HomeMy WebLinkAboutNSD_DRC_BrinnonFloodErosionRisk_030425Brinnon Flood and Channel Migration Risk
Dosewallips River, River Miles 0-1.2
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DRC | March 4, 2025
Lower Dosewallips River Assessment
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Today’s Discussion Topics
Identify and evaluate flood risk to Brinnon and adjacent public and private infrastructure
Evaluate channel migration and avulsion risk
Climate change projections – how will flood risk change with climate change?
Future Discussion: Findings related to aquatic habitat conditions and recommended actions to restore habitats critical to chinook and chum salmon populations.
DRC | March 4, 2025
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Elevations are relative to water surface during lidar flight (October 2023)
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Channel Migration and Erosion Risk
Evaluated channel migration using aerial photos from 1938 through 2023 – post US 101 construction.
Findings:
Channel is generally stable upstream of RM 0.9
Dynamic sub-reach upstream of the US-101 bridge
History of channel avulsion and migration downstream of the US-101 bridge
DRC | March 4, 2025
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Previous Studies - Areas of Channel Migration Risk
Modified from US BOR study (2004)
“High” and “Moderate” indicate risk of channel migration
DMA = Disconnected Migration Area
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WDNR-Mapped Deep Seated Landslide
Risk of future slope failure increases with channel migration and hillslope toe erosion
Landslide would likely cause immediate change to channel alignment and would increase flood and erosion risk towards Brinnon.
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Tim – landslide risk potential?
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Flood Risk – Areas of Heightened Risk
Jefferson County | January 31st, 2025
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Overflow channel north of fire station
Low Area near Community Center
Berm area (RM 0.4-0.7)
Low Area between State Park Campground and US 101.
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Culverts/Bridges
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Culverts/Bridges
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Overflow channel north of fire station
Low Area near Community Center
Berm area (RM 0.4-0.6)
Low Area between State Park Campground and US 101.
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Minor inundation, from high tide
Activated at 100-year flood
Impoundment of water near Schoolhouse Road and at US-101. No crossing documented at Schoolhouse Road.
Tidal/sea level influence downstream of Schoolhouse Road.
136 cfs diverted from mainstem into overflow channel
Not activated during common flood levels
Flood risk during 2-year and 100-year events
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Flow is diverted from mainstem into overflow channel at 100-yr flow, dry during 2-year flow.
Flow during 100-year flood is contributing to impoundment near Schoolhouse Road during flood events
May be contributing to impoundment at US-101
Relatively low avulsion risk
Inlet is on inside of a bend
Inlet is 5-7’ above low flow water surface elevation
Mature mixed forest on floodplain between mainstem and channel inlet.
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Overflow channel north of fire station
Low Area near Community Center
Berm area (RM 0.4-0.6)
Low Area between State Park Campground and US 101.
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100-Year: Impoundment of water along US-101, water comes from a combination of flood flows from the west and tidal inundation..
Flow through US-101 crossings
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2- Year: Minor inundation, influenced by sea level and high tide.
Tidal inundation from Dabob Bay during MHHW
Significant flooding and impoundment during 100-year event
Source of impounded water is a mix of water sources from Dabob Bay and from localized runoff
Two Box culverts drain the impoundment behind US-101
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Crossing 1
(Looking Upstream)
Crossing 2
(Looking Upstream)
Both Crossings 1 and 2 are connected tidal channels
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Source: Google Maps
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Overflow channel north of fire station
Low Area near Community Center
Berm Area (RM 0.4-0.7)
Low Area between State Park Campground and US 101.
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200ft
flow
flow
Extent of berm from the design drawing
Additional berm built at a different time
Small segment with different shape than design, seems intact
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Halfway House
Fire Station
Washington State Department of Waterways - 1957
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Berm was likely constructed to prevent channel migration/bank erosion towards the town of Brinnon, rather than flood prevention
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Mid-berm (RM 0.6)
Upstream end of berm (RM 0.7)
-Berm riprap face is intact
-Areas of slumping where flood flows first overtop
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Breach on eastern end of berm
Berm
Flooding behind length of berm. Berm is overtopped and inundated from upstream
Berm
Flood risk during 2-year and 100-year events
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Flooding is present but limited behind berm at 2-year event
Limited/no function for flood protection at 100-year event
Serving channel migration mitigation as designed.
Potential for channel migration/flanking upstream of berm.
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Berm
Berm is serving purpose of mitigating channel migration towards town and US-101 north of the bridge.
Potential channel migration risk upstream of berm extent into old channel scar.
Berm
Potential Channel Migration Risk
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Overflow channel north of fire station
Low Area near Community Center
Berm area (RM 0.4-0.6)
Low Area between State Park Campground and US 101.
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Some flooding during 2-year event, and significant flooding and impoundment of water during 100-year event.
One box culvert (Crossing 6) at southern end of campground
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Flood risk during 2-year and 100-year events
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Some flooding from runoff during 2-year event
Significant flooding and impoundment during 100-year event
Source of impounded water comes from a mix of river flow and tidal surge.
Examine with proposed culvert improvement project near park entrance.
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Climate Change Projections
Impacts from climate change:
Sea level rise – ESA (2023)
Increased magnitude and frequency of peak flow events
Resulting influence on flood risk:
Higher flood depths and slightly larger extents, which have a higher magnitude closer to where sea level rise has an influence.
The majority of existing buildings and the main transportation route in and out of town (US-101) are likely to be impacted by sea level rise over the next century.
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Jefferson County | February 26, 2025
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2060
2100
Figure projecting sea level rise at 100 year storm event; remember that these are estimates and that there is a range of probability
Increased magnitude of peak flow events
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The majority of existing buildings and the main transportation route in and out of town (US-101) are likely to be impacted by sea level rise over the next century.
2.5’ increase at the community center
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Overview of Findings
Areas of Flood Risk
Overflow channel north of fire station
Impoundment behind Highway 101 at north end of town/around Community Center
River left berm upstream of HWY 101 bridge
Impoundment between campground and HWY 101 road prism
Area of Channel Migration Risk
Low risk of channel avulsion at upstream end of flood channels
Moderate risk of erosion and flanking at the upstream end of the berm
Landslide potential raises risk of significant channel change
Climate Change Projections – how will flooding change with climate change?
Sea level rise will increase flood depths impacting Brinnon infrastructure and US-101.
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DRC | March 4, 2025
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