Thin-layer sediment addition to an existing salt marsh to combat sea-level rise and improve endangered species habitat in California, USA

Current tidal marsh elevations and their accretion rates are important predictors of vulnerability to sea-level rise. When tidal marshes are at risk, adaptation measures, such as sediment addition to increase elevations, can be implemented to prevent degradation and loss. In 2016, wildlife managers prescribed a thin-layer sediment addition of locally sourced dredged material from Anaheim Bay to mitigate plausible future impacts of sea-level rise to Seal Beach National Wildlife Refuge, an urbanized, subsiding, sediment limited, and low elevation tidal marsh in southern California, USA. In this study, our objectives were to determine how suspended sediment concentrations (SSC) and fluxes associated with sediment application affected an adjacent eelgrass bed and a more distant deep tidal channel at different time periods throughout the project and how sediment application affected tidal marsh elevation. Due to the sediment addition the tidal marsh elevation increased by 25.4 cm across 3.06 ha. Mean SSC levels at the eelgrass site increased from 4 mg/L during pre-augmentation to 16 mg/L during sediment application. SSC levels also increased up to 40 mg/L with the installation of sediment barriers. Winter storms during construction also had an influence on SSC, with an extreme storm event increasing SSC from 9 mg/L pre-storm levels to 16 mg/L during the storm. However, SSC at the deep site remained constant during all time periods with a mean of 7 mg/L. Our results show that little to zero sediment was exported out of Anaheim Bay during the study period, illustrating that any impacts were localized to the application area. These findings suggest that although long-range export of applied dredge material is unlikely from future thin-layer sediment application at this site, adjacent habitats could experience impacts if erosion control measures are not implemented effectively. Future projects will benefit from similar pre- and post-addition monitoring to establish the long-term efficacy of sea-level rise adaptation measures.