Navigation at the Amazon River Mouth: Sand Bank Migration and Depth Surveying

The Amazon River mouth represents a critical pass for navigation at one of the most important waterways in Brazil. Besides the economic importance of diminishing the uncertainties of establishing the navigation depth, there is a strong concern about the threat of oil spills and its environmental impact in this particular system. This work addresses to two related issues: the migration of sand banks and the difficulties related to sounding reduction. At the mouth region, the current loss of transport capacity propitiates sand deposition which will contribute to the reduction of depths and the formation of banks. Bathymetric records were analyzed and it was observed sand bank movement of approximately 250 m/year. However, anything could be concluded about the periodicity of the bank change of migration direction and its causes. In addition to the bottom variability, there is an intrinsic difficulty for establishing the reference level due to the region's great dimensions, variability and magnitude of the tides and river discharge which influences mean sea water level and tidal amplitudes. The sounding reduction is currently done by the Brazilian Navy through a simplified method. In order to estimate the expected depth differences through the use of a more realistic method, a hydrodynamic numerical model was implemented in the area. The mean differences observed, for low river discharge, were 1.14m and maximum differences reached almost 3m (both negatives and positives), which are significant values for such a critical area.