Attachment K Geotechnical Reports.pdf

10250 SW Greenburg Road, Suite 111 | Portland | Oregon 97223 Page | 1 of 9 Main (503) 452-1100 | CornforthConsultants.com

April 12, 2023 3051

Mark Wilcox Senior Project Manager – Levee Ready Columbia Multnomah County Drainage District 1880 NE Elrod Drive Portland, Oregon 97211

Supplemental Geotechnical Report MCDD Columbia Slough South Levee Portland, Oregon

Dear Mr. Wilcox, This letter summarizes the work completed at a marginally acceptable reach of the Columbia Slough South Levee, within the levee system administered by the Multnomah County Drainage District (MCDD). This supplemental geotechnical study includes a review of the 2017 FEMA accreditation study results in this area, updated seepage and stability analyses, refined damage extents, and evaluation of mitigation concepts. Results of this study are summarized in the following report.

BACKGROUND

MCDD has been working with multiple stakeholders over the last decade to complete the FEMA levee certification process for their levee system. Cornforth Consultants, Inc. (CCI) has assisted in the subsurface investigation and engineering analysis of the existing levee embankments. During this process, CCI identified a segment of the MCDD levee along the Columbia Slough (Reaches M-23 and M-24) which does not meet compliance criteria within the FEMA guidance document 44 CFR 65.10.

The calculated factor of safety (FS) for sliding stability at the riverward slope within these two reaches is below the minimum of 1.4 during a 100-year flood event, primarily due to an over-steepened riverward levee embankment and apparent loss of material from the original levee toe.

These levee reaches are currently close to meeting the FS criteria and are documented as providing adequate freeboard at the 1-percent annual-chance flood elevation. This levee section was designated Project IGA 5 by the Levee Ready Columbia (LRC) Intergovernmental Agreement (IGA) Board. A project overview with conceptual mitigation options, quantities, and cost estimates was developed for the IGA Board. Based on the recommendations of this project overview, MCDD retained Westlake Consultants to conduct a ground survey of the levee and a bathymetric survey in the Columbia Slough completed in 2022. This updated survey information was provided to CCI for use in refining the original 2017 levee stability analyses and evaluating the various mitigation options developed for the IGA board.

Attachment K Supplemental Geotechnical Reports

Columbia Slough South Levee Supplemental Geotechnical Report 3051 Portland, OR

April 12, 2023 Page | 2 of 9

SCOPE OF WORK

This supplemental report for the MCDD Columbia Slough South Levee includes work on the following tasks:

1) Review the additional survey data provided by Westlake. Update the 2017 analysis typical cross sections in reaches M-22, M-23, and M-24.

2) Review additional cross sections every 25 feet through the area where new bathymetry/survey data has been obtained to help refine mitigation extents.

3) Perform seepage and stability analyses at three additional cross sections to supplement the conceptual mitigation design work.

4) Review the mitigation alternative proposed as part of the of the project overview/business case for the site and provide input including potential limits of mitigation, approximate dimensions of mitigation necessary to achieve FS requirements, and review of long-term monitoring approach.

5) Summarize the results in a supplementary geotechnical memorandum.

Following review of the initial bathymetry, a re-analysis of the 2017 M-24 section was cut from the project scope because revised bathymetry collected by Westlake did not extend to the M-24 typical section. In this area, the levee has no backslope on the landward side due to fill placement over time.

Because the leveed area here is at the same elevation as the levee crest, the risk of flood impacts following a slump on the riverward slope is low.

EXISTING SITE INFORMATION

The Columbia Slough in the project area is bordered by the Peninsula Drainage District 2 (PEN 2) levee to the north and the MCDD levee to the south. Investigations in support of FEMA accreditation of both these levees have taken place over many years and in multiple phases. The results of the various work components related to the current study are summarized in the following subsections.

More detailed information can be found in the 2017 MCDD Levee Engineering Assessment Report and in the business cases developed for the IGA board.

2017 FEMA Accreditation Study Analysis The 2017 study used GeoStudio programs to assess seepage and stability at selected typical sections.

Seepage was calculated under saturated, steady-state conditions using the 1-percent annual-chance flood level recommended by WEST Consultants. As recommended by the SEEP/W manual, the exit gradient at the landward toe of the levee was then averaged over 3 mesh units and compared to the USACE’s recommended maximum exit gradient of 0.5 (EM 1110-2-1913).

The seepage analysis results were then used to supply pore water pressure conditions for the slope stability program, SLOPE/W. This program uses conventional limit equilibrium theory to calculate the factor of safety (FS) for the embankment slopes. A minimum failure surface depth of 10 feet was set as a limiting parameter in the model to ensure that minor, surficial failures, which would not significantly effect freeboard or crest width, would be excluded from the analysis results. Although

April 12, 2023 Page | 3 of 9 shallow failure surfaces may have lower calculated factors of safety, they are not expected to threaten levee integrity and can easily be addressed as part of good maintenance practices. The calculated critical FS for both riverward and landward slopes was compared to the USACE’s recommended minimum FS of 1.4 (EM 1110-2-1913).

The 2017 engineering assessment identified two reaches in the MCDD Columbia Slough South Levee

– M-23 and M-24 – that had a calculated FS less than 1.4 on the riverward slope during the 1-percent annual-chance exceedance flood event, as shown in Table 1 in the Engineering Analysis section. In both reaches, the riverward slope appears to be over steepened, with some loss of material at the levee toe. However, both reaches were very close to meeting the FS criteria and the inboard ground elevation was at or above the 1-percent annual-chance flood elevation, which severely limits the inundation extents in the event of a levee failure. Based on these results, CCI recommended additional monitoring of the area during flood events in the 2017 report.

IGA5 Analysis Following completion of the draft engineering assessment and USACE Portland District’s Feasibility Study of the Portland Metro Levee System, LRC’s IGA Board sponsored development of potential mitigation options for several marginally deficient levee segments. This would allow MCDD to take a more proactive approach to meet levee FS criteria. Reaches M-23 and M-24 were identified as IGA5.

The team developed two conceptual mitigation options, both of which included a 15-ft tall, 2-ft thick riprap blanket constructed at the levee toe to increase stability. Option A included riprap only and stayed within the original levee footprint. Option B included construction of a 10-ft wide bench outside of the riprap blanket to provide space for riparian habitat. This option requires placement of fill beyond the projected design levee toe into the Columbia Slough. The estimated extent of mitigation work in both alternatives was approximately 2,400 linear feet. Aquatic environmental and Section 408 permitting is expected to be required for any construction, particularly construction beyond the original levee toe.

2022 Survey Data Following up on the results of the IGA5 analysis, MCDD authorized collection of ground survey and bathymetry in the potential mitigation area to help evaluate the conceptual mitigation options. This work was completed in 2022. New ground survey was collected from approximately Station 640+00 to 656+50. Bathymetry in Columbia Slough was collected in two phases from approximately Station 641+00 to 665+58. MCDD provided this survey data to CCI for use in the engineering analyses described in the following section. The initial project area in the IGA5 analysis was reduced to approximately 649+50 to 664+25 (approximately 1,500 linear feet) following consideration of levee backslope conditions and available bathymetry area.

ENGINEERING ANALYSIS OF EXISTING CONDITIONS

Updated Seepage and Stability Analyses for Typical Sections Before evaluating the mitigation options, CCI re-ran the 2017 analysis sections using the updated surface information to confirm that the calculated factor of safety was still less than the minimum

April 12, 2023 Page | 4 of 9

USACE recommended FS. As shown in Table 1 below, the change in the calculated FS on the landward side of the levee and on the waterward side of the M-22 section was negligible. To run these analyses, CCI had to import the 2017 sections into an updated version of GeoStudio, which may also account for some of the variation. The calculated factor of safety on the waterward side of M-23 decreased slightly since the surveyed over steepening of the toe was greater than that estimated in 2017. The waterward slope at section 653+30 was identified as the critical geometry for this area.

Table 1: Summary of 2017 and 2022 Analyses along MCDD Columbia Slough South Levee

Reach Station Slope

Factor of Safety

Change (%) 2017 2022

M-22 646+70 Landward 2.26 2.27 0.7

M-22 646+70 Waterward 1.81 1.83 0.9

M-23 653+30 Landward 1.92 1.96 2.0

M-23 653+30 Waterward 1.29 1.23 -4.8

Updated Stability Analysis of Mitigation Areas CCI reviewed new cross sections within the updated ground and bathymetry surfaces space every 25 feet. These cross sections were compared to the typical analysis section at 653+30 to help bookend the likely extents of mitigation work. Following this exercise, an additional section of concern STA 656+80, was identified and included in the seepage and stability analyses for the mitigation options.

Between Station 640+50 to Section 649+50, the levee prism has a similar cross-section. The riverward slope does not appear to have lost material at the levee toe. Levee improvements are not necessary to meet the seepage and stability analysis requirements. Between Stations 649+50 to 655+00, levee geometry is similar to the cross section cut at 653+30, which shows steepening and apparent scour at the riverward levee toe. Downstream of Station 655+00, the apparent scour decreases but the riverward levee slope becomes steeper. A section cut at 656+80 was used as the typical section when evaluating mitigation of the levee between Station 655+00 and 664+20. The additional cross-sections overlaid with the typical critical section are compiled in Appendix A.

EVALUATION OF MITIGATION ALTERNATIVES

After reviewing the new data and selecting the two typical critical sections at 653+30 and 656+80, CCI evaluated the mitigation concepts proposed in the IGA5 business case and recommended modifications as necessary to meet FEMA accreditation requirements. The ground survey sections, the updated bathymetry data received from Westlake Consultants and the hydraulic survey data from West Consultants have been used to develop analytical models in SEEP/W and SLOPE/W from the GeoStudio suite. Generalized soil properties developed in the 2017 field investigation and laboratory testing program were used in seepage and stability modeling. Material properties used in the analysis are summarized in Table 2.

April 12, 2023 Page | 5 of 9

Table 2: Summary of Estimated Soil Properties for Seepage and Stability Analyses

Material Description Unit

Weight γ (pcf)

Friction Angle ф' (degrees)

Cohesion Intercept c'

(psf)

Permeability k (ft/sec)

Permeability Ratio, kh/kv

FILL - silty SAND to sandy SILT 108 30 0 6.7E-06 4

FILL- clean SAND 106 34 0 8.8E-4 4 ALLUVIUM – clayey SILT to silty CLAY 114 27 0 1.1E-05 4

ALLUVIUM – slightly clayey, slightly sandy

SILT

113 28 0 1.7E-05 4

ALLUVIUM- silty SAND to sandy SILT 118 30 0 2.6E-05 4

ALLUVIUM- clean

SAND 116 34 0 9.4E-05 4

RIPRAP 135 40 0 0.03 1

Notes: Material parameters estimated based on available information.

The mitigation alternatives were evaluated using the same methodology described in the section on the 2017 FEMA Accreditation Study Analysis. The 1-percent annual chance (100-year) flood elevation in this section of the Lower Columbia Slough is 31.9-feet (NAVD 88). This flood elevation was developed by WEST Consultants as part of the H&H studies for the FEMA accreditation project.

Evaluation of IGA5 Initial Alternatives The initial IGA5 mitigation concepts and CCI’s modifications to these initial concepts are discussed in the following subsections. All concepts were modelled for both sections 653+30 and 656+80, the two critical sections selected during the cross-section review. The modified mitigation concepts are slightly different for Station 649+50 to 655+00 and 655+00 to 664+20, reflecting the change in levee geometry within the mitigation area.

IGA5.A – Riprap Inlay A 24-inch-thick riprap inlay is constructed on the riverward slope, from the existing levee toe to a height of 15 feet above the existing levee, toe. This would increase resistance to failure at the levee toe, and also reduce the risk of future scour. When analyzed at the critical sections, this mitigation option does not meet the minimum factor of safety requirements. Results of this analysis are shown in Table 3.

IGA5.B – Riprap Inlay with Riparian Bench A rock inlay with a 10-foot bench of levee fill over the 24-inch-thick riprap inlay was evaluated. This bench would support planting and habitat restoration. This fill is susceptible to scour damage over time and is not envisioned as a structural component of the levee. Any short-term support from the

April 12, 2023 Page | 6 of 9 habitat bench should therefore be disregarded in analyzing long-term the seepage and stability. Even assuming short-term support from the habitat fill IGA5.B, this option does not meet the minimum Factor of Safety requirements. Results of this analysis are shown in Table 3.

IGA5.C – Continued Monitoring The area receives a programmed increase in monitoring activity during and after storm events, so that incipient failures in the riverward slope can be addressed promptly. CCI is not aware of any slope stability failures in this area that have required an emergency response. Beyond loss of material via scour at the levee toe, no ground surface deformation associated with slope stability failures were observed in the lidar data. Based on the site history of long-standing good performance during flood events and the cost of any of the other mitigation options, this option remains a reasonable management option in these levee reaches. The factor of safety is the same as the existing conditions shown in Table 1.

Modification of IGA5 Alternatives Using the 2-foot rock inlay mitigation alternative as a starting point, CCI modified the conceptual mitigation for the two critical sections to provide a calculated FS of 1.4. The updated conceptual mitigation options are referred to as IGA5.A.1 and IGA5.B.1

IGA5A.1 – Expanded Riprap Inlay A thicker riprap inlay and a keyed in riprap toe was analyzed to meet FS criteria. The existing levee geometry is not uniform throughout the mitigation area and the riprap inlay geometries transitions slightly downstream of approximately Station 655+00.

Between Station 649+50 and 655+00 the typical mitigation geometry is construction of a riprap inlay into the riverward slope to a height of approximately 23 feet above the existing levee toe. The complete riprap inlay will have a slope of 2H:1V. The inlay is coupled with riprap key extending 5-feet below the existing levee toe. The base of the key is 8 feet wide. The temporary excavation slopes are 1:1 at the front of the key, and 1.5:1 at the back of the key and riprap inlay. The thickness of the riprap inlay varies from 10-feet at the base of the excavation to approximately 5-feet at the top of the repair, measured perpendicular to the levee slope. A 6-inch layer of quarry spalls would be placed at any contacts between the riprap and the levee prism material. The typical geometry for this portion of the mitigation is shown in Figure 1.

Downstream of Station 655+00, the mitigation geometry changes slightly. The riprap inlay height decreased to 22 feet above the existing levee toe. The depth of the key remains unchanged (5-feet), but the key width increases to 10.5-feet. To stay within the projected levee toe and address the risk of a failure in weak levee foundation materials, the angle of the excavation backslope is not uniform. The excavation backslope angle is 1.5H:1V for a height of 8-feet above the base of the riprap key. From there the excavation backslope angle transitions to 1H:1V to the top of the excavation. The thickness of the riprap inlay varies from approximately 11-feet at the base of the blanket, to approximately 2.5 feet at the top of the repair, measured perpendicular to the levee slope. A 6-inch layer of quarry spalls would be placed at any contacts between the riprap and the levee prism material. The top of the inlay forms a 2.5 foot bench projecting into the slough on the outside of the riverward slope but remains

April 12, 2023 Page | 7 of 9 within the projected levee toe. The typical geometry for this portion of the mitigation is shown in Figure 2.

Based on our current analysis, this mitigation option will remain within the projected levee toe between Station 649+50 and 664+20. Class II/Class III riprap has been assumed to be appropriate for this inlay. Further H&H design is recommended to confirm the riprap sizing.

IGA5B.1 – Expanded Riprap Inlay with Riparian Bench This modification adds the 10-foot planting bench to the expanded riprap inlay described above. The 10-foot thick fill bench is placed at a 2H:1V slope outside the riprap inlay. The calculated riverward Factor of Safety for this mitigation is greater than 1.4. However, this habitat bench is not constructed of compacted structural fill, and the additional Factor of Safety benefit of the fill bench should be disregarded when modeling long-term levee performance for FEMA accreditation purposes.

The results of the slope stability analyses have been summarized in Table 3. The minimum Factor of Safety recommended by USACE in EM 1110-2-1913 (USACE 2000) is 1.4 for static, steady state seepage conditions. The conceptual mitigation recommendations are designed to meet this minimum threshold for Factor of Safety. The conceptual excavation and fill volumes for both options are presented in Table 4.

Table 3: Results of Seepage and Stability Analyses

Levee Station Case Description Riverward FS1 Compliance with 44 CFR 65.10 Regulations

653+30

IGA5.A 1.21 No

IGA5.B 1.37 No

IGA5.A.12 1.41 Yes

IGA5.B.13 1.51 Yes

656+80

IGA5.A 1.21 No

IGA5.B 1.39 No

IGA5.A.12 1.42 Yes

IGA5.B.13 1.54 Yes Notes:

1 Minimum recommended value, FS = 1.4, per USACE 2 IGA5.A.1 is a modified version of IGA5.A with different riprap inlay geometry 3 IGA5.B.1 is a modified version of IGA5.B using the riprap inlay geometry created for IGA5.A.1

April 12, 2023 Page | 8 of 9

Table 4: Conceptual Excavation and Fill Quantities for Modified IGA 5 Alternatives

RECOMMENDATIONS

Based on the seepage and stability analysis completed under this task order, we have evaluated the initial mitigation concepts from the IGA Board and made modifications to those concepts to achieve the target Factor of Safety for FEMA Accreditation. The ultimate mitigation solution in these reaches must satisfy the needs of diverse stakeholders, many of whom have varying priorities. In advancing towards a final plan in this area, we recommend that MCDD consider the following advantages and disadvantages of each of the mitigation options.

IGA5A.1 – Expanded Riprap Inlay The riprap inlay in IGA5A.1 will provide an estimated Factor of Safety of at least 1.4 during a 100-year flood event. This meets the stability goals for FEMA accreditation. The mitigation remains within the projected toe of the levee, which reduces permitting requirements. However, it will still require in-water work, disposal of excavated fill and development of staging and access areas to support construction. It also does not include any possibilities for habitat enhancement beyond a minimum 12-inch cap of silty sand over the riprap buttress. If selected, costs for this mitigation option are expected to include design work, permitting, USACE coordination, and public outreach, in addition to the costs of construction.

IGA5B.1 – Expanded Riprap Inlay with Riparian Bench Adding a habitat bench outside of the riprap buttress expands options for improving habitat within the Columbia Slough. The bench would extend beyond the projected levee toe and is expected to require additional permitting and associated permitting costs. However, expanding the habitat bench into the slough and deepening the remaining channel to preserve flow could support other habitat benefits, such as thermal refugia for fish. Benefits of enhanced habitat should be discussed with other project stakeholders and relevant experts.

From a purely geotechnical perspective, the habitat fill bench is not expected to be compacted like structural fill or protected against scour damage and therefore should not be considered an integral structural component of the levee mitigation project. The structural levee mitigation will be the same as option IGA5A.1 above. If selected, costs for this mitigation option are expected to include design work, permitting, USACE coordination, and public outreach, in addition to the costs of construction.

IGA5.C – Monitoring Approach IGA5.C is the functional monitoring alternative currently applied by MCDD at this site. Since the site history does not include any known failures or near-failures, this increased monitoring option

Case Excavation

Quantity (yd3) Structural Fill Quantity (yd3)

Non-Structural (Habitat) Fill Quantity (yd3)

IGA5.A.1 16,500 21,400 2400

IGA5.B.1 16,500 21,400 17,000

April 12, 2023 Page | 9 of 9 remains a viable option. It eliminates the cost of additional design permitting for the development of any constructed mitigation along the Columbia Slough, but it does require ongoing costs for increased observation effort during flood events. It also does not offer any options for improving habitat in the Lower Columbia Slough. If this area is mitigated, man-hours currently expended here during flood events could potentially be employed more effectively elsewhere in the district.

CLOSING

We appreciate the opportunity to assist MCDD with this challenging project. If you have any questions, please call us at (503) 452-1100.

Sincerely, CORNFORTH CONSULTANTS, INC.

Aine Mines, P.E.

Associate Engineer

Devina Borwankar, E.I.T Engineer

CORNFORTH

CONSULTANTS

10250 S.W . Greenburg Road, Suite 111

Portland, Oregon 97223

Phone 503-452-1100 Fax 503-452-1528

MAR 2023

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PROJ. 3051

MCDD Columbia Slough Levee Stability Analyses

Portland, Oregon

Site Plan

MD 6

40+

MD

5+

M D

M D

M D

M D

NORTH

0'

Scale 1" = 120'

120'60' 240'

20 20

MD 6

40+

MD

5+

M D

M D

M D

M D

MAR 3051

PROJ. 3051

FIG.

IGA.5.A.1 Seepage & Stability at 653+30

MCDD No. 1 Levee Assessment Portland, Oregon 2

Riprap

Fill LOOSE to STIFF, silty SAND to SANDY SILT

Fill LOOSE to MEDIUM DENSE, fine to medium SAND

Foundation SOFT to MEDIUM STIFF, clayey SILT to silty CLAY

Foundation VERY LOOSE to MEDIUM DENSE, silty SAND to sandy SILT

Foundation VERY SOFT to STIFF, slightly sandy, slightly clayey SILT

Foundation LOOSE to MEDIUM DENSE, fine to medium SAND

-20-20

-40-40

MCDD WEST

GEOLOGIC/ANALYSIS SECTION - STA. 653+30

1.41

1.42

1.96

YES

YES

YES

SLIP SURFACE CALCULATED FS MEETS REQUIRED FS*

SCALE IN FEET

0 4020

*MINIMUM ALLOWABLE FS=1.4, USACE, EM 1110-2-1913

CALCULATED EXIT GRADIENT AT THE LANDWARD TOE IS <0.1

**RECOMMENDED MAXIMUM EXIT GRADIENT = 0.5, USACE,

EM 1110-2-1913

*MINIMUM ALLOWABLE FS=1.4, USACE, EM 1110-2-1913

NOTES:

1. TYPICAL SECTION ANALYZED FOR REACH M-23 IN DRAFT

ENGINEERING ASSESSMENT REPORT

2. HABITAT BENCH DISREGARDED IN SEEPAGE AND

STABILITY ANALYSIS

Fill Habitat bench material

FEMA 1% ANNUAL-CHANCE FLOOD LEVEL: 31.9 FT

MD-CC-160

(PROJ. 36 FT)

MD-CC-161

(PROJ. 35 FT)

P2-CC-47

(PROJ. 70 FT)

1.5 8 FT

Projected Toe

Original Ground Surface

FIG.

MAR 3051

PROJ. 3051

IGA.5.A.1 Seepage & Stability at 656+80

MCDD No. 1 Levee Assessment Portland, Oregon 3

Fill Habitat bench material

Fill LOOSE to STIFF, silty SAND to SANDY SILT

Fill LOOSE to MEDIUM DENSE, fine to medium SAND

Foundation SOFT to MEDIUM STIFF, clayey SILT to silty CLAY

Foundation VERY LOOSE to MEDIUM DENSE, silty SAND to sandy SILT

Foundation VERY SOFT to STIFF, slightly sandy, slightly clayey SILT

Foundation LOOSE to MEDIUM DENSE, fine to medium SAND

Riprap

-20-20

-40-40

MCDD WEST

GEOLOGIC/ANALYSIS SECTION - STA. 656+80

1.42

2.05

YES

YES

SLIP SURFACE CALCULATED FS MEETS REQUIRED FS*

SCALE IN FEET

0 4020

NOTES:

1. ADDITIONAL CRITICAL SECTION SELECTED FOR ANALYSIS WITHIN REACH M-23

2. HABITAT BENCH DISREGARDED IN SEEPAGE AND STABILITY ANALYSIS

1.5 12.5 FT

FEMA 1% ANNUAL-CHANCE FLOOD LEVEL: 31.9 FT

MD-CC-160

(PROJ. 315 FT)

MD-CC-161

(PROJ. 315 FT)

P2-CC-47

(PROJ. 280 FT)

1.5 10.5 FT

2 1

*MINIMUM ALLOWABLE FS=1.4, USACE, EM 1110-2-1913

CALCULATED EXIT GRADIENT AT THE LANDWARD TOE IS <0.1

**RECOMMENDED MAXIMUM EXIT GRADIENT = 0.5, USACE,

EM 1110-2-1913

Projected Toe

Original Ground Surface

APPENDIX A

ADDITIONAL CROSS SECTIONS

Reach M-23 “typical section” analyzed in 2017. The levee cross section from 653+30 was compared to cross sections cut every 25 feet between Stations 649+75 and 664+25 to help determine approximate mitigation extents

End of cross sections compared to the 653+30 critical section

Downstream of approximately Station 655+00, there is no apparent scour at the riverward toe. The critical section 656+80 was used to mitigation extents instead

10250 SW Greenburg Road, Suite 111 | Portland | Oregon 97223 Page | 1 of 19 Main (503) 452-1100 | CornforthConsultants.com

December 8, 2023 3149

Mark Wilcox

Senior Project Manager – Levee Ready Columbia

Multnomah County Drainage District

1880 NE Elrod Drive

Portland, Oregon 97211

Supplemental Geotechnical Letter Report MCDD Salmon Creek Slough Animal Burrows Portland, Oregon

Dear Mr. Wilcox, This letter summarizes the work completed at a levee reach of the Multnomah County Drainage

District (MCDD) levee impacted by animal burrows along Salmon Creek Slough. Water levels in the slough are controlled by outflow from the Sandy Drainage Improvement Company (SDIC) pump station and backwater from the Columbia River during high flow events. This supplemental geotechnical study includes a review of the 2017 FEMA accreditation study results in this area using updated animal burrow impacts, updated seepage and stability analyses, refined damage extents, and evaluation of mitigation concepts. The results of this study are summarized in the following letter report.

BACKGROUND

MCDD has been working with multiple stakeholders over the last decade to complete the FEMA levee certification process for their levee system. Cornforth Consultants, Inc. (CCI) has assisted in the subsurface investigation and engineering analysis of the existing levee embankments. During this process, CCI identified a potentially deficient segment of the MCDD levee along the Columbia

River/Salmon Creek Slough (Reach M-2). Salmon Creek Slough runs from the SDIC pump station to the Columbia River. It is separated from the Columbia River by a low-elevation peninsula, which is the site of Chinook Landing Marine Park. Extensive animal burrowing has occurred on the riverward slope, potentially damaging the levee embankment such that it does not meet compliance criteria within the FEMA guidance document 44 CFR 65.10. The calculated factor of safety (FS) for sliding stability at the riverward slope within this reach is slightly below the minimum of 1.4 during a 100-year flood event due to removal of embankment material by beaver and nutria. The location of the concentrated animal burrowing along Salmon Creek is shown in the vicinity map in Figure 1.

The levee geometry meets FS criteria in undamaged areas and provides adequate freeboard at the 1-percent annual-chance flood elevation. The slough also provides critical off-channel habitat. Oregon

Metro, which manages the marine park and adjacent property, has expressed the desire to maintain critical habitat in this area without undermining levee integrity. This levee section was designated

Supplemental Geotechnical Letter Report 3149

MCDD Salmon Creek Slough Animal Burrows

December 8, 2023 Page | 2 of 11

Project IGA 6 by the Levee Ready Columbia (LRC) Intergovernmental Agreement (IGA) Board. A project overview with conceptual mitigation options, quantities, and cost estimates was developed for the IGA Board. The conceptual repair cross sections developed for the IGA Project Business Cases

Report are summarized in Figure 2.

SCOPE OF WORK

This supplemental report for the MCDD Salmon Creek Animal Burrows includes work on the following tasks:

1) Review the additional work performed by ESA, which included animal burrow locations, conceptual burrow cross sections, and a tree survey. Update the 2017 analysis typical cross sections in reach M-2.

2) Perform a limited literature review to synthesize readily available guidance on modelling the impact of animal burrows on earth embankments.

3) Perform seepage and stability analyses on two cross sections to identify the critical section and supplement the conceptual mitigation design work.

4) Review the mitigation alternative proposed as part of the of the project overview/business case for the site and provide input including potential limits of mitigation, approximate dimensions of mitigation necessary to achieve FS requirements, and review of long-term monitoring approach.

5) Summarize the results in a supplemental geotechnical memorandum.

EXISTING SITE INFORMATION

This section of the levee has been impacted by burrowing animals, particularly beaver and nutria, for many years. Investigations in support of FEMA accreditation of both these levees have taken place in multiple phases. The results of the various work components related to the current study are summarized in the following subsections. More detailed information can be found in the 2017 MCDD

Levee Engineering Assessment Report (revised in 2019) and in the IGA6 business case developed for the IGA board. Two levee sections, located at station 22+50 (M-2) and 35+00 (M-3) were analyzed for this study. The M-2 section did not meet the FEMA accreditation standards in 2017 when animal burrow impacts were incorporated into the model.

Past Analysis of Beaver Den Encroachments

In 2017, CCI completed a Levee Encroachment Analysis as part of the Phase 5 MCDD Levee

Engineering Analysis. In this encroachment study, potential failure modes were identified and analyzed for all structural encroachments as identified by MCDD. Within the Salmon Creek Slough area, representative sections were selected and analyzed for seepage and slope stability. Beaver dens were modelled as a saturated soil layer with a very high conductivity of 1ft/s, unit weight of 51pcf, zero cohesion and phi value of 12.2 degrees. The dens were assumed to extend 27 feet into the levee based on information provided by MCDD at the time. A twofold mitigation approach was then recommended for “medium” and “high” risk beaver den encroachments. The initial step was to

December 8, 2023 Page | 3 of 11 relocate the beavers to a safe location away from infrastructure. Once beavers were relocated, it was recommended that the dens be backfilled with grout.

LITERATURE REVIEW

Before starting work on seepage and stability analysis, CCI performed a targeted literature review of the currently available guidance on burrowing animal encroachments. This guidance can be divided into two categories: modelling and mitigation. Most of the available literature focuses on mitigation.

The results of the literature review are briefly summarized in the following subsections. The bibliography containing all sources included in the literature review is located at the end of this letter report.

Seepage and Stability Analyses

Only two papers were identified that specifically discussed seepage modelling of burrowing animal encroachments, and neither examined potential stability impacts (Calamak, et al. 2017, Palladino, et al.

2019). Both papers used transient conditions to model seepage and used a soil layer with very high permeability to approximate the burrow. In Palladino, et.al., the burrow sizes were set at 10 cm (4 inches), a factor of magnitude smaller than the 4-foot diameter burrows described by ESA in the

Salmon Creek levee. Calamak, et al. set a maximum beaver burrow size at 30cm (12 inches) in diameter and used that for seepage modelling. As expected, both analysis methods show that burrowing increases seepage through earth embankments. Neither study evaluated the impact of unrepaired animal burrows on slope stability.

Mitigation Options

The remainder of the reports and guidance documents consulted in the literature review focus on mitigation options. USACE and FEMA recommend relocation of beavers to a safe location away from the levee/dam followed by regular inspections to prevent beavers from returning to the site (FEMA

2005, USACE 2006). Once the beavers have been removed, the standard guidance is to backfill burrows with bentonite cement grout (USACE 2006). Fencing may also be used to control development of new animal burrows (USDA (NRCS) 2022, Cobos Roa, Diego 2015). In all sources reviewed, regular inspections, ideally on an annual basis, and timely mitigation are the preferred method to limit burrow animal impacts.

ENGINEERING ANALYSIS OF BURROW IMPACTS

CCI reviewed older sections developed as part of the 2017 work and new cross sections by ESA between Station 21+00 and Station 35+00. A site plan is shown in Figure 3, while cross sections developed by ESA through the study area are shown in Figure 4. Section 3, developed by ESA, is 37 ft east of the “typical section” that was used to model seepage and stability for Reach M-2 in the 2017 report. CCI opted to replace the 2017 typical section with the ESA section 3 and use the three ESA cross sections for this geotechnical work. These three sections are located in areas with the highest density of animal burrows. The as-built levee geometry is relatively uniform throughout this reach.

Animal burrows were modelled as drawn on each cross section. The three sections were analyzed for landward and riverward slope stability and exit gradient at the downstream toe.

December 8, 2023 Page | 4 of 11

Seepage and strength of animal burrows were assigned based on our previous experience and the literature review described above. The beaver dens were modelled with a high hydraulic conductivity to represent free flowing water through the model, and with a reduced strength during the slope stability modelling. The seepage and stability methods, and the associated assumptions made to support modelling, are described in this section.

Analysis Procedure

Seepage Analysis Methods and Adjustments

CCI modelled seepage through the foundation and embankment soils using SEEP/W, a finite element analysis software. Seepage through the foundation and embankment soils was calculated under saturated, steady-state conditions. The finite element mesh for this project was set to two-feet by two-feet, based on typical values for seepage analysis on larger embankments. Following SEEP/W’s design guidance, the exit gradient at the landward toe of the levee was averaged over an area that was approximately 4 to 6 foot in each direction to limit edge effects in 2D space. The 1- percent annual-chance flood level was developed by WEST Consultants for the 2017 engineering study. The exit gradients from these analyses were then compared to the recommended maximum exit gradient suggested by the USACE in EM 1110-2-1913 (USACE, 2000), (exit gradients should be limited to less than 0.5 at the toe of the landward slope).

The embankment and foundation material seepage parameters are taken from the 2017 study. The seepage parameters for the beaver dens were modelled as tunnels with very high permeabilities. This procedure closely follows the approach (Palladino, 2020) and our previous studies (CCI, 2019).

Seepage properties for all the materials are show in Table 1 below.

Slope Stability Methods and Adjustments

Slope stability was modelled using the SLOPE/W program, incorporating the seepage results from

SEEP/W. The critical slip surfaces on both landward and riverward slope were determined using an entry and exit search routine with factors of safety calculated for multiple circular-slip surfaces. A minimum failure surface depth of 10 feet was set as a limiting parameter in the model to ensure that minor, surficial failures would be excluded from the analysis results. These types of shallow failure surfaces may show lower calculated factors of safety; however, they are considered to be less significant to levee integrity. It is assumed that MCDD maintenance operations would be able to treat any shallow failure areas as they occur to prevent them from developing into much larger failures that could endanger the levee.

Analysis Methodology/ Material Properties

Material properties were estimated from the lab work and drilling activities described in our 2017 report. Material strengths for the beaver den were developed from several components. The ESA study collected animal burrow locations over two site visits in June 2022 and September 2022. These time periods correspond to different water levels in Salmon Creek Slough -- a higher water level of elevation 21.6 feet and a lower water level of 10.6 feet (NAVD88). Material properties for the animal burrow material were calibrated to satisfy marginally stable conditions at each of these water levels. At the same time, CCI assumed that approximately 12 feet of a 30-foot slide would contain burrowed

December 8, 2023 Page | 5 of 11 embankment. Based on these two observations, the modelled average strength in beaver den areas was assigned a strength of 60% of the undamaged surrounding material.

Table 1 shows the material properties selected for the analyses. Cohesion was added to the

ALLUVIUM – clayey SILT to silty CLAY based on review of triaxial tests performed on this material, and the observed behavior of this material near the beaver dens.

Table 1: Summary of Estimated Soil Properties for Seepage and Stability Analyses

Material Description Unit

Weight γ (pcf)

Friction Angle ф' (degrees)

Cohesion Intercept c'

(psf)

Permeability k (ft/sec)

Permeability Ratio, kh/kv

FILL- clean SAND 106 34 0 8.8E-4 4

ALLUVIUM – clayey SILT to silty CLAY

114 27 200 1.1E-05 4

ALLUVIUM – slightly clayey, slightly sandy SILT

113 28 0 1.7E-05 4

ALLUVIUM- silty SAND to sandy SILT

118 30 0 2.6E-05 4

Sheet pile 450 NA NA 1E-07 1

Beaver Den Material 93 16.2 120 100 1

Repaired Beaver Den Fill 95.1 25.5 0 8.8E-4 4

RIPRAP 140 35 0 1 1

Notes: Material parameters estimated based on available information.

CCI performed seepage and slope stability analyses for the three ESA cross sections for the existing ground geometry using the revised seepage and stability parameters developed for the beaver dens.

These analyses modelled the beaver dens and used the FEMA 100-year flood level of 34.4 ft, developed by WEST for the 2017 engineering study. The waterward slope of ESA Section 1 was identified as the critical geometry for this area and is shown in Figure 5. The seepage gradient at the landward slope is below the critical exit gradient because the existing bike path is acting as a seepage berm along the reach.

Table 2: Summary of Stability Factors for ESA Sections

Section Station Exit

Gradient

Factor of Safety Compliance with 44 CFR

65.10 Regulations

Landward Riverward

ESA 1 33+56 0.07 3.22 1.24 No

ESA 2 27+15 0.16 3.91 1.27 No

ESA 3 22+13 0.08 4.90 1.45 Yes

December 8, 2023 Page | 6 of 11

EVALUATION OF MITIGATION ALTERNATIVES

This section discusses the various conceptual mitigation options proposed for this area. CCI evaluated four concepts in total: three developed during the initial IGA TASC work, and an additional levee setback concept. The initial mitigation concepts supplied by MCDD and CCI’s modifications to them are discussed in the following subsections. All concepts were modelled for ESA Section 1, the most critical section under current slope stability conditions. Due to the estimated size of the beaver dens, CCI assumed that the strength and unit weight of the backfill in the beaver den nest would be reduced by 25% to account for difficulties with compaction, or unfilled voids. In addition, fill placed above riprap layers was assumed to also have a reduced strength in the case of future burrowing activity.

IGA6.A: Grade 3H:1V and Install Rip Rap

Work for the IGA6.A mitigation option consists of regrading the slope and installing rip rap on the riverward side of the levee. In this option, the riverward slope is rebuilt at a 3H:1V slope. The original

IGA6 concept extends beyond the existing levee toe. The levee crest is expanded by 10 feet, and the existing beaver dens are backfilled to the extent possible during construction. To prevent future burrowing activity, a layer of riprap or welded wire would be placed behind the new fill. A 4-foot layer of welded wire or riprap was modelled approximately 1 foot below the ground surface. The riprap would extend up to an elevation of 38 feet, approximately 27 feet above the levee toe. This mitigation effort would increase the factor of safety on the riverward side, and reduce the exit gradient at the level toe, as shown in the results summary in Table 3. It meets the minimum factor of safety requirements but would have significant impacts on existing habitat and require temporary traffic control measures on Marine Drive.

This mitigation extends beyond the existing levee toe, and will likely require additional permitting, However, the concept could be re-designed to stay in the existing levee footprint. It will require in-water work, disposal of excavated fill and development of additional staging and access areas for construction. In addition, this mitigation requires backfilling of the existing animal burrows in the area and removing over 85 trees from the levee prism area. Environmental impacts would need to be assessed separately. Due to construction staging, lane closures on Marine Drive are likely to be required, including occasional full closures of the road. If selected, costs for this mitigation option are expected to include design work, permitting, and public outreach, in addition to the costs of construction.

IGA6.B: Grade 2H:1V with Habitat Toe and Install Rip Rap

Similarly to IGA6.A, work for IGA6.B also occurs on the riverward side of the levee but reduces the encroachment into Salmon Creek. IGA6.B maintains the current benched shape of the riverward slope. As in IGA6A, the levee toe would be extended riverward of the current conceptual toe.

However, instead of constructing a 3H:1V slope from the new toe to the levee crest, the riverward slope will be rebuilt at 2H:1V to an elevation of 34. This lower slope will include a buried riprap blanket to prevent future burrowing into the levee prism. This blanket will be 3.5 feet tall and placed roughly 5 feet below the final ground surface. At elevation 34, a 15-foot-wide habitat bench will be constructed. Above the habitat bench, the upper slope will be rebuilt at a 1.5H:1V slope, tying into a new 10-foot-wide access road that will replace the existing maintenance bench 4.5 feet below the levee

December 8, 2023 Page | 7 of 11 crest. The geometry of the design in the IGA document was modified to fit the critical section, while maintaining the access road and design slope angles. Results of this analysis are shown in Table 3.

This mitigation uses steeper slopes than the recommendations in USACE EM 1913. When analyzed at the critical sections, this mitigation option meets FEMA factor of safety requirements for failures that impact the levee crest, but not for the newly constructed lower bench. If redesigned to satisfy

FEMA factor of safety requirements for the entire levee slope, the final design would be similar to

IGA6.A.

The implementation of this levee mitigation design has many of the same construction and permitting considerations as discussed in IGA6A. At least 85 trees would need to be removed for construction, in addition to other environmental impacts. Traffic impacts on users of both Marine Drive and the adjacent Chinook Landing part would also be significant.

IGA6.C: Minimize Cost and Install Sheet Pile Cut Off Wall

This option capitalizes on the fact that the majority of the riverward slope is technically overbuild of the design levee prism. For this option, sheet piles would be driven from the existing bench on the riverward slope to prevent future borrowing activity. This option avoids the excavation and tree removal required to fully rebuild the slope and place riprap or welded wire. Existing burrows would not be addressed. This mitigation was modelled assuming a sheet pile cut off that penetrated 35 feet into the subsurface, below the beaver dens. Slope stability was assessed for the toe below the sheet pile as well as a global failure surface running through the levee prism. When analyzed at the critical sections, this mitigation option meets the minimum factor of safety requirements. Results of this analysis are shown in Table 3 and Figure 6.

This mitigation remains inside the existing toe of the levee, which reduces permitting requirements.

This mitigation avoids the challenges of backfilling the existing beaver dens. In addition, the limited area required for this mitigation results in this mitigation having the lowest environmental demands of the proposed mitigation options. An estimated 20 trees would be removed to develop this mitigation or die because the sheet pile will cut through more than a third of their estimated root wad.

Additional environmental impacts beyond tree loss, such as noise impacts from driving sheet piles, should be considered in a more thorough environmental study if this mitigation option is selected.

One lane of traffic on Marine drive is likely to be influenced during parts of construction for staging and access, although ideally the majority of sheet pile installation work would occur on the bench below the road. The sheet pile installation will require nearby staging areas and construction access.

Additional Mitigation Concept: Levee Setback

Unlike the three IGA options, all work on this additional concept would occur on the landward side of the levee. A setback levee, constructed with a 3H:1V landward slope and a conceptual 12-ft crest width and 2H:1V riverward slope, would tie into the landward side of the current levee. Fill material would be placed between the conceptual levee crest and the existing levee crest, to prevent development of drainage issues. A sheet pile would be installed on the landward shoulder of Marine

Drive to prevent animal burrows from extending into the new conceptual levee. Conceptual sheet pile depths are approximately 45 feet. No excavation on modification of the existing levee prism is

December 8, 2023 Page | 8 of 11 expected, beyond the clearing and grubbing required to tie in the setback levee embankment. The strength of the existing levee material riverward of the sheet pile was reduced in the stability model to account for the undermining caused by future borrowing activities. A cross section of this conceptual mitigation option is shown in Figure 7. When analyzed at the critical sections, the proposed Setback

Levee Prism meets the required factor of safety. Results of this analysis are shown in Table 3 and

Figure 8.

This mitigation is designed to minimize impacts to the critical riverine habitat on the riverward levee slope and toe. It avoids all areas where burrowing activity is occurring and any in-water work. Tree removal would be confined to the landward side on park property, however an estimate on the amount of required tree excavation on the landward side is not in the scope of this project. The beaver habitats would also remain in the area undamaged. CCI assumed that the bike path would be extended approximately 20 feet on the landward side to accommodate the setback levee, which may require additional permitting and coordination with various stakeholders. This mitigation avoids backfilling the existing beaver dens but requires a significant amount of earthwork construction on the landward side of the levee prism, and a large staging area for fill storage. A temporary shoulder closure on

Marine Drive will be required during the installation of the sheet piles and occasional lane closures will be required during construction for staging and final grading

Evaluation of Mitigations

Table 3 shows the slope stability results analyzed for each of the mitigation concepts. Global stability was assessed for IGA6.A, IGA6.B and IGA6.C. Historically, the design levee in the USACE design documents showed a 12-foot crest with 4H:1V side slopes. The side slopes on the riverward side are already slightly steeper than this in the overbuilt sections. The upper portions of the levee embankment, primarily above the beaver dens, are part of this overbuilt section and considered non-critical for levee performance. Localized slide surfaces with lower factors of safety in the riverward side of IGA6.B and IGA6.C may not be considered critical to levee performance.