Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 10-3
Presentation Time: 9:00 AM-6:00 PM


ALFVING, Cameron J.1, REECE, Joshua S.2, BRADY, Mara1, MOSHIER, Shelby2 and WEINMAN, Beth1, (1)Earth and Environmental Sciences, California State University, Fresno, 2576 East San Ramon Ave M/S ST24, Fresno, CA 93740, (2)Biology, California State University, Fresno, 2555 East San Ramon Ave M/S SB73, Fresno, CA 93740

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 an onshore dumpsite (e.g., a beach) is not well understood. To help elucidate these impacts, we use the recent 2016 dredging operation in Morro Bay, CA, which placed hundreds of thousands of cubic meters of sediment onto the nearby beaches. During the operation, the dredged sediments appeared to be much darker than the in-situ beach sand and emitted a diesel odor, leaving questions about ecosystem and sediment impacts to the original beaches. Spatial-temporal analyses of grain size, mineralogy, biota, and stratigraphy were used to assess the impacts of the dredging event to the dumpsite and surrounding area. Grain size results indicated that the dredge material was a unique facies with a median grain size smaller than anything else observed in the study area. Mineralogy results supported the grain size 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 indicated that there was a greater diversity of bird species nearer in time to the dredge event as well as at the dumpsite and beach immediately adjacent to it. Invertebrate species diversity remained consistent throughout the study. Seasonal stratigraphy profiles dug across the study area post-dredge showed no buried dredge layers, suggesting enough rapid transport and/or mixing occurred in the study area to prevent any discrete stratigraphic preservation. Despite the initial sediment incompatibility introduced by the dredging, these findings lead to the conclusion that the dredge event had no discernable long-term impacts on the study area. Applying the principle of the Littoral Cutoff Diameter (LCD), we posit that approximately 40% of the dredge material was naturally deposited offshore while the remaining 60% was transported back to the dredge site via longshore currents and deposited there.