guest post by Jim Steele
published in the Pacific Tribune July 24th
California’s spectacular coastline attracts tourists from around the world. Headlands of granite or basalt resist erosion, defiantly jutting out into the sea. Pocket beaches form where focused wave energy bites into softer sandstones and uncemented stream sediments. Relentless waves undermine and steepen cliffs bordering 70% of California’s shoreline. Over hundreds and thousands of years, natural erosion sculpted our awe-inspiring undulating coast.
But beauty is in the eye of the beholder – likewise the magnitude of a “coastal crisis”. The Los Angeles Times recently published ‘California coast is disappearing under the rising sea. Our choices are grim’. They inaccurately painted natural erosion as a recent crisis due to CO2 induced climate change. However, California’s erosion “crisis” must be understood within a greater timeframe.
Since the end of the last ice age, sea level has risen 400 feet. Over 18,000 years, San Francisco’s regional coastline marched 25 miles inland, advancing 7 feet a year – more than twice California’s average. My beautiful home town of Pacifica was featured in that Times’ article because it lost several homes unfortunately built on loosely cemented sand and gravel deposited 100,000 years ago when sea level was 20 feet higher. Although the ocean’s landward march has slowed over the past 5000 years, northern Pacifica’s fragile coastline still retreated by over 7 feet per year between 1929 and 1943. Despite a warming world, the average rate of cliff retreat then markedly declined since 1943.
The ill-fated Ocean Shore Railway, initiated in 1905, foreshadowed California’s erosion problems. To give tourists awesome views, tracks were laid on a ledge dug into steep coastal cliffs. But landslides were common, and costly repairs forced the railway to close. Today, only 25% of the railway ledge built by 1928 still exists. Undeterred, designers of California’s scenic Pacific Coast Highway hoped to give automobile travelers similar breath-taking views. Again, landslides were common. Only 38% of the highway constructed by 1956 still remains. Geologists tell us such landslides constantly altered California’s modern coastline for hundreds of years.
There are few straight lines in nature. Our coastlines undulate. Likewise, our climate oscillates, and coasts erode episodically. Between 1976 and 1999 (the warm phase of the Pacific Decadal Oscillation), California experienced more frequent El Niños. Over 70% of California’s 20th century disappearing coastline eroded during El Niño events. El Niños bring more storms and more destructive waves. El Niños bring more rains that saturate soils and promote landslides. The Pacific Decadal Oscillation then switched to its cool phase. It brought more La Niñas and more drought, but fewer winter storms and less erosion. In 1949, also a time of less erosion, Pacifica’s government believed homes setback 65 feet from the edge of a bluff would be safe. They never suspected a single El Niño event would move the cliff edge 30 feet landward 50 years later.
There are some who see human structures as a blight on California’s natural coastline. In response to natural erosion, they suggest we abandon the coast. They argue California’s only choice is “managed retreat” versus “unmanaged retreat”. Although well engineered seawalls can protect homes and businesses, some environmentalists called seawalls a coastal “crisis”. California’s Coastal Commission recently pledged seawalls will “only be permitted if absolutely necessary”. But the Commission’s policy only fosters a mishmash of emergency fixes. Randomly armored properties deflect destructive waves downstream, accelerating erosion in a neighbor’s unprotected property. Coastal cities must construct well-engineered sea walls, without any gaps.
Because sea walls prevent erosion, the Commission ill-advisedly fears local beaches will be lost if denied locally eroded sand. The Times parroted that belief writing, ‘for every constructed seawall, a beach is sacrificed’. But is that true? San Francisco’s O’Shaughnessy sea wall built in 1929 prevents erosion of the fragile sand dunes supporting Golden Gate Park. Yet SF’s north ocean beach continues to grow. Without a seawall, San Francisco’s south ocean beach rapidly eroded, and threatened infrastructure now requires a sea wall.
Sources of beach sand fluctuate, and simplistic sea wall analyses are very misleading. Sand is stored and transported to beaches in many ways. Streams and rivers supply the most sand needed to nourish a beach, but mining SF bay’s sand has deprived nearby coastal beaches. Furthermore, ocean oscillations shift winds and the direction of currents that transport sand. Beaches grow for decades then suddenly shrink. Although some argue our beaches face a rising sea level “crisis”, archaeologist determined that despite more rapidly rising sea levels 5000 years ago, many California beaches grew when supplied with adequate sand.
Lastly, it’s interesting to note scientists suggested Pacific islands also face an erosion crisis due to rising sea levels. But the latest scientific surveys determined 43% of those islands remained stable while land extent of another 43% has grown. Only 14% of the islands lost land. So, I fear exaggerated crises only erode our trust in science.
Jim Steele is director emeritus of the Sierra Nevada Field Campus, SFSU
via Watts Up With That?
July 30, 2019 at 11:00PM