When designing a culvert or storm drainage system, you must evaluate aspects of structural design, hydraulic design, and durability design. The first two disciplines are quite familiar to most civil engineers. Durability design, however, is generally beyond the scope of civil engineering and is more closely aligned with the field of chemistry. Experience has shown that culverts most frequently fail as a result of durability problems. This is usually due to improper selection of materials to meet the project design life and site conditions.