3.1 Geology and Soils
Sediment core samples were taken at 125 locations in January 1999. Subsamples from the sediment cores
were later studied in a laboratory to determine grain-size and chemicalmake-up for evaluating compatibility
between borrow site material and sand at the receiver sites.
Tables 2-3 and 2-5 shows various construction characteristics of the borrow sites. The discussion below
includes additional information about the dredge sites regarding geology and soils, including the volume and
thickness of silt overburden and the thickness of the sand layer.
SO-9
Site SO-9 is located north of Oceanside Harbor and offshore of the Santa Margarita River. The original
area of investigation was over 340 acres, and the project dredge area had a surface area of 63 acres in 45
to 50 feet of water. This dredge area has been reduced in the Final EIR/EA by approximately 25 percent
R
to provide a greater buffer from previously unmapped artificial reefs. CDFG review of the side-scan sonar
R
imaging at SO-9 revealed that some targets of unknown origin were remnants ofan artificialreefexperiment
R
that had been mismapped after placement. The artificial reef had not been included in published sources
R
because it had not been re-located.
R
At the SO-9 borrow site, three layers of sediments were found. The top (surficial) layer consists of sandy
silt. The grain-size analyses showed this surficial layer to be approximately 0 to 2 feet deep over the dredge
site, and unsuitable for beach nourishment material. Beneath the surficial silt is a layer of fine- to medium-
grained sand that is suitable for beach replenishment. This sand layer is 2 to 13 feet thick, and is exposed
on the seafloor surface along the central region of the site. A third layer, consisting of fine-grained silty
sand, was found under the sand layer. The grain-size analyses showed this layer to be unsuitable for beach
nourishment material.
SO-7
Site SO-7 is located offshore of Batiquitos Lagoon. The area of investigation covered an area of
approximately 280 acres but the project dredge area would be approximately 70 acres in 60 to 85 feet of
water. If SO-7 is expanded and SO-9 and/or SO-6 are eliminated, then the dredge area would increase
R
up to 150 acres with 1.5 million cy of material removed. Most of the site is predominantly well-sorted
R
medium- to coarse-grained sand with occasional pebbles and shell fragments. This material is ideal for
Page 3.1-10
Regional Beach Sand Project EIR/EA
99-69\SANDAG EIREA 3.4.wpd 7/17/00
Previous Page
