FLOOD08407

• County: Jefferson
• County: Adams
• Community: Arvada, Westminster
• Stream Name: Little Dry Creek
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 4/1/1979
• Prepared For: Jefferson County / Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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V.
DESIGN CONSIDERATIONS AND CRITERIA

The master plan presented in this report was developed using criteria set forth by
the local entities and the Urban Drainage and Flood Control District. This chapter
describes the design criteria used and considerations observed in arriving at the final
Phase "B" plan for flood control.

Channel Design
General - The preliminary design of the proposed channel and related improvements
is based on criteria set by the Sponsors of the Study. These criteria are generally based on
the Urban Storm Drainage Criterial Manual, reference E, and on the Notice to Proceed,
Appendix A, and have been adapted to fit the local needs and conditions. Listed in Table
V-l are the criteria on which the hydraulic design of the Little Dry Creek Storm Drainage
facili ties improvements have been based.
Table V-1
Design Criteria

Grass-lined
Channels
.030
7.5 fps
2.0 fps
4:1
100' radius
1'min.

Concrete-lined
Channels
.015
15 fps
2.0 fps
2:1 or verticle
70' radius
1'min.

Underground
Conduits
.013

2.0 fps

Mannings 'n'
Max Permissible Vel.
Mini Permissible Vel.
Side Slopes
Minimum Curvature
Free board

2 x diameter
Hydraulic G.L.
Below Surface

These criteria and that established by the Sponsors should be carried over into the
final design.
Grass-Lined Channel - Where environmental and aesthetic aspects were considered
of prime importance, a grass-lined channel with a rock-lined trickle channel is proposed to
convey flood waters of the varying design storms. To facilitate mowing, channel side
slopes should be no steeper than 4 to 1. Drop structures are incorporated into the reaches
of this type of channel to keep velocities of flow less than 7.5 feet per second. The
modified channel is to be seeded with natural, drought resistant grasses that require only
occasional mowing. Each local entity will have the option of using a grass type that would
require irrigation and extensive mowing. Upon completion of final grading and seeding,
the channel banks may require a temporary bank stablization material such as jute mesh.
Concrete-Lined Channel - Concrete lined open channels are proposed in Reaches 1
and A1B where right-of-way limitations preclude the use of grass-lined channels. The use
of this type of channel will result in higher flow velocities which may require that the

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channels be fenced to restrict access. Special consideration must be given in the final
design to design of curves, lining and transition sections to minimize waves and to insure
the integrity of the channel lining during times of high flow. Where concrete channels are
used, consideration should also be given during the final design to the freeboard of the
channel, which should not be less than 1 foot at the minimum.
Concrete-Walled Channel - This channel section is used where right-of-way
constraints dictate the use of a channel width less than that of a grass channel of
equivalent capacity. The channel provides a hydraulically more efficient section by
utilizing vertical concrete side walls and a natural grass bed. Channel flow velocities are
limited to 7.5 feet per second by incorporating drop structures into reaches of this type of
channel.
Trickle Channel - Trickle flow is considered to be the runoff from events such as
ground water, snowmelt, lawn watering, and short duration low intensity storms. This
flow is approximated at 1% of the design storm flow. A trickle channel, constructed of
rock riprap, is proposed in all grass-lined channels. Where concrete-lined channels are
being constructed, the channel base should be properly notched to convey the trickle
flow.

Structures
Various hydraulic structures are proposed to control storm water. Suitable channel
transitions are to be provided upstream and downstream of all structures and appropriate
maintenance access should be provided so that each structure can be inspected and
repaired regularly to assure its integrity. The following structures are proposed for use
on Little Dry Creek:
1. Bridges - Bridges are proposed at a majority of the crossings to be improved on the
main stem of Little Dry Creek to convey flows beneath streets and railroads. The
bridge opening sizes designated in the preliminary design drawings are the
dimensions perpendicular to flow, and are the minimum required to convey the
design flow. If desired, opening heights could be enlarged to 8 feet of clearance for
maintenance and pedestrian access. An inclined drop, starting 50 feet upstream
from the bridge opening and varying from 1 to 2 feet vertically, is placed at some
bridges to increase inlet efficiency. Riprapped aprons are provided at bridge inlets
and outlets to minimize scour caused by the higher headwater depths and turbulent
flows. The rock riprap trickle channel should continue through each structure.