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San Diego Regional Beach Sand Project

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San Diego Regional Beach Sand Project SAN DIEGO REGIONAL BEACH SAND PROJECT F I N A L R E P O R T 24 Hour Vibracoring Off Southern California Coast OFFSHORE SAND INVESTIGATIONS Prepared For: San Diego Association of Prepared by: Sea Surveyor, Inc. Governments (SANDAG) 960-C Grant Street 401 B Street., Suite 800 Benicia, CA 94510 San Diego, CA 92101 TEL: (707) 746-1853 (619) 595-5300 FAX: (707) 746-0184 APRIL 1999 Table of Contents SECTION TITLE Page 1. EXECUTIVE SUMMARY 3 2. SURVEY METHODS, LABORATORY TESTING, AND ANALYTICAL TECHNIQUES 8 2.1 Field Survey Methodology 2.1.1 Geophysical Survey Methods 2.1.1.1 Survey Vessel 2.1.1.2 Navigation & Hydrographic Equipment 2.1.1.3 Side Scan Sonar and Magnetometer 2.1.1.4 Shallow Seismic Subbottom Profilers 2.1.2 Vibratory Coring 2.1.2.1 Vessel 2.1.2.2 Vibratory Corers 2.1.2.3 Penetration Recorder 2.2 Laboratory Testing 15 2.2.1 Grain Size Analyses 2.2.2 Chemical Analyses 2.3.2 Petrographic Analyses 2.3 Analytical Techniques 17 2.3.1 Trackline Chart and Seafloor Features Map 2.3.2 Subbottom Isopach Maps 2.3.3 Volume Computations 3. RESULTS AND SITE DESCRIPTIONS 20 3.1 Regional Results 3.1.1 Sediment Chemistry 3.1.2 Lithologic Analyses 3.1.3 Tide Data 3.2 Site Descriptions 23 3.2.1 Site SO-9 3.2.2 Site SO-8 3.2.3 Site AH-1 3.2.4 Site SO-7 3.2.5 Site SO-6 3.2.6 Site SO-5 3.2.7 Site SO-4 3.2.8 Site MB-1 3.2.9 Site SS-2 3.2.10 Site SS-1 4. BIBLIOGRAPHY 63 A. TRACKLINE CHARTS B. GRAIN-SIZE and CHEMISTRY TABLES C. GEOTECHNICAL CORE LOGS. D. PHOTOGRAPHS OF SEDIMENT CORES. E. GRAIN SIZE DISTRIBUTION CURVES SECTION 1 EXECUTIVE SUMMARY San Diego County is experiencing a net loss of sand from numerous beaches along its coastline. To address and remedy this problem, the San Diego Association of Governments (SANDAG) instituted the San Diego Regional Beach Sand Project to evaluate the possibility of using offshore sand borrow sites to replenish the beaches. SANDAG has established the following criteria for selecting the offshore sand borrow sites: • The offshore borrow sites should be close to the beaches requiring sand nourishment. • Based on guidelines specified by the US Army Corps of Engineers (USEPA/USACE, 1991), sand suitable for beach replenishment shall be fine- to coarse-grained with a narrow size gradation. The mean grain size diameter (d50) shall range from between 0.2 and 0.6 mm. A maximum of 10% by weight can be silt or clay (0.074 mm), and a maximum of 10% by volume of the material may be larger than sand and no larger than cobble. To add an intermediate level, this report classifies as "marginal" any material that contains a silt or clay content of 10-15% . • The offshore sand borrow sites should be located deeper than 30' to 50' of water because in the San Diego region the "depth-of-closure" for seasonal bathymetric profile changes occurs between these depths (SANDAG, 1998). Dredging shallower than the depth-of- closure merely relocates material already within the littoral zone; conversely, dredging deeper than the depth-of-closure results in introducing new material on to the beaches. • The offshore boundaries for the proposed borrow sites should be no deeper than approximately 80'-90', which is believed to represent the limit for offshore dredge operations (Moffatt & Nichol, 1999). Based upon these criteria and review of the historical data collected along the San Diego coastline, SANDAG has preliminarily selected 10 offshore borrow sites (Figure 1) that are located adjacent to beaches requiring sand nourishment between Oceanside and the US-Mexico Border. To update and expand upon the historical investigations, SANDAG contracted Sea Surveyor, Inc. of Benicia, California to conduct offshore field investigations at the 10 proposed borrow sites. The objective of this offshore sand investigation is to: 1) map the horizontal and vertical extent of suitable beach sand within the 10 proposed borrow sites, and 2) compute the volume of suitable beach replenishment material contained within each site. During January 1999, geophysical surveys were conducted at 9 of the 10 proposed borrow sites and vibracore sediment samples were collected at all 10 sites. The geophysical surveys used differential GPS navigation to map each site with a sidescan sonar, a marine magnetometer, Page 4 FIGURE 1: Location of the 10 Proposed Offshore Sand Borrow Sites. TABLE 1: Borrow Site Dimensions, Extent of Field Investigations, and Sand Volumes. Surface Geophysical Number of Sand Volume Site Location Area (acres) Survey Area Vibracores (million cubic yds) SO-9 Offshore of Santa Margarita River 344 3,000' x 5,000' 12 0.9 SO-8 Offshore of San Luis Rey River 459 4,000' x 5,000' 23 4.7 AH-1 Offshore Agua Hedionda Lagoon 275 3,000' x 4,000' 10 0.0 SO-7 Offshore of Batiquitos Lagoon 287 2,500' x 5,000' 20 1.1 SO-6 Offshore of San Elijo Lagoon 230 2,500' x 4,000' 5 0.8 SO-5 Offshore of San Dieguito Lagoon 275 3,000' x 4,000' 10 6.2 SO-4 Offshore Los Penasquitos Lagoon 230 2,500' x 4,000' 10 1.5 MB-1 Offshore of Mission Beach 413 4,000' x 4,500' 10 26.0 SS-2 Offshore of Imperial Beach 482 3,000' x 7,000' 10 0.9 SS-1 Adjacent to US-Mexico Border 631 None 15 7.6 Page 5 2 types of subbottom profilers, and a survey-grade depthfinder. After interpreting the geophysical data, a recommendation was presented to SANDAG regarding the number and location of sediment core samples that should be collected in each site. After SANDAG accepted the vibracoring plan, a total of 125 sediment cores were collected in the sites using a 20' ALPINE vibratory corer. The collected core samples were transported to Oceanside Harbor on a daily basis, where geotechnical personnel split, logged, photographed, subsampled, and archived the sediment cores. The subsamples from the vibracores were analyzed for grain-size, lithology, and chemical constituents by MEC Analytical Systems of Carlsbad, California. Table 1 summarizes the location and dimension of the sites, the field investigations that were conducted in each, and the estimated volume of suitable beach nourishment material which they contain. Chemical testing of the composited sediments collected at the offshore borrow sites were conducted to determine the suitability of the material for beach replenishment. The results from the chemical testing were compared to sediment samples collected in the tidal and intertidal zones of 6 receiver beach sites (Imperial Beach, Torrey Pines, San Dieguito Lagoon, Batiquitos Lagoon, Buena Visa Lagoon, and Oceanside), with no significant difference found. Total organic carbon concentrations in the proposed borrow site sediment ranged from 0.017 to 0.216%. Total sulfide concentrations ranged from 0.2 to 1.1 mg/Kg and dissolved sulfides were mostly non-detectable. Concentrations of pesticides, polychlorinated biphenyls, polynuclear aromatic hydrocarbons, or phenol were also not found. Concentrations of metals, arsenic, chromium, copper, lead, nickel, selenium, and zinc were detected. Results from the chemical analyses are expressed in dry weight, and presented in Appendix B. The following sections present a brief description of the proposed offshore borrow sites. SITE SO-9: Site SO-9 is located north of Oceanside Harbor and offshore of the Santa Margarita River in 50' to 80' water depths. This site is the northern-most sand borrow site, and it lies within the Oceanside Littoral Cell. Eight (8) artificial reef habitats, comprised of piles of quarry rock, are located near the center of the site and may present an obstruction to dredging. Three (3) layers of sediments occur inshore of the artificial reefs within Site SO-9: 1. The top (surficial) layer consists of sandy silt that is unsuitable for beach nourishment material. The thickness of this silt layer measures about 12' in the central region of the site and gradually thins out before reaching the nearshore boundary of the site. This surficial layer of unsuitable sandy silt has an estimated volume of 0.4 million cubic yards. 2. Buried beneath the surficial layer of sandy silt is a layer of fine- to medium-grained sand that is suitable for beach nourishment. This sand layer is 3' to 23' thick, and is exposed on the seafloor surface along the nearshore boundary of the site. An estimated 0.9 million cubic yards of suitable beach nourishment material is contained within this layer. 3. A third sediment layer, consisting of fine-grained silty sand that is unsuitable for beach nourishment, was found under the sand layer. SITE SO-8: SO-8 is located south of Oceanside Harbor and offshore of the San Luis Rey River in 50' to 90' water depths. The seafloor within the site is covered by a 4'-13' thick layer (average 10') of sandy-silt or very fine-grained silty sand that is unsuitable for beach nourishment purposes. In the northeast corner of the site, buried beneath an estimated 2.5 million cubic yards of unsuitable material, lies approximately 4.7 million cubic yards of fine-grained sand that, although quite silty, has been determined to be suitable for beach nourishment material. Page 6 SITE AH-1: This site is located offshore of Aqua Hedionda Lagoon in 60' to 130' of water. Approximately 50% of Site AH-1 is deeper than 90', which may be too deep for dredging with conventional equipment.
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