A wide spectrum of flood flows with associated probabilities occurs at the culvert site during its service life. The benefits of constructing a large capacity culvert to accommodate all of these events with no detrimental flooding effects are normally outweighed by the initial construction costs. Therefore, an economic analysis of the trade-offs should be conducted.

The initial cost is only a small part of the total cost over the lifetime of the culvert. Understanding how the culvert operates at discharges other than the design discharge can help you define some of the longer-term operational costs.

The cost of traffic detours can be the most important if factors such as the cost of emergency vehicle response time or detour distance and cost of operation per vehicle mile are considered, especially if there is a large average daily traffic rate.

Reduced to an annual cost on the basis of the anticipated service life, the long-term costs of a culvert operation include the following:

For minor stream crossings, the use of the Design Flood and Check Flood Standards table may preclude the need for a detailed economic analysis (see Chapter 4). A more rigorous investigation, such as a risk analysis may be needed for large culvert installations or when deviations from recommended design frequencies are indicated. Refer to Chapter 3 for discussion design by Evaluation of Risk assessment.