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

2.

Longitudinal ChannEll Slope (Major Drainageway)

Grass lined channe:~. slopes are dictated by velo(~ity and Froude
Number requirements. Where the natural topography is steeper
than desirable, drop structures shall be utilized to maintain
design velocities ,31nd Froude Numbers.

3.

Freeboard (Major Drainageway)

Except where locali;~ed overflow in certain areas is desirable
for additional ponding benefits or other reasons, the
freeboard for the 100-year flow shall be as follows. The
minimum freeboard ishall be 1. 0 foot.

HfB = 0.5 + V2/2g

(Equation 5.2)

Where:

Hra
V
g

= freeboard height (feet)
= average channel velocity (fps)
= acceleration of gravity = 32.2 ft/sec2

4.

Horizontal Curvatu:l:e (Major Drainageway)

The center line curvature shall have a radius twice the top
width of the design flow but not less than 100 feet.

5.

Roughness Coeffichmt (Major Drainageway)

The variation of Nanning's "n" with the, retardance and the
product of mean velocity and hydraulic radius, as presented in
Figure 5-1, shall be used in the capacity ,::omputation.

Retardance curve C shall be used to determine the channel
capaci ty, since a mature channel (substan.ti,:il vegetation with
minimal maintenanCE)) will have a higher J!1a:rming I s "n" value.
However, a recent::.y constructed channel will have minimal
vegetation and the retardance will be less than the mature
channel. Therefore, retardance curve D sha:Ll be used to
determine the limit::ing velocity in a chann'~l.

For the purpose of floodplain definitioni, only the higher
Manning's "n" valuHs need to be considered in the hydraulic
analysis.

6.

Cross Sections (Major Drainageway)

The channel shape may be almost any typE! sui table to the
location and to the environmental conditions. Often the shape
can be chosen to suit open space and recreational functions.
Representative cross sections are presented in Figures 5-2, 5-
3, and 5-4. The limitations within which the design must fall
for the major stornl design flow include:

5-6

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