Dynamics of a submarine dune field.

GÓMEZ E.A.; CUADRADO D.G.; PIERINI J.O.; RANIOLO A.

Santa Fe

Simposio; 6th Symposium on River, Coastal and Estuarine Morphodynamics; 2009

International Association on Hydraulic Engineering and Research (IARH)

In the Bahia Blanca estuary is located the most important deep-harbour complex of Argentina. The inner section of the complex access channel should be shortly dredged to allow Panamax vessels navigation during the entire tidal cycle. At this section, an important submarine dunes field is recognized. The objective of this contribution is to determine the dune field formation mechanisms to establish its dynamics. In order to determine a detailed dune field morphology and measure dune’s migration rate during six months, a Swath Bathymetry System “GeoSwath Plus” from GeoAcoustics Lt. was employed. Also, on two sections perpendicular to the channel, an ADCP was employed obtaining speed and current direction in the water column during a complete tidal cycle.   The dunes field exhibit bed forms ranging from small to very big, reaching up to 4 m height and more than 100 m of spacing. Those dunes classified as “very large” occur in the middle of the navigational channel, close to Belgrano harbour access channel, where further dredging will take place. Usually, this kind of bed forms occurs in high dynamic environments as tidal inlets and macrotidal estuaries, not under a mesotidal regime as in the Bahia Blanca estuary. Results show two different and well defined zones characterized by a particular dunes morphology each and different hydrodynamic profile, with important lateral and vertical variations in tidal currents velocity and direction. Dunes migration also shows differential rate depending on the bed form size.   It is concluded that differences in speed and direction of tidal currents between both channel sides are responsible for the dunes field formation. Such asymmetry would be related to a geological control caused by the occurrence of hard materials generating a channel narrowing southward and the presence of a submarine bank northwards.