise. People walking or cycling on a public right of way is part of the general noise within an urban area, the WHO1999 guidelines call this ‘Community Noise’ and provides guideline values for daytime and night time periods. It was considered that the most significant effect would be that of the impulsive ‘maximum noise level’ if people were walking along the bridge at night.
Using the plan of the bridge and nearby housing we calculated the noise level at the façade of the nearest flat based on the measured data. Then, using information provided for the façade construction and layout of the flat we determined that noise within the flat from the use of the bridge will not adversely affect the occupants and thus could allow the planning condition to be discharged.
The project involved a lot of planning and literature review although could be presented in a simple, clear and concise format based on accurate measurements that could be understood by any interested parties and was able to satisfy the council that noise was not of concern.
We believe that is the skill in environmental assessment of unique problems. A project can involve a lot of work including measurement and prediction but ultimately the assessment has to be simple enough for everyone to understand so people are confident in the conclusions it draws.