Flood Plains and River Terraces


A floodplain is the low-lying ground adjacent to a river, stream or drainage subject to flooding.  A river terrace is the bench or step that extends along the side of a valley and represents a former level of the valley floor.

Related resource topics for county planning include the following:

 


Map of Data


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The ESRI mxd file of the services used to create the above map.


Resource Information

Rivers are dynamic systems. River channels can migrate laterally as a result of bank erosion and deposition, and vertically as a result of bed aggradation or degradation. Floodplains, terraces, and other features are formed by these processes, and are therefore essentially part of the river system.

Floods occur when a river channel reaches its maximum capacity, often during times of heavy rain or snow melt, water overflows the river’s streambanks and floods into nearby areas that would otherwise be dry. This is especially true when water is delivered at a rate faster than the associated soils can absorb it. Floods also occur when a dam or water impoundment gives way and large amounts of water are released suddenly. For the most part, flooding is a natural process that supports channel maintenance, ecological processes, and riparian vegetation. Nevertheless, floods can cause severe human impacts and therefore must be among resource planning considerations.

Within the MAG region, flooding most often occurs from two distinct event types: (1) spring runoff from melting snowpack at high elevations, and (2) summer rainstorms. While either event can trigger flooding, the dynamics of each are different. Snowmelt is a relatively predictable occurrence dependent on the amounts of winter snowpack and rising spring temperatures. Large accumulations of snowpack melting in spring contributes to some localized flooding of floodplains of stream and river channels. In contrast, summer cloudburst events, especially those driven by monsoonal moisture, cause sporadic and localized flooding events on otherwise dry washes and canyons. Thunderstorm-triggered floods are exacerbated in locations recently affected by wildlife fires where vegetation cover is absent and soils are more exposed to erosion and channeling water down slope.

Planning for major flood events should occur at all levels of government. At the federal level, the Federal Emergency Management Agency (FEMA) provides flood data that classifies areas based on their different flood hazards through the National Flood Hazard Layer (NFHL) and National Flood Insurance Program (NFIP). This enables community officials, emergency responders, and the general public to be informed and plan accordingly to avoid or reduce impacts from floods, guide development, and reduce risk of floods by excluding areas shown as flood hazard areas. Within the MAG region, only Summit and Wasatch Counties are currently mapped by NFHL. Many of the remaining communities are covered by the NFIP, which uses paper maps to assess floodplain locations. A list of communities participating in the National Flood Program can be found here.

County and local planning efforts include the establishment of special zoning regulations in flood-prone areas.  However, these are not commonly used in the region.


Best Management Practices

Best management practices (BMPs) related to floodplains typically focus on avoiding structures and other development within these environmentally sensitive areas. Such practices seek to maintain the natural water storing capacity of floodplains, avoid future damage to human development in these areas, and prevent publicly funded cleanup and restoration of flooded communities [1]. While FFHL and FIRM mapping can be used to direct development away from defined floodplains, many flood-prone areas in the region are not mapped. For flood hazards in these areas, planners often resort to administrative buffers (setbacks of 100 feet or more) between waterways or alluvial fans and the built environment [2].


Economic Considerations

Major economic considerations for floodplains include higher development costs to mitigate flood risks. Costs include earthen fill to raise building footprints above flood elevations and other flood-control structures on private lands. Flood-control costs may also be passed on to municipal and county governments during flood emergencies.

Another economic consideration is the cost of floodplain insurance to homeowners. Development in areas subject to floods should meet additional flood-proofing requirements. Laws and regulations regarding floodplain management usually vary between communities.

 


Impact Considerations

Floods are the leading cause of natural disaster deaths worldwide. Floods also have the potential to cause significant financial impacts in the form of severe damage to structures, transportation systems, and other infrastructure.

Wildfire is a secondary cause of flooding because when vegetation is burned, soils are exposed to erosion. Debris flows below fire scars is a considerable risk until vegetation is reestablished. Planning for revegetation through seeding and other mitigation efforts after fires should be addressed in resources management documents.


Data Download
  GIS Data Map Service Web Map Document  Tabular Data  Website
Data NameData ExplanationPublication DateSpatial AccuracyContact
FIRM list
Community
Communities Participating in the National Flood Insurance ProgramcurrenttabularU.S. Federal Emergency Management Agency
National Flood Hazard Layer (NFHL)
NFHL data incorporates all Flood Insurance Rate Map (FIRM) databases published by FEMA, and any Letters of Map Revision (LOMRs) that have been issued against those databases since their publication datevariable

Summit County,
Wasatch County
1:12,000U.S. Federal Emergency Management Agency

References

  1. Summit County. 2015. Snyderville Basin General Plan.
  2. Wasatch County. 2016. Wasatch County General Plan.