FLOOD10385

• County: Jefferson
• Community: Golden, Lakewood
• Stream Name: Upper Lena Gulch
• Basin: South Platte
• Title: Major Drainageway Planning - Upper Lena Gulch - Phase B Report
• Date: 3/1/1994
• Prepared For: UDFCD
• Prepared By: Boyle Engineering Corporation
• Floodplain - Doc Type: Floodplain Report/Masterplan

RECOMMENDED ALTERNATIVE PLAN

VI.

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These reaches are similar in that they have relatively low damages, and the B/C ratios of all
alternatives are very low. The recommended alternative in both reaches is to construct erosion
control check structures at regular intervals. In addition, routine maintenance of the channel to
remove debris and monitor channel blockage is recommended.

L-IO

L-ll

The recommendation for this reach is to do nothing in terms of constructing improvements. The
concrete channel is an efficient conveyance mechanism. Regular maintenance and inspection to
identify and repair any structural problems which may occur over time and to remove accumulated
debris from the channel is recommended. Some overbank areas are slightly below the water
surface and while protected by the channel banks, may be subject to periodic flooding. These areas
should be filled if materials become available.

L-9

The recommended plan to address flooding problems on Upper Lena Gulch is based on both subjective and
objective factors. For this study, the principal objective criteria was economic. The primary economic criteria for
selecting an alternative plan is the benefit-cost (B/C) ratio of the alternative. As shown in Table 8, many of the
alternatives evaluated during the Phase A portion of the study have a B/C ratio less than one. Also, other non-
tangible considerations must be factored into the decision-making process and in some cases may make an
alternative which has a low B/C ratio more favorable.

Phase A Recommended Plan

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In developing a recommended plan for improvements to Upper Lena Gulch, several non-tangible factors were
included in the analyses and contributed to the selection of the alternatives. One such factor was compatibility with
basin improvements which have already been constructed. Other factors included public perception, prevention of
loss of life due to flooding, and inconvenience caused by nuisance flooding.

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The recommended alternative consists of enlarging the existing natural channel to have capacity for
the 10-year flow. Because of the required channel top width for a 10-year capacity, it will be
necessary to acquire a limited amount of property from the Mountain Side Mobile Estates Trailer
Park. This will involve relocating two mobile homes. Approximately 40 feet of channel right-of-
way is required for this alternative. Approximately 10 feet of "A" Street and 10 feet of Mount
Vernon Road will be required to facilitate installing the required channel section. This would leave
a roadway width of approximately 20 feet of "A" Street for access to the single row of mobile
homes. Mount Vernon Road would then be moved 10 feet to the northwest to maintain total
driving surface. The total length of the enlarged channel is approximately 1,700-feet.

L-8

Improvements which enhanced or, as a minimum, did not further degrade water quality were also favored during the
recommended plan development. The Lena Gulch basin is almost entirely developed and is relatively stable. As
such, existing runoff water quality is not expected to change greatly. The primary water quality issues of concern
relate to erosion and deposition of channel sediments. One of the objectives of the recommended plan is to control
channel erosion by providing stable channels of adequate capacity or by stabilizing those currently eroding or
having a high potential for erosion. In addition, an aggressive erosion control program for construction activities,
as well as requirements for on-site detention storage for new developments, should be incorporated into the
recommended plan so that future floodplain management guidelines help to maintain water quality in the watershed.

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Although detention storage is an option that many property owners have voiced an interest in, it
does not appear to be a technically or economically viable alternative for improving flooding
problems in the Upper Lena Gulch basin. The main drawback to the detention storage alternative
is that no sites exist in which enough storage volume can be developed to have a major impact on
the total volume of runoff coming from the basin. The estimated storage volume available at the
Jefferson County Open Space site is about 120 acre-feet. The volume of the 100-year flood
hydrograph at the Jefferson County Open Space site is approximately 210 acre-feet. The inflow
volume downstream of the Jefferson County Pond site for the 100-year flood is 410 acre-feet,
greater than half the total. The hydrologic model was used to evaluate the impacts of this proposed
pond for the 2-, 5-, 10-, 50- and 100-year flows. The impacts the Jefferson County Pond has on
the 2- through 10-year flows is negligible. Based on these findings the recommended alternative
for this reach is to maintain the existing condition. However, regular channel maintenance to
remove accumulated debris should be done.

L-7

Two alternatives are recommended for this reach. As shown in Table 8, several alternatives have a
B/C ratio greater than one. Based on B/C ratios alone, the 10-year replacement culvert is the
preferred option. However, in considering the number of undersized culverts in this reach, the
width of the floodplain, and the potential for damages to structures, a more global approach to
improving the conditions in this reach was selected. The recommended alternative is to construct a
complete system of enlarged channels and crossing structures which can handle the 10-year flows
without bank or road overtopping. In addition, the culvert under South Golden Road should be
enlarged to a 100-year capacity since this road is a major access to the Pleasant View area in

L-6

This upper portion of the Lena Gulch basin is largely undeveloped and, based on zoning and land
use projections, is not likely to change. The B/C ratios for all alternatives studied in this reach are
very low. Damages in this reach are very low and consist mainly of minor pavement damage and
debris removal and cleanup. As mentioned previously, erosion control and sediment deposition are
important water quality considerations. Therefore, the recommended alternative for this reach is to
construct erosion control check structures in those portions of the reach where velocities exceed 6-
feet per second. This alternative. will help prevent future problems associated with erosion damage.

The recommended alternative plan is composed of a selection of single alternatives from each reach as described
below: The components of the Phase A Recommended Alternative Plan are shown schematically in Figure II

Selected Alternative

Lena Gulch

Reach

L-13

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Similar to Reach L-13, the recommended option for this reach is to install erosion control check
structures at regular intervals. The previous Master Plan recommended a debris/detention basin
for this reach. Due to the location high in the basin and the relatively low damages in this and the
next reaches immediately downstream, the expense of the debris/detention basin is not justified.

L-12

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