Channel Migration Zones
Uncertainty and Risk
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.
Levels of Risk
Bankline migration and channel avulsion processes both present some level of risk to property within stream corridors. Although the quantitative probability of any area experiencing either migration or an avulsion during the next century has not been determined, their association with specific river process allows some relative comparison of the type and magnitude of associated risk. The Historic Migration Zone indicates the historic footprint of the river channels as observed in aerial images (typically since 1950’s to current time). There is a high likelihood that the river will reoccupy this historic channel area. In general, the Erosion Hazard Area delineates areas that have a moderate to high risk of channel occupation due to channel migration over the next 100 years. Such bank erosion can occur across a wide range of flows. As such, the risk is not solely associated with flood events, as channel migration commonly occurs as a relatively steady process. Avulsion tends to be a flood-driven process, and as such, risks identified by the Avulsion Hazard Zone are typically associated with infrequent, relatively rapid shifts in channel course that are commonly very difficult to predict.