Title

Identifying Sediment and Woody Debris Deposition Following Peak Flows on the Elwha River, Washington

Presenter Information

Ethan Frenzel
Nick McCausland
Bryon Free

Document Type

Oral Presentation

Location

SURC Ballroom B/C/D

Start Date

21-5-2015

End Date

21-5-2015

Keywords

ArcGIS, Sediment, Woody Debris

Abstract

The purpose of this study is to identify relationships between peak flow, sediment redeposition, and woody debris transport on the Elwha River in western Washington during and following the removal of the Glines Canyon dam. Dam removal started on June 1, 2011, and was completed on August 26, 2014. We hypothesized that, as portions of the Glines Canyon dam were removed, the woody debris and sediment from behind the dam would create logjams, fill in existing bars, and decrease the average sediment size in the channel downstream. Our research was conducted two to six kilometers downstream from the dam site. Sediment sizes were surveyed bimonthly and/or following large flood events at four sites during the study period. Surface sediment size was recorded every meter along a cross section for a minimum of 100m. ArcGIS was used to map large woody debris and log jams on Mosaicked aerial photos of the entire study area, which allowed us to track the transportation of woody debris after peak flows and dam removal stages. Woody debris below the Glines Canyon Dam generally increased during the study period. Sediment size initially rapidly decreased as cobble bars were buried by sand released from the reservoir, then increased slightly as the new sediment was reworked. This research will provide valuable information on sediment transport and river dynamics during and after dam removal, which is relevant for planning for the effects of future dam removal projects.

Poster Number

13

Faculty Mentor(s)

Lisa Ely

Department/Program

Geological Sciences

Additional Mentoring Department

Geological Sciences

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May 21st, 8:30 AM May 21st, 11:00 AM

Identifying Sediment and Woody Debris Deposition Following Peak Flows on the Elwha River, Washington

SURC Ballroom B/C/D

The purpose of this study is to identify relationships between peak flow, sediment redeposition, and woody debris transport on the Elwha River in western Washington during and following the removal of the Glines Canyon dam. Dam removal started on June 1, 2011, and was completed on August 26, 2014. We hypothesized that, as portions of the Glines Canyon dam were removed, the woody debris and sediment from behind the dam would create logjams, fill in existing bars, and decrease the average sediment size in the channel downstream. Our research was conducted two to six kilometers downstream from the dam site. Sediment sizes were surveyed bimonthly and/or following large flood events at four sites during the study period. Surface sediment size was recorded every meter along a cross section for a minimum of 100m. ArcGIS was used to map large woody debris and log jams on Mosaicked aerial photos of the entire study area, which allowed us to track the transportation of woody debris after peak flows and dam removal stages. Woody debris below the Glines Canyon Dam generally increased during the study period. Sediment size initially rapidly decreased as cobble bars were buried by sand released from the reservoir, then increased slightly as the new sediment was reworked. This research will provide valuable information on sediment transport and river dynamics during and after dam removal, which is relevant for planning for the effects of future dam removal projects.