FLOOD06660

• County: Douglas
• County: El Paso
• County: Elbert
• Community: Franktown, Parker
• Stream Name: Cherry Creek Watershed
• Basin: South Platte
• Title: Franktown Parker Tributaries fo Cherry Creek Watershed
• Date: 4/1/1960
• Prepared For: State of Colorado
• Prepared By: USDA Soil Conservation Service
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Range site and condition classes were mapped for the watershed
by the Area Range Specialist. These classes were converted to hydrologic
condition classes with the assistance of the Range Specialist and the
Work Unit Conservationist. Hydrologic soil groups and weighted values
were determined from data obtained from a soils map prepared by
the Soil Conservation Service. Runoff curve numbers were computed for
each subwatershed from weighted soil-cover complex data. The curve
numbers for floodwater storage range from 72 in the upper portion of
the watershed to 82 in the lower part.

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Engineering surveys were made on 121 channel and valley cross
sections. These were selected to adequately represent the stream
hydraulics and floodplain areas, Three to five cross sections were
surveyed per mile of stream channel.

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Hydrographs were developed by using methods described in
Engineering Memorandum EWP-l and as described in Part 3.16. National
Engineering Handbook, Section 4, Supplement A.

The damaging floods are caused by summer thunderstorms of
short duration and high intensity. These thunderstorms are the
controlling factor for flood detention storage. Flood detention
storage to control runoff varies from 0.52 inches to 0.87 inches
and represents a 25-year frequency floodwater volume for class "au
structures. Floodwater detention storage to control 1.34 inches
of runoff above structure number FP-Pl represents a lOa-year frequency
floodwater volume for the class uc" structure.

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The Wilson Method of flood routing was used throughout the project
in determining area, by depth increments, that would be inQ~dated
by each storm in the evaluation series for the following conditions:
(1) present, (2) future with planned land treatment measures installed,
(3) future with planned land treatment and structural measures
installed.

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Sedimentation Investigations

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Previous records of sediment production in the watershed include
suspended load measurements at mainstem gaging stations and several
reservoir sedimentation surveys. Measurements of suspended sediment
load were made by the U. S. Corps of Engineers over a period of 5
years at the Franktown and Melvin gaging stations. The average
sediment load at the Franktown station was 0.155 acre-foot per square
mile per year, The average load at the Melvin station was 0.456
acre-foot per square mile per year.