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

I

5

I

I

Major problems in this subbasin include the overland flow from across
Wadsworth Boulevard and onto the large commercial auto sales area,
sharp channel turns, the disappearance of the channel just before the
golf course, and the Meadow Creek subdivision below the golf course.

SECTION III
HYDROLOGICAL SUMMARY

I

GENERAL COMMENTS

I

The problem of overland flow on the auto sales lot is compounded by a
further constriction of the underground pipe to a 3.5-foot diameter at
its outlet, while its inlet upstream of Wadsworth Boulevard is 4 feet
in diameter. These factors combine to produce shallow flooding in this
commercial area south of Wadsworth Boulevard, which can readily be seen
on Sheet 5, Drawings. The channel then turns abruptly just downstream
of the underground pipe, and courses through an apartment complex with a
well-defined grass-l ined cross section and periodic check structures.
Passing through a large box culvert under Sixth Avenue, the channel then
crosses several backyard areas and then disappears just before the
golf course. In this area, severe inundation of several single-family
residences would result from a large storm.

The extent of present flooding is determined by the magnitude of runoff.
The floodplain del ineation must be based on adequate and thorough
hydrological studies if the delineation is to have a significant value
and establ ish a firm factual basis. Hydrological studies must take into
consideration volume of runoff, as well as peak rates of flow. It is
when the volume characteristics are known that one realizes the value of
existing detention storage, both pond type and channel storage, to reduce
the peak rates of flow.

1

1

I

The golf course at the lower end of the South Lakewood Gulch basin provides
a natural and effective conveyance for flood waters, A residential sub-
division at its lower end however, is subject to flooding from the golf
course discharge. Just upstream of the subdivision, the golf course
channel makes an abrupt right-angle turn, which would divert low flows
from the subdivision. Higher flood flows would tend to overshoot the
turn however, and inundate many of the multifamily units in Meadow Creek
subdivision, The outlet to Lakewood Gulch would be either upstream of
the subdivision, or an alternate course through the subdivision, depend-
ing on the magnitude of flow.

The magnitude of a flood can be judged by its maximum rate of flow, even
though the maximum rate may last for only 10 minutes. The frequency
of a flood is judged by its return period. The return period is defined
as the average interval of time within which a given event will be equalled
or exceeded once. Thus, a flood having a return period of 100 years has
a I percent probabil ity of being equalled or exceeded in any given year.

1

Runoff flows were determined at many locations for return periods of 5,
10, 50, and 100 years so that a determination could be made concerning
the storm frequency for which drainage facil ities should be designed.
This section highl ights basic information; details of the computational
methods are presented in the Hydrological Appendix.

I

1

I

Pertinent hydrological characteristics for each of the five subbasins are
summarized below in Table 11-1:

The approach taken by the Denver Regional Council of Governments (DRCOG)
Urban Storm Drainage Criteria Manual in the determination of runoff flows
is:

I

TABLE I 1-1

1. Determination of rainfall that would most 1 ikely occur during an
event with a given return period by use of isohyetal (1 ine of equal
depth rainfall) depth-duration-frequency maps that were synthesized
during the formation of the Manual. This provides a basic input
for the runoff computation. This information was analyzed for the
South Lakewood Gulch watershed and found to be relatively constant,
considering the relatively small size of the basin.

SUBBASIN CHARACTERISTICS

I

I

Area Stream Length Slope Percen t Impervious
Subbasin (Acres) (Feet) (Mi 1 es) (Feet/Feet) Present Future
1 170 6,500 1.23 0.04 31 32
2 350 4,000 0.76 0.04 25 37
3 170 3,500 0.66 0.025 33 40
4 150 2,200 0.42 0.02 40 40
5 200 6,600 1.25 0.02 38 38

2.

Field inspection
impervious cover,
to runoff flows.

and analysis of data pertaining to stream lengths,
slopes, and other physical phenomenon pertaining
This information is presented in Table 11-1.

I

1

I

I.