The best resource we've found for rainfall intensity data is NOAA's Hydrometeorological Design Studies Center Precipitation Frequency Data Server.
Start by choosing your state from the U.S. map. Depending on your state, there are two different step-by-step processes. Blue-colored states allow you to search the database, and provide easy-to-read data tables. Gray-colored states require you to determine the value for your site by reading isopluvial maps. We’ll explain both, step by step.
Step 1. Choose “precipitation intensity” under Data Type
Step 2. Select location by either selecting a data site close to your property from the dropdown list, entering the latitude and longitude of your property, or clicking on the map.
The below example shows use of the drop-down list for a project site near Chicago’s Midway Airport, where a climate station is located. Entering coordinates or clicking on the map allows selection of precise locations for project sites that are not near to the climate monitoring stations listed in the drop-down menu.
Step 3. In the Precipitation Intensity Estimates (in/hr) table, find the 2-year, 24-hour intensity. Enter this value into the LEED Online credit form.
Step 1. After choosing your state, download the document listed under “1 hour – 24 hour.”
In the example below for Wisconsin, this is Technical Paper 40, which provides data for a range of rainfall frequencies of 1–24 hours.
Step 2. Locate in the downloaded document the isopluvial map for your state that shows 2-year, 24-hour rainfall in inches.
In the Wisconsin example, page 51 of the document (55 in a PDF reader) provides an isopluvial map of the U.S. that provides the needed information.
Step 3. Find your project site on the map, and interpolate the 2-year, 24-hour rainfall value (inches).
The curved contour lines on the map represent a rainfall value; any point located directly on a line receives that specific amount of rainfall in total inches over the 24-hour storm.
In the example below, the project site is located in Dane County, Wisconsin, and falls between the 2.5- and 3-inch lines. Because the site is not quite at the midpoint between the two lines, the rainfall value will be between 2.75 and 3 inches. The interpolated value is determined to be 2.8125 inches for the project site.
In the event that interpolation is not easily performed for a given project site, assuming a higher rainfall value is the more conservative approach and may allow you to show LEED compliance while saving you time in performing the calculations. For example, assuming a rainfall value of 3 inches per 2-year, 24-hour storm for the Dane County project site is not perfectly accurate, but uses a conservative assumption because this value primarily factors into determining the portion of a rainfall event that can be managed by onsite collection facilities such as cisterns and detention ponds. For projects that exclusively use pervious surfaces to management stormwater through infiltration, the rainfall intensity value will not affect the credit outcome.
Step 4. Divide the total rainfall over the storm by 24 hours to determine the average intensity over the course of the storm.
In this example: 2.8125 inches / 24 hours = 0.1172 inches / hour
Enter this value into the LEED Online credit form.
This is the best resource we've found for finding rainfall intensity data for LEED-EBOM SSc6: Stormwater Management.
Can someone help me with how to determine the total site area for calculations on a lot with an existing residence, and a future LEED certified accessory dwelling.
How do I determine how much of the total lot to use in calculations for credits like rainwater mgmt., heat island reduction, etc.
I am trying to determine what numbers to use for the "average rainfall event" and "rainfall event interval" to calculate the volume captured via our collection facilities. Since the volume of runoff is calculated by using the 2 year 24 hour rainstorm INTENSITY (inches/hour), should I use the 2 year 24 hour rainstorm PRECIPITATION DEPTH (inches) for the "average rainfall event"? If so, what, then, is my rainfall event interval? 24 hours? 2 years? I need to convert this to seconds to calculate the minimum drawdown rate, but don't know what the interval is. With real numbers, the 2 year 24 hour intensity is 0.13 in/hr and the 2 year 24 hour depth 3.14 inches.
Based on the research I've done and the example calculation within SSc6 in the Reference Guide 2009 Ed., Updated April 2010, this is based on the 'watershed type" your site is located within. The 'average rainfall event' is based on the 90th percentile of average rainfall within the type of watershed. There are three watershed types: humid (1" / >40" annually), semi-arid (0.75" / 20-40" annually), and arid (0.5" / <20" annually). The design storm figures you provided are used instead for Equation 3: Determining Rainwater Runoff.
Would someone be willing to share their narriate on the stormwater management strategies in place, including a summary of how and to what extent infiltration, harvesting or evapotranspiration strategies contribute to the mitigation volumes reported? I would never copy and paste the narriate I'm just having a hard time getting started on this. I know someone eles' example would help me create my own. Thanks.
Mark, you may find our Documentation Toolkit for SSc6.1 to be helpful. Although it doesn't have exactly what you're looking for, there is stormwater management outline that can provide a start.
I am working on a Federal facility project in Honduras that we are designing to LEED Silver. Trying to qualify for Credit SS6.1, but have been unsuccessful in finding rainfall data. Has anyone located a source for this information for Central America?
Michelle, may I suggest that you re-post this question to the NC SSc6.1 forum? You might have better luck there with people working on this credit seeing your question.
I have a similar dilemma. I’m working on a project in Russia and having a difficult time finding much information outside of typical monthly rainfall. I've seen a discussion on the NC forum regarding a suggested approach for an international project, but I'm wondering if anyone has a go-to calculation to help determine a 2 yr, 24 hr rainfall intensity if this hasn't been calculated for a particular region.
Did you manage to figure out the solution to your above dilemma? We are facing the same dilemma with a project in South Africa.
Please let me know if you figured out a way to get international 1yr and 2yr storms.
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