FLOOD02900

• County: Adams
• Community: Commerce City
• Basin: South Platte
• Title: Lower First Creek and Direct Flow Area 0055 Major Drainageway Planning
• Date: 5/1/2002
• Prepared For: UDFDC
• Prepared By: Turner Collie & Braden Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

Time of Concentration

In accordance with the USDCM, the modified CURP procedure was used for subwatersheds less
than 90 acres (Reference 14). The modified CUHP procedure produces peak flows similar to the
rational method and requires a time of concentration value for each subwatershed modeled (for
example, subwatersheds less than 90 acres). Equations 3-2 and 3-3 from the Runoff chapter of the
USDCM were used to determine the time of concentration (Reference 14). The velocity paramcter
used to detennine the travel time variable in Equation 3-2 was estimated from Figure 3-1 of the
Runoff chapter of the USDCM using the curve for short grass pasture and lawns (Refcrence 14).

Under urban conditions, Equation 3.4 from the Runoff chaptcr of the USDCM was used to
determine the time of concentration since it yielded shorter concentration times than Equation 3-2
and 3-3 and Figurc 3-1 of the USDCM (Reference 14).

Retention Stora!!e Losses

Retention storage losses wcre dctermined according to Table 2-1 of the Runoff chapter of the
USDCM (Reference 14). Retention storage losses were assumed to be 0.05 inch for impervious
areas (representing sloped roof areas) and 0.40 inch for pervious area (representing wooded areas
and open fields). For urban conditions, the retention storage losses were assumed to be 0.05 inch for
impervious arcas (representing sloped roof arcas) and 0.35 inch for pervious area (representing lawn
grass).

Infiltration Losses

The infiltration parameters (initial infiltration, final infiltration, and decay coefficient) were
computed from Table 2-2 of the Runoff chapter of the USDCM based on a weighted average of
hydrologic soil type (Reference 14). The weighted average of hydrologic soil type for each
subwatershed was computed based on superimposing hydrologic soils maps onto the subwatershed
maps.

F.

UDSWM 95 MODELING

A UOSWM 95 model was created for the watershed to route the CURP generated hydrographs
through the stream network. Conveyance element parameters required for the UOSWM 95 model
include the conveyance element identifier, channel bottom width, length, invert slope, side slopes,
hydraulic roughness, and routing connectively. A summary of these characteristics is shown in
Appendix B and the methods for estimating these characteristics is described in the following
sections.

Conveyance Element Identification Number

Conveyance elements were numbered from downstream to upstream for each stream as shown in
Appendix B. All conveyance elements were identified with numbers. Design points were located

at the upstream ends of each conveyance element number and identified by the appropriate numbers.

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Stream Geometrv

All streams were modeled as trapezoidal cross sections. Bottom widths, lengths, invert slopes and
side slopes were estimated from the I :24,000 USGS quadrangle maps. In some cases, streams were
represented by a base flow channel and an overflow floodplain channel.

Rou!!hness

The roughness or Manning's "n" value for each conveyance element was determincd using Equation
2 from the UOSWM 95 Users Manual (Reference 19), which is shown below.

n = 0.393 x (S)OJ8 x (R) -{).16
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 coefficient for the conveyance elements ranged from 0.03 to 0.06.

Routin!! Connectivity

The routing connectivity of the conveyance elements, design points, and subwatershed hydrographs
was incorporated into the UDSWM95 models according to the connectivity diagrams provided in
Appendix C.

G.

RESULTS

Using the physical subwatershed hydrologic parameters and rainfall information, along with the
drainage system characteristics, peak flow rates were determined for each outfall design point.
Table /11-5 presents peak flow rates at select design points along the First Creek drainage system for
the existing and future development as well as the base model conditions for the 2-, 5-, 10-, 50-,
100-, and 500-year storm events. Figure 111-2 illustrates typicallOO-year runoff hydro graphs at
primary design points for the base model condition. For the First Creek mainstem, the reported
flows are adjusted flows below design point 934. Upstream of design point 934 and on the
tributaries the reported flows are unadjusted flows. Unadjusted flows are reported for design points
with tributary areas less than 10 square miles (Reference 4).

Table /11-6 provides a comparison of the base model peak flow rates, at locations along the
mainstem of First Creck, with those in the Martin/Martin study. The flow rates calculated in the

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