FLOOD01161

• Designation Number: 289
• County: Douglas
• Community: Larkspur
• Title: Flood Insurance Study - Larkspur, Douglas County, Colorado
• Date: 9/1/1987
• Designation Date: 12/1/1987
• Floodplain - Doc Type: Floodplain Report/Masterplan

maps of Douglas County, Colorado (References 9 and 4, respecti-
vely). Values of 10-, 50-, 100-, and 500-year peak discharges
were obtained using the TR-20 computer program, developed by the
SCS (Reference 10). The computed peak discharges compare favorably
with the peak discharges estimated using USGS Technical Manual No.
1 (Reference 11).

Peak discharge-drainage area relationships for East Plum Creek are
shown in Table 1.

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding from the
sources studied were carried out to provide estimates of the eleva-
tions of floods of the selected recurrence intervals.

For East Plum Creek, the water-surface profiles for the 10-, 50-,
100-, and 500-year floods were developed using the SCS WSP-2 computer
program (Reference 12). These profiles and elevations were compared
with historic floods, and then the flood stages were determined.

Cross section data were obtained by field measurements. All bridges,
culverts, and other structures were surveyed to obtain elevation
data and structural geometry.

Locations of selected cross sections used in the hydraulic analyses
are shown on the Flood Profiles (Exhibit 1). For stream segments
for which a floodway was computed (Section 4.2), selected cross
section locations are also shown on the Flood Insurance Rate Map
(Exhibit 2).

Roughness coefficients (Manning's On") were estimated by field
investigation and from pictures of the stream and its flood plain
using Rouqhness Characteristics of Natural Channels (Reference 13),
Open-Channel Hydraulics (Reference 14), and Handbook of Applied
Hydraulics (Reference 15). The On" values for East Plum Creek
ranged from 0.03 to 0.06 in the main channel and from 0.04 to 0.12
for the overbanks.

For East Plum Creek, the starting water-surface elevation was deter-
mined using the stream slope at a starting valley cross section.

The hydraulic analyses for this study were based on unobstructed
flow. The flood elevations shown on the profiles are thus con-
sidered valid only if hydraulic structures remain unobstructed,
operate properly, and do not fail.

The methods used in estimated elevations of streams studied by
approximate methods came from USGS Technical Manual No. 1 (Refer-
ence 11). This manual has curves of drainage area-versus-discharge

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