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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Also the peak rainfall intensity using the Corps' rainfall modeling would be 2.7
inches per hour for 30 minutes where Boulder City, County, and UDFCD criteria can
have five minute peak duration intensities over 8.0 inches per hour. While these
peak intensities are not likely to increase major floodplain flows significantly they
certainly would increase local basin runoff and possibly West Valley Overflows.

The Corps' rainfall modeling applies different rainfall over five zones to reflect the
wide range of elevations which we generally agree with. We would question the
raw NOAA data used at the highest elevation zone, but any error here would be of
low significance because of the routing effect of Gross.

Also, a realistic analysis of operating levels and flood hydrology effects in Gross
may reveal that Gross is even more effective in reducing the overall flood runoff
volume downstream than as modeled by the Corps. On the other hand, peak
discharges below Gross, particularly in the urban areas below Highway 93 are not
significantly affected by runoff upstream of Gross.

The Corps reduces the rainfall for the magnitude of the basin area by approximately
12.7%. This correction accounts for the phenomena that the rainfall gauge data
analyzed reports point values, and such peak point rainfall will not occur uniformly
over a watershed. We generally agree with the concept of point rainfall reduction
for the area below Gross Reservoir where modeling the effects of South Boulder
Creek spills, however, local area runoff modeling also needs to be simulated
without major area reduction effects.

The Corps also uses and Expected Probability Correction of about 4.3 to 4.7%.
This correction increases the base rainfall. In simplistic terms, probability theory
indicates that raw gauge data will tend to miss or misreport the greatest events.
We understand the Corps currently does not often use Expected Probability
Corrections. Few local jurisdictions do.

We believe it is important in the case of South Boulder Creek and the West Valley
Overflow Area to simulate both a realistic regional storm that has the correct NOAA
statistics, and a local storm, without large area correction factors. This procedure
will identify which type of event creates the worst situation at each location.
Comparisons made in Section IV indicate that different types of events control at
various locations in the lower watershed.

The current UDFCD and Boulder manuals have specific procedures for the rainfall
arrangement in the peak 2 hour period, and where this peak period should be placed
in a longer duration event. This procedure should be reviewed in future work.

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