Abrasion causes a loss of section thickness due to impacts by the aggregate carried by stream flow. Protection from abrasion generally takes the form of providing a sacrificial thickness of the structural material, whether it be a thicker sheet of steel or concrete paved invert for metal pipe, or more concrete cover over the reinforcement for reinforced concrete pipe. Alternatives to providing for a thicker section include using debris control structures to prevent the abrasive material from reaching the culvert, and providing metal planking longitudinally along the invert as a separation between the bed load and the bottom of the culvert.
Abrasion can be considered in four levels of severity as categorized by streambed velocity and general aggregate size. Protective measures, particularly in the invert, should increase with increasing levels of abrasion as discussed subsequently. (See Project Development and Design Manual, Federal Lands Highway, FHWA.)
Level 1, termed nonabrasive, has very low flow velocities and no bed load.
Level 2, low abrasive, has flow velocities of 5 ft/s (1.5 m/s) or less and light bed load consisting of sand.
Level 3, moderately abrasive, has flow velocities of between 5 and 15 ft/s (1.5 and 4.5 m/s) and moderate bed loads consisting of sand and gravel.
Level 4, severely abrasive, has flow velocities exceeding 15 ft/s (4.5 m/s) and heavy bed loads consisting of sand, gravel, and rock.
The projected velocities should be based upon a typical flow and not upon the design flood for which the culvert has been designed. The bed load size may be determined by visual inspection of the surrounding environment and the upstream channel. Sampling of the aggregate for a gradation analysis is not necessary.
