Master of Science (M.Sc.)
Virginia Institute of Marine Science
A time-series of erodibility measurements and physical and biological sediment properties was used to evaluate spatial and temporal variability in cohesive bed erodibility and controls on erodibility in the York River estuary, VA. Two sites near Clay Bank displayed dramatic seasonal variations in bed erodibility while a third near Gloucester Point displayed a more consistent level of low erodibility. Total bed solids fraction and cohesive sediment grain size were not correlated with bed erodibility. The surficial sediments were characteristically composed of 2% to 50% sand supported in a mud matrix. The total solids fraction of the bed was shown to be a function of sand fraction, complicating its use as a measure of bed consolidation. The solids fraction of the mud matrix was calculated to evaluate changes in consolidation independent of sand content. An evaluation of data from multiple studies available in the literature demonstrated that when a large range of data was considered, the solids fraction of the mud matrix provided a better correlation to erodibility than did the total solids fraction. However, the range in solids fraction of the mud matrix in York River sediments was quite small and no significant relationship was found locally between this parameter and bed erodibility.
Common proxies for bioadhesion including colloidal carbohydrate concentration, extracellular polymeric substance concentration, and organic content were measured to evaluate the influence of biostabilization on bed erodibility. In contrast to the results of many published works studying erodibility on mudflats, these bioadhesion proxies did not exhibit systematic variation in either space or time and were not correlated to bed erodibility. The seasonal timing of erodibility maxima and presence of laminations during periods of highest erodibility indicated that bioturbation was not responsible for increases in erodibility during the winter and spring at Clay Bank. Although not quantified in this study, anecdotal evidence from sediment cores and seabed camera images suggest that biological reworking and repackaging may alter bed cohesivity and structure potentially influencing the relationship between erodibility and bed solids fraction. Periods of rapid deposition, as indicated by the presence of thick sequences of laminated sediments, suggest that periodic sediment flux convergence processes are responsible for the observed seasonal cycle in bed erodibility at Clay Bank. High bed erodibility was associated with rapid deposition in the likely presence of the mid-estuary turbidity maximum while low erodibility was associated with sediment bypassing and decreased deposition in the absence of a local turbidity maximum. Finally, a consistent, low erodibility condition was observed at the Gloucester Point site for the majority of the study period and at Clay Bank in the summer and fall. A comparison of these data to other published data from the Chesapeake Bay indicated that the low erodibility background state may be characteristic of an “equilibrium” bed condition often present in many regions of the Chesapeake Bay. In the absence of recent, rapid deposition, the data set presented from the York River reasonably characterizes both the range and the profile information of the bed critical erosion stress for mid-depth, muddy regions in other similar estuaries.
© The Author
Dickhudt, Patrick J., "Controls on Erodibility in a Partially Mixed Estuary, York River, Virginia" (2008). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539617867.