Quantifying the Importance of Ice-Rafted Debris to Salt Marsh Sedimentation

Salt marshes are vulnerable to changes in sea level rise, sediment supply, and storm frequency. To keep pace with these changes, marshes must accrete sediment at a rate greater or equal to sea-level rise. Ice Rafted Debris (IRD) has been identified as a potentially significant source of inorganic sediment to northern-latitude salt marshes, but little is known about the magnitude of this source. This study quantifies the volume and mass of ice-rafted sediment deposited on Plum Island Estuary (PIE), Massachusetts, after a major coastal storm, and evaluates the significance of IRD to the overall sediment budget. To determine the area of post-storm IRD within the study site, we used pixel-based land cover classification of aerial imagery and Digital Elevation Models (DEMs). Pre- and post-storm DEMs and aerial images were produced with Structure-From-Motion photogrammetry. We then classified the images and identified areas within the domain that contained IRD. The area of IRD determined from the classification, storm frequency data, and field measurements of patch thickness were used to estimate annual sediment accretion and mass from IRD. Deposits were localized in three areas, and estimates show that IRD contributes an annual sediment accretion rate of 0.57 ± 0.14 mm/year to the study site. New England salt marsh accretion rates typically vary between 2-10 mm/year, and the average PlE sediment accretion rate is 2.5-2.7 mm/yr, therefore IRD is a significant source of sediment for vertical accretion at this location. This study shows that pixel-based classification can be a useful tool for identifying sediment deposits from remotely-sensed imagery. Additionally, it suggests that IRD is an important sediment transport pathway and contributes a significant amount of sediment to salt marshes.