I have a low spot in the back of my yard that pools excessive water after a rain. The only lower spot than that is my neighbor's yard (there is a brief upslope between my yard and his) so I cannot drain there. The back of my yard faces a sidewalk which is actually higher than the low spot by nearly 6 inches. The problem is the yards in this development were not properly graded. How do I drain this water? I don't think the classic French drain answer will work as all areas of the yard slope to this one spot, and I can't drain against the grade.
All you need is a spade and a large plastic bucket like you buy drywall or paint in.
This is a variation on a french drain and will not provide relief against the "one in a hundred year storm" but should work for most normal rainfalls.
- get a clean bucket of about twelve inch (thirty centimeter) diameter
- remove the bottom
- mark and remove the top six inches of grass as a single unit the diameter of the bucket
- dig a hole the height of the bucket plus six to eight inches ( fifteen centimeters)
- place the bucket in the hole and fill to the top with large diameter rocks( pea stone gravel in size of 5/8" or 7/8' or larger)
- place a layer or two of good quality landscape fabric or geotextile on top of the bucket so earth cannot fill the bucket
- replace the plug of sod you removed back in the hole
- tamp and water
- job done!
One other option would be to work with what you have, rather than trying to drain the water away.
You could put in a tree or a bed of plants that can tolerate having wet feet after a rain. They will absorb the water faster than your turf could. I don't know where you live and what plants would be appropriate for your location, but birch, dogwood and iris are all examples of plants that could handle occasional standing water.
I've also seen people put a pond in a spot like that. You would need to make sure you sized the pond properly to accommodate the volume of water you typically get after a rain, and you'd also have to design the edge of the pond to allow the rain water to drain into it.
It's really hard to say without knowing how much rain you get and the actual topology but dry wells and rain gardens are generally two good approaches to deal with this particular problem. A dry well is good if you don't want to fill up the space with plants. You'll just have a little drain in the lawn.
Another thing to consider is why is the water running off and is there something you can do to keep it where it lands. Is your soil compacted or does it have poor water absorbing capacity? There's info here on how to do a percolation test to see how quickly your soil absorbs water. Also a video here https://www.youtube.com/watch?v=e6VV6OU3ssA
If your soil doesn't absorb or retain enough water I would recommend doing a very heavy aeration (about 20 holes per sq ft) then topdressing with good compost in the fall. It's a good time to overseed then too. It should help retain some of the rain so that there is less runoff.
Of course you can drain against the grade w/ a submerged French drain. Mine is 6' down at the end of the run, and drains a whole acre just fine. leave the soil on top of the drain porous--i.e. mixed w/ sand-- and it works fine. a ditch or swale stays at the surface; a French drain works at any depth as long as the surface water can soak down to it.
An idea of size and photos would be useful, but if the part that abutts the sidewalk isn't 50 yards long or more, I'd build in a small, raised brick wall, or concrete edging mortared together, a few inches above the level of the sidewalk, to keep the water off, if you're allowed to do that. I'm willing to bet that you're getting pooling there because of run off from the sidewalk surfacing, which is, as you say, 6 inches higher than your garden. This still leaves the fact that its the lowest point in your garden, and may still be pretty damp if all the rain ends up there eventually from surrounding soil. If it is, and the area's not too large, import topsoil and level off, or, as Michelle says, plant a willow, create a bog garden or make a pond.
Standing water is always a concern, primarily for mosquitoes. A dry well would be the best solution if you have the space. All you have to do is dig a big hole where the water is draining to and fill with drainrock. It should be deep enough to get through any blocking soil/rock/clay layers. This gets the water away from plants and enough time to percolate back into the ground.
Not knowing the problem at root, other options are available depending on your budget. There is a great move to green roofs and storing water, rather than just dumping the problem onto someone else, these solutions actually slow the runoff down to just a trickle and look quite good too- lots of advantages of having a system like this- cooler garden for one during the summer and I would also mulch and over do your soil in your garden with a ridiculous amount of organic matter, this in one way will act as a sponge- I did this paris where I lived many years ago- the vege plot I was doing was on a clay/sand hill and all the rain would drain directly into the house at the bottom of the hill and have no where to go, I firstly dug a trench to redirect the flow of water and secondly added horse manure- tons and tons of it to the soil then adding a foot deep layer of mulch on top mostly leaf mulch from a local pond- changed the issue dramatically! the vege went mad, the house stayed dry every time we had a major rain shower and I got told off for growing too much! first time for everything...