Channel Migration Assessment

Step 4 (continued)

These figures illustrate the relationship between the erosion setback and geo-technical setback. (a) The ES is determined by estimating an erosion rate, dx/dt, for the design life of the CMZ. The ES does not always begin along the active channel, but can form along the margins of the area that encloses the HMZ or the AHZ. (b) The GS is intended to account for a vertical bank composed of erodible material that will adjust to a more stable configuration from mass wasting processes (slumping, landsliding, etc.), even if river erosion ceases (that is the channel moves away from the bank).

The ES - as part of the EHA - accounts for floodplain and terrace banks that are too high in elevation to be at risk of flooding or avulsions, yet are still susceptible to channel erosion [A Framework for Delineating Channel Migration Zones, pp. 40-45].

Bank erosion can be episodic when caused by floods. The analysis period should include the entire length of the aerial photograph record or cover at least a pre and post-large flood series to evaluate response to larger flood events. USGS gage daily and peak flow data provide information on flood magnitude, timing and duration. This data is useful to evaluate potential causes of bank erosion.

Techniques include measuring the rate of lateral migration of meander bends where there is a distinct observable and measurable trend. The ES is based on the calculation of the ES coefficient CE in Equation [1]:

CE=ER*(TE/TR). [1]

Where ER=rate of erosion (calculated by channel migration rates, TE=average time channel is expected to erode at one location (based on meander growth and cutoff), and TR=average time for river to reoccupy same location (based on historical data):

TR=(Tr1+2Tr2)/2

TR is estimated by taking the average between the time it takes a meander to move downstream one wavelength, Tr1, and the time it takes the channel to move across its valley bottom and back.

The ES is calculated by multiplying the coefficient CE by the predetermined prediction timeline of the CMZ of ≥100 years. Refer to (A Framework for Delineating Channel Migration Zones, pp 40-44] for limitations and detail and examples from completed analyses (see Examples-EHA Methods.pdf).

Generally, a GS determination is not necessary for vertical embankments composed of sound, well-indurated rock (such as a bedrock canyon), but it is potentially needed for vertical embankments composed of poorly indurated or fractured rock, and it is essential for embankments composed of unconsolidated materials (such as glacial outwash).

All methods—Quantify measurement errors and define data uncertainty

Determining channel migration zones is not just a matter of tracing historic channels on overlays or in GIS. Measurement, method and procedural errors influence erosion and migration rates and other quantification information. Measurement and photo rectification produces errors that need to be quantified in order to evaluate the magnitude of migration. A conservative approach, that is widening the CMZ, should be followed when rectification is uncertain. In determining the historic migration zone, soil and geology information can provide additional data. (See example from Jefferson County, (go to page 35) - pdf). The metadata files are required for all GIS analyses and data layers.

Previous Page l Next Page