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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V. FLOOD HAZARD AREA DELINEATION

Reach

L-13

General

The delineation of flood hazard areas for the 100-year flood has been performed for this study.
The results of this analysis are found in the attached plan and profile drawings. The 10-year
profile has also been computed and plotted on the plan and profile drawings.

100- Year Floodplain Description

The 100-year water surface elevations and floodplain limits determined in the study represent
estimates based on future development and the current channel geometry in the basin. As a
result, they may differ from the floodplain limits identified in earlier studies.

The following is a reach by reach description of the tOO-year floodplain beginning in the upper
portion of the basin and moving downstream. The floodplain of the main stem of Upper Lena
Gulch will be described first, followed by Pleasant View Tributary and Apex Gulch. Refer to
Figure 2 for reach locations.

L-12

13

Floodplain Description

Sta. 424+70 to 470+00: Sheets 1-3

The floodplain in this reach is narrow. Channel slopes and flow velocities are
high. No structures are located in the floodplain in this reach, therefore damages
are primarily due to channel erosion. Some pavement damages are also
expected due to road overtopping.

Near the intersection of U.S. Highway 40 and State Highway 93, the flow
overtops the roadway and splits. The split flow option of HEC-2 was used to
model Highway 40 as a weir in order to compute the discharge lost from the
main channel. The lost flow follows an independent flow path unrelated to the
main channel profile, until it re-enters the main channel downstream. The
downstream discharge in the main channel below the flow split was reduced by
90 cfs to account for lost flow.

The two culverts under the Mobile Premix Driveway and the driveway to the

south are both of inadequate capacity to convey the 100-year discharge. The

roadway at both locations is overtopped during the 1 OO-year flood.

Sta. 414 + 70 to 424 + 70: Sheet 3

The floodplain in this reach is mostly in an undeveloped open space with the

exception of one residential property located on the fringe of the floodplain. The

culvert under Zeta Street at the lower end of the reach is undersized.