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NRCS curve numbers and green infrastructure

  • 1.  NRCS curve numbers and green infrastructure

    Posted 05-24-2016 01:16 PM

    Hello Everyone,

    I can't remember where I heard it, but at the Congress I heard a brief mention about somebody or some task committee working on/interested in developing curve numbers for either green infrastructure or low impact development. Did I hear that correctly? Is there a group working on this? If so please let me know, and maybe provide a very brief overview of the work that is being done. Thank you!

     - Glenn

    Glenn Moglen Ph.D., P.E., F.EWRI, F.ASCE
    Professor and Laboratory Director
    Virginia Tech
    Manassas, VA 20110
    (703)361-5606 EXT 142

  • 2.  RE: NRCS curve numbers and green infrastructure

    Posted 05-25-2016 12:53 PM

    Hi Glen,

    Nashville's manual has good explanation of how they calculate adjusted curve numbers for retained volume (see section 3.2.5) https://www.nashville.gov/Portals/0/SiteContent/WaterServices/Stormwater/docs/SWMM/2016/Vol5LID/2016_FullVol5LIDManual.pdf


    Rebecca Stack A.M.ASCE
    Deputy Executive Director
    Washington DC

  • 3.  RE: NRCS curve numbers and green infrastructure

    Posted 05-25-2016 12:54 PM
    Talk to Karen Karvazy

    Sent from my iPhone

  • 4.  RE: NRCS curve numbers and green infrastructure

    Posted 05-25-2016 12:54 PM

    Hi Glenn,

    I don't know about that study but here is some relevant information. Several years ago I Developed equations and charts to evaluate allowable Curve Number reduction of impervious and pervious areas due to implementation of various LID techniques. Here is the reference to it (it should be available online):

    Zomorodi, K. 2004 “Curve Number and Groundwater Recharge Credits for LID Facilities in New Jersey”, Proceedings of the Conference “Putting the LID on Stormwater Management!” The Inn and Conference Center- Marriott, College Park, Maryland, September 21-23, 2004. Proceedings published electronically on a CD, to reach this paper on the CD first select “Program and BIOS” and then “Read BIOS and papers”. pp. 508-514.

    Kaveh Zomorodi, Ph.D., P.E., M.ASCE
    Senior Hydrologist
    Fairfax VA
    (703) 849-0336

  • 5.  RE: NRCS curve numbers and green infrastructure

    Posted 05-25-2016 12:55 PM
    Hi Glenn,
    Yes, there is a committee working on curve numbers. The contact link is
    I understand from hearsay that the 2009 publication in this link is being updated
    Also the committee is attempting to get the Dept of Ag's Soil. Conservation. Service involved.
    The best contact for reliable information is probably the committee chair, who is listed on the contact link.
    Cheryl Signs, P.E., F.ASCE

  • 6.  RE: NRCS curve numbers and green infrastructure

    Posted 05-25-2016 12:56 PM
    Hi, Glenn.  I believe that the answer is "yes".  Claudia Hoeft (NRCS) has the details.  She is probably on the collaborative email list, but I have cc'd her above just in case.  Hope you are well.  -Robert

    Robert R Mason, Chief, USGS Office of Surface Water      
    and Delaware River Master                 Phone: 703-648-5305
    12201 Sunrise Valley Drive                       Cell: 703-405-5823  
    415 National Center                                   Fax: 703-648-6693        
    Reston, VA 20192                          Email: rrmason@...

  • 7.  RE: NRCS curve numbers and green infrastructure

    Posted 05-25-2016 12:57 PM

    What are curve numbers?

    Johnny Lee P.Eng, CP, M.ASCE
    Kitchener ON
    1519 8948591

  • 8.  RE: NRCS curve numbers and green infrastructure

    Posted 06-02-2016 02:47 PM


    Curve Numbers are approximations of interception and infiltration based upon land use and soil characteristics. It's a popular rainfall-runoff methodology in the United States. The methodology was developed by the US Department of Agriculture's Natural Resource Conservation Service (though when developed it was the Soil Conservation Service or SCS). The paper which outlines the methodology and is still in use today is TR-55 (Technical Release 55) and can be found here: TR-55 Online. It's a little more difficult to implement outside of the United States as some of the input data is developed by the NRCS (Hydrologic Soil Groups being the one I'm thinking of right now).

    Hope this helps!



    Joseph Thomas P.E., M.ASCE
    Engineering Manager
    Sacramento CA

  • 9.  RE: NRCS curve numbers and green infrastructure

    Posted 05-25-2016 12:58 PM


    I suppose that there may be a number of folks working on this.

    One approach that you may be aware of is the procedure documented by the Maryland Department o the Environment (MDE) in their ESD manual. This approach is based on formulas developed by Dr. Rick McCuen back in 1983 when we were developing the Maryland Stds and Specs for design of infiltration practices.  I later adapted the approach in 2000  to estimate the effective CN value when volume based GI practices are used for LID design. I documented this approach in the Queen Anne's County,  LID/ESD design manual and also the Charles County LID/ESD design manual.  MDE liked the approach and adopted it in their ESD  guidance.  The way I do it is slightly different from MDE. I just solve the quadratic equation for an adjusted CN based on McCuen's equations while MDE prefers to use tables and interpolation, which I find to be cumbersome and less accurate.

    I can provide a copy of the documentation to supplement the description provided in the MDE ESD manual.

    Michael Clar, P.E., DWRE

    Chair, LID Committee

    Ecosite, Inc.

    Ellicott City, MD 21042



  • 10.  RE: NRCS curve numbers and green infrastructure

    Posted 06-01-2016 11:13 AM

    Michael -

    Would you be willing to post for all of us the documentation supplement to the MDE ESD manual you mentioned in your reply?


    Jeffrey Rice P.E., P.Eng, M.ASCE
    Water Resources Engineer
    Langley WA

  • 11.  RE: NRCS curve numbers and green infrastructure

    Posted 05-25-2016 12:58 PM

    I was at a state stormwater conference in March were there was discussion on changes to the SCS method.  Here are some notes I took from the conference (nothing specific regarding CNs and green infrastructure though):

    • NRCS revisions to the SCS method
    • Nrcs has revised the method and testing it on sites less than 2 square miles
    • SCS Type II is officially dead
    • SCS is better for design storms but not as great if you are trying to model historical storms
    • SCS not great for longer, low intensity storms
    • For the revised
      • winTR-55 has new method…download
    • Distribution
      • Nrcs used altas 14
      • Nrcs established 6 different distributions…michigan has only 2 of the 6…#3 and #4…our areas will #3 mostly
      • Distributions are pretty similar…the new #3 is actually more intense than SCS type 2. example from 0.14 to 0.17 between scs to #3. Appears to be about 10% difference in #4 area (with #4 10% lower).

    Perhaps downloading the WinTR-55 and playing around with the CNs will answer your question...???

    WinTR-55 Watershed Hydrology | NRCS

    Jennifer Morreale P.E., M.ASCE
    Project Engineer
    Hubbell Roth & Clark
    Detroit MI
    (248) 454-6828

  • 12.  RE: NRCS curve numbers and green infrastructure

    Posted 05-25-2016 01:06 PM

    I am part of a Standards Committee working on the ASCE Manual "Design, Construction, and Maintenance of Permeable Interlocking Concrete Pavement".  Attached is DRAFT text on defining CN for this type of pavement system; any feedback is appreciated.

    Glenn Herold P.Eng, M.ASCE
    Director Commercial Solutions
    Cambridge ON