Morphodynamic consequences of dredging and dumping activities along the lower Oka estuary (Urdaibai Biosphere Reserve, southeastern Bay of Biscay, Spain)
Highlights
► 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.
Introduction
The Oka estuary is located in the southeastern Bay of Biscay and, together with its hydrographic basin, was declared by UNESCO in 1984 as a Reserve of the Biosphere. This estuary is a drowned fluvial valley type (Pritchard, 1952, Pritchard, 1960), meso-macrotidal (Hayes, 1975) with semidiurnal tides (tidal range 4.5 m on springs and 1.5 m on neaps), total mixed (Dyer, 1973) and tide dominated (Dalrymple et al., 1992). The maximum width is approximately 1000 m and its length is 12 km, and it has an intertidal area of about 2 km2 (Fig. 1). The local wind intensity and direction show both north and south components, reaching average daily velocities of 1–2 m/s (period May–October). Occasionally within this period isolated peaks with velocities up to 6 m/s occur. On the other hand, the period November–April is characterized by predominance of northerly winds with an average daily velocity of around 4 m/s or higher; the maximum monthly values can reach up to 10 m/s (Cearreta et al., 2004).
The most significant human activity which has occured during the last 50 years in the lower Oka estuary has been the dredging and dumping of sediment to deepen and maintain the navigation route from the Murueta shipyard, constructed in 1943, to the open sea (Monge-Ganuzas et al., 2008).
The effects of dredging and dumping in the environment are variable and depend on the estuarine area and other factors such as: the magnitude and frequency of dredging, the dredging method, the form, length and depth of dredging, the grain size, the density and composition of dredged material, the intertidal area of dredging, the quality of water and sediment, the tidal range, the direction and intensity of tidal currents, the water mixing, the seasonal variability, the proximity to coastline and the presence of biological communities (IADC/CEDA, 1998).
Prediction of potential adverse effects of dredging and dumping in estuaries cannot be evaluated properly if the previously mentioned parameters are unknown. Moreover, long- and short-term effects on dredged areas must be taken in account. As well, when dredging is undertaken periodically it is possible that this provokes accumulative effects. Lindeman (1997) stated that the most devastating environmental effects are not a product of particular activities but a combination of multiple single and individual effects throughout time.
The aim of this study is to analyze the dredging and dumping which have occurred in the lower Oka estuary during the period 1957–2005, determine the consequent morphological changes in the estuary and finally to establish morphodynamic criteria for suitable management of the latter.
Section snippets
Vertical aerial photographs and ortophotos
During the last 50 years various private companies and public institutions have photographed the lower Oka estuary extensively. These images have been used in this study to describe the temporal evolution of its sedimentary environments. The information sources used here were: the photo-archive of the Urdaibai Biosphere Reserve Governing board (1957 flight, scale 1:7000; author: U.S. Army), the photo-archive of the Biscay Province Council (1965 flight scale 1:20000; 1971 flight scale 1:7000;
Period 1957–1972
During this period no dredging or dumping was undertaken in the lower Oka estuary and only some limited, small scale sand extraction was carried out. Therefore, the sedimentary evolution of the estuary during this time interval was dominated by natural dynamics. Furthermore, no significant changes in the distribution of estuarine sedimentary environments were detected (Fig. 2). This situation can be considered as typical of the original estuarine conditions and may be used as a reference for
Discussion
During a tidal cycle currents redistribute the sandy sediment that has been introduced into the estuary through the inlet by waves and flood tidal currents. Depending upon the main tidal direction in different parts of the estuary Van Veen (1936) suggested the use of the terms flood channel and ebb channel. Flood channels are deep at their seaward part and progressively become shallower landwards. Ebb channels are the estuarine extension of the fluvial channel and are slightly deeper in the
Conclusions
The use of GIS combined with oblique photographs and historical information has proven to be a useful tool for monitoring and evaluating the anthropogenic-morphodynamic evolution of the lower Oka estuary.
The analysis carried out shows that the Laida beach area has varied throughout the study period. Between 1957 and 1995 its morphology was dependent on the balance between sediment availability, and wave, wind and tidal energy. Conditioned by these sedimentary transport processes, the ebb-tidal
Acknowledgments
Murueta shipyard kindly supplied information of historical bathymetric surveys (March and June 2003) in the lower Oka estuary. This research has benefited from the funding provided by the following projects: TANYA (MICINN, CGL2009-08840), K-Egokitzen II (GV, Etortek 2010) and Harea-Coastal Geology Research Group (GV, 80IT365-10) and Unidad de Formación e Investigación en Cuaternario (UPV/EHU, UFI11/09). Prof. Antonio Cendrero (University of Cantabria, E) and Prof. Alfredo Arche
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