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

4. Roughness Coeffici.ent (Small Drainageway)

A Manning's "n" value range shall be established by the
manufacturer's da1::a, with the high value used to determine
depth/capacity requirements and the low value used to
determine the Froude Number and velocity restrictions.

5. Cross Sections (Small Drainageway)

Same as for grass lined channels (see Se,cti,)n 5.5.2).

5.6 Roadside Ditches

The criteria for the design of roadside ditches is similar to the
criteria for grass lined channels with modificat:lon for the special
purpose of initial stol~m drainage. The cri.te,ria is as follows
(refer to Figure 5-,6):

1. Capacity - Roadsidl. ditches shall have adequate capacity for
the initial storm runoff peaks. Dur ing the initial storm
runoff event, encroachment shall not extend beyond the street
right-of-way. Where the storm runoff exceeds the capacity of
the ditch, a storm sewer system shall be required.

2. Flow Velocity - The maximum velocity for the initial storm
flood peak shall r:.ot exceed 5 feet per second for a type I
ditch, and 7 fps for type II or type III ditches.

3. Longitudinal Slope - The slope shall be limited by the average
velocity of the initial storm flood peaks. Check drops may be
required where street slopes are in excess of 2%. Maximum
permissible slope is 3%.

4. Freeboard - Freeboard shall be equal to the velocity head, or
a minimum of six inches.

5. CUrvature - The minimum radius of curvature shall be 25 feet.

6. Roughness Coefficient - Manning I s "n" values presented in
Figure 5-1 shall be used in the capacity computation for
roadside ditches.

7. Grass Lining - The grass lining shall bE~ in accordance with
USDCM, Section 2.3.2, "Major Drainage - Grass :Lined Channels
-Grass. It

8. Driveway Culverts -. Driveway culverts shall be sized to pass
the initial storm ditch flow capacity without overtopping the
driveway. The mi.nimum si.ze culvert shall be a 12-inch
diameter pipe (or equivalent) with flared end s'~ctions. More
than one culvert may be required.

5 - 18

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