**5.3 Gyre extension assessment**

Gyres expanded and contracted around reefs as the forcing conditions changed (Figure 8). As the gyre on one reef arm strengthened the other weakened. Wreck Bay had its largest extension (Lc = 112 %) during the falling tide phase and when the sea-breeze emanated. The largest extensions were produced by the east reef circulation and coincided with the greatest current component speeds flowing out of the channel. This channel current formed the

The Hydrodynamic Modelling of Reefal Bays –

Placing Coral Reefs at the Center of Bay Circulation 169

Fig. 9. Diagrams depicting closed and open circum-reef circulation (CRC) simulated from RMATRK discrete particle tracking modelling. The closed CRC displaying recirculation were evident for Wreck Bay west reef (A) and east reef (B) arms, as well as Sand Hills Bay south reef (C) and north reef (D) arms particularly during wind calms. Open CRC is also displayed in Wreck Bay west reef (E) and east reef (F) arms, and again around Sand Hills Bay south reef (G) and north reef (H) arms particularly during increased channel flow.

western edge of the east gyre. When the west reef circulation emanated, gyre extensions were smaller and did not exceed 75 %. West reef gyres were most developed at low-to-rising tide during land-breeze emanation and coincided with the lowest current component speeds recorded in the channel at that time. The longest duration of this closed western gyre was observed during 15 hrs of some of the smallest tidal changes recorded.

Sand Hills Bay had its largest extension at 198 % during the combination of a rising tide and when the sea-breeze emanated. This was due to the south reef gyre that also tended to be more closed than the north reef's. The north reef gyre was most developed at the rising-tohigh tide (also when the sea-breeze emanated) and had its largest extension at 154 %. In the absence of large tidal changes and developed wind regimes, the south gyre dominated the extension.
