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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IV. HYDRAULIC ANALYSIS

the analyses for other recurrence intervals tabulated, The floodway has been computed for the

1 DO-year event based on a one foot and a one-half foot rise in the energy grade line elevation,

General

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The 100-year floodplain and floodway have been delineated along Upper Lena Gulch, Apex
Gulch and Pleasant View Tributary, The limits of the 100-year floodplain and f100dway
represent the area in which development is often regulated to prevent increases in water levels
and increased damages during a flood, Federal and state agencies such as the Federal
Emergency Management Agency (FEMA), the Colorado Water Conservation Board (CWCS),
and the Urban Drainage and Flood Control District (UDFCD) as well as the local municipalities
of Jefferson County, Lakewood and Golden use the floodplain and floodway delineations as a
mechanism to identify areas particularly prone to damage from flooding and to oversee
development activities in these floodplains,

UDFCD provided 1-=100' mapping with a 2-foot contour interval for base mapping, The
mapping coverage extended from the inlet of Maple Grove Reservoir, located immediately
downstream of 20th Avenue, upstream along Lena Gulch for 4,2 miles, Mapping was also
provided for Apex Gulch and Pleasant View Tributary from their confluences with Lena Gulch,
upstream for approximately 0,5 and 0,7 miles, respectively, Mapping was performed by
Landmark Mapping, Inc, based on aerial photography taken on April 4, 1991,

Approximately 200 digitized cross-sections were provided for use in the hydraulic analysis,
These represent the basin conditions at the time of the aerial photography, Cross-sections
were provided in a digital format suitable for input into the selected hydraulic model. Cross-
section numbering corresponds to drainageway stationing, A third decimal place in the cross
section number indicates tributary sections with 1 representing Pleasant View Tributary and 2
representing Apex Gulch,

Hvdroloav

The basis for this fioodplain and f100dway delineation is the hydrology which has been
summarized in an earlier section of this report, The delineation assumes full basin
development, based on land use and zoning projections provided by the affected communities,
and existing channel conditions,

Supplemental structure surveys were also provided by UDFCD to assist in the modeling of

drainageway crossings, These field surveys were conducted during May of 1991. Elevations,

measurements of structure openings and other physical features were identified and provided

for use in the analyses,

A hydraulic analysis has been performed for the 100-year discharge, the results of which were
used to plot the 100-year floodplain and profile, Analyses were also performed for the 10-year,
50-year and 500-year discharges, The 1 O-year flood profile has been plotted and the results of

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