FLOOD01589

• County: Clear Creek
• Community: Georgetown
• Stream Name: Clear Creek, South Clear Creek
• Title: Flood Hazard Identification Report
• Date: 2/1/1987
• Prepared For: Georgetown
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Hydraulic Analysis

An analysis of the hydraulic characteristics of Clear and South
Clear Creeks was performed in order to determine water surface elevations
for the 10-year and 100-year floods on each stream. Cross-section data
was taken from a 2-foot contour map of the Town of Georgetown (Ref. 10).
Building areas were "blocked out" of the cross-sections to determine the
effective flow through the floodplain. This cross-section data became
input data for the U.S. Army Corps of Engineers HEC-2 step-backwater
computer program (Ref. 13). Bridge elevations and sizes of openings were
surveyed and measured in the field by Black and Veatch (B & V) Consulting
Engineers (Ref. 1) for the Flood Insurance Study. The data was reviewed
and field-checked by the Colorado Water Conservation Board.

A comparison of bridge cross-section elevations surveyed by B & V
Consulting Engineers with those obtained from the 2-foot contour map led
to the conclusion that there was a significant problem with the datum of
one of the two sources. Further study of the datum problem was thus
undertaken. Two other sources of elevation data were used in this
analysis: 1. a I-foot contour map of the vicinity of Georgetown Lake
(Ref. 11), and 2. field surveys of May, 1979 by Hayes and Soucie of
selected cross-sections along Clear Creek (Ref. 9). A total of 60 points
from the three described sources were compared with the corresponding
points on the 2-foot contour map. From this, it was determined that the
2-foot contour map had a datum that was, on average, 1.5 feet high (it
was 1.5 feet higher than the mean sea level datum of 1929). An
adjustment was made to the flood profile to make it conform to the 2-foot
contour map of the Town of Georgetown since the contour map was the map
upon which the flood boundary would be drawn.

The Colorado Water Conservation Board's field surveys (Ref. 4) were
used to determine channel bottom elevations in areas where there was
recent dredging undertaken by the town. Dredging was done on Clear Creek
downstream from the 15th Street Bridge and on South Clear Creek
downstream from the Rose Street Bridge. These surveyed values were
adjusted to the map datum (by raising the values 1.5 feet).

Roughness coefficients (Manning's "n" values) were selected for the
HEC-2 program. They were obtained by the CWCB staff field survey.
Roughness values for Clear Creek varied from 0.035 to 0.055 for the
channel and from 0.030 to 0.090 for the overbank areas. Roughness values
for South Clear Creek varied from 0.040 to 0.050 for the channel and from
0.035 to 0.080 for the overbank areas.

The locations of the cross-sections taken from the 2-foot contour
map are shown on the "Flooded Area Maps" (see Plates 2 to 7) and the
hydraulic data for the cross-sections is displayed in Tables 1 and 2.

The computed 100-year flood levels are outlined
Area Maps"; plotted on the "Flood Profile" sheets; and
"Cross-section and Water Surface Elevation Data" tables.

on the "Flooded
tabulated in the

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