Shoreface morphodynamics, back beach variability, and implications of future sea-level rise for California's sandy shorelines.

摘要

California's sandy shorelines are important resources in terms of ecological habitat, recreation and tourism, and aesthetic beauty. They also serve as important natural buffers, often protecting back-beach development and infrastructure from wave attack. Encroachment or fixing of the back beach in conjunction with sea-level rise will reduce the accommodation space of beaches and subsequently increase the vulnerability of the back beach to extreme water level events associated with storms. Understanding the present state of California's sandy shorelines in terms of landward migration potential, vulnerability to extreme water events, and local morphodynamic variability is important for coastal policymakers and managers to make informed decisions regarding the future of the California coastline. In this study, the landward migration potential of California's beaches was assessed through a statewide cataloging and classification of all sandy shorelines in the context of back beach immobility and restrictions to landward migration. Beach inundation vulnerability from increased frequency of high water events with rising sea level was determined with extreme water level exceedance distributions from historic tide gauge records and sea-level rise projections of 18 cm, 79 cm, and 140 cm by 2100. Finally, alongshore variability and the influence of high wave events and berm overtopping on beach profile morphodynamics were analyzed through seasonal beach surveying of a coastal lagoon pocket beach. Fifty-eight percent of the total length of sandy shoreline in California is restricted in the back beach from landward migration. This is primarily due (in Central and Southern California) to anthropogenic coastal development and infrastructure. Beach inundation potential from extreme high water events is highly variable and is a function of berm height and backshore morphology. Backshore morphology influences foreshore and berm morphodynamics by modifying swash sediment transport. The growth and persistence of a berm on a bar-built coastal lagoon beach is dependent on berm overtopping; however, overtopping beyond a threshold frequency becomes destructive and can lead to berm erosion. While statewide more than half of California's sandy shorelines are being threatened by reduced accommodation space through terrestrial and marine encroachment, understanding local variability is essential for determining the future of these highly valued resources.