FLOOD10350

• Designation Number: 578
• County: Delta
• Community: Orchard City
• Stream Name: Surface Creek, Cedar Run and Tongue Creek
• Title: Floodplain Information Report: City of Orchard City, CO
• Date: 9/1/2004
• Prepared For: Orchard City and CWCB
• Prepared By: Love and Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hvdrolo2:ic Analvsis

The update of the hydrologic analysis for Surface Creek, Cedar Run, and Tongue Creek
for the FIR uses the most recent update of USGS regression equations and methods. The
original study (MSM, 1979) predicted flood peaks based on regional frequency analysis
of USGS streamgaging records (USGS, unpublished) from fifteen similar watersheds.

The stream basins within Orchard City were divided into catchments in the FIS as shown
on Figure 2. Design points were defined at various points in the City. Regression
equations from Vaill (2002) developed by the USGS for the Northwest Region of
Colorado were used to predict flood peaks for Surface Creek, Cedar Run, and Tongue
Creek for the 10-, 50-, 100-, and 500-year events at the design points. The equations are
presented in the Appendix.

The USGS study reports flood peak discharge estimates for various events for USGS
stream gages within the State of Colorado, including the USGS stream gage upstream of
Orchard City along Surface Creek. Flood peak estimates for this gage are found in the
Appendix. Using these predictions, flood peak estimates along Surface Creek at the
Design Points can be calculated using the area ratio equation suggested by Vaill,

QI = Q2 {A I / A 2 }O.64 , 0.5 < {A 1 / A 2 } < 1.5

(Vaill, 2000)

where;

Q 1 = projected flood peak, cfs,
Q 2 = known flood peak, cfs,
A I = drainage area at projected site, mi2, and,
A 2 = drainage area at known site, mi2.

This approach results in higher 100- year flood peaks at the Design Points than those
calculated using the regression equations. However, flood peaks predicted from the
USGS gaging station are skewed toward flood peaks resulting snowmelt events from
higher elevations in the Surface Creek basin. Since snow is uncommon within the basin
in the vicinity of and downstream of the gaging station, the regression equations are
selected for Surface Creek as better representatives of flood peaks at the Design Points
within Orchard City.

Updated flood peak discharge estimates for the 10-, 50-, 100-, and 500-year events for
Surface Creek, Cedar Run, and Tongue Creek are summarized in Table 2. Given there is
a standard error of prediction in the USGS regression equations of between
approximately 70 and 85 per cent as indicated by Vaill (2000), the updated flood peaks
are not significantly different than those reported in the FIS. The very large standard
error indicates flood peak estimates obtained from these equations should be used with
caution.

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