FLOOD01280

• Designation Number: 93
• County: Jefferson
• Community: Lakewood
• Stream Name: McIntyre Gulch
• Title: Flood Hazard Area Delineation - McIntyre Gulch, Lakewood, CO
• Date: 10/1/1977
• Designation Date: 1/1/1978
• Floodplain - Doc Type: Floodplain Report/Masterplan

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CHAPTER III
HYDROLOGIC ANALYSIS

The slopes of the sub-basins varied from 1.7% to 11% with an average
of about 4.6%. The average imperviousness for the basin is about 48~ for
the future development. A summary of all the basin characteristics is

presented in Table 2 for the future development conditions.
2. Design Rainfall
The design rainfall for the study area was obtained from the
Rainfall/Runoff Information report by the Denver Regional Council of
Governments (D.R.C.O.G.) for the 2, 5, 10, and 100 year return period storms.
The data was obtained for the area labeled Township 4 South and Range 69
West and compared to the storm isohyetals presented in the Urban Storm
Drainaqe Criteria Manual. The resulting design total rainfall (2 hour
duration) used for this analysis is presented in Table 3 for 5 minute

A. Basin Hydrology
A detailed investigation of the flow patterns of the basin area, using
the City of Lakewood maps and several field inspections resulted in the
overall basin boundary as shown on sheet 2 in the appendix. The overall
basin was subsequently subdivided into smaller sub-basins to define the
hydrology at the critical design locations along the drainageway.
1. Sub-basin Characteristics

The entire study area was divided into 18 sub-basins ranging in
size from 20 acres to 571 acres (see sheet 2 of Appendix and Table 2). For

each of the sub-basins, the area, length, centroid length, slope, percent

increments.

imperviousness, Cp and Ct values were determined for the future development

conditions. The future development parameters were defined by superimposing

3. Storm Runoff and Hydrographs
The storm runoff peak values and total volume are dependent upon
various factors, which include total precipitation, incremental precipitation
pattern, infiltration rates, antecedent soil moisture conditions, and basin
characteristics. The total precipitation and incremental pattern chosen
for this analysis is based upon a thunderstorm type occurrence, since a
thunderstorm precipitation arrangement yields higher peak flows. The peak
runoff values are generally the critical design element for urban drainage
facilities.

the projected future land use on the undeveloped area of the basin, then

computing the basin characteristics. The projected future land use was

determined from the existing zoning maps for the City of Lakewood and from

the "Concept Lakewood" land use plan. A small portion of the zoned area

was modified for a different future land use, per direction from the Lakewood

Planning Department.

The Cp and Ct parameters for each sub-basin were determined using the

updated criteria (May 15, 1975) presented in the Urban Storm Drainage Criteria

Manual and in the June 1977 issue of "Flood Hazard News". Due to the lack of

The infiltration rates used for this analysis were based on the
information presented in the Urban Storm Draina~Criteria Manual and the
results of the soils survey by the S.C.S. Infiltration rates of 0.4 in/hr.

significant lengths of storm sewer in the sub-basins, the Cp and Ct parameters
were only adjusted for sub-basin slope.

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