Channel Design for Stormwater Conveyance

Open channels are one of the most common and cost-effective ways to convey stormwater. From roadside ditches to regional flood control channels, getting the design right means balancing capacity, velocity, erosion protection, and constructability.

Channel Types

Lined Channels

Lined channels use concrete, riprap, or other hard surfaces to resist erosion. They can handle higher velocities and steeper slopes than unlined channels.

Common lining materials:

• Concrete (cast-in-place or precast)
• Riprap (loose stone)
• Grouted riprap
• Gabion baskets
• Articulated concrete blocks

When to use lined channels:

• High velocities (above 5–6 ft/s)
• Steep slopes where erosion is a concern
• Limited right-of-way requiring compact cross sections
• Areas where maintenance access is limited

Unlined (Earthen) Channels

Unlined channels rely on vegetation and soil stability to resist erosion. They are less expensive to construct but require more land and careful velocity control.

When to use unlined channels:

• Mild slopes (typically < 2–3%)
• Rural or suburban settings with available land
• Where infiltration and water quality treatment are desired
• Low to moderate flow velocities (below 4–5 ft/s depending on soil and vegetation)

Grass-Lined Swales

Swales are shallow, vegetated channels commonly used in residential and commercial developments. They combine conveyance with water quality treatment and infiltration.

Size a swale with the Swale Calculator →

Design Parameters

Cross-Section Geometry

The most common channel shapes in stormwater design are:

For trapezoidal channels, typical side slopes range from 2H:1V to 4H:1V depending on soil type and safety requirements. Steeper than 2:1 is generally only used with structural lining.

Manning’s Equation

Channel capacity is calculated using Manning’s equation:

Where:

• Q = discharge (cfs)
• n = Manning’s roughness coefficient
• A = cross-sectional flow area (ft²)
• R = hydraulic radius = A/P (ft)
• S = channel slope (ft/ft)

The hydraulic radius (R) is the ratio of flow area to wetted perimeter. A channel with a large hydraulic radius is hydraulically efficient — it carries more flow for a given cross-sectional area.

Calculate channel flow with Manning’s Channel Calculator →

Freeboard

Freeboard is the vertical distance between the design water surface and the top of the channel bank. It provides a safety margin for:

• Wave action from wind or vehicles
• Uncertainty in flow estimates
• Debris accumulation
• Superelevation at bends

Typical freeboard requirements:

• Small ditches: 6 inches (0.15 m)
• Medium channels: 1.0 foot (0.3 m)
• Major flood control channels: 2.0 feet (0.6 m) or more

Velocity Constraints

Maximum Velocity

Maximum permissible velocity depends on the channel lining:

Exceeding the maximum velocity causes erosion, channel degradation, and ultimately failure. When the required capacity demands higher velocities, you must either use a more resistant lining or reduce the slope with grade control structures.

Minimum Velocity

A minimum velocity of about 2.0 ft/s (0.6 m/s) at design flow is recommended to prevent sediment deposition. Channels that flow too slowly accumulate sediment, reducing capacity over time and increasing maintenance costs.

Design Procedure

A typical channel design follows these steps:

Step 1: Determine the design flow. Use the Rational Method or SCS method based on the contributing drainage area and design storm frequency.

Step 2: Select the channel shape and lining. Consider site constraints, available right-of-way, soil conditions, and maintenance access.

Step 3: Choose a channel slope. This is usually controlled by the site topography and the slope needed to connect inlet and outlet elevations.

Step 4: Select Manning’s n. Base this on the channel lining material, vegetation, and expected maintenance condition.

Step 5: Size the channel. Use Manning’s equation to solve for the required depth and width that convey the design flow. Iterate on geometry as needed.

Step 6: Check velocity. Verify the flow velocity is within the permissible range for your lining type. Adjust lining or geometry if needed.

Step 7: Add freeboard. Add the required freeboard to the design water surface elevation.

Step 8: Check Froude number. Verify subcritical flow (Fr < 1.0) for stability. Supercritical flow in earthen channels is generally unacceptable.

Common Design Issues

Channel Bends

Flow accelerates on the outside of bends and can cause significant erosion. For bends:

• Use a minimum centerline radius of 3× the top width
• Provide additional freeboard or lining on the outside of curves
• Consider superelevation of the water surface

Confluences and Transitions

Where channels merge or change shape, energy losses can cause localized flooding. Design transitions with gradual geometry changes (expansion angles of 12.5° or less) and provide additional freeboard.

Outlet Protection

Channel outlets to receiving waters need protection against scour. Common approaches include riprap aprons, energy dissipators, and stilling basins. The required protection depends on the outlet velocity and downstream conditions.

References

1. Federal Highway Administration. (2005). Design of roadside channels with flexible linings (Hydraulic Engineering Circular No. 15, 3rd ed.). U.S. Department of Transportation.

2. Federal Highway Administration. (2013). Urban drainage design manual (3rd ed., Hydraulic Engineering Circular No. 22). U.S. Department of Transportation.

3. Chow, V. T. (1959). Open-channel hydraulics. McGraw-Hill.

4. American Society of Civil Engineers. (2006). Sedimentation engineering (ASCE Manual of Practice No. 110). ASCE Press.