Robert A. Morton and Asbury H. Sallenger, Jr....Journal of Coastal Research 560-573 West Palm Beach, Florida Summer 2003 Morphological Impacts of Extreme Storms on Sandy Beaches and

Journal of Coastal Research 560-573 West Palm Beach, Florida Summer 2003

Morphological Impacts of Extreme Storms on SandyBeaches and Barriers

Robert A. Morton and Asbury H. Sallenger,Jr.

U.S. Geological SurveyCenter for Coastal and Regional Marine StudiesSt. Petersburg, FL 33701, [email protected]

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ABSTRACT

MORTON, R.A. and SALLENGER, A.H., Jr., 2003. Morphological impacts of extreme storms on sandy beaches andbarriers. Journal of Coastal Research, 19(3), 560-573. West Palm Beach {Florida}, ISBN 0749-0208.

Historical extreme storms that struck the Gulf Coast and Atlantic Coast regions of the United States caused severaldifferent styles of morphological response and resulted in a wide rahge of washover penetration distances. The post-storm erosional responses included dune scarps, channel incisions, and washouts, whereas depositional responsesincluded perched fans, washover terraces, and sheetwash lineations. Maximum inland extent of wash over penetrationranged from approximately 100 to 1000 m and estimated sediment volumes associated with these deposits rangedfrom about 10 to 225 m3/m of beach. Unusual styles of morphological response (sheetwash lineations and incisedchannels) and maximum washover penetration distances are closely correlated, and they also correspond to stormintensity as defined by the Saffir-Simpson wind-speed scale.

The regional morphological responses and washover penetration distances are controlled primarily by the interac-tions among heights and durations of storm surge relative to adjacent land elevations, differences in water levelsbetween the ocean and adjacent lagoon, constructive and destructive interference of storm waves, and alongshorevariations in nearshore bathymetry. For barrier segments that are entirely submerged during the storm, impacts canbe enhanced by the combined influences of shallow water depths and organized flow within the wind field. The greatestwashover penetrations and sediment accumulations are products of shallow water, confined flow, and high wind stress.

Transport and deposition of washover sediments across barrier islands and into the adjacent lagoon are commonprocesses along the Gulf of Mexico but not along the western Atlantic Ocean. This fundamental difference in stormimpact underscores how microtidal and mesotidal barriers respond respectively to extreme storms, and providesinsight into how different types of barrier islands will likely respond to future extreme storms and to a relative risein sea level.

ADDffiONAL INDEX WORDS: Washover, sediment transport, hurricane, extra-tropical storm, wind stress.

INTRODUCTION

The large, rapid changes in coastal landscapes caused byextreme storms have long captured the attention of geologicalobservers. For example, the flooding, beach erosion, and re-sulting property damage from the 1938 hurricane in NewEngland were so profound that several reports were pub-lished describing the same event (BROWN,1939; HOWARD,1939; NICHOLSand MARSTEN,1939; WILBYet ai., 1939). Inthe past few decades there have been many investigations ofthe impacts of individual mid-latitude storms in the U.S. (e.g.HAYES, 1967; WRIGHTet ai., 1970; DOLANand GODFREY,1973; LEATHERMANet aZ., 1977; NUMMEDALet aZ., 1980;KAHNand ROBERTS,1982; MORTONand PAINE,1985; STONEand WANG, 1999 among others), and entire volumes havebeen dedicated to the diverse geological and biological effectsof recent extreme storms (FINKL and PILKEY, 1991; STONEand FINKL, 1995).

Washover deposits are one of the most commonly observeddepositional responses to extreme storm events. Consequent-ly, hundreds of papers have been published on such topics asoverwash processes, textural grading and stratification of

02124 received and accepted in revision 22 November 2002.

washover deposits, beach erosion and overwash potential,and the role of overwash in the aggradation and lateral mi-gration of barrier islands (see previous lists of references).The wealth of post-storm data provides an opportunity to syn-thesize the regional impacts of extreme coastal storms, andto develop a basis for further understanding the physical con-ditions that produced the morphological changes.

OBJECTIVES AND METHODS

The primary purposes of this study were to document mor-phological impacts of extreme storms in the Gulf of Mexicoand western Atlantic Ocean and to evaluate ground and flowconditions that influence washover penetration distances,styles of washover deposition, and the associated sedimentvolumes stored in these nearly ubiquitous coastal features.To meet these objectives, post-storm aerial photographs wereused to classify the types of morphological impacts and tomeasure washover penetration for the shores (Figure 1) im-pacted by Hurricanes Carla (1961), Camille (1969), Frederic(1979), Alicia (1983), and Hugo (1989), and the Ash Wednes-day storm (an extreme Atlantic northeaster in 1962). Wash-over penetration on aerial photographs was measured inlandfrom the beach scarp or berm crest for the extreme storms

Impacts of Extreme Storms on Sandy Beaches and Barriers 561

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Figure 1. Locations of areas impacted by extreme storms in the Atlantic Coast and Gulf Coast regions. Storm impacts were analyzed using both aerialphotographs and field surveys.

identified in Table 1, except for the measurements in coastalLouisiana, which were calculated from data reported by RIT-CHIEand PENLAND(1990). The Louisiana data were includedto compare washover penetration for low-intensity hurri-canes with washover penetration for more intense storms.The thicknesses of wash over sediments deposited by Hurri-canes Carla, Alicia, and Gilbert were measured in the fieldto estimate the volumes of sediment deposited onshore (Table2) and rapidly removed from the littoral system.

MORPHOLOGICAL RESPONSES TOEXTREME STORMS

The erosional and depositional responses observed on post-storm photographs and in the field included dune scarp ero-

sion, channel incision, and washout, and construction ofperched fans, washover terraces, and sheetwash lineations(Figure 2). Terms for the erosional features (scarps and chan-nels Figures 2A and 2B) are well established, but those forwashout and the depositional features may require brief def-initions. Morton (2002) presented photographs illustratingeach of the different morphological responses.

Washout involves channel erosion across the beach andforedunes (Figure 2C) as a result of floodwaters flowing fromthe lagoon to the ocean. The term washout is used becausethe process is opposite to that of overwash (MORTONandPAINE, 1985). This relatively rare phenomenon occurs wherethe lagoon is higher than the ocean and also higher than theforedunes (EL ASHRY and WANLESS, 1968; PIERCE, 1970;WRIGHTet al., 1977).

Table 1. Minimum, maximum, and mean washover penetration distances for the areas most affected by selected storms. Hurricane intensity at landfallfrom the National Hurricane Center, intensity of the 1962 storm from Dolan and Davis (1992).

Journal of Coastal Research, Vol. 19, No.3, 2003

Intensity Number of Range MeanStorm and Year Category Location Measurements (m) (m)

Hur. Carla (1961) 4 Southeast TX 168 30--927 1931962 northeaster 5 U.S. mid Atlantic coast 200 123-750 287Hur. Camille (1969) 5 Gulf Islands, MS 30 82-752 220Hur. Frederic (1979) 3 Dauphin Is., AL 31 250--770 425Hur. Alicia (1983) 3 Galveston Is., TX 74 3-106 251985 hurricanes 1 Caminada, LA 84 15-226 114Hur. Hugo (1989) 4 Central and eastern SC 134 15-204 83

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Perched fans (Figure 2D) are small lobate to elongatewashover features that are oriented perpendicular to theshore. The fans can be either isolated or regularly spacedalongshore. Isolated fans are constructed when wave runupsuperimposed on the storm surge exceeds the lowest duneelevations, but elsewhere the surge is blocked by higher duneelevations. Most studies of washover deposits on the AtlanticCoast of the u.s. are of small isolated perched fans(SCHWARTZ, 1975; DEERY and HOWARD, 1977; LEATHERMAN

et ai., 1977; KOCHELand DOLAN, 1986; LEATHERMANandZAREMBA,1987). Morphological criteria that favor construc-tion of regularly spaced fans include a narrow barrier island,low dunes, and minor alongshore differences between dunegaps and dune crests. At some locations, perched fans are soclosely spaced that they merge to form a washover terrace(MORTON, 2002).

Washover terraces (Figure 2E) are elongate deposits ori-ented parallel to the shore (SCHWARTZ,1975; MORTONandPAINE, 1985). Terraces form where land elevations are rela-tively uniform alongshore and lower than the maximumstorm surge. They may form a uniformly wide band, or theirlandward margins may be highly irregular, depending on theinteractions between breaking waves and currents duringwashover deposition.

Sheetwash involves laterally unconfined flow where sedi-ment transport is continuous across the barrier island. Sheet-wash may result in either deposition of sand eroded from theadjacent beach/dune system or redistribution of sand erodedlocally. Common bedforms resulting from sheetwash are nar-row elongate zones of erosion and deposition that form line-ations parallel to the direction of flow (Figure 2F).

WASHOVER PENETRATION AND VARIABLEALONGSHORE PATTERNS

Washover Penetration Distances

Washover penetration distances associated with the ex-treme storms span at least two orders of magnitude and ap-proach one kilometer (Table 1, Figures 3-5). SALLENGERetai. (in press) also reported that sand bodies of the Isle Der-nieres in Louisiana migrated about 1 km during HurricaneAndrew. The maximum washover penetration for hurricanesoccurs in the right-front quadrant at landfall in a zone about20 to 50 km from the eye (Figures 3 and 4). This alongshoreposition coincides with the inner eyewall where the higheststorm surges and onshore wind velocities typically occur(SIMPSONand RIEHL, 1981). Beyond this first approximation,washover penetration depends on factors other than storm

surge elevations. For example, washover penetration awayfrom the eye of Hurricane Camille was generally greaterwhere storm surge elevations were lower (Figure 4), andwashover penetration for the March 1962 storm was highlyvariable despite relatively uniform storm surge elevations(Figure 5). Hurricane Hugo generated a maximum open-coastsurge of 5 m near Bulls Bay, South Carolina (SCHUCK-KoL-BEN, 1990), but high foredunes and forested beach ridges pre-vented the surge from transporting sand across the nearbybarrier islands. However at Myrtle Beach, about 120 km



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Figure 2. Types of erosional and depositional features produced by extreme storms. A. Dune Erosion, B. Channel Incision, C. Washout, D. Perched Fan,E. Washover Terrace, F. Sheetwash Lineations.

northeast of the maximum surge, washover sediments weredeposited more than 100 m from the shore (Figure 6).

The frequency distributions and averages of wash over mea-surements (Figure 7A and Table 1) show that even extremestorms, such as Hurricanes Carla, Camille, Hugo, and Fred-eric and the 1962 Ash Wednesday storm, cause different lev-els of impact. The greatest average washover penetration(425 m) was associated with Hurricane Frederic because itovertopped the entire western two-thirds of Dauphin Island.Average washover penetration for the 1962 storm (287 m)was only slightly greater than average penetration for Cam-ille (220 m) but the number of sites where penetration ex-ceeded 300 m was substantially greater for the 1962 stormthan for any of the other storms (Figure 7A). By comparison,washover penetration was relatively low for Hurricane Hugo,which caused less than 100 m of washover penetration atmost sites (Figure 7A). Hurricane Carla caused greater than600 m of washover penetration at more sites than any otherstorm, however, most of Carla's washover penetration wasless than 300 m (Figure 7A).

Combining the washover penetration data for all sevenstorms provides a basis for estimating the potential washoverimpact for an extreme storm that falls within the intensity

range of those studied (categories 3-5). The combined datashow a progressive decrease in probability as penetration dis-tances increase (Figure 7B). The function that best fits thedata roughly describes an exponential decline curve. Wash-over penetration of at least 100 m has a probability of 67%,whereas there is a 50% chance of at least 160 m of washover

penetration. Penetration distances of at least 200 m have aprobability of about 42% and washover penetration greaterthan 400 m has a probability of less than 10% (Figure 7B).These results for both Gulf Coast and Atlantic Coast settingsare consistent with the general observations of LEATHERMAN(1983), who concluded that Atlantic Coast barriers less than200 m wide should be overwashed frequently.

Different storms can generate similar patterns of wash overpenetration along the same coastal segment even though theactual penetration distances may differ from storm to storm.This was demonstrated by RITCHIEand PENLAND(1990) whocompared the impacts ofthree hurricanes in 1985 that causedextensive washover ofthe Caminada deltaic headland in Lou-isiana (Figure 8). The washover penetrations for storms be-fore 1978 and combined affects of Hurricanes Danny, Elena,and Juan in 1985 were greatest along the beach separatingBay Champagne from the Gulf of Mexico and least where the


564 Morton and Ballenger

Figure 3. Maximum washover penetration and storm surge elevations in areas of Texas most impacted by Hurricane Carla (1961). General locationshown in Figure 1.

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Impacts of Extreme Storms on Sandy Beaches and Barriers

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beach intersects the high elevations of the chenier ridges(Figure 8). MORTONand PAINE (1985) presented an exampleof repeated patterns of wash over penetration for differentstorms that impacted the barrier island at Galveston, Texas.At Galveston, Hurricanes Carla (category 4), Allen (category3), and Alicia (category 3) all produced maximum wash overpenetration where interior barrier elevations were low anddunes were poorly developed.

Styles of Washover Response

Extreme storms can cause the same morphological respons-es even though each storm may have substantially differentcharacteristics. In the Gulf Coast region, extreme stormssuch as Hurricanes Camille (WRIGHTet al., 1970), Frederic(KAHN and ROBERTS, 1980), and Georges have repeatedlycreated the same morphological changes on the ChandeleurIslands of Louisiana by eroding narrow, closely-spaced chan-

nels across the barriers (Figure 2B). Hurricanes Camille andFrederic also caused repeated morphological changes on westDauphin Island, even though the peak surge during Frederic(3.6 m) was higher than during Camille (2.8 m). Camille con-structed regularly spaced perched fans between sheetwashlineations to the east and wash over terraces to the west (Fig-ure 4). Hurricane Frederic (Figure 9) caused essentially thesame styles of wash over response as Camille. The sheetwashlineations produced by Frederic were laterally more extensivethan those produced by Camille, but their positions and con-struction between wash over terraces were the same for bothstorms.

The most common responses for the March 1962 north-easter and Hurricane Hugo along the Atlantic Coast wereeither wash over terrace construction or dune erosion. Forboth storms, washover penetration was greatest across theends of low-lying spits near inlets or across narrow barriersegments that were subjected to sheetwash (Figures 4 and5). HOSIER and CLEARY(1977) presented maps showingwhere Hurricane Hazel (1954) and the March 1962 storm re-peatedly constructed washover terraces and perched fans atthe same locations on Masonboro Island, NC.

INFLUENCES OF TOPOGRAPHY, WATER DEPTH,AND WIND STRESS

Topography, Bathymetry, and Flow Depth

Topography plays an important role in controlling wash-over penetration and washover patterns. Low, relatively uni-form backbeach elevations promote unconfined flow and con-struction of washover terraces or sheetwash deposits (Figures2E and 2F), whereas highly irregular backbeach elevationspromote confined flow and construction of individual perchedfans or excavation of subtidal washover channel and fan com-plexes (Figures 2D and 2B). Less certain are the influencesof nearshore bathymetry, overwash flow depth, and proxim-ity to open water on these same parameters.

Overwash flow depth determines the water column avail-able for current generation and inland sediment transport.Overland flow depths were approximated for Hurricane Carlaby subtracting the highest dune elevations (contoured onUSGS 1:24,000 topographic maps) from the maximum open-coast storm surge elevations (field surveys by the Corps ofEngineers). Comparing washover penetration with estimatedflow depths for Carla (Figure lOA) shows a reasonably closecorrelation between the two parameters where washover ter-races were deposited, but the trend reversed where flowdepths decreased dramatically and the morphological re-sponse was incised channels. Although there is considerablescatter in the paired data (Figure lOB), the trends suggestthat washover penetration was directly proportional to flowdepth up to about 350 m and 2.5 m respectively, but greaterinland penetration of washover deposition depended on aninverse relationship associated with shallower flow depths.This relationship suggests that a third factor was responsiblefor the enhanced sediment penetration distances in the zoneof channel incision. Field observations indicate that the sub-tidal depths of erosion in the channels were at least 1 m. Abetter correlation between washover penetration and flow


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depth is obtained for the zone of channel incision (Figure 11)if the estimated subtidal depths of erosion in the channels (1m) are added to the land elevations to estimate total flowdepths.

The washover styles and washover penetrations on Dau-phin Island also demonstrate how topography, bathymetry,and flow depths influence overwash processes. There ground

elevations are generally 2 to 3 m and the maximum open-coast surge during Hurricane Camille was about 2.8 m, con-sequently flow depths during the storm were extremely shal-low across most of the barrier. The washover styles on Dau-phin Island produced by Hurricane Camille (Figure 4) grad-ually changed westward from closely-spaced lineations, toperched fans, to a washover terrace with irregular landwardmargin, to a washover terrace with a uniform landward mar-gin. These morphological changes coincided with the along-shore trends of nearshore shoaling and differences in over-wash flow depths. High dunes and the ebb-tidal delta protectthe eastern end of Dauphin Island from storm waves andprevent overwash (NUMMEDALet al., 1980). To the westwhere island orientation changes and dunes are substantiallylower, bathymetric gradients are steeper, and deeper wateris close to the shore (see distance to offshore bar in Figure 9).The deeper water waves produce greater washover penetra-tion in the zone of shoreline reorientation. Because barrier

elevations decrease to the west, overwash flow depths gen-erally increase in the same direction (Figure 9), which alsocontributed to the westward change from perched fans to awashover terrace.

The styles and alongshore patterns of storm impacts onDauphin Island were essentially the same for Camille andFrederic, but the inland sediment transport distances weremuch greater for Frederic (compare Figures 4 and 9), reflect-ing the greater flow depths. Both storms produced sheetwashlineations where the barrier is narrow, dunes are uniformlylow, and the shoreface is moderately steep. Minor differencesin flow depths may have also contributed to the contrastingstyles of washover response. Dauphin Island has a history ofbeing breached repeatedly (HARDIN et al., 1976) and thesheetwash lineations formed where breaching previously oc-curred. A wave refraction analysis by NUMMEDALet al.(1980) showed that the zone of prior breaching was also thezone of bathymetric wave focussing and highest wave energy.The washover terraces, on the other hand, formed where theisland core was slightly higher and flow depths were slightlyshallower. The alongshore changes in washover morphologiesalso reflect the flow structure in the washover currents. Thesheetwash lineations were formed by highly organizedstreamlines of shore-normal currents that probably were gen-erated by wind stress, whereas the terrace deposits wereformed by shore-parallel fronts of breaking waves that pro-duced essentially uniform shore-normal flow.

The influence of overland flow and distance to open water(large-scale surface roughness) on washover penetration canbe illustrated for Hurricane Carla (Figure 3), the 1985 hur-ricanes (Figure 8), and the March 1962 northeaster (Figure12). In all of these examples wash over penetration generallyincreases where the ocean is close to inland water (narrowbarrier), and penetration decreases where the land is widerand surface friction is greater. DOLANand HAYDEN(1981)reported that washover penetration from the March 1962storm correlated well with average decadal rates of shorelinemovement. Although shoreline movement can be an indicatorof wash over potential, the significance of the correlation hasto do with the fact that shoreline movement commonly cor-relates directly with dune construction. Dune construction is


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minimized or prevented along beach segments that are erod-ing rapidly, whereas dune construction is facilitated wherethe beach is stable or accreting slowly. The distances of wash-over deposition presented by DOLANand HAYDEN(1981) cor-relate equally well with the width of adjacent backbarriermarsh and proximity to open water (Figure 12).

The plots of washover penetration (Figures 3-6) exhibittwo primary scales of variability. The large first-order scalingappears to be related to overland flow distances and prox-imity to open water as described in the proceeding para-graphs. The smaller secondary scaling is related to morphol-ogy of the washover deposits, which is partly determined by

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constructive and destructive wave-to-wave interference thatenhance and suppress washover fan construction. Whereshoals and storm waves combine to produce peak breakerheights the beach segments commonly experience the great-est beach erosion and overwash (NUMMEDALet al., 1980).CARTERand ORFORD(1981) suggested that zones of wash-over on a coarse-grained barrier in Ireland correlated closelywith areas of wave convergence and attendant increasedbreaker heights. ORFORDand CARTER(1982) also speculatedthat edge waves produced the closely-spaced washover chan-nels. Although other studies have attempted to link the spac-ing of wave-runup maxima with edge waves, there are nofield data for storm conditions that support the edge wavehypothesis for washover channel construction.

Wind Stress and Sediment-Bypass Distances

Wind stress can augment overwash processes by acceler-ating currents and generating water velocities that otherwisewould not be obtained by wave runup alone. The greatestenhancement of flow velocity occurs when the wind directionand the angle of wave approach are both directed onshore.These optimum conditions ITequently occur in the Gulf ofMexico during the landfall of hurricanes that track at highangles to the shore (MORTON,1979). On the other hand themaximums winds of extreme northeasters commonly blowparallel to the shore and reduce the coupling between windand the water flowing over land.

Where breaking waves and runup are the predominant cur-rent-generating processes, the wash over sediments are de-posited immediately landward of the berm, erosional scarp,or foredunes. However, in those cases where high wind stressis the driving force, there commonly is a zone landward ofthe backbeach across which sediments are transported, butdeposition is either absent or minor (MORTON,1979). Thisbypass zone separates the washover deposits ITom the topo-graphically high areas adjacent to the backbeach. Onshore


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Figure 13. Variations in sediment bypass along the 120 km of Texasshore most impacted by Hurricane Carla. Compare with Figures 4 and1tA

sediment transport and minor local reworking characterizethe zones of sediment bypass, which coincide with areas ofmaximum storm surge and highest wind speeds.

Wind-driven currents during Hurricane Carla (category 4)transported sediment 400 to 550 m inland from the beach(Figure 13) before surface roughness decelerated the flow andinitiated deposition. The striking effects of wind stress alsowere observed on Dauphin Island after Hurricanes Camille(category 5) and Frederic (strong category 3). Frederic con-structed sheetwash lineations, flame-shaped fans with nar-row levee-like margins, a narrow bypass zone, and a few in-cised channels. The surface lineations observed after Hurri-canes Carla, Camille, and Frederic are comparable to theflow-aligned patterns of ridges and intervening troughs pro-duced experimentally as a result of unstable pulsating flow(KARcz and KERSEY, 1980). Although the shore-normalalignment and close spacing of sheetwash lineations arguefor high velocity flow generated by wind stress, there are nosurface wind data available to directly support that interpre-tation. Furthermore, the coarse temporal and spatial scalingof storm wind observations, and time averaging methods ofanalysis (POWELL, 1982), prevent close correlation betweenthe observed morphological patterns and the interpreted sur-face wind structure.

Morphological changes caused by extreme coastal stormstypically have spatial scales of 10s to 100s of meters and theyare nearly instantaneous events (minutes to hours) becausethe non-cohesive sediments respond rapidly to the high watervelocities. This fine-scale resolution cannot be compared di-rectly with the widely spaced sites used to reconstruct thehorizontal low-level hurricane wind fields, but there is evi-dence from the wind field analyses and wind damage surveysat the sedimentological scales of interest. Some hurricanesare characterized by alongshore velocity gradients that pro-duce an inner eyewall and an outer secondary maximum inwind velocity farther from the eye (SIMPSON and RIEHL,

1981). Some of the alongshore variations in washover mor-phologies could be related to the reported alongshore vari-ability in wind speed and direction at hurricane landfall. Themost detailed near-surface wind field reconstructions comefrom the orientations and distributions of blown down treesand structural debris. After Hurricane Andrew, fuJITA(1992) and WAKIMOTOand BLACK(1993) reported organizedpatterns of damage with length scales of a few hundred me-ters caused by microscale vorticity. Further evidence of thefine-scale high velocity coupling between wind and water isprovided by photographs of the sea surface during HurricaneEloise, which show linear foam streaks that are aligned withthe surface wind (POWELL,1982).

WASHOVERSEDIMENTVOLUMES

Thicknesses and widths of washover deposits are rarely re-ported because the deposits are typically large and the fieldmeasurements can be highly variable. Consequently, there isa general lack of information regarding volumes of sedimenttransported inland of the beach and deposited during ex-treme storms. The few published data for washover volumes(Table 2) are mostly single, generalized values without anyindication of alongshore or crosshore variability. Therefore, itis unknown how representative these data are in terms ofwashover sediment volumes.

Table 2 suggests that normalized washover volumes of afew 10s of m3/m of beach are common, whereas sediment vol-umes of more than 100 m3/m of beach are rare. The largestnormalized sediment volumes are associated with confinedflow where landward sediment transport is laterally restrict-ed by higher elevations either as a result of channel incisionor breaching of the dune complex. Furthermore, the largestsediment volumes are related to the broad areas covered bywashover deposition, rather than great thicknesses of the de-posits.

Few attempts have been made to estimate the total volumeof sediment removed from the active beach and dune systemby an extreme storm and stored as washover deposits. MOR-TON and PAINE (1985) used field measurements at 62 sitesto characterize washover volume along the 30-km segment ofGalveston Island most affected by Hurricane Alicia. They re-ported that thicknesses of Alicia terrace deposits ranged from2 to 69 em and averaged about 23 em. The total volume ofAlicia washover sediments deposited on west Galveston Is-land was approximately 184,000 m3, or only about 12% ofthetotal volume of sediment eroded from the adjacent beachesand dunes.

Both erosional and depositional impacts of Hurricane Carlaare well preserved along the southeastern Texas coast wherethey still can be observed more than 40 years after the storm.Measurements on post-storm aerial photographs indicatethat wash over deposits covered about 18.4 million m2 alongthe 200-km stretch most impacted by Carla. Field measure-ments at six representative sites show that thicknesses ofCarla proximal washover deposits ranged from 60 to 126 em,whereas distal deposits associated with the flame-shapedfans were relatively uniform in thickness (26-31 em). Usingthe minimum (26 em) and average (56 em) washover thick-


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nesses to estimate total washover sediment volume, between4.8 and 10.3 million m3 of sand and shell were stripped fromthe beaches and dunes by Hurricane Carla in a few hours.Considering the moderate wave energy of the Texas coast,instantaneous losses from the sediment budget of that mag-nitude are equivalent to many years of net longshore sedi-ment transport.

There should be a high correlation between the type ofstorm impact and the percent volume of sediment erodedfrom the beaches and dunes that is transported onshore andstored in washover deposits. Where the morphological re-sponse is erosion of a dune scarp, 100% of the eroded sedi-ment is transported alongshore or offshore and no sediment(0%) is transported onshore (Figure 2A). Washover terracedeposits constructed where Hurricane Alicia partly inundat-ed Galveston Island represented only about 12% of the totalvolume of sediment eroded from the adjacent beaches anddunes (MORTONand PAINE, 1985). But where Santa RosaIsland, Florida was completely inundated during HurricaneOpal, 95-99% of the sand eroded from the beach and duneswas conserved in the wash over terrace and sheetwash depos-its (STONEet al., 1996). These data indicate that the volumeof eroded sediment that moves beyond the berm crest ordunes as washover sediment progressively increases from theperched fans (small percent) to the washover terraces, and tothe sheetwash deposits that can account for most of the sed-iment in a mass balance between beach/dune erosion andwashover deposition.

DISCUSSION AND CONCLUSIONS

The regional morphological impacts of seven extremestorms were analyzed to compare impact responses and pat-terns of wash over penetration for beaches and barriers of theAtlantic and Gulf Coasts of the United States. These analysesindicate correlations among storm intensity, morphologicalstorm impacts, and inland sediment transport distances as-sociated with overwash. There is no overwash and inland sed-iment transport when the storm response is strictly beachand dune erosion, but sand transport tends to progressivelyincrease with greater morphological impacts associated withoverwash and complete inundation of the landscape. Theanalyses also indicate that the inland penetrations of wash-over terraces are typically less than those of perched fans,and perched fan penetrations are typically less than those ofsheetwash or incised channels.

Inland sediment transport distances of storm washover canbe greatly augmented by shallow water, confined flow, andhigh wind stress. Normally washover penetration and groundelevation are inversely correlated, but when coupled withwind stress, they can be directly correlated. Under those con-ditions, slightly higher ground elevations contribute to chan-nel incision, increased flow depths, higher wind-driven flowvelocities, and greater inland sediment transport distances.

There are reasonably good qualitative correlations amongwashover penetration distances, washover styles, and theSaffir-Simpson storm intensity scale, which is defined on thebasis of wind speed (SIMPSONand RIEHL, 1981). Average andmaximum wash over penetration distances (Table 1) indicate

that only the most intense storms are capable of transportingsand more than 300 m inland (Figure 7B) for long segmentsof coast. Hurricanes Camille (category 5), Carla (category 4),and Frederic (strong category 3), and the 1962 northeaster(category 5) all deposited sand more than 300 m from thebeach but washover penetration for Hurricane Alicia (weakcategory 3) and the 1985 Louisiana hurricanes (category 1)was generally less than 125 m from the beach (Figures 7Band 8). Not all extreme storms, however, cause washover pen-etration > 300 m. High elevations and dense vegetation canlimit or prevent washover penetration for even the strongeststorms. This happened along the South Carolina coast duringHurricane Hugo (Figures 6 and 7B). In fact the average andmaximum washover penetration for the1985 Louisiana hur-ricanes (category 1) and Hurricane Hugo (category 4) are verysimilar (Table 1 and Figure 7B) despite the great differencesin storm intensity.

The storms that caused washover penetrations> 300 m(Carla, Camille, Frederic, and the 1962 northeaster), werealso the only storms that constructed extensive areas of ei-ther sheetwash lineations or incised channels (Figures 3, 4,5, and 9). Wash over penetration distances> 300 m were gen-erally associated with the sheetwash features and incisedchannels (Figures 3, 4, 5, and 9). Many storms can causebeach and dune erosion, and minor overwash, but only ex-treme extratropical storms and hurricanes with intensities ofcategory 3 and higher are generally capable of producing geo-logically preserved regional morphological responses that areassociated with complete inundation of the coast. The dataalso suggest that only category 3 and higher hurricanes havesufficiently well organized wind fields and wind speeds ca-pable of constructing closely-spaced lineations and incisingclosely-spaced channels across barrier islands (Figures 3, 4,5, and 9).

Determining accurate thicknesses and volumes of stormdeposits for regional sediment budgets are inexact becausethe deposits exhibit substantial three-dimensional variabilitythat depends on pre-storm topography and overwash flowdepths. Thicknesses of wash over sediments deposited by asingle storm typically range from <1 m to 1.5 m (Table 2).Field observations after Hurricanes Carla, Beulah, Eloise,and Alicia indicate that the thickest storm deposits are as-sociated with washover terraces or fans emerging from in-cised channels. Normalization of wash over sediment volumeson the basis of a unit beach length demonstrates that thelargest washover volumes are associated with extensive in-land penetrations rather than great thicknesses, and they aredeposited by confined flow (Table 2).

The similar patterns of wash over penetration regardless ofspecific storm characteristics (Figure 8 and MORTON andPAINE, 1985) suggest that some coastal sectors are precon-ditioned to overwash processes and relative magnitudes ofwashover penetration. The spatial persistence of washoverstyles (Dauphin Island Figures 4 and 9) is further evidenceof the preconditioning of some coastal segments not just tooverwash processes, but to specific morphological responsesbecause of their geological setting. The linkages between on-shore storm responses and the offshore geologic frameworkare poorly understood but there is some evidence that sub-



merged features can influence the onshore impact of stormprocesses. GAYES(1991) illustrated how coastal processes in-teracted with coastal development during Hurricane Hugo toconstruct shallow closely-spaced channels and lineations onthe shoreface of South Carolina. Although these were exam-ples of onshore structures influencing offshore processes, thereverse would also be true. Submerged shore-normal chan-nels, scour depressions, hardgrounds, rock ledges and othertypes of antecedent topography could alter storm waveheights and cause focussing of wave energy. Locally increasedwave heights in concert with local variations in backbeachtopography are probably responsible for the observed repeat-ed patterns of wash over penetration and repeated styles ofwashover response.

The 1962 northeaster had the greatest potential of any his-torical storm to cause extensive barrier migration along theAtlantic Coast. This is because the prolonged storm surgeeroded the beach, destroyed the dunes, and promoted over-wash of long segments of the shore (U.S. ARMYCORPS OFENGINEERS, 1963). Within the 510 km of storm-impactedcoast between New Jersey and North Carolina, no entire bar-rier island experienced migration as a result of the 1962storm (MORTONet al., in press). Despite optimum topograph-ic and oceanographic conditions for overwash, only a few bar-rier segments experienced sheetwash (26 km) or incision ofclosely-spaced channels (19 km) that conveyed sediment en-tirely across the barrier (MORTONet al., in press). The mostcommon morphological response to the 1962 storm was de-position of a washover terrace (400 km), which aggraded thebarrier islands but did not cause them to become transgres-sive landforms. The lack of barrier island migration fromsuch an extreme storm is consistent with the observations ofLEATHERMAN(1983) who, on the basis of historical morpho-logical studies of Hatteras Island, North Carolina and FireIsland New York, concluded that most Atlantic coast barrierswere either relatively stable narrowing landforms or migrat-ing landforms that move episodically and infrequently as aresult of inlet construction.

In the Gulf of Mexico, entire barrier islands have migratedfrequently as a result of repeated overwash by hurricanes.Well-documented examples of migrating barriers or barriersegments are South Padre Island and Matagorda Peninsula(Figure 3) in Texas (MORTON, 1994), Isles Dernieres, Tim-balier Islands, Breton Islands, and the Chandeluer Islandsin Louisiana (PENLAND and BoYD, 1985; KAHN and ROB-ERTS, 1986), west Dauphin Island (Figure 9) in Alabama(NUMMEDALet al., 1980), and Santa Rosa Island, Fl. (STONEet al., 1996). The number of migrating barriers in the Gulf ofMexico is high because many of them are associated witheither abandonment and foundering of lobes of the Mississip-pi delta (PENLANDand BoYD, 1985), or late Quaternary re-ductions in sediment supply of major coastal plain rivers(MORTON,1994). Apart from the issues of vertical stabilityand sediment supply, there appear to be other factors thatinfluence the susceptibility to complete overwash and barriermigration. The microtidal range and relatively low wave en-ergy of the Gulf of Mexico construct backbeach elevationsthat are generally low « 1.5 m) compared to the surge ele-vations of major hurricanes (Carla 3.7 m, Camille 4.9 m, Fig-

ures 3 and 4). In contrast, the mesotidal range and moderatewave energy of the Atlantic Ocean construct backbeach ele-vations (> 2 m) that are closer to the surge elevations ofextreme storms such as the 1962 northeaster (2.4 m, Figure5) and Hurricane Hugo (5 m, Figure 6). The fundamentaldifferences in storm processes between these two regions un-derscore how microtidal barriers of the Gulf Coast and me-

sotidal barriers of the Atlantic Coast respond respectively toextreme storms. These different responses on a historicaltime scale provide insight into how the different types of bar-riers will likely respond to future extreme storms and to arelative rise in sea level.

ACKNOWLEDGMENTS

The first author analyzed the impacts of Hurricanes Carlaand Alicia while at the University of Texas at Austin Bureauof Economic Geology. We thank Skip Davis, Al Hine, andTonya Clayton for their constructive comments on a draftversion of this paper.

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