How to Calculate Swale Capacity for Water Retention?

Measuring Swale Capacity

To calculate swale capacity, start by measuring the surface area of the swale in square feet or square meters. For example, a swale with a width of 3 feet (0.91 meters) and a length of 50 feet (15.24 meters) has a surface area of 150 square feet (13.9 square meters). Next, determine the depth of the swale, which is the distance from the surface to the bottom of the swale. This can range from 1 to 3 feet (0.3 to 0.9 meters) depending on the design.

Calculating Water Retention

Once you have the surface area and depth of the swale, you can calculate the water retention capacity by multiplying the surface area by the depth and the infiltration rate of the soil. A typical infiltration rate for soil is 0.1 to 0.5 inches per hour, which can be converted to feet per hour by dividing by 12. For example, an infiltration rate of 0.25 inches per hour is equivalent to 0.021 feet per hour. If you assume a swale depth of 2 feet (0.6 meters), the water retention capacity would be 150 square feet x 2 feet x 0.021 feet per hour = 6.3 cubic feet per hour.

Accounting for Contour Lines and Slope

When designing a swale, it’s essential to take into account the contour lines and slope of the land. Ideally, the swale should be built on a gentle slope, with a contour line that follows the natural curve of the land. This will help to slow down the flow of water and increase the water retention capacity of the swale. If the land is too steep, you may need to build a berm or a series of berms to slow down the flow of water and prevent erosion.