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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RALSTON CREEK FROM ABOVE PIERCE STREET TO BELOW STATE HIGHWAY 121 - Station
60+00 to 70+00

RALSTON CREEK FROM ABOVE WADSWORTH BOULEVARD TO BELOW GARRISON STREET -
Station 95+00 to 140+00

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A grass-l ined channel is to be used in this reach of Ralston Creek, Currently
there is an existing grass-l ined channel in this reach that was constructed
by the land developer. The channel was designed for the Corps of Engineers'
original flow projection of approximately 7,200 cfs, This channel can
accommodate the design discharge of 8,400 cfs with minor inundation of the
apartment buildings. The cost to flood proof the apartment buildings is
greater than the expected damages, therefore, no additional work is antici-
pated in this area for the master plan. The building owners are encouraged
to obtain flood insurance and to flood proof. The 100-year main channel
average velocity of 9.3 feet per second will produce certain damages to the
channel, however, the extent of these damages would not warrant additional
channel work in this area,

The master plan includes no governmental improvements to be used in this
reach of Ralston Creek except at Carr Street. The 100-year improved channel
flood plain is smaller than the existing channel flood plain due to the de-
creaSe in the flood peak in Van Bibber Creek for future conditions. Justi-
fication for the lower flows may be found in the Van Bibber Creek Phase B
Master Plan Report. Currently, this reach of channel is a grassed linear
park. The 100-year main channel average velocity of 9.6 feet per second
will produce certain damages to the channel, however, it is felt that the
extent of these damages would not warrant additional channel work in this
area.

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This reach of channel is presented on Sheet 22.

Carr Street will have the existing pipe replaced with three structural
steel plate arches, Transitions to the existing channel will be provided
upstream and downstream of this structure.

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RALSTON CREEK FROM BELOW STATE HIGHWAY 121 TO ABOVE WADSWORTH BOULEVARD -
Station 70+00 to 95+00

The master plan improvements for this reach of channel are presented on
Sheets 20 and 21. These improvements wi 11 decrease the area of the flood
plain in the vicinity of Carr Street and will alleviate the overtopping of
Carr Street-

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A European-type channel is to be used in this reach of Ralston Creek. For
the 100-year flow, this channel will have an average velocity of 7.6 feet
per second, a base width of 125 feet, a major channel depth of flow of 8
feet and textured concrete side walls that slope at 0.5 to 1. It will
follow the alignment of the existing channel except where the existing
channel makes a 90 degree bend, In these areas, the European-type channel
will bend in a gentle curve. Transitions to the existing channel wi 11 be
provided upstream and downstream of this length of channel with a berm located
adjacent to the upstream transition to guide overbank flows into the main
channel. A total of seven residences and seven separate garages will be
removed in order to build this channel. It is anticipated that most of the
houses will be purchased as they come on the market in order to minimize
the sociological impact of this portion of the master plan.

The homeowners left in the 100-year flood plain are encouraged to obtain
flood insurance and to flood-proof their dwellings.

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RALSTON CREEK FROM BELOW GARRISON STREET TO BELOW BROOKS DRIVE - Station
140+00 to 152+00

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State Highway 121 will have the existing double box culvert replaced with a
150 foot long bridge and Wadsworth Boulevard will have the existing pipe
replaced with a 140 foot long bridge.

A short length of European-type channel is to be used below Garrison
Street adjacent to the North Jeffco Metropolitan Recreation and Park
District's Ice Arena. Due to the constraints of Brooks Drive, north of
the channel, and the ice arena on the south, the European-type channel will
be sized to carry 4,500 cfs, which is approximately the 5-year flow. For
the 5-year flow, this new channel will have an average velocity of 10 feet
per second, a base width of 50 feet, a major channel depth of flow of 9
feet, and vertical concrete walls.

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The master plan improvements for this reach of channel are presented on Sheets
21 and 22. These improvements will decrease the area of the flood plain in
this reach and will alleviate the overtopping of Colorado Highway 121 and Wads-
worth Boulevard during a 100-year event,

A grass-l ined channel will be used in the remainder of this reach. The
channel will be sized to carry 4,000 cfs, which is approximately the 5-year
flow above Van Bibber Creek, For this design flow, this new channel will
have an average velocity of 8.0 feet per second, a base width of 20 feet, an
overbank depth of flow of 9 feet and 4 to 1 side slope. A transition from
the European-type channel to the existing channel will be provided at the
ice arena and a transition from the grass-l ined channel to the European-type
channel will be provided below Garrison Street.

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Garrison Street will have the existing box culvert replaced with a 100-foot
long bridge and the roadway embankment will be stabilized with rock riprap.

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