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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Present Condition Peak Flows

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3. Determination of runoff flows based on the data established in 1 and
2 above. The primary method applicable in the Manual is the Colorado
Urban Hydrograph Procedure (CUHP) which is based on a synthetic unit
hydrograph developed by Snyder in 1938.

Presented below in Table 111-1 is a summary of runoff flows under present
conditions for the 5, 10, 50, and 100-year frequency events.

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TABLE 111-1

SOILS, GEOLOGY AND INFILTRATION

PEAK RUNOFF FLOWS
PRESENT CONDITIONS
(All Values in Cubic Feet per Second)

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Soils in the South Lakewood Gulch basin are made up mostly of the Denver,
Englewood, and Nunn series as defined by the Soil Conservation Service.
These soils are generally deep and well-drained, formed either in clayey
alluvium or loamy sediments on terraces, fans, and uplands. The typical
surface layer is from 6 to 10 inches of silty clay or clay loam. The sub-
soil is clay loam or clay, from 23 to 37 inches thick, and overlies silty
clay or clay loam that extends to 60 inches or more. Slopes generally vary
from 0 to 12 percent, and the natural vegetation is mostly grass. The bed-
rock geology upon which most of the surficial deposits 1 ie is the Denver
Formation. It is composed of conglomerate, sandstone, siltstone, and
compact clay, and is about 400 to 500 feet thick.

Subbasin

Peak Flow for Various Frequencies
5-Year 10-Year 50-Yea r lOO-Year

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1 @ Adden brooke Pond 120 180 210 240
2 @ Rockmont Pond 340 500 600 670
3 @ 1st Avenue Pond 400 610 730 810
4 @ Wadsworth Blvd. 490 730 870 970
5 @ Lakewood Gulch 580 860 1,040 1,150

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For the purpose of computing subbasin hydrographs by the Colorado Urban
Hydrograph Procedure (CUHP), the infiltration values recommended in the
Urban Storm Drainage Criteria Manual were used. The soils as described
above are typical of the general soil types in the Lakewood area, and
correlate well with soil types used in previous studies on Lena Gulch
and Ralston/Leyden Creeks. For those studies, both SCS runoff curves
and actual infiltration tests indicated saturated soil infiltration rates
of about 1/2 inch per hour. (Ref. 3).

In contrast to the flows presented above, the 1968 study by the USACE (Ref. 4)
utilizing basin correlation techniques indicated a 100-year flow of about
1,600 cfs at the mouth of South Lakewood Gulch. This corresponds to the
1,150 cfs figure in Table 111-1, which is about 28 percent lower. There
are several reasons for this apparent discrepancy which are discussed as
follows:

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1. The shape of the South Lakewood Basin is extremely long and thin,
with a length to width ratio of about 8. Whereas most typical
basins in the Denver metropolitan area are relatively shorter and
consequently route flood flows much quicker, basin correlation
techniques would tend to overestimate flows on South Lakewood Gulch.

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SOUTH LAKEWOOD GULCH HYDROLOGY

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The CUHP was used to prepare runoff hydrographs for each subbasin in South
Lakewood Gulch. Individual hydrographs were then routed and combined by
the use of the Massachusetts Institute of Technology Catchment Model
(MITCAT). The combination of these two hydrological methods provides for
an economical and efficient analysis of the hydrologic response of the
basin. The various stream network and reservoir routing capabil ities of
MITCAT allow for a flexible approach in analyzing future runoff conditions,
while the widely-accepted CUHP provides the basic input data for the rout-
ing. A more detailed description of both CUHP and MITCAT is presented in
the Hydrological Appendix (Ref. 2).

2. This hydrological study involved the detailed analysis of five
separate subbasins, as well as the actual stream routing characteris-
tics of the basin. This combination of hydrology and hydraulics
carefully considered percent of impervious area slope individual
bb . , ,
su aSln shape, stream cross sections, and roughness factors. This
resulted in a more complete, specialized study of this particular
local area. Larger, basin-wide studies tend to be oriented towards
overall basin effects and do not usually include the detail required
for a refined analysis in one local area.

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3. The basin studied by the USACE was approximately 20 percent larger
than the 1,040 acres analyzed here. By delineating the basin boundary
on standard U.S. Geological Survey maps, surface contours generally

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