Channel Migration Assessment

Table 4: Summaries of minimum standards of practice are organized by scale and level of effort. Methods are numbered to correspond to watershed characterization and management objective matrices outlined in Figure 13. The scale, e.g., basin scale, reach scale, and level of effort, e.g., high, moderate, are listed in the first column. The approach and methods are described in the second column. Examples of completed CMZ assessments in Washington and other projects are provided in this table and in Step 4 (see step 4, table 6). All levels of effort require descriptions of limitations and level of confidence in the assessment and subsequent CMZ delineation. The channel migration zone is the sum of identifiable components.

  Minimum Standard of Practice
Basin/watershed scale Most applicable to basin or watershed Scale: Continuum of stream types
Approaches

Example:
Prairie Creek Channel Migration Study (pdf)

  • Synthesize available data to understand control or relevant basin-scale processes such as geology, soils, topography/gradient, hydrology, land use, vegetation.
  • Contrast study basin with adjacent basins.
  • Develop the basin history.
  • Evaluate the influence of past disturbances on channel response.
  • Reach-scale analysis on selected reaches.
  • Can be primarily GIS based with limited field verification.
1. Low Level
Minimum Level of Effort1`
  • Identify the purpose of the CMZ delineation and questions to answer.
  • Use available tools such GIS, DEM, LiDAR and hydrologic data to minimize costs.
  • Determine the characteristics and processes operating within the basin, including sources of sediment, climate and land use changes by researching FEMA floodplain maps [evaluations in Washington indicate that the FEMA 100-year floodplain is not a surrogate for the CMZ], geologic and soils maps, and historic stream flow and climate data.
  • Identify possible hazard areas using DEM, LiDAR or aerial photographs.
    Consider current / future land use and growth projections.
2. Moderate-high Level


See example:
Quinault River CMZ - (pdf)

Minimum Level of Effort—Above plus the following
  • Determine possible response and associated hazards by channel pattern and process (identified in Step 1) for past, current and future conditions.
  • Past, current and future probable disturbances may influence response.
  • Fires, landslides, timber harvest, development, hydromodifications.
  • Past, current and future climate.
  • For example, are glaciers in system and are they retreating (thereby introducing lots of sediment)?
  • General sediment budget changes: past, current and future.
  • Erosion hazard area=average rate of bank erosion per year x 100 years.
  • CMZ Hazard zone.
  • Meander and braided patterns: The probable migration area over 100 years plus the erosion hazard width and AHZ or average meander amplitude as measured from OHWM, or the FEMA 100 year floodplain, whichever is greater.
  • Anastomosed pattern: The probable migration area over 100 years for the mainstem and each anabranch that falls outside the mainstem migration area + AHZ +EHZ + half the average migration rates as buffers, or the FEMA 100 year floodplain, whichever is greater.
  • Wandering pattern: The extent of all probable channel migration over 100 years plus + AHZ +EHZ + half the average migration rates as buffers, or the FEMA 100 year floodplain, whichever is greater.
Reach Scale Most applicable to assessments by reach: Evaluate for geomorphic reaches displaying similar physical and land use characteristics
Approaches The minimum standard for more stable channel conditions such as meandered and constrained braided patterns; and anastomosed patterns where there is no evidence of avulsion in last 100 years and no evidence of potential for avulsion (link to photo examples and indicators).
  • Reach is defined by tributaries and changes in channel pattern to less stable systems.
  • Define HMZ from high resolution DEM or LiDAR, topographic maps and aerial photos.
  • Calculate migration rates for each individual bend and develop a composite for the reach.
  • Where downstream migration is apparent, attempt to correlate with hydrologic events or other factors
  • Determine whether high erosion rates correlate to high flow events.
  • Determine if erosion rates correspond to disturbance or land cover or use.
  • Consider land cover (natural versus human) when assessing channel stability and response.
  • As in Approach 1, past, current and future disturbances and changes in climate, hydrology, and land use need to be considered in channel response.
  • Construct a buffer measured out from the edge of the CMZ and/or potential avulsion zones.
Limitations
  • Assumes that these channel patterns are somewhat stable.
  • Assumes that channel response is relatively predictable.
  • Assumes that the classic meander belt application is appropriate where amplitude can be measured.
  • If any part of reach is less stable than need to use a higher level effort.
3. Low level
Minimum Level of Effort
  • Data requirements aerial photos, DEM and/or LiDAR.
  • Define meander belt, but take into account how conditions have changed.
  • Hazard area is meander belt plus average amplitude distance or HCMZ plus half the width of average amplitude as a buffer or the FEMA 100 year floodplain, whichever is greater.
  • Conduct sufficient field verification site visits to confirm assumptions and research findings.
4. Moderate level

see example: Hoh River - (pdf)

Minimum level of effort

In addition to Low level requirements above:

  • What past, current and future disturbances and changes may influence response (refer to Approach 1)
  • Map channel changes including bar forms, LWD, hydromodifications from at least 2 aerial photos series of the same scale or geo-referenced.
  • If only 2 years used years separated by a flood or at least 10 years apart.
  • Key in on local effects and calculate for each individual bend and composite for the reach.
  • Define CMZ over last 100 years (if data not available then average maximum migration rate per year x 100 years).
  • Where migration has occurred evaluate in terms of
  • Physical factors—hydrology (flow regulation, channel reactions to hydrologic events), increased or decreased sediment loads.
  • Land use changes including hydromodifications.
  • Identify causes of local instability.
  • Hazard area.
  • Entire HMZ over 100 years +EHZ+AHZ plus buffer that is equal to 1 meander amplitude, or the FEMA 100-year floodplain, whichever is greater.
5. High Level
see examples (all pdf):
This approach applies to less stable channel conditions or high level of effort determined for the shoreline designations (Step 2b)
  • Develop a “spaghetti” map showing historic channels to define the documented Historic Migration Zone (HMZ) over 100+ years where data are available (if data not available then average migration rate per year x 100 years).
  • Develop an erosion coefficient = probability x rate .
  • Measure historic rates of channel movement over documented periods of time and apply the calculated distance from a selected point (i.e., an existing OHWM).
  • Map potential avulsion channels both inside and outside HMZ. Consider changes in floodplain elevation based on topography or LiDAR. Also consider avulsion hazard zones (AHZ) based on hydraulic analysis of erosion potential.
  • Construct a meander amplitude map or envelope.
Assumptions
  • Assumes that wandering channels, extensive braiding, avulsions, high sediment load or substantial changes in hydrologic regime are relatively unpredictable so require a higher minimal standard of practice.
  • Reach is defined by geomorphic characteristics such as changes in confinement and channel pattern.
  • Assumes that the classic meander belt application is appropriate.
  • As in Basin approach, past, current and future disturbances and changes in climate, hydrology, and land use need to be considered in channel response.
Minimum Level of Effort
  • Use at least 3 sets of aerial photos; most recent, the oldest, and a middle set of photos.
  • Determine maximum or average migration rate or meander amplitude for the reach. Apply that distance from the OHWM to delineated historic migration zone (HMZ).
  • Identify potential avulsion zones (link to some indicators).
  • Use erosion coefficient to calculate buffer for erodible banks or terraces (EHZ).
  • Erosion hazard (EHZ)=erosion setback (ES) + geotechnical setback (GS).
  • Downstream migration and potential causes must be considered at the time of the delineation.
  • Float or walk entire reach and identify and map bank erosion and materials, hydromodifications and geomorphic characterization.
  • Hazard zone is [HCMZ+EHZ+AHZ plus maximum migration rate], or FEMA floodplain, whichever is greater.
  • This type of analysis can be expensive, and there may be limitations in data; e.g., limitations where bank hardened.
6. Very High level
see example:
Grays River - (pdf)
The above plus one or more of the following approaches:
  • A probability of occupation map within CMZ using GIS.
  • Map avulsions using topographic maps, LiDAR, shear stress analysis.
  • Calculate reach scale characteristics of meanders and other landforms using amplitude and radius of curvature of meanders and estimate future probability of migration.
  • Geotechnical evaluation of erosion hazard.
Site Scale
  Site specific analysis is based on minimum level of effort as identified in the level of effort matrix plus the following approach/minimal standard of practice
  • Evaluate historical conditions for context.
  • The CMZ approach will differ by channel type.
  • If meandering, then a more site-specific approach is required, incorporating the direction of migration.
  • If braided, then apply average migration rates based on historic data.
  • "Ignore" structures such as revetments and levees (i.e., it should not be assumed that they prevent migration or would be rebuilt if damaged or failed).
  • Don’t exclude areas outside of structures for consideration in the analysis.
  • More sophisticated analysis, such as modeling or probability analysis, may be more appropriate at this scale.

The evaluation must include side channels. Use elevation, morphology, and substrate to make inferences.

 
Minimum Level of Effort
  • Evaluate watershed context - reach and basin conditions.
  • Identify historic CMZ (HMZ) through all available data for site.
  • Identify other erosion and avulsion hazards.
  • EHZ= ES + GS.
  • Identify land surface deposits.
  • Identify soil erosion potential.
  • Identify location and elevation of site with respect to high flows (must select recurrence interval).
  • Identify rates.
  • CMZ Hazard = [HMZ+EHZ+AHZ plus maximum migration rate], or FEMA floodplain, whichever is greater.

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