Current Pattern in the Wax Lake Delta

The simulated near-surface velocity vectors, surface elevation, and spatially uniform wind fields are presented (Figure 6.10-6.16) for 7 cold front events from December 15, 2012 to January 13, 2013. The first cold front on December 17 was not very strong, where the magnitude of 4 m/s southeaster wind can be read from the wind field map. Basically the currents offshore the Wax Lake delta followed the prefrontal wind, moving southerly. Some of the flow might enter into the Wax Lake delta bifurcating channels, but would not shift the flow direction due to stronger (0.7 m/s) and persistent discharge input from the upper stream. Instead the channel outflow pushed the wind-induced currents offshore, resulting in a clockwise pattern, which could be clearly identified during the frontal passage phase (Figure 6.10 lower right). That was also a result of

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wind shift by blowing easterly and an obvious water setdown would serve as a verification of offshore circulation. The second cold front (December 20, 2012) was a typical one due to clockwise shift in wind direction and stronger and persistent postfrontal offshore wind. The prefrontal moderate wind set up the water level and drove a westward surface field. It was observed from the model results that the current on the east side of the domain tended to deflect to the right, which believed to be the Coriolis effect. Strong northwest wind generated an offshore clockwise current pattern, especially in the inland area that was inundated during the prefrontal phase. The 3rd prefrtonal wind (December 25, 2012) induced a larger water level surge as well as stronger onshore currents than those of the first two events, due to the larger wind magnitude of 8 m/s. The current velocity was comparable to the outflow in the channel, resulting in a reverse flow and big water intrusion in the subaerial area adjacent to the distributaries in the Wax Lake delta. Again, a counterclockwise current pattern formed on the west of the delta while a clockwise circulation on the east side. Four hours later, the wind changed direction to blowing north to northeast. The flow field in the domain shifted accordingly too, with a weaker strength. When the wind changed to westerly, the water level was set down and strong southeast flow pattern were formed. The postfrontal wind lasted for another 16 hours until becoming persistent northwest on December 26, 20 UTC. On December 28, 2012, 20 UTC, the fourth cold front initiated a southeast wind of 6 m/s, where stronger offshore currents, return flow in the channel and water intrusion on inland areas inside the Wax Lake delta were clearly visible. Especially a anti-cyclonic gyre were formed on the east boundary. The winds were then weakened (3 m/s) by blowing northeast and persisted as a postfrtonal northwest wind with magnitude of 8 m/s in 8 hours, followed by the similar circulation pattern in current flow. The 5th _{cold front persisted} about 7 days from January 1, 2013, where the westward currents were generated by southeast

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wind. When the wind changed from southerly to northerly, a clockwise gyre was formed on the west of the delta. It was noticed that the flow direction diverted prior to the wind shift. Since the postfrontal offshore wind dominated for about 6 days, the inundation and reverse flow were weak compared to previous frontal passages. The 6th event started with an easterly wind that induced a westward flow field. During the frontal passage, the current flow changed in advance by intruding the inland area of the delta. When the wind blew southerly, a stronger eastward current field was generated on the south of the delta. For the 7the cold front, it started with westward flow induced by prefrontal onshore wind. Since the wind shift from south to west a clockwise pattern on the offshore west of the Wax Lake delta was formed.

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Figure 6.10 Plan views of the spatially uniform surface wind field, simulated surface elevation contours and near-surface velocity fields during the prefrontal, frontal, and postfrontal stages of

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Figure 6.11 Plan views of the spatially uniform surface wind field, simulated surface elevation contours and near-surface velocity fields during the prefrontal, frontal, and postfrontal stages of

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Figure 6.12 Plan views of the spatially uniform surface wind field, simulated surface elevation contours and near-surface velocity fields during the prefrontal, frontal, and postfrontal stages of

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Figure 6.13 Plan views of the spatially uniform surface wind field, simulated surface elevation contours and near-surface velocity fields during the prefrontal, frontal, and postfrontal stages of

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Figure 6.14 Plan views of the spatially uniform surface wind field, simulated surface elevation contours and near-surface velocity fields during the prefrontal, frontal, and postfrontal stages of

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Figure 6.15 Plan views of the spatially uniform surface wind field, simulated surface elevation contours and near-surface velocity fields during the prefrontal, frontal, and postfrontal stages of

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Figure 6.16 Plan views of the spatially uniform surface wind field, simulated surface elevation contours and near-surface velocity fields during the prefrontal, frontal, and postfrontal stages of

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