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Goal: Identify stream reaches within your jurisdiction which require some level of channel migration assessment and reaches that may be exempt from the SMP channel migration regulations.
Product: Map of reaches that meet exemption criteria listed in SMA regulations
Information sources: Inventory data such as hydromodifications, levees, revetments, and transportation corridors that are publicly maintained and restrict movement
The following flow chart illustrates the tasks associated with Step 1a. In some cases, constructed structures and other shoreline modifications or constraints prevent river channels from normal or historic migration. Any existing legal artificial channel constraints (for example levees, public transportation corridors) that may limit channel migration could exempt those reaches from being in the regulated channel migration zone. These areas are classified as the Disconnected Migration Area (DMA) in this document.
The DMA is exempt from the Shorelines planning rules that require identification of Channel Migration Zones (CMZs). The following will help you determine the DMA and as such, what is exempt. Unless otherwise demonstrated through scientific and technical information, you should consider the following characteristics when identifying the DMA:
- Only within incorporated municipalities and urban growth areas, channel migration areas separated from the active river channel by legally existing, constructed channel constraints that limit channel movement, but not built to the 100-year flood standard, could be considered within the DMA.
- All other areas separated from the active channel by legally existing constructed structures that are likely to restrain channel migration and are built above and certified to remain intact through the one hundred-year flood, including transportation facilities, should be in the DMA.
- For legally existing structures not meeting the above criteria, require scientific and technical data to demonstrate that they are barriers to migration.
To identify stream reaches that may be in the DMA:
Step 1a, Task 1: Use existing data to identify constraints to channel migration
Make use of existing data on known hydromodifications, maintenance, and certification. State, County, City GIS layers, aerial photographs, orthophotos, or USGS topographic maps can be used to identify other areas not listed in inventories. Identification of constraints from aerial photography depends on the visibility of the structure and resolution of the data. Portions of a migrating channel that are narrow, deep, and have not migrated over time may indicate constraints.
Step 1a, Task 2: Do a simple field survey
We recommend you do a reconnaissance field survey for the constrained reaches not related to geologic controls. The survey identifies the type, extent, and composition of the channel hardening. The information gathered can be used to determine which structures constitute a legitimate barrier to channel migration under SMA criteria.
Effective barriers to channel migration (Part of DMA):
- Structures that will endure beyond the design life of the CMZ (100+ years).
- Structures that have a public commitment to keep them intact and that protect populated areas.
Ineffective barriers to channel migration (Not part of a DMA):
- Constructed structures with no public commitment for maintenance.
- Structures made of erodible materials (such as sugar dikes).
- Areas that may be restored by removing levees, revetments, or other infrastructure.
- Within incorporated areas, structures built below the 100-year flood elevation.
- Within unincorporated areas, structures built above the 100-year flood elevation but not certified to remain intact during a 100-year event.
Step 1a, Task 3: Map Disconnected Migration Area (DMA)
- Those areas meeting the exemption criteria above can be eliminated from the SMA channel migration zone regulations.
While these constraints or hydromodifications may be limiting channel migration, this does not guarantee they will always limit channel migration.
Assessing the risk associated with such structures is the responsibility of the jurisdiction.
A component of risk determination is the jurisdictions future ability to maintain the structures so they withstand migration.
Consideration should be given to the effect the artificial controls have on up- and downstream -development as well as to the opposite side of the river.
Given the potential risks, some local jurisdictions are not applying exemptions to areas landward of constructed structures.
Step 1a - Associated Policies
WAC 173-26-221(3) (b): Flood hazard reduction, Principles subsection
Shoreline Mater Program (SMP) should limit development and shoreline modifications that would:
- Result in interference with the process of channel migration.
- May cause significant adverse impacts to property or public improvements.
- Result in a net loss of ecological functions.
However, this section of the WAC states that local governments may remove areas from the Channel Migration Zone under specific conditions:
- In some cases, river channels are prevented from normal or historic migration by human-made structures or other shoreline modifications. The definition of channel migration zone indicates that in defining the extent of a CMZ, local governments should take into account the river's characteristics and its surroundings. Unless otherwise demonstrated through scientific and technical information, the following characteristics should be considered when establishing the extent of the CMZ for management purposes:
- Within incorporated municipalities and urban growth areas, areas separated from the active river channel by legally existing artificial channel constraints that limit channel movement should not be considered within the channel migration zone.
- All areas separated from the active channel by a legally existing artificial structure(s) that is likely to restrain channel migration, including transportation facilities, built above or constructed to remain intact through the one hundred-year flood, should not be considered to be in the channel migration zone.
- In areas outside incorporated municipalities and urban growth areas, channel constraints and flood control structures built below the one hundred-year flood elevation do not necessarily restrict channel migration and should not be considered to limit the channel migration zone unless demonstrated otherwise using scientific and technical information
Before removing these areas from the CMZ, the following public safety factors should be considered:
- Flood control constraints should be publicly maintained.
- Flood control constraints can be certified to FEMA standards.
- Potential for avulsions to occur by breaching structures.
The first step is to determine if and where rivers migrate in your jurisdiction based on readily available physical attributes. Subsequent steps address assessment approach and subsequent analysis and mapping methods. The approach and methods are determined by spatial scale, physical characteristics, and local governmental objectives.
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