Author

Jesse Lopez

Date

2-2017

Document Type

Dissertation

Degree Name

Ph.D.

Department

Division of Environmental and Biomolecular Systems

Abstract

This study aims to characterize the suspended sediment dynamics of the Columbia

River estuary and describe the impact of suspended sediment on the aquatic light

field, using field observations and a numerical sediment model.

The model used for this study, SELFE, was optimized to improve computational

efficiency and strong-scaling. The sediment model coupled to SELFE was then

validated against a benchmark of idealized test cases and a realistic application to

the Columbia River estuary. A yearlong simulation of the Columbia River was then

skill assessed and analyzed to describe suspended sediment dynamics, dominant

transport processes, and sediment pathways. An empirical predictive model for

the light attenuation coefficient was derived from observations of

photosynthetically active radiation and water quality variables. Results from the

numerical sediment model were then used as inputs to the light model to describe

the estuarine light field.

Results from the sediment model benchmark show that it reproduces analytical,

semi-analytical, and laboratory results in idealized barotropic open channel and

trench migration test cases. In an idealized baroclinic estuary, SELFE reproduces

sediment dynamics representative of an idealized estuarine turbidity maximum

(ETM) in a manner similar to other models and in agreement with theoretical

estuarine circulation. In a realistic Columbia River estuary application, the model

reproduces the variability and magnitude of suspended sediment concentrations

with a bias in the vertical location when stratification is under-predicted. Analysis

of yearlong simulations shows that mean advection dominates suspended

sediment transport, but tidal pumping periodically produces residual upstream

transport when the estuary is partially mixed. Suspended sediment transport is

predominantly in the along-channel direction with minor periodic lateral pulses,

the largest of which occur in the North Channel. Integrated suspended sediment

fluxes show that the main channels constitute the dominant sediment pathway,

while secondary pathways cut through Cathlamet Bay and intertidal shoals

upstream of the North Channel. A conceptual model of system-wide sediment

dynamics describing the ETMs reveal a four stage tidal pattern modulated by

stratification where erosion and transport largely occur during ebb and flood tides,

and deposition occurs during high and low water stages. The empirical light

attenuation model indicates that suspended sediment and CDOM are the dominant

varying light attenuation factors. Applying numerical sediment model results to

the light attenuation model suggests that the estuary is generally within the

euphotic zone except in the deepest sections of the main channels. However, the

extent and intensity of light limitation shows strong tidal, tidal month, and

seasonal variability as a result of fluctuating circulation and suspended sediment

dynamics.

The findings show that the SELFE sediment model is validated and capable of

approximating estuarine sediment dynamics including those associated with the

ETMs in the Columbia River estuary. Observations and model results indicate that

suspended sediment concentrations depend largely on tidal range and riverine

suspended load, but that the transport of suspended sediment follows a regular

pattern driven by river discharge, tidal conditions, and stratification. The

processes associated with this cycle occur in a region roughly collocated with the

chemical estuary and constitute an ETM zone.

Identifier

doi:10.6083/M43B5Z7T

School

School of Medicine

Available for download on Sunday, February 23, 2020

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