Presenter: Hannah Glover (University of Washington)

Description:
Many deltas are densely populated agricultural centers that are threatened by sea-level rise (SLR). Local SLR can be amplified by anthropogenic damming, fluid extraction, coastal deforestation, and levee/polder construction. In the Ayeyarwady Delta, Myanmar there has been extensive deforestation for agricultural development but minimal levee construction, providing an opportunity to compare the resilience of remaining mangrove forests to un-leveed agricultural fields. Sediment dynamics in Meinmahla Island, a mangrove-forest preserve, and an adjacent, 50-year-old agricultural field were investigated in 2018–2019. Water velocity and turbidity were measured in dead-end channels of the forest and field to evaluate sediment fluxes; topography was mapped with a GPS-RTK; and sediment cores were collected to measure accumulation rate with ²¹⁰Pb and ¹³⁷Cs geochronology. At both sites, there were tidally driven, net-landward sediment fluxes of 0.01–0.04 kg/m/s despite greater ebb velocities, likely due to sediment trapping by dense vegetation. Accumulation rates on the aggrading land surface of the mangrove forest were 0.3–0.8 cm/y, and ¹³⁷Cs indicated similar accumulation rates in the field. The land-surface elevation was comparable in both environments, further suggesting that the field is aggrading at the same rate as the forest. Overall, both the mangrove forest and un-leveed field appear to be keeping pace with SLR by tidally retaining sediment. Crops such as rice may provide similar stabilizing and trapping as mangrove or salt-marsh vegetation. Consequently, un-leveed agricultural fields are not as susceptible to rapid subsidence as leveed fields, and the impacts of deforestation and levee construction on SLR should be considered separately. Policies to promote coastal resilience require a fundamental understanding of the impacts of anthropogenic activities on sediment dynamics. The continued absence of levees may be critical to the stability of the Ayeyarwady Delta.