Surface channels, natural or man-made, are usually the most economical means of collecting and disposing of runoff in highway construction if concentration of flows cannot be avoided. A well-designed channel carries storm water without erosion or hazard to traffic and with the lowest overall cost, including maintenance. To minimize erosion and avoid a safety hazard, channels should have mild side slopes and wide rounded bottoms. Such channels can be protected from erosion by lining them with materials such as grass, rock, or concrete.

Chutes generally are applied to steep slopes and carry water at high velocities. Pipe chutes are preferable to open chutes because the water cannot jump out of the chute and erode the slope. Dissipation of energy along the chute or at the outlet is usually necessary. In highly erosive soil, watertight joints may need to be provided to prevent failure of the facility.

Variations in channel alignment should be gradual, particularly if the channel carries flow at high velocity. Whenever practical, changes in alignment should be located on the flatter gradients to prevent erosion caused by the overtopping of the channel walls and the associated erosion. Although rectangular channel sections are usually more expensive, they are preferred on bends of paved channels to give a more positive control of the flow.

If the bank and bed material will erode at the prevailing velocities, channel lining should be considered. Protective linings for channels and streams can be very expensive. A special effort should be made to develop the most cost-effective erosion protection, including maintenance, for the particular location.

Several applications are effective for both channel and bank protection, including spur dikes, permeable spur jetties, gabions and revetment mattresses, and sheet piling. For many of these protective appurtenances, no rigorous design is available, and experience or intuition is the best guides for their consideration and application. See Chapter 7, , for more information.

Culverts and bridges generally constrict the floodway and increase velocities, thus developing higher erosion potential. In many instances, erosion and scour at these locations damage the highway embankment, the structure itself, or the downstream channel. The energy of high velocity flow should be dissipated at the outlet of culverts and chutes where necessary, or the area protected by riprap or other types of protection. Some velocity control devices and methodologies are illustrated in Chapter 8, .