Figure 1. Summary of observed annual mean beach width change since 1998. Lines are linear best fit beach width trends between El Nino years for individual beaches. Beaches with negative trends since the 2016 El Nino year (upper panel) are grouped as currently eroding on a multi-year time scale. Beaches with positive trends since 2016 are grouped as currently accreting (lower panel).

San Diego County beach change over the last several decades can be conceptualized as the interplay between two primary variables:

• Available nearshore sand supply.


• Wave-driven beach erosion and accretion in response to the El Nino-Southern Oscillation (ENSO) climate pattern, with each multi-year period between El Nino winters being a distinct "beach recovery phase" (trend lines, Figure 1).

Unusually high winter waves and severe beach erosion are often associated with episodic El Nino winters, which typically occur every 2-7 years. Natural beach widening, or recovery from El Ninos, happens more gradually over the course of the milder winter years between El Nino events.

During the recovery phase, the beach does not necessarily increase monotonically in annual width, because there is still a modest range of winter and summer wave variability from year to year. However, the net change, or trend, is positive over the recovery years assuming there is a sufficient supply of nearshore sand to migrate shoreward onto the subaerial beach during the summer high tides.

South Torrey Pines State Beach (yellow squares, lower panel) is considered the closest thing to a well-monitored "control beach" in the County. It is long (3.5km), wide (70m), and relatively stable. It hasn't been nourished directly, but is located at the southern boundary of the Oceanside Littoral Cell. As a result, the beach's sand supply is likely enhanced by the overall southward littoral drift in the Cell, and the beach remains mostly sandy year-round.

The last three beach recovery phases (post-2003,2010,2016 El Ninos) at South Torrey Pines show a consistent pattern that begins with an El Nino winter erosion year of varying severity (2-10+ m), followed by multiple years of beach recovery with an average rate of approximately +1m/year (yellow trend line slopes, lower panel, Fig. 1). It is a useful reference beach when interpreting the erosion-recovery phases of the other beaches in the County.

• The 2020-2021 winter was notable for a singular extreme wave event on January 25-26, 2021, in what otherwise was a relatively modest winter for waves. It was one of San Diego County's most powerful events (wave height plus duration) in the last several decades, exceeded only by the largest event of the 2015-16 EL Nino winter, and similar to, but not as extreme as, the January 17-18, 1988 "Storm of the Century" (Seymour, 1989). As a result, the 2020-2021 winter beach width loss, and the resulting annual beach width decrease, was larger than typically seen during an ENSO recovery phase year.


• North County State Beaches north of the San Elijo Lagoon Inlet: San Elijo, Moonlight, Leucadia, South Carlsbad, and Carlsbad continue to have negative trends in annual beach width (-1m to -4m/year) since the 2015-16 El Nino (Eroding Beaches, upper panel, Fig. 1).


• County State Beaches south of the San Elijo Lagoon Inlet: Cardiff, Torrey Pines, Silver Strand, and Border Field State Park continue to have have positive beach width trends (now +1m to +2m/year rather than +1m to +4m/year in the previous report) since the 2015-16 El Nino (Accreting Beaches, lower panel, Fig. 1). The multi-year positive recovery trends at Silver Strand, Torrey Pines South, and Cardiff are now lower owing to the January 25-26, 2021 storm event.


• The 2021-2022 Winter Waves and Beaches Outlook favors below average waves and beach loss, and 2022 will likely be a 6th year in the beach recovery phase of the current ENSO cycle.

References

Seymour, R.J. 1989, "The Great Storm of January 1988," Shore and Beach, Journal of the American Shore and Beach Preservation Association, Vol. 57, No. 2. http://cdip.ucsd.edu/share/shore_and_beach_Oct_1989.pdf