(A) Characteristics of a stable channel are:
(1) It neither aggrades nor degrades beyond tolerable limits.
(2) The channel banks do not erode to the extent that the channel cross section is changed appreciably.
(3) Excessive sediment bars do not develop.
(4) Excessive erosion does not occur around culverts, bridges, storm drains or channel outfalls, or elsewhere.
(5) Gullies do not form or enlarge due to the entry of uncontrolled flow to the channel.
(B) Channel stability shall be determined for an aged condition and the velocity shall be based on the design flow or the bank fill flow, whichever is greater, using appropriate "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.
(C) Channel stability must be checked for conditions immediately after construction. For this stability analysis, the velocity shall be calculated for the expected flow from a ten-year return period storm on the watershed, or the bank full flow, whichever is smaller. The "if' value for newly constructed channels in fine-grained soils and sands may be determined in accordance with the National Engineering Handbook 5, Supplement B, Soil Conservation Service and shall not exceed 0.025.
(D) The allowable velocity in the newly constructed channel may be increased by a maximum of 20 percent to reflect the effects of vegetation to be established under the following conditions:
(1) The soil and site in which the channel is to be constructed are suitable for rapid establishment and support of erosion controlling vegetation.
(2) Species of erosion controlling vegetation are adapted to the area, and proven methods of establishment are shown.
(3) The channel design includes detailed plans for establishment of vegetation on the channel side slopes.
(Ord. 1999-05, passed 6-8-99)