There is a tidal sluice on the Holderness Drain in Hull, see web site, that consists of side hung gates. Unfortunately these appear to be stuck in the silt. This idea could be adapted in a different way and someone may find the following of interest. This idea is way beyond my expertise but I know structural engineers and can find suppliers to carry it further if anyone is interested and perhaps give some idea of cost.
How about using a stack of box culverts, each with a top hung lightweight door, positioned across a tidal river near to the sea outlet. These would act as valves allowing river water out but stopping sea water.
The length of each box to be the width of the sluice and a road on top.
The stack would stretch from shore to shore and be keyed into the bank.
The depth would be at the river bed (determined by the next bridge upstream) but above low tide level.
The height would be to the surrounding ground level or sea defence level.
The individual boxes would be 1m square - to be determined with advice.
The door would have a weight / density a little more than the weight of water, hinged at the top, on the outlet side.
These would be set in a frame work of poured reinforced concrete with piles driven into the river bed.
With clever excavation, design, supports and shuttering these could even be set underwater with waterproof concrete.
Silt will be a problem - if any doors do get blocked most will still be working.
The doors could be lifted off with a crane for maintenance.
Doors in the lower culverts would be lockable to retain water level upstream if necessary.
The total cross section will be similar to the river in flood.
The river banks on the up and down stream will need reinforcing to maintain the river width.
The individual units should cost no more than £10,000 ?
A barrage 40m wide by 4m high would need 160 units.
Triple the cost for installation, piling , etc gives a total of £5M
Having read the 2009 Parrett report we had better double the cost to allow for the politics ie £ 10M
Plan and elevation.