The medium-term analysis showed that along the 43,230 m of the Isla Salamanca coastal barrier, most of it (37,850 m) is in a process of frank transgression, with average annual rates of -4.8 m·yr− 1, with a maximum erosion of -277.6 m from the shoreline, between March 2004 and January 2021. On the other hand, adjacent to the mouth of the Magdalena River, at Boca de Ceniza, the western 5,535 m of the coastal barrier presents a maximum deposition of 499.8 m in extent (Fig. 2A and Table 1). The two specific areas of the coastal barrier studied are the areas with the highest erosion rates near the road connecting the cities of Barranquilla and Pueblo Viejo. At km 19 the average erosion rate was − 10.0 m·yr− 1, attaining a maximum erosion rate of -284 m and with the lowest retreat of -25.8 m from the shoreline (Fig. 2B and Table 1). These lower erosion rates are in the central portion of km 19, in which is due to the containment works with the construction of the seawall from the year of 2014. While at km 29 erosion rates reach a maximum of -6.2 m·yr− 1 with a shoreline retraction of -96 m (Fig. 2C y Table 1).
Sheet piling is a technique used to control coastal erosion and protect coastal infrastructure, such as Highway, bridges, and homes, from the damaging effects of storms and rising sea levels (Griggs and Reguero 2021). This technique involves placing large rocks or concrete blocks along the shoreline to reduce the force of waves and stabilize the beach (Isla et al. 2018). Fencing can be a temporary solution to protect coastal infrastructure as it does not address the underlying cause. Coastal erosion processes are affected by different causes, from eustatic or isostatic processes, to natural or anthropogenic decrease in sediment supply, to the construction of coastal infrastructures that alter the dynamics of the coast, which can act together or in isolation. However, seawall can be a quick and effective solution to reduce coastal erosion and protect coastal infrastructures in the short term (Hosseinzadeh et al. 2022). At first, the proposed containment work to protect the RN90 highway accelerated the erosion processes in the western portion of the seawall, where the highest erosion rates are found in this sector.
Compared to other coastal areas with erosional processes the Isla Salamanca coastal barrier area has a significantly elevated rate. For the case of the southern coastal zone of India a study on coastal erosional processes between 1980 to 2020, using DSAS provided a detailed assessment of coastal migration rates (Chrisben Sam and Gurugnanam 2022). The results of the study indicate that coastal erosion rates are relatively much lower than those found in this study, with an average annual erosion rate of -0.28 m·yr− 1. Meanwhile the researchers show that the rate of coastal erosion has increased significantly over the last decade, indicating that coastal erosion is an increasingly serious problem for this region of India. The researchers warn that these rates of coastal erosion can have a significant impact on coastal communities, infrastructure, and biodiversity in the region (Chrisben Sam and Gurugnanam 2022).
Table 1
Statistic summary of shoreline migration rates using the NSM (m) and EPR (m·yr− 1) methods, subdivided into total area; km 19 area; km 29 area; (see Fig. 1 for location of each sector).
Area | Total | Km 19 | km 29 |
Total number of transects | 4,323 | 2,436 | 1,814 |
Total transects that record erosion | 3,785 | 2,436 | 1,814 |
Total transects that record accretion | 535 | 0 | 0 |
Shoreline length (m) | 43,230 | 2,436 | 1,814 |
NSM | Mean mobility shoreline change (m) | -82.8 | -172.5 | -81.2 |
Maximum mobility shoreline change (m) | -277.6 | -284.0 | -96.0 |
Minimum mobility shoreline change (m) | 499.8 | -25.8 | -59.3 |
Standard deviation of mobility (m) | 88.1 | 119.1 | 1.58 |
EPR | Mean mobility shoreline change (m·yr− 1) | -4.8 | -10.0 | -4.7 |
Minimum mobility shoreline change (m·yr− 1) | -16.1 | -16.4 | -6.2 |
Maximum mobility shoreline change (m·yr− 1) | 29.0 | -5.2 | -3.4 |
Standard deviation of mobility (m·yr− 1) | 5.1 | 3.5 | 1.3 |
For the coast of Venice, Italy, the study of (Fogarin et al. 2023) show that the coast is experiencing a significant erosion rate in a short period of time and that this is due to a combination of factors, such as sea level rise and human activities. In addition, it reveals a stability in certain sectors, correlated with the large presence of coastal protection structures that stabilize the beaches, enhancing sediment deposition processes. In detail, with respect to the total length of the considered shoreline (about 83 km), 5 % of the coast is erding, 36 % is stable, 52 % isaccretionary an 7 % is not assessable.Thus, it highlights the need to take appropriate measures to address coastal erosion in the region and protect the coast and its communities from the adverse effects of shoreline change.
Also, in the study developed in Libertador Bolivar on the coast of Ecuador a mean rate of erosion of 0.64 m·yr− 1with the net transport, influenced by waves, was calculated in 470 m3·day− 1, in which littoral transport is proportional to the longitudinal component of the wave energy flow (Nativí-Merchán et al. 2021).
In this regard, as can be seen in Fig. 2A and 3, substantial accretion is occurring adjacent to the mouth of the Magdalena River. Considering the study period, the westernmost sector of the Isla Salamanca coastal barrier, next to Boca de Ceniza, there was a shoreline regression of ≈ 490m, with a maximum progradation rate of 29.0 m·yr− 1 for this sector of the barrier.
Part of the progradation in this area is linked to the demobilization of sediments along the Isla Salamanca coastal barrier by erosional processes, in which a portion of the sediments is transported by the longshore current in the direction of Boca de Ceniza. Another component that may contribute to the supply of sediment for this rapid deposition is the Magdalena River itself (Restrepo and López 2008; Torres-Marchena et al. 2023). To estimate the volume of sediment that may be being transported in the direction of Boca de Ceniza, it was calculated through the decrease between the two DSM obtained. For the Km 29 sector, 1,000 m of the total length were analyzed, obtaining a difference of -1,922.8 m3, i.e., there was a frontal erosion of the berm of -6,792.1 m3 and a deposition of + 4,869.3 m3 in the backshore (see Table S5 Supplementary material). This deposition occurs mainly in areas devoid of vegetation, creating Washover deposits (Fig. 4).
In areas with the presence of vegetation, berm erosion is less pronounced (Gedan et al. 2011; Powell et al. 2019), but no washover deposits are formed on the backshore (Figs. 5 and 6). At km 19, the quantification of the volume of demobilized sediment was performed in two different sectors, adjacent to the seawall (Fig. 2B). In sector 1 of km 19, 600 m of beach length was analyzed, with a sediment demobilization of -7,585.2 m3, being the totality of berm erosion, without any sediment accumulation in the backshore (see Table S3 Supplementary material). In contrast, sector 2 at km 19, with 560 m of beach length, there was a demobilization of -16,925.9 m3, being − 18,982.5 m3 of berm erosion and + 2,056.6 m3 of backshore deposition (see Table S4 Supplementary material).
Although the seawall is keeping the shoreline stable, it is notoriously visible that it is causing a progressive increase in erosive processes downstream of the seawall. The placement of large boulders and concrete blocks can alter the natural dynamics of the beach and negatively affect marine and coastal biodiversity. The seawalls can also impede beach recharge and coastal sediment erosion, which can affect water quality and the health of coastal ecosystems (Hosseinzadeh et al. 2022). In general, seawalls can be an effective solution to control coastal erosion and protect coastal infrastructure in the short term. However, the environmental impact must be considered and long-term solutions that address the underlying cause of coastal erosion, such as restoration of coastal ecosystems and implementation of sustainable coastal management practices, must be explored.
Loss of shoreline and risk to coastal marine biodiversity
Shoreline loss is a major threat to coastal marine biodiversity. Coastal erosion can negatively affect coastal marine biodiversity by destroying habitats and reducing water quality (Martens, 1995). Species that depend on coastal ecosystems for their survival may lose their habitats and face increased hunting and fishing pressure. In addition, the loss of the shoreline may negatively affect water quality and the health of marine fauna. The study area is part of the protected area " Isla Salamanca Highway Park", a region with great biodiversity. It has been declared an Important Bird Conservation Area and, in addition to the Ciénaga Grande de Santa Marta Flora and Fauna Reserve (contiguous area), it was declared a Ramsar Site of World Importance in 1998 and a Biosphere Reserve by UNESCO in November 2000. Salamanca is a group of small islands formed by the sediments of the Magdalena Delta, at the bottom of an ancient gulf; they are connected by small passages that form a barrier between the Ciénaga Grande de Santa Marta and the Caribbean Sea (Parques Nacionales 2023).
The maintenance of the shoreline in this region is of great importance for the preservation of the wetland ecosystem present in this sector. In addition, protected areas play a key role in sustainable development if they are managed effectively. The land-sea portion faces a wide range of impacts resulting from human activities and natural events. Climate change, for example, including the stressors of increasing average temperature and extreme weather events result in impacts that affect all dimensions of the Sustainable Development Goals (Singh et al. 2019) and can amplify anthropogenic impacts.
The construction of the road and the preventive measures taken to protect it are other causes of habitat degradation. Crossing a region of great environmental importance, such as the wetland where the study area is located, directly results in impacts from terrestrial pressures and human activities, including increased erosion and sedimentation. In addition to the anthropogenic impact, the increase in extreme meteorological events, such as the increase in storm surges, results in higher erosion rates. Thus, sea level rise is associated with flooding and loss of intertidal habitat, which negatively affects erosion prevention functions.
In addition to this, the high erosion rates observed in the study area and the predicted increase in extreme events and sea level rise, there is significant evidence of critical environmental losses, as already occurs in other parts of the Colombian Caribbean coast and in other regions (Bolívar-Anillo et al. 2019; Villate Daza et al. 2020b; Winterwerp et al. 2020; Veettil et al. 2023).
Actions to protect the coast and its ecosystems.
Rising sea levels, intensifying storms and increased human activity on the coast are among the factors that can accelerate erosion and cause the loss of beaches, dunes, and coastal wetlands, as well as the degradation of coral reefs and other marine ecosystems (Pranzini and Williams 2013). To protect the coast and its ecosystems from erosion, several actions have been implemented around the world. One option along a receding shoreline is some form of shoreline stabilization or protection. Stabilization is essentially maintaining the shoreline. Shoreline protection can be based on two types, structural such as a breakwater; or some form of shoreline protection based on the ecosystems present in the region. In conjunction, one can work with a combination of the following techniques (Kamphuis 2020).
Coastal ecosystems, such as mangroves, salt marshes, coral reefs, beaches, and dunes, provide important roles on the coast, including protection against coastal hazards such as storm surges, flooding, and erosion (Martinez et al. 2011; Portz et al. 2014; Strain et al. 2022). The role of mangroves in coastal protection is specific to the site and the local threat context (Spalding et al. 2014), and further research is needed to determine under what ecological and environmental conditions mangroves can provide coastal protection services comparable to man-made coastal infrastructure (Brathwaite et al. 2022; Strain et al. 2022).
The most appropriate solution to the problem of roads threatened by coastal erosion is to relocate the roads. Over the decades, several roads have been moved or abandoned. Yes, there are several examples of road relocation due to coastal erosion around the world. Some of the cases include: The Great Ocean Road in Victoria, Australia, which stretches for more than 240 kilometers along the southern coast of Australia, has been affected by coastal erosion on several occasions. In 2016, a relocation of a section of the road in the Great Otway National Park was carried out due to coastal erosion. The Pacific Coast highway in California, USA has been affected by coastal erosion on several sections, leading to road closures and relocations. A critical section of the highway near Big Sur was closed for more than a year due to a landslide caused by coastal erosion. The coastal road in County Kerry, Ireland that winds along the cliffs of the Atlantic Ocean in southwest Ireland, in 2014, a section of the road was relocated due to coastal erosion and landslide risk. Washington State Highway 105 in the Cape Shoal water area, coastal U.S. This road was moved several times as this area has experienced some of the highest rates of long-term coastal erosion in the U.S. (FHWA 2023).
A major problem when considering the relocation of the highway in the study area is the new design. The logical location is further inland, retreating to the coast. However, these areas also have large tracts of wetlands and mangroves. Considering these problems and the historical impacts due to the interruption of the natural connections in the hydrological system of the region, the construction of an elevated highway is proposed as an option.
Another important action is shoreline management, which involves regulating construction and land occupation along the coast to minimize human impact on coastal ecosystems. Construction regulations, the creation of protection zones, and the promotion of sustainable practices are some of the measures that can be implemented to reduce erosion and protect the coast and its ecosystems.
It is also crucial to involve local communities and coastal users in the conservation and protection of coastal ecosystems. Environmental education, the promotion of sustainable activities, and the promotion of ecotourism can help raise awareness of the importance of protecting the coast and its ecosystems (Kao et al. 2023). The question is, when will we adapt our human development to natural processes? We must be aware of the forcings acting in the coastal zone and have the habit of living in changing coastal environments. We must consider that the artificial structures present at the land-sea interface are temporary and only as a last resort consider the construction of engineered structures for coastal protection or preservation, only in heavily populated areas.