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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for the 100 year storm and 0.6 in/hr. for the 2, 5, and 10 year storm were
used for this study due to the loamy-clay type soils which are characteristic
of the basin.
The rainfall-runoff model used for this analysis is the Colorado Urban
Hydrograph Procedure (CUHP) presented in the Urban Storm Drainage Criteria
Manual, with May 15, 1975 revisions to the basic data and procedure. In
addition, the revision to the Cp and Ct calculation for basin imperviousness
and slope presented in the June 1977 issue of "Flood Hazard News" was also
used. A computer program was used to generate the unit hydrograph, the
excess precipitation, and resulting storm runoff hydrograph for each of the
sub-basins and four design storms.
B. Flood Routing
The next step in the hydrologic analysis consisted of routing and
combining the individual sub-basin storm hydrographs to determine the
peak flows and runoff volumes at the hydrologic points. The process of
routing consisted of channel routing, storage or detention routing, and
hydrograph separation procedures for special applications of flow splitting
(separation of channel).
The Muskingum method of.channel routing was used in this analysis.
Due to the relatively steep channel which is narrow and incised throughout
most of the basin, channel routing had very minimal effect (less than 1%)
on the peak flows.
The Modified Puls method of flood routing through a reservoir or an
inadvertent detention area was used for this analysis. Detention routing
was performed for the detention ponds within the North Branch of the study
area. Significant reductions in the peak runoff values were achieved by

the detention ponds in the Union Square development. The pre-development
condition (prior to development of Union Square) 100 year peak storm runoff
for the North Branch area was estimated at 610 cfs as compared to a 100 year
peak runoff value of 480 cfs (Table 4) for the future developed condition
with the effects of detention.

One of the common effects of urbanization of smaller natural drainageways

is the separation of flow or flow splitting, which is caused by the inability

of a drainage facility, such as a storm sewer or culvert, to pass the entire

storm flows, or by splitting of flows at a street intersection. The result

of the flow splitting is that portions of the runoff are carried in more than

one direction.

Some flow splitting occurs in the South Branch of the study area along
Alameda Parkway, where the street culverts are inadequate to pass the major
storm flows, and the runoff overtops Alameda Parkway and flows to the south.
The flow splitting condition is particularly severe along the south side of
Alameda Parkway, about 800 feet east of Union Blvd., where the box culvert
under Alameda Parkway is incapable of passing the 100 year storm flows,
and the flows split off to the east along Alameda Parkway leaving the main
channel. The storm flow reconverges in the main channel within the Federal

Center.

The results of the hydrologic analysis are presented as storm runoff

peaks and volumes for the hydrologic points of interest in Table 4. Flood

hydrographs for several of the hydrologic design locations were plotted

and are presented in Figures 2 to 5.

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