Date: 09 Sep 2011

Principles of Sediment Transport Applicable in Tidal Environments

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Abstract

Physical processes of sediment transport in tidal environments are extremely complicated and are influenced by numerous hydrodynamic and sedimentological factors over a wide range of temporal and spatial scales. Both tide and wave forcing play significant roles in the entrainment and transport of both cohesive and non-cohesive particles. Present understanding of sediment transport is largely empirical and based heavily on field and laboratory measurements. Sediment transport is composed of three phases: (1) initiation of motion (erosion), (2) transport, and (3) deposition. In tidal environments, the coarser non-cohesive sediments are typically transported as bedload, forming various types of bedforms. The finer cohesive sediments tend to be transported as suspended load, with their deposition occurring mostly during slack tides under calm conditions. Rate of sediment transport is generally proportional to flow velocity to the 3rd to 5th power. This non-linear relationship leads to a net transport in the direction of the faster velocity in tidal environments with a time-velocity asymmetry. Due to the slow settling velocity of fine cohesive sediment and a difference between the critical shear stress for erosion and deposition, scour and settling lags exist in many tidal environments resulting in a landward-fining trend of sediment grain size. The periodic reversing of tidal flow directions results in characteristic bi-directional sedimentary structures. The relatively tranquil slack tides allow the deposition of muddy layers in between the sandy layers deposited during flood and ebb tides, forming the commonly observed lenticular, wavy, and flaser bedding.