8-LakeSwanoDamStudyAlternativeAnalysisReport.pdf

April 2026

Prepared for:

Department of Enterprise Services Grays Harbor College

Prepared by:

ALTERNATIVE ANALYSIS REPORT

Steve Schmitz, PE Jessica Martin, PE

Tyler Giles, PE

Lyle Stone, PE GE Becca Miller, PE

David Conlin, PWS

GHC – LAKE SWANO DAM STUDY

DES Project #: 2026-053

GHC – Lake Swano Dam Study (2026-053) Alternatives Analysis Aberdeen, Washington April 1, 2026

CONTENTS

1 Project Overview

1.1 Introduction

1.2 Project Team

1.3 Site Location

1.4 Background

2 Overall Considerations

2.1 Site Access

2.2 Utilities

2.3 Geologic Setting

2.4 Sensitive Areas and Habitat

2.4.1 Water Resources and Fish Use

2.4.2 Flood Hazards and Floodplains

2.4.3 Wetlands

2.4.4 Wildlife

2.5 Hydrologic and Hydraulic Considerations

2.5.1 Alder Creek Watershed and Hydrology

2.5.2 Alder Creek

2.5.3 Lake Swano Dam

2.6 Community Involvement

3 Alternative 1 - Dam Rehabilitation

3.1 Options for Providing Seismic Slope Stability

3.1.1 Ground Improvement

3.1.2 Buttress

3.2 Spillway Capacity

3.3 Permitting

4 Alternative 2 - Dam Removal

4.1 Structural Options

4.1.1 Crossing Size

4.1.2 Three-sided Culvert

4.1.3 Long Span Bridge

4.1.4 Short Bridge

4.2 Restoration

4.2.1 Channel Restoration

4.2.2 Scour and Channel Adjustment

4.2.3 Downstream impacts

4.3 Permitting

5 Alternatives Comparison

5.1 Risk Matrix

5.2 Opportunity Matrix

5.3 Cost Comparison

6 Conclusion 7 References

FIGURES

Figure 1: Lake Swano Embankment Figure 2: Vicinity Map Figure 3: Detour Route Figure 4: Existing Access Road Figure 5: Proposed Access Improvements Figure 6: Existing Utilities Figure 7: Sensitive Areas Figure 8: Lidar Elevation Figure 9: Alder Creek Watershed and Site Photo Locations Figure 10: Alder Creek Longitudinal Profile Figure 11: Alder Creek Upstream of Lake Swano Dam Figure 12: Alder Creek Primary Spillway Outfall Figure 13: Alder Creek Upstream of Beaver Dam Figure 14: Alder Creek at Huntley Street Figure 15: Alder Creek at Fordney Street Figure 16: Confluence of Alder Creek and Chehalis River Figure 17: Ground Improvements Figure 18: Deep Soil Mixing Equipment Figure 19: Buttress Figure 20: Three-sided Culvert Example Figure 21: Long Span Bridge Example Figure 22: Short Bridge Example Figure 23: Channel Restoration Option 1 Figure 24: Channel Restoration Option 2 Figure 25: Stream Restoration Plan

TABLES

Table 1: Peak flows for Alder Creek at Lake Swano Dam

APPENDICES

Appendix A – Geotechnical Report Appendix B – Sensitive Areas and Habitat Exhibits Appendix C – Hydraulics and Hydrology Exhibits Appendix D – Structure Alternatives Exhibits Appendix E – Dam Rehabilitation Geotechnical Figures Appendix F – Examples of Dam Removal Appendix G – Rough Order of Magnitude Alternatives Cost

1 PROJECT OVERVIEW

1.1 Introduction

KPFF Consulting Engineers and GeoEngineers have been contracted by Department of Enterprise Services (DES) and Grays Harbor College (GHC) to provide an alternatives analysis for the rehabilitation or removal of Lake Swano Dam. The purpose of this report is to evaluate feasible options for each alternative and present an options matrix for the determination of the preferred alternative. The two alternatives are summarized below:

Rehabilitation of the Dam Options investigated include soil improvement through deep soil mixing, buttressing, and the full removal and reconstruction of the dam to the required level of safety.

Full Dam Removal Complete removal of the dam and replacing it with a large culvert or bridge structure.

This alternative will result in draining and restoring Lake Swano to its natural creek condition.

1.2 Project Team

Owner: Grays Harbor College & DES Hydraulic Engineer: GeoEngineers Geotechnical Engineer: GeoEngineers Environmental: GeoEngineers Structural Engineer: KPFF Consulting Engineers Civil Engineer: KPFF Consulting Engineers

1.3 Site Location

Lake Swano Dam is located on the campus of Grays Harbor College in South Aberdeen, Washington. The Lake Swano Dam is an earth embankment, 24-feet high and 300-feet long, within the Alder Creek watershed. The dam serves as a road embankment with a paved road on the Dam Crest.

Figure 1: Lake Swano Embankment

Figure 2: Vicinity Map

1.4 Background

Department of Ecology’s (DOE) Dam Safety Office (DSO) performed a conditions assessment in 2019 as part of a Periodic Inspection Report for the Lake Swano Dam. Based on their observations, DSO classified the dam to be in Poor Condition, primarily due to insufficient spillway capacity. A downstream hazard classification of Hazard Class 1A-High downstream hazard potential was assigned to the Dam due to the extents of the downstream flood inundation areas. The dam does not have sufficient spillway capacity to pass the runoff from the design step 8 event as required by DSO. The flows resulting from this event would result in overtopping of the embankment and ultimately dam failure causing downstream flooding. The flood area from a dam failure would travel north and northeast approximately 1.2 miles toward the Chehalis River and potentially flood as many as 190 residences.

KPFF was contracted in 2020 to prepare an alternatives analysis for overtopping protection of Lake Swano Dam by either installing additional spillway capacity or by armoring the downstream face of the dam. Armoring the dam resulted in the preferred alternative. Design of this alternative and geotechnical explorations were conducted in 2022 to support the armoring of the downstream face of the dam. Initial observations indicated that the soil was very loose and in poor condition.

A stability analysis was conducted in 2023 for Lake Swano Dam. Field investigations and lab results indicated that the soil of the earthen dam was loose and soft. The analysis concluded that the dam does not meet the standard factor of safety for seismic events or slope stability and is at risk of failure.

The dam was constructed in 1948 by the landowner at the time to access a gravel pit. Given the nature of the original construction of the dam and lack of engineering design, the core of the dam consists of different materials with inadequate compaction, which ultimately do not meet design standards. To address this issue, rehabilitation of the dam and full dam removal have been evaluated as alternatives and addressed in this report.

2 OVERALL CONSIDERATIONS

2.1 Site Access

Lake Swano Dam is the main access point to two buildings (Buildings 1800 and 1900) at Grays Harbor College and is crucial to the overall connectivity of the campus. Regardless of the alternative chosen, access via the dam will not be feasible during construction and an alternative route is necessary to maintain access to the buildings. There is an existing gravel road to the north that connects to West Griffith Street. A detour route utilizing this existing access will be created for staff, faculty, and emergency vehicles during construction. This proposed detour is depicted in Figure 3 below.

Figure 3: Detour Route

The condition of the existing gravel access is not suitable for this level of use, and improvements will need to be made prior to beginning work on the dam. Improvements to the existing gravel road would include straightening the road alignment, adjusting the vertical profile, and widening the road section to at least 24-ft to accommodate two-way traffic and emergency vehicles. Figure 4 shows the existing condition of the access road, and Figure 5 shows the conceptual improvements proposed. There are existing overhead powerlines and power poles located along the access road. These will need to be relocated as part of the road realignment to avoid the steep slopes and critical areas along the west side of the access road. A portion of this access is within City of Aberdeen right-of-way and design will need to be coordinated with the city. Depending on disturbance limits, temporary construction easements may be required for the reconstruction of this access road.

2.2 Utilities

The dam and associated roadway provide a pathway for utilities to serve the eastern portion of the campus. These utilities include an 8-inch water main owned by the City of Aberdeen, a 4-inch gas line owned by Cascade Natural Gas, a 3-inch conduit for power owned by Grays Harbor PUD, and telecommunication lines. Regardless of the alternative chosen, maintaining the existing utilities in the dam will not be feasible during construction, and an alternative route is necessary to maintain required service. Temporary utility connections will include an 8-inch water main constructed through the project site to provide domestic water and fire protection for the buildings. This line is anticipated to be used only during construction and a new watermain installed within the rehabilitated dam or connected to a proposed bridge structure. Likewise, dry utilities including telecommunication and power will require temporary relocation and reestablishment following reconstruction/removal of the dam.

Sanitary sewer serving the 1800 and 1900 buildings will be unaffected by either alternative.

Currently sewer for the buildings is collected at a centralized septic tank and sent north to the City of Aberdeen's municipal system via the existing access road discussed above.

Figure 4: Existing Access Road Figure 5: Proposed Access Improvements

Figure 6: Existing Utilities

2.3 Geologic Setting

The project site is located at the south side of the Chehalis River Alluvial Delta which extends to Grays Harbor and forms the flat lowlands of South Aberdeen. The hillsides south of the delta are mapped as sedimentary rock and denser terrace deposits. The areas at the project site are mapped as alluvial soils.

Our current understanding of the soils at the site are based on review of two borings advanced by South Sound Geotechnical in August of 2022, which are consistent with the geologic mapping. These borings indicate that the existing dam embankment consists of loose or soft sand and clay. The dam foundation bears on soft alluvial soils (clay). Below the alluvial soils, one of the two explorations encountered sandstone bedrock at a depth of 26 feet.

The embankment soils are loose and do not appear to have been placed or compacted with engineering controls. The alluvium consists predominately of soft clay. This material would be relatively impermeable but also provide little bearing resistance.

The bedrock is relatively shallow when considering how deep the alluvial soils can be within the Grays Harbor area. Although only one exploration encountered the bedrock, both explorations were relatively shallow and the geologic mapping suggests that bedrock would be present across the site. While it is unlikely that bedrock is shallow enough to practically support a dam foundation, it is located such that it could provide efficient support to drilled bridge foundations.

2.4 Sensitive Areas and Habitat

Sensitive areas, including streams and other water bodies, flood hazard areas and floodplains, and wetlands, documented by resource agencies and/or confirmed in the field are depicted on Figure 7. The stream alignment of Alder Creek downstream of the Lake Swano Dam appears to be inaccurately mapped by most data sources we consulted.

Figure 7: Sensitive Areas

Figure 8 includes a lidar elevation model of the Alder Creek area from Lake Swano to its mouth, which provides a useful visual aid to identify the actual Alder Creek alignment based on topography; a distinct stream channel alignment can be seen in this figure, represented as lower elevations depicted in greens and blues in contrast to the surrounding floodplain areas in yellow and higher ground in red.

Figure 8: Lidar Elevation

2.4.1 Water Resources and Fish Use

Alder Creek and Lake Swano are considered fish-bearing waters, though anadromous fish are not able to pass upstream of the existing dam (WDNR 2026a). Lake Swano may have been historically and/or recently stocked with gamefish, and resident trout could also occupy the lake and the reach of Alder Creek upstream of the dam. According to the Washington Department of Fish and Wildlife (WDFW) fish passage database, the tidegate at the mouth of Alder Creek is a total fish barrier, there are several culverts of unknown barrier status and/or partial fish barriers between the mouth and Lake Swano dam, and the dam itself is not included in the database but is unpassable by fish (WDFW 2026a).

Downstream of the dam, Alder Creek is considered documented rearing habitat for coho salmon (Oncorhynchus kisutch) and has presumed presence of rainbow trout (Oncorhynchus mykiss) (WDFW and NIFC 2026). Upstream of the dam, Lake Swano and/or Alder Creek are considered documented spawning habitat for coho salmon and documented presence of largemouth bass (Micropterus nigricans) (IBID).

2.4.2 Flood Hazards and Floodplains

Within the alluvial delta of the Chehalis River, most of the land surrounding Alder Creek is considered Zone X due to existing levee protections (FEMA 2026). There are a few Zone AH, 1 percent annual chance flood, areas near the lower 0.3 miles of Alder Creek, prior to its confluence with the Chehalis River through a tidegate. There are no flood hazards or floodplains at the site of the existing dam nor further upstream.

2.4.3 Wetlands

There are several freshwater forested/shrub wetlands documented in the general vicinity of Alder Creek within the developed portions of the City of Aberdeen (USFWS 2026).

Most of these wetlands are either in undeveloped parcels surrounded by urban residential development or appear to be relict data and have since been developed and are likely no longer present.

Much of the area immediately downstream of the Lake Swano Dam is forested with a few mapped wetlands. Field observations confirm that a large, forested floodplain wetland area is present along the 0.65 miles of Alder Creek immediately downstream of the dam. This floodplain wetland area was flooded during fieldwork in December 2025 and is bounded by South Lewis Street to the east and State Route (SR) 105 to the west.

The forested floodplain is dissected by West Huntley Street, West Harriman Street and Fordney Street road fill prisms and becomes notably narrower and more confined downstream of Fordney Street. From there to the mouth of Alder Creek at the Chehalis River, the riparian corridor of Alder Creek is variable in width, typically between 40 to 150 feet, surrounded by residential development, and characterized by limited shrub and forested floodplain and riverine wetlands.

2.4.4 Wildlife

No specific wildlife use of the site is identified by WDFW (WDFW 2026b). The site is likely used by a variety of forest- and pond/lake-dwelling species. Due to the lake’s location near the transition between undeveloped forest lands to the south and urban development to the north, the site is likely used by both human-commensal species (such as deer, squirrels, robins, etc.) as well as native species more adapted to undeveloped areas. The lake itself may host native or introduced species adapted to lacustrine habitats, such as turtles, ducks and amphibians. Turtles have been reportedly observed in the lake; it is unlikely these are the Northwestern Pond Turtle (Actinemys marmorata), currently proposed for listing under the Endangered Species Act, and more likely to be either native painted turtles (Chrysemys picta) or non-native and invasive red-eared slider (Trachemys scripta).

2.5 Hydrologic and Hydraulic Considerations

2.5.1 Alder Creek Watershed and Hydrology

The Lake Swano Dam impounds Alder Creek to create the Lake Swano reservoir. Alder Creek originates approximately 1.4 miles southeast of the dam, in the foothills above South Aberdeen and Cosmopolis (Figure 9). Downstream of the dam, Alder Creek flows north through South Aberdeen approximately 1.2 miles to its confluence with the Chehalis River.

Figure 9: Alder Creek Watershed and Site Photo Locations

We obtained 2019 Olympic Peninsula LiDAR data from the Washington State Department of Natural Resources Lidar Portal (WDNR 2026b) which was used to delineate the watershed upstream of Lake Swano Dam. The resulting watershed has a drainage area of 0.57 square miles. Land cover within the watershed is primarily forested, accounting for approximately 83 percent of the basin area. The remaining 17 percent of the basin is a mix of open pasture and residential neighborhoods. The watershed receives 86.4 inches of annual rainfall (USGS 2026).

The project site is located within United State Geologic Survey (USGS) Hydrologic Region 4 for the state of Washington (southwest coastal and lower Columbia River) (Mastin et al. 2017). Region 4 regression-based estimates for peak flows for Alder Creek are included in Table 1.

Table 1: Peak flows for Alder Creek at Lake Swano Dam

Recurrence Interval (Years) Peak Flowrate (cfs)

2 39.2

10 73.5

25 91.2

50 105.1

100 120.4

500 153.4

2.5.2 Alder Creek

The longitudinal profile of Alder Creek is shown in Figure 10. Alder Creek originates in forested headwaters upstream of Lake Swano Dam and flows toward its confluence with the Chehalis River through a progressively lower-gradient system.

Figure 10: Alder Creek Longitudinal Profile

Upstream of Lake Swano Dam, Alder Creek flows through a steep, forested headwater reach with the steepest channel slopes averaging approximately 3.8 percent. As the creek approaches the reservoir, the valley widens to roughly 60 feet and the channel slope decreases to approximately 1 percent. This 1 percent reach provides the primary flow and sediment supply to the impounded section of Alder Creek.

Figure 11: Alder Creek Upstream of Lake Swano Dam

Lake Swano Dam impounds Alder Creek for approximately 1,200 feet upstream of the dam, creating an approximately 4-acre reservoir. Nice Creek, a tributary to Alder Creek, enters the system within the reservoir footprint. The historic channel slope and morphology within the impounded reach are not known at this time. Based on the longitudinal profile, the reach-average slope spanning the upstream and downstream extents of the reservoir is approximately 1 percent.

The design team conducted a field visit on January 22, 2026 to collect depth soundings in areas accessible by boat (approximately the downstream 500 feet of the reservoir).

Water depths ranged from approximately 12 feet near the downstream end of the reservoir to about 7 feet approximately 500 feet upstream. A sediment depth measurement was also collected at the upstream extent of the depth measurements, and sediment depth was estimated to be approximately 5 feet. Based on the field measurements and the reach-average longitudinal profile, sediment depths within the reservoir are preliminarily estimated to range from approximately 5 to 10 feet.

Alder Creek exits the dam through a primary spillway through an outlet culvert that discharges into a concrete energy dissipation structure (Figure 12). Downstream of this structure, Alder Creek flows approximately 1,500 feet through a forested corridor before reaching a road crossing at Huntley Street.

Figure 12: Alder Creek Primary Spillway Outfall

The design team walked the reach downstream of the dam on December 11, 2025 to observe channel and floodplain conditions. The first 1,000 feet downstream of the dam consists primarily of second-growth forest and includes a trail that runs parallel to the creek and a pedestrian footbridge crossing approximately 200 feet downstream of the outlet. Channel slopes average approximately 0.6 percent. Evidence of beaver activity, including a channel spanning beaver dam, was observed within this reach (Figure 13).

Bankfull channel widths ranged from approximately 14 to 15 feet, with a representative width of 15 feet used as the starting point for alternatives analysis (Figure 10). Bankfull channel depths varied between 1.5 and 2 feet. The channel appeared somewhat disconnected from the adjacent floodplain, although localized areas of greater connectivity were observed along the left bank and floodplain.

Figure 13: Alder Creek Upstream of Beaver Dam

Channel slopes decrease downstream of Huntley Street, transitioning to a slope of approximately 0.1 percent over the 1.2-mile reach approaching the Chehalis River. In this low-gradient reach, Alder Creek exhibits markedly different channel conditions and appeared backwatered by downstream conditions (Figure 14).

Figure 14: Alder Creek at Huntley Street

Downstream of Huntley Street, Alder Creek flows through residential neighborhoods and 11 additional road crossings before reaching its confluence with the Chehalis River (Figure 15 and Figure 16). These neighborhoods were developed within the Chehalis River floodplain but are now separated from the river by a 4.2 mile-long levee system along the left bank of the Chehalis River managed by the City of Aberdeen. A tidegate at the confluence with the Chehalis River allows water to leave Alder Creek when downstream water levels are low and closes as tidal or river stages increase. The nearest National Oceanic and Atmospheric Administration (NOAA) tide gauge (Station 9441187 in Aberdeen) records tidal fluctuations ranging between +8.5 feet at mean higher high water to −1.6 feet at mean lower low water (NAVD88). Downstream of Huntley Street, the LiDAR profile of Alder Creek ranges from approximately 5 to 7 feet (NAVD88), suggesting that the reach may be subject to backwater from tidal conditions, elevated stages in the Chehalis River, or closure of the tide gate. Evidence of this was observed during the December 2025 site visit, which coincided with high tide and elevated Chehalis River stages. Standing water was observed in the roadside ditches adjacent to Alder Creek downstream of Huntley Street.

Figure 15: Alder Creek at Fordney Street

Figure 16: Confluence of Alder Creek and Chehalis River

Dam removal alternatives will need to consider potential sediment release from the reservoir, grade transitions at the former dam site, and the influence of tidal and backwater conditions on downstream channel stability and water surface elevations.

2.5.3 Lake Swano Dam

Lake Swano Dam is an earth embankment that is 24 feet high and 300 feet long. Under typical water surface elevations, when only the primary spillway outlet culvert is flowing, the reservoir covers approximately 4.0 acres and stores 60 acre-feet of water. When flow begins to spill through the auxiliary outlets, the reservoir has a surface area of 4.2 acres and a volume of 64 acre-feet. When the reservoir is filled to the dam crest elevation the reservoir would cover approximately 5.2 acres and hold 78 acre-feet (DSO, 2020).

Lake Swano Dam includes a primary spillway outlet and an auxiliary outlet. The primary spillway outlet is located within a concrete weir drop structure on the west side of the reservoir. The primary outlet is a 150-foot-long corrugated metal pipe (CMP) slip-lined with a 24-inch HDPE culvert that outfalls into a concrete energy dissipation structure.

The primary outlet flow rate ranges from 25 cfs at low pool elevation to 47 cfs at the maximum pool elevation (DSO 2020).

The auxiliary spillway consists of three 100-foot long 48-inch CMPs slip-lined with 39-inch HDPE culverts that outfall onto a riprap pad at the base of the dam. The auxiliary outlets are located in the center of the dam and begin to flow when lake levels are approximately 1.8 feet above the top of the primary spillway concrete weir. The flow rate of the auxiliary outlet reaches 338 cfs at the maximum pool elevation (DSO 2020).

In addition to the primary and auxiliary outlets there is a 12-inch polyvinyl chloride (PVC) pipeline located near the center of the dam that provides controlled withdrawal of water below the principal outlet elevation. The pipeline conveys water 150 feet to a concrete vault at the toe of the dam, allowing operators to lower the reservoir below the elevation of the primary spillway weir crest when needed.

Due to the dam’s hazard classification and the population at risk downstream, the outlets must pass the runoff from a Design Step 8 inflow design flood (DSO 2020). Hydrologic analysis conducted by the DSO resulted in an inflow design flood of 437 cfs. Flood routing through the reservoir provides minimal attenuation, resulting in a combined peak flow of 434 cfs through the outlets and over the dam (DSO 2020). This is greater than the capacity provided by the outlets and would result in flow overtopping the dam embankment. To be in compliance with DSO, a dam retrofit alternative would need to provide additional outlet capacity or armor the downstream face of the dam to withstand overtopping flows.

Dam removal would require evaluating the fate of sediment stored within the reservoir that has accumulated over the 78-year life of the dam. We estimated sediment accumulation using January 2026 field measurements and the longitudinal profile of Alder Creek. These data indicate approximately 5 to 10 feet of impounded sediment stored in the reservoir. We divided the reservoir into eight polygons and used LiDAR-derived side slopes to estimate the cross-sectional area of the sediment deposit. Using an average end-area method, we roughly estimated the stored sediment volume at about 22,000 cubic yards. This estimate is preliminary and intended for planning purposes only and is not to be used for analysis of downstream consequences or design.

2.6 Community Involvement

Grays Harbor College and KPFF hosted a public meeting on Monday March 16, 2026, to inform interested individuals about the project and learn how the area is used. During the meeting KPFF presented a brief history of the dam and an overview of proposed alternatives discussed in this report. Following the presentation of engineering alternatives the design team asked for community feedback. Items of note include a strong desire by attendees to maintain and improve the trail around the lake. Additional concerns related to wildlife and aquatic species, particularly a pair of domestic turtles that were reportedly released into the lake several years ago. There were also several comments expressing the desire to maintain the lake from an aesthetic perspective.

The Lake Swano interpretive trail is an important part of the college and the community. This was reaffirmed during the public outreach meeting. As noted in other sections of this report, GHC and KPFF assured the community that maintaining access to the area via the existing trail or a new improved trail is a consideration in all alternatives. During construction there may be opportunities to improve sections of the trails.

3 ALTERNATIVE 1 - DAM REHABILITATION

Rehabilitation of the dam would require modifications to bring the dam up to current design standards. Design standards and design criteria are established by the Washington State Department of Ecology, Dam Safety Office.

Lake Swano Dam is currently classified as Hazard Class 1A – High Hazard due to the potential for a dam break to impact as many as 190 residences downstream. This is the highest hazard category considered by DSO and therefore it requires the highest design criteria. Using criteria from DSO this is likely a Design Step 10 indicating that the structure must consider the highest loading possible, in other words, the system must pass the largest rainstorm theoretically possible and withstand the largest earthquake theoretically possible. Given the local climate conditions and the proximity to Cascadia Subduction Zone offshore, these are both relatively high loading conditions.

Our geotechnical analysis of the dam indicates that it meets stability criteria under normal conditions. Under seismic conditions our analysis calculates a factor of safety below 1.0, which indicates that the dam would start to deform during an extreme event. Additional analysis would be required to quantify the deformation and determine if it would be likely to result in a breach.

However, based on the high loading, the type of failure predicted (through the foundation) and our experience with similar analyses, in our opinion it is unlikely that the results of additional analysis would be limited to “safe” levels of deformation.

The 2019 Inspection report from DSO analyzed the spillway at a Design Step 8 and concluded that the spillway outlet was undersized and that under a design storm the embankments would be overtopped. Approximately 100 cfs of additional outlet capacity is required to pass the design flow.

This analysis at Design Step 8 does indicate that an overall lower design level could be proposed. However, based on our review it is unlikely that considering this lower design level for seismic stability would have significantly different results.

3.1 Options for Providing Seismic Slope Stability

Methods to achieve seismic slope stability are to either limit total deformation of the embankment during a seismic event or to create an embankment large enough so that the predicted deformation is unlikely to result in a breach and release of the reservoir. Two common techniques are discussed below.

3.1.1 Ground Improvement

The purpose of ground improvement is to strengthen soil in place. This can be effective in situations where the natural foundation soils are relatively weak. The soils in the foundation are clays, which limits the type of effective ground improvement to cementitious methods such as Deep Soil Mixing. The DSM technique involves using large equipment to mix cement into the existing soil. Once the cement has set, this creates a soil-cement mixture that is significantly stronger than the in-situ soil greatly increasing the stability of the embankment foundation.

Our preliminary analysis indicates that a treatment area approximately 40 feet below the surface and extending about 80 feet from the dam centerline would be required to provide the required stability. Based on geotechnical assessments showing variable depth to bedrock, adjustment may be necessary once additional geotechnical data is obtained. The extents of the ground improvement along the cross section of the dam are shown by the gray area in Figure 17. Examples of specialized equipment required for deep soil mixing are depicted in Figure 18.

Figure 17: Ground Improvements

Figure 18: Deep Soil Mixing Equipment

We estimate a cost on the order of $2.6M for the deep soil mixing subcontractor. This would be in addition to the cost of replacing the spillway and all other ancillary improvements that would still be required. In addition to this base material cost, the site would need to be flattened to create a temporary work surface, all utilities through the embankment would need to be relocated during construction, and given the caustic nature of the soil cement mixture, all assumed temporary erosion and sediment control (TESC) costs should be increased by a factor of two or more.

3.1.2 Buttress

Constructing a buttress increases the width of the embankment. Based on our analysis the buttress may be of limited benefit as it does not address the underlaying weak foundation soils. Our analysis considered a buttress that was roughly equivalent in size to the existing dam structure. This had only a limited improvement to the overall factor of safety as it provided no improvement to the foundation soils. In order to function efficiently, the buttress may require additional improvements including excavation and replacement of some foundation soils.

One advantage that a buttress could have that is not captured in this level of analysis is the ability to withstand larger embankment deformations without initiating a full breach.

This is difficult to quantify and beyond the scope of this study. It’s worth noting that a solution that is based primarily on buttressing the embankment is likely to still require some level of foundation improvement as well.

Costs associated with buttress fill are the same as those for fill placement and compaction on any large earthwork project. Additional costs associated with lengthening the outlets, downstream impacts, and foundation excavation would also need to be considered.

The cross-sectional view for the extents of buttressing needed along the embankment is shown in Figure 19.

Figure 19: Buttress

3.2 Spillway Capacity

If the dam is reconstructed to address seismic stability through ground improvements or buttressing, the existing spillways would likely need to be replaced or extended to match the reconstructed embankment limits. Given the extent of the anticipated improvements, and the fact that the spillway culverts have already been slip-lined once, replacing the existing spillways would likely be the most practical approach. Replacement would also provide an opportunity to increase spillway capacity by upsizing the culverts or adding an additional auxiliary capacity to meet the additional 100 cfs capacity requirements identified by DSO.

3.3 Permitting

Dam rehabilitation is likely to require in-water work during construction and may necessitate additional embankment fill either upstream into Lake Swano or, more likely, downstream, displacing a portion of Alder Creek and its associated forested floodplain. We find it unlikely that the forested wetland area or any FEMA-mapped floodplains would be affected, but this would need to be verified during design advancement.

In-water work associated with this option is anticipated to require the following permit approvals:

U.S. Army Corps of Engineers (USACE) Section 404 Clean Water Act Permit WDFW Hydraulic Project Approval State Environmental Policy Act (SEPA) Determination City of Aberdeen Critical Areas Permit

For projects requiring a federal permit, or if federal funding is obtained for the project, the following additional reviews/approvals may be required:

National Environmental Policy Act (NEPA) Approval Section 7 Endangered Species Act (ESA) Consultation Section 106 National Historic Preservation Act (NHPA) Review Section 401 Clean Water Act Water Quality Certification (WQC) Coastal Zone Management Act (CZMA) Consistency Review

Coordination with the City of Aberdeen has not been completed at this time and additional City permits may be required to authorize construction activities and ensure public safety.

In general, permitting for the dam rehabilitation option is anticipated to be relatively straightforward as the proposed project would maintain the existing condition for the most part.

Temporary impacts to water resources during construction and permanent displacement of existing stream with additional embankment fill would likely require compensatory habitat mitigation for compliance with the federal Clean Water Act and City’s Critical Areas Ordinance, which may complicate permitting somewhat. Mitigation would incur additional project costs and it may be challenging to identify appropriate mitigation in the same watershed.

Because the existing dam provides limited functional utility – i.e., it is not providing water supply or power generation and only limited flood control function – and currently acts as a fish passage barrier, this option is anticipated to be met with more resistance from resource agencies and local tribes when compared to the dam removal option. Limited outreach and coordination with resource agencies and tribes has been completed thus far during alternatives analysis but, if opposed, obtaining environmental permits for a project that resource agencies and tribes do not support could be the most significant permitting hurdle for this alternative.

4 ALTERNATIVE 2 - DAM REMOVAL

Full dam removal would result in draining of Lake Swano and restoration of Alder Creek through this reach. Although the dam will be removed, road access will need to be maintained, therefore requiring the placement of a structure at the creek crossing. Various structure alternatives have been evaluated and summarized in the following sections. Each option allows for full restoration of the creek and is in compliance with WDFW fish passage guidelines. Evaluation of scour countermeasures has not been performed and analysis would be required at future phases.

The preferred option for structure selection is a long span bridge. As noted in the following sections, this structure provides several advantages. These include stream lateral migration flexibility, overall constructability, and the ability to better manage lake levels and sediment transfer during construction.

4.1 Structural Options

4.1.1 Crossing Size

Field indicators suggest that the downstream bankfull width of Alder Creek is approximately 15 feet. Based on the Washington Department of Fish and Wildlife (WDFW) Water Crossing Design Guidelines, a preliminary minimum hydraulic opening (MHO) can be estimated as 1.2 times the bankfull width plus 2 feet. Using this approach, an initial MHO of approximately 20 feet is suggested as a starting point for crossing size.

This value should be considered a preliminary reference point. Following dam removal, the channel is expected to adjust as stored sediments are redistributed, and natural geomorphic processes are reestablished. Potential adjustments may include lateral migration, aggradation, and localized scour. Scour depths and channel migration limits have not yet been quantified and may ultimately influence the final structure span and foundation design. Depending on the selected structure type and foundation approach, maintaining foundations outside anticipated scour or adjustment zones may require a span larger than the preliminary MHO. Any required scour countermeasures would also need to be located outside the hydraulic opening, which could further influence crossing size.

4.1.2 Three-sided Culvert

A 20-foot wide, three-sided steel arch or precast concrete box with a length of 55 feet is proposed for the culvert option. The max height of the steel arch would be approximately 12 feet, while a precast box height would be approximately 10 ft. The arch or precast box would be supported on cast-in-place concrete stem walls approximately 7 ft high.

Wingwalls and headwalls would be constructed at each end of the culvert to support the roadway fill prism. The structure is designed to be perpendicular to the road centerline.

Due to the poor nature of the surrounding soils, the stem walls will be supported on a series of driven piles extending to competent material below. Preliminary estimates assume piles spaced between 4 and 5 feet apart.

This option requires extensive excavation to reach the bottom of footing. Excavation includes removal of the existing dam material as well as excavation of sediment buildup behind the dam. A suitable location to deposit the excavated fill would need to be provided. Competent backfill would be placed above the culvert. Lake Swano must be drained and have stream bypass in place before construction of the culvert can occur.

Scour depth and lateral migration have not yet been determined for this site. Ultimately the culvert and wingwall lengths will be determined by the scour requirements.

Figure 20: Three-sided Culvert Example

4.1.3 Long Span Bridge

The long span bridge alternative proposed is approximately 110-feet long, 24 feet wide single-span bridge over the restored stream channel. Precast concrete girders are recommended for the superstructure due to their low cost compared to steel girders, ease of construction, and lower long term maintenance costs. A 25-foot approach slab will be located at each end of the bridge.

The bridge has a short abutment wall placed on deep foundations (e.g. drilled shafts or driven piles) extending down to the firmer soil below. Poor soil conditions on the dam make shallow foundation alternatives (e.g. spread footings) infeasible.

Extensive excavation is required to construct the bridge and restore the stream channel.

However, the excavation for this option could be done in stages that could help reduce overall construction costs (i.e. building the bridge before having to drain Lake Swano).

A suitable location to deposit the excavated fill would need to be provided. Competent backfill would be placed behind the abutment walls. The wider opening under this bridge may help mitigate some of the scour and lateral migration potential. Scour depth and lateral migration have not yet been determined for this alternative. Potentially scour countermeasures would be required.

Figure 21: Long Span Bridge Example

4.1.4 Short Bridge

The short bridge alternative proposed is a 50 foot long, 24 feet wide single-span bridge over the restored stream channel. Precast concrete girders are also recommended for the superstructure due to their low cost compared to other girder types, ease of construction, and low long term maintenance costs. A 25 feet long approach slab will be located at each end of the bridge.

The short bridge is proposed to be placed on tall walls roughly 12 to 15 feet high. The benefit of the tall walls is the lower amounts of dam/embankment excavation required compared to the culvert and long bridge options. Soldier pile walls with tiebacks are proposed as the wall type, as they can be constructed without having to excavate the fill behind the walls. Tiebacks are required due to the poor nature of the existing dam material. Potentially the tieback length may be quite long to develop sufficient strength.

Walls will need to be constructed prior to the placement of the superstructure.

Excavation of the restored stream channel will be done in conjunction with wall construction. A suitable location to deposit the excavated fill would need to be provided.

Exact wall placement is subject to scour and lateral migration considerations. Scour countermeasures may be required.

Figure 22: Short Bridge Example

4.2 Restoration

4.2.1 Channel Restoration

Removal of the Lake Swano Dam would restore longitudinal connectivity along Alder Creek and reestablish natural processes through the existing reservoir footprint.

Following dam removal, the former lakebed would transition to a stream and floodplain, requiring some degree of channel reconstruction through the accumulated reservoir sediments.

The design of the restored channel and crossing structure must consider both the initial channel configuration immediately following construction and the geomorphic adjustments expected to occur as the creek reestablishes equilibrium conditions. These adjustments may include channel incision through the stored sediment, lateral migration, and localized scour. Restoration design must therefore balance the level of channel construction performed during dam removal with the role of natural processes in shaping the channel over time.

Preliminary evaluation of the longitudinal profile suggests two potential approaches for reconstructing the channel through the former reservoir footprint following dam removal.

These alternatives primarily differ in the amount of channel excavation performed during construction and the extent to which natural channel adjustment is allowed to occur after dam removal.

Option 1 – Short, Steep Regrade

Under this approach, the reconstructed channel would connect the existing downstream channel to the upstream sediment deposit using a short, relatively steep transition, minimizing excavation within the reservoir sediments (Figure 23). The initial channel would be graded to meet fish passage criteria, with a slope not exceeding 6 percent.

Following dam removal, natural sediment redistribution and channel adjustment would play a larger role in shaping the long-term channel form.

The primary benefit of this approach is reduced construction disturbance. Minimizing excavation would reduce turbidity generation, simplify dewatering requirements, and limit sediment disturbance during construction. However, because the channel would still be adjusting after dam removal, greater uncertainty would remain regarding the final channel geometry at the crossing location and in the abandoned reservoir. Structure sizing would therefore need to account for potential channel adjustments, including lateral migration, aggradation and degradation, and scour.

A variation of this approach could involve deferring installation of the permanent crossing structure for one season following dam removal. Allowing the channel to naturally begin to adjust toward a post-removal morphology could reduce uncertainty in structure sizing and channel geometry. However, this approach would require maintaining temporary road access for a longer period and would likely require a second construction mobilization. Another variation could involve shifting the crossing location slightly downstream to a reach with greater channel stability. These variations were not evaluated further to maintain a simplified comparison among alternatives but could warrant additional consideration during design if the dam removal alternative advances.

Figure 23: Channel Restoration Option 1

Option 2 – Longer Regrade Following Reach-Average Gradient

This option would reconstruct the channel through the former reservoir footprint using a slope consistent with the reach-average gradient observed upstream and downstream of the dam (Figure 24). Constructing a longer and shallower slope regrade would create a more stable channel profile immediately following construction and provide greater predictability in channel geometry and structure sizing.

This approach would likely reduce some of the uncertainty in estimating channel hydraulics post-construction, anticipated scour depths, and the required hydraulic opening of the crossing structure. However, the longer regrade would require substantially greater excavation of reservoir sediments, resulting in higher turbidity potential during construction, more complex dewatering requirements, and a larger disturbance footprint within the former reservoir, and require more upfront site restoration. The reconstructed channel would still undergo adjustment over time as sediment transport processes are reestablished.

Figure 24: Channel Restoration Option 2

For purposes of developing preliminary rough order-of-magnitude (ROM) cost estimates, Option 2 was carried forward for the alternatives evaluation (Figure 25). This approach provides a more defined channel alignment and grade through the former reservoir footprint, allowing excavation quantities, channel dimensions, and structure sizing to be estimated with greater confidence at this stage of design. In contrast, Option 1 relies more heavily on natural sediment redistribution and post-removal channel adjustment, which introduces greater uncertainty in predicting final channel geometry and potential maintenance needs within the crossing and regraded reach as well as within the downstream drainage. Evaluating the long-term performance of Option 1 would require a more detailed sediment transport analysis and reservoir sediment characterization, which is beyond the scope of this alternatives evaluation.

Figure 25: Stream Restoration Plan

4.2.2 Scour and Channel Adjustment

Detailed evaluation of scour depth and lateral migration limits has not yet been completed and will need to be addressed during subsequent hydraulic and geomorphic analyses. These factors will influence foundation embedment depths, wingwall and abutment wall lengths, and the potential need for scour countermeasures.

4.2.3 Downstream impacts

Removal of Lake Swano Dam would alter downstream flow and sediment dynamics.

While we did not observe areas of localized flooding or debris accumulation during our field reconnaissance, potential impacts to downstream neighborhoods would need to be evaluated. Sediment released during dam removal could deposit at downstream road crossings as it moves through the low-gradient, backwater-influenced reach. Potential mitigation strategies could include expanding the downstream floodplain and its associated storage capacity and placing engineered wood structures to capture and retain sediment before it reaches downstream neighborhoods. Establishing a temporary maintenance budget to manage sediment flushed following removal could also help address localized deposition.

4.3 Permitting

Dam removal would require in-water work during construction, restoration of Alder Creek through the extents of stream regrading, and construction of a new crossing structure. Permits that would be required for this option would largely be the same as those required for the dam repair alternative, with a few exceptions noted below. Although the same permits would generally be required, the nature of the permit reviews would be notably different because the end result of this project would be stream restoration as opposed to stream displacement.

Additionally, effects on downstream flood hazards are anticipated to be one of the focal concerns and may trigger additional analyses and permit reviews.

This option is anticipated to require the following permit approvals:

U.S. Army Corps of Engineers (USACE) Section 404 Clean Water Act Permit WDFW Hydraulic Project Approval State Environmental Policy Act (SEPA) Determination City of Aberdeen Critical Areas Permit City of Aberdeen Flood Hazard Permit

For projects requiring a federal permit, or if federal funding is obtained for the project, the following additional reviews/approvals may be required:

National Environmental Policy Act (NEPA) Approval Section 7 Endangered Species Act (ESA) Consultation Section 106 National Historic Preservation Act (NHPA) Review Section 401 Clean Water Act Water Quality Certification (WQC) Coastal Zone Management Act (CZMA) Consistency Review

If hydraulic modeling indicates that the project would change FEMA flood mapping downstream of the existing dam, a Conditional Letter of Map Revision (CLOMR) and/or Letter of Map Revision (LOMR) may also be required.

Coordination with the City of Aberdeen has not been completed at this time and additional City permits may be required to authorize construction activities and ensure public safety.

In general, permitting for the dam removal option is anticipated to be slightly more complex for this alternative as compared to the dam rehabilitation option, primarily due to the potential downstream impacts.