Small rivers, such as those that line the Oregon coast, have been shown to deliver significant concentrations of suspended sediments and nutrients to estuaries and the coastal ocean during wet seasons during which winds are predominantly downwelling-favorable. The fate of the buoyant source water and of any suspended or dissolved materials depends on the speed at which it travels through the estuary and propagates in the coastal ocean as well as the amount of mixing it undergoes along the way. Discharge comes largely in episodic pulses that can be an order of magnitude greater than background values and tidal ranges are large. The combined impact of bathymetry, winds, tides, and freshwater flow rates on the advection, dispersion, and fate of riverine water as it transits the estuarine-coastal system have not been well characterized. This thesis provides insight into the residence time and transport pathways of freshwater important for understanding the connected coastal and estuarine ecosystem. Using two modeling frameworks the impact of these forces on the temporal and spatial scales of the transport and transformation of buoyant water is investigated.
Hydrodynamic modeling with the Finite Volume Community Ocean Model (FVCOM) is used to compare bulk turnover time estimates in Yaquina Bay to those directly calculated using particle tracking across the local range of tidal amplitudes and river flow conditions. During high discharge, the Yaquina Bay timescale is on the order of 2-5 tidal cycles, but during low discharge turnover time estimates vary across methods due to the increased importance of spatial variability. The bulk methods, including a new method using the total exchange flow at the estuary mouth, were inconsistent with particle tracking measures due to the underlying assumption in applying them that the estuary is well mixed. Also due to the spatial heterogeneity within the estuary the freshwater fraction timescale varied by 12-140% across the range of forcing for two different sampling methods.
A second idealized model of the coastal ocean in the Regional Ocean Modeling System (ROMS) was developed to study the alongshore transport of the buoyant plume across a suite of winter wind, tide, and discharge conditions. With high flows, over half of the buoyant discharge accumulates in a recirculating bulge near the river mouth and the rest is transported downstream in a geostrophic coastal current with only small losses due to mixing. With increasing downwelling-favorable winds increased mixing which leads to entrainment of ambient water; the coastal current is wider and saltier; downstream velocities and freshwater transport increase. Tides have little effect on these tidally-averaged trends. Under lower, more typical winter discharge conditions, the plume is more susceptible to mixing by both tides and winds which can dilute and erode the coastal current and slow down its alongshore propagation by a greater margin than with high discharge. The plume response as discharge is decreased is also increasingly dependent on estuarine adjustment times. This research demonstrates how discharge rates can impact coastal current dynamics and transport.
This model is extended to investigate the impact of pulses of high freshwater discharge on the buoyant coastal plume. The effect of discharge on the fate of the plume is explored, including identifying three timescales associated with downstream freshwater transport. The first is a fast timescale where a mass of buoyant water is advected downstream as a coherent signal for long distances (>100 km) alongshore. The second timescale is associated with the downstream advection of the recirculating bulge that initially retains 46-70% of the freshwater volume from the pulse near the river mouth. Following the pulse this eddy transits alongshore; in its wake the buoyant plume conditions return to their initial steady-state values the preceded the pulse event. As the bulge eddy transits away from the river mouth, it leaks freshwater volume such that the timescale over which the coastal current readjusts to its steady-state conditions further downstream is slow relative to the timescale of the initial peak in freshwater transport associated with the pulse. This research demonstrates how episodic pulsing of high discharge can lead to rapid transport of freshwater volume long distances alongshore and also cause elevated freshwater transport for an extended period of time due to the process of bulge retention and leakage.