§ 35-29.3. Open channels.

(a)

Open channels, including all natural or structural channels, swales, and ditches shall be designed for the flood mitigation storm event.

(b)

Open channels shall be designed with multiple stages. A low flow channel section shall contain the streambank protection flows and a high flow section shall contain the conveyance and flood mitigation storms to improve stability and better mimic natural channel dimensions.

(c)

Trapezoidal open channels shall have a minimum channel bottom width of eight (8) feet.

(d)

Open channels with bottom widths greater than eight (8) feet shall be designed with a minimum bottom cross slope of twelve (12) to one (1) or with compound cross sections.

(e)

Open channel side slopes shall be stable throughout the entire length and the side slope shall depend on the channel material. Open channel side slopes and roadside ditches with a side slope steeper than 3:1 shall require detailed geotechnical and slope stability analysis to justify slopes steeper than 3:1. Any slope that is less than 3:1 must be approved by the city engineer.

(f)

For vegetative open channels, flow velocities within the channel shall not exceed the maximum permissible velocities given in Tables 35-29C and 35-29D of this ordinance.

(g)

If relocation of a stream channel is unavoidable, the cross-sectional shape, meander, pattern, roughness, sediment transport, and slope must conform to the existing conditions insofar as practicable. Energy dissipation is necessary when existing conditions cannot be duplicated.

(h)

Streambank stabilization shall be provided, when appropriate, as a result of any stream disturbance, such as encroachment, and shall include both upstream and downstream banks as well as the local site.

(i)

HEC-RAS, or similarly capable software approved by the city engineer, shall be used to confirm the water surface profiles in open channels.

(j)

The final design of artificial open channels shall be consistent with the velocity limitations for the selected channel lining. Maximum velocity values for selected lining categories are presented in Table 35-29C of this ordinance. Seeding and mulch shall only be used when the design value does not exceed the allowable value for bare soil. Table 35-29D of this ordinance shall be used for velocity limitations of vegetative linings. Vegetative lining calculations and stone riprap procedures must comply with Section 3.2 of the NCTCOG-iSWM Hydraulics Technical Manual.

(k)

For gabions, design velocities may range from ten (10) fps for 6-inch mattresses up to fifteen (15) fps for 1-foot mattresses. Velocities of twenty (20) fps are allowable for basket installations if indicated by the manufacturer. The design of stable rock riprap lining shall be determined on a case by case basis depending on the intersection of the velocity (local boundary shear) and the size and gradation of the riprap material. Unless otherwise approved by the city engineer, all acceptable riprap velocity limits shall comply with section 3.2.7 of the NCTCOG-iSWM Hydraulics Technical Manual.

Table 35-29C - Roughness Coefficients (Manning's n) and Allowable Velocities for Natural Channels
Channel Description	Manning's n	Max. Permissible Channel Velocity (ft/s)
MINOR NATURAL STREAMS
Fairly regular section
1. Some grass and weeds, little, or no brush	0.030	3 to 6
2. Dense growth of weeds, depth of flow materially greater than weed height	0.035	3 to 6
3. Some weeds, light brush on banks	0.035	3 to 6
4. Some weeds, heavy brush on banks	0.050	3 to 6
5. Some weeds, dense willows on banks	0.060	3 to 6
For trees within channels with branches submerged at high stage, increase above values by	0.010
Irregular section with pools, slight channel meander, increase above values by	0.010
Floodplain - Pasture
1. Short grass	0.030	3 to 6
2. Tall grass	0.035	3 to 6
Floodplain - Cultivated Areas
1. No crop	0.030	3 to 6
2. Mature row crops	0.035	3 to 6
3. Mature field crops	0.040	3 to 6
Floodplain - Uncleared
1. Heavy weeds scattered brush	0.050	3 to 6
2. Wooded	0.120	3 to 6
MAJOR NATURAL STREAMS
Roughness coefficient is usually less than for minor streams of similar description on account of less effective resistance offered by irregular banks or vegetation on banks. Values of "n" for larger streams of mostly regular sections, with no boulders or brush	Range from 0.028 to 0.060	3 to 6
UNLINED VEGETATED CHANNELS
Clays (Bermuda Grass)	0.035	5 to 6
Sandy and Silty Soils (Bermuda Grass)	0.035	3 to 5
UNLINED NON-VEGETATED CHANNELS
Sandy Soils	0.030	1.5 to 2.5
Silts	0.030	0.7 to 1.5
Sandy Silts	0.030	2.5 to 3.0
Clays (Bermuda Grass)	0.030	3.0 to 5.0
Coarse Gravels	0.030	5.0 to 6.0
Shale	0.030	6.0 to 10.0
Rock	0.025	15

 

Table 35-29D - Maximum Velocities for Vegetative Channel Linings
Vegetation Type	Slope Range (%)	Maximum Velocity (ft/s)
Bermuda Grass	0-5	6
Bahia		4
Tall fescue grass mixtures	0-10	4
Kentucky bluegrass	0-5	6
Buffalo Grass	5-10	5
	>10	4
Grass Mixture	0-5	4
	5-10	3
Sericea lespedeza, Weeping lovegrass, Alfalfa	0-5	3
Annuals	0-5	3
Sod		4
Lapped Sod		5

 

Do not use on slopes steeper than 10% except for side-slope in combination channel.

Use velocities exceeding 5 ft/s only where good stands can be made.

Mixtures of Tall Fescue, Bahia, and/or Bermuda.

Do not use on slopes steeper than 5% except for side-slope in combination channel.

Annuals - used on mild slopes or as temporary protection until permanent covers are established.

(Ord. No. 2017-10008, § II, 10-26-17)