Morphodynamic evolution of the macrotidal Sittaung River estuary, Myanmar: Tidal versus seasonal controls

Abstract

The inner part of a tide-dominated estuary is often typified by a tight meander bend, where bedload transport from both upstream by river flows and downstream by flood tidal currents merges to establish a bedload convergence (BLC) with the weakest energy condition in the estuary. Despite the potential significance in evaluating relative importance between tidal and river processes, the morphodynamics of the inner estuary remains relatively unexplored in terms of their response to governing processes. This study, based on satellite imagery and field observation, demonstrates that the inner part of the Sittaung River estuary in Myanmar has evolved via active morphodynamics over various temporal scales ranging from monthly to centennial cycles. Channels, including swatchways and flood barbs, migrate rapidly in response to mutually evasive tidal currents on monthly time scales, leading to an increase in the channel sinuosity. Annually, seasonal river floods accelerate channel migration, causing a tight meander bend. A chute cutoff of the point bar at the tight meander bend occurs during the river floods every 6 to 9 years, triggering substantial down-estuary changes in the position of the main channel, bars, tidal flats, and saltmarshes. Over centennial time scales, the main channel formed a large-scale, tight meander bend with a sinuosity exceeding 5, near the downstream end of the inner estuary, which corresponds to the BLC of the estuary. A chute cutoff of the large-scale tight meander bend during the high river flows resulted in the morphologic changes throughout the fluvial-marine transition zone of the Sittaung River estuary: widening of the main channel and the estuary via accelerated channel migration to accommodate an increase in river flows and tidal prism caused by an increase in the hydraulic gradient due to the shortening of the channel length. The tight meander bend has migrated seaward at centennial time scales, suggesting that the river influence has increased over the time, and the estuary is in the progradational phase to become a sediment-exporting delta. The recurrence of the bend tightening after the chute cutoff, however, indicates that flood-tidal currents drive the morphodynamic evolution of the estuary, and the gradual transition from an estuary to a delta is unlikely. Rapid channel migration with bar growth, frequent cutoff, and subsequent infilling of the abandoned channels over various temporal scales all typify the tide-dominated estuary is charged with high sediment loads imported from both rivers and offshore. Despite the construction of dams and reservoirs that reduced sediment supply into the estuary, other anthropogenic activities such as mining activities and deforestation appear to drive an increase in sediment loads, promoting the morphodynamics of the inner part of the Sittaung River estuary.