Forth road bridges new and old (courtesy of Scottish government)
I was intrigued to hear that the opening of the Queensferry crossing had been delayed for five months by high winds in April and May this year. I’m not going to get into the politics of the whole thing on how delays during just two months can set the whole project back five months defies any sensible kind of logic, but I won’t dwell on that side of things.
Forth Replacement Crossing (courtesy of Wikipedia)
The weather station at Gogarbank
I am sure that there are any number of wind vanes and anemometers affixed to the bridge and the cranes that are helping build the bridge, but I am not privy to that data, or the anemograph traces that they produced during April and May of 2016, so all that I can do is look for the closest SYNOP station reporting hourly wind speeds and that happens to be from Gogarbank in Edinburgh, in fact if you look at the map below, Gogarbank is remarkably only just over 5 miles to the SE of south Queensferry.
Queensferry – Gogarbank (courtesy of Google maps)
Gogarbank is a strange location to site a new weather station because it looks like a lot of money has been spent setting up a duplicate station to the one that must have existed for many years at Edinburgh airport. It’s an estimated WMO class 4 site as far as temperatures are concerned, but must be more representative of temperatures that the airport was. I won’t go into the obvious question of why the Met Office duplicated all the sensors at Edinburgh airport with a new site just 1.7 miles to its S’SE – perhaps they plan to do the same at other airports around the UK such as Heathrow? Gogarbank is 57 metres above sea level and looks well exposed to most directions.
Edinburgh airport and Gogarbank (courtesy of Google)
03166 – Edinburgh Gogarbank (courtesy of Google)
Below is a graphic showing how wind speed increases with height, and as you can see a 16 MPH surface wind would equate to a wind of over 30 MPH at crane height of 200 metres (x 1.875) . The safe working limits are between 25 and 31 MPH, so using the winds from Gogarbank we are looking at surface winds of no more than 13 to 17 MPH, either as a mean speed or in any gusts. I’m not going to try adjust the winds for height, but obviously they must be a little higher at Gogarbank than places closer to the Forth, but how do you factor in winds that are blowing off an estuary rather than over farmland and allow for all the added friction?
Forth wind speed profile (courtesy of Scottish government)
Weather impacts on the Queensferry crossing
Above is what the Forth Crossing Bridge Constructors [FCBC] have said about the delays caused by the wind and weather during April and May of 2016. And below is the anemograph trace from hourly SYNOP data that I’ve put together for the two months in question. Just note that the anemographs show wind speeds in knots and not MPH, so the limits using these graphs are roughly 11 to 15 knots, which is just a little bit less than Beaufort force five.
Winds in April 2016
03166 Edinburgh Gogarbank – United Kingdom 57 AMSL 1 April-30 April 2016
April was I would have thought a very typical month with winds generally from the west but with easterlies for short spells (but not exclusively) between the 2nd-4th, 10th-15th, 21st and the 23rd. At a quick glance I would have thought that the 1st, 6th, 11th-12th, 17th-18th, 25th-26th, and the 29th would have been out of limits, at total of around 9 days.
Winds in May 2016
03166 Edinburgh Gogarbank – United Kingdom 57 AMSL 1 May-31 May 2016
In May there were prolonged spells of easterlies between the 6th-14th and the 23rd-31st. I should imagine an easterly and the longer sea fetch may have caused then more problems than other directions, but I could be wrong. Wind speed were out of limits at a guess for the 1st-5th, 7th, 11-12th and the 19th-21st, a total of 10 days by my rough and ready reckoning.
So my figures are for 9 and 10 days out of limits are a little lower than FCBC’s 13 and 12 days, a little lower but no real conspiracy theory. Reasons for the differences are the fact that my wind speed data is for a site 5 miles to the S’SE and over land and not water, and of course the fact that I don’t have to worry about anyone’s life when I do my guessing. All that I can say is with these low limits, is that they have done a pretty good job of building a fantastic bridge with 35,000 tonnes of steel and 150,000 tonnes of concrete in the timescales that they’ve achieved, I can’t wait to drive over it in the future.