Coastal Erosion at Spratt Bight Beach, San Andrés

Abstract

San Andrés is a Colombian-Caribbean Island located 800 km from the Colombian coast. On its Eastern side there is a barrier reef formation protecting the island from offshore incident waves. Due to the protected environment created by the coral reef, sandy beaches can be formed on the East side of the island.
During normal conditions, waves come predominantly form the East (90% of the wave climate), have significant wave height of 2 m and period of 8 s. Besides, San Andrés is situated on the Caribbean hurricane route, which can cause an enormous damage to the island. The storm season at San Andrés is between October and December, which is also when major erosion events take place.
The economy of San Andrés is mostly built upon tourism, specially related to its biodiverse ecosystems and Caribbean beaches. The island’s ecological environment is composed by mangroves, seagrasses, and coral reefs, attracting a wide spectrum of fauna and flora to its ecosystems.
During the Masterplan for Coastal Erosion (PMEC), San Andrés was pointed out as a location in which coastal erosion is problematic. In a follow-up of this Masterplan, the island was elected to be part of a program in which solutions against coastal erosion would be presented. This research is part of this project, as a parallel trajectory to get a more profound understanding of the system and the possible mitigation measures that could be applied on the island.
With increasing urbanization and frequency of extreme weather events, erosion is becoming a problem with which San Andrés and its residents are repeatedly having to deal. Erosion is specially problematic for the Northern part of the island, called Spratt Bight. This region is not only the most densely populated area of the island, but also economically and touristically very important. Its beach presents periodically eroding patterns during storm seasons, when wave action drives the sediment towards the East, decreasing its beach width almost to none. A decreasing beach width has a direct negative impact on tourism, making coastal erosion in Spratt Bight not only a coastal safety problem, but also an economic issue.
This study aims to look into the main hydro- and morphological processes driving coastal erosion in Spratt Bight and, using the Building with Nature philosophy, propose a set of solutions to mitigate this problem. To reach this objective data analysis and literature research has been carried out, after which different environmental conditions were modelled using the numerical model Delft3D.
During these activities it was found that independently of its direction, waves approaching San Andrés break upon the coral reef and induce a water level set up inside the coral lagoon. The difference in water level in- and outside the lagoon generates a current and sediment transport, which is directed towards the western opening in the coral reef.
When the Northern waves approach the island (1.5% of the wave climate), the same water level set-up phenomenon is observed. However, as waves are approaching form the North, they not only break upon the reef, but are also able to enter the sheltered lagoon through the western opening in the coral reef. These waves are able to bend around the reef reaching the shore and the headland on the Northern part of the island, inducing a longshore current and a sediment transport that is southeastward directed. The result is that Northern waves are mostly responsible for a strong westward and erosive sediment transport pattern. These waves are mostly observed between October and March, which coincides with the storm season in San Andrés.
Besides, it was found that the Eastern waves are responsible for restoring the (dynamic) equilibrium profile of Spratt Bight Beach. However, this restoring force has a less strong intensity, taking more time to restore the beach than to disrupt its equilibrium.
The solutions proposed include seagrass restoration to enhance ecology, restrain sediment transport and attenuate wave heights; the beneficial reuse of dredged material, to nourish Spratt Bight Beach; and finally, the implementation of artificial coral reefs as breakwaters to prevent the newly nourished sediment to be lost from the system. Besides, artificial coral reefs enhance the ecosystem by attracting fauna and flora increasing biodiversity. All proposed solutions have a positive impact on the beaches and therefore on tourism and the economy of the island. This makes them multifunctional solutions, serving the main goal of protecting the beach while at the same time creating benefits for other functions and values in the area. Following in this way the prescriptions of the Building with Nature design approach by van Eekelen and Bouw (2020).