Section 6: Special Applications
Temporary culverts are usually installed in detours or emergency replacements of permanent culverts and bridges. The design must include consideration for soil protection to prevent erosion of the embankment and silting of the stream (Waters of the U.S.). Figure 8-31 shows and example of both problems on one temporary structure.
Figure 8-31. Temporary or Detour Culverts
Stream crossings for detours are normally built to higher AEP (lower flow) flood events than crossings designed for the highway. This may be good practice both hydraulically and economically. It follows that the hydraulic design of a detour stream crossing should be based on risk factors. Risk factors that should be evaluated include the probability of flooding during the anticipated service life of the detour (the construction period for the bridge or culvert), the risk to life and property from excessive backwaters and washouts, traffic service requirements, school bus routes, and emergency routes. Common sense and sound engineering judgment should prevail in making decisions.
Equation 8-31 describes the risk of the occurrence of a given AEP flood that a project incurs during its construction life.
R = Risk (probability of occurrence) in decimal form. Æ = AEP of the flood event in decimal form. n = duration in years of the project or the time requirement of the detour.
Equation 8-31 was used to generate the curves in Figure 8-32 for a family of curves for project lengths versus flood AEPs on the y axis and the resultant risk of occurrence on the x axis. Figure 8-32 demonstrates that during a one-year construction period, the odds are 4 to 1 against the occurrence of a flood as large as a 20% AEP event, and the chances are even (1 to 1) that the mean annual event will not be exceeded. The odds are 9 to 1 against the occurrence of a 10% AEP flood during a one-year construction and 2.7 to 1 against such an occurrence in a three-year construction period.
However, caution should be used with the graph. A one-year project which is to be started during one rainy or potential flood season and will finish during the next rainy or potential flood season really should be considered a two-year project for risk assessment, which means the odds are really even for risk instead of 4 to 1 against!
Figure 8-32. Graph was generated from Equation 8-31. The higher n values are not realistic, but were included for a sense of proportion.
Detour culverts are not a contractor item, but shall be designed by a licensed Professional Engineer on sealed sheets which are part of the plan set. The Contractor shall not design a detour culvert as part of the construction project unless the design has been signed and sealed and submitted to TxDOT for approval.