Rigid versus Flexible Lining

Engineers may design roadside channels with rigid or flexible linings. Flexible linings in channels conform better to a changing channel shape than rigid linings. However, a rigid lining may resist an erosive force of high magnitude better than a flexible one.
The following types of rigid linings are common:
  • cast-in-place concrete
  • soil cement
  • fabric form work systems for concrete
  • grouted riprap.
Rigid channel linings have the following disadvantages when compared to natural or earth-lined channels:
  • Initial construction cost of rigid linings is usually greater than the cost of flexible linings.
  • Maintenance costs may also be high because rigid linings are susceptible to damage by undercutting, hydrostatic uplift, and erosion along the longitudinal interface between the lining and the unlined section.
  • Inhibition of natural infiltration in locations where infiltration is desirable or permissible.
  • Smooth linings usually cause high flow velocities with scour occurring at the terminus of the sections unless controlled with riprap or other energy dissipating devices
  • Contaminants may be transported to the receiving waters in areas where water quality considerations are of major concern. A vegetative or flexible type of lining may filter the contaminants from the runoff.
Permanent flexible linings include the following:
  • rock riprap
  • wire enclosed riprap (gabions)
  • vegetative lining
  • geotextile fabrics.
Flexible linings generally have the following advantages:
  • less costly to construct
  • have self-healing qualities that reduce maintenance costs
  • permit infiltration and exfiltration
  • present a more natural appearance and safer roadsides.
Various species of grass may be used as permanent channel lining if flow depths, velocities, and soil types are within acceptable tolerances for vegetative lining. The turf may be established by sodding or seeding. Sod is usually more expensive than seeding, but it has the advantage of providing immediate protection. Some type of temporary protective covering is often required for seed and topsoil until vegetation becomes established.
The following are classified as temporary flexible linings:
  • geotextile fabrics
  • straw with net
  • curled wood mat
  • jute, paper, or synthetic net
  • synthetic mat
  • fiberglass roving.
Temporary channel lining and protective covering may consist of jute matting, excelsior mats, or fiberglass roving. Straw or wood-chip mulch tacked with asphalt is usually not well suited for channel invert lining but may be used for side slopes. Geotextile materials, known as soil stabilization mats, may be used for protective linings in ditches and on side slopes. These materials are not biodegradable and serve as permanent soil reinforcement while enhancing the establishment of vegetation.