§ 10.5.2. Risk

The detour stream crossing is usually design to a lower frequency storm. The design engineer should consider a number of risk factors for the hydraulic design of the culverts. The risk factors to be considered during the design should include the probability of flooding during the use of the detour, the risk to life and property from backwater and washouts, traffic requirements, school bus routes, and emergency routes.

The following equation relates the probability of occurrence to the flood event.

(Equation 10.5.2)

R = 1 −(1 − AEP) ⁿ

R = Risk - probability of occurrence
AEP = Annual Exceedance Probability of the flood event
n = length of time required for the detour (year)

The above equation generated the curves in Figure 10.5.2. The figure represents the risk versus flood event. As an example, if you were to design the detour culverts for a five (5)-year storm (20% AEP) and the project construction length was one (1)-year, the odds are four to one (4:1) against the occurrence, or twenty percent (20%) risk. If you design the detour culverts for a ten (10)-year storm (10% AEP) with the same project length, then the odds are nine to one (9:1) against the occurrence or ten percent (10% risk). Designing to a higher frequency storm will lower the risk of flood occurrence.

Figure 10.5.2 - Flood Frequency vs. Risk
(Source TxDOT, Hydraulic Design Manual, 2011)