PROPOSED REVISION OF NRCS CURVE NUMBERS · PROPOSED REVISION OF NRCS CURVE NUMBERS CWEA Storm water...
Transcript of PROPOSED REVISION OF NRCS CURVE NUMBERS · PROPOSED REVISION OF NRCS CURVE NUMBERS CWEA Storm water...
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PROPOSED REVISION OF NRCS CURVE NUMBERS
CWEA Storm water Committee MITAGS
Linthicum Heights MDDec 2017
Donald WoodwardCo‐Chair ASCE‐NRCS Task Group
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• In 1989 ARS/SCS group was established to • Determined if there was data available to reestablish the original documentation.
• If there was data available to establish curve numbers to develop curve numbers for
• the cultivation new practices.• Regional values• Monthly values
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• In 2004 the work group report indicated• The Ia/S should be less than 0.2• The asymptotic method should be used to determine CN• There is limited data to support regional curve numbers• There is some data to support monthly curve numbers
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• Given the findings of the ARS/SCS curve number work group the next step was the revision of the various chapter of NEH‐630 Hydrology
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• Natural Resource Conservation Service (NRCS) signed a Cooperative Agreement with the American Society of Civil Engineering (ASCE) in Sept 2105.
• An ASCE Task Group was formed.• With the expressed purpose of developing proposed revisions to Chapters 8, 9, 10 and 12 of NRCS National Engineering Handbook Part 630, Hydrology. (NEH 630)
• The effort involved engineers, researchers, and scientists from across the United States all outside NRCS.
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• The CN Task Group provided NCRS via ASCE drafts of each chapter by the end of September.
• It is my understanding the copies of the chapters furnished NRCS are available on the ASCE Collaborate internet site.
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CHANGES
• Ia =0.05S from Ia =0.20S• This is based on the analysis of data from over 300 small watersheds in the United States and research from other countries
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CHANGES
• A system or equation was developed to convert the CN20 to CN05values.
• The conversion equation is CN05 = CN20/(1.42‐0.0042CN20
• The original CNs can not be used with the new runoff equation
• Q = (P‐0.0S05)2/(P+0.95S05) for P>0.05S05, Q = 0 otherwise
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CHANGES
• The method to determine Curve Number (CN) from rainfall runoff data was modified in two ways. The first way was how to select the data to plot
• Use of ordered or frequency based data rather than natural data.• Ordered data is matching the largest runoff with the largest rainfall• Natural data is matching runoff with the causing rainfall
• This is the way the initial curve numbers were determine
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CHANGES
• The second change is how to obtain the CN for a particular soil‐cover complex
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CHANGES
• The current method is to plot P versus Q • The annual events using natural were used.• The median value was selected.
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226 Ordered and Natural rainfall-runoff events for a 11.3 acre ARS watershed located at Riesel, TX (WS42040, 1969-1981)
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P (INCHES)
Q (I
NC
HE
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P = QNATORD
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CHANGES
• The proposed method is to plot CN versus P for all events• Either ordered or natural events can be used.• An analysis indicated that there was not a significant difference between order or natural data in the computed CN.
• The Technical Group (TG) recommends using ordered data.• This procedure is known as the asymptotic approach
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Coshocton, Ohio#26004 pasture
1939-1986
6065707580859095100
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P (in)
CN
CNORD CNO CNfit CN inf
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CHANGES
• An equation has been developed to define the curve shown in the prior slide
• The equation has the form• CN(P) = CN∞. + (100‐ CN∞)exp(‐kP)• Which is normally fitted by trial and error• The new chapter 9 provides detail procedures for determining k• This procedure tends to make the selection of the representative CN very easy.
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CHANGES
• Analysis of many watersheds has lead to the conclusion that there are basically three types of watershed based on their responses.
• Complacent
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Complacent
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Cur
ve N
umbe
r 05
Rainfall P (in)
CNo=100/(1+2P)
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• In a complacent watershed the major source of runoff is the channel flow or it has very depth porous soils
• There is no apparent asymptotic value
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Standard
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• These are the normal Midwest agricultural watersheds that were investigation in the beginning.
• These watersheds occur in many places in the world.
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Violent
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• These watershed have an initial response like classic watershed or even a complacent watershed and then major runoff occurs.
• These would be shallow soils with impervious sublayer• Such as
• Green roofs• Porous pavement• Steep forested watershed with a shallow soil profile
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CHANGE
• The chapters will add information on• Forested watersheds
• Curve Numbers do not work in some cases• National Land Classes Data (NLCD)
• Curve Numbers have been added for a nation wide classification system• How to determine Curve Numbers for all three types• Selected Urban Curve Numbers have been added
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CHANGES
• It has been recommended that the chapters be reordered• Chapter 10 curve number science• Chapter 9 actual curve numbers• Chapter 8 land use and class information• Chapter 12 Influences of and treatment
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• It is my understanding the NRCS will complete the field review of the proposed chapters by Feb 2018
• The next step will to decide if the chapters should be used with NRCS comments or rejected.
• If accepted it will require revision of the standard NRCS computer programs Win TR‐20, WinTR‐55, EFM2, SITES.
• Other computer programs using the new CN procedure will also need to be reprogramed to use the new equations and CN values.