Monday, 9 January 2012

Anoraks & Bridges

Here's one for the nerdy "Anoraks" amongst us!

Sunday's paddle was programmed to be Eastney to Portchester around the north of Portsmouth via Portsea Creek. The route would take us alongside the very noisy M27 motorway under three of the bridges connecting Portsea Island to the rest of the UK! The railway bridge to the east and the two bridges under the A3 Hilsea Roundabout to the west.

HW Portsmouth on Sunday 8th Jan was due to be around 1030 (possibly 20-30mins later at the top of the harbour?) As this would be around the time we arrived I thought I'd better cycle over the day before and check the air draught (Clearance) under the bridges.

So here it is:-
HW Portsmouth 0948
Height 4.4m
Weather: Southern England under a ridge of high pressure approx 1020mb (high pressure depressing tidal height and possibly delaying the time of high water?)

What do you think? Should we go or not?

Railway bridge at the east- HW +20mins.
Plenty of clearance under the north span. No real tidal flow.

Eastern Hilsea East Road bridge - HW +30mins.
Clearance touch and go sitting upright in a sea kayak? If it was low pressure the water would be higher. As a guide, if the plinth in the low foreground is just covered it's a definite no go (water 25cm higher)

Same as above - close up. Note the west going ebb already starting at the right hand edge pillar.

West Hilsea Road Bridge - HW+ 40mins.
Good clearance on this newer and higher bridge. Note the strong west going ebb already producing a big eddy at the right hand edge pillar. (I estimated it to running at 1.5 - 2knots when I went back past the bridge 5mins later!)

What did I do?
I changed the club paddle to a much quieter outing (no M27 motorway) in Chichester Harbour! One of my fellow paddlers on Sunday reckoned that a tide height of 4.2m was the highest tide you could get away with - I think he was right.


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