Channel Migration Assessment

Steps 3-5 – Analysis

Step 4: Analyze migration, erosion, and avulsion components and map CMZ, erosion and avulsion hazards

Product: Analyses and interpretations of migration, erosion and avulsion data and maps locating the channel migration zone and associated hazards.

Table 6 (below) provides a list of completed CMZ assessments from Washington for reference:

TIP: Always include appropriate disclaimers on maps depicting channel migration zones. For example, "Channel migration zone boundaries depicted on this map are approximate. The boundaries have not been surveyed and are intended for planning purposes only. Additional site-specific evaluation may be needed to confirm/verify information shown on this map.” The lower the level of effort, the more important this disclaimer becomes. Generic map disclaimer language is provided under the Shorelines management jurisdiction web page.

Table 6: Only completed CMZ assessments are available. More will be added as they become available.

Scale/level of effort Examples and links
Method I.D. Effort level  

Basin/watershed

1 Low Spokane County
2 Moderate Quinault River (PDF, 84 mb: Note: This is a large file!)
5 High Upper Nisqually River (8.8 mb PDF), Pierce County (PDF, 4 mb), Jefferson County (PDF, 10 mb), Hoh Basin (PDF, 27 mb: : Note: This is a large file!)

Reach

3 Low  
4 Moderate Hoh Basin (PDF, 27 mb: Note: This is a large file!)
5 High Jefferson (PDF, 10 mb), Pierce (PDF, 4 mb.), King (PDF, 2.6 mb.), Hoh Basin Reach 7 (PDF, 6.6 mb.)
6 Very High Gray’s River (PDF, 123 mb: Note: This is a large file!)

Site

7 High Gray’s River (PDF, 123 mb: Note: This is a large file!)

Determine data needs as per chosen approach

Obtain historical and recent aerial photographs, maps, and other information for the channel reaches. You can obtain historical and contemporary aerial photos, maps, and Digital Elevation Models (DEMS) from a number of Federal, State, and local agencies. The Internet provides numerous sites with links to data resources and sites having searchable databases pertaining to maps and aerial photography. Some data sources are listed in Table 1 and data links (pdf).

The earliest aerial photography in Washington is from 1920-30’s (see data links pdf). Photos before 1940 are not available for all shorelines in Washington. In most cases, the earliest aerial coverage is from the 1940’s. A USGS Fact Sheet provides information on looking for older aerial photographs.

Other useful resources

The General Land Office (GLO) surveys conducted in mid-1800 to early 1900 may also provide useful historical maps (see data links pdf). The GLO survey field notes sometimes provide information on channel widths, wetlands, soils and vegetation type and size. The cadastral surveys when compared to aerial photographs provide an indicator of channel stability. Where the surveys can be rectified, they can be used as part of the historical analysis. Older USGS topographic maps and river surveys for civil structures also can be useful in the analysis.

There maybe situations where only a few years of aerial photographs are available or there is a large gap in years between sets. An analysis covering recent years can provide information on existing migration rates and conditions. An older set can provide information on past conditions. In these cases, it is highly recommended that field assessments accompany the analysis especially in reaches that may be critical for human health and safety and ecological elements.

Analyze current and historic channel migration and map historic and current channels, define HMZ boundaries and measure migration rates as per appropriate approach and methods

Analysis methods vary but generally, there are three planimetric analyses used in Washington to determine HMZ boundaries and migration rates. Analysts may use a combination of methods. For example, the Jefferson County channel migration study used both manual and GIS methods.

  1. Manual measurement (Steps, see Manual Method.pdf)
  2. Polygon analysis (Steps, see Polygon Analysis.pdf)
  3. Transect measurements (Steps, see Transect Method.pdf)

Refer to A Framework for Delineating Channel Migration Zones, Section 4 (Rapp and Abbe 2003) for more discussion on methods 1-3 and limitations.

  1. A fourth less frequently used method to measure migration rates is using the radius of curvature to measure channel migration. This method is more applicable to meandering patterns as demonstrated for the Gray’s River (see Methods Radius of Curvature.pdf).

At a minimum, the active channel(s) and the Holocene floodplain should be mapped for each reach over the historical period of the study. The investigator can also use stereo-pairs or orthophotos of sufficient resolution to digitize bars, islands, man-made structures, and terraces, in addition to the active channel.

Mapping these other channel features can be useful for identifying causal mechanisms on channel response. Mapping bars, islands and large woody material also help identify potential areas of vertical channel change such as aggradation. Use of stereoscope is recommended for all methods to identify relevant features.

Note: Field visits are highly recommended to verify or clarify mapping.

Analyze the avulsion potential and map the Avulsion Hazard Zone boundaries

The Avulsion Hazard Zone (AHZ) includes the areas of the river landscape, such as secondary channels, relic channels, and swales that are at risk of channel occupation outside of the Historic Migration Zone (HMZ). The purpose of delineating avulsion hazard zones is to anticipate possible shifts in channel location that may threaten infrastructure (see Overnight Avulsions.pdf) and necessitate bank protection. An avulsion is a sudden switching of the river channel to a different channel. The two channels are often separated by a vegetated, not recently eroded island. Avulsions often occur where there is aggradation and sudden loss in stream power.

Surfaces that are part of the AHZ commonly have relict, subdued channels that are both visible on LIDAR and aerial photos (see Comparison.pdf) as well as in the field. Topography is irregular with higher islands of mature vegetation and lower areas of channels and gravelly bars with younger vegetation. Where LIDAR or recent channel cross-sections or hydraulic models exist, the elevations of floodplain and relict channels can be assessed relative to active channel bed elevation. LIDAR, with an elevation tool, can be used to determine the elevations of floodplain, relict channels, LIDAR-based DEMS, cross-section survey data, and active channel.

Other indicators of potential avulsions that can be determined from aerial photographs include:

Many CMZ assessments in Washington include avulsion hazard analyses. A recent assessment, Upper Nisqually River assessment, provides examples.

Analyze erosion hazards and map the Erosion Hazard Area (EHA)

The EHA delineates the areas outside of the HMZ and AHZ that may be susceptible to bank erosion from the following:

The EHA has two components:

  1. Erosion Setback (ES), the area at risk of future bank erosion by stream flow
  2. Geotechnical Setback (GS)
    1. The GS is defined by channel and terrace banks that are at risk of mass wasting from bank toe erosion. The GS projects from the ES at a side slope angle that forms a stable bank configuration.

The EHA equals ES + GS (see Examples-EHA Methods.pdf).

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