Processes and records of coastal sediment dispersal in contrasting deltaic systems
Eidam, Emily Faye
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Rivers are responsible for most sediment delivery to the ocean, and at the river-ocean transition, a complex set of processes and pathways shape the ultimate fate of particulates. Conceptual and numerical models of fluvial-marine dispersal have become increasingly sophisticated through several decades of observational and modeling studies. However, many key processes remain poorly constrained, such as particle clearance from river plumes and episodic sediment-gravity flows, and connections between these types of discrete processes to subaqueous-delta evolution. This study investigates sediment-dispersal processes and their products in two deltaic systems: the small mountainous Elwha River delta (Washington State), and the large Mekong River delta (Vietnam). Sediment clearance from buoyant river plumes is complicated by flocculation dynamics, which remain difficult to quantify. One option for characterizing sediment-clearance rates is estimation of an “effective settling velocity” (we), i.e., a single settling velocity which describes the clearance rate of all flocculated and unflocculated sediment in a plume. This study tests the validity and key assumptions of a published 1D clearance model for we using measurements from the small Elwha River plume during and after a dam removal project. The difference between effective settling velocity (we) and total effective settling velocity (we’) is also explored, based on previous studies suggesting that we’ is the sum of we (representing gravitational settling) and an additional term representing enhanced removal at the base of a plume. Four variations of the 1D clearance model are applied, using both point and depth-integrated measurements of suspended sediment concentrations, as well as estimated and modeled plume-water residence times. For depth-integrated measurements and modeled residence times, we’ are ~0.13–3.5 mm/s, and are greater than we values obtained from point measurements (~0.041–0.33 mm/s). The depth-integrated results are interpreted to be a reasonable approximation of we’ based on measured grain-size distributions and the scale of the plume. The differences between we’ and we are attributed to concentration gradients and turbulence-induced removal of sediment at the base of the plume within a few kilometers of the river. During extreme sediment-loading events, near-bed sediment-gravity flows may dominate dispersal pathways, rather than buoyant surface plumes. These events occur episodically, and thus their mechanics are not well-constrained due to sparse in situ measurements. Instruments deployed near the Elwha River mouth for ~3.5 y during dam removal recorded a gravity flow associated with a river flood. The event lasted ~10 h and resulted in rapid deposition of >15 cm of sand within a few hundred meters of the mouth, and was interpreted to be a hyperpycnal plume or hyperpycnal flow. After a few hours, the flow deteriorated through rapid deposition of sand and vertical mixing of muddy water. The extinction dynamics were characteristic of a hydraulic jump and lofting plume at the base of the steep delta front, resulting in a neutrally buoyant plume filling most of the water column. The muddy-sand deposit was entirely eroded within three weeks, leaving little record of the event. The sandy composition, short runout distance, and rapid extinction of the gravity flow highlight the challenges to forming and maintaining hyperpycnal flows. Post-event erosion of the deposit suggests that in similarly energetic environments, such events may be under-represented in the stratigraphic record. In contrasting large-river systems, sediment dispersal processes tend to be modulated by seasonal shifts in river discharge and ocean energy, rather than by storms and other extreme events. These seasonal processes coupled with abundant sediment supply generate subaqueous deltas near many of the world's largest rivers. The geometry of these subaqueous deposits is generally controlled by wave and current energy, which produce a topset-foreset transition (or “rollover point” between zones of moderate and rapid accumulation) at 25–40-m water depth. Instrument measurements, cores, and a simplistic wave model are used to evaluate morphodynamics of the subaqueous Mekong Delta, which has an unusually shallow rollover at 4–6 m depth. Results suggest that the foreset experiences rapid accumulation and exhibits internal structures characteristic of many subaqueous deltas. However, based on the wave model, the foreset is not energy-limited and does not exhibit a classic subaqueous-delta-front stress refuge. During the high-discharge season, sediment is delivered to the topset and foreset, but further seaward dispersal is limited by landward return flow under the plume and regional circulation patterns. During the windy monsoon season, landward currents driven by regional circulation (at greater depths) and by winds (at shallow depths) serve to retain sediment near shore. Thus, persistent landward sediment fluxes in both seasons likely help shape the subaqueous delta, allowing a shallow topset to exist, despite sufficient transport energy to erode muds and fine sands during much of the year. These results highlight the importance of considering both transport energy and transport direction (leading to sediment convergence) when interpreting the evolution of large subaqueous deltas.
- Oceanography