Multi-decadal Landform Evolution in the Sittaung River Estuary, Myanmar

Abstract

 At the mouth of the Sittaung River in Myanmar, a tide-dominated estuary has developed, with a typical funnel-shaped planform. Tidal bores are formed during the spring tide. This study of the interaction of flows, sediment transportation, and landforms in the estuary, reveals the mechanism of multi-decadal estuarine landform evolution. Our analyses, using time-sequence topographic maps and satellite images, as well as field surveys, found that the main channel has been shifting its course for at least the last 100 years, which has caused alternating retreats and advances of the right and left bank lines, in cycles of around 20 years. This periodicity is evidenced by the developments of a smooth bank line due to erosion and a saw-tooth bank line due to adjuncts of bars and formations of rills to drain surface and subsurface water, and by changes in locations of villages. The main causes of active bank erosion are tidal bores and associated flood currents, which undercut bank bottom sediment and induce toppling failures of upper sediment blocks, causing an active bank line retreat and the supply of sediment into the estuary. The main-channel shifts and bank line changes can be schematically explained by two elementary processes: meander deformation and channel bifurcation. Channel shifts occur where sediment transport capacity decreases rapidly under the effect of fluvial flow and tidal currents. As a consequence, the main propagation route of tidal bores changes and erosion is newly promoted, which brings about further main-channel shifts and bed evolution of the estuary.