Abstract
Spurs and grooves (SAG) are coral structures of the fore reef slopes that can be found worldwide. SAG are composed by a series of shore-normal ridges (spurs) covered by coral that are separated by channels (grooves) covered by sand. Despite being common, there are relatively few studies that have assessed the hydrodynamics on SAG.
Measurements of wave transformation and circulation over a fringing reef with SAG system is examined at Xahuayxol, Quintana Roo, Mexico. Xahuayxol system consists of a SAG zone with larger spurs than in previous studies, a reef crest that holds a shallow reef lagoon, a beach with high amounts of marine litter and a freshwater coastal lagoon connected with the reef lagoon by a mangrove channel. At this location, an array of eight pressure sensors and twelve current meters sampling at high frequencies and in a depth range from 2 to 10 m were deployed over 8 days.
Results demonstrate that SAG structures are more important for wave transformation than previously reported, since a high rate of wave energy flux dissipation (up to 0.077 kW/m2) was found and the corresponding wave dissipation friction factor, f_w (1.01), was four times higher as compared to studies on other reefs. This research found that wave dissipation rate over the spurs can be up to three times higher than the adjacent grooves (0.098 vs 0.028 kW/m2). In total, the SAG zone and the reef crest dissipate more than 99% of the arriving wave energy.
The most prominent feature was a three-vertical layer circulation pattern with the seaward mid-current flowing in an opposite direction than the top and bottom currents. This pattern is similar to the circulation cell Type C. This circulation cell has been recently found by means of numerical modeling. However, there are two main differences between the previous numerical results and our field work results. First, our results show that wave height can stop circulation cell development when wave-induced flow is high enough to counteract the seaward mid-layer flow. Second, the main condition predicted by the model was that waves should be breaking over the spurs and grooves zone, a condition that was not present during the field work at the zone where the circulation cells were observed. Thus, the mechanism of development may be different from the previously described.
In terms of the reef lagoon circulation, this research found that wave height is the main parameter that controls the wave reef lagoon circulation. As waves suffer dramatic transformation in SAG system, it has been argued that the SAG at Xahuayxol has a crucial role in the reef circulation processes.
This research demonstrates that SAG play a central role in wave dissipation over coral reefs and, in turn, in the reef lagoon circulation. It also demonstrates that there may be other circulation cell patterns on the SAG systems that have not been reported yet, and that some patterns previously identified may have a different mechanism of development from those originally proposed.