What is a Channel Migration Zone?
Channel Migration Zone mapping is based on the understanding that rivers are dynamic and move laterally across their floodplains through time. As such, over a given time period, rivers occupy a corridor area whose width is dependent on rates of channel shift. The processes associated with channel movement include lateral channel migration and more rapid channel avulsion. The fundamental concept of CMZ mapping is to identify the corridor area that a stream channel or series of stream channels can be expected to occupy over a given timeframe. A 100-year CMZ is a typical timeframe.
In general, a Channel Migration Zone is composed of the following:
- Historic Migration Zone (HMZ) – the area of historic channel occupation, usually defined by the available photographic record. This can be thought of as the cumulative footprint of the channel across as seen in the historic imagery.
- Erosion Hazard Area (EHA) – the area outside the HMZ susceptible to channel occupation due to channel migration or mass wasting. This is the area that, based on historic rates of migration, the river may occupy over the period of the CMZ.
- Avulsion Hazard Zone (AHZ) – floodplain areas geomorphically susceptible to abrupt channel relocation. These are often swales, historic channels, or bendways that are not captured by the EHA.
- Restricted Migration Area (RMA), areas of CMZ isolated from the current river channel by constructed bank and floodplain protection features (also known as the Disconnected Migration Area, or DMA).
Rapp and Abbe (2003) define the CMZ as:
CMZ =HMZ + AHZ + EHA – RMA
This general definition allows for some flexibility in terms of both component definitions and the component inclusion in the CMZ. For example, one approach identified by the State of Washington is to use meander belt width and bendway amplitude to define the EHA, rather than measured erosion rates. This approach would be appropriate in channelized reaches where natural migration is largely inhibited. In addition, whether or not the RMA is included in the CMZ requires a decision as to whether bank armor should be considered effectively managed, stable, and permanent. In our experience, project stakeholders have been inclined to highlight the RMA, but not to exclude it from the CMZ as Rapp and Abbe (2003) propose. This is why the areas behind armor are called “restricted” migration areas rather than “disconnected” migration areas.