Coastal Change Hazards: Hurricanes and Extreme Storms

1997-98 El Niño

Historically, the net longshore sand transport direction along the central California coast has been to the south driven by winter swell waves. In contrast, during the El Niño winter of 1997-98, sand was transported from south to north and accumulated on the south sides of headlands bordering pocket beaches (see examples from Montara State Beach). This redistribution of beach sand resulted in significant beach erosion at the south ends of pocket beaches where sea cliffs were preferentially exposed to wave attack and eroded as much as 14 meters (see examples from Pacifica).

Montara State Beach

Above: Looking north along Montara Beach; note the headland in the distance. Photo taken from headland in the south. [larger version]

Above: Map of central California showing the locations of Montara and Pacifica.

Below is a shaded-relief map, derived from lidar data, of the Montara, California area (Figure 1). Shown in color are beach changes that occurred between October 1997 and April 1998, the El Niño winter. Vertical beach erosion is shown in red; vertical beach accretion is shown in blue. It can be seen that during this period of large storms, sand was transported from south to north along the beach.

Beneath the shaded-relief map is a graph of shoreline change vs. beach volume change, aligned for comparison with the shaded-relief map (Figure 2). The shoreline along Montara State Beach eroded nearly 60 meters in the south, while accreting nearly 40 meters in the north, during the El Niño Winter.

Figure 1. Lidar-derived elevation change in Montara, CA due to the 1997-1998 El Niño storms.

Figure 2. Beach volume change and shoreline change.

Pacifica

Above: Looking north along Pacifica; photo taken from headland in the south. [larger version]

Above: Map of central California showing the locations of Montara and Pacifica.

Below is an oblique airphoto of Pacifica. The cliffside photo above was taken from the subtle headland noted by the yellow arrow.

Beneath the airphoto is a three-dimensional view (derived from lidar data) of the Pacifica area. Draped on the 3-D plot are colors that indicate erosion (red) and accretion (blue). The maximum erosional area represents 14 meters of horizontal sea cliff retreat. The red arrow denotes the location of the same building in the airphoto and on the 3-D plot. The yellow arrow shows the location of the cliffside photo.