Connecting sediment dynamics to coastal morphology over linked spatial and temporal scales in Myanmar and New Zealand

Abstract

Low-lying coastal environments are threatened by sea-level rise (SLR), which is often locally exacerbated by anthropogenic alterations that accelerate subsidence. Managing and protecting these valuable regions requires understanding the fundamental sedimentary processes that build coastal landforms. This thesis presents four process-based studies of the transport and fate of sediment at the river-ocean interface. The goal of this work is to 1) connect natural sediment dynamics to resulting morphology over linked spatial and temporal scales and 2) evaluate the impact of mangrove removal on these coastal processes. Three of the studies are focused on the Ayeyarwady Delta, Myanmar while the fourth examines an embayment in New Zealand.The Ayeyarwady River currently delivers the third largest sediment load to the coastal ocean, and the tide-dominated Ayeyarwady Delta is less altered than similar megadeltas, providing a rich opportunity to connect natural processes to morphology. The first study compares sediment dynamics and retention in the tidal-to-estuarine reach of three representative distributaries of the delta. The velocity, suspended-sediment concentrations (SSC), and bed texture were evaluated in the eastern Yangon, central Bogale, and western Pathein distributaries during the high-flow (September 2017) and low-flow (March 2018 and 2019) seasons. During high flow, the lower distributaries were freshwater tidal rivers with comparable SSCs. During low flow, there was little net discharge, however, the SSC increased to >1 g/L in the Yangon, remained constant in the Bogale, and decreased in the Pathein distributary compared to high-flow conditions. This shift in conditions was driven by offshore sediment supply; distributary SSC was strongly influenced by deposits of unconsolidated mud near the mouth of the Yangon distributary. Overall, this study demonstrates that processes observed in one distributary may not represent the entire system and that the seasonal timing of discharge will impact nearshore sediment recycling. The second study examines sediment dynamics and morphology in Meinmahla Island, a mangrove preserve in the Bogale distributary. This type of vegetated mid-channel island plays an important though poorly understood role in modulating fluxes of sediment, carbon, and nutrients through the tidally influenced regions of deltas. Water velocity, salinity, and SSC were measured in low-connectivity (dead-end) and high-connectivity (flow-through) channels, and morphologic evolution was evaluated using sediment cores, aerial imagery, and channel-network surveys. Ebb-dominant, low-connectivity channels along the island exterior did not import enough sediment to maintain the observed ~0.8 cm/y accretion rates. Most of the sediment delivery occurred via interior channels, where water had a long residence time. An interior, high-connectivity channel has shoaled in response to a drainage-network change while the island has aggraded and prograded. This work demonstrates that mid-channel islands are active filters for sediment and dissolved constituents at the river-ocean interface. The third study compares sedimentary processes in the relatively undisturbed Meinmahla Island to an adjacent, un-leveed agricultural field. The Ayeyarwady Delta has been deforested for agriculture but has relatively few embanked fields, providing an opportunity to isolate the impact of land-cover change on resilience to subsidence. Sediment dynamics, accumulation rate, and elevation were compared in the field and forest. There were equivalent sediment import rates and land-surface elevations at both sites, indicating that the field is aggrading and keeping pace with SLR. This comparison highlights the relative resilience of un-leveed fields to subsidence. The future stability of the Ayeyarwady Delta depends on the continued absence of dams and levees. The final study explores the impact of mangrove removal in a quiescent, embayment in New Zealand. In many New Zealand estuaries, expanding mangrove forests are cleared in an effort to flush out fine sediment and restore bivalve habitat. In Waikaraka Estuary, mangroves expanded from the 1940s until a clearing program in 2005. The sediment dynamics and morphology were evaluated in 2019, and a Delft3D numerical model was used to further examine the impact of varying mangrove extent. In 2019, flow in the lower estuary was ebb dominant resulting in net sediment export, while flow in the upper estuary was weakly flood dominant resulting in sediment retention. Due to these persistent hydrodynamic patterns, fine sediment is unlikely to be flushed out of the upper estuary despite mangrove removal. The hydrodynamics were not significantly altered by varying the mangrove extents and were instead controlled by abiotic tidal processes. Overall, mangrove removal is an ineffective method for flushing fine sediment out of quiescent, infilled estuaries.