It is a very clever engineering feat. Each open section has a trough like shape which sits on the bed of the Thames, connected at each end to the Silver/Yellow up right sections. When London is about to be flooded by an exceptionally High Tide, the troughs are all swivelled upwards, the barrier is closed, and London is safe!
The closure gave a calm flat area of water above the barrier and Greenwich Sailing Club were making full use of it. There is a car park, cafe and visitor centre south of the Thames and entry is charged, but on the open day it was free. Look out for Next year's closure