FLOOD01787

• County: Boulder
• Stream Name: South Boulder Creek
• Title: South Boulder Creek/ Flatirons
• Date: 12/5/1997
• Prepared For: Boulder
• Prepared By: Consultants
• Floodplain - Doc Type: Community File

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The area and rainfall discrepancies appear to warrant a reevaluation of South
Boulder Creek hydrology and perhaps other watershed hydrology by the Corps that
is still being used. such as Boulder Creek.

CORPS' MODELING CALIBRATION AND SENSITIVITY TESTS

From review of the information provided, it is apparent that the Corps' work was
strongly oriented toward calibration to a Boulder Creek gaging station two miles
upstream from its confluence with the St. Vrain. The tributary area to the
confluence was reported as 440 square miles. The gaging record encompassed 28
years, from 1927 to 1949 and from 1951 through 1955. The Corps conducted a
statistical analysis of this data using procedures outlined Bulletin No. 1 7 of the
Federal Water Resources Council and corrected for expected probability. The
watershed SWMM model and lower basin stream routing modeled was apparently
"calibrated" to result in a discharge frequency curve similar to the statistical data, as
evidenced by a comparative plot of the model results.

Some of the key parameters settled upon by the Corps in comparison with values
typically used in UDFCD and Boulder manuals are given in Table 111-1. As stated
before, the rainfall used by the Corps is low for the 1 hour and shorter incremental
peak rainfall, leading to an under prediction of peak flows.

The use of steady state infiltrations of 0.5 inches per hour in urban areas and 1.0
inches per hour in the mountains, the significant increase in urban percent
impervious that has occurred, and the rather unusual parameters used for
depression storage, may result in somewhat compensating effects.

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