I note that both proposed sites suggest retaining the Severn to low tide then start releasing the water – surely this is just as the tide turns so the available head will be rapidly reducing and the flow slowing, in fact creating a half-tide condition and this gives me a concern that the stored water will create waterlogging or even flooding problems in the Somerset Levels.
Second, the Severn carries a large quantity of silt. By holding the exit flow back, surely it will cause sediment to build up in the upper reaches. I believe a Canadian river with similar tidal pattern was equipped with a barrier and became useless quite quickly due to silting. Restoration took many years.
Lastly has one of the recognised hydrological laboratories modelled the effects of the silting or that on the adjacent low lying countryside?
Geoff Lott (M), Shropshire, firstname.lastname@example.org