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

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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. Click the state where your project is located, or choose the state from the drop-down menu. In this example we’ve selected Illinois.

Step 2. Choose “precipitation intensity” under Data Type

Step 3. Select location by either entering the latitude and longitude of your property, selecting a weather station close to your property, entering an address, or clicking on the map. When clicking on the map, you can either move the red crosshair or double click to set the location.

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 4. 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. Click the state where your project is located, or choose the state from the drop-down menu. In this example we’ve selected Oregon. 

Step 2. Download the document listed under the “1 hour – 24 hour” heading. In the example below for Oregon, this is NOAA Atlas 2, Volume 10, which provides data for a range of precipitation frequencies. 

Step 3. The downloaded document includes the isopluvial map that shows the 2-year, 24-hour rainfall quantity. Be careful – the map may show this quantity in inches, or in tenths of an inch. Be sure to convert values to inches so that your calculation is accurate.

In the Oregon example, page 33 of the document (30 in a PDF reader) provides the correct isopluvial map for the 2-year, 24-hour rainfall in tenths of an inch.

Step 4. Find your project location on the map, and interpolate the 2-year, 24-hour rainfall value.

The curved contour lines on the map represent a rainfall value; any point located directly on a line receives that specific amount of rainfall over the 24-hour storm.

In the example below, if the project is located in Albany, Oregon, it falls on the the 25 tenths of an inch line, which equals 2.5 inches.

For an example project located in Lebanon (just south of Albany), the location falls between two lines (the 25 tenths of an inch line and the 30 tenths of an inch line). Therefore, the rainfall value will be between 2.5 and 3 inches. Based on the location, the rainfall value is estimated to be 2.7 inches for the project site.

When it’s necessary to estimate the rainfall value for a project, the more conservative approach is to use a higher rainfall value. While this approach is not perfectly accurate, it can help you demonstrate LEED compliance while saving you time. Remember that the rainfall intensity 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 5. Divide the total rainfall over the storm by 24 hours to determine the average intensity over the course of the storm.

For the Albany example: 2.5 inches / 24 hours = 0.10417 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.

9 Comments

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Jean Marais b.i.g. Bechtold DesignBuilder Expert
Jul 07 2017
LEEDuser Member
11145 Thumbs Up

International 2-year, 24-hour rainfall intensity values

Does anybody know of a common resource for international rainfall data with the granulanity to produce duration, depth curves needed?

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American University Sustainability American University
Sep 27 2012
Guest
1591 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 42 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
72 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, Executive Editor – 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
367 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, Executive Editor – 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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Samantha Longshore Sustainability Advisor, Transwestern Apr 21 2014 LEEDuser Expert 674 Thumbs Up

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.

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Meghan Ward Architectural Technologist, Sustainable Design Consultant, Aurecon South Africa Jan 29 2015 Guest 232 Thumbs Up

Hi Samantha,

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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Aug 22 2017
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