Beach response to the removal of dams on the Elwha River
Presentation Abstract
Conceptual models of sediment distribution from the mouth of the Elwha River developed prior to dam removal suggest two responses on the intertidal beach; 1) the cessation or reversal of chronic erosion, especially east of the river mouth, due to the addition of new sediment volume to the beach and 2) a reduction in the average bed grain size of the beach due to the composition of the expected sediment load from the reservoirs. Furthermore, the documentation of strong alongshore transport directed to the east along this shoreline suggests that the two responses described above should vary in space and time, with a response occurring first near the river mouth and progressing to the east over time. To track the intertidal response and test this conceptual model, profile and grain size measurements have been collected at four cross-shore oriented transects on the Elwha River delta, one to the west of the river mouth and three to the east. Measurements were made intermittently between 2008 and 2011, and approximately bi-weekly since February 2011. These high frequency measurements supplement bi-annual large scale topographic and bathymetric surveys conducted by the US Geological Survey. At the measurement site immediately to the east of the river mouth patterns of profile accretion and a reduction in the mean grain size of the beach suggests that, by July 2012, sediment associated with the dam removal was recruiting to the foreshore and nourishing what has been a coarse eroding beach. In November 2012 a large pulse of sediment arrived at the river mouth which built portions of the shoreline seaward >200 m and initiated a transition of some portions of the formerly intertidal and sub-tidal shoreline to back-shore habitat. The profile to the west of the river mouth has also added sediment volume since the dam removal started, and our data also suggest a reduction in the mean grain size of the beach there. By contrast, the two profiles furthest to the east on the delta have continued to erode, though possibly at a declining rate. There is little evidence for a reduction in grain size at these sites, though, suggesting that a reduction in the erosion rate may not be due to the alongshore transport of sediment from the river mouth and the addition of sediment volume. Rather, we hypothesize here that the changing morphology of the river mouth and/or the sea-floor may be altering the pattern of wave energy delivery to the beach and reducing alongshore energy flux.
Session Title
Session S-06F: Elwah River Restoration: Evolution of Habitats and Ecosystems During a Dam Removal Project
Conference Track
Restoration
Conference Name
Salish Sea Ecosystem Conference (2014 : Seattle, Wash.)
Document Type
Event
Start Date
1-5-2014 1:30 PM
End Date
1-5-2014 3:00 PM
Location
Room 602-603
Genre/Form
conference proceedings; presentations (communicative events)
Contributing Repository
Digital content made available by University Archives, Heritage Resources, Western Libraries, Western Washington University.
Subjects – Topical (LCSH)
Beaches--Environmental aspects--Washington (State)--Elwha River; Sedimentation and deposition--Washington (State)--Elwha River; Dam retirement--Environmental aspects--Washington (State)--Elwha Dam
Geographic Coverage
Salish Sea (B.C. and Wash.); Elwha River (Wash.); Elwha Dam (Wash.)
Rights
This resource is displayed for educational purposes only and may be subject to U.S. and international copyright laws. For more information about rights or obtaining copies of this resource, please contact University Archives, Heritage Resources, Western Libraries, Western Washington University, Bellingham, WA 98225-9103, USA (360-650-7534; heritage.resources@wwu.edu) and refer to the collection name and identifier. Any materials cited must be attributed to the Salish Sea Ecosystem Conference Records, University Archives, Heritage Resources, Western Libraries, Western Washington University.
Type
Text
Language
English
Format
application/pdf
Beach response to the removal of dams on the Elwha River
Room 602-603
Conceptual models of sediment distribution from the mouth of the Elwha River developed prior to dam removal suggest two responses on the intertidal beach; 1) the cessation or reversal of chronic erosion, especially east of the river mouth, due to the addition of new sediment volume to the beach and 2) a reduction in the average bed grain size of the beach due to the composition of the expected sediment load from the reservoirs. Furthermore, the documentation of strong alongshore transport directed to the east along this shoreline suggests that the two responses described above should vary in space and time, with a response occurring first near the river mouth and progressing to the east over time. To track the intertidal response and test this conceptual model, profile and grain size measurements have been collected at four cross-shore oriented transects on the Elwha River delta, one to the west of the river mouth and three to the east. Measurements were made intermittently between 2008 and 2011, and approximately bi-weekly since February 2011. These high frequency measurements supplement bi-annual large scale topographic and bathymetric surveys conducted by the US Geological Survey. At the measurement site immediately to the east of the river mouth patterns of profile accretion and a reduction in the mean grain size of the beach suggests that, by July 2012, sediment associated with the dam removal was recruiting to the foreshore and nourishing what has been a coarse eroding beach. In November 2012 a large pulse of sediment arrived at the river mouth which built portions of the shoreline seaward >200 m and initiated a transition of some portions of the formerly intertidal and sub-tidal shoreline to back-shore habitat. The profile to the west of the river mouth has also added sediment volume since the dam removal started, and our data also suggest a reduction in the mean grain size of the beach there. By contrast, the two profiles furthest to the east on the delta have continued to erode, though possibly at a declining rate. There is little evidence for a reduction in grain size at these sites, though, suggesting that a reduction in the erosion rate may not be due to the alongshore transport of sediment from the river mouth and the addition of sediment volume. Rather, we hypothesize here that the changing morphology of the river mouth and/or the sea-floor may be altering the pattern of wave energy delivery to the beach and reducing alongshore energy flux.
