Ocean Friendly Gardens
January 22 2010

Calculate Rainwater Harvesting Potential & Area Needed To Absorb It

Objective: To Capture and Clean The “First Flush”
The “First Flush” is the first ¾”-1” of rain after a dry spell, when the majority of pollutants are flushed from a hard surface (e.g., roof, driveway, pathway) into the street, storm drains and ocean.

Simple Rainwater Harvesting Formula - For every 1” of rain and 1,000 square feet of impermeable surface (roof, driveway, etc), about 620 gallons are generated. Meaning, you can capture about .62 gallons per sq ft. To figure out the square footage of a surface, multiply the width x length. The square footage of a roof is the same whether it is pitched or flat.

Permeable Area Needed to Absorb the Rainwater
Assuming a depth of 12" to your absorption area, divide the total gallons captured by 7.48 (gallons in a cubic foot of area). For example:

620 gals/7.48 = 83 cu. ft. Double that size if it's 6" deep.

Example roof and and design:

400 square foot roof, draining to 1 raingutter downspout

  • 400 sq foot roof x .62 = 248 gallons
  • 248/7.48 = 33 sq. ft. at 12" of depth, or 66 sq. ft. at 6".

12" depth - A basin or dry stream bed that is approximately 6 feet by 6 feet, or a dry stream bed that is 3 feet wide x 11 feet long.

6" depth - Double the size.

For Roofs Without Raingutters and Downspouts - slope the soil underneath the roof toward the landscape, directing it to a catchment area like a dry creekbed or allow it to spread throughou the landscape. For further design details, see the "Retention" section of the OFG sign criteria.

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