Sediment dynamics in tidal environments spanning a range of fluvial influence
Nowacki, Daniel J.
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A full understanding of the transport of sediment from terrestrial source to marine sink requires knowledge of dynamics in a multitude of intermediate environments with both fluvial and marine influences. Two of these environments, tidal flats and tidal rivers, are dramatically different in scale and setting but are unified by common sedimentary processes. This dissertation presents observations of flow and sediment transport on the tidal flats of Willapa Bay, Washington, USA; tidal channels along the tidal floodplain of the Amazon River, Brazil; and the mainstem of the tidal Mekong River, Vietnam. These three study sites are ideal locations to improve understanding of where, when, and how the annual worldwide total of 13 billion metric tons of sediment is delivered from land to ocean. The muddy tidal flats of southern Willapa Bay, Washington are tidally dominated and receive little direct freshwater input. Observations in channels of different size and their adjacent flats illustrate the hydrodynamics and sediment dynamics of each morphological setting under a range of seasonal and meteorological conditions, including rain and wind events. Interaction between the morphology of the channel/flat complex and tidal water-level variations produces well-defined velocity pulses during both flooding and ebbing tides. These pulses represent about 27% of the total along-channel water transport and 35% of the suspended-sediment transport of the system. Wind alters the typical flow regime in channels and on the flat, increasing the over-flat ebb flow in this study location while decreasing the ebb-pulse intensity. Wind speed was positively correlated with background suspended-sediment concentration (SSC). Residual along-channel water transport in channels and on nearby flats was flood dominant under all seasonal conditions sampled. Sediment flux was flood dominant during winter and spring deployments. In contrast to channels incising the Willapa flats, the channels connecting the mainstem Amazon River to its tidal floodplain have considerably more fluvial influence. Water level, flow velocity, temperature, and SSC were measured in floodplain channels along the tidal Amazon River, Brazil. Eleven deployments were made at four locations during low, rising, high, and falling seasonal river level. These observations are the first of their kind on the Amazon. In nearly all cases, tidal channels import water with high SSC from the mainstem to the tidal floodplain on flood tides and export water with low SSC back to the mainstem on ebb tides. The importance of sediment resuspension in transferring sediment to the tidal floodplain decreases with distance from the mainstem river mouth. Overbank flow in strongly tidal regions is an important characteristic of the flow regime and is controlled both by the seasonal water level and tidal conditions. A synthesis of the observations indicates that tidal-floodplain channels are important conduits of water and sediment along the tidal river, and a measurable but small percentage of the total mainstem water and sediment discharge is exchanged with the tidal floodplain. Export of sediment from intertidal environments, such as tidal-floodplain channels, to the coastal ocean is strongly influenced by mainstem tidal-river dynamics. Flow velocity, salinity, and SSC were measured for 25 hours at three cross-sections in the tidal Song Hau distributary of the Mekong River, Vietnam. Estuarine conditions varied dramatically between high and low seasonal discharge periods. The system transitioned from a tidal river with an ephemerally present salt wedge during high flow to a partially mixed estuary during low flow. The changing freshwater input, sediment sources, and estuarine conditions resulted in sediment export during high flow and import during low flow. The Dinh An channel of the Song Hau distributary exported sediment to the coast at a rate of about 1 t/s during high flow and imported sediment in a spatially varying manner at approximately 0.3 t/s during low flow. These values scale to a yearly sediment discharge of 40 Mt/y for the entire Mekong River, about 65% less than a generally accepted estimate of 110 Mt/y. Fluvial advection of sediment was primarily responsible for the high-flow sediment export, while exchange-flow and tidal processes, including local resuspension, were principally responsible for the low-flow import. The resulting bed-sediment grain size was coarser and more variable during high flow and finer during low flow. The residual flow patterns supported the maintenance of mid-channel islands.
- Oceanography