I’m afraid I am sticking my head over the parapet this time but this is my opinion.
The OS map on the left has the 5m contour outlined. The grey areas are above 5m. The numbers are spot heights taken from OS maps. This shows the low lying land stretching towards Dunball (on the western end of KSD) and the raised land carrying the two rivers. For a better map click here.
The parts of the moor that are drained appear to have sunk - as happens on all peat moors - and left the Rivers Parrett and Tone in their own strip of high ground, contained by embankments as necessary.
The only ways for water to drain is via the rivers or the KS Drain.
Eastover bridge in Bridgwater was full during the last flood so cannot take any more. This leaves the KSD (as wide as the Parrett).
If the bed of KSD was dredged from Parchey Bridge to Dunball down to below half tide level and to original width and the tidal sluice at Dunball was big enough the moor would drain by gravity for half the day. I cannot find the half tide level (mean sea level) at Dunball but Wikipedia says KSD works by gravity.
If this is correct then the Parrett could be allowed to “leak” into the Sowy just downstream of Stathe (with a sluice) and also via the Chedzoy New Cut etc at Westonzoy pumping station (not then needed, are they pumping the wrong way?) into the KSD. These would need to be kept clear and opened up where necessary.
Obviously this will cost a lot of money depending on the maintenance state of the various waterways but it should be less than £29M. Apart from Dunball sluice (built 1972) the work is mainly straightforward channel clearance that could be done far more economically by local contractors, as pre 1996.
Again by Wiki the maximum Parrett discharge in flood is 173cm/s. If half could drain via Dunball the sluice opening would have to be 86 sqm.