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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P-3

P-l,2

Sta. 31 +00 to 40+00: Sheet 13

AG

Sta. 0+00 to 29+00: Sheets 15-16

The floodplain through this reach on the Pleasant View Tributary is relatively

narrow and fairly well contained at the upstream limits of the study but quickly

broadens as flow travels overland across undeveloped property to reach the

improved channel just upstream of Orchard Street. Damages in this reach occur

to a limited number of residential structures located immediately adjacent to the

floodplain. Erosion and minor pavement damage is also expected in this reach.

The floodplain in Apex Gulch is complicated by a multi-directional flow split
caused by the undersized culvert through the upper access road to Heritage
Square. Approximately 100 cfs leaves the main channel and enters the Heritage
Square development to the south. About 250 cfs leaves the main channel in a
second flow path to the north of the main channel which crosses undeveloped
property and re-enters the main channel approximately 1500 feet downstream.

Sta. 0+00t031+00: Sheets 13-14

The 100 cfs flow split into Heritage Square travels through the development as
mostly shallow flow, with average depths less than 1 foot, out the main entrance
and across the parking lot as sheet flow. The flow re-enters the mainstem of
Lena Gulch in the vicinity of U.S. Highway 40 and Highway 93, just upstream of
the confluence of Apex Gulch with Lena Gulch. The split flow option in HEC-2
was used to compute the discharge leaving the channel in both paths near the
upper access road to Heritage Square.

The lower two reaches in the Pleasant View Tributary have the floodplain

completely contained within the banks of the improved channel to the point In

reach P-l where the improved channel terminates. At this point, a small tributary

flow enters the main Pleasant View flow from the southwest. This flow comes

primarily from local drainage within the Pleasant View Subdivision. In addition,

flow from the main channel on Lena Gulch splits and joins with the Pleasant View

Tributary flows. As previously mentioned, the floodplain in this area widens to

cover an area of undeveloped land on Camp George West. From there, flow

travels overland across the undeveloped property in Camp George West to the

confluence with Lena Gulch. Damages in this reach are primarily due to minor

erosion and debris cleanup and removal.

At the lower end of this reach, the sheet flow from the Heritage Square parking
lot and the main Apex Gulch channel flows overtop Highway 93. In addition,
upstream overflows from Lena Gulch combine with the overflows crossing
Highway 93. Floodplain water surface elevations in this complicated flow area
were computed by balancing the upstream energy of the Lena Gulch and Apex
Gulch overflows.

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