3.0 REGION 8 APPLIED APPROACH
In accordance with Regional preference and the FEMA Guidelines & Standards the following section outlines the Region 8 process for determining whether Flood Profiles will be required for 2D modeled Zone AE floodplains produced for Risk MAP studies. In order to standardize the processing steps, ensure repeatability, as well as minimize errors, Region 8 has developed a standalone ArcPy Toolbox which mapping partners can use to review their data before submitting to the FEMA Region 8 Enhanced Validation Review. Figure 2 outlines the workflow behind the tool and the decision model to determine if an FIS Profile is required. For more detailed instructions on how to run the tool and explore output files, please refer to the Appendix.
Figure 2. 2D Flood Profile Decision Model
Note on Flood Profiles: The development of a flood profile for a study should be discussed ahead of the project with community officials and the FEMA Project Officer. It could be possible a community would prefer to have a profile generated from a 2D model even if the placement of the BFE and evaluation lines satisfies the requirements.
STEP 1 | After contractors place BFEs and evaluation lines in accordance with FEMA’s Guidelines & Standards, the tool will compare a water surface elevation (WSEL) grid generated from the mapped evaluation lines and BFEs to the raw WSEL grid output from the model to determine if the BFE and evaluation line spacing is adequate (i.e., that interpolated water surface elevations fall within 0.1 foot of the modeled results).
STEP 2 | The mapped WSEL grid is compared to the modeled WSEL grid by extracting WSEL values to “Test Points” and calculating the difference between the two surfaces. The test points are created by first generating an evenly spaced mesh of 50-foot resolution then creating points within the mesh centroids. The contractor has the option to change the size of the mesh based on the average width of the floodplain. Refer to Table 1 for optional resolutions.
Table 1. Mesh resolution options
STEP 3 | Once the difference in the WSEL grids at the test point locations has been calculated, the points are categorized as passing or failing depending on if the difference is within the 0.1-foot tolerance. Multiple streams can be run at once with the tool, however the passing percentage should be calculated on a stream-by-stream basis. See Table 2 to determine the passing percentage required for your study, which depends on the scoped level of detail of the 2D model. If the stream passes and the community does not have a preference on developing a Flood Profile, no further action is needed besides the documentation mentioned in the following section.
Table 2. BFE Accuracy Standard for 2D Floodplain Mapping
STEP 4 | If the stream is not passing, the BFEs and evaluation lines should be supplemented until the 0.1-foot tolerance is achieved. The points can be symbolized by the unique pass/fail values in order to clearly see areas within the floodplain that are failing and need additional BFEs. Please refer to the Mapping Base Flood Elevations on FIRMs Guidance for further details on placing BFEs.
In some situations, it will not be possible to provide the required detail using BFE and evaluation lines due to spatial constraints and/or steep slope in the water surface profile. In these situations, especially if the water surface profile is not linear between mapped BFE or evaluation lines, the contractor and CTP should contact the Region 8 Engineer and PTS to determine whether Flood Profiles and/or FIS inserts will be needed to provide the required detail for the product user. If a Flood Profile or FIS insert is considered inappropriate for the study reach, an exception to working standard ID 128 may be considered.
Note on FIS Inserts: If an FIS insert is needed to better convey the results of the 2D model, the appropriate FIS grid insert should be discussed with community officials and the FEMA Project Officer. (FEMA Flood Profiles Guidance Document, December 2020)
The final passing results of the tool shall be documented in the “2D MAPPING SID 128 AUDIT REPORT” (ATTACHMENT A). Any failing points that do not meet tolerance and are categorized as “excluded” should be documented and justifications or explanations for these discrepancies should be provided. For example, some factors could involve backwater or stillwater areas, terrain irregularities, or model constraints.
- This documentation, including spatial data inputs and outputs used for the analysis, should be provided in the Floodplain Mapping Data Capture “Supplemental_Data” folder under a subfolder called “SID_128_Check”.
ATTACHMENT A – 2D MAPPING SID 128 AUDIT REPORT
Repeat this table for each 2D flowpath studied
