4.1 Geology and Soils
North Carlsbad. Alternative 1 would involve placing approximately 240,000 cy of dredged sediment at
the proposed North Carlsbad receiver site. Previous placement of fills on beaches in Carlsbad have not
shown dramatic changes in the littoral process. Over 12 million cy of fill has been placed onshore in
Carlsbad as a result of maintenance dredging of Agua Hedionda Lagoon and enhancement of Batiquitos
Lagoon. No adverse impacts to littoral transport have occurred. These past beach fills were in the same
range or less than the proposed fill quantity. Therefore, based on past fill events, placement of sediment
onshore at North Carlsbad would not change the littoral transport process.
Remaining Oceanside Littoral Cell Receiver Sites. No significant impacts would occur to the littoral
process. For the other receiver sites in the Oceanside Littoral Cell, (i.e, South Carlsbad North, Batiquitos,
Leucadia, Moonlight Beach, Cardiff, Solana Beach, Del Mar, and Torrey Pines). The existing offshore
sand berm would increase some amount after the first year, but that thickness would be less than a foot and
typically in the range of less than one-half foot. Only minor increases in thickness to the respective offshore
bars is anticipated. No significant impacts are anticipated. This alternative would also serve to temporarily
stabilize fragile bluffs near the South Carlsbad North, Batiquitos, Leucadia, Moonlight Beach, and Torrey
Pines receiver sites.
Mission Beach. Alternative 1 would involve placing approximately 100,000 cy of dredged sediment at
the Mission Beach receiver site. The length of the coast affected by the beach fill increases each year. The
initial beach fill length is approximately 1,590 feet. At post-project equilibrium, the length of beach
widening increases to 15,000 feet, and at the end of the first year, the length of coast affected increases to
20,000 feet based on an analytical modeling method. The fill remains perceptive along approximately
20,000 feet through the fifth year. Approximately 16 percent of the post-construction beach width at the
project's centerline remains after five years (Moffatt & Nichol 2000a). No significant impacts are
anticipated due to the replenishment action proposed under Alternative 1.
The net longshore sediment transport rate at Mission Beach is predominantly to the south. The beach fill
will likely spread in both directions, but primarily to the south toward Mission Jetty. A sand berm would
be expected to form in the shallow subtidal areas as a result of sediment transported into these zones, but
no significant increases are expected as a result of implementation of Alternative 1.
Imperial Beach. Alternative 1 would place approximately 120,000 cy of dredged sediment at the Imperial
Beach receiver site. The length of the shoreline that is affected by the beach fill increases each year. The
Regional Beach Sand Project EIR/EA
Page 4.1-7
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