Master of Arts (M.A.)
Virginia Institute of Marine Science
Quantitative evidence supplied by bottom sediment textural analysis, Fourier grain-shape analysis and Q-mode factor analysis indicate that river-borne sand-sized sediment originating in the upper reaches of the Rappahannock is actively transported downstream and ultimately delivered to the estuarine sediment regime. Current velocity observations in the upper estuary as well as suspended sediment concentrations measured at stream gaging stations, indicate that short-term extreme hydrological events such as periodic river flooding provide a plausible transport mechanism to move river-borne sands into the estuarine sediment regime.Events of this nature can disrupt average partly-mixed estuarine circulation patterns by displacing the salt-wedge to a more seaward position, increase stratification and create a net-seaward river-type flow within the affected portions of the estuary; thus, allowing high concentrations of river-borne sediments associated with the high freshwater inflow to move into the estuary and become incorporated into the estuarine sediment regime.
Bottom sediment textural analysis indicates that the two major landward sources of sand-sized sediment to the Rappahannock Estuary are the Piedmont-derived river sands and sand-sized sediment derived from the constant denudation of fastland bluff sediments which directly outcrop along certain reaches of the Rappahannock River. Sand-sized sediment is consistently present within all the bottom sediment samples taken from the estuary channel as well as along its flanking shoals.
Fourier grain-shape analysis serves to differentiate the Piedmont-derived river sands from the fastland bluff sands in that the sand-sized sediment derived from each of these provenances possess highly contrastable shape attributes. Based upon the distribution of Fourier harmonic amplitudes of ninety-four sand-shape samples over a defined range Fourier harmonic amplitude class intervals, it is found that the river sands and fastland bluff sands represent two statistically non-similar sand-shape populations. The distribution of Fourier harmonic amplitudes also suggests that these two non-similar sand-shape populations mix together within the river's active transport system landward of the Rappahannock Estuary. The proportional mixing of these two sand-shape populations and subsequent downriver transport results in the delivery of both shape populations into the estuarine sediment regime where they may become deposited and/or redistributed within the estuarine sediments.
Q-mode factor analyis is employed in order to determine the relative extent of the proportional mixing of the two non-similar sand-shape populations within the active transport system of the Rappahannock River-Estuary via the grain-shape information supplied by Fourier analysis. Qmode analysis determined that three compositionally distinct end-members, or factor components, are sufficient enough to encompass 98.5% of the total grain-shape variance contained within the distribution of Fourier harmonic amplitudes for the ninetyfour sand-shape samples. Based upon the distribution these samples within the defined factor (variable) space, it is quantitatively determined that various percentages of the Piedmont-derived river sands are present within the bottom sediments of the Rappahannock Estuary both in the estuary channel as well as along its flanking shoal areas. Thus, Fourier grain-shape analysis proves as a useful geological tool in that it quantitatively determines that river-borne sand-sized sediment is present within the Rappahannock estuarine sediment regime.
© The Author
Natale, Charles Joseph, "An investigation of sand transport phenomena in the Rappahannock River estuary, Virginia" (1982). Dissertations, Theses, and Masters Projects. Paper 1539617525.