BREACH MORPHODYNAMICS AND SEDIMENTARY DEPOSITS, CEDAR ISLAND, VIRGINIA, USA

The morphodynamics and geologic framework of an island breach on Cedar Island, VA was investigated using historical shoreline-change analysis (1852-2010), sediment vibracores, aerial photography, and trenching. Cedar Island is an undeveloped, mixed-energy, wave- and washover-dominated, transgressive barrier island located along the Virginia Eastern Shore of the southern Delmarva Peninsula. Mean spring tidal range is 1.7 m, and average significant wave height is ~0.6 m. Net longshore sediment transport is southerly.

Cedar Island’s southern end has experienced breaching at least three times over the past 60 years: 1956, 1993, and 1998. Each time, the breach typically opened, migrated south in the net littoral transport direction, and closed within four to seven years. The breach had an average channel width of ~750 m and typically expressed a wave-dominated inlet morphology with a well-developed flood-tidal delta, a well- to poorly-defined inlet throat, and a small to non-existent ebb-tidal delta.

For the past 158 years, the average retreat rate of Cedar Island’s outer shoreline was 5.5 m/yr. However, once the breach closed in early 2007, the former breach-zone shoreline advanced at an average rate of 30 m/yr. Cores collected from the former inlet throat confirmed the maximum channel depth of about 2.6 m. From bottom to top, the sedimentary deposits are characterized by three primary facies. The lowermost unit, Facies 1, is a dark grey silty clay (estuarine mud) that is truncated by a disconformity (channel base). The disconformity is overlain by a distinctive shell layer that lies at the base of Facies 2. The shell layer represents a channel lag in the thalweg of the inlet throat where flow regime was higher and finer sediments were winnowed out. Facies 2 is dominated by shelly fine to medium sand or fine to medium sand with shell fragments (active breach fill). The sand-size fraction shows a subtle coarsening upward succession, whereas the gross stratigraphy shows a fining upward succession with a coarse shell lag at the channel base and fine to medium sand at top, indicating inlet shoaling as tidal prism was lost in response to increasing hydraulic inefficiency eventually causing breach closure. Facies 2 grades upward into Facies 3, which is dominated by horizontally stratified fine sands (washover).