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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L-S

Sta. 371 +50 to 385+00: Sheets 4-5

the two paths was computed by balancing the total energy at the upstream

section. The flow down the main channel was computed to be approximately

300 cIs.

This reach consists entirely of an improved channel section. The concrete-lined
rectangular channel has adequate capacity for the 100-year flow. In this reach,
the floodplain limits are predominantly coincident with the channel limits except
for an area near the inlet to the culvert under 6th Avenue. Here, the channel
walls are not of sufficient height to fully contain the flow and some localized
overbank ponding could be expected. However, no structures are located in the
floodplain at this location and no significant damages are expected.

L-7

Sta. 325+50 to 353+00: Sheets 5-7

The floodplain in this reach covers mostly undeveloped open space with a
limited number of low density residential properties located on the edge of the
floodplain. The main channel in this reach is deep and well incised and shows
evidence of past erosion damage. The upstream portion of this reach contains
the split flow as described above for the previous reach.

L-8

Sta. 353+00 to 371 +50: Sheet 5

The channel in this reach has been reiocated from its historic path and has been

heavily encroached by development of the Mountain Side Mobile Estates. The

channel as it now exists has inadequate capacity. The culvert under the

driveway to Mountain Side is also undersized causing overtopping into the trailer

park. The floodplain in this reach is relatively wide, covering most of the mobile

home park. Because of the large number of structures located in the floodplain,

the damage potential in this reach is high.

The culvert under Orion Street at the downstream end of this reach is undersized
and causes water to back up and overtop the roadway for a length of
approximately 600 feet. This overtopping causes the flow to split into two
separate hydraulically disconnected flow paths. The split flow is described below
in the next reach. Damages in this reach are primarily due to erosion and
pavement overtopping with some damages to structures and contents expected
as well.

The downstream end of this reach is characterized by a split flow around an
island to the southeast of the main channel along Mt. Vernon Road. This flow is
divided and hydraulically disconnected. Downstream and upstream sections
were identified where the flow combined, and the analysis included the balancing
of energy losses along both flow paths. Energy profiles for a range of
discharges were computed along each flow path. The flow distribution between

L-6

Sta. 312+00 to 325+50: Sheet 7

Below Orion Street and upstream of South Golden Road, the flow is divided.
Downstream and upstream sections were identified where the flow is combined.
The same procedure as described previously was used to balance the energy
along both flow paths. This analysis resulted in a computed flow of SOO cfs for

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