Stormwater Management

How to calculate run off on your site, and rainwater storage

Patrick Chopson avatar
Written by Patrick Chopson
Updated over a week ago

Stormwater/Rainwater use and management is derived using local rainfall data, the run-off coefficient for each surface on site, the total area of each surface type, and the amount of onsite stormwater storage.

  1. Rainwater Storage (ft³): the total volume of onsite storage, either a cistern or a retention pond. To manage 100% of onsite stormwater, the user would need to size a collection unit which can hold return 100% of total runoff.

  2. Impervious Area (ft²): land coverage that repels rainwater and does not permit moisture to infiltrate ground. This term typical refers to surfaces found in urban and suburban landscapes such as roads, parking lots, driveways, sidewalks and roofs. The runoff coefficient for Impervious surfaces is 0.9.

  3. Turf Area (ft²): grassed soil, also known as natural land coverage held together by its roots. The runoff coefficient for Turf is 0.2.

  4. Native Plants Area (ft²): natural land coverage of indigenous vegetation. The roots of native species increase soil strength and stability, as they are typically drought tolerant and over the most beneficial plants life for the environment. Runoff coefficient for Native Plants surfaces is 0.05.

  5. Semi-Previous Area (ft²): land coverage that permits moisture to infiltrate ground. Previous pavement is the most common semi-previous strategy as it allows percolation or infiltration of stormwater through to the soil below. Runoff coefficient for Semi-previous surfaces is 0.5.

Other variables being considered to derive stormwater management include:

  1. Roof Area (ft²): Total roof area of the building. Runoff coefficient for Roof surfaces is 0.9. Roof surface's are included in the final calculation of total onsite runoff before storage.

  2. Rainfall Data - 24H 2Ye Event (in): Also known as Rainfall intensity, refers to the maximum amount of precipitation that will occur in a 24 Hour event on the average of once in 2 years. 2yr 24h Storm Events information can be found at the NOAA's (National Oceanic and Atmospheric Administration) NCEI Online Database here.

Calculation Method and Results

The Stormwater Management calculation methodology follows AIA's COTE TopTen Toolkit: Measure 4 - Design for Water tool, section 3 - Stormwater managed on-site, and (BD+C) LEED v4.0 SS c4 - Rainwater Management credit.
The two stormwater related results are Stormwater Managed Onsite % and Total Runoff. Total runoff is how many gallons of rainwater per year will not infiltrate the ground or get collect in onsite storage. Some precincts require that 100% of rainwater is manage onsite utilizing some of the retention strategies discussed above. The Stormwater Managed Onsite % tool is therefore calculating 1) how much annual rainfall infiltrates the ground, 2) how much rainfall can be collected in onsite storage, and 3) what percentage of rainfall has been managed because of the design of those two systems.


  1. What is a Runoff Coefficient?
    The runoff coefficient (C) is a dimensionless coefficient relating the amount of runoff to the amount of precipitation received. It is a larger value for areas with low infiltration and high runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flatland).

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