From the Society pages: Botox on the Beach?

July 13, 2013 § 10 Comments

2012AugBroad

Broad Beach in 2012 with stone revetment wall, Google Earth

1994JuneBroad

Broad Beach in 1994 with wider beach, Google Earth

A recent article in the society pages of Vanity Fair details the woes of property owners along Broad Beach in Malibu, where the narrowing of a beach by 60 feet over the last decade has alarmed wealthy residents. Property owners built a 13-foot high stone revetment wall to protect their houses. Now, they are planning to spend $20 million out of their own pockets to import 600,000 cubic yards of sand, hoping to widen the beach by 100 feet.

Apparently even the residents understand the addition of sand (“beach nourishment”) is at best a temporary solution. To maintain the width of the artificial beach, nourishment would have to be supplemented every 5-10 years—a cosmetic solution that JPL climatologist Bill Patzert called “botoxing the beach.” (Cohan and Grigoriadis 2013)

Nor are revetment walls a real solution. Though they appear to protect property immediately behind them, they actually reflect wave energy to other parts of the coast, where erosion is then accelerated.

Some attribute the erosion of Broad Beach to winter storms. Impending  sea level rise certainly will not help. Patzert and the Vanity Fair authors make the case that houses cannot be maintained forever on eroding beaches.

In this discussion, though, let’s not forget where beaches come from. In the large picture, beach replenishment is part of a natural cycle involving sediment, which is transported to the ocean via….guess what…. rivers and streams.

In a natural condition, rivers and streams carry suspended sediments toward the ocean. Wave action and the process of longshore transport then deposit these sediments along the coast.

A beach’s form is a dynamic equilibrium between erosion and deposition. There are natural seasonal cycles within this equilibrium. In the winter, storms and strong waves pull sand away from beaches, depositing sediments in offshore sandbars. In the summer, gentle swells push sand back onto shore, building up wide and gently sloping beaches. If waves remove sand faster than it is replenished, then the beach erodes. If there still existed sources of sediment, one would expect a beach eroded by winter storms to be gradually replenished through such processes over time.

Rivers and streams are the origin of 70-90% of beach sand. (California Beach Restoration 2002) However, with the damming of rivers, building of debris basins, and channelization of streams, sediment supply to Southern California coasts has been reduced by up to two-thirds. (California Beach Restoration 2002)

This might lead one to wonder, if we were somehow able to undam our rivers, would natural processes replenish our beaches without human intervention?

Complicating the answer to this question is the fact that many of our many of our popular beaches are themselves the result of artificial nourishment projects dating back to the early 1900s. Before the 1960s, beach nourishment was actually a secondary use for spoils from large scale infrastructure projects. (CSMW 2010) From 1940 to 1990, 100+ million cubic meters of sand (Flick 1993) have been artificially deposited on Southern California beaches. Beach nourishment has been so common that it is suggested that little ‘pristine’ beach material can be found in California south of Santa Barbara. (CSMW 2010)

In Santa Monica, natural sediment supplies were only enough to maintain beaches about as wide as Malibu’s. High rates of longshore transport, the loss of sediments to Redondo Submarine Canyon (California Beach Restoration Study 2002), and the fact that no major river supplies sand to the Santa Monica littoral cell (Flick 1993) are all factors that limited the width of Santa Monica’s natural beach.

SM1889 Security Pacific National Bank Collection SM00028345

Santa Monica beach in 1889, before beach nourishment, Security Pacific National Bank Collection, Los Angeles Public Library

Yet, in the second half of the 20th century, Santa Monica’s beach width tripled— from 30m to 150m (Flick 1993). This reflected the addition of 23 million cubic meters of nourishment, most of which came from large scale infrastructure projects: the expansion of Hyperion wastewater treatment facility (1947) and the dredging of Marina del Rey (1960s). (Flick 1993) Such artificially wide beaches are unlikely to have remained in place over the decades if not for the numerous breakwaters, jetties and groins that limit longshore transport, compartmentalizing Santa Monica’s shoreline. (Flick 1993)

The iconic wide beaches of Santa Monica generate recreation opportunities and tourism revenue. However, they do not reflect any natural condition, which according to Flick (1993) would have been characterized by dramatic fluctuations over time. Because of our local climate, where prolonged periods of low rainfall regularly occur, years may go by where very little sediment reaches the coast–  a pattern established even before flood control structures interrupted the cycle. During such years, one might expect beaches to thin. Episodic severe storms in the Pacific may also generate wave energy that, combined with other factors, have been known to entirely strip some beaches of sand. (Flick 1993)

The processes which replenish beach sands are also highly variable. The bulk of sediments are delivered to the coast during severe floods that occur every decade or so. During such flood events, the volume of sediment transported may be dozens of times greater than during an average year. (State of California 2002) The episodic nature of the factors responsible for both erosion and replenishment means that the appearance of Southern California’s beaches would be subject to dramatic long term fluctuations, if not for the elaborate measures taken by humans to stabilize the shoreline.

Does the Los Angeles River contribute sediments to local beaches? One would expect the beaches just south of Long Beach to receive a generous supply of sediment from the combined contributions of the Los Angeles, San Gabriel and Santa Ana Rivers. Yet, flood control has reduced the total amount of sediment contributed by these three rivers by about two thirds. (Flick 1993) The relatively modest volumes of sediment from the LA River watershed that manage to make it past debris basins and dams can  no longer even reach the ocean. Instead it is deposited in the harbor, where it must be dredged regularly. (Flick 1993)

The processes linking sediment production and transport to the hydrological cycle have been interrupted by development. To maintain the wide beaches we have grown to think of as natural, vast energy and economic inputs are needed. To maintain the flood and debris control capacity of dams and debris basins in the foothills, vast energy and economic inputs are needed.  Isn’t there something a little inelegant about these mirror image expenditures?

Thankfully, there are people working on this issue. In a 2002 study, State Coastal Conservancy and Department of Boating and Waterways conclude: “[B]each nourishment has been a short-lived engineering solution to a long-term engineering problem: sediment impoundment by dams.” The study recommends bypassing dams and removing dams that are obsolete, to supplement beach nourishment projects with an estimated 1.5 million cubic yards of material accumulating behind dams and debris basins annually.

The California Coastal Sediment Management Workgroup is seeking to establish a model of integrated sediment management whereby sediment issues can be resolved on a regional (littoral cell) level, rather than the local case by case basis that has been the default in the past.

In the meantime maybe some larger scale inspiration can be had by looking at some ambitious projects currently underway that involve restoring sediment transport processes…

In Washington state, researchers and environmentalists have been observing the restoration of natural processes after the removal of two dams on the Elwha River with great interest.

“So far, about 17 percent of the sediments have been eroded out of the upper reservoir, where they range from 30 to 50 feet thick… Where the river flows into the Strait of Juan de Fuca, the rapid formation of gravel bars since December has been gaining a lot of attention, Ogston said. Researchers can now walk out onto sandy beaches in areas previously under water… Meanwhile, around the edges of the dewatered reservoirs… Young trees are now growing alongside giant stumps that have emerged from the eroding sediment for the first time in decades. The stumps signify how a majestic forest was cut down to make room for the dams…” (Dunagan 2013)

Closer to home, in Ventura County, a plan for the dismantling of the Matilija Dam has been in the works for years. At 198 feet tall— almost twice the height of Elwha Dam, the Matilija Dam is one of the largest slated for removal. There is consensus that benefits to wildlife (the endangered steelhead trout) and restoration of sand-starved beaches can result from carefully managed removal of the dam. In a mere half a century of use, the dam’s capacity has been reduced by 90%. Now, to make progress on removal, a consortium of agencies must agree on how to release the six million cubic yards of silty sediment entrapped within, with minimal environmental damage.

Obviously taking down dams is not something that can be done casually. But the Elwha River shows how so many dynamic natural processes have inherent beauty and capacity to astonish those of us who might otherwise believe that rivers originate from behind concrete dams, that sediment is for dredging, and that beaches come from slurry that is pumped from barges….

SOURCES

California Coastal Sediment Management Workgroup. 2009. The California Coastal Sediment Master Plan Status Report.

California Coastal Sediment Management Workgroup. 2010. Results from CSMW Task III.

Cohan, W. D., and Grigoriadis, V. From Coast to Toast. 2013. Vanity Fair. Vanityfair.com/society/2013/08/end-of-malibu-nantucket-erosion

Dunagan, C. 2013. Dramatic changes following Elwha Dam removal. Kitsap Sun. March 2, 2013.

Flick, R. E. 1993. The Myth and Reality of Southern California Beaches.

State Coastal Conservancy and Department of Boating and Waterways. 2002. California Beach Restoration Study.

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