748.2 Roadway Design Criteria
On Major Routes, for bridges and boxes, the water level shall be no deeper than 1 foot below the lowest shoulder point during a 50- to 100-year event. For bridges and boxes on interstates use the 100-year event. For pipes, the water level shall be no deeper than 1 foot below the lowest shoulder point during a 25- to 50-year event. For pipes on interstates use the 50-year event.
On Minor Routes, for bridges and boxes, the water level shall be no deeper than 1 ft. below the lowest shoulder point during a 25- to 50-year event. For pipes, the water level shall be no deeper than 1 ft. below the lowest shoulder point during a 10- to 25-year event.
For structural replacements, the upper end of the hydraulic frequency ranges is used for high volume routes and sites requiring only minimal roadway work. The lower end of the hydraulic frequency ranges is used for low volume routes and sites requiring extensive roadway work. For new locations, consider the expected traffic volumes and length of roadway to be constructed in selecting an overtopping design frequency. When rehabilitating an existing structure it is not required that the adjacent roadway be raised to meet the roadway overtopping standards for new roads. The replacement structure should be designed such that it does not increase the frequency of roadway overtopping.
The overtopping discharge is the lowest discharge that overtops the lowest point on the roadway. The overtopping frequency is the recurrence interval of the overtopping discharge. The roadway overtopping frequency is documented in the files and labeled on the plans as the “overtopping frequency”. For overtopping frequencies > 500 years, simply report "> 500 years" on the plans. The files include a note indicating that improving the roadway overtopping frequency would have been outside the scope of the project.
High Water Marks
The districts shall keep a record of the elevation of high water marks during floods for future design purposes. High water marks should be indicated on roadway structures if possible and labeled with the year of the occurrence.