7.7.1 Final Hydrologic Design
Hydrologic analysis is the most important step prior to hydraulic design to establish stormwater flow rates, flow volumes, and locations of inflow and outflow to the highway facility for significant drainage areas. Recommendations from these analytical studies can affect such major items as roadway alignments, bridge lengths, bridge lateral restraints, bridge foundations, and channel design.
The initial hydrologic study is typically prepared during preliminary engineering (see
). During final design, a refinement of the original study may be all that is needed. This refinement is usually needed to reflect detailed field survey data or a change in a basic design condition, an assumption, or to reflect revised methodology, or if there has been a significant delay between preliminary engineering and final design development.
If the project is in a tidal area, consider storm surge (tide) analysis.
Specific tasks to be performed include:
- Identify any new, relevant data;
- Verify validity of previous hydrologic study and determine if the study method used is still appropriate;
- Evaluate any existing hydrologic data/results from previous studies and update as appropriate or perform new hydrologic analysis for proposed hydraulic structure locations;
- Based on the model, determine whether watershed revisions change stream water surface elevations; and
- Coordinate with the local FEMA floodplain administrator (FPA) for changes to water surface elevations and flood maps.