Can Unleveed Agricultural Fields in Deltas Keep Pace With Sea‐Level Rise?

Effective coastal management requires a fundamental understanding of the impacts of anthropogenic activities on sediment dynamics, yet it is challenging to isolate individual impacts in heavily altered regions. The Ayeyarwady Delta, Myanmar has been extensively deforested for agriculture but has few levees/polders. In this study, the relative resilience to subsidence was compared between a 45‐year‐old agricultural field and a nearby mangrove‐forest preserve. At both sites, water velocity and turbidity were measured in tidal channels, topography was mapped, and sediment cores were collected during 2018–2019. There was net sediment import at both sites due to sediment trapping by vegetation. Relative elevations were equivalent, suggesting that the field has aggraded at rates similar to the forest (0.7 cm/year). Unleveed fields may be less vulnerable to subsidence than leveed fields. However, uncertainties remain and the decision to replace mangroves with agricultural fields should weigh all the benefits provided by each environment. Plain Language Summary: Many tropical river deltas are densely populated agricultural centers, and these coastal landscapes are threatened by sea‐level rise. In these managed regions, farmers remove mangrove forests to expand fields and construct levees to control the annual inundation of fields. Unfortunately, levees prevent sediment delivery and can contribute to rapid sinking of the land surface. In the Ayeyarwady Delta, Myanmar, there are relatively few leveed agricultural fields. Here, we assess the resilience to sinking of a deforested but unleveed rice field with respect to a nearby, natural mangrove‐forest preserve in the center of the Ayeyarwady Delta. Land‐surface elevation, sediment transport, and sediment accumulation were measured at both sites in 2018–2019. The land‐surface elevation and average flooding per tide were equivalent in the field and forest. There were similar sediment retention rates, suggesting that rice crops can help trap sediment in a manner similar to mangrove trees. Consequently, unleveed fields are less likely to sink than leveed fields, despite mangrove removal. However, mangrove forests can provide additional benefits and should be preserved where possible. Key Points: Elevation and sediment import are equivalent in an unleveed agricultural field and a mangrove preserve in the Ayeyarwady Delta, MyanmarUnleveed agricultural fields in deltas are less vulnerable to subsidence than leveed or poldered fields [ABSTRACT FROM AUTHOR]