**5.2.2 Particle tracking and retention**

166 Hydrodynamics – Natural Water Bodies

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.

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 with mid-falling tide.


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.
