SWASH BARS AND SPIT GROWTH: EVOLUTION OF A RAPIDLYPROGRADING SPIT ALONG A SEDIMENT-STARVED COAST

摘要

The most striking feature of the Cape Lookout cuspate foreland is the 2.5 kmrnlong Power Squadron Spit which has developed over the past 60 years as part of a terminalrnlittoral cell. The spit has been extending to the north and widening to the west at rates thatrncan be easily characterized from aerial photographs and even from annual field surveys.rnProgradation of Power Squadron Spit is significant geologically because it is occurringrnwithin a transgressive, sediment-starved coastal setting in which adjacent barrier islandrnlimbs are rapidly eroding.rnWe document in this paper evolution of the spit by focusing on morphology andrnsedimentology from field studies, and patterns of spit progradation using photogrammetricrnanalysis. Aerial photographs from 1940 to the present were used to document changes inrnshoreline position, surficial area and coastal and nearshore morphology along the spit. Anrneight-month field survey using RTK-GPS technology to document short-term changes inrnshoreline and beachface profiles, and using sediment samples and trenches to unravel thernstratigraphy, provided definitive evidence of welding swash bars as the mechanism forrnprogradation and accretion.rnAn assessment of the various possibilities for the source of sediments to the spitrnrevealed that Cape Lookout Shoal is the primary source, contradicting long held beliefsrnthat littoral cells adjacent to cape-associated shoals converge at the apex of those capes.rnDespite being located in an exceedingly complicated geographic setting, Power SquadronrnSpit provides a textbook example of how spits grow through swash bar attachment.