Watershed

How much water should my drain flow?

Watershed refers to the amount of water flowing into the drain. We use watershed assessments to be sure we are creating a drain that will handle the water flow without overflowing. Watershed assessment is pretty straight forward. You determine a desired maximum rainfall per hour that you would like to handle. For Yarddrain.com that standard number is 2 inches of rainfall per hour. 2 inches of rainfall per hour creates 1.3 gallons of rainwater per hour. We take an aerial measurement of the watershed. This includes any rooftops that flow into the drain area. We then multiply the square footage by 1.3 and this gives us the needed gallons per hour of flow for the drain. Using Manning’s formula to determine what size pipe and at what slope it should be installed to achieve the desired flow rate. This can be done for any rainfall rate just remembering that 1 inch of rainfall per hour equates to 0.65 GPH per square foot.

the formula we use is: Square footage X Inches of rain per hour X 0.65 = Gallons per hour watershed