TITLE: Suspended sediment dispersal offshore of the Ayeyarwady delta, Myanmar: results from numerical models

The Ayeyarwady and Thanlwin Rivers deliver ~485 Mt of sediment/year to the northern Andaman Sea, the eastern portion of which includes the Gulf of Martaban, a macrotidal, shallow muddy embayment. Seasonal monsoons bring high precipitation during summer when winds are energetic and from the southwest (SW), and dry during winter when winds are moderate and from the northeast (NE). Surface circulation implies that sediment would be trapped in the northern Andaman Sea during SW monsoon and exported to the Bay of Bengal during the NE monsoon. A clinoform depocenter is found seaward of the Gulf, and a second depocenter on the northwest side of the delta in the Bay of Bengal.

A coupled numerical model was used to explore suspended sediment dispersal offshore in this system. Based on the Regional Ocean Modeling System (ROMS), it accounted for suspended sediment fluxes, and used SWAN (Shallow Waves Nearshore) for waves. The model was run for one-month cases: one each representative of the winter, and summer monsoon. Results indicated that offshore of the delta, surface currents flowed eastward during the summer monsoon and westward during the winter monsoon. Seasonal signals were less significant in bottom currents and within the Gulf. The Gulf’s turbidity was maintained by asymmetric tidal trapping. Sediment export from the Gulf was larger during the summer than the winter monsoon, and higher during spring tides that extended the turbid area to the vicinity of the clinoform. 

Suspended sediment fluxes during the winter monsoon were compared to cases where tides, waves, winds, and open boundary currents were deactivated. These runs showed that tides were more important for sediment flux than waves or winds. Though this model was for the winter monsoon, net eastward currents were calculated. When open boundary currents were deactivated the net sediment flux became westward, indicating that export toward the northwestern shelf depocenter was sensitive to conditions within the Bay of Bengal.

Zoom: Contact Susan Craig for Link. scraig@odu.edu