In order to capture and store water on the Khare property, part of their permaulture design will include swales. To figure out exactly how much water a swale will hold, the volume of the swale needs to be calculated. The volume of the swale will determine how many cubic feet of water can be held. Cubic feet can then be multiplied by 7.48 (the number of gallons in a cubic foot), to determine how many gallons of water the swale can capture and store.
The easiest way to do this is to apply the formula for the volume of a oval cylinder; because the shape of a swale (unless it has the same width and depth) is basically half an oval cylinder. (If the width and depth were the same, you would use the formula for volume of a regular cylinder).
So, to determine the volume of a swale, you would plug its dimensions into the formula for the volume of an oval cylinder and take half of that number (divide by two).
The formula for volume of an oval cylinder is:
Pi x length/2 x width/2 x height
*length= Swale’s Width, width = Swale’s Depth x 2, and height = Swale’s Length
So, determining the volume for a swale with a width of 3 feet, depth of 1 ft, and length of 20 ft would be calculated the following way.
Pi x 3ft/2 x 1ft(2)/2 x 20ft = 94.24 cubic feet
Then, because the swale is actually only half of an oval cylinder, you would divide by 2.
94.24 cubic ft / 2 = 47.12
One Cubic Foot will hold 7.48 gallons of water, so you multiply the total number of cubic feet by 7.48 to get the total number of gallons the swale will hold.
47.12 cubic ft x 7.48 gal per cubic ft = 352.45 gallons
*If you wanted to design swales large enough to catch all the run-off from a rain storm , you would first need to calculate the surface area of the land that will drain into the swale. You would then multiply the square feet by 1/12 to calculate the cubic feet of water (that would fall onto that surface area) from a 1-inch rainstorm. *If storms in your area drop more or less than one inch or rain, substitute the typical amount. (For example 2/12 for 2 inches, and .5/12 for half an inch).
The surface area of land that will drain into the Khare’s swales is approximately 12700 square feet. That would mean that just .2 inches of rain would produce 1583 gallons. See the following calculation:
12700 sf x .2/12 inches of rain = 211.66 cubic ft of rain
211.66 cubic ft of rain x 7.48 gallons per cubic ft = 1583 gallons
One swale (as calculated previously) with a 1 ft depth, 3 ft width, and 20 ft length will hold 352.45 gallons of water. To determine how many swales of this size you would need to catch all the run-off from a .2 inch rain over a 12700 sf area, you would divide the total gallons produced by the number of gallons each swale will hold:
1583 gallons / 352.45 gallons per swale = 4.49 swales (which you would round to 5)
The swales I’m designing for the Khare property are not intended to catch all possible rainfall; existing trees and structures–along with the fact that this area will have regular foot traffic) will not allow for enough swales to provide this type of catchment. Short, fish scale swales will wrap around existing obstacles to reduce storm water run-off and help to prevent marsh-like conditions on the easternmost portion of the property, while allowing water to penetrate the soil and feed surrounding vegetation.