Morphodynamic Evolution and Sediment Transport ...

Morphodynamic Evolution and Sediment Transport Processes of Cancun Beach Author(s): Mariana González-Leija, Ismael Mariño-Tapia, Rodolfo Silva, Cecilia Enriquez, Edgar Mendoza, Edgar Escalante-Mancera, Francisco Ruíz-Rentería, and Emanuel Uc-Sánchez Source: Journal of Coastal Research, 29(5):1146-1157. Published By: Coastal Education and Research Foundation DOI: http://dx.doi.org/10.2112/JCOASTRES-D-12-00110.1 URL: http://www.bioone.org/doi/full/10.2112/JCOASTRES-D-12-00110.1

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Journal of Coastal Research

29

5

1146–1157

Coconut Creek, Florida

September 2013

Morphodynamic Evolution and Sediment Transport Processes of Cancun Beach Mariana Gonza´lez-Leija†‡, Ismael Mariño-Tapia†, Rodolfo Silva§, Cecilia Enriquez§††, Edgar Mendoza§, Edgar Escalante-Mancera‡‡, Francisco Ru´ız-Renter´ıa‡‡, and Emanuel Uc-Sańchez† †

Laboratorio de Procesos Costeros y Oceanograf´ıa F´ısica Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional Unidad Mérida, Yucatań, México [email protected]

‡ AXIS Ingenier´ıa S.A., de C.V. Coastal Engineering Department Mérida, Yucatań, México

††

‡‡

Facultad de Ciencias UMDI-Sisal Universidad Nacional Autónoma de México Sisal, Yucatań, México

§ Instituto de Ingenier´ıa Universidad Nacional Autónoma de México Ciudad UniversitariaMéxico D.F., México

Instituto de Ciencias del Mar y Limnolog´ıa Universidad Nacional Autónoma de México Puerto Morelos, México

ABSTRACT ˜ Gonza´lez-Leija, M.; Marino-Tapia, I.; Silva, R.; Enriquez, C.; Mendoza, E.; Escalante-Mancera, E.; Ru´ız-Renter´ıa, F., and Uc-Sańchez, E., 2013. Morphodynamic evolution and sediment transport processes of Cancun Beach. Journal of Coastal Research, 29(5), 1146–1157. Coconut Creek (Florida), ISSN 0749-0208. Large-scale construction of tourist infrastructure on beaches around the world is consistently linked to unwanted morphological changes that lead to coastal erosion. Dune destruction, alteration of sediment sources, and the rigidisation of the coastal system are known to be the main causes of erosive behaviour on many tourist beaches. To plan sound shoreline management strategies, detailed understanding of the sediment transport processes is necessary. The present contribution focuses on the main sediment transport processes that take place at Cancun, Mexico, a large (12 km) and highly developed tourist beach. High-resolution quarterly beach profile monitoring from September 2007 to June 2009 is used to calculate volumetric changes that are reasonably well explained by the spatial patterns of modelled sediment transport potential. This parameter was calculated using the wave propagation model WAPO of Universidad Nacional Autonoma de Mexico, which explicitly solves diffraction and reflection processes that are particularly important in systems with pronounced rocky headlands, such as the northern and southern ends of Cancun beach. Results show a dominance of northward longshore transport in most of the system, and an important transport divergence with consistent southward transport at the southern end. Cross-shore transport seems to dominate the middle-north section of the beach. This behaviour is consistent with recent advances in the understanding of wave circulation in embayed beaches. The method used here is considered a good approximation of sediment transport patterns when local (surf zone) morpho- and hydrodynamic data are absent or difficult to acquire.

ADDITIONAL INDEX WORDS: Beach processes, erosion, longshore transport, wave propagation.

INTRODUCTION In today’s world, beaches are a valuable natural resource. In tropical regions, tourism is often an important source of income to such a degree that the economy of many countries that border the sea highly depends on the viability of their coastal tourist resorts. Consequently, there has been much urbanisation and construction of infrastructure in coastal zones, damaging fragile ecosystems. The structures built in these regions often change the environmental conditions, exacerbating conflicts between usage of the coastline and natural morphodynamic processes. The demand for recreational use DOI: 10.2112/JCOASTRES-D-12-00110.1 received 29 May 2012; accepted in revision 2 October 2012; corrected proofs received 1 May 2013. Published Pre-print online 29 May 2013. Ó Coastal Education & Research Foundation 2013

of tourist beaches has increased substantially in recent decades, and associated with this increased human pressure, recent studies have shown that many coastal zones around the world are threatened by coastline recession (Short, 1999). Unfortunately, there are many examples of unwanted change to coastal areas following large-scale construction of tourist infrastructure, i.e. in Brazil (Addad and Martins-Neto, 2000; Vital et al., 2003), Argentina (Codignotto et al., 2012), and Colombia (Restrepo et al., 2012), where problems of erosion and shoreline hardening have produced detrimental effects in the ecology of the region and in the tourist revenue. The beaches studied in this paper have undergone severe erosion following tourist development, which can be compared to similar sites around the world. These beaches, in the northern part of the state of Quintana Roo (Figure 1), constitute one of the primary tourist destinations and an

Morphodynamic Behaviour of Cancun

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Figure 1. Location of the area of study, indicating monitored sections of beach from north to south: I (Royal Sunset), II (Playa Marlin), III (Playa Delfines), and IV (Sun Palace). The sand bank La Ollita is indicated with the black circle, and the moored current meter location is shown with a triangle. Dotted lines represent the adjacent bathymetry (modified from Gonza´lez-Leija, 2009).

important economic resource for Mexico but were not urbanised until the early 1970s (Felix-Delgado et al., 2008). It is possible that before this urbanisation took place the beach system experienced yearly cycles of erosion and recovery that kept it in a dynamic equilibrium, with intense meteorological events (e.g. hurricanes) that severely altered the coastline position, moving it landwards (Silva et al., 2012). Then, during long periods of relative calm that promote beach growth, the beach eventually returned to its original dynamic equilibrium and previous coastline position. This idea has been proposed by Ru´ız-Martinez (2010). With the construction of the tourist infrastructure and the occupation of the coastal zone, particularly at the dune area (more than 90% at present) the equilibrium has been lost, and after extreme events, the beach fails to recover its previous dynamic equilibrium.

As a result, and given that the northern zone of Quintana Roo is a dynamic region, in the last 30 years the coastline has undergone changes that are undesirable for tourist development. Hurricane Gilbert in 1988 caused great loss of sand in the zone and serious damage to tourist infrastructure. The situation resulted in a series of human interventions (e.g. small groynes and geotextile breakwaters), which aimed to balance the recession trends of the coastline. However, these isolated coastal protection measures had adverse effects on the coast and simply transferred and increased the problems of erosion to nearby, unprotected sections of the beach. In 2004 and 2005, Hurricanes Ivan, Emily, and Wilma affected the region, resulting in important bathymetric changes due to the great losses in sand volume. SierraCarrascal (2005) and Silva et al. (2006) indicated that the loss

Journal of Coastal Research, Vol. 29, No. 5, 2013

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Gonza´lez-Leija et al.

of sand initiated by hurricanes and enhanced by the intensive use of the region has been increasing. The consequences are evident; Felix-Delgado et al. (2008) indicated that from 1999 to 2004, a decrease was registered in the economic rating for the tourist development of Cancun and as a consequence a reduction of around 20% in its economic contribution to the state. In 2005, after Hurricanes Ivan, Emily, and Wilma and their widespread devastation, ‘‘emergency beach replenishment’’ (2.7 3 106 m3) was performed along the 12 km of Cancun beach that faces SE, to restore tourist activity. After the artificial nourishment and without integral studies on technical aspects, or systematic maintenance of the system, beach loss continued. By December 2009, it was again artificially replenished. This time, the project contemplated a design for an initial dry beach of 80 m (~5,000,000 m3) and a closure structure between the coastline and the Golondrinas islet (Punta Cancun) 300 m in length, with the aim of reducing the export of sand from the system towards the north. To the best of our knowledge, these actions have been carried out without a comprehensive understanding of the patterns of sediment transport. A wave climate database (Silva et al., 2007) was consulted to identify the strongest wave storm events (Hs . 6 m). The results of this analysis are presented in Table 1. There is a direct relationship between the intensity and number of events and the erosion processes and system recovery. An analysis of aereal photographs and satellite images taken between 1970 and 2006 (Felix-Delgado et al., 2008) suggests that before 1970, the system was in dynamic equilibrium. By 1988, when Cancun was already developed, Hurricane Gilbert caused very large sand loss. During the 1990s, the lack of large hurricanes helped to partially recover the system, but during the following decade, the site was heavily affected by several hurricanes, with Ivan, Emily, and Wilma causing the most damage. One crucial factor to consider in decision making, with respect to the location of the recovery projects and infrastructure, is the capacity of a beach to maintain both its profile shape and its coastline position (Mendoza and Silva, 2004). Therefore, studies on the sediment balance of the coastal environment are necessary to define trends in morphological evolution and to assess the impact of human intervention. From 1984, there have been some sporadic studies focused on understanding the causes of the erosion of Cancun beach. Since 2005, the coastal engineering group of Universidad Nacional Autonoma de Mexico (UNAM) and the coastal processes laboratory of the Centro de Investigación y de Estudios Avanzados (CINVESTAV) del Instituto Politécnico Nacional have continuously monitored the beach of Cancun. The datasets are available for decision makers designing appropriate interventions, which contribute to resolving the different problems of erosion in Cancun. Having continuous information on the physical aspects of the morphodynamic characteristics of the Cancun coastal system has already been useful for establishing the needs and performance of the restoration projects and reducing the problems of erosion. This study presents the main sediment transport processes of Cancun beach based on the combined analysis of the local morphological and hydrodynamic characteristics. The beach morphology was monitored quarterly using detailed beach profile measurements from September 2007 to

June 2009, and the hydrodynamic conditions were obtained with the use of the wave propagation model (WAPO), which explicitly solves the diffraction and reflection processes (Silva, Mendoza, and Losada, 2006) that are particularly important in systems like this with rocky headlands, such as Punta Cancun and Punta Nizuc. Other studies related to the morphodynamics of this region include (1) descriptions of the system in general with largescale patterns of transport in 2000–01 (e.g. CFE, 2001); (2) analysis of the devastating effects of passing of hurricanes, such as Gilbert, and proposals for restoration measures (e.g. Diez, Esteban, and Paz, 2008, 2009); and (3) beach profile and shoreline modelling studies that attempt to provide information for restoration projects (e.g. Bodegom, 2004; SierraCarrascal, 2005). However, the present study uses a higherresolution monitoring program (in space and time) and explains integrally the sediment transport pattern in the embayment formed by the two headlands, Punta Cancun and Punta Nizuc.

Area of Study The area of study is a 12-km strip of beach in the hotel zone of Cancun that faces SE into the Caribbean Sea, limited by Punta Cancun to the north, Punta Nizuc to the south, and the Bojorquez-Nichupte lagoon system to the west. Four beach sections with extensions of about 1.5 km were monitored every 3 months from September 2007 to June 2009. From north to south, the monitoring sites are Royal Sunset (I), Playa Marlin (II), Playa Delfines (III), and Sun Palace (IV). The position of the monitoring sites depended on the ease of access to the beach by public paths or successful negotiation with the hotel personnel. These sections and the adjacent bathymetry obtained from a combination of measurements made during February 2008 by the coastal engineering group of UNAM and by the oceanographic department of the federal electricity agency (Comision Federal de Electricidad, or CFE) are shown in the right panel of Figure 1. This was the bathymetry used for the wave propagation exercise (as described in the next section). The dominant wind direction is from the E and SE, with speeds ranging between 4 and 6 m s1. These are fairly consistent throughout the year except from October to March, when the easterly winds are interrupted intermittently by the arrival of strong winds from the N and NE that can last several days. In addition, the region is subject to the arrival of tropical storms from June to November. The waves follow the wind behaviour closely, propagating from the Caribbean Sea with a predominant S-SE direction (Silva et al., 2006) and with relatively short wave periods (5–10 s); swell waves (12–20 s) are also present but with much less relative energy (Figure 2). The wave conditions are altered with the arrival of the strong northerly winds associated with cold fronts that transit the region during the winter and when extreme meteorological conditions of tropical storms and hurricanes hit the coast during the summer. Hurricane conditions can generate waves with significant wave heights (Hs) of more than 10 m and associated periods exceeding 12 seconds. Silva et al. (2007) reported that the Hs was 12 m during the passage of Hurricane Gilbert, and Bautista, Silva, and Salles (2003) estimated that



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Table 1. Main hurricanes affecting the northern Quintana Roo region in chronological order. Hurricane Charlie Hilda Carla Inez Beulah Allen Gilbert Roxanne Isidore Ivan Emily Wilma Dean

Date

Wind Speed (km/h)

Wave Height (m)

Central Pressure (mbar)

Aug 1951 Sep 1955 Sep 1961 Oct 1966 Sep 1967 Aug 1980 Sep 1988 Oct 1995 Sep 2002 Sep 2004 Jul 2005 Oct 2005 Aug 2007

142 107 127 117 136 189 192 127 152 120 120 176 150

9.4 6.4 10.0 6.1 11.0 11.9 12.7 8.3 11.1 7.2 6.8 12.8 5.3

991.5 1001.8 969.0 992.4 968.0 947.2 969.0 995.8 974.6 996.2 992.4 930.0 907

the mean storm surge for the event was 3.6 m in Cancun, although the maximum storm level attained reached 7 m. Locally, sea level is influenced by small amplitude astronomical tides (spring tidal range between 0.2 and 0.4 m) of a semidiurnal nature, but it can be significantly altered by atmospheric (inverse barometer) and oceanographic conditions (large-scale currents). A dramatic example of these intense variations is the sudden storm surge of about 1 m measured at a 20-m depth during the strike of Hurricane Wilma in 2005 (Escalante et al., 2009). Hurricane Wilma was one of the most intense meteorological events ever registered, with sustained winds greater than 280 km h1 and the lowest atmospheric pressure (882 mbar) recorded in hurricane history in the

Atlantic. Wilma also generated the most extreme oceanograph˜ ic conditions on record (Escalante et al., 2009; Marino-Tapia et al., 2008). Cancun beach, prior to the artificial nourishments, was composed of very fine, well-sorted sand, mainly formed from ooliths with skeletal mollusc detritus and coral fragments (Carranza, Rosales, and Pérez, 1996). The natural sediment sources for the beaches are restricted to the natural reef degradation (Felix-Delgado et al., 2008) and onshore sand transport. The artificial nourishments recently conducted provided the beach with poorly sorted material with a high shell content obtained from a submerged bank (La Ollita) located approximately 30 km to the NE (Figure 1).

MATERIAL AND METHODS

Figure 2. Comparison between offshore wave measurements from NOAA buoy 42056 at a 3000-m depth (solid line in (a) and (b)) and nearshore measurements at a 20-m depth in front of Cancun beach (black markers in (a) and (b)), showing the similarity of conditions during 2007 despite the large differences in depth. Offshore wave statistics for the period of September 2007–June 2009 from buoy 42056 show 2D scatter diagrams (the colour bar shows counts) of (c) wave height vs. direction and (d) wave period vs. direction.

Changes in the volume of beach sand were studied from the analysis of beach profiles measured with an alongshore resolution of 20 m, quarterly from September of 2007 to June 2009, along four 1.5-km beach sections. The surveys were conducted using a differential dual-frequency global positioning system (GPS; Leica 1200) consisting of base and rover that collect topographic data simultaneously every 5 and 1 seconds, respectively. The data were linked to the station of the National Active Geodetic Network (Red Geodesica Nacional Activa) of the National Institute of Statistic and Geography (Instituto Nacional de Estad´ıstica Geograf´ıa e Informa´tica) in Merida, Yucatan (x ¼ 2,322,170.7258, y ¼ 227,566.4842, z ¼ 7.912-m ellipsoid height), and referred to mean sea level height, which according to the CFE topographic surveys is found 11.894 m from the ellipsoid World Geodetic System 84. The beach profiles start at the boundary of the buildings going towards the sea up to the inner surf zone (~1-m depth). To get insight into the surveying errors, the same beach profile was measured 10 consecutive times every survey. The root mean square error (RMSE) of the profile revealed a maximum error of 0.05 m in height in regions where the profile presented abrupt changes (beach scarps, berms, and the entrance to the swash zone) and much less (0.01 m) elsewhere. This small error provides certainty of the adequate representation of the detailed morphology and quantification of the sediment budgets. From these surveys, the volumes of sand gained and eroded in each quarter were estimated from the hotel line to the mean sea level.



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To determine the influence of the waves on the sand volumetric changes, a combination of local wave measurements and data from buoy 42056 of the National Oceanic and Atmospheric Administration (NOAA) National Data Buoy Center were used to characterise the wave climate. The local wave data were recorded by UNAM (Martell et al., 2012) with an RD Instruments acoustic Doppler current profiler (ADCP) that was placed from May to September 2007 at a depth of 20 m at the location shown in Figure 1. The ADCP recorded directional wave parameters at a frequency of 2 Hz during 1024 seconds every hour. Local wave measurements were available for a limited amount of time (4 months) before the monitoring program started but were sufficient to make a comparison with the deep-water data. Significant wave heights and periods were fairly similar, with an RMSE between offshore and nearshore data of 0.4 m and 1.5 seconds, respectively (Figures 2a and b). Despite this similarity, waves measured by the deep sea buoy would not represent accurately the hydrodynamic conditions experienced at the beach (which affect the sediment transport); therefore the WAPO-UNAM model (Silva et al., 2006) was used to estimate the wave conditions (height and direction) at the breakpoint. Wave scenarios for modelling were chosen from the offshore NOAA buoy using a range of directions that characterised the annual behaviour, a standardised significant wave height (Hi) of 1 m, and an 8-second period. The 8-second period was chosen as a compromise between model efficiency and characteristic nearshore waves (Figure 2). For implementing two-dimensional (2D) WAPO, the bathymetry was discretised into a mesh of 521 3 961 nodes in the E-W and N-S directions with a regular rectangular grid of 12.5-m resolution. The modelled propagated wave fields (obtained with 2D WAPO) were used as matrices of dimensionless propagation indices (H/Hi), i.e. a ratio between the local wave height and the input height, which was always 1. This ratio was multiplied by the measured offshore significant wave height to reconstruct propagated time series for the whole period of interest (September 2007–June 2009). Using these nearshore-propagated data, time series of wave energy flux and the angle of approach at the breakpoint were reconstructed and used to estimate the longshore sediment transport potential (Pi), which is obtained from the expression presented by the Coastal Engineering Research Center (CERC, 1984): Pl ¼ ðECnÞb sinhb coshb

ð1Þ

where E is wave energy density per unit horizontal area (in joules per square metre), C is wave celerity (in metres per second), n represents the quotient between the group speed and the celerity of the waves, and h is the direction of propagation of the waves. The subscript b indicates that all these parameters are evaluated at the depth where wave breaking begins. The energy flux (EC) is expressed in terms of the measured and modelled wave characteristics: " # 1 Hb 2 qg Ho * ð2Þ sinhb coshb Pl ¼ 8 Hi where q is the density of seawater (taken as 1025 kg m3), g is the gravitational constant (9.81 m s2), Ho is the measured wave height in deep water (NOAA buoy data) at time i, and Hb/

Hi is the dimensionless index of wave propagation at the breaking point obtained from the modelled data. Apart from the longshore sediment transport potential, an assessment of cross-shore dynamics is also desirable. The crossin energy flux (CGEF) at breaking, shore gradient . ]ðECnÞb , is the net inflow of mechanical energy into the ]x surf zone (Svendsen, 2006). This parameter can be associated with the intensity of wave setup (g), radiation stresses (Sxx), and therefore the strength of undertow currents, which are capable of transporting considerable amounts of sediment seawards. Here, the modelled CGEF is proposed as an indicator of seaward sediment transport potential. 2 1 H *Cn ] qg H * o . 8 Hi ]ECn ð3Þ ¼ ]x ]x

RESULTS AND DISCUSSION Wave Climate Figure 2 shows a comparison between offshore wave parameters and those measured at a 20-m depth (Figures 2a and b). There is great similarity between these two data sets, supporting the use of NOAA offshore data as an input for the propagation model. The 2D scatter diagrams of wave direction vs. wave height (Figure 2c) show that the prevailing wave heights are between 0.5 and 1.5 m coming from the E and SE, although waves greater than 2 m were also part of the record (Figure 2c). These energetic waves occurred either during the winter season, associated with northerly winter storms, or with the passage of tropical cyclones from the Caribbean. Figure 2d shows the 2D scatter diagrams of wave direction vs. period. The dominant wave periods were primarily between 7 and 8 seconds from the E and SE (90 and 1408), although waves with higher periods from the N and NW (~3508) were also present. These scatter diagrams also provide information about the behaviour of the wave energy flux, which is a good indicator of the sediment transport and morphological change. As expected, the deep-water wave energy flux (not shown) has a predominant SE component, which suggests, as a first approximation, predominant northward sediment transport. This large-scale northward transport is supported by the observed beach width variations. Nevertheless, it is well known that local bathymetric conditions, particularly those of the breaking zone, can considerably alter the incidence angle of the waves and therefore the magnitude and direction of the currents and littoral transport, as explained in the next section.

Large-Scale Morphodynamic Behaviour of the Beach Unlike previous studies, such as that by CFE in 2000–01, the present contribution shows a higher-resolution monitoring program (in space and time), carried out from September 2007 to June 2009, which makes the calculation of volumes reliable. For the first time, these data provide a detailed characterisation of the morphodynamic behaviour of the beach, demonstrating its high complexity and great sensitivity to different wave conditions and to alterations imposed by coastal structures. Four photos taken during the field survey in June 2009, in each of the monitored sections, are presented in Figure 3, along



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Figure 3. Visual appearance and examples of beach profiles of the four monitored sections during June 2009.

with examples of corresponding beach profiles. The overall gain or loss of sand in the four beach sections during the reported period is presented in Figure 4a. Because 90% of the dunes are occupied by hotels, the beach widths and volumes were measured from the edge of the fences of the hotels to the mean sea level. It is clear that only the northern section (I) south of Punta Cancun gained material, with about þ27,000 m3, whereas the rest of the beaches showed erosive behaviour, with the greatest losses at the southernmost segment (IV), with about 25,600 m3, followed by segment II, with 13,000 m3. In general terms, the volumetric changes show that the southern beaches tend to erode while those of the north tend to have deposition. The estimated energy flux, dominantly from the SE, was therefore a reliable indicator for explaining the overall behaviour of potential longshore transport northwards, which was suggested earlier and has been reported in previous investigations (Bodegom, 2004; CFE, 2001; Diez et al., 2008; Diez, Esteban, and Paz, 2009; Sierra-Carrascal, 2005). The net

balance for these sections is loss of material, which escapes from the system either through the northern end or offshore, because this component of sediment transport is especially important in section II, as explained later. The effect of the volumetric changes on the advance or recession of the shoreline for the different sections during the period is presented in Figure 4b, which shows the difference in beach width between the last (June 2009) and the first (September 2007) surveys analysed here. Shoreline evolution at segment I shows beach growth with a maximum advance of 25 m in the central part. This behaviour coincides with the volumes of accumulation shown in Figure 4a. Segment II presented high variability, especially at its northern end where up to 15 m of beach were lost. The advance of the coastline was limited and concentrated along the southern section of this beach. It is mainly within this segment that the massive buildings prevent the development of a dry berm. A different situation was observed along segment III, where the shoreline remained approximately in equilibri-


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Figure 4. (a) Net volumetric sediment balance from September 2007 to June 2010 for the surveyed beaches (values in 103 m3). Positive values represent sand gain; negative values are losses. (b) Advance (positive) and recession (negative) of the shoreline for the same period. On the x axis, 0 represents the northernmost portion of the beach.

um in the long term, despite the large temporal variability presented (Figure 5); even though, in some locations up to 8 m of beach were lost, forming beach scarps and showing rhythmicity. Finally, segment IV had the highest beach width loss, as expected. The beach receded up to 22 m in the southern section, but towards the north it eroded less. This behaviour coincides with the presence of a submerged coastal protection structure (geotextile) installed at a 4-m depth at the northern end of section IV. It is also interesting to analyse the time evolution of the volumetric changes to better understand the nature of the sand transport. Figure 5 shows the average volumetric changes of each beach section between consecutive surveys, i.e. the value for December represents the average volumetric changes between December 2007 and March 2008, and so forth. Section I shows a consistent accumulation of sand (positive values), but this decreases gradually from September 2007 to March 2008, reaching a volume loss by June 2008. At this time of the year, the predominant SE waves should promote accretion. However, from April to May, two groynes were constructed by hotel owners, without consultation or permit, to the south of section I, interrupting the free flow of sediment northwards and causing adverse changes in the beach profile and in the neighbouring hotel infrastructure. During the winter, when NE events dominate, the groynes produced the opposite effect, causing accretion at the northern end and erosion down drift. Segments II and III show an oscillating balance with a negative (erosional) predominance, and section IV presented clear and

Figure 5. Volumetric changes between individual surveys, showing a history of volume gain or loss on each beach segment (i.e. the section IV value of 10 represents the difference between September and June 2008). The sum of all values for a given beach section should be analogous to Figure 4a.

consistent erosion except in winter 2008–09, when northerly storms generated southward sediment transport, creating considerable accretion there, while net erosive behaviour is evident in the other sections.

Sediment Transport Processes Detailed calculation of the longshore and cross-shore energy fluxes with the incorporation of modelled wave propagation patterns provide a basis for understanding with better detail the sediment transport processes and the morphological changes of the system. Figure 6b shows the temporal and spatial evolution of the longshore transport potential (Pl, Equation (2)). Positive values (red) indicate transport towards the south, and negative values (blue) show northward transport. The location of the beach sections can be seen in Figure 6c, which shows the cumulative transport potential of Figure 6b. The most prominent feature of the longshore sediment transport behaviour is the predominance of northward sediment transport in most of Cancun beach (blue in Figure 6b), as mentioned earlier. But an alongshore sediment transport process that stands out is the existence of a divergence that originates between segments III and IV, consisting of an alongshore transport increasing southwards towards Punta Nizuc (Figures 6b and c). It is expected that the



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Figure 6. (a) Temporal evolution of the deep sea wave characteristics measured by the NOAA buoy (September 2007–October 2008). (b) Temporal evolution (x axis) of the longshore (y axis) sediment transport potential estimated from the modelled wave propagation results with WAPO. Negative values (blue) represent northward transport, and positive values (red) represent southward sediment transport. (c) Cumulative longshore transport potential estimated from (b) during the studied period. (Color for this figure is available in the online version of this paper.)

sand budget in the southern region of Cancun is highly influenced by the existence and nature of the transport divergence, which is persistent yet fairly variable both in spatial extent and intensity. The transport divergence can reach as far north as the southern end of section III (Delfines beach) and be fairly strong when waves are larger than 1 m from E and NE, which occurs in winter. This enables sand from section III to be exported towards Punta Nizuc. The behaviour of the transport divergence is such that accretion might be expected up to the northern part of section IV, but as the southward transport increases in the south of section IV, this area is expected to lose sand and show erosion. This sand is likely to escape the littoral cell through Punta Nizuc and be lost to the system. Evidence of this is presented in Figure 5, because the volumetric changes between sections III and IV seem tied together at times. For example, from December 2007 to March 2008, section III loses sand while section IV shows a net gain. From December 2008 to February 2009, a similar situation occurs, but it is not as evident on the averaged volumetric changes (Figure 5) as it is in the detailed volumetric changes. Figure 7 shows results of the detailed volumetric changes between December 2007 and March 2008 for sections III and IV. Section III presents clear erosion at the southern end, while section IV shows accretion in the north, suggesting sediment

exchange between these two beach sections. Increased erosion in the south of section IV is consistent with the behaviour of the calculated longshore sediment transport (Figure 6c). Waves from the SE push the divergence point closer to the south, increasing the transport potential northwards. From Figure 6b, it is also noticeable that E-NE waves also generate a transport divergence at the middle of Cancun beach (at 5.5 km), south of section II. To the north, the alongshore sediment transport potential shows a clear decrease, which promotes sand accumulation, as observed in Figure 4. Under NE waves, a mild transport divergence occurs in a localised region north of section I, with sediment moving southwards. The alongshore sediment transport behaviour described previously, especially the existence of transport divergences that drive sediment southwards in the southern beach sections and under E-NE waves on other sections of the beach, differs from the previous concept of persistent northward transport, even under NE wave conditions, reported by Diez et al. (Diez et al., 2008; Diez, Esteban, and Paz, 2009), who justified the northward transport by the high refraction of waves from the first quadrant (NE). In this study, the use of WAPO, capable of resolving the refraction, diffraction, and reflection processes, demonstrated that most wave directions are not refracted to the point of generating northward littoral currents in the south


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Figure 7. Volumetric changes (difference) between December 2007 and March 2008 on (a) section III and (b) section IV, when the alongshore divergence extended to the north due to wave climate. Red implies erosion, and blue implies accretion. The colour bar represents metres of vertical beach loss or gain. (Color for this figure is available in the online version of this paper.)

of the beach. Numerical experiments on the morphodynamics of beaches bounded by rocky headlands show behaviour similar to that described earlier. Daly et al. (2011) show that an embayed beach subjected to a scenario analogous to Cancun, with waves approaching at an angle, would have a predominant northward sediment transport, with a clear sediment transport divergence in the south. The morphodynamic behaviour of Cancun beach adopts the characteristics of such embayed beaches. Sierra-Carrascal (2005), using the Generalized Model for Simulating Shoreline Change, reported a relative sediment balance and small coastline recession for the central southern

Figure 8. Accumulated cross-shore wave energy flux for different periods: (a) December 2007–March 2008 and (b) March–June 2008. (Color for this figure is available in the online version of this paper.)

section of Cancun beach (2–6 km from Punta Nizuc), which remains approximately constant with slight erosion. This is where segment III is located and where our data show the lowest volumetric changes. In the central-northern region (section II), the results of these investigations also show fluctuating behaviour in the volumetric changes (Figure 5), but in the case of this study, the beach is eroding consistently and considerably. To make a complete analysis of the sediment transport processes, an indication of the importance of cross-shore transport is necessary. For this purpose, the CGEF is used to show the importance of undertow currents and offshore directed sediment transport, as explained in the methodology. Figure 8a presents the accumulated CGEF calculated for a period of E-NE waves associated with winter storms (December 2007–March 2008). This figure represents regions where the CGEF accumulates during the period in question. Therefore, red shows zones where offshore transport is more likely to occur. It is evident that during this period, the central part of Cancun beach presents high values of CGEF, whereas the two ends of the beach have low values and therefore longshore transport processes dominate. However, the period from March to June 2008 shows less accumulation of CGEF and only in sections II and III. The measured morphological changes for section II (central-north) coincide with this pattern, because the beach presents considerably more erosion during December to March (Figure 9a) than during March to June (Figure 9b), suggesting that offshore transport processes are important in this beach section. This is also evident in Figure 5. In general, regions with consistently high CGEF accumulation were observed to the north of segment III, in the central part of segment II, and to the south of segment I, where two groynes were built in 2010 in an effort to control the high erosive rates. Although a clear link has been observed between the hydrodynamic processes identified here and the observed



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Figure 9. Volumetric changes (difference) for two periods in section II: (a) December 2007–March 2008 and (b) March–June 2008. Red implies erosion, and blue implies accretion. The colour bar represents metres of vertical beach loss or gain. (Color for this figure is available in the online version of this paper.)

morphodynamic response of the beach, it is important to consider the limitations of this study. The goals of estimating the potential for littoral transport Pl at the breaking point were to develop a conceptual model of longshore sediment transport and to understand its variability given the different angles of wave approach. A characteristic wave period of 8 seconds was used—which represents the wave period most often observed but can slightly overestimate the transport magnitude (celerity values would be larger than for higher-frequency waves) and slightly change the small-scale variability (higher periods would be refracted closer to shore)—but the large-scale patterns are likely to remain the same. A limitation related to the above is the spatial resolution of the bathymetric transects (200 m), which could smooth morphological features on the seafloor (e.g. submerged bars of tens of metres) capable of affecting wave propagation, especially for higher-frequency waves. Because the aim of this study is to determine the general morphodynamic patterns and trends of the system, a large-scale approach can suffice on the hydrodynamics and morphology. The same approach described in this paper was performed with a bathymetry from 2007 (1 year earlier) to test the sensitivity to bathymetric changes. It was found that similar patterns in alongshore transport potential and crossshore energy flux existed. This test demonstrates that the large-scale trends are well represented with the approach followed here, although it is recognised that the detailed morphological changes driven by the hydrodynamic conditions presented within the surf zone will not be reproduced. As presented in the previous sections, Cancun beach showed very high morphological variability, with consistent beach erosion and erosive rates of up to 20 m of shoreline recession in 2 years (section IV). This beach recession led hotel owners to implement isolated and unplanned restoration measures, which damaged the beach. Given this situation, the need for properly designed integral measures became clear. Therefore, a second beach nourishment was performed in December 2009 to January 2010, this time consisting of more than 5.2 million m3 of sand. The alteration to the beach system would probably

initially alter the sediment transport patterns presented in this article, because the new beach would be reaching equilibrium. Nevertheless, after equilibrium, the transport patterns described here are likely to remain the same. It is strongly suggested that this hypothesis be verified, because future beach maintenance should take it into account.

SUMMARY AND CONCLUSIONS Cancun is a complex coastal system with high morphological variability. It is a system that responds rapidly to changes and recovers quickly as a result of different hydrodynamic forcings. The morphodynamic patterns observed on the beach are summarised as follows. The northward wave energy flux derived from deep-water conditions in response to predominant S-SE waves with Hs between 1 and 1.5 m and T between 7 and 8 seconds explains the general sediment transport of the system, generating predominant northward transport, which showed accumulation of sand on the northern beach (section I) and erosion on the southern beaches. For the period reported here, beach recession was up to 22 m in the southern region of section IV. A clear divergence in the littoral transport potential exists between sections III and IV, which drives sediment consistently towards Punta Nizuc. The longshore transport divergence shows high variability in intensity and spatial extent, which affects the availability of sediment between the southern beaches (IV and III). Incident waves from the S-SE (e.g. 150 degrees from north) move the divergence zone closer to Punta Nizuc and cause the transport pattern to reverse intermittently close to Punta Cancun (section I), generating localised zones of accumulation. Significant attenuation of the littoral transport potential occurs in the northernmost part of section I, which induces a reduction in sediment transport, thereby promoting accretion. The cross-shore transport potential from the energy flux shows specific regions with high energy dissipation near the coast, likely to present offshore sediment transport and develop



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intense, chronic erosion. This erosion persisted in at least three of these regions throughout the monitoring period, developing under different wave conditions. These regions correspond to segments III and II and to the south of segment I. Under energetic wave conditions, the central section of the study area is prone to developing continuous zones of high-energy dissipation, which intensifies the erosion processes. The analysis presented suggests that longitudinal processes, which are highly dynamic and determine the sand availability, dominate the northern and southern ends of Cancun beach. In the central part of the system, an apparent balance between alongshore and cross-shore transport potential exists. Nevertheless, offshore transport is thought to dominate, because the sediment balance is negative. The circulation patterns found in the present study agree well with numerical results obtained for embayed beaches (Daly et al., 2011; Silva et al., 2010). Given the complexity of the system and its high sensitivity to changes in wave conditions, the building of coastal structures generates immediate variations to the morphodynamics affecting the whole system. The periodic monitoring of Cancun beach revealed a system prone to imbalance (e.g. erosion), even with the recovery projects. This imbalance originates from a combination of factors, including the massive occupation of the coastal dune by rigid infrastructure, which prevents the natural self-regulation of the coastal system; very low sediment input into the system; and high wave energy exposure.

ACKNOWLEDGMENTS The authors thank CFE for generosity in providing the bathymetric data used in this study. Special thanks to the administration of Hotel Park Royal, Hotel Royal Sunset, and Hotel Sun Palace (Mr. Winkler) for patience and cooperation in allowing us access to the beach and to install equipment on the premises. Funding is from Fondos Mixtos – Consejo Quintanarooense de Ciencia y Tecnolog´ıa (FOMIX-COQCYT) project (QROO-2003-C02-12707) and from CINVESTAV federal resources.

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