It sounds like a bit of a rant.
Well it is. Not being mean, and trying to not be rude, but I feel that I need to do this.
I saw a post about building a rain garden. Someone used a magazine article, which actually was a good article, and they did attribute and thank the magazine for letting them use their info and picture. But they inserted their words into the article, and gave advice that was incorrect, IMHO.
I am a Certified Rain Water Harvesting Specialist in Texas, having had training that is available above and beyond normal training through the Master Gardener program. Rainwater harvesting is a big deal down here, especially with the droughts we have experienced lately.
It may well be that you can do this and this and that, and install all sorts of extra rocks, gravel, soil, and whatever else. But it certainly isn't necessary, nor even desirable. Common sense is common sense.
The purpose of a Rain Garden is to manage storm water that rushes off impervious surfaces; the roof of your house, sheds, or even the road or a sidewalk. The water rushes to the storm drains, into streams, rivers and lakes, and carries many pollutants and chemicals with it, AND our precious water is gone too! What the Rain Garden does is trap the water and allow it to seep through the soil, cleansing the water and allowing it to percolate back to the aqua systems.
The Rain Garden is NOT a treatment for the wet spot in your yard! To put it there defeats the purpose of the Rain Garden; If the area doesn't drain well, it isn't going to do a bit better just because you planted a garden there. It doesn't drain for a reason; clay soil comes to mind, or heavy equipment could have been stored or parked there for a long time. Building a rain garden is work, and there are much simpler fixes for that wet spot than putting in a rain garden. Fix that low spot, either by filling or by installing drains! A Rain Garden installed in that 'wet spot' in your yard could easily become a fantastic breeding ground for mosquitoes.
A Rain Garden is a man made depression in the ground. We have clay soil here, so this rain garden is pretty shallow. The garden is 10' wide x 52' long x 12" deep. But that size and that depth, creates a garden that handles the water off this building and drains in 24 hours. If we had sandy soil that drained faster, we would have dug it out to at least 18", but not ever much deeper.You can get a good idea of the layout of our rain garden. The garden should be at least 10' away from the building. That is important. The water comes off the building very fast, down the downspouts (2 of them), and runs the 'river'. That's what the kids call it. We didn't use anything except some landscape fabric to keep our rocks and gravel from disappearing into the soil in the river bed, and we didn't dig the river bed out. We did build embankments of a few inches and really pounded them solid, so they wouldn't wash away. Lay the fabric and the rocks and gravel. The water runs under the little bridge. We dug out under the bridge so that water would fall fast enough to not be dammed at the bridge. The garden basin is dug to 12", and the soil taken from the basin is used to build a berm on the outside edge. This berm will help to hold the water, so it can trickle through the ground below. Pile soil around the basin, and whack it with the back of a shovel, then stomp it until it is very solid. You want it really compacted.
We mulched the rain garden with hardwood mulch, and planted many of the plants that grow wild in our ditches and pastures. We planted buffalo grass on our berm, but you don't have to. You can use that area to plant flowers, ornamental grass and small shrubs and trees. We are now removing patches of grass to plant flowers; drifts of black eyed Susan, coneflowers, Blackfoot daisy, native clematis, rock rose.....
A word about mulch; Do use hardwood mulch, so it won't float away, and it won't deteriorate quickly. Don't use pine, you will just wind up with a mess.
We set out to put in a Native Plant Garden, and were confronted with this rain water, rushing from the downspouts, and eroding the ground at the building; thus the Rain Garden was incorporated. This garden has grown a LOT! Kids are always in here, they love that bridge. We have birds and butterflies galore. We had so many butterflies last year that a woman was seen 'stealing' our caterpillars! Grrrrr.....
A Rain Garden doesn't have to be this size or shape, and you don't have to plant natives in it. You can plant anything you want, just be sure that what you do plant doesn't mind having wet feet every once in awhile.