[Sam85]

Hi,
I am inquiring about a rain garden. Attached is a photo of our back field. In heavy rain, it floods with surface water, which comes downhill from the left of the photo. There is an underground pipe but it gets easily overwhelmed. I am very intrigued by possibly lowering the left-hand triangle area (30m x 14m=420m2 divide by 2 = 210m2). I wonder if a rain garden would help? Basically, trying to do everything I can to prevent floodwater reaching the house & garden, which thankfully is on higher ground (attached photo).
 If we dig 20cms down, we are in heavy grey clay subsoil (gley). I wonder if we dug it out & replaced it with grit/gravel/compost would this help with infiltration? What soil/grit does it usually consist of? Sand is generally seen as a bad idea with clay subsoil. What would prevent it clogging up over time? Should it be lined at the sides & bottom to prevent it clogging? How deep should it be? We would still want the water to drain to the underground pipe to get away, so that may limit how deep we can dig.
We occasionally have sheep on the field for grazing but this is not essential. I am unsure about what plants to put on it, if we did have sheep.
Any advice or pointing us in the right direction would be greatly appreciated!

[Fleecewife]

What exactly do you mean by a rain garden?

[Kiran]

Ironically for a rain garden, in the civil engineering sense, used to managed rainfall you need well draining soil. You sound like you have heavy clay. For the purposes of managing fiel run off I'd be tempted to dig a large cut off ditch or swale along the boundary. That will allow you to connect up to wherever the existing drainage goes. A good swale is easy to mow but if you want minimal maintenance then go with a French drain. Looking at your plan, you have a decent sized area of land beyond the boundary that still falls towards your house so potentially another french drain near your house would be worthwhile to capture everything from that watershed. In construction rain gardens are considered expensive and installed as a planning requirement. You need special topsoil etc and are the size you were quiting that will be very expensive

[Sam85]

Thanks Kiran. When you say 'dig a large cut off ditch or swale along the boundary', do you mean lower the area furtherest away, so that it still drains to the main pipe? And yet still store more surface water flooding.

In some areas I would possibly dig out the heavy clay & replace with gravel/compost/soil to aid infiltration. Every little helps!