I got a question from a reader who wants to know what makes a good culvert. She has some land, and an impaired stream crossing, and a mother with a bulldozer, who is not afraid to use it. She has been offered a new culvert for Christmas. I love this.
I don’t know anything about this stream or crossing, and am surely not offering engineering advice here.
A project like this could well require permits, from the Army Corps or from the state Fish and Wildlife department.
But, I can tell you in general what you want from a culvert. What you want from a culvert is that it doesn’t get in the stream’s way. From the upstream side, the culvert should be able to pass a flood’s worth of water. If is too small, or choked with debris, water will back up behind it. You’ll get a big pool behind it, if flood pressure doesn’t wash out the obstruction.
On the downstream side, the worry is whether fish can swim upstream. A too-small culvert is bad news for that. Water constrained through it will act like a fire hose; during wet periods, it can be a velocity barrier, too fast for fish to swim through. There are lots of studies and specs on fish flows. If you want to get precise, you can find out exact velocities for different species at different life stages. If you don’t want to be precise, you can look at nearby reaches with fish. They have small pools upstream of rocks, and eddies near the edges where fish can rest between bursts of swimming against the current. Stillwater refugia. You want that.
Fortunately, the solution to both these problems is the same: big wide culverts, with floors that look like the natural stream. (Here.) As a rough recipe, put in a big section of pipe as wide as the stream, buried about a third deep. “As wide as the stream” is wider than just the water, incidentally. You should look for bankfull width, where the sides start to slope down.
There is another potential problem, one that I hope is not plaguing the reader. It is possible that the streambed below the culvert has been eroding, and the culvert is now the most hardened point along the stream. (The nick point.) The perched culvert is the only thing stopping that waterfall from marching upstream. This is a big problem for fish (because it is hard to leap into a firehose and take off swimming against the flow), and a big problem for fixing the stream. Basically what you have to do then is elevate the downstream section of the stream with a series of weirs. That’s more project than I hope the reader faces.
Good luck! Send pictures!