Uncertainty

The adoption of a 100-year period to define the migration corridor on a river system requires the acceptance of a certain amount of uncertainty regarding those discreet corridor boundaries.  FEMA (1999) noted the following with respect to predicting channel migration:

…uncertainty is greater for long time frames.  On the other hand, a very short time frame for which uncertainty is much reduced may be useless for floodplain management because of the minimal erosion expected to occur.

Dynamic streams and rivers are naturally susceptible to both lateral migration and avulsion into floodplain channels.  With potential contributing factors such as woody debris jamming, sediment slugs, or ice jams, dramatic change could potentially occur anywhere in the floodplain.  The goal of any mapping effort is to highlight those areas most prone to either migration or avulsion based on specific criteria developed from an assessment of historic channel behavior.

As predicted future migration is based on an assessment of historic channel behavior, the historic influences affecting channel migration are assumed to continue over the next century.  In the event that the conditions experienced by the stream corridor over the measured period of record change significantly over the next century, uncertainty regarding the proposed boundaries will increase.  These conditions include influences imposed by system hydrology, climate, reservoir management, riparian vegetation densities and extents, and channel stability.  Bank armor and floodplain modifications, such as bridges, dikes, levees, could also affect map boundaries.

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