Storm-driven cyclic beach morphodynamics of a mixed sand and gravel beach along the Mid-Atlantic Coast, USA

The morphodynamics of a mixed sand and gravel beach in Delaware were investigated based on 18 almost monthly beach-profile surveys at 46 locations from 2009 to 2011, 60 sediment cores, and 550 surface sediment samples collected at various alongshore and cross-shore transects. Three different atmospheric disturbances occurred within a 3-month window during the study period: 1) a distal hurricane, 2) an energetic winter storm, and 3) “NorʹIda”, a long-lasting and extremely energetic event resulting from the collision of a hurricane and winter storm. The storm-induced beach changes and post-storm recovery following each of the three storms are evaluated. A distinctive beach cycle was identified consisting of a built-up berm profile and depleted nearly-planar storm profile. The time-scale of the beach cycle relates to the frequency and intensity of storm impact and duration of inter-storm recovery instead of simple seasonality. The initiation of post-storm recovery occurs during the subsiding phase of the storm, attributable to the reduction in wave height and steepness transitioning to accretionary swells. The sediment characteristics of the storm deposit associated with NorʹIda demonstrated substantial cross-shore variation ranging from sandy-gravel and gravelly-sand within the storm swash zone (near the pre-storm dune edge) to well-sorted medium to coarse sand seaward of the storm swash zone. Storm deposits along mixed sand and gravel beaches demonstrate a variety of sedimentological characteristics. In addition, the studied beaches lacked a sandbar under all wave conditions. A new beach cycle model is proposed for the non-barred mixed sand and gravel beach.