APPENDIX B3-Pre-Dredge Eelgrass Survey - 24-103-01.pdf

Merkel & Associates, Inc.

5434 Ruffin Road, San Diego, CA 92123 Tel: 858/560-5465 Fax: 858/560-7779 San Diego CA ● Tiburon CA ● Arcata CA ● Shelton WA

April 11, 2025

M&A #24‐103‐01

Ms. Kimbrie Gobbi WSP USA, Inc.

9177 Sky Park Ct San Diego, CA 92123

Re: U.S. Coast Guard Station Cape Disappointment ‐ Baseline Eelgrass Survey

Dear Kimbrie:

This letter serves to transmit information regarding the baseline eelgrass (Zostera marina) survey completed at United States Coast Guard (USCG) Station Cape Disappointment in April 2025.

PURPOSE AND INTRODUCTION

USCG Station Cape Disappointment has experienced an extreme increase in sedimentation compared to historic rates and is in need of dredging on a recurrent basis. Dredging has been performed previously but has exceeded the currently permitted volumes. The USCG is in the process of applying for new permits to increase dredge volume allowances and to modify dredge boundaries to include the areas of sedimentation adjacent to the boat haul out. It is anticipated that at least 1 million cubic yards of material will need to be removed over a 10‐year period.

Merkel & Associates Inc. (M&A) has been retained by WSP USA, Inc. (WSP) to conduct a baseline eelgrass survey at USCG Station Cape Disappointment to support the USCG dredge permitting. 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 both the California Eelgrass Mitigation Policy (CEMP) (NOAA 2014) and Eelgrass/Macroalgae Habitat Interim Survey Guidelines (WDFW 2008). Because the CEMP standards are typically more stringent, these have been adopted except where adjusted and noted to further meet the WDFW guidelines.

PROJECT LOCATION AND SURVEY AREA

USCG Station Cape Disappointment is located at 322 Coast Guard Road, Ilwaco, Washington, at the mouth of the Columbia River (Figure 1). The survey area included an approximately half‐mile long segment of shoreline with 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 –30 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. The APE was extended to the east to include the larger shoal that has recently begun to develop.

As outlined in the CEMP, a reference area was established for the project. 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. The reference area will be used to evaluate natural variability in eelgrass to account for any naturally expected dynamics within the APE.

Eelgrass Survey - April 2025 Maintenance Dredging at USCG Cape Disappointment

Ilwaco, Washington Figure 1

M&A #24-103-01

Merkel & Associates, Inc.

µ

0 300150 Feet

Survey Area

APE

Reference Area Overwater Structures Eelgrass Vegetated Cover Eelgrass Vegetated Areal Extent Eelgrass Spatial Distribution

MAP AREA

Bathymetric contours from April 2024.

USCG Station Cape Disappointment‐ Eelgrass Survey April 2025

Merkel & Associates, Inc. #24‐103‐01

SURVEY METHODOLOGY

M&A conducted the eelgrass survey on April 5, 2025. Spatial extent data were collected using interferometric sidescan sonar (ISS), which provided an acoustic backscatter image of the seafloor within the survey area. Interpretation of the backscatter data allowed for an assessment of the distribution of eelgrass. Sidescan backscatter data were acquired at a frequency of 468 kHz scanning out 31 meters on both the starboard and port channels for a 62‐m wide swath.

The rigid hull mounted ISS system integrates motion sensors to control for heave pitch and roll, a sound velocity sensor for speed of sound correction, and a dual antenna real time kinematic global positioning system (RTK GPS) and electronic compass to control for vessel position and yaw. This rigid integration of the ISS transducers within the positioning sensors provides significantly increased precision and accuracy over conventional sidescan sonar equipment.

The survey was conducted by running parallel transects that were spaced to allow for overlap between adjoining sidescan swaths. Survey swaths were navigated until the entirety of the survey area was captured in the survey report. All data were collected in latitude and longitude using the North American Datum of 1983 (NAD 83), converted to the Universal Transverse Mercator system in meters (UTM), and plotted on a geo‐rectified aerial image of the survey area.

Following the sidescan survey, the sonar data were then ground‐truthed using a remotely operated vehicle (ROV). The ROV was operated from the surface with a computer and an operator‐held control unit. A color camera on board the ROV sent video images to the computer where images were interpreted in real time. Eelgrass density data were collected within the APE and reference area to assess the density and health of eelgrass. Data were collected by lowering the ROV to the seafloor in areas where eelgrass occurred and navigating the ROV through the beds. Once on the bottom, the ROV’s video camera was focused on an attached 1/16 square meter quadrat. Eelgrass leaf‐shoot densities were calculated by counting the number of leaf shoots within the sampled quadrats. Because the WDFW guidelines call for a minimum 1/4 square meter quadrat, four consecutive counts from 1/16‐meter quadrats were combined to make a single replicate sample.

Following completion of the survey, ISS traces were joined together and geographically registered.

Eelgrass was then digitized as a theme over and projected on an aerial image of the survey area to calculate the amount of eelgrass coverage and present its distribution. This method of eelgrass distribution calculation allows for monitoring eelgrass trends within the APE and reference area with a substantial degree of accuracy and repeatability over time.

The reported metrics for eelgrass are as follows:

Vegetated Cover – Vegetated cover is the tight boundary extent of eelgrass plants on the seafloor, prior to application of CEMP eelgrass bed definitions. The discrete mapping of plant boundaries is the basic building block for determining CEMP spatial metrics.

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.

The vegetated areal extent is defined as areas within the spatial distribution that support at least 1 turion per square meter of bottom. This is determined by performing a tight margin mapping of eelgrass plants present within the survey area and then

USCG Station Cape Disappointment ‐ Eelgrass Survey April 2025

Merkel & Associates, Inc. #24‐103‐01 4 buffering outward from the vegetated cover of plants by a distance of 0.5 meter such that any plant within 1 meter of another plant would be captured within the same contiguous vegetated areal extent boundary. The unvegetated areal extent is defined as the remainder of the spatial distribution that is not vegetated areal extent.

Spatial Distribution – The spatial distribution of eelgrass habitat was delineated by a contiguous boundary around all areas of vegetated eelgrass cover extending outward from the margins of plants by 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.

Percent Vegetated Cover ‐ The percent bottom cover within eelgrass habitat is determined by totaling the area of vegetated areal extent 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 from the survey is illustrated in Figure 1.

Table 1. Eelgrass Bed Metrics as defined under the CEMP (April 2025).

Eelgrass Beds Vegetated Cover (m2)

Vegetated Areal Extent

(m2)

Spatial Distribution

(m2)

Percent Vegetated Cover

(VAE/SD * 100)

Density (turions/m2) n=20

APE 79 167 1,107 15.1% 72.033.8

Reference Area 400 646 3,174 20.4% 68.841.6

The present baseline survey revealed a distribution of eelgrass similar to that observed in previous surveys conducted of the area (Merkel & Associates 2020, 2021, 2022, 2024). Eelgrass within the APE and reference area is 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 epiphytic loading of less than 10 percent, no sedimentation, and no signs of disease. Leaf canopy extended approximately 0.5 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 are 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 also be a factor constraining the bed vertical range. The upper margin of the beds is limited by depressed salinities.

Merkel & Associates, Inc. #24‐103‐01 5

DISCUSSION AND RECOMMENDATIONS

The present survey is a condition survey to support the permitting process for maintenance dredging. Within the maintenance dredging APE, eelgrass is restricted to the shoreward margin of to the northwest of the docks. Historically, eelgrass also occurred near the travel lift. However, with the recent increase in shoaling within this area, eelgrass has been overrun by sand and extirpated from these small beds. The rapid shoaling and steep shoal slopes preclude eelgrass from developing along the active shoal at the eastern edge of the Cape Disappointment Station.

As dredge areas are defined, it will be possible to fully assess the potential for eelgrass impacts.

Based on the results of the survey and past dredge footprints, maintenance dredging at the Cape Disappointment Station may impact eelgrass along the shoreward margin of the northwestern portion of the basin. The potential for impacts is due to possible slumping of slopes adjacent to the dredge area rather than direct excavation impacts. The risk of eelgrass impacts may be lessened by completing benched cuts along the toe of the eelgrass supporting slopes, rather than making deep cuts that result in slope destabilization and slides.

It has been a pleasure working with you on this project. If you have any questions regarding the results of this investigation, please contact me.

Sincerely, Keith W. Merkel Principal Consultant

Merkel & Associates, Inc. #24‐103‐01 6

REFERENCES

Merkel & Associates. 2020. U.S. Coast Guard Station Cape Disappointment ‐ Baseline Eelgrass Survey. September 2020.

Merkel & Associates. 2021. Eelgrass Mitigation and Monitoring Plan, USCG Station Cape Disappointment Maintenance Dredging Project, Pacific County, Washington. June 2021.

Merkel & Associates. 2021. U.S. Coast Guard Station Cape Disappointment – Pre‐dredge Eelgrass Survey. October 2021.

Merkel & Associates. 2022. U.S. Coast Guard Station Cape Disappointment – Post‐dredge Eelgrass Survey. July 2020.

Merkel & Associates. 2024. U.S. Coast Guard Station Cape Disappointment – Pre‐dredge Eelgrass Survey. May 2024.

Merkel & Associates. 2024. U.S. Coast Guard Station Cape Disappointment – Post‐dredge Eelgrass Survey. June 2024.

National Oceanic and Atmospheric Administration (NOAA) 2014. California Eelgrass Mitigation Policy and Implementing Guidelines

Washington Department of Fish & Wildlife (WDFW) 2008. Eelgrass/Macroalgae Habitat Interim Survey Guidelines. (Rev. June 16, 2008).