Communities: Know the Risk

What's Next on the Path?

  • Flood Risk Review Meeting

During the Risk Identification and Assessment Phase of a project, engineering modeling and analysis is refined to further enhance the flood risk identification.  Existing modeling may be refined to use more detailed methodology for calculating the amount of water (hydrology) expected during a storm event, include additional detail and gage analysis.  The hydraulic models to include additional refinement to cross-sections and stream crossings that may restrict flow in larger events and improve channel and structure information in existing models based on field survey. 

Engineering modeling applies the flow volume calculated for a certain storm interval and places that water into the natural channel described in the hydraulic software.  As tributaries and other drainage features add into the main stream, the flow volume increases as the model travels downstream.  The modeling calculates the peak water surface elevation determined at each cross-section and these peak values are graphically described in a profile.  These peak values are then mapped on ground elevation information to produce a floodplain delineation that identifies the expected flood extent during the analyzed storm event.

Once these hydrologic and hydraulic information is prepared, the models are used to produce a range of flood risk datasets that describe the variability of flooding within the delineated floodplain.  These flood risk datasets include:

  • Changes Since Last FIRM
  • Water Surface Elevation Grid
  • Flood Depth Grids
  • Annual Percent Chance Grid 3
  • 30 Year Percent Chance Grid

This phase of the project benefits greatly from community interaction and coordination with local technical and operations staff, providing an opportunity for FEMA and its mapping partners to engage local knowledge as the modeling is prepared.  FEMA would like to work closely with communities to identify areas where the modeling and floodplain mapping may not agree with on the ground accounts of flooding equivalent to the 1% annual chance storm event.  FEMA would like to use this phase to review community comments and include any available technical information prior to proceeding to the update of the Regulatory products (FIRM, FIS and DFIRM database).

    Flood Hazard Refinement tasks undertaken for the study streams included:
    • Provide more detailed input information for stream crossings that had not been surveyed during the GBRA or City of San Marcos analysis
    • Include the 2015 storm events in the historical analysis record
    • Analyze additional sub basins and include new gage locations into the hydrologic analysis
    • Prepare floodway analysis for selected streams