2.0 BACKGROUND (FEMA Guidelines & Standards)

Below are excerpts from FEMA Guidelines & Standards. Refer to the linked documents for the full text.

Working Standard ID 128:

• “For floodplains mapped from 2D models, evaluation lines and BFE lines on the FIRM must match modeled water surface elevations and must be plotted at intervals sufficient to interpolate accurate BFEs in between BFE or evaluation lines. If this is not possible, separate Flood Profiles for significant flow paths and/or FIS Report inserts must also be created.”

**Mapping Base Flood Elevations on FIRMs Guidance (November 2023):

• “Where BFEs and evaluation lines are the primary source of water surface elevation information on the FIRM, the mapping partner should confirm that the lines placed are sufficient to linearly interpolate the BFE at any point in the floodplain within 0.1 foot of the true value from the model water surface elevation grid.”

Floodway Analysis and Mapping Guidance (November 2023):

• EVALUATION LINES GUIDANCE:

• Evaluation lines in 2D floodway analysis may be thought of as virtual hydraulic cross sections similar to the physical cross sections used in 1D modeling and reported in the floodway data table. Evaluation lines should be placed on FIRMs where a detailed study included a floodway calculated based on 2D methods. Evaluation lines should also be used where a detailed study included a floodway calculated based on a hybrid 1D, 2D model where the cross sections do not cover the entire floodplain. In both cases, evaluation lines should be set at the critical locations as a reference point for floodway reporting and validating surcharge requirements. Where a 2D or hybrid 1D, 2D model was used but no floodway is calculated, evaluation lines should not be included.
• Evaluation lines should typically represent a single water surface elevation and may have a contoured shape. While similar to a BFE line, an evaluation line is a separate line type used to report floodway or encroachment information and used to validate that the calculated floodway or encroachment modification meets the surcharge requirements. Evaluation lines should be reported in the S_XS database layer. On the FIRM, evaluation lines can be either lettered or unlettered, and will have water surface elevation reported, similar to a lettered and mapped cross section line in a 1D model.

Figure 1. Example of BFE and Evaluation Lines Along a 2D Mapped Floodway

Mapping Base Flood Elevations on FIRMs Guidance (November 2023):

• BFE LINES GUIDANCE:

• To accurately depict the water surface elevation grid generated by the 2D or hybrid 2D model, a combination of evaluation lines (if a floodway analysis is completed) and supplemental BFEs lines should be used on the FIRM. BFE lines will be placed along the profile baseline (where applicable) at inflection points not already captured by 2D evaluation lines, or as needed in areas of backwater, ponding complex flow areas, overflow areas off the profile baseline…or other areas needed per engineering judgement.
• In a 2D modeled floodplain, water surface elevations along the profile baseline may vary significantly from nearby areas. BFE lines will be placed in areas where water surface elevations would be inaccurately estimated from the profile.
• FEMA Risk MAP standards require that BFEs (i.e., 1D cross section or 2D evaluation lines supplemented with BFE lines where needed) must be shown at appropriate locations to allow map users to accurately interpolate flood elevations, both horizontally and vertically.
• The intent is that map users can accurately interpret flood elevations on the FIRM and that BFEs are placed strategically and at reasonable intervals to enable this intent.
• Where differences exceed 0.1 foot, additional BFE lines should be placed at the required vertical interval (not to exceed a 0.1-foot resolution) to reduce the difference.