Fernando Pinheiro Andutta

Effects of mangroves and tidal flats on suspended‐sediment dynamics: Observational and numerical study of Darwin Harbour, Australia

The suspended-sediment dynamics in Darwin Harbour, Australia were investigated using field measurements and numerical modeling. The model suspended-sediment concentration (SSC) agreed well with observation; the root-mean-square error was less than 0.02 kg m−3 and the anomaly correlation coefficient greater than 0.6. Model results indicate that the tide is the dominant forcing for suspended-sediment transport: total sediment transport was seaward in the channel and landward at the East and Middle Arm entrances, dominated by the Eulerian residual current. Further numerical experiments indicate that mangroves and tidal flats play key roles in redistributing suspended sediment and affecting total sediment transport by modulating the tides and the tidal asymmetry. In Darwin Harbour, if these areas were reclaimed, there would be a significant transport of sediment into the inner harbor. However, the water in East Arm would be less turbid, with about 70% lower bottom SSC during spring tides. The landward sediment flux at its entrance would decrease by 99%, because of reduced currents in the Arm due to a weakened tidal choking effect. Tidal pumping would then dominate sediment transport in the channel and at the entrances of East and Middle Arms. Dredging for the East Arm Wharf affected the SSC upstream in East Arm. According to the model, material from dredging disposed of at a location outside the harbor will be transported back into the outer harbor, generating higher SSC values there. Although this study is site-specific, the findings may be applicable to suspended-sediment dynamics in other harbors and estuaries with extensive tidal flats and mangroves.

Hydrodynamics and Sediment Transport in a Macro-tidal Estuary: Darwin Harbour, Australia

Sediment dynamics studies were undertaken for Darwin Harbour (DH), which is a tidal dominated mangrove system in the Northern Territory of Australia. DH is located in a region with extensive mangrove and tidal flat areas, which function as trapping zones of fine cohesive sediment. Transport of sediment was estimated for the dry season, and thus river discharge was negligible. Numerical simulations were also made with two scenarios: (S1) where the numerical mesh included mangrove and tidal flats, and (S2) in which the mesh neglected these areas. For the first scenario, the formation of two Estuarine Turbidity Maxima zones (ETM) were verified, and located at the inner and outer harbour. In addition to the formation of ETM zones, for the second scenario increased tidal asymmetry was predicted, which resulted in landward sediment transport. The suspended sediment concentration within these ETM zones was modulated by spring and neap tidal conditions. From our simulations we demonstrated that the sediment transport of small particles, e.g. 2 μm particle size, in DH is driven by flood dominance, which is affected by wet/dry areas such as mangroves and tidal flats. Therefore, mangrove areas of DH may trap fine sediment for long periods, and if the trapped sediment carries pollutants one would expect conditions similar to many European estuaries, where pollutant sediment has been found to be buried for over tens to hundreds of years.

Circulation and salt intrusion in the Piaçaguera Channel, Santos (SP)

Analysis of thermohaline properties and currents sampled at an anchor station in the Piacaguera Channel (Santos Estuary) in the austral winter was made in terms of tidal (neap and spring tidal cycles) and non-tidal conditions, with the objective to characterize the stratification, circulation and salt transport due to the fortnightly tidal modulation. Classical methods of observational data analysis of hourly and nearly synoptic observations and analytical simulations of nearly steady-state salinity and longitudinal velocity profiles were used. During the neap tidal cycle the flood (v 0) velocities varied in the range of -0.20 m/s to 0.30 m/s associated with a small salinity variation from surface to bottom (26.4 psu to 30.7 psu). In the spring tidal cycle the velocities increased and varied in the range of -0.40 m/s to 0.45 m/s, but the salinity stratification remained almost unaltered. The steady-state salinity and velocity profiles simulated with an analytical model presented good agreement (Skill near 1.0), in comparison with the observational profiles. During the transitional fortnightly tidal modulation period there was no changes in the channel classification (type 2a - partially mixed and weakly stratified), because the potential energy rate was to low to enhance the halocline erosion. These results, associated with the high water column vertical stability (RiL >20) and the low estuarine Richardson number (RiE=1.6), lead to the conclusions: i) the driving mechanism for the estuary circulation and mixing was mainly balanced by the fresh water discharge and the tidal forcing associated with the baroclinic component of the gradient pressure force; ii) there was no changes in the thermohaline and circulation characteristics due to the forthnigtly tidal modulation; and iii) the nearly steady-state of the vertical salinity and velocity profiles were well simulated with a theoretical classical analytical model.

Sediment Transport Dynamics in Ports, Estuaries and Other Coastal Environments

[Extract] Given ever expanding global trade, the international economy is linked to the well-being of major coastal infrastructures such as waterways and ports. Coastal areas comprise about 69% of the major cities of the world; therefore the understanding of how coastal aquatic environments are evolving due to sediment transport is important. This manuscript discusses topics from both modelling and observation of sediment transport, erosion and siltation in estuarine environments, coastal zones, ports, and harbour areas. It emphasises particular cases of water and sediment dynamics in the high energy system of the Po River Estuary (Italy), the Adriatic Sea, the Mokpo Coastal Zone (South Korea), the Yangtze Estuary and the Shanghai Port, the Yellow Sea (near China), and Darwin Harbour (Northern Australia). These systems are under the influence of strong sediment resuspension/deposition and transport that are driven by different mechanisms such as surface waves, tides, winds, and density driven currents.

Circulation and suspended sediment dynamics in a tropical estuary under different morphological setting

Estuarine processes are directly related to the interaction of its forcing conditions with the local morphology. In this study we assess the implications of the opening of a new inlet on the hydrodynamics and suspended sediment concentration (SSC). A set of physical parameters have been measured in the Itanhém river estuary, a small, shallow and mangrove fringed tropical estuary in Northeastern Brazil. Field surveys have been conducted in August 2007 and January 2008, separated by an important morphological change. Our observations show that even shortening the lower estuary channel in 2 km, the inlet opening did not imply in changes in the estuarine circulation. However, SSC increased after the inlet opening. General estuarine circulation showed synodical modulation of tidal asymmetry and residual suspended sediment transport. The estuary showed flood dominance at spring tide and ebb dominance at neap tide. Although not directly changing the estuarine hydrodynamics, the morphological change resulted in an important increase in SSC. This increase might be related to a facilitated import of inner shelf sediment through a shorter channel, having important implications for the estuarine sedimentation processes.

Fundamentals of Estuarine Physical Oceanography

This book provides an introduction to the complex system functions, variability and human interference in ecosystem between the continent and the ocean. It focuses on circulation, transport and mixing of estuarine and coastal water masses, which is ultimately related to an understanding of the hydrographic and hydrodynamic characteristics (salinity, temperature, density and circulation), mixing processes (advection and diffusion), transport timescales such as the residence time and the exposure time. In the area of physical oceanography, experiments using these water bodies as a natural laboratory and interpreting their circulation and mixing processes using theoretical and semi-theoretical knowledge are of fundamental importance. Small-scale physical models may also be used together with analytical and numerical models. The book highlights the fact that research and theory are interactive, and the results provide the fundamentals for the development of the estuarine research.

Numerical Hydrodynamic Modelling

As estuaries are three dimensional and time dependent, numerical models have been developed to overcome the simplifications inherent to the already studied analytical models (simple geometry, steady-state) and calculate estuarine circulation and salinity distributions. These models can be numerically integrated at selected grid points spatially distributed in the system domain; the governing partial differential equations use methods of finite-difference or finite-elements in curvilinear horizontal coordinates or sigma vertical coordinates, respectively.

Hydrodynamic Formulation: Mass and Salt Conservation Equations

When hydrographic properties and motions in an estuary have spatial and temporal variation, they are termed as non-uniform and unsteady, as opposed to uniform and in steady-state.

Introduction to Estuary Studies

The word estuary is derived from the Latin word aestus which means tide, also the adjective aestuarium has tidal or abrupt high wave as a meaning, an environment highly dynamic with changes due to natural forces. This word is generally used to indicate the place where the river meets the sea, characterizing a coastal river discharge.

Physical Properties and Experiments in Estuaries

The investigation of processes and how estuarine systems function, presents distinct aspects, being conducted with various purposes and objectives.