Curve Number Problem 1

[DPAJR]

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

 

[cvg]

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

 

[DPAJR]

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

 

[gbam]

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?