Wednesday, February 22, 2017

Removing the Wandle Half-Tide Weir

Recently I visited a bit of marine engineering that combined my interest in the rivers of London with the famous Thames Tideway Tunnel.

Where the Wandle River meets the Thames there has been for many years a half-tide weir. I'd been puzzled as to quite why, though I remember it did create a rather nice waterfall at low water:
I've got to know the Wandle a bit by walking up it and also kayaking down it: its a pretty little chalk river with (apparently) quite good wildlife, though it was always let down a bit by the mess where it meets the Thames.

But now work is underway to improve it and it all relates to the famous Thames Tideway Tunnel aka the "super-sewer". This is being built under the Thames to cope with the requirements of a London that expanded more than a little since the Victorians, in particular Bazelgette, built the sewage network including the wonderful Crossness Pumping Station.

As part of the project the Wandle half-tide weir is being demolished and I had a chance to visit the works barge and see how its going. The two aims are to improve the environment and allow lighters to be brought into where one of the access tunnels to the super-sewer is being dug, so that extracted soil can be barged away rather than clogging up the roads.

The environmental gains should be extensive, as there's a lot of silting and the mud contains nasty hydrocarbons, rather than nice gravel to allow trout to lay their eggs and eels to flourish. So hopefully both will become more common and that must be a good thing.

The demolition is being undertaken in various stages: firstly dredging to get back to the 1970s level of the river, then the concrete steps (above either side) are being drilled out (as per photo at top), then the weir plates removed, then the top of the weir sides leaving just the metal pilings. These are then cut off using divers, leaving a river flowing naturally.

As to why the weir is there in the first place? Well apparently there was talk of a marina a bit like Chelsea Marina but after the weir was put in place there was the big crash of 1987 and so money ran out and it was never followed up.

So it started to fade and decay, leaving the area where the Wandle meets the Thames in sore need of a little TLC.

There's also talk of the river bank being upgraded to be part of the Thames river-side walk, so hopefully in a few years all this area will be looking much healthier.

Who knows, maybe seals will be visiting this stretch of the Thames more often in the future:
The team from Land & Water have spotted a couple of seals, many cormorants and herons, trout and elvers plus was visited by an urban fox (of course).

Thanks to Land & Water for the highly informative tour.


my2fish said...

as a structural engineer, these construction/demolition projects always fascinate me. it'll be quite interesting to see how the ecology changes in and around the river after the changes are made to the weir.

great post, JP.

JP said...

Glad you liked it - I certainly was intrigued as to how they could go about demolition such a solid structure in tidal conditions where it is often underwater. The whole super sewer project should improve the Thames water quality by avoiding overflows, and there seems to be a concerted effort to improve the Wandle.

The air quality in London and how that impact us humans here.... well that's another not so positive story!