Shoreline retreat and beach nourishment are projected to increase in Southern California

Coastal erosion is a significant concern globally, particularly in low-lying, semi-arid urban areas experiencing rapid population growth and changing hydroclimatic conditions. Southern California, with its extensive sandy beaches and rapidly urbanizing coastline, serves as a representative case study for understanding these challenges. Sea-level rise (SLR) exacerbates the vulnerability of these coasts to flooding and accelerates shoreline retreat. However, a substantial portion of shoreline retreat is also attributed to long-term coastal erosion due to sediment imbalance resulting from factors like changes in precipitation patterns, dam construction, and land-use alterations. These changes are particularly pronounced in semi-arid regions where variations in precipitation can significantly impact sediment transport to the coast. In Southern California, additional factors like subsidence due to groundwater pumping and urban infrastructure contribute to shoreline erosion. The increasing frequency and intensity of coastal hazards necessitate quantifying and forecasting morphodynamic changes to implement effective coastal management strategies and mitigate the economic and logistical pressures associated with beach nourishment.

Previous research has highlighted the acceleration of shoreline retreat in sandy beaches as an initial indicator of SLR. Studies have indicated that a significant portion of this retreat is due to long-term coastal erosion stemming from sediment imbalance, sea-level rise, and other factors. The semi-arid coastal areas of Southern California and the Mediterranean coast of North Africa, known for their temperate climate, exhibit rapid sea-level rise and shoreline erosion due to changes in sedimentation processes caused by inland aridity and land-use changes. These changes are also observed in other arid and semi-arid regions globally. The Global Scale Assessment Model (GSAM) offers insights into shoreline retreat, but its limitations lie in its incomplete treatment of arid and semi-arid coasts due to the lack of available datasets. While beach nourishment is a common mitigation strategy, its long-term environmental impacts remain debated, and its financial burden is particularly challenging for developing nations.

This study employed a multi-decadal approach using publicly available datasets and established methods applicable to other semi-arid regions. The study area, the Gulf of Santa Catalina in Southern California, and two validation sites (Corona Del Mar in Southern California and Hammamet North in Tunisia) were selected based on their representative characteristics of semi-arid coasts facing similar challenges. The Digital Shoreline Analysis System (DSAS) method was used to measure and predict coastal evolution using photogrammetric shoreline positioning from airborne and orbital scenes (Landsat and Sentinel 2B). The DSAS method involved creating cross-shore transects to assess shoreline change rates over time, providing Net Shoreline Movement (NSM) and End Point Rate (EPR) values. The Bruun rule was employed to calculate the shoreline retreat rate (SRR) associated with sea-level rise (SLR). To account for the sediment imbalance caused by land-use changes, supervised classification of Landsat imagery (1985 and 2015) was conducted to quantify land-cover components (continental water bodies, bare soils, vegetation, urban spaces, and sandy deposits). This analysis provided insights into the temporal evolution of land use and its impact on sediment transport. A conceptual model combining shoreline retreat rates with population density growth rates was developed to forecast shoreline retreat under different urban growth scenarios (minimum, average, and maximum). The cost of beach nourishment was estimated based on projected shoreline retreat rates, current sand prices, and forecasted inflation rates. High-resolution Global Surface Water Explorer (GSWE) datasets were used to validate water level changes along the transitional environment. Field visits validated observed retreat rates and land-use classifications.

Analysis of shoreline evolution from 1992 to 2018 revealed rapid shoreline retreat rates in the Gulf of Santa Catalina, ranging from -0.75 to -1.24 meters per year, averaging approximately -1 meter per year. Validation sites showed similar trends, with Corona del Mar exhibiting an average retreat of -0.25 m/yr and Hammamet North showing -2.45 m/yr. The combined average retreat rate for both sites was approximately -1.45 m/yr. Comparison with the GSAM's standard retreat rate of -0.07 m/yr highlights the severity of erosion in these semi-arid urban coasts. The Bruun rule indicated that SLR contributed less than the coastal erosion component to the observed SRR. Analysis of land-use changes from 1985 to 2015 revealed significant expansion of urban areas and a corresponding reduction in vegetation coverage in both study areas and validation sites. Forecasting shoreline retreat to 2100 under three scenarios (minimum, average, maximum) revealed that the average shoreline retreat rate could increase to -2.12 m/yr by 2050 and -3.18 m/yr by 2100. These projections are significantly higher than the GSAM's prediction for ambient sandy beaches. The required annual sand volume for beach nourishment is projected to triple by 2050 (from ~1223 m³/year/km to ~3669 m³/year/km in scenario 2), and the cost could increase fivefold, reaching ~$125,973.05 per kilometer by 2050. These increases reflect the significant impact of population density growth and land-use change on sediment transport and coastal erosion.

The findings demonstrate the crucial role of both SLR and, more importantly, anthropogenic factors in driving accelerated shoreline retreat in semi-arid urban coastal areas. The significant increase in projected shoreline retreat rates emphasizes the need for proactive and comprehensive coastal management strategies that go beyond simply addressing SLR. The dramatic increase in the cost of beach nourishment highlights the urgent need for sustainable solutions that consider the economic and logistical challenges faced by coastal communities, particularly in developing nations. The model's accuracy depends on the accuracy of inputs, including SLR projections, population growth forecasts, and sand price estimations. Future research should focus on refining these parameters and exploring alternative mitigation strategies, such as nature-based solutions and integrated coastal zone management.

This study provides a comprehensive assessment of shoreline retreat and the increasing need for beach nourishment in Southern California, highlighting the combined impacts of SLR and rapid urban growth. The accelerated erosion rates and projected costs emphasize the urgent need for integrated coastal management plans that address both natural and anthropogenic factors. Future research should investigate the long-term ecological impacts of beach nourishment and explore sustainable alternatives to mitigate coastal erosion. The methodology used in this study, employing publicly available data and established methods, can be replicated in other semi-arid coastal regions worldwide, facilitating a broader understanding of these global challenges.

The study's projections rely on linear extrapolations of observed trends, which may not accurately capture the complexities of future environmental changes and socio-economic factors. The cost estimates are based on current sand prices and inflation rates, which are subject to change. Furthermore, the environmental and ecological impacts of the increased beach nourishment required are not fully addressed in this study.