Publication Summary

A Framework for Delineating Channel Migration Zones" is a technical guidance document for planners and practitioners who are delineating Channel Migration Zones (CMZs) for purposes of erosion hazard management and outlines a methodology based on up-to-date, peer-reviewed research.

Common tools used to assess flood hazards, such as Flood Insurance Rate Maps (FIRMs), do not characterize areas susceptible to channel erosion either within or outside of the areas prone to flooding. FIRMs have limited application in planning areas safe for development because they fall short in portraying the geomorphic hazards that bank erosion may pose to land and structures. As a consequence, the costs of property lost to bank erosion are commonly transferred to the landowner.

The channel migration zone (CMZ) refers to the geographic area where a stream or river has been and will be susceptible to channel erosion and/or channel occupation. CMZ delineations help reduce risks to human communities by guiding development in and along river systems away from areas at risk of channel erosion. CMZ delineations can also provide guidance in reducing degradation and loss of critical aquatic and riparian habitats, helping assure that the river landscape is not permanently degraded or disconnected from the river by development. Because alluvial channels are rarely static through time, rivers and streams naturally migrate within their valleys. Channels respond with horizontal movement (lateral migration, avulsion, channel widening, cannel narrowing) and vertical movement (incision and aggradation) depending on site-specific circumstances and watershed conditions. Human landscape disturbance can exaggerate or constrain channel migration by affecting local and watershed processes of flooding, erosion, and deposition.

Channel migration is not limited to areas of flood inundation and can advance into landscape features above the 100-year flood water surface elevation. The time period used in the CMZ analysis is usually pre-determined by the local jurisdiction in an effort to comply with specific ordinances. However, in the instances where no timeline is given, the time period should account for long-term alteration of the landscape, given: (i) property claimed by erosion represents a permanent loss (unlike temporary impacts of flood inundation) and (ii) development within a CMZ represents a permanent loss of habitat. The CMZ is the sum of several different components of the river landscape, some which may not apply in every CMZ study: CMZ = HMZ + AHZ + EHA - DMA; EHA = ES + GS, where HMZ = historical migration zone (the collective area the channel occupied in the historical record), AHZ = avulsion hazard zone (the area not included in the HMZ that is at risk of avulsion over the design life of the CMZ), EHA = erosion hazard area (the area not included in the HMZ or the AHZ that is at risk of bank erosion from stream flow or mass wasting over the design life of the CMZ), and DMA = disconnected migration area (the portion of the CMZ where man-made structures physically eliminate channel migration). The EHA has two components: the Erosion Setback (ES) and the Geotechnical Setback (GS). The ES is the area at risk of future bank erosion by stream flow; the GS is defined by channel and terrace banks that are at risk of mass wasting (due to erosion of the toe). The GS projects from the ES at a side slope angle that forms a stable bank configuration, thereby accounting for mass wasting processes that will promote a stable angle of repose. As a tool, CMZ delineations allow planners and managers to weigh the relative risks of human development with habitat preservation over time. Accordingly, it is essential that CMZ delineation is done by qualified personnel who thoroughly document the data, methods, and interpretations upon which the delineation was made.