All open channels subject to this chapter whether private or public, and whether constructed on private or public land, shall conform to the design standards and other requirements contained herein. The waterway for channels shall be determined using Manning’s Equation.
(A) Manning’s Equation.
Q = (A)(V)
Where;
V = 1.486 R(2/3) S(1/2)
n
n
A = Waterway area of channel in square feet
Q = Discharge in cubic feet per second (cfs)
V = mean velocity of flow in feet per second
R = the hydraulic radius in feet
S = Slope of the energy grade line in feet per foot (approximated by channel bottom slope in most cases)
n = roughness coefficient
(B) Channel cross section and grade.
(1) The required channel cross section and grade are determined by the design capacity, site topographical conditions, the material in which the channel is to be constructed, and the requirements for maintenance.
(2) A minimum depth may be required to provide adequate outlets for subsurface drains, tributary/lateral ditches or streams. 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, where possible, because siltation will take place and ultimately reduce the channel cross section resulting in more frequent maintenance required. The maximum permissible velocities in vegetation-lined channels are shown in Table 4. Developments through which the channel is to be constructed must be considered in design of the channel section. The proposed channel may increase sediment load, peak discharge and velocity in downstream reaches.
(C) Side slopes. Earthen channel side slopes shall be no steeper than two to one, and shall be dependent on existing soil conditions. Flatter slopes may be required to prevent sloughing or erosion and for ease of maintenance. It is desirable to have slopes that are easily mowed to control vegetation and the growth of brush. Where channels will be lined, side slopes shall be no steeper than one and one-half to one with adequate provision made for bank drainage. Side slopes steeper than one and one-half to one may be used for lined channels; provided that, the side lining and structural retaining wall are designed and constructed with provisions for live and dead load surcharge. Maximum allowable side slopes for road side ditches are outlined in the Knox County Subdivision Control Ordinance.
(D) Channel stability.
(1) Characteristics of a stable channel are:
(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 other channel structures; and
(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 roughness coefficient “n” values for various channel linings as shown in Table 3.
(3) Channel stability must also 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 from the watershed, or the bank full flow, whichever is smaller. The roughness coefficient “n” value for newly constructed channels shall be as shown in Table 3. The allowable velocity in the newly constructed channel may be increased by a maximum of 20% to reflect the effects of vegetation to be established under the following conditions:
(a) The soil and site in which the channel is to be constructed are suitable for rapid establishment and support of erosion controlling vegetation;
(b) Species of erosion controlling vegetation adapted to the area and proven methods of establishment are shown; and
(c) The channel design includes detailed plans for establishment of vegetation on the channel side slopes, and a maintenance plan is in place to ensure vegetation is established.
(E) Drainage of waterways. Vegetated waterways that are subject to low flows of long duration, or where wet conditions prevail, shall be drained with a tile system or by other means such as paved gutters. Tile lines should be a minimum of six inches in size, be offset from the center line, and may be outlet through a drop structure at the end of the waterway or through a standard tile outlet.
(F) Appurtenant structures.
(1) The design of channels shall include all structures required for the proper functioning of the channel, the laterals thereto, and travel ways for operation and maintenance. Recessed inlets and related structures needed for entry of surface and subsurface flow into floodway channels, without significant erosion or degradation, shall be included in the design of channel improvements. The design is also to provide the necessary flood gates, water level control devices, structural grade control and any other appurtenance affecting the functioning of the channels and the attainment of the purpose for which they are built.
(2) The effect of channel improvements on existing culverts, bridges, buried cables, pipelines, utilities and other like items shall be evaluated to determine the need for modification or replacement. Culverts and bridges which are modified or added as part of channel improvement projects shall meet design and construction practice standards for the type of structure, and shall have a minimum capacity equal to the design discharge or appropriate governmental agency design requirements, whichever is greater.
(3) Utilities on or adjacent to a site being developed, or redeveloped, are the sole responsibility of the developer. The Board will not be held responsible for the existence, location, modification or damage to any utilities encountered while performing any activities related to this chapter.
(G) Disposition of spoil. Spoil material resulting from clearing, grubbing and channel excavation shall be disposed in a manner which will:
(1) Minimize overbank wash. Maintain a minimum 15-foot wide berm between the top of bank and beginning of spoil toe;
(2) Provide for the free flow of water between the channel and floodplain unless the valley routing and water surface profile are based on continuous dikes being installed;
(3) Not limit the deve lopment of travel ways along the ditch for maintena nce purposes;
(4 ) Leave the right-of-way in as good of condition or better than existed prior to construction; or in the best condition feasible for productive use by the owner, consistent with the project purposes;
(5) Improve the aesthetic appearance of the site to the extent feasible;
(6) Be approved by the IDNR or US Army Corps of Engineers (as applicable) if deposited in the floodway; and
(7) Comply with 327 I.A.C. 15-5, “Rule 5”, erosion and sediment control requirements.
(H) Brush and trees. Brush and trees may be disposed of by removing from the site, burying or burning in accordance with local and state regulations.
Table 4: Maximum Permissible Velocities in Vegetal-Lined Channels and Watercourses(6) | |||
Cover | Slope Range (%) | Permissible Velocity (1) | |
Erosion Resistant Soils (ft. per sec.) | Easily Eroded Soils (ft. per sec.) | ||
Table 4: Maximum Permissible Velocities in Vegetal-Lined Channels and Watercourses(6) | |||
Cover | Slope Range (%) | Permissible Velocity (1) | |
Erosion Resistant Soils (ft. per sec.) | Easily Eroded Soils (ft. per sec.) | ||
Alfalfa | 0-5 | 3.5 | 2.5 |
Bermudagrass | 0-5 | 8 | 6 |
5-10 | 7 | 5 | |
over 10 | 6 | 4 | |
Blue Grass (other turf grasses) | Over 10 | 5 | 3 |
Common Lespedeza(4) | 0-5(5) | 3.5 | 2.5 |
Grass Mixtures | 0-5(2) | 5 | 4 |
Kentucky Blue grass | 0-5 | 7 | 5 |
Lespediza Sericea | 0-5(3) | 5 | 4 |
Red Fescue | 0-5 | 3.5 | 2.5 |
Redtop | 0-5 | 3.5 | 2.5 |
Reed Canarygrass | 5-10 | 4 | 3 |
Smooth Brome | 5-10 | 6 | 4 |
Sudangrass(4) | 0-5 | 3.5 | 2.5 |
NOTES TO TABLE: (1) Use velocities exceeding 5 feet per second only where good cover and proper maintenance can be obtained. (2) Do not use on slopes steeper than 10%, except for vegetated side slopes in combination with stone, concrete or highly resistant vegetative center section. (3) Do not use on slopes steeper than 5%, except for vegetated side slopes in combination with stone, concrete or highly resistant vegetative center section. (4) Annuals - use on mild slopes or as temporary protection until permanent covers are established. (5) Use on slopes steeper than 5% is not recommended. (6) Adapted from Natural Resources Conservation Service, SCS-TP-61, Handbook of Channel Design for Soil & Water Conservation, 1949. | |||
(Ord. 8-2014, passed 7-21-2014)