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Contemporary shoreline changes and consequences at a tropical coastal domain

Contemporary shoreline changes and consequences at a tropical coastal domain GeoloGy, ecoloGy, and landscapes, 2018 Vol . 2, no . 2, 104–114 https://doi.org/10.1080/24749508.2018.1452483 INWASCON OPEN ACCESS Contemporary shoreline changes and consequences at a tropical coastal domain a,b a a Temitope D. Timothy Oyedotun   , Arturo Ruiz-Luna and Alma G. Navarro-Hernández a b c entro de Investigación en alimentación y d esarrollo (cIad ) a.c., Mazatlán, Mexico; Faculty of earth and environmental s ciences, l eslie c ummins Building, University of Guyana, Turkeyen c ampus, Georgetown, Guyana ABSTRACT ARTICLE HISTORY Received 4 d ecember 2017 Coastal environment is affected by diverse human and natural activities, more than any a ccepted 3 March 2018 other natural environment. The main aim of this study is to examine shoreline dynamics of the sandy beach of Mazatlán, a medium-sized tropical coastal city in north-west Mexico. This KEYWORDS paper specifically investigates the shoreline change as impacted by natural and anthropogenic c oastal processes; interferences on the Mazatlán coastline. The mean high water (MHW) shoreline positions morphodynamics; human were extracted from Landsat Images (2012–2016) and a 2016 GPS field survey data. Digital activities; erosion; dsas; Shoreline Analysis System (DSAS) was then used to investigate the dynamics of the extracted Mazatlán; coastal cities shoreline movements and the relative changes. Results showed that 96% of the coastline is −1 undergoing yearly small-scale erosion at two distinct rates. The first at −1.9 ± 0.9 m year and −1 the other at −1.4 ± 0.2 m year , which are noted at Cerritos and other sections of the coastline, respectively. Changes in the coastal behaviour, here, are attributed widely to suspected sea level rise; increasing tidal range in the region; and the lack of or inadequate accommodation space for sediment movement occasioned by landed assets alongshore. These factors are not only encouraging erosion but also causing the depreciation of landward assets. 1. Introduction different timescales and processes involved in shoreline variability make it unsolvable challenge to many coastal Intense conflicts between the natural environment and managers and policy-makers (Stive et al., 2002). The ina- human activities are reported widely in literature (e.g., bility to understand and predict shoreline variability can Brown et al., 2013, 2017; Larsen, 2016; Rhoads, Lewis, result in the misinterpretation of coastal change scenar- & Andresen, 2016; Waters et al., 2016). Perhaps no nat- ios, and ae ff ct (directly or indirectly) decision-making ural environment is more ae ff cted by human activity at and subsequent design of intervention plans (Stive et the contemporary (shorter) timescale than the coastal al., 2002). So far, the variability in shoreline position environment (Brown et al., 2017). The coastal environ- remains a reliable proxy to describe the overall coastal ments have served as a viable source of many valuable or beach changes (Absalonsen & Dean, 2011; Bouvier resources for man’s economic, social, and recreational et al., 2017; Oyedotun, 2016, 2017). Shoreline dyna- activities (Cai, Su, Liu, Li, & Lei, 2009). Consequently, micity/variability has oen b ft een used to study coastal coastal geomorphology and processes are impacted by changes at short (days to seasons, e.g., Pearre & Puleo, anthropogenic influence, directly or indirectly (Blum & 2009; Stive et al., 2002) or long (decades to centuries, Roberts, 2009; Brown et al., 2017), in addition to the e.g., Harley, Turner, Short, & Ranasinghe, 2010; Goble & mirage of natural environmental forcings, thereby pre- MacKay, 2013) timescales. Analysis of shoreline change senting acute challenges within this natural environment is, thus, a well-established field (Burningham & French, (Teasdale, Collins, Firth, & Cundy, 2011). Shoreline is, 2017). However, the understanding of the change is perhaps, the most basic indicator of changes in coastal oen m ft ade easy by provision of multi-temporal data environment and, by implication, erosion, deposition, and robust quantification of the trends in the shoreline and subsequent recovery (Bouvier, Balouin, & Castelle, behaviour (e.g., Garcin et al., 2016). Contemporary 2017; Davidson, Turner, Splinter, & Harley, 2017; Kroon shoreline change is the short-term behaviour (annual et al., 2007; Oyedotun, 2016; Phillips, Harley, Turner, to decadal timescale) of the shoreline positions, espe- Splinter, & Cox, 2017; Robinet et al., 2016). cially as it relates to either the hydrodynamics processes Our ability to understand shoreline variability (e.g., Hapke, Himmelstoss, Kratzmann, List, & Thieler, remains one of the core issues in nearshore science as 2011; Hapke, Plant, Henderson, Schwab, & Nelson, 2016; CONTACT Temitope d. Timothy o yedotun temitope.oyedotun@uog.edu.gy © 2018 The a uthor(s). published by Informa UK limited, trading as Taylor & Francis Group. This is an open a ccess article distributed under the terms of the creative c ommons a ttribution license (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. GEOLOGY, ECOLOGY, AND LANDSCAPES 105 Hapke et al., 2010) or the local sediment budgets (e.g., Peraza-Vizcarra (1986) observed the net movement of 3 −1 Bradbury, Cope, Wilkinson, & Mason, 2013; Pye & Blott, sediment of 214,609 m yr south-eastward occasioned 2016). by changes in wave regimes, while Montaño-Ley and e m Th ain purpose of this study is to analyse con- Gutiérrez-Estrada (1987) found erosional volumes of temporary small-scale changes on the sandy beach of sediment with the maximum erosion at 15.7  m per Mazatlán, a medium-sized tropical coastal city in north- metre of length of the beach and Montaño-law et al. west Mexico. Based on the recent image analyses and (1988) observed an 8.5 m of eroded sediments along the fieldwork, this paper specifically investigates the shore- beach. Mazatlán coastline is characterized by mixed tide line change as impacted by the natural interferences and with an average range of about 1.0 m, a prevailing NW examines the effects of this on anthropogenic presence wind and occasional tropical storms migrating along the at the coastal environment. Although consideration of Pacific Coast of Mexico from SW striking the Mazatlán historical changes can show long-term effects and conse - city (Montaño-Ley et al., 2008). quences of the interactions along the coastline. However, Mazatlán city is with a population of around using Mazatlán coastline as the template for tropical 400,000, a population density of 173 per km and 87% regions, the findings presented in this study strongly of infrastructures that support tourism, fishing, indus - suggest that we do not have to consider a distant past trial, and other diverse economic activities (INEGI, nor the large-scale measurement to be able to infer the 2010). The city has a high pressure on the natural envi- pattern and effects of these nature–human interactions. ronment (Camacho, Ruiz-Luna, & Berlanga-Robles, Indeed, the implications of the continuous and contem- 2016). Despite the economic well-being and financial porary small changes suggest that rapid attention to this benefits of the wetland coastal systems of Sinaloa State small-scale investigation is highly essential if we want to to the local communities (Camacho-Valdez, Ruiz- avert long-term and permanent damage in the future. Luna, Ghermandi, & Nunes, 2013), the coastline has been subjected to high risks due to increasing land use changes which affect the supply and quality of 2. Mazatlán coastline this systematically transformed coastal environment Mazatlán, a southern city in the State of Sinaloa, is located (Camacho et al., 2016). Indeed, land use and land cover on the 23°5′ and 23°19′ North and 106°17′ and 106°30′ changes in Mazatlán and the entire Sinaloa State, like West (Figure 1). The city is a mostly sandy tropical other tropical coastal cities, have grown over the last coastline oriented NW–SE, covering a length of approx- decades with noticeable urban growth at the expense imately 20 km. The earlier studies of Montaño-Ley and of natural vegetation and agriculture (Ruiz-Luna & Figure 1.  Mazatlán showing the recent shoreline positions and profile areas shortlisted for analyses in this study. Inset: Map of Mexico showing the state of sinaloa.  s ource: The data for the map was extracted from the GadM database (www.gadm.org), version 2.5, november 2015. 106 T. D. T. OYEDOTUN ET AL. Berlanga-Robles, 2003). With increase in the popula- (Red Green Blue) Colour composites of 543 (for Landsat tion of this tropical city is the increase in the demands TM and ETM+) and 652 (for Landsat OLI images) were for the use of the coast and the nearshore waters for then applied on the images to enhance the objects and recreation, tourism, commercial, and other human distinguish clearly between soil, land, and water. The uses. Interplay of the increasing human activities and distinguished shoreline position was then extracted by natural forcings, is making the coastal management in digitizing and imported into DSAS module in ArcGIS this part of the world to be problematic for the coastal environment. managers. The findings from the Mazatlán coastline Position accuracy of the 2016 shoreline (extracted are expected to serve as baseline information on the from Landsat Image) was confirmed with the global trend of this form of interactions in the State of Sinaloa, positioning system (GPS) Field Survey carried out Mexico, and the entire tropical region in providing a alongshore the study site using the Garmin GPS Model scientific guidance on coastal management and policy 120 (programmed to measure coordinates in UTM 13 N decision-making. continuously as we walk along the beach) between 25 October and 9 November 2016. Digital Shoreline Analysis System (DSAS) (Thieler, 3. Methods Himmelstoss, Zichichi, & Ergul, 2009) was then used Shoreline geometry is one of the basic indicators in eval- to investigate the dynamics of shoreline movements and uating coastal changes and this has been used exten- the relative changes at shorter scale along the Mazatlán sively in investigating historical trend/pattern of coastal beach. Although the utilization of DSAS is widely used dynamics (Oyedotun, 2014). Differences in historical in Historical Trend Analysis (e.g., Jabaloy-Sánchez et al., and recent rates of change along a coastline are mostly 2013; Oyedotun, 2014, 2016), it has also been applied reflected in shoreline movements, and here, the mean for short-term investigations of shoreline variations and high water (MHW) shoreline positions (Boak & Turner, short-term coastal changes (e.g., Hapke, Kratzmann, & 2005) were analysed for the shoreline changes. The wet/ Himmelstoss, 2013). dry lines along the beach are used as proxies for the Shoreline change analyses were performed through position of MHW (Boak & Turner, 2005). These lines, the generation of shore-normal transects at 50 m inter- perpendicular to the coast, were extracted from Landsat vals along the open coast of Mazatlán from Punta Imageries covering the period from 2012 to 2016 aer ft Cerritos to Punta Tiburón (Figure 1). A range of statis- they have been pre-processed through the application tical analysis were calculated for each of the transects at of radiometric calibration and atmospheric correction 99.5% Confidence Interval, applied to the minimum of in ESRI© ArcGIS, following the guidance described four (4) shoreline intersection threshold. Here, specifi- in USGS instruction guides USGS (2016a, 2016b). All cally, results of Shoreline Change Envelope (SCE), Net the images were collected almost at the same time in Shoreline Movement (NSM), Linear Regression Rate summer season in good quality so as to eliminate the (LRR), and End Point Rate (EPR) of change (Oyedotun, effects of sea level rise and waves (Vu, Lacroix, a Th n, 2014, 2016; Thieler et al., 2009) are presented. SCE meas - & Nguyen, 2017). The Landsat data sourced from U.S. ured the total change in position of all the shorelines Geological Survey, USGS, (www.glovis.usgs.gov) are under consideration; NSM, the distance between the Landsat 7 (Enhanced Thematic Mapper, ETM) and 2012 and 2016 shoreline positions; LRR determined the Landsat 8 (Operational Land Imager, OLI – er Th mal rate of change statistic by fitting the least square regres- Infrared Sensor, TIRS) 30 m spatial resolution images sion to all the shorelines at each of the transects while at World Reference System (WRS) path/row 31/44. For EPR in this study was derived by dividing the total shore- the Landsat 7, the image was acquired on 17 September line movement by the time period considered (5 years 2012 while that of Landsat 8 images were acquired on in this study). The Mazatlán coastline, for this study, 12 September 2013, 17 October 2014, 16 July 2015, and was divided into four sections (Figure 1) to allow for a 04 September 2016, respectively. more systematic analysis and interpretation of the rates e Th imageries were transformed to Geographic of shoreline dynamics. At the recent (contemporary) Projection (Universal Transverse Mercator (UTM) timescale, erosion trends are presented as negative val- WGS84 Datum, Zone 13N) before being preprocessed ues and the depositional/accretional trends as positive in ESRI© ArcGIS 10.3 for image enhancement (includ- values. The shoreline change analyses in this study are ing radiometric calibration, atmospheric correction, used as metric to evaluate the modifications alongshore gap filling, pan-sharpening) and geometric rectifica- the coastline of Mazatlán on a local scale. Assessment tion to eliminate imageries defects (e.g., wedge-shaped and interpretation of the broader influence of shoreline gaps, radiometric distortion, presence of noise, etc.) movements was modified aer des ft cription by Hapke et (Lillesand, Kiefer, & Chipman, 2008). To extract the al. (2011) while the consequence of this influence on the exact shoreline from the images, a non-linear edge-en- system and adjoining anthropogenic presence on the hancement technique was performed in MATLAB to coast, is from the field observation and insights provided delimit the land–water boundary of the images. RGB by Cooper, O’Connor, and Mclvor (2016). GEOLOGY, ECOLOGY, AND LANDSCAPES 107 shoreline movement (landward) is mostly between 5 4. Results and discussion and 1  m (Figure 2(a)). There was, also, interruption 4.1. Contemporary shoreline dynamics of low volume of (~ −1  m) erosion at a few discreet The shoreline change metrics and trends (NSM, locations especially in areas where the envelope of var- SCE, and time-averaged LRR) are shown in Figure 2. iability (SCE) were substantially moderate (2.1–5  m) Although the net shoreline change in some areas along (Figure 2(b)). the coastline is minimal, the overall trend of shoreline Results of the geomorphic shoreline assessment indi- movement is erosional (Figure 2(a)) with 96% of the cate that the most erosional dynamic part of Mazatlán system undergoing erosion at smalle scale (>−1 m). The coast is at Cerritos. This was the section that exhibits subsection with highest negative erosion net rate, for the alternate stretches of high yearly rates of erosion of −1 the last five years, is between Punta Cerritos and El between −0.5 and 1.8  m  year (Figure 3) which cor- −1 Sábalo, to the north. This is followed by the coastline responds to maximum net retreat rate of −2  m  year between El Sábalo and Punta Camarón. At this contem- (Figure 2(c)). The retreat at this stretch of the beach porary timescale, the spatial pattern of shoreline vari- could be attributed to the availability of sediments, the ability along Mazatlán coast corresponds closely to the reshaping of available berms at the northern end of the net rate of shoreline movement (Figure 2(a) and (b)), beach or the limited coastal defence. Pocket of shoreline that is, NSM and SCE correspond. This indicates that progradation sparsely occurred around El Sábalo, Punta continuous and persistent change in shoreline position Camarón, and Punta Tiburón sections (Figure 2(c)) but is prevalent in the last five years. The consistent ero- these did not match the overall magnitude of persis- sional rates of the shoreline are significant here as the tent erosion rates of change in almost these sections. NSM and SCE are exactly and effectively the same for e p Th ockets of shoreline accretion rates, here, can be the erosion hotspots, which are predominantly between associated with the lateral alongshore movement and Punta Cerritos and El Sábalo section than other sec- deposition of eroded sediment at the spot of occur- tions of the coastline. Across much of the Mazatlán rence. However, the 20 km stretch coastline was largely coast, the aggregate amount of shoreline erosion was marked by erosion, although with variation of yearly between 5 and 7 m where the MHW shoreline positions rate (Figure 3). Progradation are sparsely localized at the −1 shifted landward. This notable change was highest in rate of < 0.5 m year which did not match the overall Cerritos area. In other section of the beach, the total erosion observed. Figure 2.  shoreline change metrics and trends for Mazatlán coastline, showing (a) net shoreline Movement (nsM), (b) shoreline change envelope (sce), and (c) average rate of change, linear Regression Rate (lRR).  s ource:  Georeferenced l ocation data was extracted from the GadM database (www.gadm.org), version 2.5, november 2015. 108 T. D. T. OYEDOTUN ET AL. Figure 3. yearly rate of change (epR) along Mazatlán coast, from punta c erritos to punta Tiburón. Comparison of retreat rates at this contemporary rise (e.g., Zhang, Douglas, & Leatherman, 2004). The scale (Figure 4) showed signic fi ant die ff rences in behav - insights from the examination of temporal Mazatlán iour between all the sections. The envelope of variability shoreline dynamics at the recent times (Figures 2 and (SCE) is highly obvious in Punta Cerritos (Quartile 1 3) are pointers to the influence of strong coastal forcings (Q1) where the 25% of the distribution is 7 m; Q3 (75% which are obviously compelling the landward movement of the distribution) is 10 m; median is 8 m and the high- of shoreline positions (e.g., Castelle et al., 2018; Hapke est value is 15.2 m). SCE at El Sábalo is also high (Q1 is et al., 2016). Although this study did not examine the 6.3 m; Q3 is 8.4 m; median is 7 m; and the highest value exact coastal forcing evidence compelling the observed is 9.6 m), moderate at Punta Camarón (Q1 is 2.7 m; Q3 change, the recent investigations of sea level rise in the is 6.2 m; median is 3 m; and highest value of 9.6 m) and region (e.g., Kopp et al., 2016; Lluch-Cota et al., 2010; low at Punta Tiburón (Q1 is 3  m; Q3 is 5  m; median Páez-Osuna et al., 2016; Ruiz-Fernández et al., 2016) is 4  m; highest value of 8.7  m) (Figure 4(a)). Despite suggest the ostensible rise in the sea level and storm the envelope of variability being strong in the far north surge. sections of this system, the yearly erosional rates are also Sea level rise is an important forcing along the Pacific higher at this area than the other three sections (that coastline of both North and South America (Enfield is, at El Sábalo, Punta Camarón, and Punta Tiburón, & Allen, 1980; Páez-Osuna et al., 2016). Indeed, the respectively) (Figure 4(b)–(d)). increase in water levels or surge in sea levels is docu- Two distinct yearly rates of erosion are evidence in mented significant drivers of sedimentary shorelines −1 this coastline. The first is at −1.9 ± 0.9 m year and the movements, whether at short- or long-term scale (Páez- −1 other which hovers at −1.4 ± 0.2 m year are the main Osuna et al., 2016). The coherence in the consistent ero - bulk of the erosional distribution. The first is noted sional behaviour of Mazatlán coastline can possibly be at Cerritos while the later at the other sections of the linked to a gradual rising sea level and relative stormy system (Figure 4(b)). Alongshore sequence of shoreline swell as the localized analyses presented here suggest. movement at the central and southern parts indicated This kind of assertion should, however, be made with that the average shoreline rates of erosion spread over caution as there are many other forces (waves, weather, −1 −0.5  ±  1  m  year (Figure 4(c)) at El Sábalo, which etc.) for example, elevate surge levels (e.g., Woodworth, −1 is lower than at Punta Camarón (−0.5 ± 0.0 m year ) Flather, Williams, Wakelin, & Jevrejeva, 2007), high −1 and Punta Tiburón (−0.7 ± 0.7 m year ), respectively. waves (e.g., Dodet, Bertin, & Taborda, 2010), strong There is a minimal accretion at El Sábalo and Punta winds (e.g., Burningham & French, 2013), etc. which Tiburón. This observed average rates of shoreline have the potentials in enforcing coastal change in this movement corresponds with the total shoreline migra- shoreline or any other similar tropical shorelines of tion, with only El Sábalo having a cumulative accretion the world. This caution is highly needed based on the of ≥1.8  m. The envelopes of change associated with observation by Schumm and Lichty (1965) that shorter migration of shoreline movement at Punta Camarón timescale changes are intrinsically and essentially linked are far wider than the one experiences at other sections with stringent cause–effect association at smaller spatial (Figure 4(a)). scales, like Mazatlán, than at a broader and wider spatial Changes in coastal behaviour are attributed widely to scale or at longer term. As an example, in the 1970s, long-term tidal cycles (e.g., Gratiot et al., 2008), climate anomalies of monthly sea level, coastal sea surface tem- change (e.g., Nicholls & Cazenave, 2010) or sea level perature and alongshore wind force were modifying the GEOLOGY, ECOLOGY, AND LANDSCAPES 109 the impacts of coastal forcing obvious. These anthro- pogenic instances are leading to shoreline planform squeeze with glaring effects on the coastline and the landed assets alongshore. This study did not, however, investigate whether there is (or there is no) any evidence of geological control on the shoreline behaviour as it is the norm with similar studies (e.g., Burningham & French, 2017). 4.2. Anthropogenic influences and consequences As common with any coastal areas with beehive of tour- ism activities, coastal defences are the main key struc- tural control on the coastal morphodynamics along Mazatlán coastline. One of the main objectives of this study is the examination of the influence of the contem - porary changes in the modifications of anthropogenic presence in this study area. Most Punta Cerritos – El Sábalo section of the coastline is characterized as having moderate development (aer H ft apke et al., 2011) as there are moderately spaced, privately owned landed/family properties and houses which are not clustered in this sec- tion. Here, there is little to limited tourist infrastructures, no massive commercial development projects except sparse hotels and there are some open spaces between the communities of family homes. e a Th nthropogenic infrastructural development levels between El Sábalo and Punta Camarón, on the other hand, can be described as dense (aer H ft apke et al., 2011, 2013) as there are a sizeable number of single-family houses, hotels, continuous communities of buildings, sizeable hotels, a good number of tourist infrastruc- tures and commercial buildings. Developments between Punta Camarón and Punta Tiburón can be described as heavy as there are conspicuous and predominant mul- ti-story buildings, hotels, diverse condominium com- plexes, various tourist infrastructures alongshore, and visible commercial properties at this section of the coast- line. e Th visual assessment of the influence and conse- quence of the yearly rates of change along the different sections of Mazatlán coastline are obvious. The expo- sure of this coastline to the influences of sea level rise and other coastal forcings like tidal influences and wave energy (e.g., Dickson, Walkden, & Hall, 2007; Nicholls & Cazenave, 2010; Woodroe ff & Murray-Wallace, 2012 ) results in the landward movement of the shoreline – the effects of which are visible on not only to the landed Figure 4.  classification of shoreline statistics and behaviour properties along the coast but also the coastal defence based on each of the four sections of the coastline. I – punta c erritos; II – el s ábalo; III – punta c amarón; IV – punta Tiburón. structures (Figure 5). The consequences of these struc- (a) – sce; (b) – epR; (c) – lRR, and (d) – nsM. tures are not only observable in the inhibition of the natural response of the beach and coastline to natural structures and dynamics in coastal zones in this Pacific processes like storms, they also increase erosion and dis- region (Enfield & Allen, 1980). ruption of alongshore sediment movement (Figure 5), e co Th ntrasting mode of coastline behaviour in this the phenomena which are also common in other coastal area is amplified by the presence of anthropogenic influ - areas (e.g., Hapke et al., 2013). This is also causing the ence exacerbating the contraction and thereby making wearing and tearing of the coastal defence structures and 110 T. D. T. OYEDOTUN ET AL. Figure 5. examples of effects and consequences of shoreline erosion on landward assets alongshore Mazatlán coast. (a)–(c) (p unta c erritos), (d)–(f ) (el s ábalo), (g)–(i) (punta c amarón), and (j)–(l) (punta Tiburón). the overflow of landward assets with coastal sediments This suggests that other assets not yet ae ff cted, may (Figure 5(a)–(c) and (e)). be in danger of erosional force in a period not far from Shorelines erode principally by natural imprints like now. e Th results of this study have, however, shown that coastal storms, sudden weather-related events, sea level human modifications along coastlines influence shore- rise, changes in sediment supply, and human imprints/ line change both historically and contemporarily, and modifications like beach nourishment, engineering are also influenced by the coastal processes. From this structures (e.g., Absalonsen & Dean, 2011; Dickson study, it can be inferred that the rates of shoreline change et al., 2007; Nicholls & Cazenave, 2010; Woodroe & ff are highly dependent on the level of human interference Murray-Wallace, 2012). The consequence of human along the coastline. Between El Sábalo, Punta Camarón, activities on rates of shoreline migration encourages and Punta Tiburón where anthropogenic develop- beach erosion. Because most of the structures erected ments are heavy, the yearly erosion rates are minimal −1 along the coastline are permanent, this indicates that −0.5 ± 1.0 m year (Figure 4) but the wear and tear of the impacts of these activities will be persistent and the land-based assets along these sections of this coastal long-lasting with continuous influence on both the ero- environment are obvious (Figure 5). sion of shoreline beaches and dilapidation of landward At Cerritos area, on the other hand, the yearly rates −1 assets (Figure 5(d) and (f )–(l)), if no strategic decisions of erosion are higher −1.0 ± 1.5 m year (Figure 4) are made on intervention policy. Although, alongshore but there is no evidence of wear and tear on the landed erosion is prevalent in this coastline, the impacts on assets, except the incursion of coastal sediments on coastal landed assets are localized. Some of the present the landed properties, clogging of stairways of proper- assets which bear the brunt of erosion are because of ties with coastal sediments, etc. (e.g., Figure 5(a)–(c)). cumulative years of erosion rates. However, whether the effects of shoreline migration GEOLOGY, ECOLOGY, AND LANDSCAPES 111 inland have erosional effects on the human assets or such properties are to the tide (Urbina, 2016), this not, one thing that is obvious is that the shorelines shows that the likely future depreciation or appre- in this study site are experiencing a coastal squeeze, ciation in value of any landed properties depend on reinforced by both the natural processes (e.g., sea level the closeness of such to shorelines. Key lessons here rise, storm surges, increasing wave, tidal flooding, is that the decisions and policy-makers, henceforth, etc.) and anthropogenic interventions in most areas, should be taking into consideration the future trend except the sections where there are sparse levels of in shoreline movement in the formulation of policy infrastructural developments at the nearshore. This for shoreline and coastal management, the erection of sort of human influence is known to control the geo- human infrastructures along/within the coastal envi- morphic regime along sandy tropical coastal environ- ronment. If these are not critically considered as soon ment, not only interfering in sediment movement and as possible, it is not only the coastal geomorphological budget, but in encouraging erosion through sediment system that will be permanently affected but also the starvation (e.g., Blum & Roberts, 2009; Brown et al., landed assets which contribute to the disruption of the 2017). coastal geomorphic system. As warned recently, this kind of policy consideration need to happen as fast as possible because of the possibilities of economic 4.3. Implications collapse of coastal assets in the face of sea level rise It is confirmed that seas and oceans, all over the world, and coastal erosion/flooding, which are perennially are rising faster in this and last century than at any real and could be worse than the bursting of property/ other time of the last 28 centuries, principally because financial markets crises of 2000 and 2008 (Urbina, of greenhouses gases from human emissions (Kopp et 2016). These will affect the whole economy of the al., 2016). The effects of this kind of phenomenon are tropical coastal communities, not only of the property already attracting media attention in other parts of the owners, property developers, mortgage lenders, finan- world (e.g., Gillis, 2016; Strauss, 2016). For example, it cial institutions but to all and sundry who depend on was reported that tidal flooding in places like Miami the benefits that the coastal regions offer, for example Beach, Charleston, and Norfolk are becoming a routine tourism. which are making life miserable for their inhabitants and communities (Gillis, 2016). With the projection of 5. Conclusion continuing sea level rise in this twenty-second century (Kopp et al., 2016), the rates of shoreline erosion will As no natural environment has become more affected increase and may be far worse, not only for Mazatlán, by human activity at the contemporary timescale but for many coastal communities and cities world- than the coastal environment, this study was aimed wide, likely resulting in the abandonment of those at investigating geomorphic response at the tropical cities (Gillis, 2016). Mazatlán coast is thriving today coastline of Mazatlán, Mexico, in the context of natu- because of the growth of tourism, which is encouraging ral and anthropogenic influence. Shoreline positions the developments of diverse facilities along the coast. If investigated through DSAS showed that there is a strategic actions and policies are not put in place to give continuous and persistent landward movement of room for the expected and continuous sea level rise, shoreline positions in Mazatlán in the last five years. increasing wave and storm surges, etc. and the result- Changes in coastal behaviour, here, are attributed ant landward shoreline movement, the coastal environ- widely to strong coastal forcings which are obviously ment will soon start witnessing perennial saltwater/ compelling the landward movement of shoreline posi- coastal flooding, clogging of drains with sea sediments, tions, and also the lack of/inadequate accommodation blocking of the nearby roads and streets with seawater space for sediment movement, occasioned by landed and sands, and depreciation of the landward assets. assets alongshore. These are not only encouraging One legitimate concern in support of the debate for erosion but also causing the depreciation of landed societal response for coastal defence/protection is the assets. With the projection of continuing sea level perceived and the real threat of coastal flooding and rise, increasing storm surges, etc. in this twenty-first erosion to the human infrastructure (Penning-Rowsell century, there is the urgent need for the decision et al., 2013). and policy-makers to factor-in this reality in coastal Although this fear may be justified, it is a pointer to and shoreline management, and in the allocation of the fact that many of these infrastructures are in haz- coastal land to the property developers. It is not only ardous locations and thereby have detrimental effects the geomorphic system along the coasts that will be on coastal landscape, coastal ecosystems, and coastal permanently affected, the anthropogenic structures habitat (Cooper et al., 2016). With real estate agents will not be spared also – both physically and eco- considering how close a property is to the water edge nomically – if no consideration is given to the reality before making efforts for the sales and the worth of a of shoreline changes in this area and other similar property, and the buyers now concerned on how far tropical coastal communities. 112 T. D. T. OYEDOTUN ET AL. Camacho-Valdez, V., Ruiz-Luna, A., Ghermandi, A., & Acknowledgements Nunes, P.A.L.D. (2013). Valuation of ecosystem services This work was undertaken as part of the postdoctoral provided by coastal wetlands in northwest Mexico. Ocean research programme of T. D. T. Oyedotun under the sponsor- & Coastal Management, 78(2013), 1–11. ship of Mexico’s National Council on Science and Technology Camacho, V.V., Ruiz-Luna, A., & Berlanga-Robles, A.C. (CONACYT) and The World Academy of Sciences (TWAS) (2016). Effects of land use changes on ecosystem services – for the advancement of science in developing countries; value provided by coastal wetlands: Recent and future 2015 CONACYT-TWAS Postdoctoral Fellowship Award landscape scenarios. Journal of Coastal Zone Management, at Centro de Investigación en Alimentación y Desarrollo 19, 418. doi:10.4172/jczm.1000418 (CIAD). CONACYT and TWAS are sincerely appreciated for Castelle, B., Guillot, B., Marieu, V., Chaumillon, E., Hanquiez, this opportunity. V., Bujan, S., & Poppeschi, C. (2018). Spatial and temporal patterns of shoreline change of a 280-km high-energy Disclosure statement distrupted sandy coast from 1950 to 2014: SW France. Estuarine, Coastal and Shelf Science, 200, 212–223. No potential conflict of interest was reported by the authors. doi:10.1016/j.ecss.2017.11.005 Cooper, J.A.G., O’Connor, M.C., & Mclvor, S. (2016). Coastal defences versus coastal ecosystems: A regional appraisal. Funding Marine Policy. doi:10.1016/j.marpol.2016.02.021i This work was supported by TWAS-CONACYT [grant num- Davidson, M.A., Turner, I.L., Splinter, K.D., & Harley, ber 3240287517]. M.D. (2017). Annual prediction of shoreline erosion and subsequent recovery. Coastal Engineering, 130, 14–25. doi:10.1016/j.coastaleng.2017.09.008 ORCID Dickson, M.E., Walkden, M.A., & Hall, J.W. (2007). Systemic impacts of climatic change on an eroding coastal region Temitope D. Timothy Oyedotun    http://orcid. over the twenty-first century. Climatic Change, 84, 141– org/0000-0002-3926-0358 166. doi:10.1007/s10584-006-9200-9 Dodet, G., Bertin, X., & Taborda, R. (2010). Wave climate variability in the North-East Atlantic Ocean over the last References six decades. Ocean Modelling, 31, 120–131. Absalonsen, L., & Dean, R.G. (2011). Characteristics of the Enfield, D.B., & Allen, J.S. (1980). 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Contemporary shoreline changes and consequences at a tropical coastal domain

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GeoloGy, ecoloGy, and landscapes, 2018 Vol . 2, no . 2, 104–114 https://doi.org/10.1080/24749508.2018.1452483 INWASCON OPEN ACCESS Contemporary shoreline changes and consequences at a tropical coastal domain a,b a a Temitope D. Timothy Oyedotun   , Arturo Ruiz-Luna and Alma G. Navarro-Hernández a b c entro de Investigación en alimentación y d esarrollo (cIad ) a.c., Mazatlán, Mexico; Faculty of earth and environmental s ciences, l eslie c ummins Building, University of Guyana, Turkeyen c ampus, Georgetown, Guyana ABSTRACT ARTICLE HISTORY Received 4 d ecember 2017 Coastal environment is affected by diverse human and natural activities, more than any a ccepted 3 March 2018 other natural environment. The main aim of this study is to examine shoreline dynamics of the sandy beach of Mazatlán, a medium-sized tropical coastal city in north-west Mexico. This KEYWORDS paper specifically investigates the shoreline change as impacted by natural and anthropogenic c oastal processes; interferences on the Mazatlán coastline. The mean high water (MHW) shoreline positions morphodynamics; human were extracted from Landsat Images (2012–2016) and a 2016 GPS field survey data. Digital activities; erosion; dsas; Shoreline Analysis System (DSAS) was then used to investigate the dynamics of the extracted Mazatlán; coastal cities shoreline movements and the relative changes. Results showed that 96% of the coastline is −1 undergoing yearly small-scale erosion at two distinct rates. The first at −1.9 ± 0.9 m year and −1 the other at −1.4 ± 0.2 m year , which are noted at Cerritos and other sections of the coastline, respectively. Changes in the coastal behaviour, here, are attributed widely to suspected sea level rise; increasing tidal range in the region; and the lack of or inadequate accommodation space for sediment movement occasioned by landed assets alongshore. These factors are not only encouraging erosion but also causing the depreciation of landward assets. 1. Introduction different timescales and processes involved in shoreline variability make it unsolvable challenge to many coastal Intense conflicts between the natural environment and managers and policy-makers (Stive et al., 2002). The ina- human activities are reported widely in literature (e.g., bility to understand and predict shoreline variability can Brown et al., 2013, 2017; Larsen, 2016; Rhoads, Lewis, result in the misinterpretation of coastal change scenar- & Andresen, 2016; Waters et al., 2016). Perhaps no nat- ios, and ae ff ct (directly or indirectly) decision-making ural environment is more ae ff cted by human activity at and subsequent design of intervention plans (Stive et the contemporary (shorter) timescale than the coastal al., 2002). So far, the variability in shoreline position environment (Brown et al., 2017). The coastal environ- remains a reliable proxy to describe the overall coastal ments have served as a viable source of many valuable or beach changes (Absalonsen & Dean, 2011; Bouvier resources for man’s economic, social, and recreational et al., 2017; Oyedotun, 2016, 2017). Shoreline dyna- activities (Cai, Su, Liu, Li, & Lei, 2009). Consequently, micity/variability has oen b ft een used to study coastal coastal geomorphology and processes are impacted by changes at short (days to seasons, e.g., Pearre & Puleo, anthropogenic influence, directly or indirectly (Blum & 2009; Stive et al., 2002) or long (decades to centuries, Roberts, 2009; Brown et al., 2017), in addition to the e.g., Harley, Turner, Short, & Ranasinghe, 2010; Goble & mirage of natural environmental forcings, thereby pre- MacKay, 2013) timescales. Analysis of shoreline change senting acute challenges within this natural environment is, thus, a well-established field (Burningham & French, (Teasdale, Collins, Firth, & Cundy, 2011). Shoreline is, 2017). However, the understanding of the change is perhaps, the most basic indicator of changes in coastal oen m ft ade easy by provision of multi-temporal data environment and, by implication, erosion, deposition, and robust quantification of the trends in the shoreline and subsequent recovery (Bouvier, Balouin, & Castelle, behaviour (e.g., Garcin et al., 2016). Contemporary 2017; Davidson, Turner, Splinter, & Harley, 2017; Kroon shoreline change is the short-term behaviour (annual et al., 2007; Oyedotun, 2016; Phillips, Harley, Turner, to decadal timescale) of the shoreline positions, espe- Splinter, & Cox, 2017; Robinet et al., 2016). cially as it relates to either the hydrodynamics processes Our ability to understand shoreline variability (e.g., Hapke, Himmelstoss, Kratzmann, List, & Thieler, remains one of the core issues in nearshore science as 2011; Hapke, Plant, Henderson, Schwab, & Nelson, 2016; CONTACT Temitope d. Timothy o yedotun temitope.oyedotun@uog.edu.gy © 2018 The a uthor(s). published by Informa UK limited, trading as Taylor & Francis Group. This is an open a ccess article distributed under the terms of the creative c ommons a ttribution license (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. GEOLOGY, ECOLOGY, AND LANDSCAPES 105 Hapke et al., 2010) or the local sediment budgets (e.g., Peraza-Vizcarra (1986) observed the net movement of 3 −1 Bradbury, Cope, Wilkinson, & Mason, 2013; Pye & Blott, sediment of 214,609 m yr south-eastward occasioned 2016). by changes in wave regimes, while Montaño-Ley and e m Th ain purpose of this study is to analyse con- Gutiérrez-Estrada (1987) found erosional volumes of temporary small-scale changes on the sandy beach of sediment with the maximum erosion at 15.7  m per Mazatlán, a medium-sized tropical coastal city in north- metre of length of the beach and Montaño-law et al. west Mexico. Based on the recent image analyses and (1988) observed an 8.5 m of eroded sediments along the fieldwork, this paper specifically investigates the shore- beach. Mazatlán coastline is characterized by mixed tide line change as impacted by the natural interferences and with an average range of about 1.0 m, a prevailing NW examines the effects of this on anthropogenic presence wind and occasional tropical storms migrating along the at the coastal environment. Although consideration of Pacific Coast of Mexico from SW striking the Mazatlán historical changes can show long-term effects and conse - city (Montaño-Ley et al., 2008). quences of the interactions along the coastline. However, Mazatlán city is with a population of around using Mazatlán coastline as the template for tropical 400,000, a population density of 173 per km and 87% regions, the findings presented in this study strongly of infrastructures that support tourism, fishing, indus - suggest that we do not have to consider a distant past trial, and other diverse economic activities (INEGI, nor the large-scale measurement to be able to infer the 2010). The city has a high pressure on the natural envi- pattern and effects of these nature–human interactions. ronment (Camacho, Ruiz-Luna, & Berlanga-Robles, Indeed, the implications of the continuous and contem- 2016). Despite the economic well-being and financial porary small changes suggest that rapid attention to this benefits of the wetland coastal systems of Sinaloa State small-scale investigation is highly essential if we want to to the local communities (Camacho-Valdez, Ruiz- avert long-term and permanent damage in the future. Luna, Ghermandi, & Nunes, 2013), the coastline has been subjected to high risks due to increasing land use changes which affect the supply and quality of 2. Mazatlán coastline this systematically transformed coastal environment Mazatlán, a southern city in the State of Sinaloa, is located (Camacho et al., 2016). Indeed, land use and land cover on the 23°5′ and 23°19′ North and 106°17′ and 106°30′ changes in Mazatlán and the entire Sinaloa State, like West (Figure 1). The city is a mostly sandy tropical other tropical coastal cities, have grown over the last coastline oriented NW–SE, covering a length of approx- decades with noticeable urban growth at the expense imately 20 km. The earlier studies of Montaño-Ley and of natural vegetation and agriculture (Ruiz-Luna & Figure 1.  Mazatlán showing the recent shoreline positions and profile areas shortlisted for analyses in this study. Inset: Map of Mexico showing the state of sinaloa.  s ource: The data for the map was extracted from the GadM database (www.gadm.org), version 2.5, november 2015. 106 T. D. T. OYEDOTUN ET AL. Berlanga-Robles, 2003). With increase in the popula- (Red Green Blue) Colour composites of 543 (for Landsat tion of this tropical city is the increase in the demands TM and ETM+) and 652 (for Landsat OLI images) were for the use of the coast and the nearshore waters for then applied on the images to enhance the objects and recreation, tourism, commercial, and other human distinguish clearly between soil, land, and water. The uses. Interplay of the increasing human activities and distinguished shoreline position was then extracted by natural forcings, is making the coastal management in digitizing and imported into DSAS module in ArcGIS this part of the world to be problematic for the coastal environment. managers. The findings from the Mazatlán coastline Position accuracy of the 2016 shoreline (extracted are expected to serve as baseline information on the from Landsat Image) was confirmed with the global trend of this form of interactions in the State of Sinaloa, positioning system (GPS) Field Survey carried out Mexico, and the entire tropical region in providing a alongshore the study site using the Garmin GPS Model scientific guidance on coastal management and policy 120 (programmed to measure coordinates in UTM 13 N decision-making. continuously as we walk along the beach) between 25 October and 9 November 2016. Digital Shoreline Analysis System (DSAS) (Thieler, 3. Methods Himmelstoss, Zichichi, & Ergul, 2009) was then used Shoreline geometry is one of the basic indicators in eval- to investigate the dynamics of shoreline movements and uating coastal changes and this has been used exten- the relative changes at shorter scale along the Mazatlán sively in investigating historical trend/pattern of coastal beach. Although the utilization of DSAS is widely used dynamics (Oyedotun, 2014). Differences in historical in Historical Trend Analysis (e.g., Jabaloy-Sánchez et al., and recent rates of change along a coastline are mostly 2013; Oyedotun, 2014, 2016), it has also been applied reflected in shoreline movements, and here, the mean for short-term investigations of shoreline variations and high water (MHW) shoreline positions (Boak & Turner, short-term coastal changes (e.g., Hapke, Kratzmann, & 2005) were analysed for the shoreline changes. The wet/ Himmelstoss, 2013). dry lines along the beach are used as proxies for the Shoreline change analyses were performed through position of MHW (Boak & Turner, 2005). These lines, the generation of shore-normal transects at 50 m inter- perpendicular to the coast, were extracted from Landsat vals along the open coast of Mazatlán from Punta Imageries covering the period from 2012 to 2016 aer ft Cerritos to Punta Tiburón (Figure 1). A range of statis- they have been pre-processed through the application tical analysis were calculated for each of the transects at of radiometric calibration and atmospheric correction 99.5% Confidence Interval, applied to the minimum of in ESRI© ArcGIS, following the guidance described four (4) shoreline intersection threshold. Here, specifi- in USGS instruction guides USGS (2016a, 2016b). All cally, results of Shoreline Change Envelope (SCE), Net the images were collected almost at the same time in Shoreline Movement (NSM), Linear Regression Rate summer season in good quality so as to eliminate the (LRR), and End Point Rate (EPR) of change (Oyedotun, effects of sea level rise and waves (Vu, Lacroix, a Th n, 2014, 2016; Thieler et al., 2009) are presented. SCE meas - & Nguyen, 2017). The Landsat data sourced from U.S. ured the total change in position of all the shorelines Geological Survey, USGS, (www.glovis.usgs.gov) are under consideration; NSM, the distance between the Landsat 7 (Enhanced Thematic Mapper, ETM) and 2012 and 2016 shoreline positions; LRR determined the Landsat 8 (Operational Land Imager, OLI – er Th mal rate of change statistic by fitting the least square regres- Infrared Sensor, TIRS) 30 m spatial resolution images sion to all the shorelines at each of the transects while at World Reference System (WRS) path/row 31/44. For EPR in this study was derived by dividing the total shore- the Landsat 7, the image was acquired on 17 September line movement by the time period considered (5 years 2012 while that of Landsat 8 images were acquired on in this study). The Mazatlán coastline, for this study, 12 September 2013, 17 October 2014, 16 July 2015, and was divided into four sections (Figure 1) to allow for a 04 September 2016, respectively. more systematic analysis and interpretation of the rates e Th imageries were transformed to Geographic of shoreline dynamics. At the recent (contemporary) Projection (Universal Transverse Mercator (UTM) timescale, erosion trends are presented as negative val- WGS84 Datum, Zone 13N) before being preprocessed ues and the depositional/accretional trends as positive in ESRI© ArcGIS 10.3 for image enhancement (includ- values. The shoreline change analyses in this study are ing radiometric calibration, atmospheric correction, used as metric to evaluate the modifications alongshore gap filling, pan-sharpening) and geometric rectifica- the coastline of Mazatlán on a local scale. Assessment tion to eliminate imageries defects (e.g., wedge-shaped and interpretation of the broader influence of shoreline gaps, radiometric distortion, presence of noise, etc.) movements was modified aer des ft cription by Hapke et (Lillesand, Kiefer, & Chipman, 2008). To extract the al. (2011) while the consequence of this influence on the exact shoreline from the images, a non-linear edge-en- system and adjoining anthropogenic presence on the hancement technique was performed in MATLAB to coast, is from the field observation and insights provided delimit the land–water boundary of the images. RGB by Cooper, O’Connor, and Mclvor (2016). GEOLOGY, ECOLOGY, AND LANDSCAPES 107 shoreline movement (landward) is mostly between 5 4. Results and discussion and 1  m (Figure 2(a)). There was, also, interruption 4.1. Contemporary shoreline dynamics of low volume of (~ −1  m) erosion at a few discreet The shoreline change metrics and trends (NSM, locations especially in areas where the envelope of var- SCE, and time-averaged LRR) are shown in Figure 2. iability (SCE) were substantially moderate (2.1–5  m) Although the net shoreline change in some areas along (Figure 2(b)). the coastline is minimal, the overall trend of shoreline Results of the geomorphic shoreline assessment indi- movement is erosional (Figure 2(a)) with 96% of the cate that the most erosional dynamic part of Mazatlán system undergoing erosion at smalle scale (>−1 m). The coast is at Cerritos. This was the section that exhibits subsection with highest negative erosion net rate, for the alternate stretches of high yearly rates of erosion of −1 the last five years, is between Punta Cerritos and El between −0.5 and 1.8  m  year (Figure 3) which cor- −1 Sábalo, to the north. This is followed by the coastline responds to maximum net retreat rate of −2  m  year between El Sábalo and Punta Camarón. At this contem- (Figure 2(c)). The retreat at this stretch of the beach porary timescale, the spatial pattern of shoreline vari- could be attributed to the availability of sediments, the ability along Mazatlán coast corresponds closely to the reshaping of available berms at the northern end of the net rate of shoreline movement (Figure 2(a) and (b)), beach or the limited coastal defence. Pocket of shoreline that is, NSM and SCE correspond. This indicates that progradation sparsely occurred around El Sábalo, Punta continuous and persistent change in shoreline position Camarón, and Punta Tiburón sections (Figure 2(c)) but is prevalent in the last five years. The consistent ero- these did not match the overall magnitude of persis- sional rates of the shoreline are significant here as the tent erosion rates of change in almost these sections. NSM and SCE are exactly and effectively the same for e p Th ockets of shoreline accretion rates, here, can be the erosion hotspots, which are predominantly between associated with the lateral alongshore movement and Punta Cerritos and El Sábalo section than other sec- deposition of eroded sediment at the spot of occur- tions of the coastline. Across much of the Mazatlán rence. However, the 20 km stretch coastline was largely coast, the aggregate amount of shoreline erosion was marked by erosion, although with variation of yearly between 5 and 7 m where the MHW shoreline positions rate (Figure 3). Progradation are sparsely localized at the −1 shifted landward. This notable change was highest in rate of < 0.5 m year which did not match the overall Cerritos area. In other section of the beach, the total erosion observed. Figure 2.  shoreline change metrics and trends for Mazatlán coastline, showing (a) net shoreline Movement (nsM), (b) shoreline change envelope (sce), and (c) average rate of change, linear Regression Rate (lRR).  s ource:  Georeferenced l ocation data was extracted from the GadM database (www.gadm.org), version 2.5, november 2015. 108 T. D. T. OYEDOTUN ET AL. Figure 3. yearly rate of change (epR) along Mazatlán coast, from punta c erritos to punta Tiburón. Comparison of retreat rates at this contemporary rise (e.g., Zhang, Douglas, & Leatherman, 2004). The scale (Figure 4) showed signic fi ant die ff rences in behav - insights from the examination of temporal Mazatlán iour between all the sections. The envelope of variability shoreline dynamics at the recent times (Figures 2 and (SCE) is highly obvious in Punta Cerritos (Quartile 1 3) are pointers to the influence of strong coastal forcings (Q1) where the 25% of the distribution is 7 m; Q3 (75% which are obviously compelling the landward movement of the distribution) is 10 m; median is 8 m and the high- of shoreline positions (e.g., Castelle et al., 2018; Hapke est value is 15.2 m). SCE at El Sábalo is also high (Q1 is et al., 2016). Although this study did not examine the 6.3 m; Q3 is 8.4 m; median is 7 m; and the highest value exact coastal forcing evidence compelling the observed is 9.6 m), moderate at Punta Camarón (Q1 is 2.7 m; Q3 change, the recent investigations of sea level rise in the is 6.2 m; median is 3 m; and highest value of 9.6 m) and region (e.g., Kopp et al., 2016; Lluch-Cota et al., 2010; low at Punta Tiburón (Q1 is 3  m; Q3 is 5  m; median Páez-Osuna et al., 2016; Ruiz-Fernández et al., 2016) is 4  m; highest value of 8.7  m) (Figure 4(a)). Despite suggest the ostensible rise in the sea level and storm the envelope of variability being strong in the far north surge. sections of this system, the yearly erosional rates are also Sea level rise is an important forcing along the Pacific higher at this area than the other three sections (that coastline of both North and South America (Enfield is, at El Sábalo, Punta Camarón, and Punta Tiburón, & Allen, 1980; Páez-Osuna et al., 2016). Indeed, the respectively) (Figure 4(b)–(d)). increase in water levels or surge in sea levels is docu- Two distinct yearly rates of erosion are evidence in mented significant drivers of sedimentary shorelines −1 this coastline. The first is at −1.9 ± 0.9 m year and the movements, whether at short- or long-term scale (Páez- −1 other which hovers at −1.4 ± 0.2 m year are the main Osuna et al., 2016). The coherence in the consistent ero - bulk of the erosional distribution. The first is noted sional behaviour of Mazatlán coastline can possibly be at Cerritos while the later at the other sections of the linked to a gradual rising sea level and relative stormy system (Figure 4(b)). Alongshore sequence of shoreline swell as the localized analyses presented here suggest. movement at the central and southern parts indicated This kind of assertion should, however, be made with that the average shoreline rates of erosion spread over caution as there are many other forces (waves, weather, −1 −0.5  ±  1  m  year (Figure 4(c)) at El Sábalo, which etc.) for example, elevate surge levels (e.g., Woodworth, −1 is lower than at Punta Camarón (−0.5 ± 0.0 m year ) Flather, Williams, Wakelin, & Jevrejeva, 2007), high −1 and Punta Tiburón (−0.7 ± 0.7 m year ), respectively. waves (e.g., Dodet, Bertin, & Taborda, 2010), strong There is a minimal accretion at El Sábalo and Punta winds (e.g., Burningham & French, 2013), etc. which Tiburón. This observed average rates of shoreline have the potentials in enforcing coastal change in this movement corresponds with the total shoreline migra- shoreline or any other similar tropical shorelines of tion, with only El Sábalo having a cumulative accretion the world. This caution is highly needed based on the of ≥1.8  m. The envelopes of change associated with observation by Schumm and Lichty (1965) that shorter migration of shoreline movement at Punta Camarón timescale changes are intrinsically and essentially linked are far wider than the one experiences at other sections with stringent cause–effect association at smaller spatial (Figure 4(a)). scales, like Mazatlán, than at a broader and wider spatial Changes in coastal behaviour are attributed widely to scale or at longer term. As an example, in the 1970s, long-term tidal cycles (e.g., Gratiot et al., 2008), climate anomalies of monthly sea level, coastal sea surface tem- change (e.g., Nicholls & Cazenave, 2010) or sea level perature and alongshore wind force were modifying the GEOLOGY, ECOLOGY, AND LANDSCAPES 109 the impacts of coastal forcing obvious. These anthro- pogenic instances are leading to shoreline planform squeeze with glaring effects on the coastline and the landed assets alongshore. This study did not, however, investigate whether there is (or there is no) any evidence of geological control on the shoreline behaviour as it is the norm with similar studies (e.g., Burningham & French, 2017). 4.2. Anthropogenic influences and consequences As common with any coastal areas with beehive of tour- ism activities, coastal defences are the main key struc- tural control on the coastal morphodynamics along Mazatlán coastline. One of the main objectives of this study is the examination of the influence of the contem - porary changes in the modifications of anthropogenic presence in this study area. Most Punta Cerritos – El Sábalo section of the coastline is characterized as having moderate development (aer H ft apke et al., 2011) as there are moderately spaced, privately owned landed/family properties and houses which are not clustered in this sec- tion. Here, there is little to limited tourist infrastructures, no massive commercial development projects except sparse hotels and there are some open spaces between the communities of family homes. e a Th nthropogenic infrastructural development levels between El Sábalo and Punta Camarón, on the other hand, can be described as dense (aer H ft apke et al., 2011, 2013) as there are a sizeable number of single-family houses, hotels, continuous communities of buildings, sizeable hotels, a good number of tourist infrastruc- tures and commercial buildings. Developments between Punta Camarón and Punta Tiburón can be described as heavy as there are conspicuous and predominant mul- ti-story buildings, hotels, diverse condominium com- plexes, various tourist infrastructures alongshore, and visible commercial properties at this section of the coast- line. e Th visual assessment of the influence and conse- quence of the yearly rates of change along the different sections of Mazatlán coastline are obvious. The expo- sure of this coastline to the influences of sea level rise and other coastal forcings like tidal influences and wave energy (e.g., Dickson, Walkden, & Hall, 2007; Nicholls & Cazenave, 2010; Woodroe ff & Murray-Wallace, 2012 ) results in the landward movement of the shoreline – the effects of which are visible on not only to the landed Figure 4.  classification of shoreline statistics and behaviour properties along the coast but also the coastal defence based on each of the four sections of the coastline. I – punta c erritos; II – el s ábalo; III – punta c amarón; IV – punta Tiburón. structures (Figure 5). The consequences of these struc- (a) – sce; (b) – epR; (c) – lRR, and (d) – nsM. tures are not only observable in the inhibition of the natural response of the beach and coastline to natural structures and dynamics in coastal zones in this Pacific processes like storms, they also increase erosion and dis- region (Enfield & Allen, 1980). ruption of alongshore sediment movement (Figure 5), e co Th ntrasting mode of coastline behaviour in this the phenomena which are also common in other coastal area is amplified by the presence of anthropogenic influ - areas (e.g., Hapke et al., 2013). This is also causing the ence exacerbating the contraction and thereby making wearing and tearing of the coastal defence structures and 110 T. D. T. OYEDOTUN ET AL. Figure 5. examples of effects and consequences of shoreline erosion on landward assets alongshore Mazatlán coast. (a)–(c) (p unta c erritos), (d)–(f ) (el s ábalo), (g)–(i) (punta c amarón), and (j)–(l) (punta Tiburón). the overflow of landward assets with coastal sediments This suggests that other assets not yet ae ff cted, may (Figure 5(a)–(c) and (e)). be in danger of erosional force in a period not far from Shorelines erode principally by natural imprints like now. e Th results of this study have, however, shown that coastal storms, sudden weather-related events, sea level human modifications along coastlines influence shore- rise, changes in sediment supply, and human imprints/ line change both historically and contemporarily, and modifications like beach nourishment, engineering are also influenced by the coastal processes. From this structures (e.g., Absalonsen & Dean, 2011; Dickson study, it can be inferred that the rates of shoreline change et al., 2007; Nicholls & Cazenave, 2010; Woodroe & ff are highly dependent on the level of human interference Murray-Wallace, 2012). The consequence of human along the coastline. Between El Sábalo, Punta Camarón, activities on rates of shoreline migration encourages and Punta Tiburón where anthropogenic develop- beach erosion. Because most of the structures erected ments are heavy, the yearly erosion rates are minimal −1 along the coastline are permanent, this indicates that −0.5 ± 1.0 m year (Figure 4) but the wear and tear of the impacts of these activities will be persistent and the land-based assets along these sections of this coastal long-lasting with continuous influence on both the ero- environment are obvious (Figure 5). sion of shoreline beaches and dilapidation of landward At Cerritos area, on the other hand, the yearly rates −1 assets (Figure 5(d) and (f )–(l)), if no strategic decisions of erosion are higher −1.0 ± 1.5 m year (Figure 4) are made on intervention policy. Although, alongshore but there is no evidence of wear and tear on the landed erosion is prevalent in this coastline, the impacts on assets, except the incursion of coastal sediments on coastal landed assets are localized. Some of the present the landed properties, clogging of stairways of proper- assets which bear the brunt of erosion are because of ties with coastal sediments, etc. (e.g., Figure 5(a)–(c)). cumulative years of erosion rates. However, whether the effects of shoreline migration GEOLOGY, ECOLOGY, AND LANDSCAPES 111 inland have erosional effects on the human assets or such properties are to the tide (Urbina, 2016), this not, one thing that is obvious is that the shorelines shows that the likely future depreciation or appre- in this study site are experiencing a coastal squeeze, ciation in value of any landed properties depend on reinforced by both the natural processes (e.g., sea level the closeness of such to shorelines. Key lessons here rise, storm surges, increasing wave, tidal flooding, is that the decisions and policy-makers, henceforth, etc.) and anthropogenic interventions in most areas, should be taking into consideration the future trend except the sections where there are sparse levels of in shoreline movement in the formulation of policy infrastructural developments at the nearshore. This for shoreline and coastal management, the erection of sort of human influence is known to control the geo- human infrastructures along/within the coastal envi- morphic regime along sandy tropical coastal environ- ronment. If these are not critically considered as soon ment, not only interfering in sediment movement and as possible, it is not only the coastal geomorphological budget, but in encouraging erosion through sediment system that will be permanently affected but also the starvation (e.g., Blum & Roberts, 2009; Brown et al., landed assets which contribute to the disruption of the 2017). coastal geomorphic system. As warned recently, this kind of policy consideration need to happen as fast as possible because of the possibilities of economic 4.3. Implications collapse of coastal assets in the face of sea level rise It is confirmed that seas and oceans, all over the world, and coastal erosion/flooding, which are perennially are rising faster in this and last century than at any real and could be worse than the bursting of property/ other time of the last 28 centuries, principally because financial markets crises of 2000 and 2008 (Urbina, of greenhouses gases from human emissions (Kopp et 2016). These will affect the whole economy of the al., 2016). The effects of this kind of phenomenon are tropical coastal communities, not only of the property already attracting media attention in other parts of the owners, property developers, mortgage lenders, finan- world (e.g., Gillis, 2016; Strauss, 2016). For example, it cial institutions but to all and sundry who depend on was reported that tidal flooding in places like Miami the benefits that the coastal regions offer, for example Beach, Charleston, and Norfolk are becoming a routine tourism. which are making life miserable for their inhabitants and communities (Gillis, 2016). With the projection of 5. Conclusion continuing sea level rise in this twenty-second century (Kopp et al., 2016), the rates of shoreline erosion will As no natural environment has become more affected increase and may be far worse, not only for Mazatlán, by human activity at the contemporary timescale but for many coastal communities and cities world- than the coastal environment, this study was aimed wide, likely resulting in the abandonment of those at investigating geomorphic response at the tropical cities (Gillis, 2016). Mazatlán coast is thriving today coastline of Mazatlán, Mexico, in the context of natu- because of the growth of tourism, which is encouraging ral and anthropogenic influence. Shoreline positions the developments of diverse facilities along the coast. If investigated through DSAS showed that there is a strategic actions and policies are not put in place to give continuous and persistent landward movement of room for the expected and continuous sea level rise, shoreline positions in Mazatlán in the last five years. increasing wave and storm surges, etc. and the result- Changes in coastal behaviour, here, are attributed ant landward shoreline movement, the coastal environ- widely to strong coastal forcings which are obviously ment will soon start witnessing perennial saltwater/ compelling the landward movement of shoreline posi- coastal flooding, clogging of drains with sea sediments, tions, and also the lack of/inadequate accommodation blocking of the nearby roads and streets with seawater space for sediment movement, occasioned by landed and sands, and depreciation of the landward assets. assets alongshore. These are not only encouraging One legitimate concern in support of the debate for erosion but also causing the depreciation of landed societal response for coastal defence/protection is the assets. With the projection of continuing sea level perceived and the real threat of coastal flooding and rise, increasing storm surges, etc. in this twenty-first erosion to the human infrastructure (Penning-Rowsell century, there is the urgent need for the decision et al., 2013). and policy-makers to factor-in this reality in coastal Although this fear may be justified, it is a pointer to and shoreline management, and in the allocation of the fact that many of these infrastructures are in haz- coastal land to the property developers. It is not only ardous locations and thereby have detrimental effects the geomorphic system along the coasts that will be on coastal landscape, coastal ecosystems, and coastal permanently affected, the anthropogenic structures habitat (Cooper et al., 2016). With real estate agents will not be spared also – both physically and eco- considering how close a property is to the water edge nomically – if no consideration is given to the reality before making efforts for the sales and the worth of a of shoreline changes in this area and other similar property, and the buyers now concerned on how far tropical coastal communities. 112 T. D. T. OYEDOTUN ET AL. Camacho-Valdez, V., Ruiz-Luna, A., Ghermandi, A., & Acknowledgements Nunes, P.A.L.D. (2013). Valuation of ecosystem services This work was undertaken as part of the postdoctoral provided by coastal wetlands in northwest Mexico. Ocean research programme of T. D. T. Oyedotun under the sponsor- & Coastal Management, 78(2013), 1–11. ship of Mexico’s National Council on Science and Technology Camacho, V.V., Ruiz-Luna, A., & Berlanga-Robles, A.C. (CONACYT) and The World Academy of Sciences (TWAS) (2016). Effects of land use changes on ecosystem services – for the advancement of science in developing countries; value provided by coastal wetlands: Recent and future 2015 CONACYT-TWAS Postdoctoral Fellowship Award landscape scenarios. Journal of Coastal Zone Management, at Centro de Investigación en Alimentación y Desarrollo 19, 418. doi:10.4172/jczm.1000418 (CIAD). CONACYT and TWAS are sincerely appreciated for Castelle, B., Guillot, B., Marieu, V., Chaumillon, E., Hanquiez, this opportunity. V., Bujan, S., & Poppeschi, C. (2018). Spatial and temporal patterns of shoreline change of a 280-km high-energy Disclosure statement distrupted sandy coast from 1950 to 2014: SW France. Estuarine, Coastal and Shelf Science, 200, 212–223. No potential conflict of interest was reported by the authors. doi:10.1016/j.ecss.2017.11.005 Cooper, J.A.G., O’Connor, M.C., & Mclvor, S. (2016). Coastal defences versus coastal ecosystems: A regional appraisal. 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Journal

Geology Ecology and Landscapes – Taylor & Francis

Published: Apr 3, 2018

Keywords: Coastal processes; morphodynamics; human activities; erosion; DSAS; Mazatlán; coastal cities

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