decreasing water velocity. As the water discharge decreases, the water in the embayment rises to
a level that floods private property used for cattle grazing and crop production. To alleviate the
flooding, landowners breached the berm to allow their fields to drain. The breaches rapidly
drained the embayment, destroyed fish habitat, and prematurely flushed young fish into the
ocean (Hofstra and Sacklin 1987).
To control water level in the estuary without destroying fish habitat, a process referred to as
controlled breaching was developed and implemented in the early 1980s. Controlled breaching
releases water at a rate in which fewer fish are entrained in the outflow and the embayment and
fish habitat is maintained. Hofstra and Sacklin (1987) provide a more comprehensive description
of the controlled breaching management plan. Controlled breaching of the creek mouth was
typically accomplished with hand tools or conventional earth moving equipment. A controlled
breach increases the flow gradient and flow velocity by reducing the length of the outflow
channel before the mouth is completely closed by the sand spit. The breach was controlled by
carefully selecting the location along the prograding spit that the breach cut was made. The
closer the breach cut is made to the embayment, the faster the outflow and the lower the resulting
embayment water level.
The controlled breaching method was successfully implemented from the early 1980s to the late
1990s to control flooding, maintain embayment integrity, and minimize fish losses. The permit
required to breach the Redwood Creek mouth expired in 2002, because changing physical and
political conditions have not required purposeful breaching in recent years.
San Dieguito Lagoon, Southern California.
San Dieguito Lagoon is a 140-acre wetland
located just north of the city of Del Mar in San Diego County, California. The lagoon is located
at the mouth of the San Dieguito River and forms an inlet to the Pacific Ocean. San Dieguito
Lagoon is typical of lagoons located on the southern California coast, providing a fish nursery
and endangered species habitat as well as food sources for migrating birds. Many coastal
lagoons in southern California are only marginally stable. Tidal flow and natural flooding from
rainwater runoff are often insufficient to keep coastal lagoon inlets open. After the inlet closes,
water quality decreases, and increased water levels create flooding problems (Elwany et al.
1998).
On time scales longer than a few years, river flooding is the primary process determining
whether San Dieguito Lagoon Inlet is open or closed. Over shorter time periods, the inlet
condition is controlled by river and lagoon bathymetry, the available tidal prism, and sand
transport along the beach (Elwnay et al. 1998). Plans for maintaining healthy lagoons have often
been based on increasing the tidal prism with large-scale dredging to deepen the lagoon and
constructing jetties to stabilize the inlet. An alternative inlet maintenance strategy applied at San
Dieguito is to purposefully breach the inlet, as necessary. The plan, coupled with scouring from
seasonal rainwater runoff, properly maintains tidal flushing (Elwany et al. 1997).
The lagoon inlet is breached with conventional earth-moving equipment and timed to coincide
with the spring tide to achieve maximum tidal flushing of the lagoon. The sand removal is
conducted with a front-end loader, excavators, and scrapers. The volume of sand removed
ranges from 5,000 to 15,000 cu yd and is typically placed on the beach south of the inlet so that
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