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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7. If a natural channel is to be utilized as a major drainageway
for a developed area, then the applicant shall meet with the
City to discuss the concept and to obtain the requirements for
planning and design documentation including the completion of
a detailed channel stability analysis.

8. Plan and profile drawings of the floodplain shall be prepared.
Appropriate allowances for known future bridges or culverts,
which can raise the water surface profile and cause the
floodplain to be extended, shall be included in the analysis.
The applicant shall contact the city for information on future
bridges, culverts or other planned improvements.

with most natural waterways, erosion control structures should be
constructed at regular intervals to decrease the thalweg slope and
to control erosion. However, these channels should be left in as
near a natural condition as possible. For that reason, extensive
modifications should not be undertaken unless they are found to be
necessary to avoid excessive erosion with subsequent deposition
downstream.

The usual rules of freeboard depth, curvature, and other rules
which are applicable to artificial channels, do not apply for
natural channels. All structures constructed along the channel
shall be elevated a minimum of one foot above the lOO-year water
surface level. There are significant advantages which may occur if
the designer incorporates into his planning the overtopping of the
channel and localized flooding of adjacent areas which are laid out
and developed for the purpose of being inundated during the major
runoff peak.

5.4.2

Grass Lined Channels (Major Drainagewayl

Key parameters in grass lined channel design include velocity,
slope, roughness coefficients, depth, freeboard, curvature, cross-
section shape, and lining materials. other factors such as water
surface profile computation, erosion control, drop structures, and
transitions also play an important role. A discussion of these
parameters is presented below.

1. Flow Velocity (Major Drainageway)

The maximum normal depth velocity for the lOO-year flood peak
shall not exceed 7.0 feet per second (fps) for grass lined
channels, except in sandy soil, where the maximum velocity
shall not exceed 5.0 fps. The Froude Number shall be less
than 0.8 for grass lined channels. The minimum velocity,
wherever possible, shall be greater than 2.0 fps for the
initial storm runoff.

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