Geologic Publications for Mount Rainier
A framework for delineating channel migration zones
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Cygnia F. Rapp
, Timothy B. Abbe
Washington State Department of Ecology
Approximately one-third of the nation's streams experience severe channel erosion; more than half of these events occur in the Northwest and the Southwest (FEMA 1999). Rivers in Washington State are especially prone to unanticipated channel erosion due to the legacy of glaciation and the effects of large woody debris (LWD) on channel bed dynamics. In many instances, outwash terraces are assumed to be "topographic constraints" because they lie well above the 100-year floodplain. However, terraces composed of erodible material (such as glacial outwash or alluvium) can erode more rapidly than floodplain surfaces due to landslides initiated by riverine erosion of the hillslope toe. Additionally, stable log jams initiate aggradation of the channel bed, much like a small dam, thereby causing the channel to flood and erode greater portions of the floodplain. (Fluctuations in LWD loading also contribute to the formation of floodplain surfaces often mistaken as terraces. In fact, these surfaces are made of alluvial materials that are prone to erosion, just as outwash terraces or other floodplain banks).
Common tools used to assess flood hazards, such as Flood Insurance Rate Maps (FIRMs), are based on fixed-bed hydraulics and do not characterize areas susceptible to channel erosion either within or outside of the areas prone to flooding. As a result, many floodplain and floodway boundaries on FIRMs are reliable for only short periods after their production. Given their short-term reliability and focus on inundation, FIRMs fall short in portraying the geomorphic hazards that bank erosion may pose to land and structures. This limits their usefulness in planning areas that are safe for development. As a consequence, the costs of property lost to bank erosion are commonly transferred to the landowner.
The principal goal of delineating the Channel Migration Zone (CMZ)—the area where a stream or river is susceptible to channel erosion—is to predict areas at risk for future channel erosion due to fluvial processes. CMZ delineations help reduce risks to human communities by guiding development in and along river systems away from such areas. Limiting development within CMZs also reduces the costs of repairing or replacing infrastructure and major civil works that might otherwise be threatened or damaged by channel migration. Additionally, CMZ delineations can provide guidance in reducing degradation and loss of critical aquatic and riparian habitats, helping assure that fluvial process are accommodated and that the river landscape is not permanently degraded or disconnected from the river by development.
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In Text Citation:
Rapp and Abbe (2003) or (Rapp and Abbe, 2003)
Rapp, C.F. and T.B. Abbe, 2003, A framework for delineating channel migration zones: Publication #03-06-027, Washington State Department of Ecology, 66 p..