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CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual MeetingPages 19 - 27SHOREFACE PROCESSES IN ONSLOW BAYE. Robert ThielerDuke University, Dept. of GeologyProgram for the Study of Developed ShorelinesDurham, NC 27708-0228THE SHOREFACE ENVIRONMENTThe shoreface is the interface between the continentalshelf and the subaerial coastal plain. The shoreface canbehave as a source, barrier, filter, or conduit for the bi-directionalexchange of materials between the land and the sea.The shoreface of barrier islands is the generally concaveupward surface extending from the surf zone to the pointwhere the slope becomes the same as the very gentle slope ofthe inner and central continental shelf. By this definition, thebase of the shoreface off southeastern North Carolina islocated at 10-12 m water depth (Figure 1).Figure 1. The shoreface is defined as the region between thesurf zone and the inner continental shelf. Off southeasternNorth Carolina, the base of the shoreface is located between 10-12 m water depth. (Modified after Wright et al., 1991).Oceanographic and geologic processes in this environmentdetermine how a shoreline will respond to storms, tosea-level rise and to human-induced changes in sand supplyover time scales from hours to years to millennia. Understandingshoreface processes is also critical to understandingthe behavior of replenished beaches, which provide manybeachfront communities with storm protection, recreationareas, and an important tourism resource. Sediment transportacross the shoreface is a key factor affecting 1) short- andlong-term fluctuations of beach and surf zone sand storage(Wright et al., 1985) ; 2) the morphology and stratigraphy ofthe shoreface (Niedoroda et al., 1985); and 3) the nature ofthe inner shelf sand sheet (Swift, 1976). On retreating barrierisland coasts, the shoreface is also a major source of newsediment to the coastal system, via the erosion and release ofpreviously deposited lagoonal and fluvial sediments. Thisprocess, termed "shoreface bypassing" by Swift (1976), is allthe more important on the southeastern U.S. Atlantic coastdue to the absence of a modern fluvial contribution. Curray(1969) suggested that the present is a unique moment in geologictime with regard to shoreface evolution. Specifically,the relative stillstand in sea-level along most of the U.S. EastCoast since about 4500 BP has allowed shoreface environmentsto mature and steepen as they seek an equilibriumform. If true, this process would minimize the amount ofsediment available to beaches from the continental shelf, andperhaps increase the rate of shoreface retreat.The shoreface is one of the most complex and leastunderstood coastal environments (Wright, 1987; Nummedal,1991). Geologists, oceanographers and engineers are onlyjust beginning to understand that nearly all shoreface environmentsare different, where processes and controls vary inimportance both spatially and temporally (Niedoroda et al.,1985; Kraft et al., 1987; Wright et al., 1991). The shorefaceis also the interface that couples the beach to the shelf. Theoryand empirical observations have done much to identifyshoreface sediment transport rates under various conditions.Presently, however, we can neither identify nor predict thenet transport of material on the shoreface (Wright, 1987;Pilkey, 1993; Nittrouer and Wright, 1994). An appliedunderstanding of shoreface processes is also needed todesign coastal engineering projects, as well as to evaluateand improve models used in coastal engineering to predictthe behavior of beaches. On a millennial time scale, sedimentationon coastal plain shelves during a time of risingsea-level such as the Holocene is driven by the bypassing ofsediment onto the shelf via the shoreface; fluvial sedimentsare trapped in the estuarine system. Swift (1976) describedthis process as "shoreface bypassing." This mechanism providesthe primary source of new sediment to the shelf as theravinement surface bevels previously deposited coastal plainmaterial. Thus, shoreface bypassing regulates sediment sup-19
E. Robert Thielerply, and in effect the rate and character of shelf sedimentation.The shoreface maturation process postulated by Curray(1969) seems to have peaked in Onslow Bay. The regressivebarrier islands (e.g., Shackleford Banks, Bogue Banks andBear Islands) are no longer prograding seaward; they appearto have accumulated all of the available inner shelf sand.This process of "inner shelf sweeping up" appears to havebeen very efficient; much of the inner shelf shows little or noevidence of modem sedimentation (Hine and Snyder, 1985).The transgressive islands (e.g., Topsail Island, WrightsvilleBeach and Masonboro Island) exhibit much the same shorefacecharacteristics, although they have substantially lesssediment volume. Nearly all of the sediment in the TopsailIsland barrier system, for example, is contained within thebody of the island landward of the beach; the shoreface sedimentvolume is very small.Over shorter time frames (e.g., individual storm eventsto several weeks), a variety of processes operate on theshoreface and inner shelf (Figure 2). These processes createhigh bottom stresses that mobilize sediment. As described byGrant and Madsen (1979), the bed agitation required for sedimentresuspension is furnished primarily by gravity waves,but sediment exchange is accomplished by quasi- steadystatemean flows. It is generally recognized that along-shelfflows predominate over across-shelf flows, but that acrossshelftransport gradients are relatively high (Nummedal,1991; Nittrouer and Wright, 1994). Several types of shorefaceand inner shelf currents have been recognized, including:1) storm- driven pressure gradient currents (e.g. windinducedupwelling and downwelling currents); 2) tidal currents;3) storm surge ebb currents; and 4) turbidity currents.Ekman (1905) predicted the presence of inner shelf currentsand Sverdrup, Johnson and Fleming (1942) noted the theoreticalbasis for currents related to pressure fields in theirclassic textbook. Shi and Larsen (1984) and Dean and Perlin(1986) suggested that cross-shore transport on the shorefacecould also be affected by forced long-period (infragravity)waves associated with groupy incident waves.Our contemporary understanding of short-term shorefaceand inner shelf processes is derived from both sedimenttransport modeling and field measurements. Early work onbottom boundary layer models was done by Jonsson {1966).Bijker {1967), and Sternberg {1972). Today, a number ofdifferent models {see review by Dyer and Soulsby. 1988) areused to examine boundary layer structure and sedimenttransport. In the past few years the Grant and Madsen {1979)and Glenn and Grant {1987) models for combined wave andcurrent flow have come into wide use {Cacchione et al.,1994; Nittrouer and Wright. 1994). These models coupledwith field measurements of the bottom boundary layer, haveidentified a number of important factors that influence shorefacesedimentary processes. The rates and directions of sedimenttransport on the shoreface and inner shelf are governedFigure 2. Schematic diagram of the major interacting components of the shallow-water bottom boundary layer. (Modified afterWright et al., 1989).20
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Page 8 and 9: OVERVIEW OF THE MARINE TERTIARY AND
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Page 46 and 47: William J. ClearyPHOLOGYFigure 2. A
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Page 50 and 51: William J. ClearyFigure 9. Shorelin
Page 52 and 53: William J. Clearymajor reduction in
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Page 56 and 57: Lynn A. LeonardFigure 2. Stratigrap
Page 58 and 59: Lynn A. LeonardFigure 5. Deposition
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WILLIAM J. CLEARY AND ORRIN H. PILK
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Appendix 6. Post Hurricane Fran (9/
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