**6.5 Bathymetric characteristics necessary for promoting CRC**

The topography of the reefal bay allows it to produce signature dynamics driven primarily by over-the-reef flow, wind and tidal forcings. Waves break over the reef and the generated flow feed reef-parallel currents that in turn supply a major channel outflow. The channel (Figure 13) features significantly in this system and its prominence is the main bathymetric difference from other more popularly studied reef systems such as atolls, platform and

The Hydrodynamic Modelling of Reefal Bays –

**7. Conclusion** 

loop (open CRC);

regime.

expansion.

**8. Summary** 

bay system:

of wind and tide regimes to driving gyre emanations.

giving rise to a persistent circulation;

was primarily responsible for bay extension;

Placing Coral Reefs at the Center of Bay Circulation 175

bay circulation around the reef, revealing signature patterns, and deriving the contributions

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

A characteristic bathymetry comprised of reef arms broken by prominent channel,

 A reef-centered circulation that continuously looped the reef (circum-reef circulation or CRC) to form either a closed gyre (closed CRC) or to flow along the fore-reef as open

The dominance of the CRC alternating between reef arms and dictating which reef arm

The persistence of particularly one reef arm's CRC regardless of the wind or tidal

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

Research on reefal bays revealed that inner bay waters exiting the channel between reefs recirculated into the back-reef, and that this circulation was localized and permanent around reefs as the signature circulation. The distinctive topology of reef arms subtending the headland and separated by a prominent channel induced particles to circulate the reef in expanding and contracting gyres. Gyres expanded by as much as 98% of the horizontal distribution, with expansion and contraction linked to cyclical wind and tidal regimes, giving the reefal system a signature pulse in circulation. Strengthening of the circulation around the reef resulted in closure of looping circulation, increased recirculation rate of particles, increased gyre current speeds and broadening of the circulation's spatial extent.

A reef-centered circulation driven by wind, tides and over-the-reef flow;

A CRC that was persistent because it is always present and localized;

A CRC with a spatial pulse indicated by cycles of expansion and contraction;

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.

### **6.6 Reliability of the hydrodynamic model**

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 bay circulation around the reef, revealing signature patterns, and deriving the contributions of wind and tide regimes to driving gyre emanations.
