Hi, I'm doing a research project on swales for a long and narrow property and am trying to figure out the depth and width the swales should have. I've read the swales section in Rainwater Harvesting for Drylands and Beyond Volume 2 by Brad Lancaster, but that doesn't answer my question. Basically the property is 1300m in length, with a slope of around 3˚and a field width of 250m. It's a pretty bare, sandy, stony soil and doesn't get much rainfall, but usually once a year there is a heavy precipitation event that can be up 100mm in one go. I'm looking at putting 4-5 swales in, for example under one plot that's 100m in length and 250m in width. How big should this swale be? All the literature I've read so far seem to be for small swales, but am I right in thinking these would need to be very wide and deep, like several meters? Thanks for the advice!
I would say that you should dig your swales wide and deep enough so that they capture 100% of the largest rain event that you might experience in a normal year. Since the grade on your slope isn't very steep, you could dig swales that are quite wide. Obviously, the wider and deeper they are, the more water they will hold.
One inch of rain falling on 1 acre of ground is equal to about 27,154 gallons. There are 7.48 gallons in one cubic foot. Thus, you need 3630 cubic feet of "storage" in your swale to capture an inch of water per acre.
But that stat assumes that 100% of the water that falls will accumulate in the bottom of the swale, which ignores the fact that with infiltration, a significant percentage of rain will soak into the soil profile long before it washes down to the bottom of the swale. The healthier your soil, the faster water will infiltrate. If you've got a cover crop growing, mulch on the soil surface, and you practice no-till, I would imagine that even in a heavy rain event of 100mm (as you stated -- almost 4 inches), you'd see at least half of that soak in quickly.
So . . . lets assume that for a massive rain event of 100mm/4 inches, you need about 7000 cubit feet of rainwater "storage" per acre; a swale with a 7000 cubic foot storage capacity.
250 meters long (820 feet) x 2 meters wide (6.5 feet) x .5 meters deep (1.5 feet) would yield a storage capacity of about 8000 square feet, or enough to absorb 60,000 gallons of water. You'd need to figure out how far apart to space such swales. Obviously, if you didn't want to build such a deep swale, then just make more swales that are not as deep, fitting them closer to one another.
Hope that helps.
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