**5.2 RMA model simulations 5.2.1 Current flow**

Velocity results from S4 current meters compared well with RMA model results (depth averaged) for the dominant north (Y) component of the channel site at Wreck Bay (Figure 5), giving no significance for difference by t-test. For the month of August (2000) , S4 north component readings averaged -7.8 cm s-1 while the RMA model averaged slightly lower at - 8.2 cm s-1 (Table 2). The north component was used to represent the channel flow given its high cross-correlation value of -0.99 with the channel flow magnitude.

The Hydrodynamic Modelling of Reefal Bays –

Placing Coral Reefs at the Center of Bay Circulation 165

Fig. 6. Depth-averaged current field maps for (a) Wreck Bay and (b) Sand Hills Bay during a dominant rising tide combined with sea-breeze regime. Current vectors depict well-formed, closed looping circulation on the down-shore reef arm (circled), causing both bays to be

expanded beyond the reef.

Depth-averaged velocity results from hydrodynamic modelling showed that currents circulated the reef arms constantly. This circling of the reef was strongest during the combined condition of a rising tide with prevalent sea-breeze (Figure 6). This particular condition generated some of the strongest currents on the west reef of Wreck Bay (the 28 to 32 cm s-1 category) and the corresponding south reef of Sand Hills Bay. Back reef current highs by the model, however, were less than measured in the field. Field-measured monthly average for the Wreck Bay east back reef current magnitude was 22 cm s-1 and agrees with model averages, however, the variation in flow is not replicated and spikes in back reef speeds (up to 38 cm s-1) not captured. In Sand Hills Bay, model currents strongly circulated the south reef at up to 28 cm s-1 on the southern curve of the gyre. Engine Head Bay showed no formation of looping currents. The combination of a prevalent sea-breeze with falling tide strengthened the east reef circulation in Wreck Bay (Figure 7). Horizontal current fields depicted velocities of up to 32 cm s-1 in this gyre, the fastest speeds occurring on the western side of the gyre. For Sand Hills Bay, the north reef gyre was pronounced with a central inner gyre showing closed circulation. Horizontal current fields depicted velocities of the 18 to 20 cm s-1 category around the north reef. Engine Head Bay again showed no formation of horizontal circulatory currents.

Fig. 5. RMA model and S4 field north component current data comparisons for the Wreck Bay Channel area. A t-test reported no significance for difference when both current data sets were input as independent samples (t = 1.46; p = 0.15).


Depth-averaged velocity results from hydrodynamic modelling showed that currents circulated the reef arms constantly. This circling of the reef was strongest during the combined condition of a rising tide with prevalent sea-breeze (Figure 6). This particular condition generated some of the strongest currents on the west reef of Wreck Bay (the 28 to 32 cm s-1 category) and the corresponding south reef of Sand Hills Bay. Back reef current highs by the model, however, were less than measured in the field. Field-measured monthly average for the Wreck Bay east back reef current magnitude was 22 cm s-1 and agrees with model averages, however, the variation in flow is not replicated and spikes in back reef speeds (up to 38 cm s-1) not captured. In Sand Hills Bay, model currents strongly circulated the south reef at up to 28 cm s-1 on the southern curve of the gyre. Engine Head Bay showed no formation of looping currents. The combination of a prevalent sea-breeze with falling tide strengthened the east reef circulation in Wreck Bay (Figure 7). Horizontal current fields depicted velocities of up to 32 cm s-1 in this gyre, the fastest speeds occurring on the western side of the gyre. For Sand Hills Bay, the north reef gyre was pronounced with a central inner gyre showing closed circulation. Horizontal current fields depicted velocities of the 18 to 20 cm s-1 category around the north

reef. Engine Head Bay again showed no formation of horizontal circulatory currents.

Fig. 5. RMA model and S4 field north component current data comparisons for the Wreck Bay Channel area. A t-test reported no significance for difference when both current data

Average: -8.2 -7.8 Maximum: -0.7 0.3 Minimum: -24.5 -24.7 Range: 23.8 25.0 Table 2. RMA model and S4 field north component current data statistics and comparisons

Y-COMP VELOCITY RESULTS (cm s-1) RMA Model Data S4 Field Data

sets were input as independent samples (t = 1.46; p = 0.15).

for Wreck Bay Channel.

Fig. 6. Depth-averaged current field maps for (a) Wreck Bay and (b) Sand Hills Bay during a dominant rising tide combined with sea-breeze regime. Current vectors depict well-formed, closed looping circulation on the down-shore reef arm (circled), causing both bays to be expanded beyond the reef.

The Hydrodynamic Modelling of Reefal Bays –

**5.2.2 Particle tracking and retention** 

with mid-falling tide.

**5.3 Gyre extension assessment** 

Placing Coral Reefs at the Center of Bay Circulation 167

Under only the rising tide regime, 19 % particles remained in Sand Hills Bay after 9 hrs. The rising tide combined with land-breeze regime increased the remaining particles to 22 % after 9 hrs. When the sea-breeze dominated, however, combined with the rising tide the retention dropped to 2 % in 9 hrs. Therefore particles were likely to remain trapped in Sand Hills Bay the longest when introduced at the beginning of the rising tide cycle during a land-breeze regime and were likely to be flushed out the quickest if introduced during the sea-breeze

Fig. 8. Reef gyre extension measurements for Wreck Bay and Sand Hills Bay during 18 hrs (1.5 tidal cycles) of highest Y-component current speeds recorded in Wreck Bay. Tracks are displayed as time progresses in 3-hr increments for new particles introduced into the bay every three hours. Gyres undergo expansion and contraction but are always present.

Under only the falling tide regime, 36 % particles remained in Wreck Bay after 6 hrs. The falling tide combined with land-breeze or sea-breeze regime decreased the remaining particles to 6 % and 10 % respectively after 6 hrs. Therefore particles were likely to remain trapped in Wreck Bay the longest if introduced at the beginning of the falling tide cycle and were likely to be flushed out the quickest if introduced at the beginning of the rising tide.

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

Fig. 7. Depth-averaged current field maps for (a) Wreck Bay and (b) Sand Hills Bay during a dominant falling tide combined with sea-breeze regime. Current vectors depict well-formed, closed looping circulation on the up-shore reef arm (circled), causing both bays to be expanded beyond the reef.
