Channel Migration Assessment

Step 5: Develop hazard rating zones (recommended but optional)

This step is still under construction

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)
  • Severe CMZ hazard boundary: Distance measured from HCOT (≈HMZ) landward boundary where distance length represents the greatest bank loss or corridor movement associated with a single storm event over the period of the photo record
  • 5 steps were outlined for determining the distance measurements and location of boundary:
    • The historical channel occupation tract (HCOT≈HMZ) is identified.
    • The ‘severe CMZ base width’, defined as the boundary representing the greatest bank loss in the most erodible material within a geomorphic reach observed over a short period of time from the aerial photography record, is determined.
    • Depositional reaches for potential avulsion sites and routes are identified and mapped.
    • Terrace recession in the less erodible valley is located, described and measured to determine terrace potential to recede in storm events.
    • Map fixed boundaries that do not appear to have eroded over the course of the photo record, and will not likely erode in the future.

Revisit channel migration assessment:

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.

Field assessments

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:

Where the AHZ is not certain, four primary field-based tasks are recommended:

Fieldwork (on-the-ground data collection) primarily consists of describing and mapping geologic, geomorphic, and vegetative conditions in the given area.

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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