**5. Results**

#### **5.1 General current flow description based on field measurements**

Results from fixed S4 current measurements in Wreck Bay (Figure 4) showed water flowing through the channel generally exited in a south-south-eastward direction, with a deflection southwards when current speeds were high.

Mean speed values for channel currents peaked at 22 cm s-1, and flow directions were southward from 173° to 181°. On the western arm speeds averaged 28 cm s-1 with a mean flow direction of 102°, and on the eastern arm mean speed was 22 cm s-1 with a flow direction of 290°. Flow persisted southwards out through the channel from the back-reef currents continuously, except during very rare occasions of in-flow at mid-depth when velocities were at their lowest (mean of 2.9 cm s-1). Channel currents in Wreck Bay were greatly influenced by the back-reef feeder currents, more than the direct influence of wind and tides. Correlations of channel and back reef flow components showed that the western arm current magnitude was almost five times more strongly correlated (cross correlation r =

 Indefinite tracks: where one set of particles was tracked for as long as they remained in the reefal bay system - their paths plotted for every three hours they remained. Hourly plotted tracks were used to predict the duration of a reef gyre. Three-hourly tracks

Fig. 3. Diagram of the reefal bay dimensions used in calculating the circulatory extent of the bay. The extent (BC) is calculated as a fraction of the AC distance normal to a line (DE) joining the land projections. AC is derived from an elliptical approximation of the outer,

Extents were measured from these plots as a proportion of the linear distance, from the shore to the elliptical arc, normal to a straight line joining the land projections at the ends of the bay indentation (Figure 3). The ellipse best approximates the seaward edge of the gyre. The elliptical major axis is always equal to or greater than the length of the straight line joining the land projections. Therefore, the reef circulation lateral extension, *Lc*, is given as the percentage:

> 100 *<sup>c</sup> BC <sup>L</sup>*

Indefinite tracks allowed predictions of the retention ability of gyres. The number of

Results from fixed S4 current measurements in Wreck Bay (Figure 4) showed water flowing through the channel generally exited in a south-south-eastward direction, with a deflection

Mean speed values for channel currents peaked at 22 cm s-1, and flow directions were southward from 173° to 181°. On the western arm speeds averaged 28 cm s-1 with a mean flow direction of 102°, and on the eastern arm mean speed was 22 cm s-1 with a flow direction of 290°. Flow persisted southwards out through the channel from the back-reef currents continuously, except during very rare occasions of in-flow at mid-depth when velocities were at their lowest (mean of 2.9 cm s-1). Channel currents in Wreck Bay were greatly influenced by the back-reef feeder currents, more than the direct influence of wind and tides. Correlations of channel and back reef flow components showed that the western arm current magnitude was almost five times more strongly correlated (cross correlation r =

particles remaining around the reef was counted after each 3-hr track run.

**5.1 General current flow description based on field measurements** 

*AB* (3)

were plotted to capture the full horizontal extent of the circulation.

seaward curve of the looping currents.

southwards when current speeds were high.

**5. Results** 

0.62) than the eastern arm currents (cross correlation r = 0.18) with channel currents. The west reef feeder currents therefore contributed much more to channel flow than the east reef. Multiple regression values showed that the back-reef currents combined accounted for 47% of the variability in the channel currents, compared to wind and tides accounting for 29%.

Fig. 4. Current component plots are shown for the east back-reef (a), the west back-reef (b) and channel (c) of Wreck Bay, collected from long-term deployment of S4 current meters moored at all three sites at the same time. This field data compared favorably with RMA model results.

Accountability by winds and tides of the overall variability in the current magnitude decreased from highs of 55-56% for the spring and winter data to 29% for the summer currents. During this summer period the lowest recorded mean channel current speed (7.7 cm s-1) was observed as well as an equality of the relative contributions of tides and winds to the overall variability.
