FLOOD06450

• Designation Number: 92
• County: Jefferson
• Community: Arvada
• Stream Name: Ralston, Leyden Creeks
• Basin: South Platte
• Title: Major Drainageway Planning - Ralston/Leyden Creek, Arvada, Jefferson County, CO
• Date: 8/1/1977
• Designation Date: 1/1/1978
• Floodplain - Doc Type: Floodplain Report/Masterplan

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V 111-2

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This includes both preventive and corrective actions, the latter encompas-
sing flood control works such as retarding basins and channelization along
with flood-proofing and land use changes.

The 1974 Water Resource Development Act in Section 73 puts nonstructural
alternatives on a par with structural approaches and requires all U.S.
Federal agencies to consider nonstructural alternatives in their planning
process.

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Nonstructural flood plain management seeks to mitigate the effects of a
flood event in lieu of altering the flood event. Because of its relatively
recent acceptance as a viable alternative, it is separately discussed.
This is done to increase awareness of the nonstructural approach.

Additional information on nonstructural (modified) channels can be found
in Section VI of this Report.

Structures. Structures used in this master plan will be designed to
blend into the local environment. Appropriate channel transitions will be
provided upstream and downstream of all structures and maintenance access
will be provided either around or through all structures. If the structure
is located along a recreation belt, pedestrian and bicycle routes will
also be provided at the structure, The following structures will be used
in this master plan.

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Land use management is the keystone of nonstructural flood plain management.
In its broadest terms it involves both land use and runoff controls. Land
use management is used in this report to describe policies of land manage-
ment for prudent and productive use of hazardous areas. It involves a set
of actions at the local, regional and state government levels which can be
reI ied upon to guide the wise use of public and private flood prone land.
These actions include acquisition, legislative controls, land valuati.ons, in-
vestments, and the provision of information.

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Nonstructural flood plain management is not new. For instance, Empress
Maria-Theresa (1740-1780) of the Austro-Hungarian Empire decreed that a
wide flood plain be left undeveloped ~djacent to Vienna for the Danube
River. Current public works engineers for Vienna are grateful for her
fores i ght.

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Great flood plains are left undeveloped and undiked for 50 kilometers up-
stream from Vienna to temporarily store flood peaks, thus further protecting
Vienna even today by providing flood plain storage.

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Ancient Egyptians managed the Nile flood plain for agricultural enhancement
through the entire length of the country. Management included an early
warning and flood forecast system upstream of Aswan at the first cataract.

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Modern articulation of flood plain management in the United States dates to
1945 when Dr. Gilbert White prepared his paper entitled "Human Adjustment
to Floods: A Geographical Approach to the Flood Problem in the United States."
Progress was slow until 1966 when the Task Force on Federal Flood Control
Policy chaired by Dr. White reported to the President of the United States
in a document entitled "A Unified National Program for Managing Flood Losses,"
after which President Lyndon Johnson issued Executive Order 11296 deal ing with
Federal policy on flood plain development.

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Since 1966, the cause of flood plain regulation and management has proceeded
rapidly, spurred on by major flood disasters even where flood control proj-
ects had been completed. It is now a well accepted fact that structural
works for flood control manage the smaller and more frequent floods, but
sometimes set the stage for greater disasters from large floods. The ob-
jective of flood plain management is to optimize the balance between correc-
tive and preventive steps and the flood hazard from all sizes of floods,
Larger floods will occur, the only conjecture is when.

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1. Bridges - Bridges will be used in this master plan for large
design discharges in areas where minimum channel obstructions
are warranted and for skewed crossings.

Bridges will be designed with raised earth benches along the
channel for future maintenance and pedestrian access under
the structure. Bridge spans will be designed for a maximum
length of 50 feet. Debris walls will be incorporated upstream
of piers where appropriate. The water surface of the design
discharge should leave a minimum of 12-inches freeboard to the
low chord of the bridge for debris purposes.

2.

Concrete Box Culverts - Concrete box culverts will be used in
this master plan for moderate design discharges in areas where
greater channel obstructions are allowed. For multiple box
installations, one box will have a raised invert for maintenance
and pedestrian access through the structure. Where possible,
concrete box culverts will be designed with a minimum of 8 feet
clearance for maintenance and pedestrian access. Debris walls will be
incorporated upstream of common box walls where appropriate.
The water surface of the design discharge should leave a minimum
of 12 inches freeboard to the low chord of the box for debris
purposes. When design velocities through the box exceed 10 feet
per second, riprap will be provided upstream and downstream of
the box.

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Structural Steel Pipe Arches-Structural steel pipe arches will be
used in lieu ot bridges and-concrete box culverts where feasible
because of economics. The backwater effect of these structures
must be consistent with channel requirements and adjacent topog-
raphy. For multiple-structure installations, one pipe arch will
have a raised invert for maintenance and pedestrian access
through the structure. Decorative stone walls will be incorporated