Culvert design is an iterative process that starts with reasonable assumptions and culminates with a final selection of material, shape, and placement that satisfies the requirements of function and safety. Culvert analysis is a straight forward process that determines the functioning status of an existing culvert structure or a proposed design.

Culvert analysis includes determination of flow rates, velocities, and water surface elevations for the full range of probabilities (50%, 20%, 10%, 4%, 2%, and 1% AEPs) for both the existing and the proposed conditions. A complete list of the requirements for design can be found in the in Chapter 3.

Differences exist between computer programs. Simple culvert computer programs have limitations such as how they handle roadway overtopping and upstream momentum. More complex hydraulic programs are not so limited because they include features such as backwater calculations and more data input capabilities. Unless the culvert is hydraulically simple, the more complex hydraulic programs are recommended for use. For situations where the roadway is overtopped at the structure, the simplified computer methods become unstable when overtopping occurs. These errors can be critical when a FEMA NFIP analysis of water surface elevation is required. (See Chapter 5, .)