This assumes you are not allowed to install a pump of course.
Are you suggesting bury a large collection tub from the french drains and install a sump pump to discharge into the drains? Also an idea. To be fair utilising the drains is in the first place not allowed, the fact that I have had a pump running (80 Litres a minute or so), for 3 days and the water level is still at the same.
If you're pumping water from below the level of the water table, surely the water will just keep filling up to try to reach/stay at the level of the water table? Would it be possible to have a pond and/or bog garden at the lowest point and run your drainage into there rather than into the (not allowed) drains? It's not a problem I've ever had, being on sandy soil with a low water table and not close to any rivers, canals etc.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
This assumes you are not allowed to install a pump of course.
Are you suggesting bury a large collection tub from the french drains and install a sump pump to discharge into the drains? Also an idea. To be fair utilising the drains is in the first place not allowed, the fact that I have had a pump running (80 Litres a minute or so), for 3 days and the water level is still at the same.
Thats a cubic metre every 12 minutes!! Thats a ridiculous amount to be dealing with for that space. I suspect JennyJ may be right in that your lower garden is in effect already a sump for the surrounding land. In theory there must be some capacity of pump that could start to out-compete the inflow (and combined with gravel beds & french drains might give you a conventional flower border) but i wouldn't want to be paying the electricity bill. You are sure there isn't something else going on? like a hidden leak from another drain or somewhere else !!?
It might be time to start convincing yourself that what you really wanted all along was a pond & bog garden.
You said your neighbours garden was all at the level of the red line in a previous post. How wet is their garden especially on the side furthest from yours? Because the other option if you dont want a pond is to raise the whole area as Loxley suggested earlier. Expensive of course.
Looking at the image here, there seems to be an area on the right where the water is trickling to. If you increase the level in the raised bed by 15-30cm, fill with sharp sand or similar, and drill weep holes along the lower edge, at least the bed will be usable.
I don't know what's happening on the right, but the OP seems most interested in this bed, so assume it's OK for water to carry on seeping away there.
"What is hateful to you, do not do to your neighbour".
Good spot. Re-reading the OP more carefully it does say that this is not the lowest point of the garden, so there presumably is somewhere for the water to drain to. But the OP also mentions "rising ground water that is currently about 30cm above the level of the tiles." Those two statements can't both be true ... Unless this area is being permanently pumped out Or the tiles ARE normally underwater & the photos show the result of pumping out at 80L/min for three days!!! @lesinnl could you clarify please?
If its the latter then the bed needs raising more than 30cm I'd suggest, possibly up to the level of the lawn.
Finally i am also unsure of the reason for wanting to lay new tiles if this area is either underwater or regularly flooded. I'm sure you can lay foundations on waterlogged ground but normal methods won't work or will quiclky fail i would guess.
So, Friday I took advise from a water management company. They suggest creating a negative void deep under the bed and below the tile level all the way along. This will create a a sink away that can then have a bed planted above. This water will soak naturally away as the ground water falls. Then under the tiles I lay a sort of rockwool system that will hold more water volume than sand or soil again acts as a buffer and water soaks back out into the ground below when levels naturally fall. Adding a small drain will not overload the drainage system and therefore I can get a subsidy. Its a lot of digging thick, wet clay, to 60 cm deep, lets hope it works.
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It might be time to start convincing yourself that what you really wanted all along was a pond & bog garden.
Because the other option if you dont want a pond is to raise the whole area as Loxley suggested earlier. Expensive of course.
I don't know what's happening on the right, but the OP seems most interested in this bed, so assume it's OK for water to carry on seeping away there.
@lesinnl could you clarify please?
If its the latter then the bed needs raising more than 30cm I'd suggest, possibly up to the level of the lawn.
Finally i am also unsure of the reason for wanting to lay new tiles if this area is either underwater or regularly flooded. I'm sure you can lay foundations on waterlogged ground but normal methods won't work or will quiclky fail i would guess.