Morphodynamic consequences of dredging and dumping activities along the lower Oka estuary (Urdaibai Biosphere Reserve, southeastern Bay of Biscay, Spain)

Dredging and dumping in the lower Oka estuary (southeastern Bay of Biscay) during 1973–2003 have modified its pattern of sedimentary transport and morphology. An analysis of these activities through time and morphodynamic response of the estuarine system is presented. The relationships between both processes have been established.A Geographical Information System (GIS) has been used to create a temporal cartographic series of the changing patterns of estuarine sedimentary environments, identify the anthropogenic changes generated and observe the consequent responses of the estuarine system. The GIS has proven to be a very useful tool to monitor and evaluate the natural and human induced morphological evolution of the lower Oka estuary during the last 50 years. In the absence of dredging and dumping (1957–1973), the estuary had a distinct pattern of flood and ebb channels. Flood channels are deeper than ebb channels at their mouth, and progressively becomes shallower in the direction of the flooding tide. On the other hand, ebb channels form a seaward extension of the fluvial main channel. Both types of channels are prone to be evasive and braiding was common. During the period of study, the channels showed low natural variability relative to their location and spatial extension. The dredging and dumping carried out between 1973 and 2003 altered the natural flood/ebb channel distribution and modified sedimentary dynamics. Dredging caused the isolation of meanders of the ebb channel, the rapid infilling of the new dredged areas, and affected the flood channels thus producing the necessity for regular dredging, approximately every 5 years.Observation of the natural response of the estuarine system after the dredging and dumping makes it possible to state that, at present, the lower Oka estuary is not in a state of morphodynamic equilibrium and has a tendency to lose its capacity gradually. Dredging and dumping have accelerated this process and have increasingly unbalanced sedimentary regime.

► We analyzed the dredging over the Oka estuary and its morphodynamic response.
► In the absence of dredging the estuary presented a pattern of flood and ebb channels.
► The dredging altered the channel distribution and modified the sedimentary dynamics.
► At present, the lower Oka estuary has a tendency to lose its capacity gradually.
► The dredging and dumping have increasingly unbalanced its sedimentary regime.