Beach analysis paints a pricy picture for Del Mar

Combating sea-level rise by spreading sand on Del Mar’s beaches will likely suffice for a few decades, then become an exorbitantly expensive and uncertain undertaking, according to the first comprehensive analysis of sediment migration patterns in Del Mar.

Known as “sand nourishment,” placing sand on Del Mar’s beaches emerged over the past six months as the primary strategy for maintaining beach widths as the ocean rises, after the city’s task force on sea-level rise took a much-disputed turn away from “managed retreat.”

The Sea-Level Rise Stakeholder-Technical Advisory Committee (STAC) has been awaiting the sand management plan since last summer. The committee’s environmental consultant gave a brief outline on Feb. 22 before releasing its draft of the plan on March 8. STAC is set to review the sand plan at its March 22 meeting. See the draft here.

The report’s broad assessment: while beach nourishment should suffice in keeping Del Mar’s iconic beaches from disappearing into the Pacific over the short term, that effectiveness will plummet once sea-level rise begins to accelerate, eventually costing more than $20 million every decade for Del Mar to maintain its beaches’ widths.

Because STAC turned to beach nourishment as Del Mar’s primary strategy for combating sea-level rise, sand management will be an integral part of the city’s broader plan for adapting to the impacts expected to come with higher water levels. The city council is scheduled to hold its first discussion of the Adaptation Plan on April 16.

For the sediment management plan, ESA (Environmental Science Associates) developed a sand “budget” based on extensive data explaining how sand moves among the San Dieguito River, its lagoon, onto Del Mar’s beaches, and out into the broader sediment system known as the Oceanside Littoral Cell, which runs from Dana Point to Point La Jolla.

One of the report’s most disheartening—though unsurprising—findings is that Del Mar beaches are losing sand even as neighboring beaches gain it. Under current conditions, more than 17,000 cubic yards of sand needs to be placed each year in Del Mar just to keep beach widths, at an expected annual cost of $500,000.

Once sea-level rise takes full hold, Del Mar’s sand shortage steepens sharply.

At 1 foot of sea-level rise, Del Mar will need roughly 52,000 cubic yards per year to maintain beach width, with a cost of around $850,000. At 2 feet of sea-level rise, the need climbs to roughly 70,000 cubic yards and $1.3 million per year. If the Pacific rises 5.5 feet, Del Mar could need nearly 100,000 cubic yards of sand every year at an annual expense of more than $2 million.

A Scripps study published last year—which looks at tide gauges in Crescent City, San Francisco and La Jolla—expects minor changes in ocean levels through 2050. But after that, global warming and melting Antarctic ice sheets will combine to accelerate sea-level rise, the study says. Its lowest estimate predicts the ocean at La Jolla will rise 1.1 feet by 2100. The highest estimate predicts 7.1 feet.

Finding the necessary sand will be another matter altogether. Even Del Mar’s immediate needs exceed the available supply. The existing annual deficit of 17,000-cubic yards cannot be met by dredging the San Dieguito Lagoon. Under the terms of the lagoon’s $90 million restoration, Southern California Edison (SCE) is required to keep the inlet open and maintain a channel. SCE has needed to dredge the lagoon only twice since 2011, each time placing roughly 16,000 cubic yards of sand on North Beach.

A bonanza of sand awaits beneath Lake Hodges, where the century-old dam choked off nearly all of Del Mar’s natural sand supply. Roughly 4 million cubic yards of sand has built up behind the dam, but accessing it will be a fraught political endeavor.

Del Mar’s best bet, according to the report, will be to partner with regional efforts to mine offshore sources.

Other options include sand retention structures such as reefs and groins; more frequent dredging the mouth of the San Dieguito River, possibly more than once a year; and dredging an unknown quantity of sand that appears to have accumulated in the river between Jimmy Durante Boulevard and the railroad.

Over the longer term, sea-level rise will likely make river dredging untenable. If so, Del Mar may have to build levees along the perimeter of the lagoon to protect its neighborhoods against the increasingly frequent and devastating floods that would result from higher water levels.

Copyright © 2018, Del Mar Times
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