Locating 2-year, 24-hour rainfall intensity values for EBOM SSc6

Content Scope: 
Use Tabbed Form
Bird's Eye view

Start by using NOAA’s data

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.

Hydrometeorological Design Studies Center

For blue states

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. 

For gray states

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.

Web Tools

NOAA's Hydrometeorological Design Studies Center Precipitation Frequency Data Server

This is the best resource we've found for finding rainfall intensity data for LEED-EBOM SSc6: Stormwater Management.

6 Comments

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American University Sustainability American University
Sep 27 2012
LEEDuser Member
735 Thumbs Up

"Average rainfall event" for calculting volume captured

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.

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Bradley Buck Dec 15 2012 Guest 5 Thumbs Up

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.

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mark cunningham
Oct 26 2010
Guest
37 Thumbs Up

Narriate on the stormwater management strategies

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.

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Tristan Roberts LEED AP BD+C, Editorial Director – LEEDuser, BuildingGreen, Inc. Oct 29 2010 LEEDuser Moderator

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.

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John Klein Sr. Architect HDR Engineering
Jul 18 2010
Guest
273 Thumbs Up

Trying to locate 2yr, 24hr rainfall info for Central America

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?

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Tristan Roberts LEED AP BD+C, Editorial Director – LEEDuser, BuildingGreen, Inc. Jul 27 2010 LEEDuser Moderator

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.

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