**4.2 Environmental problems at Santa Marta: Barranquilla area**

About 19 km west of Barranquilla (km-19 site) (for location, see **Figure 13**), severe beach erosion has threatened a section of a major coastal highway (**Figure 17**). Presently, the highway is protected by recently installed riprap revetment. However, continued beach erosion is undermining the foundation of the riprap and likely the road in the near future. Furthermore, the erosion may continue to expand westward and propagate the problem over a larger area.

#### **Figure 17.**

*Severe beach erosion at 19 km site threatening a major highway. The recently installed riprap is experiencing toe scour due to continued erosion.*

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

Analysis of time-series aerial photos from Google Earth has shown that the beach along the km-19 coast is experiencing persistent rapid erosion from 2009 to 2015, at over 15 m per year. The sediment transport gradient along this stretch of coastline is caused by the shadowing effect of the Santa Marta headland, which creates a westward increasing wave height along the km-19 site. This results in more sediment being transported westward from the km-19 site than being transported into the site. A major coastal road to the city of Barranquilla is now practically at the shoreline (see **Figure 17**). Originally, the road was several hundred meters from the shoreline. In 2009, the distance was only 115 m. The waves along this section of the coast is quite energetic nearly all year round, and the longshore sediment transport gradient resulting is induced by natural morphological conditions.

### **4.3 Environmental problems at Riohacha coastline**

The Riohacha coastline (for location, see **Figure 13**) provides an example of beach accretion and erosion associated with artificial interruption of the persistent westward longshore sand transport. **Figure 18** shows the aerial view of the Riohacha coastal line with a groin field. The groins were installed with the perspective to allow accretion of beaches.

Effectively, it is apparent that the groins at the updrift end have impounded a large amount of sand and resulted in a quite wide beach there (**Figure 19**). Several small rivers to the east and updrift of the beach contribute to the sand supply.

However, the field of these 150-m long groins nearly completely interrupted the westward longshore sand transport and resulted in severe beach erosion along a long and extending stretch of downdrift coast. **Figure 20** illustrates the erosion along the beach downdrift of the groin field. In addition, due to the depleted sand supply, the groins downdrift of the large beach failed to impound any sand. Similar sand bypassing patterns around and through a groin were found by [27] in a largescale laboratory study on interaction of a groin with longshore sand transport.

#### **4.4 Environmental problems at Costa Verde Cienega**

Costa Verde Cienega (for location, see **Figure 13**) is located within the greater shadow zone of the Santa Marta headland and at the edge of the secondary shadow zone. The waves along this stretch of the coast are low due to the shadowing effect of the Santa Marta headland. A wave-height gradient exists along this stretch of the coast due to the secondary shadowing by the headland, although the rate of

#### **Figure 18.**

*Aerial view of the Riohacha shoreline. Note the large amount of sand accumulation at the east-most three groins and little to no sand accumulation at the rest of the groins and erosion further downdrift.*

#### **Figure 19.**

*Large amount of sand accumulation updrift of the first groin. The groin is considerably higher than the beach.*

#### **Figure 20.**

*Severe beach erosion downdrift of the Riohacha groin field, exposing a crucial infrastructure to direct wave attack.*

#### **Figure 21.**

*Severe beach erosion along the Costa Verde Cienega coast, exposing a crucial pipeline (yellow marker). Note the low-profile sandbag groin and the earth-moving machine at the top of the picture. The earth-moving machine was moving sand from the upland to nourish the beach and attempting to control the erosion.*

longshore sand transport should be much smaller due to the low wave. Beach erosion here has exposed a crucial pipeline along the coast (**Figure 21**). Various low-profile groins made of sand bags were installed in an effort to control the erosion. Based on field observations, the sandbag groins have very limited and localized effect.

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

In addition to the longshore sand transport gradient induced by natural processes, anthropogenic activities also play a significant role in the beach erosion in the area. The westward longshore sand transport and a transport gradient were induced by a 140-m long groin installed between January 2011 and May 2012. Rapid sand accumulation east and updrift of the long groin occurred. By May 2012, the updrift beach had grown to about 35 m landward of the tip of the long groin. Only 5 months later in October 2012, the updrift of the long groin was filled to the tip of the groin.

The impoundment at the groin created a transport gradient at the downdrift, which further contributed to the beach erosion problems caused by the natural wave-height gradient associated with the secondary shadowing of the Santa Marta headland. Therefore, similar to the long groins at Riohacha, the downdrift beach did not benefit from sand bypassing even when the updrift beach reached the tip of the groin.

### **4.5 Coastal mitigation in northern Colombia**

The above examples, along with numerous other cases in the northern Colombia coast, strongly suggest that caution should be carefully exercised in artificially interrupting the persistent westward longshore sand transport. Impounding the longshore moving sand by engineering structures, many times by groin field, is quite easy to do. The often very rapid beach accretion at the updrift side of the structure may lead to construction of longer and more groins. This can quickly turn into a very destructive practice along the tropical trade wind-dominated coast, where longshore sand transport is westward almost 100% of the time.

Well-planned regional scale study should be developed in order to avoid these types of problems in the future. The study should investigate (1) the source and availability of the sand to the beach, (2) rate of longshore sand transport, and (3) most importantly, existing natural and anthropogenic causes of longshore sediment transport gradient. The study should have adequate temporal and spatial scales. Spatially, the study should include a long stretch of beach, particularly along the downdrift coast, to ensure that the downdrift effect induced by the shore protection structures is fully considered over a long period.
