*3.2.1 Coastal mitigation in the metropolitan region of Fortaleza*

As erosion travels west, real estate owners along the shoreline try to protect their property by building coastal protection with their own resources or by pushing public power to build structures with public resources. In this way, several interventions were made in the last decades, of the rock-fill and bag-wall types.

The rock-fills protect the coast from erosion; however, it results in the loss of the beach area and in increased erosion downdrift (**Figure 6A**). The bag-wall, built in the 2010s, on the other hand, has already been destroyed by the action of the waves (**Figure 6B**), showing it completely inefficient for solving the kind of problems created by the construction of the Mucuripe harbor and by the unsustainable type of urbanization of Fortaleza city.

#### **Figure 6.**

*(A) Erosion in Icarai Beach, 25 km downdrift of Mucuripe harbor, resulting from the interventions in Fortaleza city and in the area around the harbor. The rising of the sea must also be a factor producing the erosion. (B) Bag-wall installed in Icarai Beach in the 2010s, already destroyed by the waves.*

*Environmental Problems and Coastal Mitigation in South America: Examples from Northeast… DOI: http://dx.doi.org/10.5772/intechopen.87959*

It seems clear that the erosion in the area will not cease with the kind of intervention that has been taking place in the last six decades. Effectively, it is necessary to consider that the waves are very active and that sand is missing in the shoreline. In addition, the sea level is rising. In [19], it is pointed out that the sands which are dredged from the harbor's basin could be used to mitigate the erosion, by means of artificial nourishment of the beaches, as it takes place in other countries (e.g., [20]). Vasconcelos [16] advanced in this point of view, considering that the sand of the dredging could be dispersed along the littoral of Fortaleza, at the bathymetry of 5 m. Effectively, at 5 m deep, the longshore current would naturally transport the sands to the beaches downdrift, rebuilding coastal dynamics. This measure would waive nourishment of beaches, as well as the installation of rock-fills, bag-walls or other engineering structures and actions, implicating in less cost and less disturbance of the shoreline and of the social life related to it.

Nevertheless, these suggestions have not yet been considered by public authorities. In fact, a new dredging took place in August/September 2018, and as before, the sands were transported offshore, to be deposed and lost in areas far away from the shoreline. Meanwhile, the aggressive, dramatic, disastrous, and degrading erosion and destruction of the north part of Ceará State coastal area keep taking place.

### **3.3 Environmental problems in Canoa Quebrada beach**

The town of Canoa Quebrada is located 140 km east of the city of Fortaleza and represents one of the most visited tourist destination in the coastal zone of the Northeast Brazil. It is characterized by the presence of active sea cliffs modeled in the Barreiras Formation, with the occurrence of a narrow beach in its foothills, plus beach rocks nearshore, which partially protects the sea cliffs (**Figure 7**). Clays and friable sands make up the sediments of the sea cliffs, which are bare or only partly protected by vegetation [21].

The morphodynamics in the area is very active: the strong rains that characterize the rainy season cause ravines and acute ridges, resulting in transport and deposition of sediments in the foothills, in the form of cones of dejection (**Figure 8**). The rain also contributes to the softening of the material of the cliff and to the increase of the water table, which produces dissolution in the slope and increases the internal pressure in the sediments, thus increasing the fragilization of the feature (e.g., [22]).

An important anthropogenic factor of the dynamics of the sea cliffs is the installation of numerous tourist facilities on their top and slope. These structures cause trampling and compacting of the sands, increasing ravines and generating gullies, which contributes to the acceleration of the erosive processes caused by rainwater (e.g., [23]) (**Figure 9**).

The cliffs of Canoa Quebrada have an average height of 15 m and can be considered as small- to medium-sized cliffs. However, the volume of material that is

**Figure 7.** *Reef line at Canoa Quebrada beach (adapted from Google Earth image, 2018).*

#### **Figure 8.**

*Grooves at the top and cone of dejection at the base of the sea cliffs of Canoa Quebrada.*

#### **Figure 9.**

*Occupation of the top and bottom of the sea cliffs of Canoa Quebrada.*

removed from these features is not negligible due to the high frequency of landslides. The occurrence of successive episodes of landslides is due to the combination of three factors: the friable nature of the clay-sandstone material, high anthropogenic presence, and strong tropical rainfall.

The sea cliffs present slope angle usually under 45° and are characterized mostly by the occurrence of retreat controlled by milder gravitational sliding more than by abrupt collapses (**Figures 10** and **11**). Together with the type of wave breaking, sea level rising, tidal amplitude, and sediment granulometry, the inclination directly influences the erosive process that characterizes the sea cliffs today [24–25].

The analysis of the set of points where topographic profiles were made indicates that from east to west (profiles 1,2,3, **Figure 11**), the slope presents relatively smooth landslides, while in the westernmost portion of the beach, where the profile assumes an angulation of more than 45° (profiles 5 and 6, **Figure 11**), landslides occur. The analysis of satellite images containing a time series of the years 2004, 2008, and 2017 indicates that the velocity of retreat of the Canoa Quebrada sea cliffs is very high when compared to hard rock cliffs.

Retreat velocity was calculated in three distinct sections, with the following results: in the east, the retreat was the smallest, of the order of 90 cm between 2004 and 2017, representing an average retreat of 8.2 m per century. In the central areas, which have many tourist developments, the retreat was slightly faster, of the order of 1.2 meters in 13 years, representing approximately 10.9 m per century. To the west,

*Environmental Problems and Coastal Mitigation in South America: Examples from Northeast… DOI: http://dx.doi.org/10.5772/intechopen.87959*

#### **Figure 10.**

*(A) The cliff on January 16, 2016, and to the right on February 20, 2016. (B) The retreat of the sea cliff by gravitational slip is visible in the 2016 picture.*

**Figure 11.** *Topographic profiles of Canoa Quebrada sea cliffs.*

less sheltered area in relation to the attack of the waves due to smaller presence of beach rocks, the cliff receded 1.7 m in 13 years, equivalent to 15.5 m per century. The average speed of retreat of the cliff for the studied area is of 11.5 m per century.

It is also observed that the line of rupture of the sea cliff in the image of 2017 presents recesses, with gaps and gullies that did not exist in 2004, indicating morphogenetic processes acting differently over the years. In 2004, the performance of the marine processes at the base of the cliff was dominant, with more intense and uniform retreat of the slope. The presence of retreat in the image of 2017 indicates that there was a change in the relation of forces between the active processes, being now the subaerial processes more intense than the marine ones. In synthesis, it is concluded that the cliffs rapidly recede by the combined action of wave attack, rising sea levels, rainfall, and human action.

**Figure 12.** *Tourist equipment installed very close to the unstable sea cliffs, running the risk of being buried by landslides.*

The average retreat velocity of the Canoa Quebrada sea cliff is compatible with the soft nature of the sediments that compose it. However, it is a very high speed if we consider the presence of an important urban nucleus and many tourist facilities located near or on the edge of the cliffs.

Environmental risk prevention was never present in the urban planning of Canoa Quebrada. Natural and anthropic factors were not taken into account in the expansion of the village on the cliffs. However, the cliffs of Canoa Quebrada are fragile and present strong risks of landslides, as indicated by the survey realized and exposed above. These factors must be taken into account in planning the future use and occupation of the area, under penalty of material damages, environmental degradation, and unsafety to the public. It is also noted that in the central portion of the beach, there are ventures located very close to the foot of the cliff, running at great risk of burial by materials from the top, through the frequent landslides (**Figure 12**).

In this central area, it is strongly advisable to remove part of the existing tourist structures, thus ensuring the use of this equipment safely by tourists and inhabitants of the town.
