MODELING BACK BEACH EROSION AT BEVERLY AND MOOLACK BEACH, OREGONBY: GILBERT BERNHARDT AND SISAVATH VIRASAK

This study addresses the critical need to predict erosion levels at Beverly Beach and Moolack Beach, situated between Yaquina Head and Devil's Punchbowl on Cape Foulweather. The proximity of these beaches to Highway 101, along with the presence of homes and businesses on terraces above the back beach outcrop scarp interface, necessitates a reliable forecasting tool. The coastline in this area is prone to mass movements, such as slumping and rockfalls, posing continuous safety risks to the highway, residential areas, businesses, and their occupants.

To meet this need, a robust mathematical model was developed using real-world data from the National Oceanic and Atmospheric Administration (NOAA). The model incorporates information on Highest High Tide (HHT), High Tide (HT), and average mean sea level fluctuation (MSL). Real-time and historical swell data from the National Buoy Data Center (NBDC) was also integrated into the model, initially designed in Excel and later translated to Python for improved usability.

The model offers flexibility for long-term historical and predictive analyses on various time scales, including multiyear annual steps, yearly monthly steps, or monthly daily steps. Average swell periods from NBDC data are considered in each time step to determine key wave characteristics, such as deepwater wavelength, shallow water wave depth, celerity, breaker height, and swash zone depth.

Incorporating swash zone depth, HHT, and HT data (NOAA), along with computed predictions of sea-level rise from NOAA's sea level curves and mean sea level fluctuation, the model establishes water height at the back beach interface. This information is then utilized to calculate the high tide water energy density at the back beach interface. The depth of erosion is derived by dividing the high tide water energy density by the unconfined rock strength of the back beach outcrop interface.

Model validation is performed through a comparison of historical photographs and cross-referencing with US Geological Survey (USGS) topographic maps alongside current survey data. Additionally, ongoing monthly erosion measurements are conducted using strategically placed erosion pins at six sites near or at the intersection of the back beach with the outcrop of the Astoria Formation along Beverly and Moolack Beaches.