Assessing the Environmental Impacts of a Beach Nourishment Operation, Morro Bay, Ca

Dredging is carried out worldwide to maintain navigable water channels and to source sediments for beach nourishment operations. As sea level continues to rise throughout the 21st century, dredging for beach nourishment is poised to become increasingly prevalent, and increasingly important in California, where as much as 86% of the coast is erosional. In repurposing dredge to nourish beaches, the impact of placing dredge material at a dumpsite (e.g., a beach or nearshore zone) is not well understood, as most studies on the impact of dredging focus only on the dredge pit from which material is excavated. To help understand impacts of using dredge material to nourish beaches, we use the recent 2016-2017 dredging operation in Morro Bay, CA, which placed between 3.9 x 105 to 7.6 x 105 cubic meters of sediment onto the nearby beaches. During the operation, the dredged sediments appeared to be significantly darker than the in-situ beach sand, leaving questions about ecosystem and sediment impacts to the original beaches. Spatial-temporal analyses of grainsize, mineralogy, biota, and stratigraphy were used to assess the impacts of the dredging event to the dumpsite and surrounding area. Grainsize results indicated that the dredge material was a unique facies with a median grainsize smaller than anything else observed in the study area. Mineralogy results supported the grainsize results in showing that the dredge material was a unique facies with a different percent abundance of quartz, intermediates, and lithics to the in-situ beach sand. Biology results suggested a correlation between finer sediments and increased biodiversity and organic matter. Stratigraphy results indicated that the dredge material remained buried at the dumpsite as a distinct facies for approximately one year before becoming visually undetectable. Applying the principle of the Littoral Cutoff Diameter (LCD), we posit that approximately one third of the dredge material was naturally deposited offshore while the remaining either remained at the dumpsite or was dispersed throughout the study area. Despite the initial sediment incompatibility introduced by the dredging, these findings lead to the conclusion that the dredge event had no discernible negative long-term impacts on the study area.