FLOOD03617

• County: Summit
• Community: Silverthorne
• Basin: Colorado Mainstem
• Title: FIS - Silverthorne
• Date: 8/9/2001
• Prepared For: Silverthorne
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

2.4 Flood Protection Measures

The discharges of the Blue River at Silverthorne are regulated by the Dillon Dam and
Reservoir located approximately 0.5 mile upstream of the southern corporate limits. The
reservoir storage significantly reduces the peak discharges of the frequent floods.
However, it is observed in the Blue River hydrology report (Reference 8) that, for the
infrequent flood events under consideration here, the reduction in flood peaks due to
reservoir attenuation is only approximately 10 percent of the peak discharge.

3.0 ENGINEERING METHODS

For the flooding sources studied by detailed methods in the conununity, standard hydrologic and
hydraulic study methods were used to determine the flood-hazard data required for this study.
Flood events of a magnitude that are expected to be equaled or exceeded once on the average
during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having
special significance for floodplain management and for flood insurance rates. These events,
commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and 0.2-percent
chance, respectively, of being equaled or exceeded during any year. Although the recurrence
interval represents the long-term, average period between floods of a specific magnitude, rare
floods could occur at short intervals or even within the same year. The risk of experiencing a rare
flood increases when periods greater than 1 year are considered. For example, the risk of having
a flood that equals or exceeds the lOO-year flood (I-percent chance of annual exceedence) in any
50-year period is approximately 40 percent (4 in 10); for any 90-year period, the risk increases to
approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based
on conditions existing in the community at the time of completion of this study. Maps and flood
elevations will be amended periodically to reflect future changes.

3.1 Hydrologic Analyses

Hydrologic analyses were carried out to establish peak discharge-frequency relationships
for each flooding source studied by detailed methods affecting the conununity.

For the original study, the discharge-frequency data for the Blue River within Silverthorne
were developed in a report entitled "Hydrology Report, Blue River Downstream of Dillon
Reservoir, Summit County, Colorado", (Reference 8). In brief, the approach used
considers the total inflow to the Dillon Reservoir and its routing through the reservoir to
obtain the 10-, 50-, 100-, and 500-year discharges on the Blue River downstream of the
Dillon Dam. The contribution of the local inflow or the tributaries' flows to the Blue
River discharges is also described.

To evaluate the peak flows for the Blue River below Dillon Dam, the current gage record
(through water year 1994) was compared to the flood frequency values used in the original
study (Reference 9) by a statistical analysis of the USGS gage (09050700) below the dam
(Reference 10) using the HEC-FFA program. It was found that there was not a significant
difference between the values so the peak flows from the original study flood frequency
values were used.

The peak flows for Willow Creek and Straight Creek were developed using the USGS
regional regression equations (Reference 11).

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