Many methods exist to determine a culvert’s correct size. The easiest is one in which field measurements are taken of the stream’s cross section when the stream flow is at its lowest, in late summer or early fall. To take the measurements, you will need a tape measure, four stakes, a hammer, string and a notepad.

1. Minimum diameter = 18 inches; culverts under roadway approaches have a minimum diameter of 12 inches 2. No minimum velocity 3. No minimum pipe slope; 10% max for concrete, 20% max for corrugated metal and thermoplastic pipe 4. The headwater and tailwater conditions determine which equations (nomographs) are used for design 5.