**6. Conclusions**

82 Hydrodynamics – Natural Water Bodies

Fig. 3a illustrates the pre-processing step for simulation of pollutant dispersion and demonstrates the complex geometrical configuration required to represent the turbulent flow. There are six continuous pollution sources in the cities of Macapá and Santana, from the mouth of the Matapi River (southern area) to the north of the city, the upper area of the figure. The natural flux of flows passes from the bottom (left) to top (far right, in the north and northeast). The continuous point sources of pollutants are represented by red circles along the coast, which represent the main release points of untreated pollutants into the waters in Macapá and Santana cities (Pinheiro & Cunha, 2008). The same representation

Fig. 4 shows the results of the simulations of pollutant dispersal plumes (light blue and reddish margins) during a tidal cycle. These maps show that the plumes tend to stay close to the shore. From left to right (top row) is the initial phase of a simulated low tide (approximately 7 hours). Again, from left to right (bottom row) begins the high tide phase (approximately 5.5 hours). During the tidal cycle it was possible to simulate the complex interactions between hydrodynamics and a coupled scalar (hypothetical pollutant), with an emphasis on the dynamic plumes between mainland Santana and the island of Santana. Case study 2 (Fig. 5), shows the phases of the dispersion of pollutant plumes (hypothetical tracer) in the Matapi River during a tidal cycle. The flow pattern (streamlines) changes significantly over a period of the semi-diurnal tide. Simulating the dispersal of pollutants indicates a remarkable complexity in the flow, depending on the geometry of the river

In Fig. 5, from left to right depicts changes in pollutant plumes during low tide (approximately 7 hours), during a complete semi-diurnal tidal cycle, where the natural flux

In Fig. 6, from left to right, there are three different flow fields indicated: a) velocity vectors, b) streamlines (paths of constant speeds), c) dispersion pattern of the scalar from two

Fig. 5. Lines of transient currents in the Rio Matapi: a) low tide at t = 1h, b) end of the ebb at

occur in Rio Matapi indicated by Fig 3b (Cunha, 2008).

channel and the timing of the reversal of the tidal cycle.

(hypothetical) continuous point sources of pollutants.

t = 5.5 h, c) reversal of the tide at t = 6h.

of the tide flows from top to bottom. The reverse shows the rising tide.

The main conclusions of this research are:

In the estuarine region of the Lower Amazon River, in the state of Amapá, the measurement of net discharges of large tidal rivers is only feasible with the use of devices such as ADCP to integrate hydrodynamic processes and water quality variables (biogeochemical cycle and interaction between the plume of the Amazon River and Atlantic Ocean).

Relevant hydrodynamic parameters such as velocity profiles, stress and identification of background turbulent flow velocity components need to be determined with the aid of modern equipment whose operation must be efficient and economic for hydrometric quantification in complex estuarine environments.

Bathymetric analyses, at the scales of interest, have been a difficult hurdle to overcome because of the intricate system of channels in the Amazon River.

The logistics required for experimental studies in large rivers is a major obstacle that has inhibited research interest in this poorly studied area.

A major challenge to be overcome in systematic studies of water quality parameters is the generation of local physical parameters, such as rating curves, rates of sedimentation and resuspension of sediments, etc, which are a fundamental input for complex numerical models of water quality.

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The modelling of water quality in the Amazon estuary is complex due to the absence and / or inadequacy of data describing different physical characteristics. The drainage system imposes enormous difficulties in this area. An example is the absence of long-term time series to obtain necessary parameters and coefficients to build numerical models.

Hydrodynamic simulations of flow and dispersion of pollutant plumes released into the environment are difficult to implement, and require calibration and verification with local data that are not always available.

Existing techniques in numerical modelling can become strong allies in informing public policy and the management of regional water resources.

Among the parameters of interest from numerical models, the generation of 3D computational meshes is potentially the most important source of novel information.

The development of local expertise constitutes one of the biggest challenges in the area, since the best and most efficient option for development of experimental studies in hydrodynamics and computer simulation, is the formation of local human resources. The main advantages are lower operating costs for complex experimental campaigns.

The implementation of a database accessible to the interested user would also be an important technological challenge for the systematic studies of the hydrodynamics and water quality at this region. Thus, would be possible to improve our understanding about the ecosystem functioning and to evaluate the complexity of the Amazon estuary and the role of carbon cycle in these environments.
