Coastal sand dunes play an important role in the littoral processes of California’s coastline. Functioning as both source and sink for sand, these dunes often protect coastal infrastructure and provide a buffer between anthropogenic development and coastal forces. At Toro Creek Beach, north of Morro Bay California, recent extreme winter storms have raised concerns about the erosion of the sand dunes that front California State Highway 1. This thesis attempts to characterize the historical growth and recession patterns of the Toro Creek dunes since 1977, the dune movement over the 2023-2024 winter seasons, the wave forcings behind recent high erosion events, and potential impacts of sea level rise.
Four topographic surveys of the beach were conducted through the winter of 2024 and showed evidence of -11.5 ft and -12.8 ft of erosion north and south of the Toro Creek bridge. Historical aerial and satellite imagery was used to delineate the extent of the dunes at Toro Creek and estimate dune movement during this period. From 1977-2023, the dunes moved seaward 37.4 ft and 126.6 ft north and south of the bridge. Throughout this period, temporal erosion and accretion is seen on both sides of the bridge. The winters of 2021-2024 display dune erosion on at least one side of the Highway 1 bridge that approaches or exceeds yearly retreat rates developed by the USGS for shoreline (-4.9 ft/yr) and coastal cliffs (-1.6 ft/yr) in the Morro Bay area. Of these winters, 2023 witnessed the most erosion: -33.3 ft north of the bridge and ‑1.1 ft south of the bridge.
This study utilized wave data from the Diablo Canyon Waverider buoy to develop a significant wave height climate specific to the orientation of Toro Creek Beach. Using tide data from the National Oceanic and Atmospheric Administration (NOAA) tide gauge in Port San Luis, I estimated total water level—the summation of significant wave height and tide—for Toro Creek. Subsequently, I evaluated wave runup using an empirical equation developed by Stockdon et al. (2006). I analyzed the total water level and runup height—the summation of runup and tidal stage—along with other coastal processes like wave direction and refraction during the peak events of the 2021-2024 winters. Among these years, 2023 experienced total water levels as high as 16.3 ft, and runup values as high as 14.3 ft—some of the highest since the start of analyzed wave data.
Additionally, I calculated the impact sea level rise (SLR) would have on runup using our wave data, under an extreme sea level rise scenario of 6.4 ft. The number of 24-hour days per year during which runup height exceeds the extreme runup height reached during the December 28, 2023 storm increases from 0.01 to 95.06 with 6.4 ft SLR. The exceedance of the 17 ft FEMA Coastal Flood Zone VE elevation for Toro Creek increases from 0 to 6.60 24-hour days per year with 6.4 ft SLR.
Overall, this study shows that dune erosion accompanies high wave runup heights, and that future SLR will increase the likelihood of dune erosion at Toro Creek beach.
| Identifer | oai:union.ndltd.org:CALPOLY/oai:digitalcommons.calpoly.edu:theses-4577 |
| Date | 01 September 2024 |
| Creators | Emerson, Owen D |
| Publisher | DigitalCommons@CalPoly |
| Source Sets | California Polytechnic State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Master's Theses |
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