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Home My WebLink AboutSUP 06-10X2A; AGUA HEDIONDA OUTER LAGOON MAINTENANCE; POST-DREDGE EELGRASS SURVEY IN SUPPORT OF THE ENCINA POWER STATION 2017-2018 DREDGING PROJECT; 2018-05-23 Merkel & Associates, Inc. 5434 Ruffin Road, San Diego, CA 92123 Tel: 858/560-5465  Fax: 858/560-7779 e-mail: associates@merkelinc.com May 23, 2018 M&A# 14‐051‐06 Mr. Shane Beck Ms. Sheila Henika MBC Aquatic Sciences Cabrillo Power I LLC 3000 Red Hill Avenue Encina Power Station Costa Mesa, CA 92626 4600 Carlsbad Boulevard Carlsbad, CA 92008‐4301 Re: Post‐dredge eelgrass survey in support of the Encina Power Station 2017‐2018 Dredging Project at Agua Hedionda Lagoon (Outer Lagoon), Carlsbad, CA Dear Shane and Sheila, PURPOSE AND INTRODUCTION Merkel & Associates Inc. (M&A) has been retained by MBC Aquatic Sciences to perform pre‐dredge and post‐dredge eelgrass surveys for the maintenance dredging operations in the Outer Lagoon at Agua Hedionda Lagoon in Carlsbad, CA (Figure 1). The dredging is part of an ongoing effort by Cabrillo Power to prevent intruding sands from forming sandbars in the Outer Lagoon. The work was performed under U.S. Army Corps of Engineers permit number 200100328‐RRS. The permit allows maintenance dredging operations to impact eelgrass within the 13.1‐hectare (32.4‐acre) area dredge limits delineated in the permit. Dredging was initiated on February 19, 2018 and was completed on April 13, 2018. Dredged sand was discharged along the beach. The purpose of this report is to provide information regarding the post‐dredge distribution of eelgrass in the Outer Lagoon and reference area. The survey work was completed in conformance with the California Eelgrass Mitigation Policy (CEMP) (NOAA 2014. PROJECT STUDY AREA Agua Hedionda Lagoon is a 95.1‐hectare (235‐acre) coastal estuary located within the City of Carlsbad, in northern San Diego County, California (Figure 1). The Outer Lagoon is approximately 21.5 hectares (53.1 acres) and generally ranges in depth from ‐34 feet MLLW at the channel mouth just west of the railroad trestle, to about 0 feet MLLW along the top edge of the sandbar in the northern portion of the lagoon. Dredging occurred within the northern portion of the lagoon where the sandbar was cleared. All dredging during the present dredge cycle occurred north of the channel that connects the Outer Lagoon to the Middle Lagoon. As in prior dredging cycles, the Middle Lagoon was surveyed to act as a reference area for the project. This reference area is 9.5 hectares (23.5 acres) and allows comparison of the dredged and non‐dredged areas. Project Vicinity Map Encina Power Station - Agua Hedionda Lagoon 2017-2018 Agua Hedionda Lagoon Maintenance Dredging Figure 1 14-051-06 Merkel & Associates, Inc. µ 0 100 20050Meters Agua Hedionda Lagoon Outer Lagoon SAN CLEMENTE CARLSBAD SAN DIEGO Map Area Inner Lagoon Middle Lagoon Outer Lagoon Post‐Dredge Eelgrass Survey May 2018 Merkel and Associates # 14‐051‐06 3 SURVEY METHODOLOGY The post‐dredge survey of the Outer and Middle Lagoons was conducted on May 1, 2018 by M&A biologists, Daniel Kahl and Daniel Hartsook. Data were collected using interferometric sidescan sonar, which provided an acoustic backscatter image of the seafloor within the survey areas. 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 interferometric sidescan system integrates motion sensors to control for heave pitch, and roll as well as 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 interferometric sidescan transducers within the positioning sensors provides significantly increased precision and accuracy over conventional towfish sidescan sonar equipment. The survey was conducted by navigating parallel tracklines, spaced to allow for overlap between adjoining sidescan swaths. Survey swaths were navigated until the entirety of the survey areas were 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 project site. Following completion of the survey, sidescan sonar traces were joined together and geographically registered. Eelgrass was digitized as a theme over an aerial image of the project site to calculate spatial metrics defining the extent and distribution of eelgrass. Metrics determined for eelgrass from the acoustic survey include: spatial distribution, areal extent, and percent vegetated cover within the areal extent of the beds. Eelgrass density data were collected within the APE and reference areas to assess the density and health of eelgrass. Data were collected by randomly placing a 1/16th square meter quadrat on the seafloor in areas where eelgrass occurred. Eelgrass turion (leaf‐shoot) densities were calculated by counting the number of turions within the sampled quadrats. 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 project site to calculate the amount of eelgrass coverage and present its distribution. This method of eelgrass distribution calculation allows for monitoring eelgrass trends at the project site with a substantial degree of accuracy and repeatability over time. The reported metrics for eelgrass followed the definition of survey parameters to be included as discussed under the CEMP. These 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 includes vegetated cover and extent of unvegetated habitat that defines a coalesced bed with gaps of less than 1 meter across being considered part of the defined bed. Outer Lagoon Post‐Dredge Eelgrass Survey May 2018 Merkel and Associates # 14‐051‐06 4 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 eelgrass habitat area. 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. The spatial distribution of eelgrass habitat was then determined by extending a consistent 5‐meter (16‐ foot) buffer outward from all mapped eelgrass and then refining the buffered area to exclude areas where existing overwater infrastructure, unsuitable depths, steep slopes, or substrate conditions would preclude eelgrass establishment. POST‐CONSTRUCTION EELGRASS SURVEY RESULTS Eelgrass spatial distribution from the survey is illustrated in Figure 2. The total areal extent of eelgrass inside the dredge limits (APE) amounted to approximately 470 m2 (0.1 acre), and the total areal extent of eelgrass within the reference site amounted to approximately 36,018 m2 (8.9 acre). Bed spatial and density metrics are summarized in Table 1. Eelgrass in both the Middle and Outer Lagoon was healthy in appearance and showed no sign of disease. Epiphytic loading of approximately 30 percent was observed on the eelgrass in Outer Lagoon and 20‐50 percent was observed on the eelgrass in the Middle Lagoon. Eelgrass canopy height ranged from 0.1 to 0.5 meters within the Outer Lagoon and from 0.2 to 0.6meters in the Middle Lagoon. There was no sediment accumulation noted on any of the leaves. The survey showed eelgrass occurring between 0 to –12 feet MLLW, primarily on shallow benches on the shorelines of the lagoon with deeper eelgrass occurring around the aquaculture racks. The majority of the eelgrass located within the dredge limits was found near the intake structure of the power plant (Figure 2). Table 1. Eelgrass bed metrics as defined under the CEMP. Area Of Potential Effect Eelgrass Beds Spatial Distribution (m2) Eelgrass Areal Extent (m2) Vegetated Cover (m2) Percent Vegetated Cover Density (turions/m2) (n=20) Outer Lagoon Outside Dredge Limits 54,830 35,601 31,488 88.4% Outer Lagoon Inside Dredge Limits 5,428 470 157 33.4% 176.0 ± 44.1 Reference Area Eelgrass Beds Spatial Distribution (m2) Eelgrass Areal Extent (m2) Vegetated Cover (m2) Percent Vegetated Cover Density (turions/m2) (n=20) Middle Lagoon 62,089 36,018 30,635 85.1% 113.1 ± 49.5 Figure 2 Post-dredge Eelgrass Survey - May 2018 Encina Power Station - Agua Hedionda Lagoon 2017-2018 Agua Hedionda Lagoon Maintenance Dredging Figure 2 14-051-06 µ Merkel & Associates, Inc. 0 100 20050Meters Middle Lagoon Outer Lagoon Study Area Reference Area Legend Dredge Limit Eelgrass Vegetated Cover Eelgrass Areal Extent Eelgrass Spatial Distribution Outer Lagoon Post‐Dredge Eelgrass Survey May 2018 Merkel and Associates # 14‐051‐06 6 IMPACT ANALYSIS AND DISCUSSION The pre‐ and post‐dredge eelgrass surveys were conducted to evaluate potential impacts to eelgrass associated with the project. To numerically evaluate changes in the eelgrass, the pre‐ and post‐dredge extent and density of beds was compared (Tables 2 and 3). In addition, a spatial change analysis was completed that digitally compares the pre‐dredge and post‐dredge surveys to identify area where eelgrass declined, expanded, or remained unchanged between surveys (Figure 2). Table 2. Project Eelgrass Areal Extent Summary Location Pre‐Dredge (October 2017) Post‐Dredge (May 2018) Area Difference (Post minus Pre) Percent Change Outer Lagoon APE 450 m2 470 m2 +20 m2 4% increase Middle Lagoon Reference 35,202 m2 36,019 m2 +817 m2 2% increase Table 3. Project Eelgrass Density Summary Location Pre‐Dredge Density (turion/m2) Post‐Dredge Density (turion/m2) Mean Percent Difference (Post/Pre)‐1)*100 Outer Lagoon APE 221.9118.0 176.044.1 20.7% decrease Middle Lagoon Reference 107.742.2 113.149.5 5.0% increase Within both the Outer Lagoon APE and the Middle Lagoon Reference Area, eelgrass expanded slightly from that observed in October 2017. During the present dredge cycle, none of the eelgrass in the APE of the authorized maintenance dredging area was impacted by dredging, although such impacts have previously been mitigated should they occur. No impacts to eelgrass were noted outside of the dredging limits. Mean densities within the Outer Lagoon eelgrass decreased by 20.7 percent while concurrently increasing by 5.0 percent in the Middle Lagoon Reference Area. However, the exceptionally high variance in eelgrass densities, particularly within the Outer Lagoon during October 2017, made the differences in density not statistically meaningful despite the apparent differences in change in mean density at the two sites. This survey completes your post‐dredging eelgrass survey obligations. In conclusion no adverse impacts to eelgrass were determined to have occurred in association with the shoal maintenance dredging. If you have any questions or need additional information, please do not hesitate to contact me at (858)‐ 560‐5465. Sincerely, Keith Merkel Principal Consultant Outer Lagoon Post‐Dredge Eelgrass Survey May 2018 Merkel and Associates # 14‐051‐06 7 REFERENCES Merkel & Associates, Inc. 2017. Pre‐dredge eelgrass survey in support of the Encina Power Station, 2017‐2018 Dredging Project at Agua Hedionda Lagoon (Outer Lagoon), Carlsbad, CA. Letter Report; November 14, 2017. NOAA Fisheries, West Coast Region. 2014. California Eelgrass Mitigation Policy and Implementing Guidelines. October 2014.