Category:748 Hydraulics and Drainage

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To produce the most practical and flexible design, each hydraulic aspect and the resulting design frequencies must be considered and established based upon the relative risks and costs. The hydraulic frequency establishes a design storm and associated discharge and the criteria establish the acceptable level of performance during that storm. Instead of the simple term “design frequency”, use the terms “backwater design frequency”, “roadway overtopping design frequency”, “freeboard design frequency”, etc. The plans are to show the frequency to which each aspect of the project is designed.

Roadway Overtopping. 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 roadway overtopping frequency is documented in the files and labeled on the plans as the “overtopping frequency”. The files include a note indicating that improving the roadway overtopping frequency would have been outside the scope of the project.

Freeboard. When the drainage area produces unusually large debris, additional freeboard to protect the structure is desirable. If the drainage area produces very little debris the freeboard criteria may be reduced.

Backwater. For projects with moderate or high upstream property impacts, analyze and quantify backwater impacts so that design decisions can be made. This applies even when the District does the design. In some cases, the District may choose to document reasons why backwater impacts do not need to be estimated instead of performing the calculations. The Bridge Division will continue to assess the backwater impacts as part of most hydraulic designs performed by Bridge. Impacts to areas with no existing improvements or anticipated potential for development would be considered minimal, even for relatively large areas. Impacts to agricultural land can be considered minimal if only a small area is affected, or considered moderate to high for a large area.

Consider the extent, depth, duration and frequency of flooding when evaluating backwater impacts. The depth and frequency of flooding are the most critical, but MoDOT can be held responsible for unreasonable increases in any of these.

If it can be assumed that the road won’t be raised in the foreseeable future, consider the flow over the road when sizing the structure. Otherwise, size the structure to handle the full flow so that the structure won’t need to be replaced to raise the road in the future. In this case, the backwater must be specifically calculated for the event that just overtops the road in addition to the standard frequencies. This event may produce the maximum backwater impact.

Choose the upper end of the allowable backwater range only when it is appropriate and reasonable. For new structures the backwater impacts are determined cumulative with bridges left in place. When extending a culvert, a complete hydraulic analysis or documentation of why a complete analysis is not necessary is required. When the existing culvert can be shown to be adequate and impacts from the lengthening are minor, design exceptions can be sought to allow the use of reduced criteria to avoid replacing the culvert. However, a hydraulic analysis is still necessary to determine the impacts.