Flood Hazard Zones

The maps on this website show two types of flood hazard areas FEMA inundation zones and Fluvial Erosion Hazard (FEH) zones created by the Vermont Agency of Natural Resources (ANR). The FEMA and the FEH flood hazard areas show two different types of flood hazards and are complementary and together they provide provide the best information about flood hazards.

The FEMA inundation hazard zones and the Vermont Agency of Natural Resources Fluvial Erosion Hazard zone shown in the maps of this project

FEMA Flood Zones

The FEMA flood zones are created by FEMA’s National Flood Insurance Program (NFIP). The NFIP makes federally subsidized insurance available to communities that participate in the program. The program is voluntary and building code and land use issues are made by the local government. If a Presidential disaster declaration is made in a community that participates in the NFIP then federal assistance can be provided for the flood recovery efforts.

Flood hazard maps made by FEMA’s NFIP show inundation hazards for the Floodway, Zone A & AE (100 year floodplain), and Zone X (the 500 year floodplain).

Floodway: The floodway is an area near the river or stream that experiences regular flooding; this zone is shown in red in the map on this website. No homes or structures should be built within the floodway.

Zone A and AE: This zone has a 1% chance of flooding in a given year and is often referred to as the 100 year floodplain. This zone is shown in yellow on the maps in this project. The “100 year floodplain” term, however, can be misleading because it does not mean that a flood will happen in this zone once in 100 years it actually means that every year there is a 1% chance of a flood of this size. In fact, statistics show that a homeowner in the A & AE zone has a 26% chance of being flooded over a 30 year mortgage.

Zone X: This zone has a 0.2% chance of flooding in a given year and is often referred to as the 500 year floodplain. This zone is shown in green on the maps in this project. Like the 100 year flood plain this term can be misleading. In many areas of Vermont Irene flooded into this zone.

 

A diagram of the Floodway and the 100 year Floodway. Source: http://www.trorc.org/pdf/wq/floodsheet1_web.pdf

For more information about FEMA flood zones visit the official site of the NFIP www.floodsmart.gov

For more information about the NFIP in Vermont visit the Agency of Natural Resources Flood Hazard Management Page

Fluvial Erosion Hazard Areas

Fluvial Erosion is erosion caused by rivers and streams, this erosion can be both gradual shifts in the path of a river as well as catastrophic erosion that rapidly changes the path of a river. The Fluvial Erosion Hazard area (FEH) is a flood hazard zone that shows areas that are prone to fluvial erosion hazards. This zone is particularly important because in Vermont’s steep watersheds that experience frequent flash flooding, most flood damage in is caused by fluvial erosion. FEMA maps only show inundation hazards and so the FEH zone is highly important for a more complete understanding of flood hazards in a community.

The FEH is created by river scientists at the Agency of Natural Resources and its adoption by communities into zoning and building codes is voluntary but highly encouraged. The creation of a Fluvial Erosion Hazard zone is a much more through process based on GIS remote sensing data and extensive field visits during a Stream Geomorphic Assessment. To learn more about this process visit the Stream Geomorphic Assessment Process page.

Rivers are dynamic systems that move and meander over time as they erode and deposit sediment. The diagram below shows how the White River near Liberty Hill Farm has meandered since the 1930’s on the left, and how a FEH zone captures this meander belt width of the river on the right.

The map on the right using historical ortho imagery shows how the White River near Liberty Hill Farm has meandered over time. The diagram on the left shows how an FEH zone captures this meander belt width of the river. Sources: http://www.middlebury.edu/media/view/422751/original/final_2012_es_irenereport_web.pdf and http://www.vtwaterquality.org/rivers/docs/rv_definingrcfactsheet.pdf

According to the ANR Limiting development within the FEH zone serves two functions, “First it will prevent development in hazardous areas, reducing costly flood losses and increasing public safety. Second, it will prevent river corridor encroachment which would increase overall fluvial erosion hazards and impede a river’s natural tendency to adjust toward a more stable, equilibrium condition” (Mike Kline and Kari Dolan, 2010).

To learn more about FEH zones read the Municipal Guide to Fluvial Erosion Hazard Mitigation and visit the ANR Flood Hazard Management site.