FLOOD00126

• Designation Number: 459
• County: Clear Creek
• Community: Empire
• Title: Flood Insurance Study - Empire, Clear Creek County, Colorado
• Date: 6/1/1976
• Designation Date: 7/1/1997
• Floodplain - Doc Type: Floodplain Report/Masterplan

The method consists of a series of regression equations from
which flood characteristics can be estimated from measurable
physical and climatic parameters of drainage basins. Drainage
area and normal annual precipitation over that area are the
two factors required to estim~te flood flows.

Results of the above described regional hydrologic analyses
were compared to statistical analyses of historical discharge
records at two gaging stations within Clear Creek County oper-
ated by the U. S. Geological Survey (References 15, 16, & 19-21).
Analysis of these two stations fol lowed the standard log-Pearson
Type III method as outl ined by the Water Resources Counci I
(Reference 22). Comparisons of Clear Creek at Lawson (29 years
of record) and at Fal I River (9 years of record) to results of
the regional hydrologic analysis showed a maximum difference in
flood flows of 29 per cent and an average difference of only 10
per cent. This error is acceptable considering the inherent
inaccuracies of the analysis and the short period of record. The
regional analysis was, therefore, adopted for use in Clear
Creek County for both gaged and un gaged areas.

Drainage area-peak discharge realtionships as developed for
Clear Creek County and adapted to Empire are shown on Figure 2.

3.2 Hydraulic Analysis

Analysis of the hydraulic characteristics of streams in the
community were carried out to provide estimates of the elevations
of floods of the selected recurrence intervals along each stream.

Water surface elevations of floods of the selected recurrence
intervals were computed through use of the U. S. Army Corps of
Engineers HEC-2 step-backwater computer program (Reference 9-1 I)
and supplemented by manual calculations. Manual calculations
were primari Iy used to determine the division of flood flows at
bridge sections and to verify water surface elevations near
critical depth. Cross sections for the backwater analyses were
field-surveyed and were located at close intervals above and
below bridges and culverts in order to compute the significant
backwater effects of these structures in the urbanized areas.
Locations of selected cross sections used in the hydraul ic
analyses are shown on the water surface profi les (Panels 0IP-04P).

Channel roughness factors (Manning's "n") , for these computations
were assigned on the basis of field inspection of flood plain
areas, comparison with streams of known roughness (Reference I)
and from computations relating to the USGS stream gage at Lawson.

Flood profi les were drawn showing computed water-surface elevations
to an accuracy of 0.5 foot for floods of the selected recurrence
intervals (Figure 2). AI I elevations are measured from mean sea
level datum; elevation reference marks used in the study are shown
on the separately printed flood insurance zone map.

8