Selected BMPs: Lined Channel

Definition: Channels that incorporate erosion- resistant linings on banks and bottoms to resist scour and erosion. Linings materials may include of vegetation, rip rapock, or gabion mattressess, or interlocking paving blocks, concrete, or synthetic fabrics. on channel banks and bottoms.

Purpose: Channel linings protect the channel banks and bottoms from erosive velocities and scour.

Application: Some type of lining should be placed in most channels. Bare earth channels should be used only as temporary interceptors channels. The maximum flow velocity under design conditions should not exceed 1.5 feet per second (fps) in bare earth channels. Vegetation Vegetation is the preferable method for stabilizing channels and will an be adequate in most instances. lining for most channels. Vegetated channels offer the ancillary benefits of reducing flow velocity and filtering runoff. The design of grass-lined channels is discussed in GRASS SWALE.

The maximum permissible velocities for bare earth and vegetation-lined channels are discussed in "Specifications and Methodology" section. In instances where steep slopes or high discharge rates preclude the design of stable vegetated channels, linings constructed from As the sloerosion- resistant materials may be considered

Lined channels are frequently used in combination with vegetated channels. In particular, lined channels can be used at grade transitions, steep reaches, or points of flow confluence to prevent scour and erosion.

pe of the channel and the flow velocities and quantities increase in a channel, riprap, gabions, or concrete lining should be used.

Recommended Design Criteria:

Requirements for Regulatory Compliance

(none specified)

The relevant requirements for using lined channels are established in 25 Pennsylvania Chapter 102. Permanent channels must convey the greater of 2.75 cubic feet per second (cfs)/acre or tributary area or the discharge rate associated with the 24-hour, 25-year return frequency storm. Channels must also be provided with stable transitions to existing watercourses.

Performance-Based Guidelines

The velocity of the flow in the channel should not exceed that considered non-erosive for the soil and planned vegetation or lining. The guidelines from the Pennsylvania Erosion and Sediment Control Manual are summarized below. An alternative design procedure, which is based on the concept of maximum permissible tractive force, is contained in the U.S. Department of Transportation’s Design of Stable Channels with Flexible Linings (HEC-15). The HEC-15 procedure requires determining the mean particle size for noncohesive soil and the plasticity index (PI) for cohesive soil. Laboratory tests are the usual source of this information, although county soil surveys also may be used.

The following five types of lined channels are described:

• Bare earth channels
• Temporary linings
• Vegetative-lined channels
• Rock-lined channels
• Gabion-lined channels

Bare Earth Channels

Typically, bare earth channels should be used only as temporary interceptor channels for conveying sediment-laden project runoff to a sediment-removal facility.

Temporary Linings

Most temporary channel linings are designed to meet two objectives:

• To stabilize and protect the seed bed
• To increase the maximum permissible channel velocity over that possible with a "seeded" bare earth channel

Whenever temporary channel linings are used, specific design calculations for demonstrating flow capacity and stability must be provided. Separate calculations for demonstrating flow capacity and stability for the vegetated condition also must be provided.

Although channels with temporary linings may be sized initially in accordance with procedures contained in the Pennsylvania manual, maximum permissible velocities are not given for temporary liner materials. Therefore, the methods in HEC-15 should be used for calculations that demonstrate flow capacity and stability of temporary linings.

Temporary linings must be installed correctly. Key trenches should be used and the linings should be staked securely. The linings should be inspected after each major storm and repaired if needed.

Vegetative-lined Channels

The design procedures in the BMP description, GRASS SWALE, are sufficient for designing most vegetative-lined channels. The methods in HEC-15 also are acceptable. Seeding types, seeding rates, and fertilizer application should adhere to the standards contained in PERMANENT SURFACE STABILIZATION.

Vegetation is the recommended lining if slopes are flat enough to maintain appropriate velocities. Maximum permissible velocities for grass swales are shown in the "Specifications and Methodology" section.

Rock-lined Channels

Rock-lined (rip rap) channels may be sized on the basis of the maximum permissible velocity and the following conditions:

• The channel alignment is straight or gradually turning.
• Channel side slopes are no steeper than 2H:1V.
• The placement thickness is 1.25 times the maximum rock size shown in Table 1.
• Rock is crushed and has a unit weight of 165 lbs/cubic foot (pcf).
• A geotextile filter underlayment is provided.
• A fFilter rock layer is sized and installed in accordance with Table 1 (recommended).

For example, if stone has a unit weight of 150 pcf, multiply weights of maximum, average, and minimum sizes in the table by a factor of 1.5.

The above procedure is applicable for designing most rock-lined channels. However, if a more conservative design procedure is desired, or if flow conditions through bends must be considered and side slopes steeper than 2H:1V must be evaluated, the procedures found in HEC-15 (USDT, 1975). are recommended.

Gabion-lined Channels

Gabion lined channels should be sized in accordance with the procedures in "Specifications and Methodology." The selection of rock fill for the gabion baskets and the initial construction, stretching, filling, and other installation and construction steps should conform strictly with manufacturers’ standards and recommendations, which must be referenced.

Other Channels

In general, concrete-lined channels are not used as part of erosion and sedimentation control plans. Concrete-lined channels are used for permanent conveyance of flows down steep slopes and are described in the BMP description, PAVED FLUME OR CHUTE.

Because other forms of permanent channel stabilization exist that are not described, the plan preparer is responsible for demonstrating that the channels have been sized in accordance with the procedures contained in the Pennsylvania Erosion and Sediment Pollution Control Program Manual (PADEP, 1996), and that appropriate engineering practices have been observed for the structural design, stability, drainage, and other design components.

Operation and Maintenance: Linings should be inspected regularly and immediately after any unusual flow event. Damage or erosion should be corrected as soon as possible.

Considerations: Bare earth channels typically should be used only as interceptor channels for conveying sediment-laden project runoff to a sediment-removal facility. The other types of linings can be considered in the following hierarchy ranging from the least expensive to the most expensive.

• Vegetative-lined channels
• Rock-lined channels
• Gabion-lined channels
• Concrete-lined channels

For each application, the minimum level of lining material that is adequate should be used. The further down the list a lining technology is, the higher the initial cost and maintenance requirements and the lower the habitat benefits.

Concrete-lined channels, in particular, should be avoided, if possible. not be used. Concrete-lined channelsIf possible, instead try redesigning the channel. Concrete-lined channels are not aesthetically pleasing and do not benefit habitat or water quality. Furthermore, hHigh velocities caused by low roughness coefficients can make it difficult to establish stable transitions to receiving watercourses. Concrete-lined channels also tdo not provide an opportunity for water to percolate into the ground. Finally, concrete channels are prone to cracking or the displacement of slabs which can interfere with proper operation of the channels and result in erosion. create erosion potential and associated problems downstream.

All rip rap, gabion, or paver-lined channels should include an underlayment of filter geotextile (see Appendix E, Approach for the Developing Material Specifications). However, rip rap or gabion-lined channels that are installed without a filter rock layer are more likely to experience failures caused by water flowing beneath the liner and geotextile. Flow beneath the liner can eventually wash away fine sediment, causing slumping of the channel liner and breakouts of flow. Therefore, it is recommended that these channels include a stable foundation layer of filter rock or sand.

Lined channels are most commonly employed on steep slopes or where high discharge rates are expected. Therefore, the use of ENERGY DISSIPATORS will generally be required to transition flow to existing watercourses.

Living plants used instead of, or in conjunction with, structures is advantageous. The degree of protection, which may be low to start with, increases as the plants grow and spread. Once self-maintaining plants are established, maintenance requirements are lower than for structures used by themselves. The protection provided by natural vegetation is more reliable and effective when the cover consists of natural plant communities adapted to their site.

References:

Commonwealth of Pennsylvania, Department of Environmental Protection. Erosion and Sediment Pollution Control Program Manual. DEP #466. April 1996.

U.S. Army Corps of Engineers Hydraulic Tables

U.S. Department of Agriculture, Natural Resources (formerly Soil) Conservation Service

U.S. Department of Interior, Bureau of Reclamation’s Hydraulic & and Excavation Tables

U.S. Department of Transportation, Federal Highway Administration. Design of Stable Channels with Flexible Linings, HEC-15. NTIS PB89-122584/AS. 1975.

U.S. Department of Agriculture, Natural Resources (formerly Soil) Conservation Service. National Engineering Handbook Washington, D.C. 1972.

Washington State Department of Ecology. Stormwater Management Water for the Puget Sound Basin. The Technical Manual. Publication #91-75. February 1992.

SPECIFICATIONS AND METHODOLOGY: Procedures for channel design are summarized in this section and detailed in the Pennsylvania Erosion and Sediment Pollution Control Program Manual (PADEP, 1996). By using the procedures, bare-earth, vegetative-lined, rock-lined, and gabion-lined channels may be designed with little additional information. Additional design considerations and material specifications for channel linings are in the manual.

An additional reference, especially helpful for designing temporary channel linings, is the Federal Highway Administration’s publication Design of Roadside Channels with Flexible Linings, (HEC-15). Copies may be obtained by contacting:

National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone (703) 487-4650

Publication Number: NTIS # PB89-122584/AS

No one procedure exists that will enable a designer to immediately select the proper channel section configuration or its dimensions because of the innumerable combinations of hydraulic conditions possible. All of the published procedures for designing channel sections requires the designer to make assumptions or determinations for hydraulic conditions. Therefore, the design of lined channels will generally will require the services of an Basically, the only method available is that of trial-and-error, which can only be minimized through the experienced of the designer. All of the published procedures for designing channel sections requires the designer to make assumptions or determinations for hydraulic conditions.

Some of the basic calculations are presented in this section under the following four headings:

• Manning’s Equation
• Flow Conditions
• Maximum Permissible Velocities
• Channel Design

Manning’s Equation

Flow capacity and velocity in open channels typically are computed by using Manning’s equation. The recommendation is to use the equation (including derivative forms). Manning’s equation is:

Where: Q = quantity of flow in cubic feet per second (cfs)

V = velocity in feet per second (fps)

n = Manning’s coefficient of roughness

A = cross-sectional area of channel (sq ft)

P = wetted perimeter of the channel (ft)

R = hydraulic radius = A/P (ft)

S = slope of the channel bottom (ft/ft)

Manning’s equation can be solved by a hand calculation or with an appropriate hydraulic table or nomograph. The velocity that is determined then is multiplied by the cross-sectional area A to derive Q. The following documents are available for solving Manning’s equation:

1. Department of Interior, Bureau of Reclamation’s Hydraulic & Excavation Tables
2. U.S. Army Corps of Engineers Hydraulic Tables
3. SCS’s Engineering Field Manual

Using the listed nomograph and referencesd tables requires basic assumptions and preliminary determinations with respect to the values n, R, and S. The values may be determined by doing the following:

n–Select the appropriate value from tables 2, 4, and 7 below.

R–Equals A/P. For uneven channel sections, A and P must be accurately measured from scale drawings. For uniform sections, standard charts are available for solving the values for A and R for given measurements of depth and bottom width. A useful reference is the U.S. Army Corps of Engineer’s Hydraulic Tables, Tables 22 through 46.

S–Bed slope, in ft/ft, as measured or determined from field measurements or a scaled map drawing.

Suitable side slopes for various types of earth and rock linings are shown in Table 3.

Flow Conditions

Channels for conveying runoff should be designed to handle anticipated flow conditions. Uniform flow at and near "critical depth" is unstable, and will frequently result in ifthe formation of waves. The undulation of the waves may exceed the top of the channel sections designed for uniform flow at the specified design flow rate.

Table 2. Recommended n Values to be Used with Manning’s Equation
Surface	Min.	Design	Max.
Asphalt Lining		0.015
Brick in cement mortar; brick sewers	0.012	0.015	0.017
Concrete-lined channel	0.012	0.015	0.018
Cement-rubble surface	0.017		0.030
Neat cement surface	0.010	0.012	0.013
Plastic-lined channel	0.012		0.014
Shotcrete	0.016		0.017
Asbestos-Cement Pipe		0.009
Concrete pipe	0.012	0.015	0.016
Vitrified-Clay Pipe	0.010	0.013	0.017
Common-clay drainage tile	0.011	0.012	0.017
Semi-circular metal flumes, smooth	0.011		0.015
corrugated	0.023	0.025	0.030
Channels and ditches:*
Earth, straight and uniform	0.017	0.023	0.025
Rock cuts, smooth and uniform	0.025	0.030	0.035
jagged and irregular	0.035	0.040
Dredged earth channels	0.025	0.028	0.033
Earth bottom, rubble sides	0.028	0.030	0.035
Natural Streams:*
1. Clean, straight bank, full stage no rifts or deep pools	0.025		0.033
2. Same as 1., but some weeds and stones	0.030		0.040
3. Winding, some pools and shoals, clean	0.033		0.045
4. Same as 3, lower stages, more ineffective slope and sections	0.040		0.055
5. Same as 3, some weeds and stone	0.035		0.050
6. Same as 4, stony sections	0.045		0.060
7. Sluggish river reaches, rather weedy or with very deep pools	0.050		0.080
8. Very weedy reaches	0.075		0.150
See tables 4 and 7, and Figure 1 for additional n values *n values for other channels and natural streams may be adjusted for bends, vegetation, obstructions, and uniformity by using SCS Engineering Handbook #5, Supplement B.

To minimize the potential for overtopping of the channels due to peak discharge. Accordingly, for the range of anticipated unstable flow conditions, freeboard must be incorporated into the channel design. The sequence of calculations for establishing the need for freeboard and for computing freeboard is:

Step 1–Compute the design channel’s critical slope as:

S_c = 14.56 n²D_m /R^4/3

where: S_c = critical slope

n = Manning’s n

D_m = mean depth of flow = A/T

A = area of channel (sq ft)

T = top width of channel (ft)

R = hydraulic radius = A/P

P = wetted perimeter

Step 2–Establish stable or unstable flow conditions as:

unstable zone = .7S_c < S_o < 1.3 S_c (S_o = channel slope)

Step 3–If unstable flow conditions exist, the required freeboard, F, shouldwill be computed as:

F = (.025 V) (3 D) = (.075 VD)

where: F = freeboard (ft)

V = velocity (fps)

D = channel depth (ft)

Note: The final design channel depth will be F + D.

Step 4–If stable flow conditions exist,, freeboard should be established in accordance with the following:

1. Freeboard for bare earth channels should be a minimum of 0.5 feet.
2. Freeboard the lined channels should be 0.5 feet, or 25 percent of the design depth, whichever is greater.

For either stable or unstable conditions, lining material should extend to the freeboard depth for lined channels.

Maximum Permissible Velocities

All temporary and permanent watercourse channels should be designed to convey flows at or below the maximum permissible (mean) velocities shown in tables 4, 5, 6, and 7.

For bare earth channels leading to sediment basins, velocities may be increased to 1.5 times the maximum permissible velocities shown in Table 4.

Instructions for Using Table 5:

1. A velocity of 3.0 ft/sec should be the maximum if only a sparse cover can be established or maintained because of shade, soil, or climate.
2. A velocity of 3.0 to 4.0 ft/sec should be used under normal conditions if the vegetation is to be established by seeding.
3. A velocity of 4.0 to 5.0 ft/sec should be used only if a dense vigorous sod can be obtained quickly or if water can be diverted out of the waterway while vegetation is being established.
4. A velocity of 5.0 to 6.0 ft/sec may be used on well-established, good-quality sod. Special maintenance may be required.
5. A velocity of 6.0 to 7.0 ft/sec may be used only on established, excellent-quality sod, and only under special circumstances in which the flow cannot be handled at a lower velocity. Under these conditions, special maintenance and appurtenant structures should be used.
6. If the vegetative lining is supplemented by stone centers, or other erosion-resistant materials, the velocity in Table 5 may be increased by 2.0 ft/sec.
7. When a base flow exists, a rock-lined low-flow channel should be designed and incorporated in the vegetative-lined channel section.

Channel Design

No one procedure exists that will enable a designer to immediately select the proper configuration of the channel section or its dimensions because innumerable combinations of hydraulic conditions are possible. Basically, the only method available is that of trial-and-error, which can only be minimized through the experience of the designer. All of the published procedures for designing channel sections require the designer to make assumptions or determinations for hydraulic conditions.

After the designer has selected a channel configuration and trial dimensions, the design must be shown to satisfy certain basic or imposed requirements. The major requirements are:

Requirement No. 1–Is the channel section stable at the calculated flow velocity?

Checks: For the type of channel surface or selected lining is the calculated velocity less than the appropriate maximum permissible velocity shown in tables 4 through 7? If the answer is no, additional trial sections must be calculated.

Will the design channel lining (as with vegetation) be established before the channel is used for its stated objective? If not, delay using the lining or use a different lining.

Special Exception: For collection ditches leading to sediment basins, velocities may be increased to 1.5 times the maximum permissible velocities shown in Table 4.

Requirement No. 2–Are flow conditions stable at design discharges?

Checks: Does S_c fall within the unstable zone? If the answer is yes, calculate required freeboard. If flow conditions are stable, establish freeboard.

Requirement No. 3–Does the design channel meet all of its objectives?

Checks: Checks will vary for different objectives.