FLOOD10385

• County: Jefferson
• Community: Golden, Lakewood
• Stream Name: Upper Lena Gulch
• Basin: South Platte
• Title: Major Drainageway Planning - Upper Lena Gulch - Phase B Report
• Date: 3/1/1994
• Prepared For: UDFCD
• Prepared By: Boyle Engineering Corporation
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The sub-basins used to model the Upper Lena Gulch basin in this study were smaller and more numerous than in
the 1975 Master Plan. In building the CUHP/SWMM model for calibration, it was necessary to estimate percents
impervious for each of the 77 sub-basins. To do this, the existing land use in 1992 was used to proportionately
disaggregate the 1975 percent impervious area to each of the 77 sub-basins of the new model.

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Basin infiltration properties were estimated using Horton's equation as outlined in the USDCM. The measured
initial and final infiltration rates reported in the 1975 Master Plan were used in this study because they are
reasonable estimates based on actual basin conditions and more detailed field analyses than the general criteria for
infiltration rates presented in the USDCM. The infiltration values reported in the 1975 Master Plan were used for
all sub-basins except for the non-forested ar~ of Apex and Jackson Gulch. The initial infiltration rates used for
these sub-basins was based on further information provided by UDFCD and the project sponsors.

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For sub-basins with areas less than 90 acres, the CUHP requires that the time of concentration for each sub-basin
be estimated. The time of concentration used as input to the CUHPE/PC model was estimated using the procedures
outlined in the USDCM. The computed times of concentration are shown in Table 1.

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taken from the

One-hour rainfall depths for the 2-,5-, 10-,50-, and IOO-year recurrence interval rainstorms were
USDCM depth-duration-frequency information and are summarized in Table 2 below.

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Table 2

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I-Hour Rainfall

Rainfall
Depth (in)

Recurrence
InterVal

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1.04
1.44
1.72
2.35
2.60

2-yr
5-yr
10-yr
50-yr
IOO-yr

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Channel routing parameters were developed from topographic mapping provided by UDFCD and field inspect
The channel length and invert slope were estimated from USGS 7.5 minute quadrangle maps. The channel
geometry was estimated from the topographic maps. The channel roughness coefficient was estimated from criteria
outlined in the USDCM, The channel routing parameters used in the model for existing major drainageway
conditions are presented in Table 3.

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