Assessing the Geologic and Biologic Responses of a Beach System to a Nourishment Operation, Morro Bay, CA

Dredging is carried out worldwide to maintain navigable water channels and to source sediments for beach nourishment operations. The impact of placing dredge material at a dumpsite is not well understood, as most studies on the impact of dredging focus only on the dredge pit from which material is excavated. 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, provides an excellent opportunity to understand impacts of using dredge material as nourishment. 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. Stratigraphy results indicated that the dredge material remained buried at the dumpsite as a distinct facies for approximately one year before becoming visually undetectable. Biology results suggested a correlation between finer sediments and increased biodiversity and organic matter. Applying the principle of the Littoral Cutoff Diameter (LCD) suggests that 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.