PERMFILE110032

• Permit No: C1981010
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Doc Name: Johnson Gulch Drainage Report & Standard Ditch Designs
• Section_Exhibit Name: Appendix P
• Media Type: D
• Archive: No

Standard Ditch Design
Page 2
Where: R= Hydraulic Radius, Ft.
Vet = Mean Velocity of Flow
S Slope of Ditch
n = coefficient of roughness
Q (Vel) x (X-Area)
Q Design = (Q) = (F.S.)
GRAPHS
This paper contains two graphs for use in selecting the proper sized ditch.
Graph 111 is for watersheds less than 500 acres in area. Graph (l2 is an extension
of Graph lfl to cover areas up to 1000 acres. To use these graphs, the area of
the watershed that drains to the ditch and average slope of the watershed must be
determined: The area of the watershed can be scaled or planimetered and the average
slope can be determined as follows: (2)
1) Select three contours located at 25, 50 and 75 percent of the
watershed height (Z)
2) Measure the length of each contour LC 25, LC 50, LC 75
3) Solve for the average slope by:
S = 0.25 Z (LC 25 + LC 50 + LC 75)
• Watershed Area
(Units are in feet or square fee[)
The average slope can also be approximated from a topographic map.
Once this information is determined, the intercept of the average slope with
the watershed area can be found on the appropriate graph. The first curve encount-
ered above the intercept is the value of D (the depth of the V-shaped ditch) that
should be used for the particular watershed.
When the depth of the ditch (D) is known the ditch dimensions and specifications
are also known.
MINIMUM DITCHES
To insure uniformity and proper drainage minimum ditch depths should be used
for areas of minimal runoff. For haulroads, ditches should be at least two feet in.
depth and for access roads, a minimum of one foot should be used.
(1). Civil EnAineerinQ Handbook, Fourth Edition, .Urquhart, L.C., McGraw-Hill Book
Company, 1962, p. 4-68
(2) Procedures for determining peak flows in Colorado, U.S. Dept. of Agriculture,
Soil Conservation service, March, 1977.