FLOOD10617 ‎(2)‎

• County: Boulder
• Community: Boulder County and Incorporated Areas
• Title: FIS - Boulder County and Incorporated Areas - Vol 1
• Date: 10/4/2002
• Prepared For: Boulder County
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

USGS (Reference 32) I was used to obtain peak discharges on the
upstream portion of Little James Creek and Unnamed Tributary to
Little James Creek.

~drologic data for flooding sources affecting the City of
Lafayette and the Town of Superior are based on the data generated
for the October 1976 study of Coal Creek and Rock Creek, performed
by the ses (Reference 19). Since there are no stream flow records
for Coal Creek and Rock Creek, the SCS used synthetic
rainfall-runoff procedures to dete~ne the flows for various
frequency storms. Analyses were based on a storm duration of
24 hours I Type II and IIA distribution, as described in the SCS
National Engineering Handbook, Section 4 (Reference 33). The
amount of rainfall was obtained from the precipitation frequency
atlas (Reference 34), and areal adjustment was applied to convert
the point precipitation values to average precipitation over the
watershed area. The studies were conducted using runoff
computations based on information regarding the type and location
of existing and planned land use provided by the BeS (Reference 5).
~drologic soil cover camplexes and associated Runoff Curve NUmbers
were extracted from the SCS reports (Reference 14) and field
checked. Values of 10-, 50-, 100-, and SOO-year peak discharges
were obtained using the SCS computer programs WSP2 and TR20
(References 35 and 361 respectively).

Frequency-discharge data for two of the streams studied in detail
in the City of LODgment are based on info~tioD published in USACE
Flood Plain Info~tion reports for Lefthand and St. Vrain Creeks
(References 7 and 8, respectively). The lOO-year flood discharges
on Lefthand Creek and St. vrain Creek are 4,250 cfs and 10,200 cfs,
respectively. The SOC-year flood discharges for these two streams
equal the discharges for the standard project floods as published
in the Plood Plain ~Dfo~tioD reports (References 7 and 8). These
relationships are based on a Log-Pearson Type IX~ analysis of peak
runoff data recorded at gages on St. Vrain Creek near Lyons and
Platteville, Colorado (Reference 37). The years of record vary
from 79 years at the Lyons gage to 47 years at the Platteville
gage.

Discharge-frequency relationships for Dry Creek No. 1 and Spring-
Gulch were computed using the USACE HEC-l Flood Hydrograph Package
(Reference 38). Synthetic flood hyrographs computed by this method
reflect the effects of characteristics of the
basin: precipitation, ground cover, slope, drainage area, and
other physical characteristics of the drainage basin. Where
available, hydrologic data were compared with other studies
completed in the area (References 39 and 40). The effects of
detention. storage near State Highway 66 and at Long" Peak Dam OD
Spring Gulch were studied (Reference 39) and found to be
insignificant for the magnitude of the floods considered in the
study. That portion of the Spring Gulch Basin located north and
east of Terry Lake is considered to be containted completely by
Terry Lake.

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