MONITORING SHORELINE CHANGES AT CANCUN ... - World Scientific

MONITORING SHORELINE CHANGES AT CANCUN BEACH, MEXICO: EFFECTS OF HURRICANE WILMA R. Silva-Casarin', I. Mariiio-Tapia2, C. Enriquez-Ortiz2, E. MendozaBaldwin1, E. Escalante-Mancera3 & F. Ruiz-Renteria3 Cancun beach is one of Mexico's most important coastal resorts and is of paramount importance for the local and national economies. Following the tendency of the time (1970's and 80's), this very large strip-development was built along 17km of coastal dunes on a barrier island on the Mexican Caribbean coast. A very large development, the concentration of buildings in a limited space and the unavailability of sand reserves, has meant that the extreme weather events that occur in the region (hurricanes) have permanently damaged the beach. The effects of hurricane Gilbert in 1988 were especially harmful, Ivan in 2004 and Emily in July 2005 weakened the system, but Wilma (October 2005) caused extreme damage to the beach and coastal infrastructure. As a consequence of the hurricanes prior to Wilma and chronic shoreline recession, the beach had been experiencing receding rates of 2 m/year since 1984. This paper describes the shoreline monitoring system implemented in Cancun in May 2005 which aims to better understand shoreline changes using field data, satellite images, video monitoring, and numerical modeling. The results focus on the effects hurricane Wilma had on the beach. Hurricanedriven erosion presented non-uniform changes along the beach with pronounced accretion in the southern section (15 m beach width gain), and intercalated zones of beach disappearance and eroded but existent beach (~15 m wide).

INTRODUCTION Development of the Tourist Resort Cancun is a 17km long barrier island located in the North East of the Yucatan peninsula in the state of Quintana Roo on the Mexican Caribbean (Figure 1). In 1967, the Bank of Mexico undertook studies to determine the feasibility of developing a tourist resort in the Cancun region, in the hope of bringing in much needed foreign currency. When the new state of Quintana Roo was created in 1973, it became obvious that the success of the new state would require some means of creating a tax base and income for its inhabitants. The national political authorities decided to adopt tourism as the financial engine for this state and their people, and developed an up-market resort designed to attract mainly wealthy North American tourists. It was on this assumption that the

Institute de Ingenieria. National University of Mexico. Cd. Universitaria. 04510 D.F., Mexico. Email: [email protected]. Fax: 52-55-56.16.21.64 2 Coastal Processes Lab. CINVESTAV Merida Km 6 Antigua Carretera a Progreso Merida, Yuc, Mexico. 3 ICMyL. Nacional University of Mexico. Av. Ninos Heroes s/n 77580, Puerto Morelos, Q. Roo. Mexico. 3491

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COASTAL ENGINEERING 2006

hotels were designed, and the shopping malls conceived. Cancun started operations in 1974. Although the government planners recognized that the white sandy beaches, the turquoise Caribbean waters and the sun were the main attractions for the tourists, they built Cancun paying little attention to the environmental consequences of their new development.

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Northern Inlet \

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Nichupte lagoon

Figure 1. Location of the site of study. Before its development Cancun was a pristine and uninhabited barrier island, lG0-400m wide, with a north to south orientation, enclosing Nichupte, a shallow lagoon of 3-4 m average depth. There are two permanent inlets between the Caribbean Sea and the lagoon: the Northern inlet and Nizuc inlet. Between these two inlets there were 17 km of beach occasionally interrupted by 3-7 smaller intermittent inlets which served to maintain the balance between the sea and the lagoon. Upon construction, the use of causeways was maximized and bridges minimized to save money. The causeways restricted the free flow of fresh seawater into some parts of the lagoon and the environment changed abruptly.


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Large parts of the Bojorquez lagoon were filled to create golf courses, amusement parks, and hotels, affecting water circulation and promoting eutrophication. Figure 2 shows the area surrounding Punta Cancun (in the north) before the development took place (1967) and the same area in 2005. Due to its topographic characteristics, this region of the barrier island was naturally prone to breaching as evidenced by the fan-like deposits in the north of Figure 2a which occurred during large storms. This natural breaching benefited the ecosystem by renewing the water inside the lagoon and by relieving storm surge pressure over the beach by equilibrating sea level inside and outside the lagoon and reducing the erosion potential. When the hotels were built, the dunes were made rigid and the breaching could no longer occur (Figure 2b). Water was not sufficiently renewed and storm surge damage increased as the flow was not regulated. Today, Cancun suffers from eutrophication in the Bojorquez lagoon and a chronic beach erosion problem.

Figure 2. Detail of the northern end of Cancun, Punta Cancun, (a) in 1967 before the resort developed (b) and in 2005.

Today Cancun measures 12,700 Ha, of which 21.8 % are urban areas, and has approximately 500,000 inhabitants. 14.3% of the area has a very intensive tourist development (mainly along the barrier island) with 25,400 rooms and an average of 3,300,000 tourists a year, which represents an average 70% occupancy throughout the year. Although a quarter of the area is a nature reserve and the site belongs to the Mesoamerican Barrier Reef System, the environment within the lagoon and in the surrounding reefs has been degraded. Oceanographic Vulnerability of Cancun

Wave climate affecting Cancun propagates predominantly from the Caribbean Sea (SSE direction) driven by the trade winds. (Data from the NOAA buoy 42056 located in the Yucatan basin at water depth of 4446 m, Figure 3). Only a very small percentage of the waves come from the north, generated in the

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Gulf of Mexico by winter cold fronts. Wave climate arriving at Cancun is generally mild, with small, locally generated waves (90% of the observations are Hs < 1m and T < 8 sec). Semidiurnal tidal oscillations are small amplitudes e.g. spring tidal ranges of 0.4 m. Thus sea level variations of atmospheric (or other) origin can significantly alter the sea level.

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Figure 3. Wave statistics at Cancun for 2005 taken from NOAA buoy 42056, showing a predominant angle of approach from the SSE.

One climatic characteristic of the region is the presence of extreme tropical storms which alter the calm that predominates. The presence of hurricanes can drive waves with Hs > 10 m and T > 12 sec as shown in a time series of significant wave heights taken from buoy 42056. The waves generated by hurricanes Emily (My 2005) and Wilma (October 2005) were of extreme heights (Figure 4). Emily

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Figure 4. Wave time series from buoy 42056 showing the effects of hurricanes Emiiy and Wilma on the Caribbean Sea.

A total of 46 tropical systems (storms and hurricanes) have affected Cancun since 1970. Table 1 presents the seven most damaging storms with Gilbert


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(1988) and Wilma (2005) having the most dramatic effects on the beach and coastal infrastructure. Apart from the effects of hurricanes, the morphology of the coast is affected by its lack of land sediment; it is a sediment starved coast. The entire Yucatan Peninsula is limestone, gently dipping towards the East. No rivers flow on the surface in Yucatan because the limestone bedrock is highly fractured and filled with solution features such as underground caverns. Water drains into cracks and sinkholes, and flows under the ground, out to sea. Table 1. Strongest tropical storms and hurricanes that have affected Cancun since 1970 in chronological order. Year 1980 1988 1995 2002 2004 2005 2005

Month 8 9 10 9 9 7 10

Name Allen Gilbert Roxanne Isidore Ivan Emily Wilma

Category when hitting land 4 4 1 2 1 1 3

MATERIALS AND METHODS

Due to the oceanographic risks, and the intensive land use at Cancun, morphodynamic studies are necessary to ensure appropriate management of the beach. Several studies have been carried out at Cancun in response to the extreme erosion events and in order to perform engineering solutions of beach recovery (beach fills). However, as yet, no morphodynamic studies which focus on whole system behavior, including the small scale sediment transport processes, sandbar behavior, and sediment exchange with the shelf have been undertaken. A research project aimed at providing the platform to start such long term studies (O(years)) started in January 2005. This research project studies morphodynamics with three complementary approaches, namely field studies, video imaging and numerical modeling. Field Studies

At this first stage and given the equipment limitations, the field campaigns were focused on performing beach profiles on the frontal 12 km of Cancun beach (from Punta Cancun to Punta Nizuc). The profiles were measured every 200 m with differential GPS (including base and rover). Data from 2 campaigns, before and after Wilma (8 August and 25 October, 2005) were used to assess the beach changes produced by the hurricane. The bases were installed on the control points administered by the National Institute of Geography, Information

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and Statistics. Several GPS points were transferred from the control points to different positions in the Cancun area to ensure the base was always within 5 km of the rover. Additionally, two ADCPs were installed at a water depth of 20 m to monitor current profiles, directional wave information and surface elevation variability. One instrument was installed off Punta Nizuc, and the another one 30 km to the south, off Puerto Morelos. Video Monitoring and Satellite Imaging Following Holman et al (1993), and based on the very positive results obtained in many parts of the world using beach video systems, we decided to develop a video system for Cancun. The aim was to monitor the time variability of beach width, sandbar migration, and beach use. The video cameras used are SONY SNC-RZ30 with combining network functionality and Pan/Tilt/Zoom (P 25x optical zoom) capability. The cameras work as FTP servers, hence with an IP address they can be accessed remotely and changes made in configuration, sampling scheme, and views recorded. The SNC-RZ30 employs JPEG compression and can be set up to transfer these JPEG images to the FTP server. Image quality is extremely high because the SNCRZ30 uses a high-resolution CCD imager and a newly designed image processor. Six cameras were installed between April and June 2005 on hotel rooftops, and camera positions and ground control points for image rectification were measured in August 2005. The cameras were programmed to take 12 different views of the beach, each one of 300 sec, 3 times a day. The images are sent through the hotel network to the image processing centre in Mexico City (IMAM). Figure 5 presents a panoramic view of the camera installed on the rooftop of Beach Palace Hotel, 3km south of Punta Cancun.

Figure 5. Panoramic view obtained with the video camera installed on the rooftop of Beach Palace Hotel.


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Certain limitations were found in the installation of the camera system. For example, it was not always possible to find hotels that provided a 180° view of add the entire beach, as neighboring buildings blocked the view. Hence four of the six cameras installed had only a partial view of the beach. Although the cameras were installed, and ground control points surveyed, the system presented problems of data loading between the hotel network and the server in Mexico City. To aid analysis of shoreline changes QuickBird rectified satellite images with resolution of 0.7 m. The satellite data includes April and October 2005 covering the entire 12 km of frontal beach at Cancun. Later, with the passage of Wilma through the site, five of our six cameras were damaged. Numerical Modeling

To evaluate the relative strength of different hurricanes that affected the region (in terms of wind speed and wave height), the HURAC model (Silva et al. 2002) was used. HURAC is a parametric model, developed at the Institute of Engineering (UNAM) which estimates pressure, wind, and wave fields generated by hurricanes. It is based on simplifications and adaptations to the HydrometRankin Vortex Model (Holland, 1980; and Bretchsneider, 1990). Input data include time (hour, day, month, and year), central surface pressure of the storm in mbar, associated geographic position (latitude, longitude), maximum sustained (8 minute) surface (10m) wind speed in km.li"1, direction of translation, and cyclostrophic radius. A 56-year database, 1949-2005, including the reports published by NOAA (http://www.nhc.noaa.gov/), was used to build a database containing 6-hourly data for each hurricane that affected the region. The HURAC Model has been validated with data from the National Data Buoy Centre (see Silva et al. 2002). An example of model validation with Wilma using buoy 42056 located in the Caribbean is presented in Figure 6. 12 10

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Figure 6. HURAC validation for hurricane Wilma.

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From Figure 6 it is clear that the parametric model performs reasonably well in reproducing the temporal behavior of wave height, slightly underestimating its values at the peak of the storm and when the cyclostrophic radius no longer affects the site.

ANALYSIS OF HURRICANE-GENERATED BEACH CHANGES

Waves generated by hurricane Wilma affected Cancun 20-23 October, 2005. Drastic erosion was observed along most of the 12 km of beach, but the erosion pattern was not uniform. Figure 7 summarizes the erosion pattern found immediately after the passage of hurricane Wilma, dividing the frontal 12 km into four sections showing different morphological responses. The results are presented by section, as shown in Figure 7.

Figure 7. Map of Cancun showing the four regions that presented different morphodynamic responses to hurricane Wilma's effects.


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Section I

The section showed considerable accretion, with the beach growing between 10-15 m. Photographs of before and after the event, and a beach profile measured in the middle of this section show evidence (see Figure 8 and 9).

Figure 8. Photographic evidence of the extreme accretion experienced in Section I, and Figure 9. Beach profile showing the same region before and after Wilma. Section II

Section II suffered serious erosion. Figure 10 shows satellite images before and after Wilma (April 28 and October 25) Wilma. The beach on April 28 was of approximately 25-28 m width, and after Wilma the beach disappeared completely, exposing the underlying bedrock, and many buildings had structural damage.

Figure 10. Satellite images showing Section II before (left) and immediately after (right) Wilma. Note that the beach has disappeared completely.

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Section III This section of the beach was eroded but the effects were less drastic than in sections II and IV). It is worth pointing out that the section starts at Playa Delfines, the only beach section with a (1 km) dune field in Cancun. Figure 11 shows a photograph of the beach two days after Wilma passed, showing the presence of a fairly large beach. The beach profile in the same figure shows that the beach suffered erosion near the structure (1 m erosion) but was barely affected in width. This section of Cancun is proved to be the most stable zone. 4.0 -,

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Figure 11. Photographic evidence of the presence of a 25 m wide beach in Section III, two days before the passage of Wilma. On the right a beach profile shows that the beach suffered erosion close to the wall but beach width was not gravely affected.

Section IV Section IV suffered the most extreme erosion. The entire beach disappeared and structural damage was experienced, in many sections the underlying rock was exposed showing soft carbonate calcium outcrops. Figure 12 shows photographic evidence of such extreme erosion and a beach profile across the section. The beach on August 28 was 35 m wide but after Wilma it was replaced by an exposure of bare rock 3.5 m high. This kind of erosion extended for 3.8 km northwards. In the extreme north serious erosion problems were observed but less accentuated.

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20

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Distance Cm)

Figure 12. Photographic evidence of the effects of Wilma on Section IV, showing the exposure of a 3.5 m outcrop. On the right a beach profile shows the extreme beach erosion.


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Historically, Gilbert was considered the hurricane that most affected Cancun, but the erosion generated by hurricane Wilma was unprecedented. Such extreme morphological behavior can be explained with the help of the numerical modeling with HURAC. The first three panels in Figure 13 show a comparison of the pressure, wind, and wave fields generated by hurricanes Gilbert (1988) and Wilma (2005). The bottom panels present the persistency of these events as hurricanes of level 1 or above. Gilbert Sep-1988 Pressure

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Wilma Oct-2005 Pressure

Figure 13. HURAC model cumulative results for hurricanes Gilbert (1988) and Wilma (2005).

Figure 14 demonstrates that hurricane Gilbert was more intense in terms of the surface pressures, wind speed and wave heights generated. The difference with Wilma was the length of time the hurricane affected the area. Slightly smaller waves, (16 m for Gilbert and 13 m for Wilma) for longer, are more damaging. Moreover at such magnitudes of wave height, differences of 3 m are insignificant as 13 m waves will probably be as damaging to the beach and infrastructure as waves of 16 m. A further factor which explains the extreme erosion associated with Wilma is the presence of hurricane Ivan (2004) and the abnormally active 2005

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hurricane season, before Wilma. Hurricane Emily in My 2005 weakened the beach considerably. Figure 14 shows two rectified video images from the southern end of Cancun (corresponding to Section II of Figure 7) before and after Emily. On My 7 2005 the beach in front of the Sun Palace Hotel (section II) was completely covered with sand, and rock outcrops on the beach were only evident in the submerged part of the beach (at image coordinates x ~ 15, y ~ 50). On My 20, after the passage of hurricane Emily the concentration of rock outcrops on the subaereal beach (20< x