Detailed methods for developing BFEs include:

• Topography
• Field surveys must reference same datum used to produce FIRM.
• Assumed datum may be used but it may not be sufficient to revise a FIRM
• One or more cross sections may be required
• Hydrology
• Methods include discharge-drainage area relationships, regression equations, TR-55, rational formula, and other hydrograph methods
• A list of hydrologic models accepted by FEMA can be found at https://www.fema.gov/hydrologic-models-meeting-minimum-requirement-national-flood-insurance-program
• Hydraulics
• Methods include normal depth, critical depth, step-backwater, and calculations for the impact of hydraulic structures
• A list of hydraulic models accepted by FEMA can be found at http://www.fema.gov/hydraulic-numerical-models-meeting-minimum-requirement-national-flood-insurance-program

Resources:

• Guidance for Flood Risk Analysis and Mapping, MT-1 Technical Guidance available at https://www.fema.gov/media-library-data/1520966727494-8c05ecc18e141449aa79665641bd9fe3/MT1_Technical_Guidance_Feb_2018.pdf
• FEMA 265, Managing Floodplain Development in Approximate Zone A Areas: A Guide for Obtaining and Developing Base (100‐year) Flood Elevations available at http://www.fema.gov/library/viewRecord.do?id=2215