Sec. 21A-71. Open watercourse.
(a) Open Watercourse Access. Access to storm drainage ditches and channels shall be by means of maintenance easements. Such maintenance easements shall be not less than twenty (20) feet in width, measured horizontally from the top of the bank, exclusive of the width of the ditch, or channel, and a maintenance easement of this type shall be provided on each side of a flood control or storm drainage ditch channel or similar type of facility. Maintenance easements are to be kept free of obstructions.
(b) Open Watercourse Information.
(1) Design Storm Frequency. Flowing full for ten (10) year storm.
(2) Design Flow.
a. Areas under two hundred (200) acres use Rational Method Q = CiA.
b. Areas between two hundred (200) and three hundred (300) acres transition between Rational Method and Technical Release 55 or industry recognized computer programs.
c. Areas over three hundred (300) acres use Technical Release 55 or industry recognized computer programs.
(3) Runoff Coefficient. Based on Table No. 1 (see elsewhere in this article).
(4) Allowable Velocities—New Ditches.
a. Five (5.0) feet per second (fps) with sod or jute mat lining.
b. Over five (5) feet per second (fps) special lining.
(5) Allowable Velocities—Existing Channels. Ability of the channel to handle the flow satisfactorily.
(6) Manning’s “n” Value.
a. Sod or jute mat lining: 0.05.
b. Paved lining: 0.015.
c. Rock protection: 0.08.
d. Existing lining: 0.025 to 0.20.
(7) Minimum Slope.
a. Desirable for new channels: 0.40 percent.
b. Absolute 0.10 percent—with a minimum velocity of two (2) F.P.S., based on a two (2) year storm.
(8) Side Slopes. Desirable four to one (4:1).