(A) Trapezoidal, concrete and nonerodible.
(1) Collect all necessary information, estimate Manning’s “n”, the design flow and select the slope.
(2) Compute the section factor Z using:
Z =AR
2/3 = nQ |
1.49 /S |
(3) Substitute into the above equation the expressions for A and R according to the shape of the channel given in the following Figure:
(4) From this equation solve the depth by iterating until the equation balances.
(5) Check for the minimum permissible velocity at which the water will carry silt.
(6) Add a proper freeboard. Freeboard may vary from 15% to 30% of the depth and be a minimum of six inches.
(B) Grassed channels; nonerodible.
(1) Design for stability.
(a) 1. Select type of grass lining. This depends on the soil, climate and the following chart of permissible velocities:
Figure II. Permissible Velocities For Channels Lined With Grass* | |||
Cover | Permissible Velocity, fps | ||
Slope Range % | Erosion Resistant Soils | Easily Eroded Soils | |
Figure II. Permissible Velocities For Channels Lined With Grass* | |||
Cover | Permissible Velocity, fps | ||
Slope Range % | Erosion Resistant Soils | Easily Eroded Soils | |
Bermuda grass | 0—5 | 8 | 6 |
5—10 | 7 | 5 | |
Buffalo grass, Kentucky bluegrass, smooth brome, blue grama | 0—5 | 7 | 5 |
5—10 | 6 | 4 | |
Grass mixture | 0—5 | 5 | 4 |
5—10 | 4 | 3 | |
Do not use on slopes steeper that 10% | |||
Lespedeza sericea, weeping love grass, ischaemum (yellow bluestem), alfalfa, crabgrass | 0—5 | 3.5 | 2.5 |
Do not use on slopes steeper than 5% except for side slopes in a combination channel | |||
Annuals - used in mild slopes or as temporary protection until permanent covers are established, common lespedeza, Sudan grass | 0—5 | 3.5 | 2.5 |
Use on slopes steeper than 5% is not recommended | |||
Remarks: The values apply to average, uniform stands of each type of cover. Use velocities exceeding 5 fps only where good covers and proper maintenance can be obtained. | |||
*U.S. Soil Conservation Service | |||
2. Decide on the length of grass and find the degree of retardance from the following table:
Figure III. Guide In Selection Of Vegetal Retardance* | ||
Stand | Average Length Of Grass (In.) | Degree Of Retardance |
Figure III. Guide In Selection Of Vegetal Retardance* | ||
Stand | Average Length Of Grass (In.) | Degree Of Retardance |
Good | 30 | A: Very high |
11—24 | B: High | |
6—10 | C: Moderate | |
2—6 | D: Low | |
Fair | 30 | B: High |
11—24 | C: Moderate | |
6—10 | D: Low | |
2—6 | D: Low | |
*U.S. Soil Conservation Service | ||
(b) The design flow is calculated using an acceptable method from the hydrology section. Assume a value n and from the vegetal retardance chart, Figure III, determine the corresponding value for VR.
(c) Select a permissible velocity from Figure II and compute the hydraulic radius.
(d) 1. Use the Manning’s formula and compute the value of VR using:
VR =
1.49 R5/3 S1/2 |
n |
2. Check this value against the value for VR obtained in step 2. Continue the trials until the computer VR value is equal to the value from Figure III.
(e) Next compute the water area using:
A = Q/V |
(f) 1. Since the correct value for A and R have been found, the section dimensions (b=base; y=depth), may now be determined by solving for:
AR =
by . by (for a rectangular section) |
b + 2y |
2. Make the appropriate substitutions in the above equation for various design shapes as per Figure I.
(2) Design for maximum capacity. This design phase is to determine the additional depth necessary to sustain the maximum capacity of a fully developed grass lining.
(a) Assume a depth y, and compute the water area A and the hydraulic radius R.
(b) Compute the velocity V by V = Q/A and compute the value of VR.
(c) From the n - VR curve of a higher degree of retardance, determine the value for n.
(d) Compute the velocity by the Manning’s formula and check this value against the value of V obtained in step 2.
(e) Make trial computations until the computed V in step 4 is equal to the computed V in step 2.
(f) Add the proper freeboard to the computed depth. Use 15% to 30% of the depth for determining the freeboard with a minimum of six inches.
(Prior Code, § 11-8-4) (Ord. 594, passed 8-5-1997)