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William J. ClearyPHOLOGYFigure 2. A. Bogue Banks. Forested beach ridges characterizeBogue Banks. Regressive barriers contain 15-25 times moresand per unit of length of coast than do transgressive barriers.B. Masonboro Island. Very low and narrow transgressive barriersare prone to overtopping during storm events. Inlet fill iscommon beneath the barrier and shoreface.Inlets) has resulted in dramatic changes along adjacentshorelines and significant morphological changes in the inletand its associated shoals (Cleary and Hosier 1987, 1989,1995; Cleary 1994).It is increasingly evident that the dynamics of inlets aresite specific, with each system exhibiting individualizedreponses to local environmental and geological factors. It isthe intent of this paper to provide a brief overview of thecoastwise variability of inlet types, morphological changeswithin the inlet system and the role the inlets play in the patternsof erosion and accretion on the adjacent shorelines. Theoverview is based on current investigations and publisheddata from studies of the majority of inlets in southeasternNorth Carolina.INLETS, EBB DELTAS AND SHORELINE MOR-Tidal inlets in southeastern North Carolina are mixedenergy (transitional) wave-influenced systems. At mixedenergy inlets (wave-dominated), a large portion of the varioussand bodies are concentrated within the inlet throat.Along southeastern North Carolina, natural inlets displaywell developed ebb deltas. Mixed energy or transitionalinlets are perhaps the most difficult to study due to the varietyof factors involved in determining the morphology of theinlet and associated sand bodies.Ebb-tidal deltas, the seaward shoals of an inlet areformed through the interaction of waves and tidal :currents.The general morphology of these features has been describedin detail beginning with the studies of Oertel (1972) andHayes, et al (1973). A number of studies have refined the initialmodels and described physical processes that shape thesefeatures (FitzGerald, 1976; Humphries, 1977; FitzGerald1984.) The overall morphology and the extent to which ebbdeltas are developed is a function of the inlet's tidal prismand the exposure to wave energy (Walton and Adams, 1976;Nummedal et al, 1977; FitzGerald, 1993; Hayes, 1994).Ebb tidal deltas along the Onslow Bay shoreline are reservoirsof good quality beach fill sand. The volume of sandcontained in these systems ranges from less than 750,000cubic meters to more than 80,000 million cubic meters.Slight changes in the size or shape of ebb deltas can have asignificant effect on adjacent shorelines (FitzGerald, et al,1978; FitzGerald and Hayes, 1980; Cleary and Hosier, 1987and 1989; Cleary 1994). Regardless of size, the offshoreshoals influence the ends of the barriers, acting as naturalbreakwaters and modifying the wave energy impinging uponthe shoreline. Waves approaching the islands are refracted insuch a manner that a region of sediment transport reversaloccurs downdrift of the inlet (Hayes, et al, 1973; Hayes,1980 and 1994).This mechanism of transport reversal had been proposedto account for the bulbous shoreline segment immediatelydowndrift of mesotidal inlets (Hayes et al, 1973). Cyclicalepisodes of complex bar- welding events account for a portionof the observed progradation (FitzGerald, 1984). Whenan inlet changes location or the symmetry (skewness) of theebb delta changes there is a concomitant change in :he patternof erosion/accretion on the adjacent shorelines (FitzGerald,.et al 1978; FitzGerald, 1984; Cleary and Hosier 1987and 1989; Cleary 1994).PREVIOUS STUDIES OF NORTH CAROLINAINLETSA number of North Carolina inlet studies exist. Theseinvestigations range in scope from the distribution and geologicsignificance of inlets (Cleary and Hosier, 1979, 1986a}to the US Army Corps of Engineers reports dealing with the42
INLET INDUCED SHORELINE CHANGES: CAPE LOOKOUT — CAPE FEARFigure 3. Brown’s Inlet. Brown’s Inlet, a stable system, islocated between Onslow Beach and Brown’s Island. Viewtoward the northeast and Cape Lookout.A series of barrier islands, spits and cuspate forelandsform the 185 km stretch of open ocean coast between CapeLookout and Cape Fear North Carolina (Fig.1 }. The OnslowBay coastal compartment is comprised of the low energyflank of the Cape Lookout Foreland and the northeasternhigh energy flank of the Cape Fear Foreland. This lowmesotidal shoreline segment is comprised of thirteen morphologicallydiverse barriers that can be grouped into twodistinct classes. The division occurs between Browns Islandand Onslow Beach where a submarine headland composedof Tertiary limestone and sandstone forms a prominent protuberancealong the coast. Onslow Beach perched atop theheadland separates the wide, high beach ridge and modifiedbeachridge barriers to the northeast from the transgressivebarriers with low narrow profiles to the southwest (Figs. 1 &2 A & 2 B).Overwash and historic inlet breaches are more commonalong the southern segment. The lagoons that back theregressive barriers, principally Bogue Banks, are shallow,open and generally free of tidal marsh. By contrast, marsheshave infilled the great majority of the southern lagoons. Thelack of tidal marsh behind Bogue Banks is directly attributableto the lack of significant inlet activity along the barrierduring its progradational phase during the past several thousandyears.Inlets along the regressive segment are generally locationallystable systems. This group includes those inletswhose throat section has remained in approximately thesame location (100 -200 m) for the past 75 years (Fig. 3).The minimum inlet width of this type varies from time totime and is related to throat expansion during storms andsubsequent constriction during storm free periods. Browns,Bear, Bogue and Beaufort Inlets (Fig. 1) are stable systemswhose locations are controlled by ancestral drainage patterns.Barden's Inlet, a small migrating inlet separatingShackleford Banks from Core Banks (Fig.1 ) is of relativelyrecent derivation having formed during a storm breach in the1920's.effects of dredging and navigation improvements such asthose for Masonboro Inlet (Vallianos, 1975; US Army Corpsof Engineers, 1982} and New River Inlet (US Army Corps ofEngineers, 1990}. Photographic reviews-currently out ofdate-depict, using sequential photographs or overlay maps,changes in inlet shorelines during a 35- 40 year period(Langfelder, et al, 1974; Baker, 1977}. An unpublished studyby Priddy and Carraway (1978} utilized a statistical analysisapproach to evaluate the recent migratory history of NorthCarolina inlets in an attempt to define an "inlet hazard zone".ONSLOW BAY SHORELINE OVERVIEWFigure 4. New Topsail Inlet. New Topsail Inlet located at thesouthern end of Topsail Beach is an unstable system that hasmigrated over nine km since the early eighteenth century.Small linear vegetated marsh islands paralleling the mainchannel record the inlet’s former positions.A second inlet type, typical of the transgressive barriersegment are locationally unstable systems that migrate atrates that range from 10- 100 m/y. Typically these systemsform during major storm events, generally seaward of smallincised coastal plain estuaries and migrate in a down driftdirection (southwest). Several of the current inlets, such asNew Topsail (Fig. 4), have been in existence for several centuries.Many others that formed subsequent to New Topsailin the eighteenth and nineteenth centuries are now closed.The position of the former inlet is now marked by extensivetidal marsh that has colonized the relict flood deltas.Several inlets in the transgressive segment, includingOld Topsail and Mason's, are in various stages of closure.New River, New Topsail, Old Topsail, Mason's, Masonboro(now stabilized) and New Inlet are migrating systems (Fig.5A-D). Rich Inlet (Fig. 5 E) is the only naturally stable systemin the southern barrier shoreline reach. Carolina Beach43
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Page 8 and 9: OVERVIEW OF THE MARINE TERTIARY AND
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