River Network Uncertainty and Coastal Morphodynamics in the Mekong Delta: Model Validation and Sensitivity to Fluvial Fluxes

Abstract:

A loose-coupling of a basin-scale hydrological and sediment flux model with a
coastal ocean hydrodynamics model is used to assess the importance of
uncertainties in river mouth locations and fluxes on coastal geomorphology of
the Mekong river delta. At the land-ocean interface, river deltas mediate the
flux of water, sediment, and nutrients from the basin watershed, though the
complex delta river network, and into the coastal ocean. In the Mekong river
delta, irrigation networks and surface water storage for rice cultivation
redistribute, in space and time, water and sediment fluxes along the coastline.
Distribution of fluxes through the delta is important for accurate assessment of
delta land aggregation, coastline migration, and coastal ocean biogeochemistry.
Using a basin-scale hydrological model, WBMsed, interfaced with a coastal
hydrodynamics/wave/sediment model, COAWST, we investigate freshwater and
sediment plumes and morphological changes to the subaqueous delta front. There
is considerable uncertainty regarding how the delta spatially filters water and
sediment fluxes as they transit through the river and irrigation network. By
adjusting the placement and relative distribution of WBMsed discharge along the
coast, we estimate the resulting bounds on sediment plume structure, timing, and
morphological deposition patterns. The coastal ocean model is validated by
comparing simulated plume structure and seasonality to MERIS and MODIS derived
estimates of surface turbidity. We find good agreement with regards to plume
extent and timing, with plumes weakest in the early spring, extending strongly
to the west in the fall, and toward the east in winter. Uncertainty regarding
river outflow distribution along the coastline leads to substantial uncertainty
in rates of morphological change, particularly away from the main Mekong River
distributary channels.