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Potter Heigham Bridge on Twitter

NorfolkNog

By NorfolkNog
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NorfolkNog

NorfolkNog

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Potter Heigham Bridge has started Tweeting! A very helpful daily update gives the current bridge height which looks to be a useful resource. The prospects at the moment don’t look too good however!

 

@PotterBridge 

 

Some of the stuff on Twitter can be a bit banal but there are some informative and interesting established contributors on there including Carol’s Broadland Memories and the Museum of the Broads which are well worth following.

 

cheersbar 

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0.32 metres on the Environment Agency's Repps gauge would be equivalent to 2.01 metres on the height gauge inside the Pilot's office at the bridge.

 

If it's still calibrated in feet and inches then that would be showing a smidgeon over 6 feet 7 inches.

 

When trying to state the bridge clearance precisely it becomes tricky because all three gauges (upstream, downstream, and Pilot's office) are not calibrated the same, with a 4" difference between them.

 

The Pilot office gauge is generally accepted as the most accurate, though even that is arbitrary, being not the exact height of the centre of the arch, but a notional square shape passing through the semi circle.

 

To get the height of that gauge in metres, simply deduct the online Repps reading from 2.33 metres. 

 

(Then multiply the result by 39.37 if you want it in inches).

 

 

 

 

post-195-0-85867300-1408639985_thumb.jpg

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Potter's arch had always intrigued me, trying to ascertain the exact height of the arch in relation to the gauges.

 

Hamilton had a stab at it, many years ago. This diagram from one of his old guides shows his calculations of the arch curve, but unfortunately he never completed the reference to the gauges, though he did mean to.

 

I mused over the different ways it could be surveyed accurately now, and the simplest was to take very high resolution photos exactly square on, and scale them by accurately measuring more easily accessible reference points.

 

Comparing it with Hamilton's, this gave a waterline width of 20 feet 8 inches to his 22 foot "datum line". He didn't give the water level height though, so maybe it was during an even lower tide back in the old days.

 

The weak link in the exercise is that the BA might reset the gauges at any time, making them (hopefully !) more accurate, or at least more consistent.

 

It does however show the radius of the arch accurately, and the effect of wider, flatter, cabin topsides on the actual air draught.

 

It shows one of the problems of modern boats getting through, such as a modern 8 foot wide cabin roof needing 4" more clearance than the more traditional 6 foot wide cabin roof.

 

(I've had to downscale the images to be able to upload to the forum, so the pixels per inch are now less, as the originals are 6000 x 4000, to get the accuracy).

post-195-0-10241200-1408691777_thumb.jpg

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