Clallam County’s overall goal is to better understand the current and future patterns of erosion and flooding along the Lower Elwha River system to incorporate risk-informed decisions into various planning processes to benefit public safety, infrastructure, and aquatic and riparian ecology.
A Detailed Channel Migration Zone (dCMZ) will provide landowners, Clallam County, tribal co-managers, and conservation partners a better understanding of hazards and future trends. Post-dam removal dCMZs will inform and guide shoreline, critical area, and land use management along these waterways, aiding both applicants and practitioners.
Clallam County doesn’t currently have a dCMZ post dam removal. The Elwha River is a dynamic river that is expected to continue to change over time, considering dam removal and climate change.
What is a CMZ?
Channel migration zones are areas in a floodplain where a stream or river channel can be expected to move naturally over time in response to gravity and topography.
How is it made?
CMZ studies analyze historical information, LiDAR and field data to interpret past and current channel conditions in order to predict future channel behavior and areas at risk of channel movement. The CMZ study takes into account trends in channel movement, context of disturbance history and changes in boundary conditions, as well as topography, bank erodibility, hydrology, sediment supply and woody debris loading.
The CMZ boundary delineates the area in which channel processes will occur over a specified period of time. Consequently, the timeline used for a CMZ delineation will affect the relative area included in the CMZ. For example, a CMZ intended to capture channel processes for 100 years into the future may be smaller in area than a CMZ intended to capture channel processes for 500 years. The boundary of the CMZ is stationary for the design life of the CMZ delineation; it does not change unless channel erosion hazards are not properly accounted for in the original CMZ delineation, leading to unanticipated erosion.
How will it be used?
The Elwha River is a shoreline of statewide significance under the Shoreline Management Act (SMA). Shoreline use and development activities are regulated locally by the Clallam County Shoreline Master Program (SMP), which establishes limitations and requirements specific to CMZs. As an early part of the recent SMP update, the inventory of shoreline conditions occurred during the active phase of removing both Elwha River dams (2011 to 2014) and did not include CMZ mapping for the Elwha system. Dam removal has dramatically affected the river environment downstream and at the delta. A growing collection of newer data and technical information documents the river’s response to the release of impounded sediment and wood, the draining of two reservoirs, and the return of natural sediment and wood transport processes.
Critical areas such as geologically hazardous areas and frequently flooded areas that are located in shoreline jurisdiction are regulated by the SMP, while critical areas located outside shoreline jurisdiction are regulated by the Clallam County Critical Areas Ordinance (CAO). Accurate CMZ mapping will help better implement both the SMA and CAO locally on a project-specific basis.
Planners and resource managers will have the reach-scale information necessary to establish jurisdictional boundaries; better protect people, development, and natural resources from flood and erosion hazards; and prevent future development in hazard areas.
