Study area and vulnerable features

The coastal area of Tabasco is located between 92o 28' and 94o 10' west longitude and 17o 15' and 18o 39' north latitude (^{Hernández et al., 2008}), with a coastline of 200 km distributed in the municipalities of Huimanguillo, Cardenas, Paraiso and Frontera, which together are home to about 616 500 inhabitants (^{INEGI, 2010}). Adjacent to its coastline is the Gulf of Mexico, one of the most complex and productive waters in the world (^{Tunnell et al., 2004}).

The coast of Tabasco is the result of significant alluvial processes (^{West et al., 1985}) caused by the river dynamics exerted by the Grijalva-Usumacinta basin, Gonzalez and Tonala River. Due to its genesis dominated by the accumulation of sediments, the coasts from Tabasco are sandy low located less than one meter above sea level, making them vulnerable to small variations (^{Ortiz and Linares, 1999}).

In this context of vulnerability the coast from Tabasco (Figure 1), host important natural resources such as the biosphere reserve Pantanos de Centla (RBPC), considered the most important unitofenergystorage in North America(^{Toledo, 2003}) as well as areas for agricultural crops like pastures, cocoa, corn, family orchards and lake systems Carmen-Pajonal-Machona and Mecoacan; giving place to commercially important fish species, of which 90% of the state fishing (^{Contreras and Castañeda, 2004}) is obtained. Associated to these coastal lagoons from Tabasco are mangrove species Rhizophora mangle (red mangrove), Avicennia germinans (black mangrove), Laguncularia racemosa (white mangrove) and Conocarpus erectus (Buttonwood mangrove) (^{Villanueva et al., 2010}) conditioned to different salinity gradients and flooding periods.

Figure 1 Vulnerable areas to rising sea level. ^{SEMARNAP (1997)}. 

According to tidal stations in the region, the tide is mixed with diurnal influence. The waves are moderate in E-W direction with maximum heights of 2m under normal conditions; however the winds from the north significantly increase wave heights up to 5 meters (^{SEMAR, s/f}; ^{Lankford, 1976}).

Some studies performed in the coast of Tabasco show the vulnerability to INM (^{Hernandez et al., 2008}; ^{Ortiz et al., 2010}), the main impact is coastal erosion, which in some places has been alarming (-3 to - 5 m / year in Sanchez Magallanes, Figure 2).

Figure 2 Impacts produced by INM. Magallanes Sanchez Cardenas, Tabasco. 

Vulnerability model: the inclusion of the term in the coastal context

For the ^{IPCC (2007)} vulnerability reflects the ability of a system or subsystem to cope with climate variability and extreme events. This conceptualization leads to expect an answer either positive or negative, to the extent in which are the coasts conditions (^{CEPAL, 2012}). However, exposure, resistance and resilience are who determined in the physical context, the vulnerability of the coastal zone (Figure 3); therefore any vulnerability study, parts from the assessment of physical impacts on the environment (^{Klein and Nicholls, 1999}).

Figure 3 Vulnerability components. Based on ^{Klein and Nicholls (1999)} and ^{Ojeda et al. (2011)}. 

According to the adopted model, the first three components indicate natural vulnerability (or relative); i.e. the impacts suffered in the physical environment. However, the ability to adapt, besides to complement the model, become the determinant factor of real vulnerability (or net) of the system. Therefore, many systems will have high natural vulnerability, but low socio-economic vulnerability (^{Ojeda et al., 2011}) if it counts with the elements or efficient adaptation strategies. Therefore, the effects of INM can impact coastal resources, but these will be higher in regions with reduced capacity of adaptation (^{Bijlsma et al., 1996}).

In Tabasco, the coastal area plays an important role in the economy of the entity, in particular in the municipalities of Cardenas, Paraiso and Centla, thus other nearby to the shoreline (Huimanguillo, Comalcalco and Jalpa de Méndez); the coastline stretches over 200 km, including an extensive network of runoff, deltaic plains, lagoon systems, marshes and swamps (^{Rodriguez-Rodriguez, 2002}). The surface of this area is approximately of 80 000 hectares, representing 3.2% from the total surface of the entity; the potential use of the coastal area has been restricted to agricultural activities, specifically coconut, ranching, vegetable crops in small areas, and more recently to logging of mangrove (^{Palma-Lopez et al., 2007}).

The effects of climate change can generate different levels of vulnerability, in the manner of how communities face them and the conditions of geographical space. According to De la Lanza et al. (1993), environmental changes and human intervention has led to a decrease in surface and changes in mangrove biodiversity in the lagoon area from Terminos, Campeche. In Tabasco the contrary situation is observed; the deltaic complex subsidence favors the advance of the coastline to the continent and the effects are seen through beaches and ports erosion, as well as the intrusion of sea water into the continent, contributing to expand flooded areas (Ortiz et al., 2005), condition that favors the growth of mangrove areas.

Evidence of the adaptation of some species to environmental changes, are modifications in the distribution patterns of these. According to ^{Guerra and Ochoa (2006)}, although in 1992 the pantanos de Centla were declared as protected area, the variation by loss in the types of vegetation and land use has not stopped; only mangrove has been able to maintain the area occupied and even in other parts of the coast of Tabasco, such surface has increased.

The measurement model adopted: coastal vulnerability index

Accurately determine the response of the coast to INM is complex due to many variables that come into play. Coastal areas are non-lineal systems, where human activities, natural processes and policies regulations determine their ability to address and mitigate adverse events; however, these play an important role in INM (^{Pendleton et al., 2010}).

The methodological framework from IVC prepared by ^{Gornitz et al. (1992)} and proposed to assess the vulnerability of the Atlantic coast of the United States of America (USA) considered indicators that influence in risks of flooding and coastal erosion, thus prioritizing the physical conditions of the coast in potential impacts by INM. This model was also applied to measure the sensitivity of Canadian waters (^{Shaw et al., 1998}), in this case, by sensitivity understand, the magnitude of change in the coast produced by sea level rise. However, ^{Thieler and Hammar (1999)} adjusted ^{Gornitz (1992)} proposal to assess vulnerability in regional and national coastal parks from USA, considering six variables (Table 1). More recently, ^{Ojeda et al. (2011)} applied the model already fit, but made adjustments in one of the variables (replacing coastal slope for topographic index) according to the requirements of the Andalusian coast in Spain.

Table 1 Variables considered in each of the countries evaluated. 

Incluye tipo de roca (litología); 1sustituida por índice topográfico2*variables descriptivas. a ^{Gornitz et al. (1992}), b ^{Shaw et al. (1998}), c ^{Thieler y Hammar (1999}) y d Ojeda et al. (2009).

This methodology uses a weighting system of 1(very low) to 5 (very high vulnerability), for each of the variables. The combination of six variables gives the degree of vulnerability of the coast (^{Ojeda et al., 2011}) as follows:

Geological / geomorphological variables: coastal geomorphology, change rate of the coastline long-term and coastal slope.

Physical / hydrodynamic variables: change rate for relative sea level, average significant wave height and average tidal range. The six variables are integrated into an equation that gives as result the IVC and consequently different degrees of vulnerability along the coast; already applied in different countries (Table 1).

Index results allow anticipating the impacts that the coast will experience by INM. However, this model only indicates the relative vulnerability of each coastal stretch in terms of their exposure (physical environment) against INM. This represents the basis to perform studies with greater detail and allows estimating the risk and taking the necessary measures to safeguard the assets on the coasts.

Although there is plenty confidence that this model is applicable to any coastal environment (^{Gornitz, 1992}; ^{Thieler and Hammar, 1999}), analyze each of the variables considered, since the model has been applied in large regions (Spanish Andalusian coast, Atlantic coast from US, Canadian coasts). This is argued because a more extensive coastline allows more diverse landscapes; however, in a "short" coastline like Tabasco (200 km) there are fewer environments, leading to reduce vulnerability classifications. On the other hand, coastal ocean processes influencing may be of greater or lesser intensity, which also affects vulnerability indicators. At the end the objective of this analysis is to ensure that the methodology is applicable to the coast of Tabasco and if not to include new variables or modify as necessary.

Methodology adopted and its relevance to the coast of Tabasco

The proposed methodology has been applied in the world to different coastal areas, therefore, to feed the model requires considering key aspects such as own characteristics of the coast under study, environmental pressures, its adaptability and available data (Figure 4).

Figure 4. Components to assess vulnerability by exposure on the coast of Tabasco. Based on ^{Gornitz et al. (1992}), ^{Shaw et al. (1998}), and Hammar ^{Thieler (1999}) and Ojeda et al. (2009).

Variables proposed vs local conditions

Low relief, the origin of its formation, the annual influence of cold fronts and excessive pressure from human activities, locate the coast of Tabasco in high vulnerability to INM. However, all natural systems have the capacity to withstand and recover from damage, which in ecology is known as resistance and resilience (^{Klein and Nicholls, 1999}) these qualities limit the impact through adaptation to meet the sea level rise. The methodology proposes to confront the effects favored by marine phenomena such as waves, tides and sea level and qualities of the coast to mitigate those effects, such as the altitude above sea level and coast resistance, according its shape and geological origin.

1. Coastal geomorphology

This variable reflects coast resilience to erosion, therefore takes into account characteristics attributed to geological origin. Between coastal landforms it can be distinguished cliffs, long beaches, spits, deltas, swamps, glacial deposits, estuaries, coastal lagoons, offshore bars, among others (^{West et al., 1985}).

In Tabasco ́s case, the combining of strong inflow of sediments and dense mangrove vegetation can mean long-term stability of a coast where the sea is growing at similar rates to global (^{Syvitski et al., 2005}). Although geology / geomorphology variable in studies performed to date has considered mangroves as a type of coastal landform, the vulnerability degree assigned is moderate to very high vulnerability (Table 2). However, due to the lower diversity of landscape and little geologic differentiation of coastal environments of Tabasco, mangroves could have a lower degree of vulnerability (low to moderate).

Table 2 Environments considered in geology-geomorphology variable. 

2. Coastal slope

Tabasco is part of the Gulf of Mexico region, where lowlands and plains predominate, since the Tabasco coast extends into a low flood plain, this could be left out. However, there are minor differences in topography same that through a LIDAR digital model will allow to obtain measurements of great value to perform a relief differentiation along the coast.

3. Sea level rise

Trends in the Gulf of Mexico range from 1.8 ± 9.2 mm/year (^{Zavala et al., 2010}). The danger becomes greater on the coast of Tabasco due to its geographical location and formation processes to which has been subject. However, the impacts cannot be uniform throughout the coast of Tabasco, as there are natural subsidence areas, other receiving sedimentary contributions, some with higher human pressure mainly by the extraction of hydrocarbons and the last protected with coastal infrastructure as geotubes, jetties and breakwaters.

4. Change in shoreline

It is considered that knowing the present and past of the coast is vital to detect risks by sea level rise on the coasts. In Tabasco, have been performed studies to evaluate this variable (^{Hernandez et al., 2008}; ^{Ortiz et al., 2010}); data found show an alarming coastal retreat, in the delta from Rio San Pedro and San Pablo and in the area from Sanchez Magallanes, according to this the coast of Tabasco is showing signs of high vulnerability.

5. Swell

Swell is a phenomenon that contributes to the conditions of maritime climate, to know and predict their behavior is critical, since its occurrence modifies the coastline (^{Pérez et al., 2008}).

The variable swell plays an important role in the vulnerability of the coast of Tabasco, on the understanding, which is of cumulative origin. The swell dynamics in Tabasco changes depending on weather conditions (storms, cold fronts and wind). ^{West et al. (1985)}) described waves ranging from 30 to 70 cm in calm weather; however, in the windy season the waves have more energy exceeding 5 m (^{Lankford, 1976}; ^{SEMAR s/f}). This combination of relative calm and aggressive storm causes the migration of sediment, especially important for a cumulative cost of origin and serious threats to INM impacts, therefore it should be considered in impact assessments (^{Hernandez et al., 2008}).

6. Tidal range

Normally coastlines are subject to tides. The coasts of Tabasco are microtidal; that is, the amplitude is small. ^{Gornitz et al. (1994)} found that a large tidal range dissipates wave energy, limiting active erosion to high tide periods but in turn can cause strong currents that can carry unconsolidated sediments. It should be noted that this variable is not relevant to the Tabasco coast, and at one point could be omitted or replaced by another.

Sedimentation

Inland watersheds are responsible for the stability of coastal areas, produce, transports and deposit the inputs for coastal environments (^{Toledo, 2003}). The annual contribution of the world's rivers is estimated between 20 billion metric tons of sediment that come under natural conditions (^{Milliman and Syvitski 1992}). However, over 100 million tons of sediment being trapped in dikes and dams built in the last 50 years (^{Syvitski et al., 2005}).

This is important, since the coast of Tabasco are the result of "powerful alluvial processes" (^{West et al., 1985}), as has been the Grijalva-Usumacinta basin, the largest and mighty of Mexico. However, in the affluent from Grijalva by the construction of the largest dams systems in the country (Angostura, Chicoasén, Malpaso and Peñitas) and more recently at the bottom of the basin a control gate, and there is a relationship between changes of land use and the supply of sediment (^{Geyer et al., 2004}).

Subsidence

The effects of human pressure on the coast contribute to higher levels of vulnerability. The settlement of major urban centers leads to overexploitation of aquifers, and construction in flood-prone areas. The coast of Tabasco is a plain with slopes lower than 10.5%; however, Grijalva- Usumacinta delta for natural conditions has a decreased subsidence resulting in higher beach erosion and shoreline retreat (^{Ortiz Linares, 1999}). On the other hand it considers that the subsidence caused by the extraction of ground fluid (gas, oil and water) (^{Gornitz, 1994}), favor a more extensive flooding area in the Tabasco coast.