FLOOD04017

• County: Jefferson
• County: Denver
• Community: Columbine Valley, Denver
• Stream Name: Dutch Creek, Lilley Gulch, Coon Creek,
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 5/1/1978
• Prepared For: Jefferson County / Arapahoe County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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illustration of the 100-year peak flow hydrographs for the present and

future conditions at Design Point D-6, Dutch Creek at Platte Canyon

Road. Peak flows for the 2-, 5-, 10-, and 50-year storms future con-

ditions are also plotted.

Routed peak at D-6

Total basin hydrograph
at D-6

7,389 cfs

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6,890 cfs

Flood Routing

A summary of the peak flows at each of the design points for each fre-
quency is presented in Table 2.

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After obtaining the storm hydrographs for the individual sub-basins,
it was necessary to combine these flows in such a manner to determine
the total flows throughout all of the channels. The travel time was
calculated for each reach, using an estimated velocity of 6 feet per
second and was then rounded to the nearest 10 minute interval. The inflow
hydrograph to the sub-basin was delayed by this time interval and combined
with the sub-basin hydrograph.

A summary of peak flows for the present and future developed conditions

for each frequency is also provided on the Discharge Probability Curves

shown on Figures BelA through BelD in Appendix B.

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Only the 100-year discharges for the present and for the future condi-

tion were routed. Flows for the 2-, 5-, 10-, and 50-year storms were

interpolated, using the subbasin hydrographs for these frequencies as

a guide to the relative magnitudes of flow.

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One hundred year discharges for the future condition were routed through

Beer's Sisters' Reservoir on Coon Creek and through Johnson Reservoir.

An attempted routing through Grant "A" Lake showed that the dam would

be overtopped, and so was not considered. Considerable enlargement of

the spillway would be required to pass the 100-year future runoff.

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The 100-year discharges for the present condition were also routed through

the same reservoirs and in this instance Grant "A" Lake was adequate;

however, the water surface was within a foot of overtopping the dam.

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For purposes of comparison, a 100-year storm hydrograph for the total

basin at D-6 was computed. The sub-basin above D-l was omitted because

of its significantly different characteristics and because it would not

contribute to the peak flow due to the long travel time.

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