FLOOD00281

• County: Larimer
• Community: Greeley
• Title: Design Criteria and Construction Specifications volume 2 of 5 Storm Drainage Criteria
• Date: 10/1/1997
• Prepared For: Greeley
• Prepared By: Department of Public Works
• Floodplain - Doc Type: Educational/Technical/Reference Information

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5.2.4

Rock Lined Channels

Riprap lined Ghannels are generally discouraged and shall be
permitted only in areas of existing development where right-of-way
for major drainageways is limited and such limitation prohibits the
use of grass lined channels. The advantage of rock lining a
channel is that a steeper channel grade can be used due to the
higher friction coefficient of the rock and a higher allowable
shear stress. Also, steeper side slopes are permitted. Rock
linings, or revetments, are permitted as a means of controlling
erosion for natural channels.

If the project constraints dictate the use of riprap lining for a
major drainageway, then the engineer must present the concept, with
justification, to the City for consideration of a variance from
these Criteria. The design of rock-lined channels shall be in
accordance with the most current revision of the USDCM, Volume 2,
"Major Drainage, section 5 - Riprap."

5.2.5

other Lining Types

The use of synthetic fabrics and slope revetment mats for major
drainageways in the city is restricted to areas of existing
development where the ROW constraints prohibit the use of a grass
lined section. A synthetic lining, such as a soil stabilization
fabric, in combination with grass lining may be acceptable in some
situations. Such use shall be allowed only upon written approval
from the City. The linings shall be restricted to channels with a
Froude Number of 0.8 or less.

5.2.6

Wetlands vegetation Bottom Channels

The selection of a particular channel can be based on many factors,
including hydraulic practice, environmental design, sociological
impact, and basic project requirements. However, prior to choosing
the channel type, the need or desire for channelization should be
established.

Once a decision is made to channelize, then investigations into the
status of the present drainageway are necessary to define the
constraints on the channel design. For instance, if the channel
presently has wetland characteristics, then the Section 404
requirements of the Clean Water Act may require that the design
maintain a wetland area. The engineer should contact the Corps of
Engineers for additional information.

The process of choosing a channel configuration and the design
criteria for a wetlands bottom channel (if this type is selected)
shall follow the latest revision of the USDCM. The engineer is
referred to these interim criteria (Section 2 of the USDCM under
Major Drainage) for the procedures and criteria for all channel
design.

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