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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

Consequently, Zeta Street is overtopped during the 100-year flood. Damages in
this reach are due to erosion and pavement overtopping

damage is expected to the permanent buildings located along the campground.
Additional damages would be expected if the campground is fully occupied with
recreational vehicles when a flood occurs.

L-tl

Sta. 396 + 50 to 414 + 70: Sheets 3-4

This reach contains a mixture of commercial and residential properties and a
range of drainage facilities. Flow discharging from the Zeta Street culvert is
conveyed along an artificial channel section. Adjacent to this portion of the
channel, a concrete retaining wall has been constructed to retain the high banks
and to act as a floodwall to prevent bank overtopping. This wall extends
approximately t50-feet downstream from the outlet of the Zeta Street culvert.
The floodplain limit is contained by this wall, and it is important that the wall is
maintained in order to prevent flooding on the east side of the wall and to the
north of Highway 40.

The GOO-foot long culvert discharges into a small natural channel. The channel
crosses under two driveways in the vicinity of West Street and the Amoco Gas
Station on Highway 40. The floodplain in the lower portion of this reach is
generally contained by U.S. Highway 40 on the south, except at the area
adjacent to the Amoco Gas Station where Highway 40 is overtopped. The flow
constriction at the eastern Amoco driveway, which occurs due to an undersized
culvert, causes overtopping of Highway 40 to the south. A weir flow calculation
was performed to determine that approximately 40 cfs is lost from the main
channel. The lost flow follows an independent path unrelated to the main
channel profile, until it re-enters the main channel downstream.

At the end of the concrete f1oodwall, the channel makes a 90-degree bend to the
east and enters an unimproved channel section which has experienced relatively
severe erosion problems. The banks are heavily scoured and evidence of bank
sloughing is readily visible.

L-l0

Sta. 385+00 to 396+50: Sheet 4

Downstream of this channel section, flows enter an approximately 600-foot long,
5-foot by 8-foot concrete box culvert. This box culvert is of inadequate capacity
for even the to-year event without overtopping. The culvert alignment is through
the East Tincup Village Campground along U.S. Highway 40. Discharge in
excess of the culvert capacity inundates the campground and is conveyed
downstream in relatively shallow overland flow. Some structure and content

Immediately downstream of the Amoco Gas Station driveway the channel is
unimproved and undersized. The channel has been encroached by two
residential/commercial structures just upstream of Violet Street and is heavily
obstructed by debris. In spite of this debris, the floodplain through this reach is
relatively narrow. The structure under Violet Street is of adequate capacity to
convey the 1 DO-year flow.

14