FLOOD05088

• County: Adams
• Community: Arvada
• Stream Name: Hidden Lake, Bates Lake
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 11/1/1975
• Prepared For: Urban Drainage and Flood Control District, Arvada and Adams County
• Prepared By: Hydro-Triad, LTD.
• Floodplain - Doc Type: Floodplain Report/Masterplan

111-4

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The floodway definition that was used for this analysis and
accepted for use by Adams County, Arvada and U.D. & F.C.D. is based
on the criteria that the water surface elevation in the floodway
would increase by no more than 0.5 feet with development in the flood
storage area. Since a significant portion of the floodplain in the
Hidden Lake and Bates Lake basins is shallow (generally less than 1.0
feet) overland flow without a defined channel, two techniques were
needed to define the actual floodway. The first technique defines
the floodway based on channel cross-section rating, where the flood-
way width is adjusted until the 0.5 foot criteria is met, This type
of floodway is referred to as a channel floodway.
The second technique used to define the floodway for the overland
flow portion of the floodplain, referred to as an overland floodway,
is defined as follows:
a) review the reach in question to determine the local slope,
Manning's n value and the average depth to the nearest 0.5
foot (typically one foot);
b) from the depth, n value and slope, define a width of channel
to pass the 100 year storms using infinite width channel
techniques;
c) compare the floodway width to the floodplain width. If
the defined width is greater than the floodplain, increase
the depth and repeat steps band c.
This method is very flexible and can be modified by adjusting the
depth of flow until a reasonable floodway width is determined. However,
the ratio of the width to the depth of flow should be greater than