2.0 Alternatives Considered
Hopper Dredge
The hopper dredge is a self-contained vessel that loads sediment from an offshore borrow site then moves
to a receiver site for sand placement. The hopper dredge contains two large arms that have the ability to
drag along the ocean floor and collect sediment. The drag heads are about 10 feet square. The hopper
R
dredge moves along the ocean surface with its arms extended, making passes back and forth until its hull
is fully loaded with sediment. The vessel can hold approximately 2,000 to 5,000 cy of sediment per load.
The hopper dredge can generally reach within approximately 0.5 mile of shore to offload, unless booster
pumps are placed along the beach that can increase its pumping distance. At this position, the hopper
dredge connects to a floating or submerged pump line from shore. The vessel then discharges a mixture
of sediment and sea water onto the receiver site. Submerged lines are encased by several large tractor tires
to prevent abrasion of the ocean floor or reefs.
The hopper dredge can also connect to a floating platform called a mono buoy, which is used to
interconnect the floating pump line with a steel sinker pipeline that would run the rest of the distance to the
beach. The mono buoy is generally anchored to the seabed at an appropriate depth and location to serve
the project needs, depending on locations of sensitive resources and engineering considerations. For this
R
project the mano bouy would be anchored in at least 25 feet of water. The contractor would also be
R
conditioned to avoid sensitive resources such as kelp, reefs, and structures such as outfalls. An anchor plan
R
would be prepared for each mono buoy for submittal to the resource agencies prior to construction
R
illustrating any or all sensitive resources and the relationship between anchors on the ocean floor.
R
Cutterhead Suction Dredge
A cutterhead suction dredge is similar to a hopper dredge in that it uses a long arm that extends down to
the sea floor to dredge sediment. A rotating head about eight feet in diameter sweeps an area about 300
R
feet wide. However, a cutterhead dredge breaks up sediment material along the seafloor, then uses a
R
vacuum mechanism to suck up sediment into an intake line and pump it directly to shore through a discharge
line. The cutterhead dredge anchors above a borrow site while its arm swings back and forth to dredge
up sediment, and pumps a mixture of sediment and sea water through a floating discharge line directly onto
the receiver site. The discharge line would either be assembled afloat and connected to the cutterhead
suction dredge to land by tugboats, or assembled on land and dragged offshore to the dredge by tugboat.
Unlike the hopper dredge, the cutterhead dredge remains at the dredge site for the entire operation while
pipelines carry the material.
Page 2-40
Regional Beach Sand Project EIR/EA
99-69\SANDAG EIREA 3.4.wpd 7/17/00
Previous Page
