Stratigraphic architecture, morphodynamics, and evolution of breaches along Cedar Island, Va: A low-profile, washover-dominated, trangressive barrier island.

摘要

Island breaching has occurred at least three times (1956, 1993, and 1998) over the past 60 years in the same place along southern Cedar Island, Virginia. Each time, the breach captured enough tidal prism to remain open, become a tidal inlet, migrated laterally to the south in the direction of net longshore sediment transport, and closed in four to nine years. Twenty-five sediment cores have been analyzed to determine the horizontal and vertical trends within the inlet throat, flood-tidal delta, and washover deposits within the repeatedly breached area. When open, the latest tidal inlet (1998--2007) had a maximum depth of 4.5 m to 2.6 m and ranged in width from 250 m to 54 m. The latest tidal inlet initially captured approximately 18% of the available tidal prism from Wachapreague Inlet, Virginia. Fifteen primary facies were identified that consist mostly of sand, shell, and mud layers. They represent four depositional environments: beach-washover-aeolian, tidal inlet, flood-tidal delta, and estuarine. The sand facies were deposited in the beach--washover--aeolian, tidal-inlet, and flood-tidal delta environments; the shell facies were deposited in the beach--washover--aeolian environment and at the base of the tidal-inlet environment (channel floor); and the mud facies were deposited only in the estuarine environment. The gross sedimentology (shell, sand, silt, and clay) of the tidal-inlet fill sediments typically showed a fining-upward succession with a coarse shell lag at the channel base and grading upward to fine sand. However, the vertical grain-size trends of some sediment cores coarsened upward based on the sand, silt, and clay fractions.;The evolution of Cedar Island Inlet was studied to determine the life stages of an ephemeral tidal inlet. From this study, eight stages were identified showing how an ephemeral tidal inlet evolves during the time from which it is breached to when it closes. The inlet begins as a shore-normal breach. If it captures enough of the existing tidal prism to remain open, it will evolve into a tidal inlet. The inlet migrates in the direction of net longshore sediment transport, which is southerly in this area. During lateral migration, the inlet begins to rotate counterclockwise with a resulting shore-oblique channel orientation to the adjacent shoreline of 47 degrees in the case of Cedar Island Inlet. Finally, the inlet loses hydraulic efficiency causing the tidal prism to decrease and the inlet closes. An eight-stage model is presented that synthesizes the morphodynamic evolution of Cedar Island Inlet from opening to closing.