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Curve Number Problem

Curve Number Problem

Curve Number Problem

(OP)
I am working on a stormwater runoff project in a desert area.  There are about 500 acres of mostly bare sandy desert.  If I look up a curve number from a list nothing is an exact match but urban desert with CN = 63 looks close.  However, when I use the "Available Water Capacity" from the NRCS map unit description with an S value of 1.8 inches and solve for CN in the equation S=(1000/CN)-10 I get a CN of about 85.  The difference is a max flow of 40 cfs vs 120 cfs using HEC-HMS.  

I was wondering which one would most of you choose?

Thanks
dpajr

RE: Curve Number Problem

63 sounds low. The last project I did which was in the Mohave Desert used the following:


Mountain Type C/D soil
cover type - Barren (rockland, eroded and graded land)
quality of cover - barren
CN = 92-93


Alluvial Plain Type B soil
cover type - Open Brush (creosote, sage, etc.)
quality of cover - poor
CN = 82-83


Lava Flows Type C soil
cover type - Barren (rockland, eroded and graded land)
quality of cover - barren
CN = 90

from SBCFCD hydrology manual:
http://www.sbcounty.gov/dpw/floodcontrol/pdf/HydrologyManual.pdf
 

RE: Curve Number Problem

(OP)
Thanks CVG,

I ran it both ways.  The difference is about 80cfs.  Either way the client can't build on it.  I am going with the higher numbers.  

The referenced hydrology manual sounds like the curve number descriptions are a lot more complete than anything I have come across before.

dpajr

 

RE: Curve Number Problem

DPAJR;
Where is your project site?  Does the agency have guidance?  Can you track down CNs for similar areas to verify the San Bernadino Manual cvg provided?

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