FLOOD11726

• Designation Number: 77
• County: Boulder
• Community: Longmont
• Basin: South Platte
• Title: FIS - Longmont
• Date: 1/1/1977
• Designation Date: 7/27/1977
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

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There have been no flood protection measures taken on Dry
Creek or in the Loomiller Basin, except for using
Loomiller Pond as a flood storage facility. Various
properties along the streets in Loomiller Basin have
low retaining walls along the street frontage to prevent
yard flooding.

In addition to the physical flood prevention measures
mentioned, the City of Longmont initiated an ordinance
in May of 1973 for flood plain regulation and has fol-
lowed with flood plain management policies outlined in
The St. Vrain Comprehensive Plan (Reference 1).

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3.0 ENGINEERING METHODS

For flooding sources studied in detail in the community,
standard hydrologic and hydraulic study methods were used to
determine the flood hazard data required for this study.
Floods having recurrence intervals of 10-, 50-, 100-, and
500-years have been selected as having special significance
for flood plain management and for flood insurance premium
rates. The analyses reported here reflect current conditions
in the watersheds of the streams.

3.1 Hydrologic Analyses

Hydrologic analyses were carried out to establish the
peak discharge-frequency relationships for floods of the
selected recurrence intervals for each stream studied
in detail in the community.

Frequency-discharge data for two of the streams studied
in detail in the City of Longmont are based on informa-
tion published in the U.S. Army Corps of Engineers Flood
Plain Information Reports for the St. Vrain Creek and
Left Hand Creek (References 3 and 4). The 100-year
flood discharges on Left Hand Creek and the St. Vrain
Creek are 4,250 (cfs) and 10,200 (cfs) respectively.
The 500-year discharges for these two streams equal the
discharges for the standard project floods as published
in the Flood Plain Information Reports (References 3
and 4). Ten-year and 50-year discharges for these two
streams were taken from the area versus discharge curves
in Figure 10. These relationships are based on a
log-Pearson Type III analysis of peak runoff data
recorded at gages on the St. Vrain Creek near Lyons,
Colorado and Platteville, Colorado (Reference 9). The
years of record vary from 79 years at the Lyons gage
to 47 years at the Platteville gage.

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