south fork skykomish river channel migration zone draft ... migration/s-fork...apr 20, 2017 ·...
TRANSCRIPT
South Fork Skykomish River Channel Migration Zone
Draft Study and Map
6:00 – 6:30 Open House
6:30 – 6:35 Welcome and Introductions
6:35 – 6:50 Public Rule Presentation
6:50 – 7:20 Technical Presentation
7:20 –7:40 Q & A
7:40 – 8:00 Open House
Public Meeting, Skykomish School, April 20, 2017
Public Meeting, Skykomish School, April 20, 2017
Department of Permitting and Environmental Review
King County Channel
Migration Public Rule
Steve Bottheim, Geologist
Public Meeting, Skykomish School, April 20, 2017
Department of Natural Resources and Parks
Water and Land Resources Division
River and Floodplain Management Section
South Fork Skykomish River
Channel Migration Zone
Draft Study and Map
John Bethel, Geologist
Technical Presentation
• Overview
• Study Area
• Types of Channel Migration
• Channel Migration Zone (CMZ) Mapping Process
• Development of Draft South Fork Skykomish CMZ
Map
• Next Steps
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Channel Migration Overview Channel Migration:
• The shifting of a river within a river valley
• A type of flood hazard
Channel Migration Zone (CMZ):
The area within which a river channel can be expected to migrate over time
Uses of CMZ Mapping:
• Inform the public about hazards
• Support flood risk reduction planning and projects
• Serve as a regulatory map in unincorporated areas
Raging River
Example CMZ Map
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Study area of South Fork Skykomish CMZ Study and Map
• Foss-Tye confluence to King/Snohomish County Line (13.1 mi)
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Skykomish
Grotto
Baring
Riverwood
Timberlane
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Types of Channel Migration
• Lateral migration: Progressive channel
movement across valley
bottom
• Avulsion: Abrupt shift of
channel location
Lateral
migration
Avulsion
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Lateral Migration Example
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RM 9.2 between Grotto and Baring - 1956
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RM 9.2 between Grotto and Baring - 2013
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150 ft
Lateral Migration Mapping Combined
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Avulsion Example
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RM 12 near Grotto - 1944
Northwest Portland
Cement Co. plant
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RM 12 near Grotto - 1970
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RM 12 near Grotto - 2013
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CMZ Mapping Process
• Evaluate historical & present conditions
1. Riverbank materials
2. Historical channels
3. Lateral channel migration
4. Avulsion pathways
5. Barriers to channel migration
• Fieldwork, GIS analyses
• Identify CMZ components
Blockage by landslides not mapped
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Natural Riverbank Materials
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Alluvium
Rock Avalanche Deposit
Bedrock
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Historical Migration Zone - RM 7.1 to 7.5
The Historical Migration Zone (HMZ) is the composite of all
historical channels based on historical aerial photos
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Lateral Migration
RM 9.2 to 10.3
Lateral migration
rate=distance/time
220 ft. / 57yrs = 3.8
ft/yr
230 ft. / 57yrs = 4
ft/yr 290 ft. / 57yrs = 5.1 ft/yr
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Avulsion Pathways
RM 10 between Grotto and Baring
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Avulsion Pathways
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Flow Depth During 10 Year Flood (42,300 cfs)
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Barriers to Channel
Migration
BNSF
Town of Skykomish Revetment
US Highway 2
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Not Barriers 24
USFS Money Creek Campground
Revetment in Baring
Revetment near Beckler
River Confluence
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Development of Draft South
Fork Skykomish Channel
Migration Zone Map
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RM 13.1 to 14.2 - 2013
S. Fk. Skykomish R.
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Historic Channels – 1956 to 2013
S. Fk. Skykomish R.
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Hazard Migration Zone (HMZ)
S. Fk. Skykomish R.
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Erosion Setbacks from HMZ
S. Fk. Skykomish R.
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Avulsion Pathways
S. Fk. Skykomish R.
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Erosion Setbacks from Avulsion Pathways
S. Fk. Skykomish R.
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Unconstrained Channel Migration Zone
S. Fk. Skykomish R.
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Draft South Fk. Skykomish Channel Migration Zone Map
S. Fk. Skykomish R.
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DNRP = KC Department of Natural Resources and Parks
DPER = KC Department of Permitting and Environmental Review
Next Steps:
SF Skykomish River CMZ Studies
Timeframe
Public meeting April 20, 2017
Public comment period ends
• Written comments to DNRP, John Bethel
May 31, 2017
Respond to comments and revise draft study and
map as necessary.
July 2017
Provide final study and map to DPER for adoption
via Public Rule process
Target - Early August
2017
Final study and map effective 30 days after
adoption (signature by DPER department director)
Target – September 1,
2017
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Questions?
John Bethel, Geologist
206-477-4645
www.kingcounty.gov/rivers
Steve Bottheim, Geologist
206-477-0375
www.kingcounty.gov/permitting