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Hypothetical Riverine Floodplain Diagram

Detailed Description

A cross-sectional view of a hypothetical river showing one possible arrangement of the three Federal Flood Risk Management Standard (FFRMS) floodplain elevations (Climate-Informed Science Approach, the Freeboard Value Approach, and the 0.2% Annual-Chance Flood Approach) above the current Base Flood Elevation, i.e., the 1% annual-chance flood elevation. The 1% annual-chance flood or 100-year flood refers to a flood event that has a 1% chance of being equaled or exceeded in any given year. The relative elevation of these three FFRMS floodplains can vary from site to site and among projects with different service lives at a particular site. Importantly, a project’s Climate-Informed Science Approach (CISA) floodplain, which is determined from the best available information about plausible future climate scenarios over the course of its service life, does not necessarily lie at a higher elevation than the floodplains determined through the Freeboard Value Approach and the 0.2% Annual-Chance Flood Approach. Under CISA, the 1% annual-chance flood elevation may change over time and, consequently, may vary for different actions in the same location. Floodplains labeled in gray are not included in the Federal Flood Standard Support Tool.  The CISA floodplain is not currently available for riverine locations due to the lack of consensus future flood scenarios reflecting anticipated climate change. 

Alternative text description of the diagram: Water-colored cross-section of a riverine landscape at the top with a line diagram version below it, representing the Federal Floodplain Risk Management Standard floodplain elevations under a hypothetical situation. The water-colored cross-section shows the main channel of the river in the center with the landscape rising gently up on both sides to create a shallow river valley. On the left of the main channel, there is an artificial-looking flat area where a house sits, about half-way up the river valley. On the right of the main channel, the landscape gently rises with some trees and an old meander scar labeled near the main channel. In the line drawing, the same main channel, elevation and house are present, overlaid by hypothetical flood elevations. The Base Flood Elevation (BFE, 1% annual-chance flood) is slightly above the main channel, and the Freeboard Value Approach (+ 2 feet for non-critical actions and + 3 feet for critical actions) is show above the BFE just below where the house sits. Above that elevation line, the 0.2%-Annual-chance flood approach elevation line is drawn so that it lands above the base of the house, and the Climate-informed Science Approach (CISA) is drawn bisecting the house.


Public Domain.