WAVE TRANSFORMATION AND WAVE-DRIVEN CIRCULATION ON NATURAL REEFS UNDER EXTREME HURRICANE CONDITIONS

Abstract

The understanding of wave transformation and circulation on coral reef environments has profound implications for the preservation of the natural environment and also for the planning of mitigation measures which protect the coastline. Most of the Mexican Caribbean coast is fronted by reefs, and it is here where coastal development is being undertaken at an alarming rate, as tourist resorts expand and multiply. The transport and dispersion of larvae (fish and coral), nutrients, pollutants, heat, and sediments are dictated mainly by circulation processes which are, to a great extent, dominated by wave processes at fringing reefs. Wave transformation on coral reefs has received considerable attention, but the 2D effects of wave reflection and diffraction have not often been addressed despite the complex geometry and steep foreshores that characterize fringing reefs. In this contribution the effects of reflection and diffraction are explored by the application of a model that considers these two processes (WAPO/COCO) and another that does not (SWAN/DELFT3D), at Puerto Morelos on the Mexican Caribbean coast. Reanalysis data of 48 years of wave parameters show that waves of 2 m height can be considered as high energy storm conditions, which are generated mainly by passing hurricanes and to a lesser extent by northerly storms in winter. These are the conditions used to run the models using a range of wave periods (8, 10 and 12 seconds) and directions (SE, NE and NNE). The results show that the reflection-solving model is able to generate rhythmic patterns in the horizontal distribution of Hs that are not reproduced by the SWAN model. Furthermore, the patterns change considerably depending on wave period and direction. These differences are attributed to the effects of full diffraction and reflection processes. While both models perform well and reproduce previously reported circulation patterns, the differences in Hs horizontal distribution generated by the WAPO model have important implications for the circulation around the reef crests. For instance, vorticity is increased and in many occasions a different pattern from that suggested by SWAN occurs, such as a clear southward longshore current along the reef crest under northerly wave forcing. Although the circulation was calculated in both models by the same method (radiation stress gradients), the version of COCO which was used generates smaller set-up inside the reef lagoon and therefore the circulation in this region and at the outlets is underestimated. Although it is necessary to corroborate the model results with measurements of the 2D wave height patterns and circulation, the magnitude of the currents suggested by the reflection-solving model are strong enough to persist even under the presence of stronger flows inside the lagoon and at the inlets due to wave set-up gradients.