FLOOD01923

• Designation Number: 462
• County: Douglas
• Community: Unincorporated Douglas County
• Stream Name: Willow, Little Willow, East Willow Creeks
• Title: Flood Hazard Area Delineation - Willow Creek, Little Willow Creek, East Willow Creek, Douglas County, CO
• Date: 12/1/1996
• Designation Date: 7/1/1997
• Floodplain - Doc Type: Floodplain Report/Masterplan

3.6.4 Time of Concentration: In accordance with the USDCM, the modified CUHPF/PC
procedure was used for subwatersheds less than 90 acres. The modified CUHPF/PC
procedure produces peak flows similar to the rational method and requires a time of
concentration value for each subwatershed modeled. Equations 3-2, 3-3 and 3-4 from the
Runoff chapter of the USDCM (Volume I) were used to determine the time of concentration.
The velocity parameter used to determine the travel time variable in Equation 3-2 was
estimated from a trapezoidal channel having the geometry used for the UDSWM386 model, a
slope determined from the enlarged (1"=1,000') USGS quadrangle map, a Manning's 'n' of
0.04, and the 5-year discharge. A comparison of the results for each equation was made and
the shortest time of concentration was used for modeling purposes. Time of concentration
values generally ranged from 19 to 50 minutes.

3.6.5 Retention Storage Losses: Retention storage losses were determined according to Table
2-1 of the Runoff chapter of the USDCM. For future conditions, the retention storage losses
were assumed to be 0.05 inch for impervious areas (representing sloped roof areas). Retention
storage losses in pervious areas were assumed to be 0.4 inch.

3.6.6 Infiltration Losses: The infiltration parameters (initial infiltration, final infiltration, and
decay coefficient) were computed from Table 2-2 from the Runoff Chapter of the USDCM
based on an area weighted average of hydrologic soil type. The weighted average of
hydrologic soil type for each subwatershed was computed by superimposing the hydrologic
soils maps (Figures 2.IE and 2.1 W) onto the subwatershed maps (Figures 3.1E and 3.1 W).

3.7 UDSWM386 Modeling

3,8

A UDSWM386 model was created to route the CUHPF/PC generated hydrographs through
the existing stream network. Conveyance element parameters required for the UDSWM386
model include the conveyance element identifier, channel bottom width, length, invert slope,
side slopes, hydraulic roughness, and routing connectivity. A summary of these
characteristics is shown in Table 3.5 (located in Appendix A) and the methods for estimating
these characteristics are described in the following sections.

3.7.1 Conveyance Element Identification Number: Conveyance elements were numbered
from downstream to upstream for each stream as shown in Figures 3.2E and 3.2W. All
conveyance elements were identified according to the upstream design point. Design points
were located at the outfall of each tributary subwatershed and numbered according to the
highest subwatershed ID tributary at that point.

3.7.2 Stream Geometry: All streams were modeled as trapezoidal cross-sections. Bottom
widths, lengths, invert slopes, and side slopes were estimated from either detailed mapping at
a scale of 1"=100' or the enlarged USGS quadrangle map at a scale of 1"=1,000', depending
on availability. In some cases, streams were represented by a base flow channel and an
overflow floodplain channel.

3.7.3 Roughness: The roughness or Manning's "n" value for each conveyance element was
determined using Equation 2 from the UDSWM386 Users Manual (UD&FCD 1985b), which
is shown below:

n = 0.303 . (S)038 . (R) .016

where:

n = Manning's roughness coefficient
S = Friction slope (feet/feet)
R = Hydraulic radius (feet)

This equation is the result of research work by Robert D. Jarrett of the USGS (Jarrett, 1984)
and is recommended for hydrologic routing in natural channels. For this study, the friction
slope was approximated using the invert slope and the hydraulic radius was approximated for
100-year flood conditions. The Manning's "n" value for the conveyance elements generally
ranged from 0.067 to 0.133.

3.7.4 Routing Connectivity: The routing connectivity of the conveyance elements, design
points, and subwatershed hydrographs were incorporated into the UDSWM386 models
according to the SWMM Routing Schematic diagram provided in Figure 3.3.

3.7.5 Existing Ponds: The existing dam and reservoir in the upper reaches of the Willow
Creek Watershed was ignored because it is privately owned. However, Little Willow Creek
was routed through the existing detention pond in the Roxborough Village Subdivision, as
well as the Platte Canyon Reservoir.

Results

Table 3.6 (located in Appendix A) presents the peak discharges from the UDSWM386 model
at various design points along the main channels for the 10-, 50-, 100-, and SOD-year storm
events. Figures 3.4, 3.5, 3.6 (located in Appendix A) present discharge profiles along the
main channels for the 10-, 50-, 100-, and SOD-year storm events. Figure 3.7 shows typical
hydro graphs at selected locations.

The following information is also provided in the separately submitted Technical Addendum
to this report.

1. Summary output tables from the UDSWM386 model for the 10-,50-, 100-, and 500-
year storm events for future development conditions.
2. Discharge profiles along the main channels for the 10-, 50-, 100-, and 500-year storm
events.
3. The routed hydrographs at roadway and canal crossings, as well as existing detention
facilities for the 10-, 50-, and 100-year storm events.

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