HomeMy WebLinkAboutNSD_JeffCo_Dosewallips_Powerlines_Preliminary_BOD_111424
Tami Pokorny
Natural Resources Coordinator
Jefferson County Public Health Department
615 Sheridan Street Port Townsend, WA 98368
1900 N. Northlake Way, Suite 211
Seattle, WA 98103
Dosewallips River – Powerlines Reach Restoration
Preliminary Design
Basis of Design Report
November 2024
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TABLE OF CONTENTS
1 Introduction ....................................................................................................................................................... 1
2 Project Setting .................................................................................................................................................... 1
3 Goals and Objectives .......................................................................................................................................... 4
4 Existing Conditions ............................................................................................................................................. 4
4.1 Geology .................................................................................................................................................... 5
4.2 Channel and Floodplain Morphology ....................................................................................................... 6
4.3 Hydrology and Hydraulics ........................................................................................................................ 9
4.3.1 Hydrology .................................................................................................................................. 10
4.3.2 Hydraulic Modeling ................................................................................................................... 12
4.4 Sediment Transport Dynamics and Channel Migration ......................................................................... 16
4.5 Instream Wood and Habitat Elements................................................................................................... 20
4.5.1 Large Wood ............................................................................................................................... 20
4.5.2 Riparian and Wetland Communities ......................................................................................... 22
4.5.3 Aquatic Habitat Conditions, Salmonid Use, and Periodicity ..................................................... 25
4.5.4 Climate Change ......................................................................................................................... 27
5 Design Overview .............................................................................................................................................. 28
5.1 Preliminary Design ................................................................................................................................. 29
5.1.1 Project Metrics .......................................................................................................................... 29
5.2 Riparian Restoration .............................................................................................................................. 29
5.2.1 Riparian Planting Associations .................................................................................................. 31
5.3 Engineered Log Jams and Large Wood Placement ................................................................................ 32
5.3.1 Instream Large and Small Apex Engineered Log Jams .............................................................. 33
5.3.2 Bank-Oriented small Apex Engineered Log Jams ...................................................................... 33
5.3.3 Low Profile Engineered Logjams ............................................................................................... 33
5.3.4 Floodplain Roughness Engineered Logjams .............................................................................. 33
5.3.5 Stability and Risk Based Design ................................................................................................. 34
5.4 Site Constraints ...................................................................................................................................... 34
5.4.1 Construction Access .................................................................................................................. 34
5.4.2 Base Flood Elevation, No-Rise, and Floodplain Permitting ....................................................... 37
5.4.3 Land Ownership and Land Use ................................................................................................. 38
5.5 Changes from Conceptual to Preliminary Design .................................................................................. 38
5.6 Proposed Conditions Hydraulic Modeling ............................................................................................. 39
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5.6.1 Hydraulic Model Setup.............................................................................................................. 39
5.6.2 Hydraulic Results ....................................................................................................................... 41
5.7 Construction Cost Estimate .................................................................................................................... 43
6 Preliminary No Rise Assessment ...................................................................................................................... 44
7 References ....................................................................................................................................................... 46
LIST OF TABLES
Table 1. Estimated Discharge Values. ...................................................................................................................... 11
Table 2. Estimated Change and Magnitude of Future Flows with Projected Climate Change Impacts for 2070-
2099 ...................................................................................................................................................... 12
Table 3. Roughness Values. ...................................................................................................................................... 13
Table 4. The magnitude of future peak flows 2070 - 2099 projected as result of warming climate....................... 28
Table 5. Dosewallips River Powerlines Reach Restoration Metrics ......................................................................... 29
Table 6. Risk based design criteria for the Dosewallips River – Powerlines Reach. ................................................ 34
Table 7. Modeled average existing, proposed, and change in water surface elevation throughout the Powerlines
reach. .................................................................................................................................................... 46
LIST OF FIGURES
Figure 1. Project Vicinity Map .................................................................................................................................... 2
Figure 2. The Lazy C and Powerlines reaches of the Dosewallips River..................................................................... 3
Figure 3. Property boundaries and landowners in the project reach. ....................................................................... 3
Figure 4. Geologic map of the Lazy C and Powerlines reaches (Polenz et al. 2014; WDNR 2021) ............................ 6
Figure 5. Braided channel morphology within the Powerlines reach near RM 1.8 looking downstream (10/9/20). 7
Figure 6. Relative elevation map of the Powerlines reach showing topography and mapped side channels and
small tributaries. ..................................................................................................................................... 8
Figure 7. Main channel avulsion path along the former side channel 1 at RM 2.2 on 01/24/24. ............................. 8
Figure 8. Abandoned main channel between RM 2.1 and RM 1.8 looking upstream from river left (01/24/24). .... 9
Figure 9. Side Channel 3 looking upstream between a gravel bar island and the northern floodplain (09/13/23).. 9
Figure 10. Drainage Areas Utilized for Hydrologic Analysis and Hydraulic Model. ................................................. 11
Figure 12. Inundation extents and depth at the spawning and outmigration flows ............................................... 14
Figure 13. Inundation extents and depth at the 2-year flow ................................................................................... 15
Figure 14. Inundation extents and depth at the 100-year flow event ..................................................................... 16
Figure 15. Newly formed gravel bar near RM 1.6. ................................................................................................... 17
Figure 16. History of channel migration within the Lazy C and Powerlines Reaches between 1939 and 2019 (NSD
2021). .................................................................................................................................................... 18
Figure 17. Continued sediment deposition in the Powerlines Reach. ..................................................................... 19
Figure 18. Stable large wood jams and large wood pieces within the Lazy C and Powerlines Reaches as mapped
on 10/9/20. ........................................................................................................................................... 21
Figure 19. Large wood accumulation on a gravel bar downstream of the channel avulsion (01/24/24). .............. 22
Figure 20. Canopy height map for the Powerlines Reach (2018 LiDAR) .................................................................. 23
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Figure 21. Typical riparian conditions in the Powerlines Reach with mixed forest on higher terraces and young
deciduous forest on gravel bars and lower terraces (10/9/20). .......................................................... 23
Figure 22. Typical red alder with conifer understory within the Powerlines Reach higher floodplain (09/13/23). 24
Figure 23. Delineated wetlands and waters within the Powerlines Reach study area (NSD 2024). ....................... 25
Figure 24. Periodicity for selected salmonids in the lower Dosewallips River (NSD 2021). .................................... 27
Figure 25. Example of future conditions in a riverine floodplain with a functional large wood cycle recruiting
large conifers as stable hard points...................................................................................................... 30
Figure 26. Access route from south via state parks and transmission line easement to Train Wreck Creek (NSD
2022). .................................................................................................................................................... 36
Figure 27. Snapshot of the effective FIRM, displayed in the FEMA National Flood Hazard Layer (NFHL) online map
viewer on 10/22/2024. ......................................................................................................................... 37
Figure 28. Conceptual Design Layout (NSD 2021) ................................................................................................... 38
Figure 29. Example of surface modifications used to represent apex ELJs in proposed hydraulic model. ............. 40
Figure 30. Example of elevated roughness areas to represent proposed ELJs in hydraulic modeling. ................... 41
Figure 31. Velocity patterns through the proposed ELJ array at the 100-year flow ................................................ 42
Figure 32. Comparison of change in modeled 100-year WSE within the project reach. FEMA XS locations shown
in purple. .............................................................................................................................................. 45
LIST OF APPENDICES
Appendix A Hydraulic Model Existing and Proposed Conditions
Appendix B Preliminary Design Plan Set
Appendix C Construction Cost Estimate
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1 INTRODUCTION
Jefferson County (County) is implementing a component of the Dosewallips River Lazy C/Powerlines Resiliency
Plan (NSD 2021) which developed a reach-scale restoration plan for projects within the lower Dosewallips River
from River Mile (RM) 1.0 to RM 3.3 located near Brinnon, WA. Specifically, the County is implementing the
Dosewallips River Powerlines Reach Restoration Project (project) on County, Washington State Department of
Natural Resources (WDNR) and private land located between RM 1.0 and 2.0. The lower Dosewallips River
supports Puget Sound Chinook (Oncorhynchus tshawytscha) and Hood Canal summer-run chum (Oncorhynchus
keta), both listed under the Endangered Species Act as federally Threatened species. The Resiliency Plan
provided an assessment of physical and biologic conditions and began to describe actions that can help preserve
and restore important ecological habitat to support these listed salmonids as well as assist landowners exposed
to flood and erosion hazards.
The Resiliency Plan identified and prioritized actions within the Lazy C and Powerlines reaches that could best
protect and restore riverine processes and in-channel, side channel, and floodplain-associated habitats that
support the recovery of listed salmonids, foster climate change resiliency, and are sensitive to local land uses
and downstream shellfish habitat. This analysis identified actions such as riparian planting, the placement of
engineered log jams (ELJ) and large wood, and the enhancement of side channel and floodplain habitats through
increased flow connectivity as priority actions. Implementation of restoration actions in the Powerlines reach
was recommended as the first phase of river restoration. Based on these recommendations, Jefferson County
pursued and obtained a Washington Salmon Recovery Fund Board (SRFB) grant (#21-1024) in 2021 to complete
preliminary designs for the Powerlines reach.
This report describes the basis of design for the restorative actions proposed for the Powerlines Reach
Restoration Project (RM 1.0 to 2.0). The project proposes to construct ELJs within the river channel and
implement riparian restoration actions to restore geomorphic, hydraulic, and habitat forming processes. This
design process follows the SRFB Manual 18 guidelines for preliminary design including a summary of changes
from the previously completed conceptual design (NSD 2021), goals and objectives, existing conditions, and the
proposed restoration actions and intended effects.
2 PROJECT SETTING
The Dosewallips River flows eastward from the central Olympic Mountains through steep forestlands into Hood
Canal just north of Brinnon, Washington (Figure 1). The basin’s headwaters are situated within Olympic National
Park and Olympic National Forest and the remainder of the basin is owned by the State of Washington
Department of Natural Resources and State Parks, private timber companies, and private landowners. The
tidelands are owned by WA State Parks and private landowners, and several areas are also leased for
commercial shellfish production. The river has a watershed area of 116 square miles (mi2) and a relief of 7,770
feet spanning from the peaks of the Olympic Crest to Hood Canal. The watershed receives an average annual
precipitation of 77.6 inches which falls as both rain and snow primarily during the fall and winter months
(October - February). The watershed’s high relief and precipitation produce a high sediment load (Labbe et al.
2005). Most of the basin is forested, with 63% of the area covered by tree canopy (streamstats.usgs.gov). Within
the Dosewallips watershed, there are reaches with high quality habitat interspersed with reaches that have
been heavily impacted by human use. Supporting factors for salmon habitat in the watershed include that much
of the land is publicly owned with Olympic National Park and Olympic National Forest owning 93% of the
watershed, with much of that in protected status (Shared Strategy 2007).
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Figure 1. Project Vicinity Map
The full reach analyzed in the Resiliency Plan is located between river miles (RM) 1.0 to 3.3 along the mainstem
of the Dosewallips River directly downstream from the Rocky Brook confluence and Dosewallips River canyon
(Figure 2). The Resiliency Plan divided this area into two sub-reaches based on geomorphic characteristics (such
as morphology and channel migration history) and current development- the Lazy C Reach (RM 2.0-3.3) and the
Powerlines Reach (RM 1.0-2.0). Termed the ‘Lazy C’ and ‘Powerlines’ sub-reaches in previous assessments (NSD
2021), infrastructure in the reach includes the Lazy C development as well as other private parcels at the
upstream end from RM 2.0 to RM 3.3 and a Bonneville Power Administration (BPA) powerline corridor which
crosses the river between RM 1.1 and 1.2 near the downstream end. The Powerlines Reach (project reach)
encompasses publicly owned parcels (Jefferson County and Washington State Parks) and privately owned
parcels (Figure 3).
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Figure 2. The Lazy C and Powerlines reaches of the Dosewallips River.
Figure 3. Property boundaries and landowners in the project reach.
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3 GOALS AND OBJECTIVES
The County and the Dosewallips River Collaborative worked with the NSD team to establish goals for the
Resiliency Plan project (NSD 2021). That effort informed the selection of the Powerlines Reach for restoration
design and the development of specific goals and objectives related to the restoration of geomorphic processes,
and aquatic habitats.
The overarching goal of the project is to improve habitat for Hood Canal Summer Chum and Puget Sound
Chinook salmon by enhancing stream channel and floodplain features, conditions, and processes. This includes
1) Increasing spawning capacity through the sorting and building of spawning-suitable gravels, 2) Increasing
adult holding habitat through the creation and maintenance of deep pools, 3) Improving juvenile salmonid
rearing habitats in floodplain and off-channel areas, and 4) Improving the form and function of the riparian
community through the treatment of invasive plant species and the interplanting of conifers. To achieve these
overarching project goals related to salmonid habitat the project proposes the following specific goals and
objectives:
Goal: Reduce channel migration rates and local bank erosion and improve aquatic edge habitat including
riparian health.
Objective: Install an array of large woody material to stabilize existing gravel bars and plant with
riparian species.
Objective: Install large woody material to slow bank migration in areas of rapid channel migration to
reduce sediment recruitment and allow establishment of mature riparian vegetation.
Objective: Install large woody material near existing wood jams to stabilize existing wood.
Objective: Install large woody material in existing side channels to increase hydraulic roughness and
reduce the risk of future channel avulsion through the side channels.
Goal: Increase adult salmon holding habitat and increase gravel sorting
Install large woody material to scour and maintain deep pool habitat.
Install large woody material to sort and retain a wide range of gravels.
Goal: Increase juvenile salmonid rearing habitat
Objective: Install large woody material to develop scour pools and improve salmonid habitat cover
Objective: Increase off-channel habitat access to provide velocity refugia for juveniles
Goal: Increase seasonal inflow and fish access to off-channel juvenile salmonid rearing habitat (side
channels and sloughs)
Objective: Install large woody material to increase flows into side channel habitats
Goal: Restore riparian habitats
Objective: Increase shading and habitat in the riparian corridor by planting native vegetation and
controlling invasive species
Goal: Ensure the continued protection of existing infrastructure while improving aquatic habitat
function.
Objective: Ensure that the project elements do not create increased rise on homes and properties in
the Lazy C.
4 EXISTING CONDITIONS
As shown in Figure 2, the Powerlines Reach is located between the upstream extent of the Brinnon Flats where a
volcanic outcrop naturally constricts the channel (RM 1.0), and the Lazy C residential development where there
is another bedrock exposure. Although the Powerlines Reach is largely protected open space, unconstrained and
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relatively intact compared to more developed reaches (Labbe et al. 2005), the quality of habitat there still
suffers from a long history of road building, logging, road failures including FSR 2610, and the operation for
nearly a decade of a major splash dam upstream at RM 3.9. These activities and events have contributed to the
loss of mature forests and large conifer in-channel wood, scouring and simplification of the river channel,
destabilization and redistribution of sediment including locations with very high aggradation, and a lack of pools,
vegetated islands, and habitat structure and complexity generally.
Impacts to summer chum salmon include reduction in the extent, quality, complexity and stability of suitable
holding, spawning and egg incubation habitat; physiological and predation stress due to lack of thermal refugia
and cover; and simplified life histories leading to lost opportunities to adapt to climate change and other
emerging and intensifying environmental changes. The identified solution provided in the Lastelle 2015 guidance
to climate change impacts is to "Maintain and promote aggressive approaches to salmon habitat restoration and
protection priorities that account for climate change.”
The Powerlines Reach, while not impaired by floodplain development, is affected by upstream factors as well as
impairments caused by a lack of large wood which has adversely affected riparian and wetland habitat
formation and puts the existing aquatic habitats supportive of anadromous salmonid use at risk. The following
impairments are present (NSD 2021):
Channel and floodplain formation, connectivity, and sediment transport processes.
Upstream channel confinement (e.g., Upstream Lazy C sub-reach, Wolcott Flats etc.) increased
sediment supply to this sub-reach.
Aggradation coupled with historical forest clearing has transformed the sub-reach from an
anabranching forested island morphology to a braided wide channel.
Frequent bed mobilization and sediment deposition in braided sections negatively impacts
salmon redds.
Channel migration processes are impaired.
Historical forest clearing and low levels of stable large wood have increased channel migration
rates above likely historical levels.
High channel migration rates increase risk of avulsion through side channels which will decrease
the amount of
Large wood recruitment, riparian, wetland and aquatic habitat formation if impaired.
Historical forest clearing and instream wood removal have reduced levels of stable large wood
in the main channel and side channels.
Channel migration rates greater than historical levels are limiting ability of riparian vegetation to
mature.
Increased sedimentation due to upstream channel confinement is encouraging the formation of
unstable morphologies (braided channels) which negatively impact salmonid habitat.
4.1 Geology
The geologic and glacial history of the area greatly influence the conditions in the Powerlines reach. The
Dosewallips watershed lies on the eastern slope of the Olympic Mountains which are composed of an
“accretionary wedge” of uplifted and folded oceanic crust that have formed (and are continuing to be formed)
by subduction of the Juan de Fuca plate under the North American plate. The upper portion of the watershed is
composed of slightly metamorphosed and highly erodible marine sedimentary lithologies deposited during the
uplift processes. Because of their highly erodible nature, these marine sedimentary materials provide the
Dosewallips River with a high sediment load (Labbe et al. 2005). The middle and lower portions of the watershed
are composed of less erodible Crescent formation basalts (Figure 4)
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Figure 4. Geologic map of the Lazy C and Powerlines reaches (Polenz et al. 2014; WDNR 2021)
Alpine glaciers likely extended from the Olympic crest downstream to at least the Rocky Brook confluence just
upstream of the project area (Polenz, 2021; Hellwig, 2010). While these glaciers may not have directly
influenced the geology and topography of the study site, the site was likely influenced heavily by continental
glaciation during the last glacial maxima. The Cordilleran ice sheet extended into Puget Sound around 15,000
years ago, covering the region in a thick layer of ice extended into the project site, covering the site in roughly
300-600 feet of ice, and carving the U-shaped valley seen today. The ice sheet likely dammed the river, forming
a glacial lake in the valley upstream. Deposits from these processes are seen in the geologic record today, with
the hillslopes surrounding the site overlain with glacial outwash and glacial lacustrine deposits – both of which
are highly erodible which makes them susceptible to mass-wasting processes such as landsliding. These deposits
are overlying Crescent formation basalt, which extrudes beyond the deposits in several areas of the project
reach –at the downstream end of the Powerlines reach where they form a bedrock constriction at RM 1.0.
4.2 Channel and Floodplain Morphology
The Powerlines reach exhibits a complex, multi-threaded channel morphology and well-connected floodplain.
The channel has expressed a full meander sequence and spans the entirety of the valley bottom from hillslope
to hillslope. The main channel exhibits a primarily meandering pool-riffle channel morphology with deep pools
at the outside of meander bends separated by steeper riffles.
There are however, two areas within the reach that exhibit braided channel morphology, one between RM 1.8
and 1.9 and the other at the downstream end between RM 1.1 and 1.2 (Figure 5). These segments contain
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broad, poorly sorted gravel bars intersected by a series of unstable braid channels. These areas provide poor
quality spawning habitat due to the unstable nature and frequent bed mobilization of the braided morphology.
The downstream braided segment also exhibits a degree of over-widening, likely due to the lack of mature
vegetation underneath the BPA powerline corridor. A bedrock constriction defines the downstream end of the
sub-reach.
Figure 5. Braided channel morphology within the Powerlines reach near RM 1.8 looking downstream
(10/9/20).
There were four side channels mapped within the reach in 2020 (NSD 2021). Since then, side channel one (SC-1),
which formerly split off from the river across from the Lazy C development near RM 2.2, was captured by an
avulsion of the main channel between 2022 and 2023 and now conveys the primary flow of the Dosewallips
River (Figure 7). The historical main channel from RM 2.1 to RM 1.8 has become a backwater area and currently
maintains perennial slow water aquatic habitat (Figure 8). Consequently, SC-1 is no longer a side channel and is
not included in the discussion of side channel conditions or actions in this report but is shown in Figure 6 below
for reference.
Side channel two (SC-2) begins near the outside of a meander bend near RM 1.8, contains a mixture of pool-
riffle and plane-bed morphology, and is fed by a small seasonal tributary channel. SC-2 exhibits signs of channel
modifications activities such as straightening and wood removal. Side channel three (SC-3) begins near RM 1.7
and is wider than the other three side channels, likely because it is the most recent to have formed. The side
channel and experiences higher energy flood flows due to its short channel length (Figure 9). All three side
channels were connected during the field visit. Side channel four (SC-4) begins near the outside of a meander
bend near RM 1.45 and did not have surface flows during the October 2020 field visit. The side channels are
surrounded by diverse floodplain landforms that have formed in the lee of migrating meanders. The floodplain
ranges from 1 to 7 feet above the low flow channel throughout the Powerlines Reach.
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Figure 6. Relative elevation map of the Powerlines reach showing topography and mapped side channels and
small tributaries.
Figure 7. Main channel avulsion path along the former side channel 1 at RM 2.2 on 01/24/24.
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Figure 8. Abandoned main channel between RM 2.1 and RM 1.8 looking upstream from river left (01/24/24).
Figure 9. Side Channel 3 looking upstream between a gravel bar island and the northern floodplain (09/13/23)
4.3 Hydrology and Hydraulics
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4.3.1 Hydrology
To model relevant hydraulic conditions in the project reach, the NSD team developed discharge estimates for
five flow scenarios: two that are relevant to salmonid life history stages and three peak flow scenarios: the 2-
year, 10-year, and 100-year floods (Table 1). Flow estimates were developed based on the historical discharge
record available from the now inactive USGS gage that was located on the Dosewallips River 5.8 miles upstream
of the project site (USGS gage #12053000). Accurately representing the magnitude of peak flows within the
project reach is challenging because the Dosewallips River gage operated for a relatively short period from 1930-
1951, and there is no other available gage in the basin. The active USGS gage on the neighboring Duckabush
River (USGS gage #120454000), which has been operating from 1938 to present, was considered as an alternate
source of peak flow data, as the Duckabush River watershed is located immediately to the south of the
Dosewallips River watershed and has similar characteristics for slope, relief, and precipitation. A Bulletin 17B
peak flow analysis of both the Dosewallips gage and the Duckabush gage data was performed and scaled to the
project area by drainage area as described in Mastin et al. (2016). However, since the drainage area ratio of the
Dosewallips project area to the Duckabush River gage area is greater than the recommended limit of 1.5, the
flow estimates from the shorter-duration Dosewallips River gage were chosen for use in hydraulic model. A flow
duration analysis was computed for the Dosewallips gage to estimate low flows relevant to salmonid life stages.
Median September exceedance flow was selected to represent typical flows for Hood Canal Summer Chum
spawning, and median daily February exceedance flow was selected to represent typical flows for Hood Canal
Summer Chum outmigration.
The hydraulic model includes an inflow for the mainstem Dosewallips River, an inflow for Rocky Brook, and an
intermediate inflow to account for flow accrued from the multiple small, unnamed tributaries within the model
domain. The drainage basin areas for the project reach and the inflows for Rocky Brook and the unnamed
tributaries were delineated using the USGS Streamstats tool (USGS 2019; Mastin et al., 2016) and are displayed
in Figure 10. The drainage area for the Dosewallips River is approximately 101 square miles while the drainage
area for Rocky Brook is approximately 9 square miles. Scaled flows for two nearby, unnamed tributaries, located
on the right and left banks of the Dosewallips River just downstream of Rocky Brook, were added to the Rocky
Brook inflow due to their relatively low magnitude and proximity to Rocky Brook. The total drainage area
included at the Rocky Brook inflow location is approximately 10.6 square miles. The drainage area associated
with the intermediate inflow location, added to the model near RM 2.8, is approximately 4.4 square miles.
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Figure 10. Drainage Areas Utilized for Hydrologic Analysis and Hydraulic Model.
Smaller tributaries were not explicitly included as separate hydrologic inputs in the hydrologic analysis and
hydraulic model but were combined into the upper Dosewallips River inflow, due to their relatively low flow
contributions at the scale of the hydraulic model. Consequently, while we understand that the local effects of
these tributaries can be significant, leading to ponding and flooding on adjacent floodplain, these impacts will
not be discernible in modeling results.
Discharge values used for current conditions hydraulic modeling and associated analyses are shown in Table 1.
The 2-, 10- and 100-year recurrence peak flows are also referred to as the 2-, 10- and 100-year floods.
Table 1. Estimated Discharge Values.
Recurrence Interval
Dosewallips River
Discharge Estimate
(cfs)
Rocky Brook
Discharge Estimate
(CFS)
INTERMEDIATE
DISCHARGE ESTIMATE
(CFS)
HCSC Spawning (median Sept exceedance) 168 18 7
HCSC Outmigration (median Feb exceedance) 327 34 14
2-year (50% probability in any given year) 4,689 412 192
10-year (10% probability in any given year) 8,776 783 352
100-year (1% probability in any given year) 14,469 1,279 589
HCSC = Hood Canal Summer Chum
Once peak flows were determined for the project site, it was necessary to estimate the impact that climate
change would have on these flows to understand and model future floods. The Columbia Basin Climate Change
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Scenarios (CBCCS) project summarizes climate change projections for many watersheds, the closest to the
Dosewallips being the Skokomish River (Hamlet 2010). For each basin, the CBCCS projects the 20-year, 50-year,
and 100-year recurrence interval floods into the future to estimate their magnitude in the years 2070-2099
based on one of two climate change scenarios. For this project the A1B climate change scenario was used, which
is the higher of the two scenarios and the closest to current climate change projections. In addition, monthly
mean model predictions for September and February were extracted from the model and compared to historic
conditions to predict change during the Hood Canal Summer Chum (HCSC) spawning and outmigration periods.
Data was downloaded from the CBCCS Project website (Hamlet 2010). The percent change in the Skokomish
River floods from 2020 to 2070-2099 as estimated by the CBCCS was determined and then applied as a
multiplier to the historical estimates of the Dosewallips floods to estimate climate change flows (Table 2Table 2.
Estimated Change and Magnitude of Future Flows with Projected Climate Change Impacts for 2070-2099). Note
that the lowest flood flow for which the CBCCS makes estimates is the 20-year flood; therefore, the multiplier
applied to the Dosewallips 2-year and 10-year floods is the CBCCS-estimated 20-year flood multiplier.
Table 2. Estimated Change and Magnitude of Future Flows with Projected Climate Change Impacts for 2070-
2099
FLOW PROJECTED
CHANGE (%)
FUTURE (2070-2099)
DOSEWALLIPS RIVER
DISCHARGE
ESTIMATE (CFS)
FUTURE (2070-2099)
ROCKY BROOK
DISCHARGE
ESTIMATE (CFS)
FUTURE (2070-2099)
INTERMEDIATE
DISCHARGE ESTIMATE
(CFS)
HCSC Spawning -30 118 12 5
HCSC Outmigration 10 360 38 15
2-year 18 5,533 487 226
10-year 18 10,355 924 416
100-year 23 17,797 1,573 724
HCSC = Hood Canal Summer Chum
4.3.2 Hydraulic Modeling
A two-dimensional (2D) hydraulic model of the Dosewallips River was developed using the U.S. Army Corps of
Engineers (USACE) Hydrologic Engineering Center modeling platform, River Analysis System (HEC-RAS), version
6.4.1 (USACE, 2023). The model was developed to support characterization of existing reach conditions and to
model proposed project actions, with model outputs including flow depth, velocity, and water surface elevation.
The hydraulic model used for this analysis is a subsection of a larger model which extends from Dosewallips RM
6.7, at the USFS bridge, to the river mouth at Hood Canal. The Powerlines reach analysed here extends from RM
1.0 to RM 2.0.
The underlying terrain for the hydraulic model was developed using 2023 bathymetric bare earth LiDAR
collected and processed by NV5 (NV5 2024). There are no known culverts or crossings in the Powerlines reach.
The hydraulic model mesh was created with a finer mesh spacing in main channels and presumed flow paths
and a coarser mesh spacing in less topographically complex areas, Figure 11.
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Figure 11. Hydraulic mesh showing a portion of the model area.
Hydraulic resistance is characterized in the model by polygons representing differing surface roughness types,
with Manning’s n roughness coefficients assigned using Chow (1959) and engineering judgment. Roughness
polygons for this model were delineated using 2023 LiDAR data and 2023 NAIP imagery. The Manning’s n values
for each land cover category are shown in Table 3.
Table 3. Roughness Values.
CATEGORY MANNING'S N
Forested floodplain 0.08
Grass floodplain 0.03
Grazed floodplain 0.03
Compacted surface 0.025
Impervious surface 0.015
Dosewallips River main channel 0.04
Side channels 0.04
Gravel bar 0.045
Forested bar 0.08
Existing wood 0.15
Buildings 0.99
Engineered Logjam 0.15
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The existing conditions hydraulic model was run in a quasi-steady state for the 2-year, 10-year, and 100-year
peak flows and for the Hood Canal Summer Chum (HCSC) spawning (median September exceedance) and HCSC
outmigration (median February exceedance) flows. There are three inflow boundary conditions in the model:
the upper Dosewallips, Rocky Brook Creek, and an intermediate inflow.The downstream boundary condition for
the larger hydraulic model is a constant stage hydrograph based on the tidal datum at Hood Canal, located
sufficiently downstream of the Powerlines reach that it will not affect the results presented in this report.
Due to lack of available data for calibration, the hydraulic model is uncalibrated at all flows.
Results
Select existing conditions depth, velocity, and water surface results are included within Appendix A.
Modeled existing conditions for the spawning and outmigration flows are very similar. However, depths and
velocities are generally slightly greater at the higher outmigration flow. Spawning flow hydraulic results are
shown in Figure 1, and outmigration flow results are shown in Figure 2 of Appendix A. At both flows, most flow
is contained within the mainstem of the Dosewallips River, although side channel SC-3 and a flow path on the
scroll bar between RM 1.4 and RM 1.5 are activated under these lower flows. The main channel now flows in
what used to be SC-1 the meander cutoff near RM 1.8. In general, there is little to no floodplain activation at
these flows. Although there are a few localized pools within the reach, mainstem flow depths generally range
from 0.25 ft to 4.0 ft, while side channel depths are typically below 2.0 ft. At the spawning and outmigration
flows, mainstem velocities are generally low and range from 1 to 5 ft/s. Modeled velocities in SC-3 and the scroll
bar side channel are lower than in the mainstem – velocities in SC-3 are generally less than 1 ft/s at both
spawning and outmigration flows, while velocities in the scroll bar side channel are lower than 3 ft/s at the
spawning flow and lower than 4 ft/s at the outmigration flow.
Figure 12. Inundation extents and depth at the spawning and outmigration flows
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Floodplain inundation increases at the 2-year flow (shown in Figures 7-8 of Appendix A). All remaining side
channels (SC-1, SC-2, SC-3, and SC-4) and multiple flow paths within the scroll bars are activate at this flow. Flow
depths in the mainstem channel range from 3 to 8 ft, with some deeper pools up to 10 ft deep. Side channel
depths are typically lower and range from 2 to 8 ft, with the greatest side channel depths in SC-3. Mainstem flow
velocities typically range from 3 to 8 ft/s, with some isolated areas experiencing velocities of up to 9 ft/s.
Velocities within side channel and floodplain areas are typically lower, with typical side channel velocities
ranging from 1 to 4 ft/s. Velocities are greatest within SC-3, with some higher velocity patches of 9 to 10 ft/s at
channel bends.
Figure 13. Inundation extents and depth at the 2-year flow
At the 100-year flow (shown in Figures 9-10 of Appendix A), model results indicate significant floodplain flow
and inundation throughout the valley. All side channels and scroll bars are inundated at this flood flow, including
the left bank floodplain area between RM 1.2 and 1.3, which was not significantly wetted at lower modeled
flows. Depths and velocities are higher at this flow, with mainstem flow depths typically between 6 and 10 ft
with deeper pools up to 16 ft deep. Mainstem velocities are generally between 4 to 9 ft/s, with some areas
experiencing velocities greater than 10 ft/s. Mainstem velocities are highest between RM 1.55 and RM 1.7.
Modeled velocities along the eroding left bank on the Coone property between RM 1.4 and RM 1.5 are
moderately high and can reach up to 7 ft/s. Flow depths within side channels vary at the 100-year flow, but
typically range between 5 to 8 feet, with some deeper areas of flow up to 10 ft. Velocities within side channel
SC-3 are typically higher and range between 3 to 8 ft/s with some higher velocity areas of up to 10 to 12 ft/s.
Velocities in side channels SC-2 and SC-4 are lower and typically below 4 ft/s.
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Figure 14. Inundation extents and depth at the 100-year flow event
4.4 Sediment Transport Dynamics and Channel Migration
The Dosewallips Resiliency Plan (NSD 2021) identified impaired sediment transport and channel migration
regimes as factors which are affecting the formation and longevity of aquatic habitat in the Powerlines Reach.
Specifically:
Upstream channel confinement (e.g., Upstream Lazy C sub-reach, Wolcott Flats etc.) has increased
sediment supply to this reach, which is encouraging the formation of unstable morphologies
(braided channels) which negatively impact salmonid habitat.
Aggradation coupled with historical forest clearing has transformed the reach from an anabranching
forested island morphology to a braided wide channel.
Frequent bed mobilization and sediment deposition in braided sections is likely to negatively impact
salmon redds.
Historical forest clearing and low levels of stable large wood have increased channel migration rates
above likely historical levels.
High channel migration rates increase risk of avulsion through side channels which will decrease the
amount of in-channel and side-channel habitat.
The Powerlines reach has a lower sediment transport capacity than the Lazy C reaches and is acting as a
depositional reach for sediment mobilized from upstream and local sources (NSD 2021). The well-connected
floodplain, wide active channel corridor, and meandering pool-riffle morphology cause flows to spread out
across the valley during floods, limiting the amount of available force within the channel capable of mobilizing
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sediment. Furthermore, the backwater created by the downstream geologic constriction further reduces the
sediment transport capacity. This creates broad areas of the reach in which the applied shear stress is below the
threshold for transporting gravels during the 1-year and 10-year flow levels – causing sediment to deposit
throughout many areas of the reach. This is especially exhibited in the upstream braid bars between RM 1.8 to
1.9 and the downstream braid bars under the powerline corridor between RM 1.1 to 1.2, the latter of which is
directly related to the backwater effect of the geologic constriction. The extensive sediment deposition
throughout the reach is responsible for the high channel migration rates and subsequent development of the
well-connected floodplain and side channels, as well as the high frequency of bed mobilization, which can
negatively affect salmonid spawning habitat.
The depositional nature of the Powerlines reach and low capacity for coarse sediment to be transported through
the downstream geologic constriction will cause sediment to be continuously deposited within the reach.
Sedimentation in the Powerlines reach is driven by the backwater depositional zone created by the geologic
constriction at RM 1.0. Through time, sediment has continued to fill the valley throughout the Powerlines reach,
encouraging the active channel migration that has formed a new floodplain surface across the valley (Figure 16)
following episodes of historical channel incision. Sedimentation propagates upstream with time. Since channel
migration results from sedimentation within the river, channel migration also propagates upstream - towards
the Lazy C reach. This process is exhibited by the up-valley meander migration that has occurred since ~1950 as
the river progressed up-stream. As deposition in the Powerlines reach continues over time it will propagate
upstream towards the Lazy C Reach, raising the channel bed and water elevations along with triggering more
channel migration (Figure 17).
Figure 15. Newly formed gravel bar near RM 1.6.
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Figure 16. History of channel migration within the Lazy C and Powerlines Reaches between 1939 and 2019
(NSD 2021).
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Figure 17. Continued sediment deposition in the Powerlines Reach.
The Powerlines reach has experienced a much greater degree of channel migration since 1939 than the Lazy C
reaches with the current meander planform extending across the entirety of the valley. The current meander
belt formed between 1939-1951 when a previous meander in the middle of the reach cut off. Since 1951 the
channel has continued to progress outwards towards the valley walls and up-valley towards the Lazy C housing
development.
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As is common with the progression of a river meander belt, scroll-bar floodplain features have formed in the lee
of each meander as point-bars formed during erosional processes have become vegetated. Furthermore, the
abandoned paths of the main channel have formed into side channels with each of the four side channels being
in a former main-channel location. Thus, the active channel migration processes within the Powerlines reach are
responsible for the high diversity of channel and floodplain habitats currently present. Additionally, the
evaluation of historical channel migration patterns reveals the widening of the active channel corridor around
the BPA powerline corridor, especially compared to other narrower sections of the reach.
4.5 Instream Wood and Habitat Elements
The Resiliency Plan documented instream and floodplain habitat conditions based on examination of aerial
photography, LiDAR data, and a field reconnaissance conducted in the fall of 2020 (NSD 2021). The results of this
analysis are summarized below.
4.5.1 Large Wood
The Dosewallips Resilience Plan (NSD 2021) identified a lack of large wood is the Powerline Reach as an
impairment which has adversely affected riparian and wetland habitat formation and puts the existing aquatic
habitats supportive of anadromous salmonid use at risk. Specifically:
Historical forest clearing and instream wood removal have reduced levels of stable large wood in the
main channel and side channels.
The lack of large wood contributes to channel migration rates greater than historical levels, which
are limiting ability of riparian vegetation to mature.
There were 18 total stable large wood jams within the Powerlines reach during the 2020 field survey with 10 of
the jams in the main channel and 8 within side channels. The total wood frequency is 26% of reference
conditions (i.e. Queets River). However, since almost half of the jams were identified as being stable within the
existing side channels, the actual frequency of stable jams is closer to that of only the main channel (14%)
because those jams would likely not be able to withstand the forces of the main channel if an avulsion occurs. A
subset of the main-channel stable log jams consisted of a large meander jam near RM 1.8 between SC-2 and SC-
3, a large bar apex jam on the left bank near RM 1.6, and a large channel spanning log jam complex within the
downstream braid bar near RM 1.2 (Figure 18). There were also many pieces of mobile wood that have
accumulated on top of bar surfaces. The amount of wood accumulation, especially within the downstream braid
bar, is reflective of the depositional qualities of the reach. The log jams within the side channels consisted of
bank deflector jams and log steps. The low level of stable large wood within the Powerlines reach compared to
reference conditions is likely due to historical stream cleaning and forest clearing activities, as well as the lack of
mature riparian vegetation capable of remaining stable once recruited into the channel.
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Figure 18. Stable large wood jams and large wood pieces within the Lazy C and Powerlines Reaches as mapped
on 10/9/20.
A subsequent field reconnaissance by NSD in January 2024 observed the avulsion of Side Channel 1 (Figure 7)
and the mobilization of large wood through this area and downstream within the lower Powerlines Reach. While
the locations and quantity of wood has changed since the 2020 survey, NSD observed that the stable logjams
noted in 2020 generally persist, and new wood recruited from subsequent channel avulsion and migration has
occurred throughout the reach (Figure 19).
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Figure 19. Large wood accumulation on a gravel bar downstream of the channel avulsion (01/24/24).
4.5.2 Riparian and Wetland Communities
Riparian communities within the Powerlines reach are characterized by mixed mature deciduous/coniferous
forest on the higher floodplain terraces, and by younger deciduous forest on the developing gravel bars (Figure
20). These floodplain features support limited wetland communities, despite the frequent connection to flood
flows. The higher flood terraces provide patches of a developing mixed deciduous/conifer “reference
community” that supports an overstory of a mixture of black cottonwood (Populus balsaminfera), big leaf maple
(Acer macrophyllum), red alder (Alnus rubra), mature grand fir (Abies grandis) and western red cedar (Thuja
plicata), with an understory of sapling grand fir, Douglas fir (Pseudotsugs menziesii), western hemlock (Tsuga
heterophylla), and Sitka spruce (Picea sitchensis), vine maple (Acer circinatum), salmonberry (Rubus spectabilis),
snowberry (Symphoricarpus albus), and sword fern (Polystichum munitum) (Figure 21, Figure 22).
The recently formed gravel bars within this reach support multiple age classes of red alder and black
cottonwood, with big leaf maple typically located on the higher elevations. Western red cedar, grand fir, and
western hemlock saplings are infrequent within the understory, and large patches of invasive Himalayan
blackberry (Rubus armeniacus) dominate the understory in the younger alder-forest areas. These gravel bars
and young riparian communities also support scattered patches of invasive butterfly bush (Buddleja davidii) and
Scotch broom (Cytisus scoparius), along with pockets of invasive knotweed (possibly Fallopia japonica). Side
channels are common through these floodplain areas. These open channels typically support Pacific willow (Salix
lucida), Sitka willow (Salix sitchensis) and salmonberry, along with red alder and cottonwood along the channel
margins.
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Figure 20. Canopy height map for the Powerlines Reach (2018 LiDAR)
Figure 21. Typical riparian conditions in the Powerlines Reach with mixed forest on higher terraces and young
deciduous forest on gravel bars and lower terraces (10/9/20).
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Figure 22. Typical red alder with conifer understory within the Powerlines Reach higher floodplain (09/13/23).
NSD conducted a wetland delineation on September 13th and 14th, 2023. The full details of this delineation are
presented in the Wetlands Delineation Report (NSD 2024). A total of 10 wetlands were delineated during this
effort all located within the floodplain area of the Powerlines Reach (Figure 23). Two primary types of wetlands
were identified– depressional and riverine. Depressional wetlands (Wetlands 1, 2, 6, 7, and 9) occur as linear
depressions situated at the base of valley walls (within the eastern and western floodplains respectively) and are
hydrologically supported by seepage from adjacent valley walls and precipitation which is captured and stored
within each depression. The riverine wetlands (Wetlands 3, 4, 5, 8, and 10) were delineated along both sides of
the river, where the lower elevation floodplain is regularly engaged by flows.
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Figure 23. Delineated wetlands and waters within the Powerlines Reach study area (NSD 2024).
4.5.3 Aquatic Habitat Conditions, Salmonid Use, and Periodicity
Aquatic Habitat Conditions
The focal species in the Dosewallips River, fall Chinook and Hood Canal summer chum, spawn in the fall and
spend hours to months rearing in freshwater before outmigrating to estuarine and nearshore habitat. Both are
listed as federally threatened species under the Endangered Species Act. The life stages of these species most
affected by the quality, quantity, and diversity of aquatic habitats are spawning, incubation, emergence, and fry
rearing. Within freshwater habitats, habitat diversity, channel stability and sediment load have been identified
as the elements most critical for restoration (Brewer et al. 2005). Loss of riparian forest has also been specifically
noted for the Dosewallips watershed.
The Powerlines reach contained higher quality aquatic habitat with little channel confinement, as well as more
common side and braided channel habitat (NSD 2021). Aquatic habitat surveys in 2020 in the Powerlines Reach
identified that riffle habitats made up 43% of the available habitat by area, and pools represented 42%, with
glides representing 15% of the channel area. A total of 9 pools were documented in the main channel, with 5
additional pools in side channels. Pools per mile in the mainstem total 9.1/mi. and average pool residual depth
was 4.26 ft. Recent surveys in the Dosewallips watershed (Labbe et al. 2005) noted large wood as being an
important factor in creating pools and scour (LWD pieces was responsible for 60-80% of pools in the reaches
surveyed). It was also noted that large bar apex logjams (Abbe and Montgomery 2003) had a significant
influence on habitat formation, but that lack of “key” pieces (large tree boles) is limiting logjam formation.
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Salmonid Use and Periodicity
Critical life stages for chum and Chinook have been identified as spawning, incubation and adult holding (Shared
Strategy 2007). Incubation and rearing success are driven by large wood and moderate peak flows, absence of
excessive fines in spawning gravels, moderate or low levels of scour, and access to off-channel and floodplain
habitats. Puget Sound Chinook, as a larger species, require larger substrate for spawning and deep holding pools
with cover for adult migration. Egg incubation is affected by scour during high flows and excessive fine sediment
which can smother redds. Juvenile Chinook remain in the river for approximately 4 months (Table 3) and depend
on low velocity habitat and cover for rearing. The majority of juvenile Chinook in the Dosewallips and other
coastal rivers exhibit the ocean-type life history and out-migrate after about 4 months of rearing in freshwater,
completing the rest of their growth in estuarine or nearshore habitat. A small proportion of Chinook maintain a
stream-type life history, spending up to a year in fresh water before outmigration. The proportion of stream-
type Chinook in the Dosewallips is unknown but expected to be small. Low velocity habitat includes river
margins, alcoves, back waters, and side channels, where small fish can escape the force of the current, have
sufficient hiding cover for protection from predators, and be able to rest and feed.
Chum salmon have similar requirements for deep pools with cover for holding, but they use slightly smaller
gravels for spawning than Chinook. While most Chinook spawn in mainstem river channels, chum salmon are
more likely to spawn in lower velocity areas with smaller substrate, which may include mainstem habitat,
smaller creeks and side channels. Incubation for chum is also limited by scour from high flows, perhaps to a
greater extent than Chinook based on the chum behavior of mass spawning and redd superimposition.
Spawning success for chum is linked to suitable spawning gravel, adequate stream flows and water
temperatures, as well as habitat quality in the form of large wood for cover and holding pools for returning
adults to rest. Excessive fine sediment is also a concern for chum eggs in terms of suffocation. Since chum fry
out-migrate upon emergence (Figure 24), their dependance on adequate riverine rearing habitat is less than that
of Chinook, but they still need safe pathways to out-migrate through the mainstem channel and have a higher
need for sufficient estuarine and nearshore habitat. Chum spawning in the Dosewallips is limited to the lower
4.3 miles of river, with the greatest concentrations below RM 2.5 (Brewer et al. 2005), which includes the study
area.
Coho salmon also spawn in the fall and have similar needs for spawning, incubation and fry rearing as Chinook.
However, coho spend 1-2 years in freshwater habitat for rearing before outmigration (Figure 24), so they are
more affected by the quality and duration of supportive riverine habitat conditions across multiple seasons.
Specifically, high flow refuge in the fall/winter, overwintering habitat, and summer low-flow rearing habitat are
necessary for coho to survive to become smolts. Like Chinook fry, coho fry and juvenile life stages are most
common in slower velocity habitat (3-6 cm/s) and moderate depths (up to 1 m). These areas are most often
found at the margins of mainstem channels, in side channels, backwaters, or floodplain channels. Figure 24
shows the periodicity for Chinook salmon, summer Chum salmon, and coho salmon in the lower Dosewallips
River.
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Figure 24. Periodicity for selected salmonids in the lower Dosewallips River (NSD 2021).
Limiting Factors for Salmonids
The Hood Canal Summer Chum Recovery Plan identifies the primary limiting factors affecting chum salmon as
(Brewer et al. 2005):
1. Climate related changes affecting streamflow patterns
2. Fishery exploitation
3. Habitat loss
Unfavorable stream flows in 1975 and 1976 caused a crash in chum populations across Washington State, but
Hood Canal populations remained low while other populations recovered. High levels of fishing in the 1980s had
further impacts on the population and coincided with a shift in ocean conditions in 1986 to patterns less
favorable to chum (Brewer et al, 2005).
Several factors have been identified in the Dosewallips River as affecting summer chum populations. For
spawning and incubation life stages, loss of channel complexity and floodplain access are key factors (Brewer et
al. 2005). Historically, much of the lower few miles of the river has been simplified, with the construction of
dikes, placement of riprap, splash damming and the removal of wood. The surrounding floodplain in the area
has also been converted to pastureland and residential development (Brewer et al. 2005). Sediment aggradation
is also noted as a problem for spawning and incubation due to sediment input from forest roads, as well as
channelization and diking. Logging of forests, specifically old growth areas, has resulted in loss of recruitment
sources for large wood into the river. The USFS rated the riparian conditions along the river as fair to poor
(Brewer et al. 2005). Loss of side channel habitat and channel instability have also been noted as limiting factors
for salmon habitat. Estuarine habitat degradation was also noted as a leading limiting factor for juvenile Hood
Canal summer chum rearing.
4.5.4 Climate Change
Scientific studies in the Pacific Northwest region have concluded that the frequency and magnitude of peak
flows will increase over the next 100 years while summer/fall low flows will diminish (Hamlet et al. 2013;
Mauger et al. 2016; Warner, Mass, and Salathé 2015; Hamlet and Lettenmaier 2007; Elsner et al. 2010). Impacts
from the warming climate on flooding are projected to occur in the coming decades, with increases in peak
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flows modeled for the 2040s (inclusive of 2030-2059; Hamlet et al. 2013), 2050s (inclusive of 2040-2069; Mauger
et al. 2016), and beyond. Numerous studies for western Washington have indicated increases in peak flows as
early as the 2020s (inclusive of 2010-2039; Elsner et al. 2010, Mantua et al. 2010). The warming climate effects
are therefore relevant to the consideration of all geomorphic and hydraulic processes related to flow duration,
frequency, and magnitude.
Projections for increased peak flow magnitudes with the warming climate are driven by projections for more
intense precipitation, particularly in the fall and winter months. Higher precipitation rates will increase the
magnitude of annual peak flows and contribute to increases in mass-wasting and sediment delivery to the river
network – both upstream and within the project area – which will impact flooding by raising bed elevations and
further increase the frequency of inundation by raising water levels. The increase in annual peak flow magnitude
will also contribute to increases in channel migration and erosion.
The warming climate will also result in lower base flows in the late summer and fall. This will diminish the
wetted channel area and depths that will decrease available habitat. Warmer air temperatures will also result in
warmer water in the river, further stressing fish (Mantua et al., 2010) and making riparian shade and in-stream
cover even more important for salmonids. While these changes were not quantitatively evaluated by the scope
of this study, they further underscore the importance of increasing the ecological resiliency of the project area.
In practice, this could be accomplished by increasing stream shade, the quantity of off-channel habitats and
deep pools, as well as increased surface water/ ground water interactions through increased floodplain
connectivity and added roughness so that the predicted increases in stream temperature can be partially offset
by temperature decreases.
Looking to the future, the impact of the warming climate on river dynamics in the project area was analyzed by
projecting peak flow estimates into the future and modeling the resulting “climate change” flows. The discharge
estimates used for climate change floods are based on the work done by the Columbia Basin Climate Change
Scenarios (CBCCS) project (Hamlet et al. 2010) and are included in Table 4.
Table 4. The magnitude of future peak flows 2070 - 2099 projected as result of warming climate.
RECURRENCE INTERVAL PRESENT DISCHARGE
ESTIMATE (CFS)
PERCENT INCREASE
ESTIMATE
FUTURE (2070-2099)
DISCHARGE ESTIMATE
(CFS)
1-Year 2,100 18% 2,480
10-Year 11,420 18% 13,480
100-Year 17,120 23% 21,060
5 DESIGN OVERVIEW
The proposed restoration actions in the Powerlines Reach address the project goals and objectives, and are
informed by the analysis of existing conditions and the findings of the 2021 Resiliency Plan.
Restoration actions within the active Dosewallips channel focus on installing large log jams to create hard points
in the channel to reduce the rates and frequency of channel migration and encourage the development of stable
forested islands. Reducing the rates and frequency of channel migration within the active channel will help
decrease the frequency of redd scour increasing the egg to fry survival rates of chum and Chinook. Sufficiently
reducing channel migration rates in the short term within the Powerlines reach is essential so that riparian
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forests have enough time (~100 years) to mature so that the wood can remain stable when eventually recruited
to the river. Placement of large wood to encourage development of velocity refuges, spawning gravel storage,
and floodplain reconnection would benefit coho, steelhead and rainbow, and Chinook, as well as pink and chum
for spawning habitat.
Within the active side channels of the Powerlines reach, restoration actions focus on increasing stable wood
loading to provide increased hydraulic complexity and woody cover and increasing hydraulic roughness to
reduce the risk of the mainstem Dosewallips River avulsing into these potential pathways. Collectively, these
actions serve to maintain the current active channel planform as the preferential route for main channel flows
and reduce the risks to aquatic habitats of an avulsion. These actions will also support and improve aquatic
habitat quality and stability for chum spawning and rearing for coho, Chinook and steelhead.
The proposed floodplain and riparian enhancements are intended to facilitate the restoration of the natural
large wood cycle where stable riparian forests can grow and develop to key sized trees so that when these trees
are naturally recruited to the river, they can contribute to stable hard points in the reach. This strategy will
eventually allow the river system to sustain itself and accomplish the habitat forming processes necessary to
sustain healthy aquatic ecosystems. All of these short-term actions that support the long-term habitat expansion
benefits for chum, coho, and Chinook in the project area. Proposed actions can be phased in a variety of ways
such as by geographic location, geomorphic setting, access points, or regulatory and landownership boundaries.
5.1 Preliminary Design
Preliminary design for the Powerlines Reach (Appendix B) includes approximately 75 acres of active channel and
floodplain across multiple ownerships including private, Washington Department of Natural Resources state
owned aquatic lands, and Washington State Parks lands. The restoration actions include 41 ELJ structures: 21
Apex ELJs (8 large and 13 small), 11 low-profile ELJs, and 9 floodplain roughness ELJs, ranging in size from small
low-profile jams to large apex jams, distributed within the river channel, the river left channel margin, and
within side channel SC-3 on river left, and approximately 18.2 acres of riparian enhancement through planting
and invasive plant treatment. This suite of restoration actions has the option to be phased or sequenced to
facilitate the development and implementation of short-term actions while building toward a cohesive
restoration vision for the Powerlines reach. Phased construction may also be required due to the narrow in-
water work window prescribed for the Dosewallips River (i.e., July 16-August 15). The primary restoration
elements proposed in the preliminary design are described in the sections below.
5.1.1 Project Metrics
Table 5 presents the Dosewallips River Powerlines Reach project metrics based on the preliminary designs.
Table 5. Dosewallips River Powerlines Reach Restoration Metrics
SPATIAL RESTORATION METRIC EXISTING PROPOSED
Reach Length 3,100 lf 3,100 lf
Acres of Riparian Zone Restored - 18.2
Wood Structures 10 41
Pools 9 21
Side Channel Enhancement - 1,075 lf
5.2 Riparian Restoration
Riparian planting and management actions include a suite of strategies to restore the riparian forest and the
related ecological and geomorphological functions of a mature riparian forest consistent with reference
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conditions (Figure 25). The strategies include removal of invasive species (e.g., Himalayan blackberry), installing
floodplain roughness structures in areas that get overtopped to rack debris and protect riparian plantings,
conifer underplanting in existing riparian forests, and plantings associated with the ELJ installations.
Historically along the Dosewallips River, mature riparian trees such as black cottonwood, western red cedar,
Douglas fir, grand fir, Sitka spruce, and big leaf maple provided a source of stable in-channel wood jam key
members. These riparian trees vary in growth rates and time needed to achieve key member, or stable wood,
sizes. Black cottonwood may attain 1-meter diameter size within 70-80-years, whereas Sitka spruce may attain
key member diameter within 100-years (Burns and Honkala 1990). In large powerful rivers such as the
Dosewallips, only these large diameter (>80-100 cm) species attain large enough size to provide stable large
wood (i.e. key members) capable of forming stable river bank and mid-channel logjams. Smaller tree species
such as red alder, or sapling or young grand fir, western red cedar, and black cottonwood, are typically not large
enough to provide these wood jam functions over the long-term. Therefore, it is important for trees to be able
to grow large enough to function properly as aquatic habitat structural components within riparian forest
ecosystems.
In certain areas of the deciduous dominated floodplain the forests within the project reach currently lack
adequate conifer components to develop into the reference target mixed coniferous/ deciduous forest
community that will provide future key member-size trees. The existing deciduous forests have varying degree
of salmonberry and Himalayan blackberry dominated understory, and generally lack the conifer saplings that are
needed to ultimately provide large key member-size trees. Without restoration of a mixed riparian forest that
can ultimately act as a source-pool of conifer LWD in the river, and without the channel stability provided by the
ELJ treatments, the trajectory of the project area’s young forests will be young alder and cottonwood forest.
Figure 25. Example of future conditions in a riverine floodplain with a functional large wood cycle recruiting
large conifers as stable hard points.
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5.2.1 Riparian Planting Associations
The preliminary plans (Appendix B) provide the plan view of the proposed riparian restoration and a plant
schedule with details for each of the proposed planting areas. Patches of mature mixed deciduous/coniferous
forest located on the upper floodplain provides the “reference community type” for the proposed actions to
restore a mixed coniferous and deciduous forest community to the floodplains of the project area. The species
composition proposed for the different planting areas while similar, is based on: the dominant and co-dominant
species characteristic of reference plant community, species likely present in the riparian communities of the
project area, and species documented as currently present in the project area. Species composition within each
planting zone also reflects differences in soils (i.e young gravel bar vs. terrace), and hydrology as noted from
field conditions and the hydraulic modeling outputs.
Conifer Underplanting
As described above, the existing floodplain forests within the project area are dominated by stands of red alder,
cottonwood, and big leaf maple trees. Conifer underplanting is intended to restore the deciduous riparian
forests to a mixed conifer deciduous forest. This will improve floodplain and streambank stability to reduce
channel migration rates, increase riparian habitat structure and diversity, and increase the recruitment of large
key pieces over time. Conifer underplanting will occur both on the higher elevation floodplain which contain
mature deciduous forest, and on the younger gravel bars that currently support young monotypical red alder
stands.
The existing deciduous forests typically have a dense understory of native shrubs (i.e. salmonberry, snowberry),
and non-native shrubs (i.e. Himalayan blackberry and Scotch broom). We recommend selective brushing in areas
where conifer saplings will be planted within dense Himalayan blackberry and Scott’s broom. This would consist
of mechanical control simply to improve planting conditions for the conifer seedlings. No thinning of the
overstory trees is required as we are recommending the planting of shade tolerant species, and thinning can
lead to colonization by Himalayan blackberry.
Three conifer underplanting “zones” or types are indicated within the preliminary plans (Appendix B). Species
composition and planting densities are consistent across the zones, with differences consisting of selective
Himalayan blackberry and Scott’s broom control, and areas that allow for installation of browse protection. Elk
browse is evident throughout the floodplain areas with severely browsed shrubs (i.e. salmonberry). Browse
protection is only proposed in areas that do not receive frequent (i.e. 2-year flows) flooding events.
The conifer underplanting zones will restore a mixed deciduous/coniferous tree community that can tolerate
seasonal inundation and predominantly dry, well-drained soils during summer months. Species were also
selected to be tolerant of shading due to the existing overstory. Tree species include: Sitka spruce, western red
cedar, Douglas-fir, grand fir, and big leaf maple. Existing species such as red alder and black cottonwood are in
abundance and are expected to recruit naturally. Tree spacing is 30 feet on center, which is moderate, and
results in approximately 50 trees per acre, which takes existing sapling and pole conifers and mature deciduous
trees into consideration.
Channel Bank Planting
Channel bank planting is intended to provide woody, water-tolerant shrub cover in association with ELJ
construction. The plant community composition reflects the existing willow community growing along the
gravel/cobble bar areas that are frequently exposed to flood flows. Proposed species include Sitka willow, Pacific
willow (Salix lasiandra), and cottonwood. Red alder is also common along this zone but we expect red alder to
easily colonize following construction so it was not included in the planting plan. No browse protection is
proposed as high velocity flows will likely mobilize any protection that is installed.
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Floodplain Planting
Floodplain planting area are located on higher gravel bar areas that are currently dominated by young red alder
stands. The plantings will occur within the footprint of construction access and intends to increase species
diversity within the generally monotypical alder stands. These areas are also frequently inundated and fall
within the modeled 2-year flow extents. Proposed species lean heavily on woody shrubs native to the area and
include Sitka willow, cottonwood, snowberry, vine maple, and sword fern. Shrub spacing ranges from 4-feet to
10-feet on center. No browse protection is proposed as high velocity flows will likely mobilize any protection
that is installed.
Upper Bank Planting
The upper bank planting area is located on the high terrace associated with left bank ELJ placement. This area is
dominated by a mixed, mature deciduous/conifer forest dominated by cottonwood, big leaf maple, grand fir,
western red cedar with an understory of vine maple and sword fern, with sparse Sitka spruce, Douglas-fir, and
salmonberry. This planting area is mostly above the extent of the 100-year floodplain and remains fairly dry, but
has deep silty floodplain soils. The plantings will occur entirely within the footprint of the construction
disturbance with browse protection to guard against elk damage. Proposed species include big leaf maple,
Douglas fir, grand fir, and cottonwood, with snowberry, vine maple, and sword fern. Tree spacing is 15 feet on
center which results in approximately 200 trees per acre. The plantings will be directly protected by the new
ELJs installed along the bank and are intended to provide increase root cohesion in association with the stability
that the ELJs provide.
5.3 Engineered Log Jams and Large Wood Placement
Engineered log jams are intended to mimic the geomorphic functions of natural log jams by increasing flow
resistance, slowing flow velocities, locally raising water surface elevations, slowing bank erosion to closer to pre-
development rates, aggrading, and sorting sediment, and connecting the channel to side channels and the
adjacent floodplain (Abbe and Montgomery, 2003; Montgomery et al., 2003). Log jams provide critical habitat
functions such as triggering and sustaining the formation of deep pools, retaining spawning gravels, and
providing in-stream cover. The goal of constructing engineered log jams is to re-initiate these habitat-forming
processes in the near-term until natural wood recruitment from the restored riparian forest sustains those
processes in the long-term.
Smaller ELJs in side channels and on the floodplain are intended to improve existing habitat conditions within
less hydraulically active sections of the floodplain and increase stable wood loading within the reach. These
structures are placed in locations where hydraulic forces (velocity and shear stress) are lower than the main
channel. These smaller structures are designed to add hydraulic roughness and complexity to the side channels
and floodplain, sort and store mobile sediment, rack mobilized small woody debris during floods, and increase
bar stability during flood events. Structure placement was optimized to minimize unnecessary disturbance to
riparian plant communities for machine access or structure installation.
The structures rely on buried rootwad posts to withstand the hydraulic forces and achieve stability. Post bases
will be excavated to be below scour depth for each structure, 8 to 10 feet below the channel thalweg, then
backfilled. Key logs will be attached to the posts with chain lashings, and racking and slash will be laced
throughout the structure, with the densest concentration at the structure front. Construction will require the
excavation into the native materials, then the placement of posts and key logs, with native material ultimately
being backfilled within the structure. No offsite export of materials will be necessary.
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5.3.1 Instream Large and Small Apex Engineered Log Jams
The project includes eight large (Type1) and seven instream small (Type 2) Apex ELJs, which are intended to
mimic the geomorphic functions of natural bar apex log jams. Arrays of these structures will be placed in the
middle of the channel and on existing gravel bars within the project reach. These structures will split flows
around and across the gravel bar, helping to “lock in” the bar location, as well as providing scour pools and
complex woody cover around the upstream edges of the jams as well as reducing overall bed mobilization
frequency and providing refugia for spawning redds. The Apex ELJs will be constructed as an array of multiple
structures so larger island areas can form and so the structures can ‘protect each other’ by reducing the overall
hydraulic forces within a particular region of the river corridor. The structures are designed to aggrade sediment
in their lee (behind them) and accelerate flow to the front and sides, eventually encouraging the development of
deep pools, split flow paths, and forested islands by reducing downstream hydraulic forces so that vegetation
can establish and flourish. Because the structures are often placed in areas with high hydraulic forces, deep
pools are scoured at the front face of the jam. The apex jams will also be planted with riparian species such as
willows to promote the development of stable vegetation communities along the gravel bars, increasing long-
term bar stability and leading to the development of a stable mid-channel forested island over time.
5.3.2 Bank-Oriented small Apex Engineered Log Jams
Six additional bank-oriented small (Type 2) apex ELJs installed along the unstable and rapidly eroding left bank,
are intended to mimic the geomorphic functions of natural flow deflection or meander bend log jams. The
structures are intended to deflect flow from one area of the river corridor to another. These ELJs can be used to
slow channel migration rates to historical levels on a particular portion of bank by deflecting flow away from the
bank, as well as cause an increase in frequency of side channel inundation by deflecting flow towards the side
channel inlet. The ELJs contribute to instream wood loading, and because they are often placed in areas with
high hydraulic forces (such as the outside of an actively migrating meander bend), large scour pools develop at
the head of the structures which can provide important holding habitat for salmonids. Bank-oriented ELJs can
also be used to protect developing riparian vegetation located behind the structures by reducing the rates of
future channel migration and erosion. The structures can be placed in groups to treat larger areas such as the
entirety of a meander bend.
5.3.3 Low Profile Engineered Logjams
Five Low Profile ELJs are placed in side channel SC-3 and six others (eleven total) are located in low points on the
gravel bars that serve as secondary flow paths as flows rise above summer base level. The low profile ELJ
structures serve to add habitat complexity and hydraulic roughness while reducing risk of channel avulsion
through these side channels. A channel avulsion through any of the treated side channels is undesirable as it
would greatly reduce the main channel length in the reach and destroy existing functional side channel habitat,
reducing the overall habitat complexity of the Powerlines reach. To reduce the risk of avulsion within SC-3,
larger and more densely placed log jams are proposed to increase the obstructions to flow, reducing flow
velocities, and partitioning shear stresses that act upon the channel bed. Collectively, these actions serve to
maintain the current active channel planform as the preferential route for main channel flows and reduce the
risks to aquatic habitats of an avulsion through either of these channels. Pools are expected to form on the
upstream faces.
5.3.4 Floodplain Roughness Engineered Logjams
The nine Floodplain Roughness ELJs are positioned on gravel bar tops or within floodplain areas that are
unvegetated or contain immature vegetation. The structures will increase floodplain roughness and reduce flow
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velocities. They also provide a local hard point and an area of hydraulic shadow where willows or other early
successional species can establish to promote bar stability and increased vegetation coverage with the eventual
goal of promoting the development of mature riparian forested islands and increasing shade and cover for the
channel. Collectively, the goal of the proposed floodplain roughness jams and the riparian enhancement is to
facilitate the restoration of the natural large wood cycle where stable riparian forests can grow and develop to
key sized trees so that when these trees are naturally recruited to the river, they can contribute to stable
hardpoints in the reach.
5.3.5 Stability and Risk Based Design
The stability criteria used in the ELJ design is based on the evaluated level of risk for the project. Conditions at
the project site and the proposed design were assessed using the US Bureau of Reclamation’s 2014 Large Woody
Material – Risk Based Design Guidelines (RBDG). Although the minimum stability design flow criteria
recommended for the ELJ structures using this methodology is the 50-year flow, the design engineers applied
the 100-year design flow to meet recent legislation in Washington exempting landowners from liability for the
LWM structures on their property if the structures are designed for the 100-year event and to provide additional
protection for downstream infrastructure. The ratings for risk are shown in Table 6.
Table 6. Risk based design criteria for the Dosewallips River – Powerlines Reach.
PUBLIC SAFETY
RISK
PROPERTY
DAMAGE RISK
STABILITY
DESIGN
FLOW
CRITERIA
FOS SLIDING FOS BUOYANCY FOS ROTATION
& OVERTURNING
High Moderate 100-year 1.5 1.75 1.5
5.4 Site Constraints
The two primary site constraints that have shaped the design of the Powerline project are FEMA floodplain
regulations which require the project to achieve no rise at the upstream Lazy C community and limited access
routes into the reach. Much of the Powerlines reach is publicly owned, with the exception of a large private
parcel on river left at the downstream end of the project, and on those parcels that are privately held there are
no structures (i.e. homes) located within the floodplain. The downstream end of the reach is a natural geologic
constriction in the river which separates any water surface elevation changes or channel migration resulting
from the restoration actions from downstream private properties. While there is no public or private
infrastructure within the reach, the proposed restoration actions must take into account any effects on the
upstream Lazy C community, including erosions risks and increases to flooding magnitude and frequency. These
constraints are discussed further below.
5.4.1 Construction Access
The reach is flanked on both banks by steep hillslopes that are not easily traversable by equipment and has a
limited number of routes down onto the floodplain. Establishing access to the proposed ELJ construction
locations with ground-based equipment involves the creation of temporary access routes, the development of
staging areas for equipment, and multiple channel crossings, each requiring long span bridges to transport
construction equipment and logs to the ELJ locations. The primary ground access will be through an existing
private driveway near the downstream end of the reach. All equipment and materials will enter the site through
this roadway. The project will take advantage of existing private roads as much as feasible to reduce impacts to
existing native vegetation. Temporary access routes through will then be established to construct the ELJs,
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including the use of temporary bridges to cross wetted areas. Refer to Appendix B Sheet 6 for the proposed
access and staging plan.
In 2021, NSD conducted an assessment of construction approaches and access (NSD 2022). Four primary access
routes for ground-based equipment were considered, as well as helicopter access. Access options 1 and 2
involved entering the project reach from the western (upstream) end via the Lazy C development. Option 1
followed Appaloosa Dive and Palamino Lane to the river, then skirted the northern edge of the valley past the
upstream-most meander bend, following the river into the project reach, avoiding any crossings. Unfortunately,
this meander bend has since progressed all the way to the steep valley wall, eliminating the low-gradient
floodplain in this area and precluding ground access from that direction.
Option 2 also starts in the Lazy C development and cuts across the floodplain at widest part of the meander,
requiring two bridge crossings to reach the project reach. This pathway is the route that the meander cutoff
took, transforming the floodplain inside of the meander into a complex network of perennial channels and
downed wood that 1) the project team does not wish to disturb and 2) no longer comprises solid access.
Options 3 and 4 involve entering the project area from the eastern (downstream) end on the left and right banks
respectively. Option 3, on the left bank, follows a private driveway from Dosewallips Road down to the riverbank
floodplain beneath the powerlines. It is the shortest of the access routes considered and uses established roads,
minimizing the need for clearing of riparian vegetation. This is the preferred access route but is only viable with
left bank landowner permission. In July 2024, the landowner agreed to participate in the Powerlines reach
restoration program. This then allowed the use of ground-based equipment to construct the ELJs and allowed
much of the downstream river left section of the reach to be treated. Current access proposes to use the
existing driveway road on river left to mobilize equipment and materials to the left bank floodplain. Temporary
access routes will be created by clearing vegetation while avoiding and minimizing impacts to mature forest and
aquatic habitats as practical. Temporary bridges will be used to span the Dosewallips River to construct the right
bank ELJ elements and mobilize plant materials in those same areas.
Option 4 patched together a series of private forest roads and BPA access roads in the general vicinity of Train
Wreck Creek (Figure 26). The route was technically feasible at that time, but was longer than might be cost
effective, would require coordination with multiple landowners, and was generally in poor condition.
Additionally, subsequent bank erosion and loss of floodplain on river right has left the end of the road hanging
more than twenty feet above the valley bottom, rendering this route even more problematic (NSD 2022).
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Figure 26. Access route from south via state parks and transmission line easement to Train Wreck Creek (NSD
2022).
Given some uncertainty associated with the participation of the left bank landowner, the NSD team also
evaluated the use of helicopters to place wood and build the proposed ELJs. Helicopter construction is a
common and effective method for constructing log jams in the Pacific Northwest especially in more remote
areas with little to no ground access. This approach is often prescribed to avoid impacts to sensitive habitats
that would result from ground-based equipment and in areas where access for ground-based equipment is not
cost effective or feasible. Helicopter constructed ELJs themselves would be designed to have the same
geomorphic and hydraulic effect as any built by ground-based equipment. The differences between the two
construction approaches would be seen in the architecture and ballasting of the structures themselves. Ground-
based structures would be built around an array of piles or posts that would require excavation or pile driving
and then the placement and lashing of key wood cross-members. Helicopter-based ELJs would be built without
any excavation, with wood and rock ballast placed in layers until the design configuration was achieved.
Helicopter construction would also eliminate the need for temporary construction access and the associated
impacts to riparian and wetland habitats that would result from the movement of materials and tracked
equipment. Constraints associated with the use of helicopter include the need to achieve structure stability
through cabled rock collars which introduces cabled materials to the stream channel; the inability to use ground
equipment to excavate pools or move existing woody material in and around the ELJ construction location to
maximize immediate habitat benefit; and constraints associated with flying near the BPA powerline corridor
which may eliminate preferred ELJ construction locations. Our understanding is that the minimum required
offset form the powerlines would be approximately 100 to 300 feet, and that the helicopter staging area would
need to be located upstream, as we would not be able to fly material above the lines.
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5.4.2 Base Flood Elevation, No-Rise, and Floodplain Permitting
Design and placement of ELJs need to both meet geomorphic and habitat objectives while not increasing
flooding or erosion hazard risks to adjacent landowners and fitting in with the floodplain management of the
river system. The Federal Emergency Management Agency (FEMA) requires all projects in the mapped
floodplain, including restoration projects, to comply with the floodplain regulations set forth in the National
Flood Insurance Program (NFIP). Through the project reach, the Dosewallips River is categorized as a Special
Flood Hazard Area, specifically a regulatory Zone AE floodplain with designated floodway. Figure 27 shows the
mapped flood zones within the Powerlines Reach. Jefferson County code section 15.15.080 describes the
limitations to development (referred to as encroachments) within regulatory floodways and requires that “the
proposed encroachment would not result in any increase in flood levels during the occurrence of the base flood
discharge” (Jefferson County, 2024).
Figure 27. Snapshot of the effective FIRM, displayed in the FEMA National Flood Hazard Layer (NFHL) online
map viewer on 10/22/2024.
As our preliminary floodplain assessment, described below in Section 6, indicates that the proposed restoration
actions are anticipated to increase the Base Flood Elevation (BFE) in the project reach, a Conditional Letter of
Map Revision (CLOMR) will need to be obtained from FEMA to document the proposed changes to the BFE and
the 100-year floodplain limits before the project could be implemented. The time frame for receiving FEMA
approvals can be on the order of 12 to 18 months, and thereby can significantly increase the design timeline and
cost for restoration actions. Additionally, the need to not increase flood risk at the nearby Lazy C residential
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structures effectively limited the farthest upstream extent of the Powerlines project, as reflected in the
preliminary design.
5.4.3 Land Ownership and Land Use
Much of the Powerlines reach is publicly owned with the exception of a large private parcel on river left that
covers the downstream half of the project area. Jefferson County owns most of the public parcels with a few
parcels on right bank held by WA State Parks (Figure 26). The reach is undeveloped and largely forested except
for the gravel bars and a large, cleared utility maintenance corridor beneath the BPA powerlines. There are no
structures or utilities located within the floodplain. The towers for the powerlines are located on the ridges
above the river, and the powerlines themselves cross at a significant elevation above the valley floor and are not
at risk of being impacted by a ground-based project.
5.5 Changes from Conceptual to Preliminary Design
The conceptual restoration plan for the Powerlines reach (Figure 28) extended all the way upstream to the Lazy
C community and included 52 apex, deflector, and low profile engineered log jams spread through the active
and side channels of the Dosewallips River, an additional 30 to 40 floodplain roughening jams, and 37 acres of
riparian enhancement through planting and invasive plant treatment.
Figure 28. Conceptual Design Layout (NSD 2021)
The biggest changes between concept and preliminary design were driven by the FEMA floodplain regulations.
Since the proposed ELJs structures are large and densely placed and no major excavation that might offset a rise
is proposed, the project is anticipated to cause a rise in the BFE within the project area, which is a desirable
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outcome from a habitat and geomorphic perspective. However, since the FEMA floodplain regulations prohibit
any rise on insurable structures, complying with these regulations required pulling the upstream end of the
project area back to the far side of the upstream meander, approximately one-half river mile from the Lazy C
community to a point where the preliminary modeling indicates that the rise from the project is unlikely to
extend upstream to the community. This significantly reduced the footprint for ELJ placement.
The other major changes to the design, reducing the extent and the density of the floodplain roughness
structures, eliminating the key log structures in the floodplain, and reducing the area of planting was guided by
further field assessment of the project area. NSD walked the project reach at low water in January 2024,
including investigating the side channels and floodplain areas where much of this work was proposed. Overall,
these floodplain and side channel areas were found to already be high quality habitat with complex floodplain
channels under the tree canopy, existing floodplain wood, and young conifers coming up through the deciduous
overstory. The design team decided that these area were already in good condition and there was no
justification for disturbing these high-quality habitats to establish machine access and install additional wood.
Additional density of bank structures was also added along the vertical bank on river left in response to
observed erosion rates over the past several winters and the lack of mature forest on that bank.
5.6 Proposed Conditions Hydraulic Modeling
The existing conditions hydraulic model was modified to represent proposed design elements, and model results
were compared to existing conditions. Model results are presented in Appendix A and discussed below.
5.6.1 Hydraulic Model Setup
The instream large and small apex ELJs and the bank oriented small apex ELJs were represented in the model
with roughness and surface edits, while low profile and floodplain roughness ELJs were modeled with roughness
changes only. The hydraulic model terrain was modified with raised surface “pickets” aligned along the face of
the proposed apex ELJs. Modeling the proposed ELJs as pickets accounts for ELJ porosity by allowing some flow
through the ELJ. Modeled ELJ height is based on the proposed ELJ type; small apex ELJs are modeled as 6 feet
above channel bed while large apex ELJs are modeled as 8 feet above channel bed. Scour pools on the front side
of the ELJs were also graded into the model terrain. Breaklines and refinements region with smaller cell sizes
were added in the hydraulic model mesh to allow for accurate representation of the pickets and scour pools.
Figure 29 shows typical surface modifications applied to represent apex ELJs in the proposed hydraulic model.
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Figure 29. Example of surface modifications used to represent apex ELJs in proposed hydraulic model.
Elevated roughness values were applied within the ELJ footprint for all proposed ELJ types. A roughness value of
n=0.15 was chosen to represent ELJs. Figure 30 depicts typical elevated roughness areas for each proposed ELJ
type.
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Figure 30. Example of elevated roughness areas to represent proposed ELJs in hydraulic modeling.
5.6.2 Hydraulic Results
Select proposed depth, velocity, and water surface results, along with a comparison to existing conditions, are
included in Appendix A.
At the spawning and outmigration flows (Figure 3 and Figure 4 of Appendix A), changes in depth and velocity are
mild. Localized depth increases of around 0.1 ft and localized velocity increases of up to 2 ft/s are visible in some
areas, generally just upstream of proposed ELJs. Localized depth decreases of 0.1 to 0.4 ft and velocity decreases
of up to 3 ft/s occur behind and adjacent to ELJs. The greatest change in depth at this flow, and at all flows,
occur within proposed scour pools, where depths range from 1 to 7 ft at the spawning and outmigration flows.
At the 2-year flow (Figure 7 and Figure 8 of Appendix A), depths are generally increased throughout the reach in
the vicinity of the ELJ placements. The greatest increases in depth range from 0.5 to 1 ft and occur in the left
bank floodplain between RM 1.5 and RM 1.7, within SC-4, and near wetlands 3 and 4. As in the lower flows,
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localized decreases in depth occur behind some ELJs. At this flow, we begin to observe more pronounced
changes in velocity, including approximately 0.1 to 2 ft/s decreases in velocity adjacent to and behind proposed
ELJ locations. Notably, model results appear show that since the bank-oriented small apex ELJs deflect flow away
from the bank, velocities are lowered by 1 to 3 ft/s on the left bank of the river, along the Coone property
between RM 1.5 and RM 1.3. This deflection of more severe hydraulics away from the bank could slow or reduce
channel migration and erosion of the bank. Velocities are increased as flows are pushed around proposed ELJs
and onto the floodplain or into side channels. Velocities are increased by 0.1 to 2 ft/s in the mainstem adjacent
to apex ELJs and within the left bank floodplain between RM 1.5 and RM 1.7. As in the lower flows, velocities are
mildly increased in SC-4 and the wetlands located in the right bank floodplain.
Figure 31. Velocity patterns through the proposed ELJ array at the 100-year flow
Trends in depth and velocity change are generally similar at the 100-year flow (Figure 9 of Appendix A). At this
flood flow, inundation occurs across a wider portion of the valley and the greatest increases in flow depth (0.5 –
1 ft) occur from RM 1.5 to 1.6. Velocity reductions are more significant in the mainstem channel at this flow,
particularly behind proposed ELJs. Areas with velocities greater than 8 ft/s are reduced in the mainstem when
compared to existing conditions. The flow deflection and velocity reduction along the Coone bank is maintained
at this higher flow. Figure 31 displays the velocity patterns created by a group of ELJs near RM 1.5. Note the low
velocity area along the Coone bank at the top of the figure and the split flow vectors around the ELJs. Similar
patterns are observed at all modeled flows, but to a lesser extent.
Climate change results showed the same overall patterns, with faster and deeper flows during high flow events,
and shallower and slower flows at the lower flow events.
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5.7 Construction Cost Estimate
A preliminary level construction cost estimate was developed for the full suite of conceptual restoration actions
shown in the Powerlines reach and is estimated to be $4,742,774 in construction costs. Cost estimates were
developed on a unit cost basis for the types and sizes of proposed log jams, assumed ground-based construction
equipment, as well as additional costs associated with mobilization, access and staging, temporary erosion and
sediment control, site isolation related to the construction of log jams within the active channel, and site
revegetation. The restoration costs in the Powerlines reach are driven primarily by the number of proposed log
jams structures within the active and side channels, 41 structures, as well as the related access requirements to
reach the structure locations and the site isolation work needed to install the log jams in the active channel.
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6 PRELIMINARY NO RISE ASSESSMENT
The FEMA base flood in the Dosewallips River, which corresponds to a 100-year recurrence interval flood event,
is estimated to be 16,600 cfs per the effective Flood Insurance Study (FIS). The FEMA base flood elevations
(BFEs) range from 34.1 to 77.2 ft NAVD88 within the project area (FEMA XS G to FEMA XS O) (FEMA 2019).
Floodplain regulations require that a hydrologic and hydraulic analysis be conducted on the proposed project
actions to assess if the water surface elevations during a 100-year flood event (referred to as the Base Flood
Elevation) would be increased as a result of the proposed actions. FEMA further requires that any rise in BFE
must not impact any insurable structures in the floodplain as defined by the NFIP (any building with a minimum
of three walls and a roof). Although there are no insurable structures within the project reach, there are many
residential buildings located just upstream of the project reach at the Lazy C Subdivision. The need to not
increase flood risk for these structures effectively limited the farthest upstream extent of the Powerlines
project, as reflected in the preliminary design.
A preliminary no-rise analysis was conducted using results from the 2D hydraulic model to evaluate magnitude
and extent of potential increases to the BFE as a result of proposed project actions. The modeled 100-year flow
developed for this project (16,337 cfs) was considered sufficiently close to the FEMA base flood (16,000 cfs) for
this preliminary analysis (FEMA 2019). Modeled water surface elevations under existing and proposed
conditions were compared within the project reach, from FEMA XS G to FEMA XS Q and are summarized in
Figure 32 and Table 7. FEMA lettered and non-lettered cross section locations within the reach were obtained
online from the FEMA Map Service Center in 2024. There are several unlettered cross sections in this reach,
which do not appear on the official FEMA maps, but are present in the FEMA model. Proposed ELJ placements
are located between FEMA XS L and FEMA XS I.
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Natural Systems Design 45
November 14, 2024
Figure 32. Comparison of change in modeled 100-year WSE within the project reach. FEMA XS locations shown
in purple.
Localized increases in water surface elevations range from 0.1 to 0.7 ft in the vicinity of the proposed ELJ
placements, from approximately RM 1.35 to RM 1.8, with the greatest increases occurring in the mainstem and
floodplain between RM 1.5 to RM 1.6. Water surface increases tend to be greater upstream of the proposed
ELJs as water stacks in front of the ELJs, which can lead to local decreases in WSE behind ELJs.
Water surface elevations were averaged across FEMA cross section locations, per FEMA guidance (2022), and
tabulated in Table 7. In this tabular analysis, the greatest increase in WSE is 0.3 ft at FEMA XS K and at an
unlettered cross section between XS K and XS J. WSE change diminishes to zero outside of the proposed ELJ
placements, upstream of FEMA XS L and downstream of FEMA XS I.
JEFFERSON COUNTY DOSEWALLIPS RIVER POWERLINES REACH RESTORATION
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November 14, 2024
Table 7. Modeled average existing, proposed, and change in water surface elevation throughout the
Powerlines reach.
FEMA XS LETTER MODELED 100-
YR WSE (FT)
RISE
(FT)
NOTES
EC PC
Q 87.2 87.2 0.0
P 83.6 83.6 0.0
O 76.7 76.7 0.0 *Last XS upstream of Lazy C development
N 70.2 70.2 0.0
M 66.8 66.8 0.0
UNKNOWN (not lettered, not mapped) 59.6 59.6 0.0 *First XS downstream of Lazy C development
L 58.6 58.6 0.0 *Last XS upstream of proposed ELJs
UNKNOWN (not lettered, not mapped) 54.9 55.0 0.1
K 51.8 52.2 0.3
UNKNOWN (not lettered, not mapped) 46.7 47.1 0.3
J 45.6 45.8 0.2
I 44.6 44.6 0.0 *First XS downstream of proposed ELJs
UNKNOWN (not lettered, not mapped) 41.9 41.9 0.0
H 39.9 39.9 0.0
G 32.5 32.5 0.0
Since our preliminary hydraulic analysis indicates that the proposed restoration actions are anticipated to
increase the Base Flood Elevation (BFE) in the project reach, a CLOMR will need to be obtained from FEMA
before the project can move to construction. Water surface changes are minimal outside of the proposed
project area, and this preliminary analysis shows no significant change in WSE at insurable structure locations
within the Lazy C Development. A full CLOMR analysis using the official FEMA model will be conducted in the
final design phase, and the design will be adjusted as necessary to ensure no rise at the Lazy C. Once the project
is completed, a Letter of Map Revision (LOMR) will need to be prepared and then reviewed and approved by
FEMA.
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Appendix A
Hydraulic Model Existing and Proposed Conditions
Appendix B
Preliminary Design Plan Set
Appendix C
Construction Cost Estimate