16.10.250 Channel stability.
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 or elsewhere.
5. Gullies do not form or enlarge due to the entry of uncontrolled surface 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 bankfull flow, whichever is greater, using “n” values for various channel linings as shown in Table 4. In no case is it necessary to check channel stability for discharges greater than that from a 100-year 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 10-year return period storm on the watershed, or the bankfull flow, whichever is smaller. The “n” 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. 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 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.
|
Cover |
Slope Range (2) (%) |
Permissible Velocity (1) |
|
|
Erosion Resistant Soils (fps) |
Easily Eroded Soils (fps) |
||
|
Bermuda Grass |
0–5 5–10 >10 |
8 7 6 |
6 5 4 |
|
Bahia Buffalograss Kentucky Bluegrass Smooth Brome Blue Grass |
0–5 5–10 >10 |
7 6 5 |
5 4 3 |
|
Grass Mixtures Reed Canarygrass |
(2) 0–5 5–10 |
5 4 |
4 3 |
|
Lespedeza Sericea Weeping Lovegrass Yellow Bluestem Redtop Alfalfa Red Fescue |
(3) 0–5 |
3.4 |
2.5 |
|
Common Lespedeza (4) Sudangrass (4) |
(5) 0–5 |
3.5 |
2.5 |
1Use velocities exceeding five feet per second (fps) only where good cover and proper maintenance can be obtained.
2Do not use on slopes steeper than 10 percent except for vegetated side slopes in combination with stone, concrete, or highly resistant vegetative center section.
3Do not use on slopes steeper than five percent except for vegetated side slopes in combination with stone, concrete, or highly resistant vegetative center section.
4Annuals – use on mild slopes or as temporary protection until permanent covers are established.
5Use on slopes steeper than five percent is not recommended.
*From Soil Conservation Service, SCS-TP-61, Handbook of Channel Design for Soil and Water Conservation.
[Ord. 12-1-97C § 13(D).]