Sediment Transport And Trapping On The Ayeyarwady-Martaban Continental Shelf

The Ayeyarwady and Thanlwin Rivers, which primarily drain Myanmar, are together one of the largest point sources of freshwater and sediment to the global ocean. Much of the estimated 600 Mt of river sediment annually carried by the combined Ayeyarwady and Thanlwin River system is delivered to a wide continental shelf in the northern Andaman Sea. Called here the Ayeyarwady-Martaban continental shelf, this area is influenced by strong tides, monsoon conditions, and periodic cyclones; however, the processes that dominate dispersal of fluvial material in the coastal ocean of this system remain poorly understood. The shelf exhibits a dramatic asymmetry of the surface morphology and sediment texture in the east–to–west direction, and recent field observations indicate that sediment accumulation rates increase and then decrease offshore of the western portion of the Gulf of Martaban. A three-dimensional coupled hydrodynamic and sediment transport model was used to explore the oceanographic processes responsible for sediment dispersal off the Ayeyarwady-Thanlwin Rivers. Model runs were developed using ROMS (Regional Ocean Modelling System) and SWAN (Simulating WAves Nearshore) to represent oceanographic conditions in the region and suspended sediment transport. The model setup was applied to two cases: one month representative of the winter northeast monsoon, and one month representative of the summer southwest monsoon. Model estimates of sediment dispersal and transport during summer and winter monsoon conditions were analyzed, and compared to spatial patterns found in field measurements, as well as satellite imaging. Within the Gulf of Martaban, both the surface and bottom currents were strongly tidally driven; therefore, seasonal signals were less prevalent. Over the Ayeyarwady delta region, the seasonal signal was large in both wave energy and surface currents, which had a distinct bidirectional pattern: flowing eastward during the SW monsoon and westward during the NE monsoon. Bottom currents over the Mouths of the Ayeyarwady had less seasonality. During the southwest monsoon, wave energy was higher over the Ayeyarwady Delta, leading to increased resuspension of sediment that was subsequently carried into the Gulf near the coast. Sediment resuspension was strongly tidal-dominated within the Gulf of Martaban, with asymmetric tidal trapping making the Gulf region conducive to high turbidity. During the NE monsoon, the Gulf acted as a mixing-bowl, where there were high sediment fluxes during flood and ebb tides, but very little net export. Meanwhile, the SW monsoon had high tidal fluxes as well as a large net export of sediment out of the Gulf