Beach changes from sediment delivered by streams to pocket beaches during a major flood

摘要

The significance of sediment delivered via storm-associated stream discharge in altering sediment characteristics, beach form, and volume is evaluated on pocket beaches with different basin characteristics and wave exposures. The focus is on changes on three beaches on Elba Island, Italy caused by a flood event in September 2002 that had an estimated recurrence interval of 200 years. Beach profiles and foreshore sediment samples were gathered in 1999 and 2000 to identify pre-storm characteristics, in September 2002 to reveal the immediate effects of the storm, and in 2003 and 2004 to reveal post-storm recovery. Foreshore sediments were finer and better sorted and contained no pebbles prior to the flood. Coarsening of the sand and granule fraction was evident after the flood, along with pebble accumulations, especially near major streams. Mean size, sorting values and percent pebbles one and two years after the flood were generally greater than pre-flood conditions but less than immediately after the flood. Beach profiles reveal conspicuous erosion immediately after the flood, when sediment delivered by streams is transported to subaqueous deltas. Thereafter, sediment moves onshore and alongshore to adjacent beaches to restore subaerial volumes. The location of streams within a compartment, relative to the location of capes and headlands, is important in determining the movement of sediment added to the beach by streams. The sites are all sheltered from the highest-energy waves from the west, facilitating longshore transport to the west. Where the largest stream is located at the east end of a compartment, sediment discharged from it is source material for the downdrift beaches to the west. Where the largest stream is at the west end of the compartment, the ability to supply sediment to the beaches to the east is restricted. Hence, broad-scale geologic controls (headlands and capes) enhance or diminish the ability of streams to influence beach change throughout the pocket. The inability of beaches on two of the sites to migrate landward, due to human development of uplands, will be an issue in the future. Lack of sediment to replenish beaches through erosion of the upland, places increased emphasis on the role of coastal streams in the beach sediment budget. Changing watershed attributes to allow more sediment discharge during high-energy, low-frequency events (e.g. devoting more land to agriculture) would be a way of helping to restore beach sediment.