January 1, 2001
El Niño, coastal erosion, and shifting sands of beaches in central California linked
By Tim Stephens
"By concentrating our population on the coasts, we have put the bulk of our civilization within a few feet of sea level," he said. "When you combine rising sea levels with El Niño on the West Coast and hurricanes on the East Coast, the result will be larger and more frequent losses due to storm damage."
In a related study, Griggs and graduate student Cope Willis found that although winter storms during El Niño can erode beaches down to bare rock, the same storms ultimately replenish the beaches by washing tons of fresh sediment from rivers and streams into coastal waters. Their preliminary results show no long-term changes in the amount of sand on central California beaches.
The studies, which involved analyses of historic records as far back as 1910, were presented December 16 and 17 at the fall meeting of the American Geophysical Union in San Francisco.
El Niño is a disruption of oceanic and atmospheric circulation patterns spawned at irregular intervals in tropical waters. It involves weakening of the trade winds and unusually warm surface waters in the eastern Pacific, leading to changes in weather in regions far from the tropics. Storlazzi used weather and oceanographic records to identify El Niño events dating back to 1910. He then ranked their intensity to create an index showing six higher intensity and 17 lower intensity El Niños in the past 90 years.
El Niño winters batter the coast harder than usual for several reasons. In a normal year, there might be one really big storm, but beaches absorb most of its force, Storlazzi said. During an El Niño year, a succession of big storms removes the protective beaches. Waves attack structures built on the beach and start hitting the seacliffs. Storms and waves also approach the coast from a more southerly direction, hitting areas normally protected from wave action. In addition, El Niño storms bring high rainfall (which saturates and weakens coastal bluffs), elevated sea levels (causing waves to break closer to shore), and bigger waves.
But not all El Niños are created equal. Storlazzi looked at equatorial conditions such as sea-surface temperature, barometric pressure, winds, and storm records to rate the intensity of El Niños, and compared those findings with conditions in California.
"With increasing El Niño intensity, there is an exponential increase in the factors that enhance coastal erosion in California, such as rainfall, wave height, and sea level," Storlazzi said. "We found that 75 percent of the shoreline erosion and damage has occurred during El Niño winters, and the moderate- to high-intensity El Niños do most of the damage."
The most recent ones, in 1982-83 and 1997-98, were among the most intense El Niños of the past 90 years, Storlazzi noted. Unfortunately for coastal dwellers, a 30-year gap between major El Niños, from 1941 to 1972, coincided with a period of intense coastal development. The result, said Storlazzi, is that a lot of people built houses in vulnerable places, such as right on the beach or on seacliffs that are now crumbling.
Beach erosion caused by the El Niño winters of 1982-83 and 1997-98 spurred calls for state-funded beach-nourishment projects. Few studies, however, have documented long-term trends in beach size or identified sites that could benefit from such projects, according to Griggs.
"There has been a big push to restore beaches in California, but before we spend millions of dollars on beach nourishment projects it's critical that we know which beaches, if any, are undergoing long-term erosion and why," Griggs said. "We also need to ask where is all the sand going to come from, how long is it going to stay on the beach, and what are the long-term costs."
Willis is studying seven beaches between San Francisco and Monterey using aerial photographs and other historic records to document changes. At the meeting, he will present results from three state beaches in Santa Cruz County indicating no net loss of beach size in this area.
"El Niño winters cause a lot of beach erosion, but the beaches recover pretty rapidly," Willis said. "The heavy rains flush out a lot of sediment, which is crucial for maintaining beach size."
In southern California, the situation is different because of the number of dams on coastal rivers that block the transport of sediment to the coast, he said. Eroding seacliffs are also an important source of sand that gets cut off when people build seawalls.
Beach nourishment is a long-term commitment because sand doesn't stay in one place but moves constantly along the coast, Griggs said. "We have to ask ourselves if beach nourishment is going to be a cost-effective long-term solution. My concern is that we're just dumping sand down a hole," he said.