APPENDIX B2-Eelgrass Mitigation Plan - 20-021-01.pdf

M&A #20‐021‐01

EELGRASS MITIGATION AND MONITORING PLAN

USCG STATION CAPE DISAPPOINTMENT MAINTENANCE DREDGING PROJECT

PACIFIC COUNTY, WASHINGTON

Prepared for:

Wood Environment & Infrastructure Solutions, Inc.

7376 SW Durham Road

Portland, OR Attn: Erin Hale erin.hale@woodplc.com/(805) 403‐4802 and

U. S. Coast Guard Civil Engineering Unit Oakland

1301 Clay Street, 7th Floor Suite 700N Oakland, California 94612 Attn: William Robinson

Prepared by:

Keith W. Merkel, Principal Ecologist

Merkel & Associates, Inc.

5434 Ruffin Road

San Diego, CA 92123

(858) 560‐5465

Prepared under:

Contract #F015600058

Wood Project #336010028

June 2021

Merkel & Associates Eelgrass Mitigation and Monitoring Plan

USCG Station Cape Disappointment Maintenance Dredging Project i

TABLE OF CONTENTS

I. INTRODUCTION

II. EELGRASS IN BAKER BAY AND THE LOWER COLUMBIA RIVER

III. PROJECT ACTIVITIES AND POTENTIAL IMPACTS TO EELGRASS

Maintenance Dredging Survey Results

IV. EELGRASS MITIGATION

Mitigation Overview Eelgrass Restoration Site Selection Transplant, Donor, and Reference Sites Transplant Site Donor Eelgrass Site Reference Sites

Restoration Methods Plant Collection Transplant Units Planting Eelgrass Units

Monitoring and Reporting Overview Survey, Mapping, and Density Determinations for Eelgrass Monitoring Mitigation Performance and Success Criteria

V. REFERENCES

LIST OF FIGURES

Figure 1. Project Location Map Figure 2. Frequency and Depth Distribution of Eelgrass Figure 3. Depth Ditribution of Eelgrass within Cape Disappointment Survey Area Figure 4. Parameter suitability indices for eelgrass in Baker Bay Figure 5. Habitat suitability index (HSI) for eelgrass in Baker Bay Figure 6. Proposed dredging and preliminary eelgrass mitigation areas Figure 7. Merkel bareroot eelgrass planting unit

LIST OF TABLES

Table 1. Eelgrass Bed Metrics as defined under the CEMP (June 2020)

USCG Station Cape Disappointment Maintenance Dredging Project 1

I. INTRODUCTION

Merkel & Associates, Inc. (M&A) has been retained by Wood Environment & Infrastructure, Inc. (Wood) to prepare and implement an eelgrass mitigation and monitoring plan in support of the U.S. Coast Guard

Station Cape Disappointment Maintenance Dredging Project in Ilwaco, WA within Baker Bay in the lower

Columbia River Estuary. Proposed dredging of the Station’s boat basin is anticipated to occur in

November 2021 and may result in minor impacts to eelgrass (Zostera marina) growing along the southwest margin of the basin. Because the potential impacts are very minor and potentially avoidable the plan provides recommendations for avoidance recommend avoidance measures if possible, while providing further guidance for impact minimization and of impact as a first priority with mitigation of impacts being proposed should eelgrass ultimately be damaged by the maintenance dredging activities.

Impacts to eelgrass generally require a 1:1 replacement within Washington State. The guidance for specific mitigation requirements is not established for the state; however information to guide surveys and mitigation projects is provided within the Washington Department of Fish & Wildlife

Eelgrass/Macroaglae Habitat Interim Survey Guidelines (WDFW 2008) and the Army Corps Seattle

District’s Components of a Complete Eelgrass Delineation and Characterization Report (ACOE 2016).

Within the Corps guidance one method for delineating eelgrass beds available is to follow the guidelines of the National Marine Fisheries Service West Coast Region’s California Eelgrass Mitigation Policy (CEMP, NMFS 2014). Final approval of an eelgrass mitigation plan will be provided by consulting and authorizing agencies, including NMFS and the WDFW Area Habitat Biologist (AHB). Under the WDFW guidance, mitigation for the loss of eelgrass typically entails providing eelgrass enhancement away from the project footprint. However, because establishment of eelgrass has historically had low success rates in

Washington, project proponents generally must increase the scope of the mitigation effort, such as planting an area larger than the project impact footprint in order to achieve the mitigation goals.

This plan is organized in a manner that first characterizes the eelgrass habitat within Baker Bay as the system is exposed to relatively unique stressors and beds have been noted to be highly dynamic through time. Understanding the drivers of eelgrass distribution within the project area will assist in structuring a mitigation program that has a high potential for success. The next section of the plan summarizes potential impacts to eelgrass and identifies recommendations to minimize potential for eelgrass losses.

The final section of this mitigation and monitoring plan provides details on the recommendations for eelgrass restoration and monitoring to meet the mitigation requirements for the project.

II. EELGRASS IN BAKER BAY AND THE LOWER COLUMBIA RIVER

USCG Station Cape Disappointment is located at the terminus of a small peninsula located along the western edge of Baker Bay, on the Washington side of the mouth of the Columbia River (Figure 1). Baker

Bay is a predominately mesohaline estuarine embayment consisting of approximately 6,500 acres of intertidal and shallow subtidal sand shoals and tidal channels. A string of small islands and Jetty A extending off of the headlands at Cape Disappointment provide a partial barrier to wave energy and directed freshwater discharge from the river’s mainstem. They also define the deeper channel systems such as the Ilwaco Channel that passes by Station Cape Disappointment. Together with Young’s Bay on the south (Oregon) side of the lower Columbia River, these two embayments support the majority of known/documented eelgrass occurrence within the lower portion of the Columbia River Estuary, where

Project Location Map Figure 1

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U.S. Coast Guard Station Cape Disappointment Maintenance Dredging, Ilwaco, WA

0 1,000 2,000500 Feet

B A K E R B A Y

MAP

AREA

Columbia River

Baker Bay

Ilwaco

Astoria

PACIFIC

OCEAN

Station Cape

Disappointment

Mitigation Planning Area contour interval: 1 foot

Eelgrass Spatial Distribution (5m buffer)

Eelgrass - June 2020

USCG Station Cape Disappointment Maintenance Dredging Project 3 overall eelgrass distribution is believed to be limited by persistently low salinity; relatively high wave and current energy; seasonally low light levels; and a strong net outflow of the river‐dominated currents resulting in poor seedling retention/recruitment and survival (Judd et al. 2009).

Prior eelgrass restoration planning within the lower Columbia River considered salinity to be a major controlling factor relative to eelgrass distribution and restoration potential (Borde et al 2008, Judd et al.

2009, Thom 2016, Merkel & Associates 2018a). In a recent study eelgrass habitat suitability was assessed within Baker Bay to support eelgrass restoration planning and site selection for the Army Corps of Engineers Portland District. Eelgrass occurrence frequency data were developed by mapping eelgrass distribution across multiple years based on available imagery (Merkel & Associates 2018a). Eelgrass distribution data were accumulated over five survey intervals including 2009, 2011, 2014, and 2016 from

Google Earth historic imagery and 2015 from NOAA IOCM imagery. The image analysis was performed only on aerial imagery that allowed identification of eelgrass beds from the imagery and it is likely that eelgrass was missed in the mapping, especially in deeper or more turbid waters. The distribution of eelgrass derived from this effort is believed to be a conservative estimate of both distribution and frequency of occurrence. The maximum extent of eelgrass in Baker Bay was determined to be 1,028 acres based on the union of all mapped eelgrass extents from the survey interval included in the analysis. The distribution of eelgrass within the bay was found to be highly variable based on the general low extent of eelgrass within high occurrence frequencies over the survey intervals. More than

58 percent of the maximum eelgrass extent occurred only once in five survey periods and eelgrass was persistent through all surveys within less than 0.02 percent of the maximum extent of beds. This frequency analysis has been updated on the basis of baseline eelgrass survey conducted in June 2020.

Along with the frequency distribution data, eelgrass depth distribution was calculated for the June 2020 study area (Figure 3). The percent of the total eelgrass distributed across the available bathymetry as defined by 1 foot depth bins has been plotted along with the overall area distribution across the depth range. The distribution of available area across the depth range is also illustrated. Finally, the eelgrass distribution adjusted by the available area is also illustrated. What is demonstrated in the depth distribution is that eelgrass occurs within a narrow depth range with 92 percent of the eelgrass occurring between ‐2 and ‐5 feet MLLW and 58 percent of the eelgrass occurring between ‐3 and ‐4 feet

MLLW. Further, of the available space within the ‐3 foot depth bin, 64 percent was occupied by eelgrass in June 2020 while a full 51 percent of the ‐4 depth bin was also occupied. What is most notable lack of eelgrass within intertidal regions of the survey area in spite of the fact that these areas have a better light environment, suitable substrate, shallower slopes, and acceptable energy regimes to support eelgrass. The principal limiting factor within the intertidal environment is believed to be the depressed surface water salinity of the fluvially dominated estuary as is discussed further below.

Eelgrass Frequency of Occurrence in Baker Bay (2009 - 2020) Figure 2

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USCG Station Cape Disappointment Maintenance Dredging Project, Ilwaco WA

B A K E R B A Y

0 1 20.5 Miles C O L U M B I A R I V E R

ILWACO

Eelgrass Frequency of Occurence 20%

40% 60% 80% 100%

Eelgrass - (2009 -2016)

USCG Station Cape Disappointment

Study Area

Bathymetry sources:

USACE

(2003-2009) Contour interval (10 feet - MLLW)

USCG Station Cape Disappointment

EAST SAND ISLAND

WEST

SAND

ISLAND

Jetty A

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Figure 3. Depth Ditribution of Eelgrass within Cape Disappointment Survey Area

The high variability in eelgrass distribution in Baker Bay is likely mediated strongly by light levels

(photosynthetically active radiation [PAR]), wave and current energy, and salinity regimes. All of these factors are strongly influenced by the bay bathymetry, morphology, and exposure. Relaxation of salinity constraints is considered to have been the single most substantial factor influencing a temporary major expansion of eelgrass within the Columbia River estuary in recent years. Eelgrass in Baker Bay and

Young’s Bay near the mouth of the Columbia River has generally been noted to be extremely limited in extent. Earlier information suggested eelgrass in Baker Bay was limited to a few acres of eelgrass, although it was noted that there was potential for more eelgrass to be established within the area

(Borde 2008). However in 2015, the Army Corps eelgrass survey noted substantial eelgrass in areas of southern Baker Bay (ACOE 2015), and in recent review of August 2016 aerial photography, eelgrass was noted to be widespread in Baker Bay and extended further up into the estuary as well (Merkel &

Associates 2018a). This prolific expansion followed low summer discharge from the Columbia during water years 2014 and 2015 and thus salinities would have been expected to substantially increase in the estuary. Similar patterns of expansion of eelgrass into inner estuary environments has been noted in other, large (San Francisco Bay) and small (e.g., Noyo River, Albion River) estuary systems during the recent drought years. Unfortunately the coarseness of spatial modeling and the lack of sampling through Baker Bay limit the full demonstration of the effect of the recent drought on salinities, although model runs for salinity as reported in Judd et al (2009) do assist in depicting the concepts.

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With respect to Station Cape Disappointment’s location within Baker Bay, the narrow, channel fringing bed subject to the present mitigation planning effort was identified during the 2018 frequency of occurrence analysis as supporting eelgrass. Further, based on the suitability analysis performed, which considered depth, salinity, wave energy, and frequency of eelgrass occurrence, the outside bend of

Ilwaco Channel where both the 2020 APE and Reference Area are located, was identified as being among the most highly suitable locations for eelgrass within Baker Bay, particularly in terms of salinity.

Based on a recent assessment of snowpack conditions within the Columbia River Basin at large and the upper Columbia River basin in particular there is considerable potential for heavy spring/summer 2021 river discharge. Snowpack is presently at 118 percent of average in the upper Columbia River basin

(NRCS 2021). This basin contributes substantial snowmelt runoff from the Canadian Rockies to late spring and summer river flows. As a result, it is anticipated that the importance of salinity as a limiting factor for eelgrass distribution is likely to increase in 2021 putting the lower Columbia River eelgrass at risk of significant declines this year. While salinity is an uncontrollable factor, understanding its importance in establishing conditional suitability for eelgrass occurrence suggests that any eelgrass restoration activities contemplated for meeting potential mitigation demand at Cape Disappointment should emphasize locations where salinity is likely to remain sufficient during extended periods of high river flows. For these reasons, conducting eelgrass restoration within close proximity to the project area where eelgrass impacts are anticipated is the preferred strategy for pursuing mitigation should it be necessary.

III. PROJECT ACTIVITIES AND POTENTIAL IMPACTS TO EELGRASS

Maintenance Dredging

The proposed project involves maintenance dredging of the boat basin at Station Cape Disappointment.

The Coast Guard is currently pursuing a new 10‐year permit to allow for multiple maintenance dredging cycles to occur at this location. The maintenance dredging design depth is set at ‐10 feet mean lower low water with an allowance for 2 feet of overdepth and is tentatively scheduled to commence in

November 2021 (USCG 2020).

Baseline Eelgrass Assessment

A baseline eelgrass survey was conducted by M&A under subcontract with Brice Engineering, LLC (BNG) on June 29, 2020 to assist the USCG with maintenance dredging prioritization. The purpose of this survey was to provide a quantitative assessment of the eelgrass communities within the area of potential effect (APE) in conformance with the requirements of the California Eelgrass Mitigation Policy

(CEMP) (NOAA 2014). Additional eelgrass surveying and mitigation guidance documents including

WDFW Eelgrass/Macroalgae Habitat Interim Survey Guidelines (WDFW 2008) and Components of a

Complete Eelgrass Delineation and Characterization Report (USACE 2016) were also consulted. As the guidance provided in the CEMP is generally more stringent and is referenced by the USACE Seattle

District (2016) guidance document as providing a suitable framework for eelgrass bed delineation and mapping, it has been selected as the primary reference for guiding this plan’s approach to eelgrass impact assessment and mitigation. Eelgrass density assessment, statistical evaluation, and monitoring timing proposed within this plan are drawn from the WDFW (2008) and USACE (2016) guidance to

USCG Station Cape Disappointment Maintenance Dredging Project 7 ensure that the approach to eelgrass monitoring and reporting are compatible with local regulatory policy and practice.

The baseline eelgrass survey area included an approximately half‐mile long segment of shoreline along the Coast Guard Station at the south end of the survey area and extending northward towards the State

Park public boat launch ramp (Figure 1). The depth at the survey area ranged from intertidal elevations to a maximum depth of approximately –20 feet mean lower low water (MLLW). The APE was defined to include all of the Coast Guard waterside facilities and shallows extending outward to the deeper waters of the adjacent channel into Baker Bay.

A reference area was established during the 2020 survey. It is located to the northwest of the APE. The reference area was selected based on similarity in physical and biological characteristics as well as proximity to the APE. Surveys of the reference area are to be conducted coincident with those in the

APE under a pre‐ and post‐construction impact assessment framework. The reference area will be used to evaluate natural variability in eelgrass to account for any naturally expected dynamics within the APE.

An additional reference area has been tentatively selected for inclusion in the monitoring program as a means of better capturing and accounting for the anticipated variability and uncertainty regarding eelgrass growing conditions, given the dynamic nature of the estuary. This is explained in further detail in Section 4.

The reported metrics for eelgrass are as follows:

Spatial Distribution – The spatial distribution of eelgrass habitat was delineated by a contiguous boundary around all areas of vegetated eelgrass cover extending outward a distance of 5 meters. The resultant spatial distribution boundary of the eelgrass habitat was then clipped to remove areas that were determined to be unsuited to supporting eelgrass based on depth, substrate, or existing structures.

Areal Extent – The eelgrass habitat areal extent is the quantified extent of the spatial distribution of the beds comprised of unvegetated and vegetated areas of the bed.

Percent Vegetated Cover ‐ Eelgrass vegetated cover exists when one or more leaf shoots

(turions) per square meter is present. The percent bottom cover within eelgrass habitat is determined by totaling the area of vegetated eelgrass cover and dividing this by the total areal extent of the bed.

Turion (Shoot) Density ‐ Turion density is the mean number of eelgrass leaf shoots per square meter within mapped eelgrass vegetated cover. Turion density should be reported as a mean ± the standard deviation of replicate measurements. The number of replicate measurements (n) is reported along with the mean and deviation. Turion densities are determined only within vegetated areas of eelgrass habitat; and therefore, it is not possible to measure a turion density equal to zero.

Survey Results

Eelgrass bed spatial metrics are summarized in Table 1, and eelgrass distribution within the survey is illustrated in Figure 1.

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Table 1. Eelgrass Bed Metrics as defined under the CEMP (June 2020).

Eelgrass Beds Spatial

Distribution (m2)

Vegetated Cover (m2)

Percent Vegetated Cover

Density (turions/m2) n=20

APE 2,334 300 12.8% 110.438.8

Reference Area 4,655 1,699 36.5% 108.053.4

Eelgrass within the APE and reference area were found to be derived from the same intermittent fringing eelgrass bed positioned along the upper margin of an outside bend in the Port of Ilwaco channel. Eelgrass throughout the survey area was determined to be healthy, with no sedimentation and no evidence of disease. Epiphytic loading of approximately 10 ‐ 20 percent occurred on eelgrass within the both the APE and the reference area. Leaf canopy extended from 0.6 to 0.9 meters off the bottom of the APE and the reference area.

The lower margins of the eelgrass bed in both the APE and the reference area were limited by steep slope rather than light limitation; however, in portions of the northerly end of the reference area, where channel slopes flatten somewhat, light limitation may have also been a factor constraining the bed vertical range. Overall, eelgrass was found to occur from approximately ‐2 to ‐7 feet MLLW during the survey.

Potential Impacts to Eelgrass

Based on results of the June 2020 baseline eelgrass survey and maintenance dredging design provided by Wood, proposed dredging activities could result in direct impacts to approximately 14 m2 of eelgrass growing along the southwestern margin of the boat basin (Figure 4). These impacts could be avoided by reducing the lateral extent of the dredge footprint at the southwest corner of the boat basin and in the area immediately northwest of the gangway leading to the docks and boathouses. It is unclear at this stage in the project whether these areas can be avoided while still achieving the necessary clearance to support desired vessel class and associated draft requirements to meet the Coast Guard’s needs at this facility.

Eelgrass impacts are to be measured in accordance with the CEMP (NMFS 2014). This includes two elements. The first is a reduction of eelgrass areal extent between pre‐ and post‐dredging surveys beyond that observed in reference areas. Pre‐dredging surveys must be conducted not more than 60 days prior to commencement of dredging and post‐dredging surveys must be conducted not more than

30 days following completion of dredging unless dredging is concluded during the low‐growth period.

Should this occur, the post‐dredging survey may be conducted during the start of the next high growth period. Guidance provided for the region is that impact assessment should be completed during the active growth period for eelgrass when above‐ground biomass is present and readily detectable (June

1st – October 1st; WDFW 2008 and USACE 2016). As a result of this recommendation for late spring through summer evaluation of impacts and a desire to conduct any required mitigation transplanting during an earlier and slightly overlapping period of time, it is likely to be necessary to either conduct earlier impact evaluations to determine if an impact requiring mitigation has likely occurred, or fully plan and mobilize for transplants prior to having certainty as to the mitigation need.

USCG Station Cape Disappointment Maintenance Dredging Project 9

IV. EELGRASS MITIGATION

Mitigation Overview

If impacts to eelgrass are found to be unavoidable based on results of the pre‐dredging and post‐ dredging eelgrass impact assessment surveys, then eelgrass mitigation would be required. Because the

CEMP standards are substantially more explicit regarding impact assessment, particularly with respect to mapping eelgrass areal extent and distribution, these have generally been adopted over the WDFW guidelines. In terms of the post‐mitigation monitoring program used to evaluate the success of the mitigation effort, the WDFW (2008) guidance regarding turion density sampling, statistical analysis, and monitoring timing have been adopted over the CEMP to conform to regional standards of practice. This approach to eelgrass impact assessment, mitigation, and monitoring capitalizes on the efficiency and accuracy of employing hydroacoustic remote sensing technologies (interferometric sidescan sonar) for mapping eelgrass distribution and abundance; leverages mitigation techniques that have been proven to be successful based on recent eelgrass restoration work in Baker Bay (M&A 2018a, 2018b, and 2019) and extensive work throughout coastal California; and establishes a framework for monitoring at a lesser frequency interval than that prescribed by the CEMP as a result of the shorter growing season and slower rate of anticipated mitigation site development under the climatic conditions of the Pacific

Northwest.

Under both the CEMP and WDFW (2008) guidance, impacts to eelgrass area or density are to be offset by restoration of eelgrass to achieve no net loss of habitat function. Under the CEMP, mitigation is required at a 1.2:1 ratio (mitigation to impact) to account for the temporal loss of function associated with the timeline of mitigation site development/maturation. This assumes that mitigation is completed immediately after project‐related impacts occur, however; both the CEMP and WDFW acknowledge that mitigation can be performed at a 1:1 ratio if the work is completed in advance of the impact, thereby avoiding any temporal loss of habitat function. Since it is not known at the present time whether eelgrass mitigation will be required and because the potential impacts are minor in nature, this plan establishes a 1.2:1 ratio for eelgrass mitigation success, with mitigation implementation occurring following completion of dredging only if impacts to eelgrass are found to have occurred.

The preferred option for mitigation has been selected early in the mitigation planning process to avoid any delays in permitting. Because the greatest risk factor in achieving mitigation success is driven by an uncontrollable factor of depressed salinity from future Columbia River flows, the employment of multiple reference sites in the monitoring program that bracket the restoration site along the channel gradient is proposed. The natural variability in reference sites will be used to scale restoration planting performance expectations.

Eelgrass Restoration Site Selection

The restoration of eelgrass in Baker Bay requires development of an understanding of where restoration is likely to be most successful. To accomplish this, M&A has made use of the Ecological Limits Viability and Sustainability (ELVS) model that was first developed for application in San Diego Bay to predict eelgrass response to changing circulation and temperature regimes in association with operations of a once through cooling power plant (M&A 2000) and which was later employed to as a predictive model for eelgrass restoration planning in San Francisco Bay (M&A 2005). The ELVS model is a spatial‐numeric

USCG Station Cape Disappointment Maintenance Dredging Project 10 model based on Hutchinsonian niche theory and the n‐dimensional hypervolume concept where the coincident occurrence of favorable conditions across many environmental parameters favors the presence and stability of a resource of interest, while lesser coincidence of favorable conditions diminishes the likelihood of sustainability of the resource.

The ELVS model was recently adapted for use in identifying suitable locations to conduct eelgrass restoration to serve as compensatory mitigation for the Mouth of the Columbia (MCR) Jetty A

Rehabilitation Project on behalf of the Army Corps of Engineers Portland District (M&A 2018a and b) and provides a strong foundation for identifying suitable locations within Baker Bay to conduct eelgrass restoration. In 2018, M&A successfully completed a 0.65 acre transplant over 10 discreet sites within

Baker Bay using the ELVS model to select the best locations within a lower intertidal swale centrally located within the bay. The project included constraints that required limiting the restoration planting to very shallow and unmodified areas within the suitable depth range for eelgrass that could be accessed without diving, which further elevated the need to make strategic decisions regarding where to prioritize restoration efforts to achieve the necessary mitigation. This is because shallower sites are more prone to both depressed salinity and wave energy stressors.

The ELVS model uses tolerance thresholds and ranges for multiple parameters to develop Parameter

Suitability Indices (PSI). The PSIs examine the distribution and frequency of occurrence of eelgrass across environmental gradients using ranges and thresholds that define soft and hard boundaries to the distribution of eelgrass. The PSIs relate the values of the parameter (e.g., depth bins, salinity concentration) to a suitability value ranging from 1 to 0 with 1 being highly suitable to support eelgrass and 0 being completely unsuitable to support eelgrass. PSIs are subsequently combined through raster analysis in a GIS application to present a spatial representation of probabilistic suitability to support an ecological resource (species, habitat, community, etc.). The combination of the PSI gradients produces an overall Habitat Suitability Index (HSI) with the highest combined value, on a scale from 1 to 0 reflecting the highest potential to support eelgrass. The model has an open architecture that allows for population of the model with as many or as few parameters as are available with more parameters being incorporated generating greater predictive capacity and model stability overall.

For the present application of the ELVS model to assist in focusing eelgrass restoration efforts a simplified application relying on the information available and intrinsic environmental parameter valuation has been adopted. More explicitly, parameter suitability indices of bathymetry, salinity, and hydrodynamic energy conditions have been applied to explore how eelgrass persists in Baker Bay across the parameter conditions present in the Bay (Figure 4). These PSIs are believed to be strong drivers of eelgrass distribution and bed dynamics in this system and it is believed at the present time that salinity is likely to be the strongest individual parameter or driver of eelgrass distribution and abundance within the bay, particularly in the vicinity of Station Cape Disappointment, which is generally protected from extremes of hydrodynamic energy (wind waves and currents) and provides suitable depths to support channel fringing eelgrass beds along the margins of Ilwaco Channel.

Notably the slope of the bottom and substrate instability may also be an important limiting factor in this area due to the characteristics of the eelgrass occurring on the margin of an outside bend in the channel. This was not explicitly evaluated as a PSI within the present modeling effort.

Parameter Suitability Indicies (PSI) for Baker Bay ELVS Eelgrass Restoration Suitability Model (2018) Figure 4

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USCG Station Cape Disappointment Maintenance Dredging Project, Ilwaco WA

BATHYMETRY PSI

SALINITY PSI

HYDRODYNAMIC

ENERGY PSI

Pacific Ocean

Washington

Oregon

Columbia River

Project Area

Parameter Suitability Index High

Low

Suitability

0 1 2 3 40.5 Miles

1 inch = 7,600 feet

Shoreline Model Boundary Cumulative Eelgrass Distribution (2009, 2011, 2014, 2015, and 2016)

Bathymetry Data

(2003/2009 USACE)

Salinity Data (1999-2006, Burla et al. 2010)

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Combining the PSIs within the ELVS model generates an HSI reflecting the degree of suitability for supporting eelgrass across the modeling area (Figure 5). For the Station Cape Disappointment maintenance dredging project, the ELVS model was updated locally within the study area in order to provide a refined model. The inset in Figure 5 depicts the HSI output for project specific runs of the ELVS suitability model using updated bathymetric data from the 2020 surveys. The model output differs from that of the broader Baker Bay due to changes in bathymetry that have occurred since the completion of the Army Corps of Engineers Portland District’s 2009 LiDAR and a 2003 single beam bathymetric survey that were used to support the original ELVS model (Merkel & Associates 2018a).

In the case of the outer bend of the Ilwaco Channel the modeled suitability depicts suitability to support a narrow fringing bed of eelgrass with increasing suitability to the northeast of the station. This increasing suitability is likely a factor being supported by decreasing slope and expanding bands of suitable depths to support eelgrass within the ambient light environment. The model predicts suitability to support eelgrass both where eelgrass occurs along the channel margin and within areas falling in the same depth zone, but not presently occupied by eelgrass. Moreover eelgrass along the margin adjacent to the Coast Guard docks is comprised of multiple non‐coalesced individual clones suggestive of establishment of eelgrass following prior complete plant losses from this area. The sparse clone distribution may be related to poor conditions for seed settling and seedling establishment, but adequate suitability for growth and expansion of mature plants. Such conditions are often well suited to transplant success because a particular portion of the natural colonization is disrupted but can be skipped over by planting.

A further factor supporting the selection of the area for planting is that the Ilwaco Channel area of Baker

Bay has been identified as retaining some of the highest maximum salinities within the bay with the average modeled salinity ranging from approximately 29 parts per thousand (ppt) at the Coast Guard

Station to 32 ppt near the public launch ramp (Figure 4). As a result, while the surface water may be highly depressed due to the seasonally strong presence of a freshwater lens, the deeper portions of the site likely retain salinities suitable to support eelgrass year‐round under normal water years.

For reasons of generally favorable site suitability, logistical considerations of conducting a transplant, and the limited scale of impact, it has been determined that eelgrass mitigation in proximity to the impact area is appropriate. The scale of the restoration should also be expanded significantly over the scale of anticipated impact in order to protect against shortfall of required mitigation area.

Mitigation Methods

Eelgrass mitigation is proposed to be conducted using bareroot planting units derived from donor beds outside of reference and restoration sites. For this project, the northern section of the channel fringing eelgrass bed on the right bank of Ilwaco Channel near the public boat launch (Figure 6) provides a good locally derived source of donor eelgrass material to support the transplanting effort.

Transplanting of bareroot planting units will be completed based on infilling around existing plants within the open gaps in the fringing eelgrass bed extending from the shoreward margin of the boat basin, northwest towards the reference site (Figure 6). The final layout and extent of the planting area will be driven by the results of the pre‐dredging and post‐dredging surveys that will ultimately

Habitat Suitability Index for Eelgrass Restoration in Baker Bay Figure 5

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USCG Station Cape Disappointment Maintenance Dredging Project, Ilwaco WA

B A K E R B A Y

0 1 20.5 Miles

C O L U M B I A R I V E R

ILWACO

EAST SAND ISLAND

WEST

SAND

ISLAND

Jetty A

USCG Station Cape Disappointment

High

Low

Suitability

Shoreline Model Boundary Cumulative Eelgrass Distribution (2009, 2011, 2014, 2015, and 2016)

Station Cape Disappointment Study Area

Eelgrass Mitigation Areas Figure 6

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0 250 500 750 1,000 Feet

Merkel & Associates, Inc.

REF 1

REF 2

Mitigation Planting Areas Limits of Dredging

Study Area

APE

Donor Area Reference Area

1 foot MLLW contours Eelgrass - June 2020

-14-11 -17

-1 1

-9

-2

-13

-22

-19

-10 -20

-21

-15

-18

-10

-21

-22

-192

REF 1

APE

STATION CAPE

DISAPPOINTMENT

USCG Station Cape Disappointment Maintenance Dredging Project 15 determine the level of compensatory eelgrass mitigation required. The transplanting is anticipated to occur predominantly within the USCG Station boat basin and the adjacent channel margin area south of the reference area within areas ranging in depth from ‐2 to ‐5 feet MLLW. Because the planting is being proposed within an unmodified site, which is itself located in an area (Baker Bay) where eelgrass occurrence is intermittent as a result of the dynamic conditions imposed by interannual variability in

Columbia River flows, this plan recommends significantly oversizing the mitigation site to reduce the risk of failure. Further, because depressed salinity is a vertically defined stressor, the deeper planting elevations and extension of planting to subtidal elevations would assist in mitigating potential risks.

Eelgrass will be harvested on the first day of the transplant. Eelgrass processing into planting units will occur at a temporary shore‐based station set‐up near the public launch ramp or, alternatively, at the

Coast Guard Station. Plants stored overnight will be stored in mesh bags hung in the water off of the

Coast Guard dock. Should surface salinities be depressed, the bags will be weighted to sink them below surface water. During the second day of work, planting of bare root units processed the previous day will be conducted. This process will continue through the completion of the planting program. After completion of the last plantings, a post‐planting mapping of the plantings and a post‐transplant memorandum will be prepared.

Transplant, Donor, and Reference Sites

Transplant Site The proposed location and extent of the eelgrass transplant site is presented in Figure 6 and has been tentatively scaled to achieve an minimum targeted transplant to impact ratio of approximately 20:1, based on the results of the baseline eelgrass survey, proposed limits of dredging, and the anticipated higher level of failure risk associated with planting in an unmodified restoration site. At the present time, 719 m2 (0.18 acre) of available planting area has been identified as available for restoration between ‐2 to ‐5 feet MLLW based on the June 2020 eelgrass distribution (Figure 6). As result the targeted planting area of 20:1 for the anticipated impact of 14 m2, or 280 m2 (0.07 acre) has been identified as available in the planting area. Planting will be performed on 1 meter centers such that the total number of planting units required is 280 (1 plant per square meter [10.76 square feet]) distributed over 3,014 square feet. This level of planting is extremely small and would require only two days to complete the work.

During planting, the transplant site will be temporarily monumented by PVC pipe corner posts and planting layout will be controlled by temporary nylon planting lines placed during the planting period to maintain planting unit spacing at 1 meter centers. Immediately following planting an area, planting lines will be removed to protect new plants from abrasion damage as the line moves in the waves and currents. The position of the planting area corner points will be determined by real‐time kinematic global positioning system (RTK GPS).

Donor Eelgrass Site Donor eelgrass for the eelgrass transplant will harvested from an established eelgrass bed identified during the June 2020 survey. This bed is substantial enough to accommodate harvest of adequate plant material to support the preparation of the required 280 planting units with no more than 10 percent of the existing donor bed shoots being removed from within the bed. This requires the harvest of approximately 1,960 viable turions with suitable rhizome material consisting of at least 3 nodes and

USCG Station Cape Disappointment Maintenance Dredging Project 16 internodes. The total donor bed area in June 2020 was 2,773 m2 (0.69 acres). With anticipated average densities of eelgrass being similar to that measured in June 2020 (108.0±53.4 turions/m2) it is roughly estimated that the defined donor bed hosts a total of approximately 299,484 turions. As a result the donor bed would supply all of the plant material for the transplant at an approximately harvest level of

0.6 percent of the available turions. In other words all of the plant material required to support the restoration could be derived from as little as 6.5 percent of the designated donor area. For this reason harvest densities are anticipated to be far lower than 10 percent of available turions to meet the plant material needs for the restoration. Following harvest, the broken rhizomes from which shoots were collected generally branch and eelgrass density within the donor sites increases for a period beyond that of the natural carrying capacity of the donor bed. However, over a short time, the beds return to their natural, but highly dynamic shoot carrying capacity.

The restriction on harvesting less than 10% of the turions from within the donor bed will be assured by completion of pre‐ and post‐transplant eelgrass surveys of the donor beds to document the area and distribution of vegetated cover and density of the donor beds immediately prior to and following the transplant. The allowable harvest level will be determined from the pre‐transplant survey data and will be implemented on a per square meter basis by moving continuously through the donor bed harvesting

10% or fewer of the mean turion density from each square meter. Managing the extraction on a square meter by square meter basis ensures even distribution of harvest at low levels of effect. Harvested areas are tracked on a daily basis to ensure that divers do not return to the same areas for subsequent harvesting activities. Harvested beds rapidly recover in density and often rise to densities higher than pre‐harvest levels due to rhizome branching at removal nodes. However, densities ultimately fall back to pre‐harvest levels, generally by the subsequent high growth season.

Factors contributing to selection of a suitable donor sites for this work include:

1) Proximity to the transplant receiver site that favors both logistical convenience and selection of appropriate plant materials for the area;

2) Suitability of donor site size and turion density to provide necessary transplant materials;

3) Recovery potential for the donor site; and,

4) Accessibility of the donor site and safety working within the site.

As a footnote to the donor bed selection and harvest level, if it was known for certain that substantial eelgrass losses would occur with the dredging project, then it would be prudent to undertake a salvage rather than donor harvest and conduct the work ahead of the dredging impact since the mitigation sites proposed are unmodified and not dependent upon the dredging or other site manipulation to prepare the areas. However, under the present circumstance, impacts are not certain to occur and the eelgrass areas potentially impacted are small and thus of an inadequate size to support the mitigation. For this reason, harvesting from a donor bed rather than conducting a pre‐dredge salvage is most appropriate.

Reference Sites Eelgrass reference sites are to be established within proximity of the transplant area to the north and south of the planting area along the channel gradient (Figure 6). Reference sites will be monitored for eelgrass spatial extent and turion density concurrent with the monitoring of the eelgrass mitigation site and the dredging APE. Reference areas are selected to be similar in character to the transplant site and

USCG Station Cape Disappointment Maintenance Dredging Project 17 will be averaged for comparison with the transplant site as the two identified reference sites bracket the transplant location.

Monitoring of the reference sites will be conducted coincident with the monitoring of the transplant areas. Changes in the reference sites over time will be considered to represent natural environmental variability when evaluating the performance of the transplant sites (see Section IV. Monitoring and

Reporting Plan).

Restoration Methods

Plant Collection Bare‐root eelgrass plant material will be salvaged from the donor beds by "raking" rhizomes out of the surface sediment layers and loosely filling a mesh bag with salvaged material. In collecting eelgrass, care will be taken to work the rhizomes free as opposed to ripping the plants free of the sediment. This will preserve as much root material as possible. Salvaged materials will consist of no less than three healthy internodal segments with well‐developed root initiates and vigorous shoots. More intact rhizome segments and roots are preferred for use in the planting unit bundles. Salvaging is a mobile exercise and harvesters will move systematically through an area and collect/groom no more than 10 percent of the plant material within a donor bed.

Collected material will be held in a flow‐through seawater source until it is processed into planting units.

No material will be stored for over 24 hours from harvesting to unit preparation. Once units are prepared, they will be stored in open water for no longer than 24 hours for a maximum total of 48 hours of storage from harvest to planting with storage generally being loose in flowing seawater or within mesh nets in the bay.

Transplant Units The proposed transplant will utilize anchored bare‐root transplant units. Bare‐root transplants are the preferred means of transplanting eelgrass in most situations, and anchored bare‐ root units are the principal planting units used in large‐scale restoration projects at the current time. The survival of such planting units has been shown to be quite high when properly prepared (Fonseca et al. 1982; Merkel

1987, 1990a). Similarly, bare‐root units have shown an ability to rapidly expand and colonize bare substrate (Merkel 1990b). In addition to offering high unit survival and rapid expansion rates, bare‐root units can be prepared with limited damage to the donor bed. Unlike plug extractions, bare‐root units can be prepared using materials collected without substantial sediment disturbance. Each transplant unit for the project work will consist of 4‐6 turions.

The anchors used in this program will be biodegradable and pliable anchors such as those developed initially for transplants in Mission Bay’s Sail Bay (Merkel 1987) and which have subsequently been used in more than 80 eelgrass restoration projects throughout California, Oregon, Washington, and Alaska, where more than 720,000 planting units have been prepared on these anchors. The Merkel planting units make use of a paraffin impregnated paper anchor extending to a cotton twine collar that is cinched around rhizomes (Figure 7).

Planting Eelgrass Units Planting will be conducted by planting along temporary planting lines. Using planting lines, the restoration sites are to be planted on 1 meter centers. This layout will allow for ease of tracking work

USCG Station Cape Disappointment Maintenance Dredging Project 18 progress and completion of quality control reviews. The plant materials will be planted by excavating a hole in the sediments with a small trowel or by hand. Each anchor will be planted parallel to the sediment surface and the root/rhizome bundle will be planted approximately 3 cm (1 inch) below the sediment surface with the anchor being placed approximately 10 cm (4 inches) below the sediment surface (Figure 7). During planting, spot checks of the plantings will be made to ensure proper planting depth and firmness of the anchoring system.

Planting unit spacing is typically determined by balancing the rate of bed establishment with the cost of the transplant project. In some instances, rapid bed establishment is required to minimize potential storm damage or scouring of unconsolidated rhizome mats. In other cases, rapid recovery rates are desirable to meet bed establishment milestone objectives. Taking into account the rate of eelgrass growth and the expanded transplant area to reduce failure risk, a planting unit spacing of one meter on center will be used for the present project. The transplant unit count required to achieve the planting density over 0.07 acre is 280 planting units.

Figure 7. Merkel bareroot eelgrass planting unit and planting configuration to be employed.

USCG Station Cape Disappointment Maintenance Dredging Project 19

Transplant Timing

Eelgrass restoration should be conducted during the first spring following dredge impacts. Timing for the transplants should be from May through June, however if significantly depressed salinities occur within the transplant or donor area during the early period of this window, transplants should be delayed until later in the transplant window. The purpose for the spring transplant is to allow plants to establish new roots and rebuild rhizome stores prior to entering the winter low growth period.

Monitoring and Reporting Overview

The proposed monitoring of the restoration area will occur at four intervals. The first is the initial planting implementation monitoring to be completed immediately upon completion of planting. This monitoring interval will document tine locations and conditions of planting and provides a baseline for the transplant and reference sites at year 0. The transplant and reference sites will subsequently be monitored at Year 1 (2023) Year 3 (2025) and Year 5 (2027) post‐planting, which conforms with the recommendations provided by WDFW‘s (2008) guidance. All eelgrass impact assessment and mitigation monitoring surveys will be completed during the active growth period for eelgrass when above‐ground biomass is present and readily detectable (June 1st – October 1st; WDFW 2008 and USACE 2016).

During monitoring intervals spatial bed metrics and plant metrics will be collected. Spatial metrics will include vegetated cover, areal extent, and spatial distribution of beds. Plant metrics will include turion density, leaf length, flowering, percent epiphytic and sediment loading, percent flowering and any evidence of plant disease or other stressors. The results of the monitoring will be submitted in cumulatively developed tracking reports within 30 days of each monitoring effort. The reports will provide mapping results as well as charts and tables summarizing the conditions and trajectory of the transplants.

Survey, Mapping, and Density Determinations for Eelgrass Monitoring Mapping will use interferometric sidescan sonar (ISS) acoustics and supplemental low altitude UAV

(unmanned aerial vehicle) orthophotography with density and plant metrics being derived by diver quadrat sampling and observation of eelgrass plants. These methods are proposed for initial surveys and the monitoring to be completed at years 1, 3 and 5.

The sidescan acoustic survey methodologies have been in use since 1984 and are the most common and precise survey and mapping methodologies motion control, and continuous sound velocity updates to reduce spatial error to sub‐meter levels.