The Locks
Richmond lock and footbridge is described as a half tide lock. In each arch of the footbridge there is a shutter known as a curtain. Two hours after high water the curtains are lowered into the river, creating a barrier to maintain the level of water above this point. While the curtains are in the lowered position, vessels are obliged to use the lock.
The time taken to transit the lock will of course vary depending on the time of tide. Two hours before the next high water the curtains are lifted, which allows vessels to navigate unimpeded through the bridge. The river above this point now becomes tidal until Teddington Lock.
The next major obstacle is Teddington Lock and although generally a faster lock to pass through Teddington is probably the busiest lock on the Thames and again in the summer months getting locked out can mean a wait of up to half an hour added to the journey time.
We have often had difficulty in the past explaining to the public why last Monday (for instance) it took two and a half hours to Hampton Court Palace but this Monday it will take over four hours. A combination of unpredictable tidal conditions, bad luck at the locks and an unusually busy river can cause unforseen delays.