On the 9th of February 2020, I set off to the M48 Severn bridge with my good friends Will and Sara, to record the speed of the wind blowing into the Severn Estuary. It was absolutely pouring with rain as we drove the hour to get there, so our hopes of a dry wind sampling session were slowly diminishing. Nonetheless, anemometer in hand, we parked up at the service station on the English side of the bridge and began the final little walk over the M48 to the southern side of the bridge. From here we had a tremendous view of the newer M4 Severn Bridge, which was actually still open thanks to a better, wind-deflecting design. Impressively, the rain stopped as we stepped out onto the bridge and about half an hour later, the sun even graced us with its presence.
The highest wind speed I sampled was recorded near the end of our day at the bridge, after the rain had stopped and the sun had come out. The value my (uncalibrated) anemometer recorded was a gust of 66.9 mph / 29.9 m/s. This was the greatest value my anemometer has ever shown and it certainly felt like the strongest wind I had ever experienced. It was difficult to stand and we had to hold on to the railing to keep us supported. The anemometer was pointing straight out to sea too, which is notable because earlier in the day the strongest wind speeds I could record were with the anemometer pointing straight down, where the compressed updraught around the bridge produced a much greater reading. This makes the 66.9 mph gust even more impressive! :)
Interesting features relating to the interaction of the bridge and the wind:
There were some really interesting things to note at the bridge that day, about the way the wind interacted with the steep cliffs and the bridge supports. There was an upward deflection of the 50mph wind along all the cliffs and the supports for the bridge, which sent the wind roaring into the underside of the carriageway. This created some impressive updraughts through the splits in the bridge that allow it to expand without destroying itself, so water was spraying up in little fountains in a few places through the path and road. I also note in the video that just over the side of the bridge, there is a very strong updraught, presumably for the same reason. Another interesting feature worth noting about the interaction of the wind and the bridge was how much stronger the wind was against the big pillars for the cable supports. This is a common effect seen in all situations where an object restricts the flow of a gas or liquid. The wind is being compressed as it's forced around the obstructing pillar and so speeds up greatly to compensate. The area just behind the pillar, sheltered from the wind, has much slower but more turbulent air, creating a very localised area of low pressure.
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