Title

Entrainment of estuarine cohesive sediments: Analysis of in situ measurements and numerical modeling

Date of Completion

January 1998

Keywords

Applied Mechanics|Physical Oceanography|Engineering, Marine and Ocean|Environmental Sciences

Degree

Ph.D.

Abstract

The primary purposes of this dissertation are (1) to analyze the entrainment of the estuarine cohesive sediments caused by natural forces and (2) to estimate the entrainment rates.^ Field data collected in central Long Island Sound are analyzed using various signal processing tools along with the Grant and Madsen (1986) model. The results show that the interaction between waves and currents enhances bottom shear stress which leads to strong sediment entrainment into the water column. Strong along-estuary winds may play a key role in enhancing the sediment resuspension and erosion. An extraordinarily large event was triggered by the passage of a frontal system when the strong winds augmented the tidal currents. The timing of the storm with respect to the tidal phase, in addition to the route and magnitude of the storm is critical to the hydrodynamic fluctuations and associated sediment entrainment.^ A sediment advection-diffusion model in combination with the model of Mellor and Yamada (1982) is used to simulate the entrainment and mixing processes. The model results show that vertical mixing was controlled by tidal currents, surface winds and heat flux. Daily solar radiation played an important role in suppressing the transfer of momentum and bottom sediments.^ The bottom sediment entrainment rate is derived from an inverse two-layer model using in-situ measurements. The computed entrainment rate can be grouped into storm and non-storm periods depending on the magnitude of bottom shear stress. A parametric relationship shows that entrainment rate is governed by a two-term power-law function of shear stress; including an ambient resuspension term due to tidal currents and a bed erosion term due to higher shear stress resulting from episodic storm events.^ The elastic properties of cohesive sediment and the time scale of erosion behavior are important factors in forming the erosion expression. Further investigation of the cohesive sediment properties and in-situ observation of the bed structure variation are encouraged. ^

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