Things we have learned (3)

Design of weirs

All weirs on the River Thames – Britain’s second longest river, are provided with mechanically operated sluices, to allow an increased flow in the event of high flow rates.  This has been gradually developed over the past hundred years or so by the Thames Conservancy (now the Thames Division of the EA), and although there is still a risk of flooding in rare events, has greatly reduced the height of any particular flood, therefore reducing consequent damage.  The design of structures consider local conditions, local housing and risk, generally by opening up the natural drainage capacity of the river, to minimise the effects of run-off from high precipitation rates upstream.  Some gates are in the weir structure itself (e.g. Henley, Marlow), others have sluices in side-streams (e.g. Sunbury), either natural or artificial cuts.  At Richmond there is a removable weir, (mentioned earlier) which comes down from a resting position above, into the water, to stop the river level going down too far as the tide goes down – and then is raised again as each tide comes up.  In times of flood it simply would not be lowered – unless the rising water is from an extra high tide rather than from up-stream.  (PLA operated). 

With this in mind, proposals for weirs on the Severn have included some form of variation – initially of a mechanical nature – tilting – which automatically lower the weir head with increasing water.  After 1996 however, the ‘inflatable type’ has been recommended, due to advantage being possible of opening up to four days in advance of a flood-peak arriving at that site.  There is also the conservation and environment advantage – the inflatable weir does not show above ground (or water) level – except perhaps for a small hut for control gear – but even this can be largely hidden. 

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