The lowest elevation in a ditch, or on the inside of a pipe or drainage structure, is referred to as the ____.

• A. Crown
• B. Invert
• C. Rim
• D. Heel

Answer: (B)

In drainage design, the invert is the lowest interior point of a pipe or drainage structure—the bottom of the inside where flow occurs. This elevation is essential for calculating the pipe’s slope and flow capacity, since flow is driven by the difference in invert elevations between connected elements. Measuring the invert relative to a project datum lets you compare elevations across structures and ensure proper gravity flow. The other terms describe different parts: crown is the topmost part of a cross-section, rim is the top edge of an opening, and heel isn’t used to denote the interior bottom point.