FLOOD05134

• County: Jefferson
• Community: Lakewood
• Stream Name: South Lakewood Gulch
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 7/1/1977
• Prepared For: Lakewood
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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depression desc:"ibed at the lower end of Subbasin 1, ending abruptly
at Garrison Street. At this point, a small inlet to the storm sewer
system under Garrison Street carries low flows, with any significant
flows going over the street. The channel itself continues on the east
side of Garrison Street, at a low point in the street about 50 yards
from the location of the upstream inlet. From this point downstream,
a small but well-defined channel carries the flows to the box culvert
at Alameda Boulevard and beyond to the lake at Rockmont College.

representing about 30 percent of the total reach length, are zoned to
be developed in the future, thus potentially causing more encroachment.
A local developer has recently constructed a grass-lined trapezoidal
channel with four drop structures in the vicinity of Brentwood Street
and Bayaud Avenue (channel not shown on Drawing). Even with this channel
in place, extensive shallow flooding will occur west of South Balsam Street,
as can be seen on Sheet 4, Drawings. An existing depression at the lower
end of this subbasin would store Some storm runoff, discharging into a
4-foot RCP. The depression has no embankment, but rather is entirely
below grade with the outlet invert several feet below surface grade. The
detention storage effect of this existing depression is minimal for the
larger flows.

SUBBASIN 4 (Pond near First Avenue and Balsam Street to Wadsworth Boulevard)

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The overall average imperviousness in this subbasin is presently about
25 percent, with a slope of about 4 percent. The existing development
is uniformly distributed throughout the basin and consists of mostly
single-family residential developments. Most open areas have been zoned
for residential development in the future. One exception to this is
the large open area between Kipling and Garrison, which is zoned as a
park.

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Two major problems are evident in this subbasin along the major drainage-
way. Firs t, in the open a rea ups tream of Ga rr i son, the channe 1 is not
well-defined and the precise flow path of storm runoff is not apparent.
This in itself is not a problem, but it could become one if this area
were to develop into something other than a park. The primary problem
arising from the wide flow of water across this area is how to handle
it once it reaches Garrison Street. The 12-inch reinforced concrete
pipe (RCP) under Garrison wi 11 essentiall,y have nO effect On large flood
flows, and the 50-yard discontinuity in the stream centerline under
present conditions would indicate a large flow of storm water down the
street itself.

This subbasin is tntally developed at the present time, with an overall
imperviousness of about 40 percent, average slope in the basin is 2 per-
cent, and the stream length is about 0.4, miles. About half the length
of the channel is a continuation of the48-inch diameter pipe which out-
lets the depression in Subbasin 3. The remaining channel, beginning at
the crossing of the Rocky Mountain Ditch near First Avenue, consists of
a low, marshy area extending to Wadsworth Boulevard.

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Several major problems exist in this reach. First, the underground pipe
for the first half of the reach leaves no defined surface drainage channel.
Consequently, discharges in excess of the pipe capacity (about 100 cfs)
would flow down First Avenue, effectively cutting off this area to traffic.
This amounts to about 600 feet of First Avenue being inundated.

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The second major problem in SubbaSin 2 is the lake at Rockmont College,
which appears to have an inadequately designed embankment and outlet
works. Modifications by the owner to the dam embankment and/or spill-
way will be necessary to meet current flood requirements as establ ished
by the State Engineer.

SUBBASIN 3 (Rockmont Ponds to pond near First Avenue and Balsam Street)

The second major problem in this area is the floodplain encroachmeAt in the
lower parr of the subbasin. /Iany houses are located along the channel in
this area.

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The third major problem is the restrictive outlet (48-inch RCP) under
Wadsworth Boulevard and the street emba~kment itself. This would cause
additional inundation in the lower reach, due to the backup of water that
could not find its way into the culvert.

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Subbasin 3 is characterized by considerable residential development,
covering about 80 percent of the area. It is about two-thirds of a mile
long, with a small but well-defined channel traversing its length. Over-
all imperviousness is presented in Table 11-1. The average slope
is about 2.5 percent. The basin will be almost totally developed in the
fu ture.

A rather severe problem exists at the low point of Wadsworth Boulevard,
which is about 200 feet north of the gulch centerl ine. The backing up
of water at the centerl ine causeS flows in excess of the culvert capacity
to overtop Wadsworth at this point, flowing onto the large commercial
area on the other side of the street.

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The main problem in this area is the obvious floodplain encroachmrnt by
existing development. The channel, about 3 or 4 feet deep typically,
makes several right-angle turns while coursing through the residential
area. In addition, houses are adjacent to the channel for almost its
entire length. The only areas where this is not evident at the present
time are at the extreme upper and lower ends of this reach. These re3ches,

SUBBASIN 5 (Wadsworth Boulevard to confluence with Lakewood Gulch)

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This subbasin must contend with locally-generated storm runoff, as well
as those flows generated upstream. The area is totally developed, with
a combination of residential and commercial areas, as well as a country
club golf course at the lower end. Overall imperviousness is about 38
percent, with a stream length of 1 .25 miles and an average slope of about
2 percent. Except for the piped area in the upper reach of the subbasin,
and a short section just upstream of the golf course, the Surface channel
is ",e II-defined.

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