SHOREFACE PROCESSES IN ONSLOW BAYdiction of beach and surf zone morphodynamics: equilibria,rates of change and frequency response. Proceedings, Co<strong>as</strong>talEngineering Conference, 19th. ASCE, New York, pp.2150-2164.Zeidler, R., 1982. Profile of equilibrium. In: M.L. Schwartz (Editor),The Encyclopedia of Beaches and Co<strong>as</strong>tal Environments.Hutchinson Ross, Stroudsburg, pp. 660-661.27
CAROLINA GEOLOGICAL SOCIETY<strong>Guidebook</strong> for 1982 Annual MeetingPages 29 - 40MORPHOLOGY AND DYNAMICS OF BARRIER AND HEADLAND SHOREFACES IN ONSLOW BAY, NORTHCAROLINAStanley R. Riggs 1 , William J. Cleary 2 and Stephen W. Snyder 31 Department of Geology E<strong>as</strong>t <strong>Carolina</strong> UniversityGreenville, NC 27858Tele: 919-328-6360; Fax: 919-328-43912 Department of Earth SciencesUniversity of North <strong>Carolina</strong> at WilmingtonWilmington, NC 28403-3297Tele: 910-395-3498; Fax: .910-350-40663 Department of Marine, Earth, and Atmospheric SciencesNorth <strong>Carolina</strong> State UniversityRaleigh, NC 27695Tele: 919-515-7912; Fax: 919-515-7802ABSTRACTP<strong>as</strong>sive margin co<strong>as</strong>tlines with limited sand supplies,such <strong>as</strong> much of the U.S. Atlantic margin, are significantlyinfluenced by the geologic framework of older stratigraphicunits that occur beneath and seaward of the shoreface. ManyU.S. e<strong>as</strong>t co<strong>as</strong>t barrier islands are perched barriers in whichthe underlying, pre-modern sediments determine the morphologyof the shoreface and strongly influence modernbeach dynamics and composition. Perched barriers consist ofvariable layers of beach sand on top of older, eroding stratigraphicunits with highly variable compositions and geometries.Along many parts of the co<strong>as</strong>tal system,stratigraphically-controlled bathymetric features on the innershelf modify waves and currents and thereby affect patternsof sediment erosion, transport, and deposition on the adjacentshoreface. It is essential to understand this geologicframework before attempting to model the large-scale behaviorof these types of co<strong>as</strong>tal systems.In North <strong>Carolina</strong>, most shoreline features are controlledby the pre-Holocene stratigraphic framework of the shoreface;the beaches are perched on top of pre-existing Pleistocene,Tertiary, and Cretaceous sediments. The surficialgeology of the co<strong>as</strong>tal zone is subdivided into two distinctprovinces resulting in different stratigraphic controls of theshoreface. North of Cape Lookout the geological frameworkconsists of a Quaternary sequence that fills a regional depositionalb<strong>as</strong>in called the Albemarle Embayment. The co<strong>as</strong>talzone south of Cape Lookout is dominated by Tertiary andCretaceous units that crop out across the co<strong>as</strong>tal plain andcontinental shelf, with very thin Quaternary units onlylocally preserved. Superimposed upon this regional stratigraphyis an ancient drainage system resulting in a series of fluvialvalleys filled with younger co<strong>as</strong>tal sediments separatedby large interfluve are<strong>as</strong> of older stratigraphic units. Thisresults in a co<strong>as</strong>tal system in which the shoreface is eithernonheadland or headland dominated, respectively. Headlanddominated shorefaces are further divided into subaerial andsubmarine categories. Nonheadland dominated shorefacesare further divided into those influenced primarily by transgressiveor regressive processes, or channel-dominated depositionalprocesses (i.e., inlet migration or stream valley fill).Examples of each of these six types of shorefaces are presentedto demonstrate the control that the geologic frameworkexhibits on shoreface morphologies and processes.INTRODUCTIONShoreface Profile of EquilibriumFenneman (1902) originally defined a shoreface profileof equilibrium <strong>as</strong> a profile that "the water would ultimatelyimpart, if allowed to carry its work to completion." Recently,many other workers have expanded upon the definition of theshoreface profile of equilibrium including the following:Schwartz (1982): "a long-term profile of ocean bed producedby a particular wave climate and type of co<strong>as</strong>tal sediment";Dean (1983): "an idealization of conditions which occur innature for particular sediment characteristics and steadywave conditions"; and Larson (1991): "a beach of specificgrain size, if exposed to constant forcing conditions, normally<strong>as</strong>sumed to be short-period breaking waves, willdevelop a profile shape that displays no net change in time:'Bruun (1962) developed a simple model to characterizethe profile of equilibrium that is <strong>as</strong>sumed to exist for allshorefaces, and which h<strong>as</strong> become the b<strong>as</strong>is for most modelsin the design of co<strong>as</strong>tal engineering projects. However, Kriebelet al. (1991) argued that "a beach profile in true equilibriumnever exists in nature because nearshore water levels,waves, and currents are constantly changing."Pilkey et al. (1993) believe that there are many "shorefaceprofiles of equilibrium" and the model <strong>as</strong> developed by29
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