**7. Conclusion**

174 Hydrodynamics – Natural Water Bodies

ribbon reef. The channel in the reefal bay is the main conduit of back-reef water exiting to the sea, and therefore sets up the hydrodynamics to produce jet currents that help complete the circum-reef current. This CRC has been shown to either close in on itself , which is when gyres are formed that cause particles to re-circulate on the reef, or to flow along the fore-reef

and join the general long-shore flow, causing particles to leave the system.

Fig. 13. Spatial 3D model of Wreck Bay (A) and Engine Head Bay (B) revealing their differences in topography. In Wreck Bay, reef arms are emergent at high tide and the deepest part of the system is its prominent channel. This is topographically more complex than the open, non-reefal Engine Head Bay (spatial 3D Models are exaggerated vertically). Bathymetric characterization includes the reef arms, where their presence localizes the CRC and relative size becomes an important factor. The larger reef arm generates the more expansive gyres and therefore greatest emanations of the bay. This geomorphology is typical of many Caribbean reefal bays. By over-generalization, however, bays have been classified geomorphologically by variations in their coastlines' indented shape (Rea & Komar, 1975; Silvester et al., 1980). This has been applied to systems for which the circulation can be persistent or temporary. Gently-sloping shorelines, for example, exposed to wave action may contain gyre circulations, similar to the CRC, that comprise a seaward rip current diffusing beyond the breaker zone and returning landward as slow mass drift under wave action (Carter 1988). Unlike the reefal-bay system, however, the stability of these gyres is heavily dependent on high energy wave action and so rip features are hardly permanent or in the same location. Ultimately, the bathymetry unique to these reefal-bay systems is principal in forming and maintaining the CRC, as seen in the simulation of the

longer-lasting gyres when both the wind and tidal contributions are reduced.

The hydrodynamic model used flow over-the-reef along a boundary line to simulate wave breaking and captured the effects of shorter period wind-wave driven flow important in driving channel currents. Current simulations in the channel were therefore in good agreement with S4 field data and are considered most important in these models since they form the main link in the CRC formation, in addition to being the direct driver of CRC emanation and contraction. Simulations, however, fell short in capturing some effects caused by the reef flat (Cetina-Heredia et al., 2008), and the contribution of longer period swell, seiching and infra-gravity waves (Lugo-Fernandez et al., 1998; Pequignet, 2008). This affected the back-reef outputs where currents were faster and less variable than simulated by the RMA model. Results from the model, however, were sufficient for simulating the

**6.6 Reliability of the hydrodynamic model** 

The hydrodynamic modelling and tracking simulations were able to reproduce field observations, allowing the following signatures to be developed for characterizing the reefal bay system:


These signatures are now identified with the reefal bay system, where the reef is shown to be central to inducing the circum-reef circulation or CRC that encourages re-circulation of inner bay waters, and that this CRC formation is not found in non-reefal bays, where there is an absence of emergent reef between headlands. Driving forces such as wind, over-the-reef flow and tidal changes were responsible for maintaining the CRC including its contractions and emanations. These findings are important in their implications for stabilizing and protecting these systems as well as the shoreline of which they are a part. Incorporating the reef-parabola geomorphology as the centre of circulation gives predictability to other bay features such as the physicochemical, geo-physical and biological dynamics, which are all affected in greater part by local circulation. Many of these bays, for example, function as nurseries for marine and terrestrial species where their planktonic stages are directly influenced by current patterns and regimes. Identifying the CRC will aid in locating and protecting habitats conducive to plankton viability and survival, including reef growth and expansion.
