Draining wet garden

During periods of heavy rain my garden has excess surface water due to a high water table and heavy clay soil . I am on high ground, not in a flood area.

I want to grow veg in rotation using raised beds, but would like to improve the drainage first. The garden is pretty well flat, but the south side has a slope draining into the garden, with two sides falling away. I would like to make a drainage trench from the first point the water collects. Is this adequate or do I need to dig individual drains for all four raised beds.

I hope the plan I have made in my album"Drainage problem" is clear to illustrate the issues.

Any thoughts appreciated.

 

Not sure of the diagram, but the advice we were given was to dig a ditch along the side where the water enters the garden. You are trying to interrupt the flow into your garden which is what a ditch would do. How you dispose of that water is more difficult. We have a very deep soak away at the end of a ditch.

 

French drain. Dig trench, put perforated drainage pipe in the bottom, add gravel (until the pipe is "just covered"), back fill with soil.

If there are convenient spots I would create a vertical "void" to the pipe and backfill them with large-ish stone - that way any serious surface water can just run straight down into the pipe, rather than having to percolate through the soil first. Those "vertical drains" are best sited where the water sits as a puddle (e.g. where you can see it still 40 minutes after a proper thunderstorm / downpour).

If the obvious route is down the right side of the garden, and the garden slopes from the top of that "proposed pipe" to the bottom, then I would be inclined to put a side-branch pipe, connecting into the main pipe, across the garden. How many of those you do will depend where water collects. I'd be inclined to do it where the path is between the beds (maybe one above the beds, one in the middle, and one at the bottom). If it is in/under the path then presumably leaving some "vertical rubble voids" down to the pipe can eb accomodated (in the middle of the path)

What's goign to happen at the lowest point on the pipe (presumably the bottom of your drawing)? Will the water be able to escape at an outfall and flow away, or will you be making a deep soakaway (big hole, filled with rubble)?

If a soakaway I suggest you leave an open void in the middle of it, big enough to put a submersible pump into, so that in really wet seasons, like th Winter just gone, when everything is saturates, and the soakaway doesn't!! then you can pump it out (via a hose - to the street, or a storm drain in the street, or similar). I've known people sink a wheelie bin (with holes drilled in the bottom / sides) into the soakaaway to create that "pumping out void".

Much easier if the bottom end of the pipe can just "flow away" but if you have that sort of fall then I doubt you would have standing water anyway.

You could:

Put perforated drainage pipe in bottom of each raised bed (suitable deep).

Arrange it so that it is level, with an uphill section of say 4" at the "exit" end

Then connect it to the main drainage pipe

When there is enough water (i.e. water table is high) the water pressure will force the water over the low point and exit.

However, you could then plumb the high end into the downpipe (and waste pipe from bath) so that the underground drainage pipes form a reservoir of water and keep the beds moist in summer.

 

Paving Expert has some good info on laying land drains:

http://www.pavingexpert.com/drain03.htm