HomeMy WebLinkAboutNSD_DRC_BrinnonFloodErosionRisk_030425 (2)Brinnon Flood and Channel
Migration Risk
Dosewallips River, River Miles 0-1.2
1DRC | March 4, 2025
Lower Dosewallips River Assessment
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 2
3
Elevations are relative to water surface during
lidar flight (October 2023)6
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 2
5
Previous Studies - Areas
of Channel Migration Risk
1) Modified from US
BOR study (2004)
2) “High” and
“Moderate”
indicate risk of
channel migration
3) DMA =
Disconnected
Migration Area
5
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.
25
Flood Risk – Areas of Heightened Risk
Jefferson County | January 31st, 2025 2
1) Overflow channel
north of fire station
2) Low Area near
Community Center
3) Berm area (RM 0.4-
0.7)
4) Low Area between
State Park
Campground and US
101.
3
2
1
4
7
Culverts/Bridges
7
Culverts/Bridges
11) Overflow channel north
of fire station
2) Low Area near
Community Center
3) Berm area (RM 0.4-0.6)
4) Low Area between State
Park Campground and US
101.
10
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
11
21) Overflow channel north
of fire station
2) Low Area near
Community Center
3) Berm area (RM 0.4-0.6)
4) Low Area between State
Park Campground and US
101.
13
1
2
1
2
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
15
2- Year: Minor
inundation, influenced
by sea level and high
tide.
Crossing 1
(Looking Upstream)
Crossing 2
(Looking Upstream)
Both Crossings 1 and
2 are connected tidal
channels
1
2
Source: Google Maps 16
3
1) Overflow channel north
of fire station
2) Low Area near
Community Center
3) Berm Area (RM 0.4-0.7)
4) Low Area between State
Park Campground and US
101.
17
200ft
Extent of
berm from
the design
drawing
Additional
berm built at a
different time
Small segment with
different shape than
design, seems intact
N
Halfway House
Fire Station
Washington State Department of Waterways - 1957
19
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
24
Breach on eastern end
of berm
Berm
Flooding behind length of berm.
Berm is overtopped and
inundated from upstreamBerm
Flood risk during 2-year
and 100-year events
21
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
23
4
1)Overflow channel north
of fire station
2) Low Area near
Community Center
3) Berm area (RM 0.4-0.6)
4) Low Area between
State Park Campground
and US 101.
25
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
6
Flood risk during 2-
year and 100-year
events
26
Climate Change Projections
Impacts from climate change:
1) Sea level rise – ESA (2023)
2) 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.
30Jefferson County | February 26, 2025
2040
2060
2100
28
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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.
30DRC | March 4, 2025