The risk of channel migration, avulsion and bank erosion is not equal within the entire mapped CMZ as illustrated in the conceptual map below. Depending on the needs of the CMZ study, it may be necessary to approximate the relative risk of these hazards.
Determinations of erosion and migration hazard can be somewhat subjective, depending on the criteria used for defining severe, high, moderate, and low risk. Methods used to quantify channel behavior, data quality, and method and procedure errors determine the level of confidence in the hazard ratings. The reasoning for the hazard levels (low, moderate, high, severe) should be clearly explained in terms of certainty, consistency with pertinent regulations, and as they relate to anticipating future channel change.
Rapp and Abbe (2003) suggest that probabilistic analysis may provide a method for quantifying hazards. Specific ranges of probabilities of channel occupation are assigned to hazard ratings. This approach has limitations and should only be applied in rivers where the HMZ captures the full extent of anticipated future channel behavior.
More often hazard ratings rely on best professional judgment and information obtained during the CMZ assessment including:
Best professional judgment implies that the analysts are well trained and experienced in channel behavior. We recommend that hazard determination be done by persons that are geomorphologists, hydrologists, or licensed geologists, hydrogeologists, engineering geologists or hydraulic engineers and have extensive experience in interpreting channel behavior. All decisions must be documented in the report and metadata where GIS is used in the assessment.
|One example of Severe hazard boundary determination—Upper Nisqually
River (GeoEngineers 2007)
Channels that are confined or controlled by not easily eroded materials are comparatively stable in terms of lateral or translational migration. Other less confined or controlled channels may also appear to be stable. Present channel conditions are a snapshot in time.
Stability is often thought to mean that the channel does not change. Stability in this context is seldom the universal condition. A presumed equilibrium may be an expression of a channel in transient recovery that appears stable because a past disturbance is no longer occurring. This does not mean the channel will remain the same. Dynamic equilibrium better describes channel behavior.
Since channels do change, the CMZ maps should be revisited and conditions evaluated. This is particularly important following larger floods or other disturbances including fires, mass wasting, substantial change in upstream land use, and sediment or hydrologic regime.
For planning purposes, limited reconnaissance field investigations may be sufficient in reaches with lower certainty on mapping; otherwise surveys may not be necessary. However, for regulatory actions field investigations are beneficial and recommended for:
- Establishing photo points and photographing relevant information.
- Documenting the characteristics and erodibility of surficial deposits within and along the stream or river.
- Writing a description of the channel and floodplain conditions which characterize:
- Bank conditions (material properties, slope, vegetation, failure mechanisms).
- Artificial structures (revetments, bridges, deflectors, abutments, levees).
- In-stream roughness components (woody debris, bars, bed material/texture).
- Side channels.
- Floodplain vegetation (actively eroding areas are highlighted as well as the locations of old channel deposits).
- Whether elevated rates of mass wasting and bank erosion in the watershed will significantly increase the quantity of sediment delivered to the project reach.
- Whether development in the watershed is likely to cause increases in the frequency and magnitude of peak flows.
Refer to [A Framework for Delineating Channel Migration Zones, Ecology Publication 03-06-027, Cygnia Rapp, R.G.; Timothy Abbe, Ph.D., R.G, 2003, Appendix B, for field forms and more detailed discussion.
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