Seasonal and tidal variability in suspended sediment dispersal offshore of the Ayeyarwady delta, Myanmar: results from a numerical model

The Ayeyarwady and Thanlwin Rivers deliver ~485 Mt of sediment/year to the northern Andaman Sea. The Ayeyarwady river mouths empty via the Ayeyarwady Delta, while the Thanlwin empties into the Gulf of Martaban located east of the delta. The Gulf of Martaban is a macrotidal, shallow embayment, and the abundant sediment supply and tidal energy make it one the world’s largest perennially turbid zones. 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 has been found seaward of the Gulf, and a second depocenter on the northwest side of the delta in the Bay of Bengal. The phasing and timing of sediment delivery to these depocenters has relevance for sediment budget, event preservation, and carbon cycling, however, the sediment delivery mechanisms to these depocenters remain a question.

To address this, a coupled hydrodynamic and sediment transport numerical model was used to quantify suspended sediment dispersal offshore of the Ayeyarwady delta and within the Gulf of Martaban. Based on the Regional Ocean Modeling System (ROMS), it accounted for suspended sediment fluxes and used SWAN (Shallow Waves Nearshore) for waves. Open boundary and atmospheric conditions were derived from available global model products to account for larger scale ocean conditions and winds. The model has been run using different versions of initial sediment bed grain size distributions, based on either simple assumptions or historical and recent grain size observations. The magnitude of suspended sediment flux shows sensitivity to the initial grain size distribution, but the overall seasonal and tidal trends are less sensitive.

Model applications to date have focused on quantifying the variability of suspended sediment flux over tidal and seasonal timescales. The model has been run for two one-month cases: one each representative of the winter and the summer monsoon. Results indicated that offshore of the delta, surface currents flowed eastward during the summer monsoon and westward during the winter monsoon. The bottom currents offshore of the delta, however, showed less dependence on seasonal signals and were westward on average for both the summer and winter model runs. Within the macrotidal Gulf of Martaban, turbidity was maintained by asymmetric tidal trapping. Sediment export from the Gulf primarily directed toward the Martaban Depression Clinoform, with very little sediment delivered westward to the Bay of Bengal.  Sediment export was larger during the summer than the winter monsoon, and especially high during spring tides that extended the turbid area to the vicinity of the clinoform.