San Diego Beach Report: 2020

State Beach: Border Field • Silver Strand • Torrey Pines • Cardiff • San Elijo • Moonlight • Leucadia • South Carlsbad • Carlsbad

Introduction

The San Diego County State Beach Report is an annual update of the physical condition of nine California State Park beaches in the County. In addition, an UPCOMING WINTER WAVES AND BEACH EROSION OUTLOOK is provided during the fall months of each year.

The report “Beach Years” begin October 1 of the previous calendar year to best capture a full annual winter (erosion) and summer (accretion) cycle. This 2020 Beach Year Report is based on observations from Oct 1, 2019 to Sep 30, 2020.

Select one of the State Beaches from the menu above to view individual beach survey products.

Overview

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.

2020 Executive Summary

The 2015-2016 El Nino was the strongest since 1997-1998. Cardiff, which has been recently nourished, and Silver Strand, are the only two beaches that have fully recovered to an annual mean width greater than or equal to their pre-El Nino condition.

North County State Beaches north of the San Elijo Lagoon Inlet: San Elijo, Moonlight, Leucadia, South Carlsbad, and Carlsbad 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 have positive beach width trends (+1m to +4m/year) since the 2015-16 El Nino (Accreting Beaches, lower panel, Fig. 1).

Several of the beaches have positive recovery rates that significantly exceed the rate of +1m/year at South Torrey Pines.
- Cardiff (+4m/year): There has been significant nourishment activity as part of the San Elijo Lagoon Restoration Project.
- North Border Field (+3m/year): The section of beach north of the Tijuana River Inlet has widened as the Inlet has migrated south in recent years. Southward migrating sand from the Imperial Beach portion of the Regional Beach Sand Project II nourishment is believed to be a contributing factor (Ludka et. al., 2020).
- Silver Strand (+4m/year): The recovery rate is similar to those at Cardiff and North Border Field, but historical surveys of this beach are limited, and it is unknown if this is natural widening or the result of beach management activity in the South Bay region.

The 2020-2021 Winter Waves and Beaches Outlook favors below average waves and beach loss, and 2021 will likely be a 5th year in the beach recovery phase of the current ENSO cycle.

References

B.C. Ludka, A.P. Young, R.T. Guza, W.C. O’Reilly, M.A. Merrifield, 2020 (submitted). Nourishing an erosion hotspot. Geophysical Research Letters.