Effective flood management begins with flood mapping: determining the extent and depth to which water will extend during a riverine or coastal flood. 

Floodplain managers define two important characteristics for rivers: the floodway and the floodplain. A floodway is the primary method of conveyance for the water in the channel during times of flood. It is also known as the hydrologic floodplain, defined as the land adjacent to the baseflow elevation that is below bankfull elevation (i.e., the elevation at which water has filled the primary channel and just begins to spill out onto the topographic floodplain). Restricting the flow of water through the floodway (e.g., via buildings or other structures) can increase the backwater in the channel, resulting in flooding upstream. As a result, development in the floodway is usually highly restricted and requires assessment by a professional hydraulic engineer to ensure there is no rise in upstream flood elevations [56]. 

A floodplain (also known as a topographic floodplain) is land adjacent to the channel including the floodway and floodway fringe that lies below the base flood elevation (BFE) [57].

For the purposes of the NFIP, the BFE is the elevation to which water rises during the 100-year flood (i.e., a flood with a 1 percent annual probability). Planning more critical structures may require the definition of the floodplain for the 500-year flood (i.e., a flood with a 0.02 percent annual probability) [58]. The region of the floodplain outside of the floodway acts as storage but not conveyance for water during a flood. Most floodplains (especially for larger rivers) are fairly flat, as the river naturally moves back and forth across the landscape over time in a process called lateral migration. However, rapidly moving streams in a steep channel may appear to have no floodplain at all [57].

Reading a Federal Insurance Rate Map (FIRM) 

FEMA has conducted flood insurance studies (FISs) for over 19,000 communities nationwide. These detailed analyses aim to combine precipitation, hydrologic, and topographic data together with hydraulic and hydrologic (H&H) modeling to allow communities to make better floodplain management decisions [58]. 

The FIS contains information on floods of record, the study methodology and data used, flow velocity data, and flood profile data. The FIS also provides information on the relevant FIRM (Flood Insurance Rate Map, print/download a FIRM) panels for the channel in the study. While the FIS typically provides channel profiles for the 10-, 50-, 100- and 500-year floods, FIRMs only show the 100-year (and sometimes the 500-year) floodplain, for the sake of clarity. 

The area below the BFE on a FIRM is known as a Special Flood Hazard Area (SFHA). SFHAs are marked on FIRMs using specific codes that relate to the amount of information available about flood depths and elevations. Although FEMA has generated FIRMs for nearly 22,000 communities, not every FIRM has an associated FIS (i.e., some FIRMs may be lacking specific elevation data). FIRMs are also created for communities impacted by coastal and lake flooding and for flooding as a result of sheet flow and ponding (the latter are created based on topographical analysis and historic observations) [58]. The codes and their corresponding meanings are listed in the table below. Following the launch of Risk Rating 2.0 in October 2021, FEMA will be phasing out the use of flood zones as the primary indicator for insurance premiums under the NFIP. However, the existing FIRMs will remain in place [55]. 

[55] Federal Emergency Management Agency (FEMA), "Risk Rating 2.0: Equity in Action," April 18, 2022. [Online]. Available: https://www.fema.gov/flood-insurance/risk-rating. [Accessed June 6, 2022].

[56] FEMA and North Dakota State Water Commission, Regulating withing a floodway: Frequently asked questions, FEMA.

[57] United States Environmental Protection Agency (USEPA), "Stream Corridor Structure: The Floodplain," [Online]. Available: https://cfpub.epa.gov/watertrain/moduleFrame.cfm?parent_object_id=637. [Accessed June 6, 2022].

[58] Federal Emergency Management Agency (FEMA), "National Flood Insurance Program (NFIP) Floodplain Management Requirements," FEMA, Washington, DC, 2005.