This paper describes the formation of barrier spits and islands via the process of channel avulsion and shoal bypassing. It draws on case studies in South Carolina (USA) where ebb-tidal deltas containing excess sediment released large volumes that rapidly accreted along the adjacent beach. Barriers involving order of 10 to 10 m were formed in just a few years. In these cases, the shoreline jumped hundreds of meters seaward as shoals merged with the beach. It is hypothesized that the position of the new shoreline can be predicted using equilibrium profiles. First, a profile of equilibrium is defined (V_(eq)) in the absence of offshore shoals, based on the particular conditions of sediment texture, closure depth, wave climate, and tidal regime. Then this beach volume is compared with the volume available in offshore shoals. Where excess sediment is deposited at river mouths and tidal deltas, shoals contain unit volumes above closure depth (V_(DS)) that may greatly exceed V_(eq). The shoals will persist offshore as long as ebb currents exceed, or balance out, landward-directed currents produced by flood tides and waves. If channels shift by avulsion, ebb flows are diminished and the shoals will tend to migrate landward and accrete vertically. Longshore spreading will occur until the new planform is in equilibrium with the adjacent shorelines. By estimating the applicable shore length over which the shoal will spread and applying equilibrium volumes, it is possible to estimate the new shoreline position. If V_(DS) is less than V_(eq), the shoal will fully merge with the beach. However, if it is large relative to the equilibrium profile, a barrier ridge can form seaward of the beach, enclosing a lagoon. Case examples suggest that some barrier island chains such as the islands of the Santee-Cooper River system near Charleston (SC) may have formed very rapidly (decadal time scales) in connection with an avulsion upstream.
展开▼
