Summer Flash Flood Response
I believe your scheme will aid by keeping these drains emptier by stopping the tidal surge. However major damage was also caused by the failure to keep the drains clear up Tower and Barbican Hill road (where the water in the gutters spurted upwards of a metre at each drain due to the pressure of the blockage).
Improving the existing drainage and increasing its capacity to drain water elsewhere (I.e not into the closed sink of the river which will already be full) would help stop the flooding.
As our shops are flooded by water seeping up from the tides into our shops the tidal barrier remains incredibly beneficial and is the only solution.