To determine how much water a swale can hold, first find its volume in cubic feet. Total cubic feet is then multiplied by 7.48 (the number of gallons in a cubic foot), to determine how many gallons of water the swale can retain.
Because the shape of a swale is basically half an oval cylinder we plug its dimensions into the formula for the volume of an oval cylinder and take half of that number (divide by two). *Note: if the width and depth of the swale are the same, use the formula for volume of a regular cylinder).
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
Next, because the swale is actually only half of an oval cylinder, you would divide by 2; your answer is in cubic feet.
Multiply your answer by 7.48 (the number of gallons held by 1 cubic ft). This will give you the total gallons of water your swale can hold.
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