Date Awarded

1995

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Jerome P.-Y Maa

Abstract

The resuspension behavior of natural estuarine sediments was studied using the VIMS Sea Carousel, a benthic annular flume. The bed shear stresses produced by the flume were measured by a hot-film sensor mounted flush on the bottom of a laboratory version of the Carousel under a clear-water and flat-bottom condition. Measurements showed a reasonably uniform bed shear stress across the channel and agreed with the relationship &\tau\sb{lcub}\rm b{rcub}=0.011\Omega\sp{lcub}1.69{rcub},& where &\Omega& = ring speed (rpm) and &\tau\sb{lcub}\rm b{rcub}& = spatially-averaged bed shear stress (N/m&\sp2&), predicted from a previous numerical study. Thus, the &\tau\sb{lcub}\rm b{rcub}& was used as a bed shear stress parameter for this study. Field resuspension experiments were conducted during each season for a full year at two sites in the lower Chesapeake Bay. All the field measurements indicated the existence of surficial fluff on top of the relatively well consolidated sediment beds. These sediment beds behaved like cohesive beds despite the sand dominance, and showed an exponentially decreasing resuspension rate with time for a constant &\tau\sb{lcub}\rm b{rcub}.& These characteristics reflect that the natural sediment beds at the study sites developed a depth-increasing erosion resistance profile. Measured critical bed shear stress, &\tau\sb{lcub}\rm cr{rcub},& was slightly larger at the Wolftrap site than at the Cherrystone site (0.13 &\sim& 0.15 vs. 0.1 &\sim& 0.12 N/m&\sp2&) because of the spatial variation of physical energy condition and biological activity. The temporal variation of sediment erodibility was observed at Cherrystone, but it was not apparent at Wolftrap. From all of the field measurements, a relationship (n = 32, r&\sp2& = 0.89) was found between initial resuspension rate (E&\sb{lcub}\rm o{rcub},& in kg/m&\sp2&/sec) and approximate excess bed shear stress (&\rm\tau\sb{lcub}b{rcub}&-&\rm\tau\sb{lcub}cr{rcub},& in N/m&\sp2&), E&\sb{lcub}\rm o{rcub}& = M(&\tau\sb{lcub}\rm b{rcub}&-&\tau\sb{lcub}\rm cr{rcub})\sp\eta,& where the constants M and &\eta& are 0.018 and 1.88, respectively. Laboratory resuspension tests using the bottom sediments collected at the field sites showed a noncohesive nature of sediment bed (e.g. ripple formation) and much smaller E&\sb{lcub}o{rcub}& (&1\sim2& orders of magnitude) than those measured in the field. These differences reflect the complex nature of sediment properties and constant biophysical reworking processes in the natural environments. The differences also indicate that the direct application of the laboratory results to predict the erodibility of natural sediment is not warranted.

DOI

https://dx.doi.org/doi:10.25773/v5-e5qc-h862

Rights

© The Author

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