Applicability of CUHP in Mountainous Terrain

[old and gray]

The Town in which I have worked for many years (located at 7500 foot
elevation on the front range) has recently had a master drainage study
done for selected basins within the Town using the CUHP/SWMM. The
intent of this study was to determine the criteria for regional
detention/water quality facilities to which upstream property owners
would have to adhere. I now have a project with a client within one
of those basins who wishes to develop a parcel of land (industrial).
The Town insists that I do this work using the CUHP. I have never
felt comfortable using the CUHP in our location because, in my opinion
(and what I have read in the CUHP manual) the procedure is not
applicable. The study which was done has basins with slopes as steep
as 0.19 feet/foot and land covers consisting of rock outcrops, typical
mountainous conifer and shrub/grassed land cover and land use ranging
from very low density residential to industrial. I have throughout
the years used SCS TR-20/TR-55 procedures as more applicable.

Since I ultimately will have to "seal" this study as a P.E., I would
be very interested in comments from those more experienced with the
CUHP than I, as to applicability of the CUHP in this application and
if it were to be used any idea as to the potential errors which might
be introduced if used and not applicable. I need to determine if I
will even do the study. Thanks much in advqnce for any comments you
might give me.

[Ben-UWLLC]

Should not be a problem and should give as reasonably accurate results as
any other method if proper 1-hr rainfall depth and initial/final
infiltration rates are used. In fact, I have seen much bigger errors when
using the TR-20/TR-55 procedures which have no calibration behind them in
this region. My observation has been that when using CUHP in the higher
elevations often the infiltration rates used are too low as the soils can be
very absorptive in their natural state until fully saturated. This is not
the case in burn areas.

Do not forget to correct the slopes per instructions in the user's manual
before applying the slope weighted equation for finding the effective slope
for each catchment (i.e., do not use the slope you get by dividing the
difference in elevations by the length).

Ben

[Ken MacKenzie]

Sent on behalf of Dr. Guo, UCD:

According to the USGS study, flood flows from the watersheds below 8000 feet
are dominated by rainfall runoff, not snowfall runoff. CUHP is a rainfall
runoff model. The UDFCD leads the effort to maintain CUHP's consistency and
accuracy. The SCS method needs a detailed calibration before a regional
consistency and user's guidance can be developed.

TR 20-SCS was developed for rural areas, and TR 55-SCS was extended into
urban areas using peaking factors and adjustments to CN. CUHP applies a
similar approach of peaking factor to model urban flows. CUHP was calibrated
for waterway slopes up to 12%. Any waterways steeper than 4% need an
adjustment. In fact, any waterways having an invert slope >10% will carry
roll waves. This is more a hydraulic problem rather than a hydrologic
problem. In practice, we separate hydrologic approaches (to calculate the
channel flow Q) from hydraulic approaches (to calculate the channel depth,
D). I am not aware of any watershed model that was calibrated for steep
mountain slopes. The closest one might be HMS with Dam Break analysis using
very steep slopes. Good luck.

Dr. James C.Y. Guo, Prof, PE, Director of H&H Program
Civil Engineering, U. of Colorado Denver
Denver, Colorado 80217 (303-556-2849)
Guo's Webpage - Ctrl key+Click to read
http://www.UCDenver.edu