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(A) This section describes the technical criteria necessary to design stormwater channels and ditches using conventional design procedures.
(1) These procedures shall be applied to roadside and rear yard ditches and highly urbanized channel.
(2) All blue line streams (especially in undisturbed areas) shall be designed using Natural Channel Design techniques, if possible.
(3) This criterion represents minimum requirements.
(B) Manning’s Equation is required, except in cases where backwater conditions are significant. All calculations must be submitted for review.
(C) Channels and ditches should be capable of conveying the ten-year storm flow within their banks. Through drainage systems (culverts, storm sewers and the like) shall generally be designed to collect and transport the post-development rate of runoff for the ten-year design storm. In all cases, the 100-year discharge elevation shall be checked to ensure that adjacent structures do not suffer flood damage.
(D) All through systems constructed must be capable of passing the 100-year design flow within the drainage easement.
(E) All open channels, whether private or public, and whether constructed on private or public land, shall conform to the design standards and other design requirements contained herein.
(F) The waterway for channels shall be determined using Manning’s Equation, where:
Q = 1.486 R 2/3S1/2A
Q = volumetric flow rate (cfs)
R = the hydraulic radius in feet
S = the slope of the energy grade line in feet per foot
n = roughness coefficient
A = cross-sectional area
The hydraulic radius, R, is defined as the cross sectional area of flow divided by the wetted flow surface or wetted perimeter.
(G) Channel cross section and grade: the design capacity, the material in which the channel is to be constructed, and the requirements for maintenance determine the required channel cross-section and grade. The channel grade shall be such that the velocity in the channel is high enough to prevent siltation but low enough to prevent erosion. Velocities less than one and one-half feet per second should be avoided because siltation will take place and ultimately reduce the channel cross-section.
(H) Side slopes: earthen channel side slopes shall be no steeper than three to one. Flatter slopes may be required to prevent erosion and for ease of maintenance. Where concrete lined channels are required, side slopes shall be no steeper than 12 to one with adequate provisions made for weep holes or subsurface drainage. Side slopes steeper than 12 to one may be used for lined channels provided that the side lining is designed and constructed as a retaining wall with provisions for live and dead load surcharge.
(I) Channel stability:
(1) All channels constructed shall have the following characteristics.
(a) It neither aggrades nor degrades beyond tolerable limits.
(b) The channel banks do not erode to the extent that the channel cross-section is changed appreciably.
(c) Excessive sediment bars do not develop.
(d) Excessive erosion does not occur around culverts, bridges or elsewhere.
(e) Gullies do not form or enlarge due to the entry of uncontrolled surface flow to the channel.
(2) Channel stability shall be determined for an aged condition and the velocity shall be based on the design flow or the bank full flow, whichever is greater, using “n” values for various channel linings. In no case is it necessary to check channel stability for discharges greater than that from a 100-year return period storm.
(Ord. 2010-11, passed 7-22-2010)