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Home My WebLink AboutSUP 06-10X2A; AGUA HEDIONDA OUTER LAGOON MAINTENANCE; PRE-DREDGE EELGRASS SURVEY IN SUPPORT OF ENCINA POWER STATION 2017-2018 DREDGING PROJECT; 2017-11-14 Merkel & Associates, Inc. 5434 Ruffin Road, San Diego, CA 92123 Tel: 858/560-5465  Fax: 858/560-7779 e-mail: associates@merkelinc.com November 14, 2017 M&A# 14‐051‐06 Mr. Shane Beck Ms. Sheila Henika MBC Applied Environmental Sciences Cabrillo Power I LLC 3000 Red Hill Avenue Encina Power Station Costa Mesa, CA 92626 4600 Carlsbad Boulevard Carlsbad, CA 92008‐4301 Re: Pre‐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 Applied Environmental Sciences to perform pre‐construction and post‐construction 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 is being performed under U.S. Army Corps of Engineers permit number 200100328‐RRS. The permit allows maintenance dredge operations to impact eelgrass within the 13.1‐hectare (32.4‐acre) area dredge limits delineated in the permit. Dredged sand will be discharged along the beach on the north and south sides of the Agua Hedionda Lagoon entrance jetty. Sand will be placed as far north as Pine Street and as far south as Cannon Road, where the beach ends. The purpose of this report is to provide information regarding the pre‐dredge distribution of eelgrass in the Outer Lagoon and control 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. Most of the proposed dredging is anticipated to target the sandbar in the northern portion of the lagoon. However, some level of maintenance dredging is expected to occur in other portions of the lagoon within the permitted dredge limits. As in prior dredging cycles, the Middle Lagoon was surveyed to act as a reference area for the project. This reference site is 9.5 hectares (23.5 acres) and will allow comparison of the dredged and non‐ dredged areas. Project Vicinity MapEncina 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 Pre‐Dredge Eelgrass Survey November 2017 Merkel and Associates # 14‐051‐06 3 SURVEY METHODOLOGY The pre‐construction survey of the Outer and Middle Lagoons was conducted on October 26, 2017 by M&A staff, Daniel Kahl and Christopher Hughes. 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 sites to assess the density and health of eelgrass. Data were collected by randomly placing 1/16th square meter quadrats 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. 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 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. 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. Outer Lagoon Pre‐Dredge Eelgrass Survey November 2017 Merkel and Associates # 14‐051‐06 4 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 mapping method applied during this investigation provides for a substantial degree of accuracy and repeatability over time. PRE‐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 450 m2 (0.1 acre), and the total areal extent of eelgrass within the reference site amounted to approximately 35,202 m2 (8.7 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 ranged from 0‐20 percent in Outer Lagoon and 10‐60 percent in the Middle Lagoon. Eelgrass canopy height ranged from 0.1 to 0.9 meters within the Outer Lagoon and from 0.2 to 0.8 meters 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. The majority of the eelgrass located within the dredge limits was found near the intake structure of the power plant with only a small amount of eelgrass occurring north of the channel extending under the railroad bridge to the Middle Lagoon (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 47,430 28,529 26,000 91.1% Outer Lagoon Inside Dredge Limits 3,076 450 215 47.7% 221.9 ± 118.0 Reference Area Eelgrass Beds Spatial Distribution (m2) Eelgrass Areal Extent (m2) Vegetated Cover (m2) Percent Vegetated Cover Density (turions/m2) (n=20) Middle Lagoon 50,333 35,202 32,920 93.5% 107.7 ± 42.2 Pre-dredge Eelgrass Survey - October 2017 Encina Power Station - Agua Hedionda Lagoon2017-2018 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 Pre‐Dredge Eelgrass Survey November 2017 Merkel and Associates # 14‐051‐06 6 IMPACT ANALYSIS Provided that dredging activities are confined to the area within the dredge limits, the proposed dredging is anticipated to impact up to 450 m2 (0.1 acre) of eelgrass that currently occurs within the dredge limit. These impacts have been previously mitigated through transplantation in the Inner Lagoon and are authorized by agency agreement and the U.S. Army Corps of Engineers permit. To avoid impacts to adjacent eelgrass outside of the dredge limits, dredge and assist vessels should operate only within the dredge limits. No impacts to eelgrass are anticipated due to the discharging activities at the beaches. This survey completes your pre‐construction eelgrass survey obligations and is valid for construction activities in the project area initiated by March 1, 2018. Dredging is expected to begin in Winter 2018. After the dredging is complete, a post‐dredge survey will be performed using the same methods described in this report. Once the survey is completed, a comparison will be made between the pre‐ and post‐dredge surveys to determine if there has been a dredge‐related loss of eelgrass outside of the permitted limits. 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 Pre‐Dredge Eelgrass Survey November 2017 Merkel and Associates # 14‐051‐06 7 REFERENCES [M&A] Merkel & Associates, Inc. 2014. Pre‐dredge eelgrass survey in support of the Encina Power Station, Agua Hedionda 2014‐2015 Dredging Project at Agua Hedionda Lagoon (Outer Lagoon), Carlsbad, CA. Letter Report; November 10, 2014. ‐‐‐‐‐‐‐‐‐‐‐2010. Pre‐dredge eelgrass survey in support of the Encina Power Station, Agua Hedionda 2010‐ 2011 Dredging Project at Agua Hedionda Lagoon (Outer Lagoon), Carlsbad, CA. Letter Report; December 28, 2010. ‐‐‐‐‐‐‐‐‐‐2008. Pre‐dredge eelgrass survey in support of the Encina Power Station, Agua Hedionda 2008‐ 09 Dredging Project at Agua Hedionda Lagoon (Outer Lagoon), Carlsbad, CA. Letter Report; December 5, 2008. ‐‐‐‐‐‐‐‐‐‐2006. Pre‐dredge eelgrass survey in support of the Encina Power Station, Agua Hedionda 2007 Dredging Project at Agua Hedionda Lagoon (Outer Lagoon), Carlsbad, CA. Letter Report; December 15, 2006. National Marine Fisheries Service. 1991 (as revised). Southern California Eelgrass Mitigation Policy. Revision 11. NOAA Fisheries, West Coast Region. 2014. California Eelgrass Mitigation Policy and Implementing Guidelines. October 2014.