FLOOD04702

• Designation Number: 347
• County: Jefferson
• Community: Unincorporated Jefferson County
• Stream Name: Upper Lena Gulch
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Upper Lena Gulch
• Date: 1/1/1993
• Designation Date: 9/1/1993
• Floodplain - Doc Type: Floodplain Report/Masterplan

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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,

represents an idealized section for routing computations and 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 are presented in Table 3.

Future Basin Conditions

One-hour rainfall depths for the 10-, 50-, and 1 OO-year recurrence interval rainstorms were taken
from the USDCM depth-duration-frequency information and are summarized in Table 2 below,

Land use and zoning maps provided by Jefferson County and the Cities of Golden and

Lakewood were used to identify projected ultimate future land use in the basin, Estimates of

the percent impervious area for the projected land use were made and entered into the

hydrologic model. Peak discharges for each of the recurrence intervals were computed for

future basin development conditions, Figure 5 shows a map of the basin which depicts the

projected future land use, Table 4 contains the peak discharges for selected design points for

each of the recurrence intervals under future basin development

Table 2
1-Hour Rainfall

Recurrence
Interval

Rainfall
Death (in)

10-yr
50-yr
100-yr

1.72
2,35
2,60

TABLE 4
Future Basin Conditions
Peak Flow Summary At Design Points (cts)

Channel routing parameters were developed for 54 conveyance elements from topographic
mapping provided by UDFCD and field inspection, These routing parameters include length of
conveyance element, channel bottom width, channel shape, channel invert slope, and a
channel roughness coefficient The channel length and invert slope were estimated from USGS
7,5 minute quadrangle maps, The channel geometry used in the UDSWM2/PC model

Location 1Q:yr QQ:yr ~
Lena Gulch at Hogback 510 1,320 1,770
Lena Gulch at 6th Avenue 800 1,740 2,270
Lena Gulch at 1-70 1,440 3,920 3,810
Inflow to Maple Grove Res, 1,580 3,080 3,980

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