dune and beach morphodynamics at cabo falso, baja california sur, mexico: response to natural,...

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Dune and Beach Morphodynamics at Cabo Falso, Baja California Sur, Mexico:Response to Natural, Hurricane Juliette (2001) and Anthropogenic InfluenceAuthor(s): Vera Camacho-Valdéz, Janette M. Murillo-Jiménez, Enrique H. Nava-Sánchez, andCuauhtémoc Turrent-ThompsonSource: Journal of Coastal Research, Number 243:553-560. 2008.Published By: Coastal Education and Research FoundationDOI: http://dx.doi.org/10.2112/05-0575.1URL: http://www.bioone.org/doi/full/10.2112/05-0575.1

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Journal of Coastal Research 24 3 553–560 West Palm Beach, Florida May 2008

Dune and Beach Morphodynamics at Cabo Falso,Baja California Sur, Mexico: Response to Natural, HurricaneJuliette (2001) and Anthropogenic InfluenceVera Camacho-Valdez, Janette M. Murillo-Jimenez, Enrique H. Nava-Sanchez, andCuauhtemoc Turrent-Thompson

Departamento de OceanologıaCICIMAR-IPN, AV. IPN S/NCol. Playa Palo de Santa RitaCP. 23080, La Paz, B.C.S., [email protected]

ABSTRACT

CAMACHO-VALDEZ, V.; MURILLO-JIMENEZ, J.M.; NAVA-SANCHEZ, E.H., and TURRENT-THOMPSON, C.,2008. Dune and beach morphodynamics at Cabo Falso, Baja California Sur, Mexico: response to natural, HurricaneJuliette (2001) and anthropogenic influence. Journal of Coastal Research, 24(3), 553–560. West Palm Beach (Florida),ISSN 0749-0208.

Cabo Falso is located on the southern coast of the Baja California peninsula. The beach is broad and wide with asteep face and tall foredunes on the backshore. Landwards, an active, predominantly parabolic dune field has devel-oped with average heights of 30 m. Part of the field is used by recreational all-terrain vehicles. The main goal of thisstudy was to identify the principal morphodynamic characteristics of the deposits influenced by these recreationalactivities and affected by Hurricane Juliette in 2001. The morphology of the study area was monitored using seasonaltopographic profiles, examination of aerial photographs from 1972 and 1993, and on-site photographs (November 1999to April 2002). The topographic profiles indicated that the erosion period for both the beach and the dunes was Juneto December. Hurricane Juliette was the responsible for most of the observed erosion. The greatest sediment accu-mulation occurred between December 2001 and February 2002, associated with the onset of the natural recoveryprocess of the beach–dune system. The photographs revealed that the dunes have lost vegetation cover because of theactivities of all-terrain vehicles, which erode paths perpendicular to the dune crests. This in turn allows increasedsediment movement, resulting in instability of the dune field system.

ADDITIONAL INDEX WORDS: Dune erosion, beach erosion, beach recovery, dune vegetation lost, recreational activi-ties, dune damage by ATVs, beach–dune system.

INTRODUCTION

Beaches and dunes are coastal environments with complexmorphodynamics, because energy from waves, currents, andwind are typically large and applied over relatively small ar-eas (SHERMAN and BAUER, 1993). Beaches are commonlydominated by the action of waves and littoral currents, whiledunes are directly affected by the dominant local winds.Beaches and dunes have traditionally been studied as sepa-rate systems. Recently, however, there has been an increasein studies focusing on beach–dune interactions (MCLACHLAN,1990; PSUTY, ALLEN, AND STARCHER, 1988), in recognitionthat the two environments are closely related. The exchangeof sediment between beach and dune is governed by a myriadof both hydrodynamic and aerodynamic variables. These in-clude incoming wave period, arriving angle and height, tidaland littoral current intensity and flow direction, dominantwind speed and direction, and local sea breeze intensity. Allhave important effects on the integral evolution of the beach–dune system (CHAPMAN, 1989).

DOI: 10.2112/05-0575.1 received 10 August 2005; accepted in revision26 July 2007.

Due to their natural beauty and recreational utility, beach-es and dunes are environments commonly subjected to in-tense human pressure (BAUER and SHERMAN, 1999). Dunesare particularly vulnerable to anthropogenic erosion, andmany of these systems have been altered extensively by hu-man activities, causing important ecological and morpholog-ical change (MAYER, 1995). Regrettably, in Mexico the use ofdune fields is poorly regulated, and their future, as properlyconserved environments, is uncertain. This inadequate man-agement is the cause of important economic and environmen-tal loss. In this context, it has become important to gathermore knowledge about the morphodynamics of these systems,with the aim of achieving adequate management of coastalzones.

Accordingly, studies have focused on the factors that influ-ence the morphodynamics of beaches and coastal dunes (BAK-KER, JUNGERIUS, and KLIJN, 1990; CARTER, CURTIS, andSHEEHY-SKEFFINGTON, 1992; GIMINGHAM et al., 1989;NORDSTROM and GARES, 1990; PYE, 1983). However, no workhas been published yet on the morphodynamics of the beachand dune field at Cabo Falso, Baja California Sur, Mexico.The main goal of this study was to identify the principal mor-phodynamic characteristics of beach and dune deposits,

554 Camacho-Valdez et al.

Journal of Coastal Research, Vol. 24, No. 3, 2008

Figure 1. Location of the Cabo Falso study area, Baja California Sur,Mexico.

Figure 2. (A) Topographic map of Cabo Falso (digitalised from INEGI,1985) and (B) schematic diagram of the topographic profile locations.

Figure 3. (A) Wind direction roses and (B) wind speed histogram forCabo Falso, June–September 2001.

which are under influence of recreational activities and wereaffected by the passage of Hurricane Juliette. This site wasselected because of the feasibility of monitoring its naturalphysical conditions within an area protected from human ac-tivities and within a neighbouring area used for recreationalpurposes by all-terrain vehicles (ATVs).

STUDY SITE

Cabo Falso is located on the southern tip of the Baja Cal-ifornia peninsula, facing the Pacific Ocean, approximately 5km west of the town of Cabo San Lucas (Figure 1). The coast-line is characterised by long, wide sandy beaches with steepbeach faces. Backshore foredunes and large parabolic dunesare present. The parabolic dunes largely extend inland andhave an average height of 30 m. Part of this dune field isused for recreational activities by ATVs. The local tide ismixed semidiurnal, with an average high tide of 0.457 m andlow tides of �0.609 m (CICESE, 1990). This has been consid-ered a high-energy coast because of the relatively low vari-ability and persistence of local winds and because of the pres-ence of large wind waves and swell (WRIGHT et al., 1973).

The area is located just within the Tropics, and tropicalstorms and hurricanes increase humidity, giving it a sub-tropical climate (INEGI, 2000). Its average annual tempera-ture is 22�C, with an average minimum of 16�C in Januaryand average maximum of 29�C in August. Average annualprecipitation is 345 mm, and the rainiest month is Septem-ber, with an average of 118 mm. The summer tropical stormsand hurricanes produce extreme precipitation of short dura-tion. The area is dominated most of the year by winds witha strong north-westerly component (MADEREY, 1975).

METHODS

Geomorphological Changes

Changes in morphology and vegetation cover of the studyarea were identified by means of comparative analysis of ae-rial photographs taken in 1972 (scale 1:75,000) and 1993(scale 1:50,000); photographic surveys of the dune field madein November 1999, June 2001, and April 2002; a series ofphotographs of the postbeach and foredune taken in June,October, and December 2001; and three topographic profilessurveyed in June 24 and December 6, 2001, and February 21,2002. The profiles, perpendicular to the coastline, were sur-veyed from the beach berm to the highest point of the pres-ent-day dune, using a Sokkia (set 3BII) Total TopographicStation. The northern profile (site 1, Figure 2) is located inthe pristine zone of the study area, where human activitiesare prohibited, while the other two profiles (Sites 2 and 3,Figure 2) were obtained within the area exposed to the pas-

555Beach–Dune Morphodynamics: Response to Natural, Hurricane, and Anthropogenic Influences


Figure 4. Aerial photographs (INEGI, 1972, 1993) and sketches, showing the dune vegetation cover in 1972 (A) and 1993 (B).

sage of ATVs. Along each transect approximately 10 pointswere measured, obtaining the distance between the topo-graphic station and a stick placed in each point and theheight for each point based on a benchmark.

Wind Measurements

Wind direction and speed were obtained using a Unidatameteorological station, model 6522A, which was installed be-side a lighthouse located in the southern portion of the studyarea, at the top of El Vigıa hill, approximately 500 m fromthe coast and 210 m above sea level. The hoped-for annualdata record was shortened to 4 months (June to September2001) because strong winds during Hurricane Juliette dam-aged the equipment. A meteorological station could not beinstalled on the beach, however, with the data obtained atthe hill, but theoretical wind speeds on the beach wear ex-trapolated from the hilltop station by using a model that pro-vides the variation of wind speed with height based upon the

effect of friction between the air and the earth (TVERSKOI,1962). The wind direction data was plotted in a rose diagram(Figure 3A), which shows for every 10� interval directiondata, the frequency percent (isolines are 4% equidistance).The theoretical wind speed was plotted in a graph (Figure3B) showing the frequency percent for each 1 m s�1 interval,up to 23 m s�1.

RESULTS

Reduction of Vegetation Cover

Comparison of the aerial photographs from 1972 and 1993revealed an important change in the vegetation cover. Withinthe recreational area, approximately 45% of the vegetationcover was lost. Vegetation patches cut by ATV traffic areclearly visible in the south-east corner of that area (Figure4). However, the 1993 photograph also showed that both theprotected and the recreational areas had new patches of veg-



Figure 5. Photographs taken in 1999, 2001, and 2002, illustrating theloss of vegetation cover induced by the passage of the ATVs along a par-abolic dune flank.

Figure 6. (A) Profile changes of beach and dune deposits, surveyed inthe protected area (site 1); (B) close-up of beach profiles; and (C) data ofbeach erosion and accumulation.

etation, which indicates that natural growth continues in cer-tain areas of the dune field despite ATV activity.

In the dune photographic surveys carried out in 1999, 2001,and 2002 (Figure 5), the most visible changes are also in thevegetation cover. In the November 1999 photograph, trackscaused by circulation of the ATVs are clearly visible, as wellas abundant vegetation next to the tracks (Figure 5). How-ever, a little more than a year later, the 2001 photographshowed that the vegetation cover at the crest of the duneflank was almost completely eliminated and the tracks werewider, especially at the base of the dune flank (Figure 5). Inthe last photograph of the sequence (April 2002), new trackswere made by ATVs where vegetation previously existed (Fig-ure 5). Such changes in less than 3 years indicate that ATVactivities have been an important factor in the reduction ofthe vegetation cover, resulting in increased instability of thedune field within the recreational area.

Profile Surveys

Protected Area, Site 1

Changes in dune and beach topography at site 1 within theprotected area are shown in Figure 6. The June 2001 surveypresented a typical summer profile, with summerlong swellconditions and low-energy waves favouring sediment accu-mulation on the beach. The profile for December 2001 showsthe effect of Hurricane Juliette, which produced an escarp-ment at the base of the dune, with erosion causing a retreatof at least 60 m from the previous position. The February2002 profile illustrates the subsequent natural recovery ofthe beach, with an increase in dune height of 15 to 20 m,although the beach face did not reach the position it had be-fore the hurricane.

The dune profile showed deposition of between 0.5 and 1m of new sediment between June and December. BetweenDecember and the following February, however, furtherchanges were minimal, with both profiles showing similarmorphology.



Figure 7. (A) Profile changes of beach and dune deposits, surveyed inthe recreational area (site 2); (B) close-up of beach profiles; and (C) dataof beach erosion and accumulation.

Figure 8. (A) Profile changes of beach and dune deposits, surveyed inthe recreational area (site 3); (B) close-up of beach profiles; and (C) dataof beach erosion and accumulation.

Recreational Area, Site 2

Figure 7 shows the dune and beach profiles obtained dur-ing summer and winter at site 2. These profiles show a smallvertical change corresponding to erosion of 0.5 to 1 m of theupper half of the dune along a distance of about 125 m be-tween the two surveys. The middle portion of the dune profileremained stable throughout all of our surveys. On the otherhand, the beach face receded approximately 70 m during theJune–December 2001 period, but the profile recovered almost20 m of this loss by February 2002.

Recreational Area, Site 3

The morphodynamics recorded by the seasonal profilesmeasured at site 3 are shown in Figure 8. The December 2001and February 2002 profiles exhibit significant changes at thecrest of the dune, with erosion of approximately 2 m betweensurveys, while the middle portion of the dune remained sta-ble, as at site 2. The beach profiles show erosion from Juneto December, with a maximum horizontal recession of 25 m.From December 2001 to February 2002 a readvance of 10 mis apparent but, as was the case at the other two sites, stillfar from total recovery to the previous summer’s profile.

Photographic Evidence of Beach and DuneMorphodynamics

A series of photographs of the beach near site 2 (recrea-tional area), taken in June, October, and December 2001,showed significant changes in morphology caused by the pas-sage of Hurricane Juliette in September 2001. In June thereis a smooth surface between the beach and the dune (Figure9A); the beach has a gentle slope and a wide backshore. TheOctober photograph (Figure 9B) shows drastic changes inmorphology, with an erosional escarpment at the base of thedune. The development of the escarpment is attributed to theerosive action of the high-energy waves during Hurricane Ju-liette, which caused the retrogressive movement of the beachface up to the base of the frontal dune in the backshore (Fig-

ure 9B). In the December photograph (Figure 9C), the erosionescarpment had smoothed considerably despite the high-en-ergy winter waves, which commonly lead to erosion of beach-es. This is an indication that sediment from the dune is beingdeposited at its base by gravity, smoothing out the beach anddune escarpments, supplying some sediment to the beach,and contributing to its natural recovery. However, the slopeof the escarpment is still steep compared to June.

Wind Records

Between June and September 2001, 78% of the winds werenorth-westerly (Figure 3A), and speeds were most frequentlyfrom 2 to 10 m s�1. According to BAGNOLD (1941), a minimumwind speed of 4.5 m s�1 is required to initiate movement ofsand-sized particles. From this, it is suggested that localwinds, which are shown in Figure 3B, can move the sand inthe area during 67% of the wind record from Cabo Falso.

The meteorological station at Cabo Falso started to registeranomalies from Hurricane Juliette on September 26, but re-cording stopped when the equipment was damaged. Averagedaily hurricane wind data, QuikScat Level 3, Product 109JPL, was downloaded from the PO.DAAC web page(PO.DAAC OCEAN ESIP TOOL, 2006) and plotted for Septem-ber 25–29 (Figure 10). The centre of the storm for September25–27 was south of the lowest latitude shown in Figure 10.The dotted line shows the track that the storm followed fromSeptember 28–30. On September 25, the dominant regionalnorth-westerly wind pattern was present, and by noon onSeptember 26 (figure not included) the winds became east towest, with speeds up to 20 m s�1, intensifying and shiftingmore towards the north-west by noon of September 27 (Fig-ure 10). By noon of September 28 winds were to the north–north-west, and by noon of September 29 the winds hadveered to the east–north-east. In the study area, in responseto dominant north-westerly winds, sediment movement isnormally from the north-west to the south-east. During thepassage of Hurricane Juliette, the dominant sediment move-



Figure 9. Photographs of the beach and dune within the recreational area (site 2, taken in 2001). The October photograph shows the beach scarp byHurricane Juliette, and the December photograph shows that the upper parts of the scarp slide by gravity, filling the scarp base and smoothing theprofile.

ment was apparently the opposite direction, from the south-east to the north-west.

DISCUSSION

Loss of Vegetation Cover

The presence of vegetation on dunes is an important factorin their development because it tends to diminish the move-ment of sediment (CARTER, CURTIS, and SHEEHY-SKEFFING-TON, 1992). Both the aerial and the on-site photographic sur-veys revealed patches of the lost vegetation. These are dueto the anthropogenic pressure exerted mainly in the recrea-tional zone, where circulation of ATVs is destroying the veg-etation. This increases the exposed surface of loose or poorlycompacted sediment, which becomes susceptible to reworkingby the wind, as has been demonstrated by TIKKANEN andHEIKKINEN (1995) in other areas.

Sediment Erosion and Accumulation on the Beach andDune

The dune profile of the protected zone behaved differentlyfrom those within the recreational zone. We observed sedi-ment accumulation in the protected zone during the June–December 2001 period (despite the impact on the beach faceitself during the passage of Hurricane Juliette), as well as inthe following period (December 2001–February 2002). A pos-sible explanation for this is that, although the hurricane ob-viously affected the entire dune field, significant sedimenttransport during the hurricane was from the south-east tothe north-west, coinciding with the wind direction when thehurricane eye was located to the west and north-west of thestudy area (Figure 10). The recreational zone was thus sup-plying sediments to the protected zone.

Photographs from before and after the hurricane show thedevelopment of an erosional escarpment at the base of the



Figure 10. Wind direction and speed during the passage of Hurricane Juliette (September 25 and 27–30, 2001).

dune and a severe reduction in the width of the beach, whichin areas was eliminated by waves that had attacked the baseof the dune. In the December 2001 photograph, the beachprofile showed recovery from the erosion caused by the hur-ricane despite the winter conditions with high wave energy.It is clear that the passage of Hurricane Juliette modified themorphology of the beach and dune system at Cabo Falso andthat formation of new paths through the vegetation cover inthe parabolic dune flank due to the passage of ATVs is animportant factor in the resultant morphodynamics of thebeach–dune system at Cabo Falso.

CONCLUSIONS

Accumulation of beach sediments occurred during the De-cember 2001 to February 2002 period, while erosion, accen-tuated by the passage of Hurricane Juliette in September,took place between June and December 2001. This patternwas not present in the aeolian deposits, however, where sed-iment loss in the recreational area occurred during both pe-riods, while sediment accumulated in the protected zone. Thebase of the dune was eroded during the passage of the hur-ricane. Afterwards, the profile of the dune began equilibrat-ing itself, with gravity causing material to fall from the faceof the escarpment. This accumulated on the adjacent beach

and contributed to its natural recuperation. Circulation ofATVs affects the morphology of the dune ridges within therecreational zone, destroys the fragile vegetation cover, andcreates new paths to a degree that most likely will initiatereactivation of the dune if it continues.

ACKNOWLEDGMENTS

We would like to thank the Instituto Politecnico Nacionalfor logistical help by the Centro Interdisciplinario de CienciasMarinas (CICIMAR-IPN) and for the financial support fromthe Coordinacion General de Proyectos de Investigacion(CGPI20010314). We are grateful for the initial review of themanuscript by Dr. Donn S. Gorsline. Special tanks to Dr.Guillermo Gutierrez de Velasco for preparing the hurricanewind data.

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