DID YOU KNOW?
by Shawna Anderson
Spring 2008

San Dieguito Lagoon Inlet

On March 16, 2008, the San Dieguito Lagoon inlet closed to the ocean, trapping water in the lagoon.

Inlet taken on April 17, 2008

This happens frequently at lagoons along the Southern California coastline because of their physical characteristics. Because our lagoon inlet channels are smaller and shallower than those of most lagoons, both nationally and internationally, they tend to fill with sediment more frequently, plugging the entrance to the ocean. The San Dieguito Lagoon inlet channel is 100 feet wide at its maximum, much narrower than a more typical inlet of 700 feet wide.

This most recent closure is affecting not only the health of the lagoon but also the 150-acre wetland restoration project currently being conducted by Southern California Edison (SCE). Once the lagoon has closed to tidal flushing, its characteristics start to change. Water levels start to rise and eventually so do temperature and salinity, harming water quality and the general health of the lagoon. In this most recent closure, brackish water has been trapped in the lagoon. Eventually if the lagoon is left in this condition, water temperatures will rise during the hot summer months and evaporation will turn the lagoon into a hypersaline environment. Water levels are much higher today than they were before the inlet closed.   These high water levels inundate the sensitive marsh habitat in the lagoon, leaving fewer places for birds to nest during the breeding season (February - September). Recently graded areas that will become wetlands and accommodate new marsh habitat as part of the SCE wetland restoration project are now partially inundated. Although salt marsh plants tolerate (and in fact thrive on) periodic inundation, long-term inundation creates unfavorable conditions that are detrimental to the habitat.

a: Dissolved oxygen and salinity values during April 2008.

Prior to this most recent closure, the lagoon inlet had actually been open since September 2006, when it had to be opened mechanically. The lagoon typically closes almost annually. Since that opening, sand moving into the lagoon from ocean waves and tides has slowly been filling the channel bottom. This sediment reduces the capacity of the channel to carry sand coming from the river back out to the ocean; thus, more sand has been deposited into the channel. Instead of moving in and out through the channel, sand settles and eventually fills the channel. Big storm events that generate large waves can also close the river mouth much more rapidly.

Luckily, the inlet can be opened mechanically before the lagoon’s health has been irreparably harmed. Working with the City of Del Mar under a Coastal Commission emergency permit, SCE’s contractor started dredging the inlet channel on April 25th to begin the process of opening the inlet. In this process, sand is removed from the channel downstream of the railroad tracks with excavators and front loaders and deposited on the nearby beach. A total of 15,000 cubic yards of sand will be removed and deposited, and the inlet is expected to be officially re-opened on May 6 th or 7 th.

Sand Placed on the Beach, south of the inlet

Timing is important to achieve the most cost-effective and successful operation. Usually, low tide conditions are best so that tidal flow pushes the remaining sand out of the channel to the beach. For this reason, the contractor is opening the lagoon at low tide. The opening should restart the regular tidal flushing that is so critical to the healthy functioning and existence of the lagoon.

Even after expending large amounts of time and intense effort to open the lagoon, costing several thousand dollars, the lagoon mouth can still close again within a day. In the past, it has only taken one large storm to fill the channel with sand again. For this reason, SCE will need to manage the river inlet so that it will stay open as far into the future as possible.

Once construction of the wetland restoration project has been completed in 2009, SCE will maintain the inlet in an open condition. The channel sediment will be cleaned out on a regular basis, about once every 8-12 months so that it does not accumulate beyond a certain level. Because removing built-up sand will occur on a regular basis, expensive emergency efforts will be avoided, resulting in less expense and work overall. Tidal flushing will also continue uninterrupted to keep the newly created and fragile wetland in a healthy condition.

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