Channel Migration Assessment

Step 2 – Determine the Minimum Standards of Practice and Level of Effort

Goal: Identify the minimum standard of practice to conduct a channel migration assessment.

Step 2 from the Channel Assessment flow chart provides guidance on determining an appropriate migration assessment approach and supporting methods. The approaches and methods are organized by assessment area size and channel pattern (Step 2a). Environmental and infrastructure values provide guidance on the level of effort that should be considered. The likely time period anticipated for geomorphic events to occur provides additional refinement to level of effort (Step 2b). In Step 2c, planning and management objectives, assessment spacial scale, and level of effort provide more detailed guidance on appropriate approaches.

Step 2a: Channel migration assessment applicable methods for SMP update characterization and inventory

Product: Descriptions and maps locating the channel migration zone as required by the SMA for the inventory and watershed characterization phase of the SMP update

Information sources: Inventory data on stream processes for the jurisdiction and the aerial photographs (at least 2 different years), maps, orthophotos and LiDAR used in Step 1.

Channel migration zones are included in list of required inventory items [WAC 173-26-201(3)(c)(vii)]. The minimum requirement for the CMZ in an inventory and characterization phase of the SMP update is to identify the general location of CMZ and floodplain.

Figure 11: The numbers identify the linked summary of approaches and methods. M=meandered pattern, B=braided pattern, A=island braided pattern, W= wandering pattern. This table provides a list of minimum standards of practice approaches and methods for watershed characterization. The methods are organized by scale and channel type - meandering, braided, Island braided and wandering. Straight channel patterns that may have potential to migrate fall under meandering pattern. Channels with avulsion potential fall under island braided and wandering channel categories. The approaches and methods are described in Table 4, Step 2c.

At this point, a channel migration assessment can be done to meet the SMA requirements for inventory and characterization element of your SMP update. But we recommend that all of Step 2 be done before choosing your assessment approach. Channel migration poses a hazard to property and life. So the more general assessments require regulations that are more restrictive.

Local governments that have completed general channel migration assessments for their SMP updates have required property owners to demonstrate through scientific and technical information that they are not in the channel migration zone before they can build or fill.

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