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Morphodynamic consequences of dredging and dumping activities along the lower Oka estuary (Urdaibai Biosphere Reserve, southeastern Bay of Biscay, Spain)
Ocean & Coastal Management 77 (2013) 40e49
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Morphodynamic consequences of dredging and dumping activities along the lower Oka estuary (Urdaibai Biosphere Reserve, southeastern Bay of Biscay, Spain) M. Monge-Ganuzas a, *, A. Cearreta b, G. Evans c a Oficina Técnica de la Reserva de la Biosfera de Urdaibai, Dirección de Biodiversidad y Participación Ambiental del Gobierno Vasco, Carretera Gernika-Lumo s/n, P.K. 130, 48300 Gernika-Lumo, Spain b Departamento de Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Universidad del País Vasco/EHU, Apartado 644, 48080 Bilbao, Spain c Department of Ocean and Earth Sciences, National Oceanographic Centre, University of Southampton, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 19 February 2012
Dredging and dumping in the lower Oka estuary (southeastern Bay of Biscay) during 1973e2003 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 (1957e1973), 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. Ó 2012 Elsevier Ltd. All rights reserved.
1. 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, 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
* Corresponding author. E-mail addresses: [email protected] (M. Monge-Ganuzas), [email protected] (A. Cearreta), [email protected] (G. Evans). 0964-5691/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ocecoaman.2012.02.006
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 1e2 m/s (period MayeOctober). Occasionally within this period isolated peaks with velocities up to 6 m/s occur. On the other hand, the period NovembereApril 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).
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2. Materials and methods 2.1. Vertical aerial photographs and ortophotos
Fig. 1. Location of the Oka estuary and image of study area. Localities and morphological elements mentioned in the text are shown.
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 shortterm 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 1957e2005, determine the consequent morphological changes in the estuary and finally to establish morphodynamic criteria for suitable management of the latter.
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; 1982 flight scale 1:18000 and 1995 flight scale 1:18000; author: Biscay Province Council) and the photo-archive of the Environment Department of Basque Government (1992 flight scale 1:5000; 1996 flight scale 1:5000; 2001 (ortophoto) 0.5 m/píxel; 2002 (ortophoto) 0.25 m/ pixel and 2005 (ortophoto) 0.25 m/pixel; author: Basque Government). Due to scale variability of the photographs and ortophotos and with the aim of homogenizing the results, a common 1:20,000 scale was chosen for this study. The images corresponding to the 1957e1996 time period were not geo-referenced so they were first scanned (300 pixel/inch) and then geo-referenced using ArcMap GIS software. During the referencing process an RMS (Root Mean Square) lower than 20 m and an error higher than 30 m for each coordinate were deemed not to be acceptable. Later, photo interpretation and identification of different sedimentary environments throughout the temporal series was made using GIS software. Existing environmental units and their sedimentary structures were first identified in the field and compared with their air photo expression. As well, the minimum unit to map (100 m2) was chosen taking into account the work-scale and image quality. The sedimentary environments chosen were: tidal channels, intertidal sandy flats, intertidal mudflats, mixed intertidal flats, salt marshes; supratidal areas and zones of human occupation. The corresponding polygons for each sedimentary environment were delineated over each image. Later, the polygons were exported as shape files into Surfer 8 software and maps of sedimentary environments were produced for each group of historical images in order to illustrate their evolution throughout time. Also, the temporal variability of the ebb and flood channels and estuarine surface sedimentary structures (eg bedforms) were mapped and interpreted in relation to dredging and dumping operations. 2.2. Oblique aerial photographs In order to complete the analysis and to obtain additional information, historical oblique aerial photos obtained by individuals, private companies and public institutions were compiled and interpreted. 2.3. Public archives Likewise, historical and technical information was compiled from the local archives of the Ministry of Environment (1973e1993 period) and the Governing Board of the Urdaibai Biosphere Reserve (1994e2005 period). This information was used in order to describe the technical details of the dredging and dumping operations carried out during the last 50 years. 2.4. Bathymetric surveys In order to analyze the effects of the most recent dredging and dumping carried out in the lower Oka estuary different bathymetric
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surveys (before, immediately after, and 39 months after dredging) were carried out of the dredged zones. The first two were produced by the Murueta shipyard and were made available to the authors. The third was carried out using the bottom tracking tool of an Acustic Doppler Current Profiler (ADCP Winriver RDI 1200 with a Trimble GPS located on board a 4 m long vessel). These data were corrected using the nearby Bilbao tide-gauge record and bathymetric maps and volumetric calculations were produced using the Surfer 8 software. 3. Results 3.1. Period 1957e1972 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 later changes. 3.2. Period 1972e2005 3.2.1. Dredging 1973/1978 It was during this interval that the dredging and dumping operations were first undertaken in the estuary. A total of 223,000 m3 of sandy sediment was extracted from the main ebb channel of the estuary over an area 2800 m long, 40 m wide and to a depth of 2 m with reference to the Bilbao ordnance datum (Fig. 2). Extracted sandy material was dumped over the San Kristobal and Axpe salt marshes (Fig. 1). These were undergoing renaturalization after reclaimed agricultural land had been re-
Fig. 2. Evolution of sedimentary environments in the lower Oka estuary throughout the study period (1957e2005). Dredging periods are indicated.
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flooded by estuarine waters. After dredging and dumping, the meandering channels of Busturia and Kanala became inactive and turned into a single straight channel. The zones that were not dredged maintained their features as existed during the 1957e1972 period with the exception of the Laida beach which was intensively eroded. During the following years the morphology of this beach continued to vary (Fig. 3). 3.2.2. Dredging 1984/1987 In 1984 new dredging was undertaken and 125,000 m3 of sand were extracted from the main ebb channel in an area 1912 m long, 40 m wide and to a depth of 1 m below the Bilbao ordnance datum. The dredged material was dumped over the San Antonio salt marshes. Subsequently, in 1987 an unknown volume of muddy sediments was dredged opposite the Murueta shipyard and dumped over an adjacent salt marsh environment (Fig. 2). 3.2.3. Dredging 1992 In 1992 further dredging was undertaken, with 56,000 m3 of sand being extracted and dumped over the Kanala intertidal mudflat (68%) and the sandy intertidal flat situated to the north of Sandindere (32%) (Figs. 1 and 2). After the operations, the dredged
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zone became a straight channel that ran from the Murueta shipyard to Kanala. Small creeks draining into the dredged channel developed along its margin. The Kanala meander maintained its morphology but the Arketa meander was abandoned. Consequently, the main ebb channel avulsed towards the main flooddelta channel. Meanwhile, in the areas of dumping, the sandy sediment was re-worked by tidal currents. In front of San Antonio, the original flood-ebb tidal channel complex became deeper and wider after the dredging of the ebb channel located to the west. Consequently, the flood-ebb complex was destabilized. 3.2.4. Dredging 1995/1996 During this interval the zones previously dredged were dredged again and approximately 45,000 m3 of sand were dumped in areas of Kanala and Sandindere and, for the first time, also on the Laida beach (Figs. 1 and 2). After these operations a sandy supratidal accumulation appeared on the Laida beach, and the rebuilt Arketa meander was abandoned once again as the flow utilized the newly dredged channel. The area in the vicinity of Sandindere and Kanala became supratidal and the main ebb channel was constituted by two straight sections with an inflexion point in front of the Axpe dumping area. Chute structures (Van Straaten, 1954) developed in
Fig. 3. Dredging and dumping activities carried out in the lower Oka estuary throughout the study period (1957e2005).
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front of Kanala. The Kanala meander was also abandoned due to the cut induced by the action of the newly excavated channel. The edge of Axpe and San Kristobal dumping areas was re-worked by tidal currents and developed eroded margins with the eroded sediment being spread-out nearby. The edges of the Kanala dumping area were also partially eroded. These sediments were transported towards the inner estuary area and formed a sandy tongue over the mudflats further inland. Finally, in the mudflats the previously described network of creeks continued its development.
completely abandoned. The tidal inlet was reduced in width and the flood-delta showed a clear development. After dredging, the main ebb-tidal channel migrated towards the northwest in an attempt to regain its previous meandering configuration. These changes were faster around the Arketa meander than around the Kanala meander. The Kanala and Sandindere dumping areas formed new supratidal zones and were colonized by vegetation. Furthermore, the creek network draining into the straight dredged channel continued to develop in the mudflats.
3.2.5. Dredging 1998/1999 During this period two new dredgings were undertaken to maintain the channel width of 40 m and depth of 1 m below the Bilbao ordnance datum (Fig. 2). In 1998 the main ebb channel was dredged through out a length of 2010 m with the extraction of 50,600 m3of sand. In 1999 this zone was dredged once again and additional 42,000 m3 of sand were extracted. The muddy sediment was transferred away from the estuary, i.e out of the system, and the sandy sediment was dumped onto the Laida beach except for a small volume that was dumped over the San Antonio dumping area. After these dumpings, the supratidal area of the Laida beach increased markedly and the Arketa and Kanala meanders were
3.2.6. Dredging 2003 The last dredging, up to present, in the lower Oka estuary was carried out in 2003 along the main ebb channel in order to maintain the 40 m width and 1 m depth described (Fig. 2). A total of 243,000 m3 of the extracted sand was dumped onto the Laida beach and 44,000 m3 of muddy sediment were disposed elsewhere and removed from the estuarine system. After dumping, Laida beach gained a large supratidal area. The Kanala meander became active once again and the main ebb channel started migrating towards the north eroding the southern edge of the dumped accumulation. Also, the main ebb channel located on the western flanks of the estuary modified its position and moved eastwards (Fig. 4). These
Fig. 4. a 1e3: views of the lower Oka estuary after the 1973/1978 dredging. Observe salt marshes filled with sandy dredged sediment in ae1 and route of dredged channel in ae2 and ae3; b: view of the lower Oka estuary after the 1995/1996 dredging. Observe the dredged channel and cutted meanders; c: view of the estuary mouth after the 2003 dredging. Observe the accumulation of sediment over the mouth area that provoked the variation of the wave breaking.
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changes modified the quality of the famous surf waves in the area for more than 3 years (2003e2005) and provoked the suspension of the surf Billabong Pro championship and had an adverse economic impact on the surrounding area (Monge-Ganuzas et al., 2008). Due to the increase in the availability of sediment which could be transported into the estuary mouth, the flood tidal delta increased its length and prograded towards the east. Furthermore, inside the estuary the limit between the intertidal sandflats and mudflats moved approximately 200 m landwards, and the network of creeks over the mudflats continued developing. Fig. 5 illustrates the morphological response of the lower Oka estuary to the 2003 dredging and dumping. It shows the situation before dredging (A: March 2003) where the ebb main channel had a variable depth above the Bilbao ordnance datum between 1 and 2 m. After dredging (B: June 2003) the whole main ebb channel showed a depth of approximately 1 m above Bilbao ordnance datum. After 39 months (C: September 2006) the depth of the main ebb channel was dramatically reduced to between 0 and 1 m with reference to Bilbao datum. Zones adjacent to the main ebb channel gained significant accumulations of sand which were related to the lateral and front lobes of the flood channels that sometimes extend inland invading the previous main ebb channel. Therefore, in just over three years after the dredging operations were completed the lower Oka estuary had a reduced depth and had evolved towards a situation worse than existing before dredging commenced. 4. 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
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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 upper than in the lower estuary. These channel types are prone to be evasive due to tidal asymmetries (Robinson, 1960). In the lower Oka estuary such tidal asymmetries exist. During the last period of the ebb phase although the flood currents have started, ebb currents continue emptying the estuary for approximately 1 h (Monge-Ganuzas et al., 2008). Hence, the morphology of the Oka estuary under natural conditions (before 1973) displays a regular repetitive pattern that consists of mutually evasive meandering ebb channels and straight channels separated by intertidal shoals and linked by connecting channels (Fig. 6). This morphology has been referred to as a multi-channel system (Jeuken and Wang, 2010) and has been described in various estuaries around the world (Van Veen et al., 2005; Toffolon and Crosato, 2007). Human interferences on this ebb-flood tidal multi-channel system can produce important morphodynamic changes. Winterwerp et al. (2001) discussed the effect of human interference in the morphodynamics systems. They used the term cells (a chain of so-called macro-cells and meso-cells, based on morphological characteristics and numerically computed patterns of tide averaged sand transports. Each macro-cell consists of a main ebb channel and a main flood channel, displaying a characteristic morphologic behavior that is associated with net sediment exchanges between the macro-cells; Jeuken and Wang, 2010; page 553) to explain the process. These authors stated that the alteration of channel cells by dredging can provoke the complete degeneration of the affected cell and then, the alteration of all the cells in the estuary. Also, they showed that dumping of sediments in one of the cell channels increases the channel transport capacity. Hence, dumped sediments can be eroded and transported. On the contrary, when sand-dumping exceeds more than 10% of the transport
Fig. 5. Temporal evolution of the lower Oka estuary bathymetry. A: before the 2003 dredging; B: immediately after the 2003 dredging; C: 39 months after the 2003 dredging (A and B diagrams from Azti-Tecnalia).
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Fig. 6. Multi-channel cells identified in the lower Oka estuary under natural conditions in 1965 before the first historical dredging.
capacity of the channel, its hydraulic resistance increases considerably, current velocity decreases and therefore its transport capacity can lead to the cell degeneration. After the 1973e1978 dredging, the Busturia and Kanala meanders were abandoned. The Busturia meander remained inactive whilst the Kanala meander recovered during the 2003 dredging. The existence of the straight channel after dredging probably increased the transport capacity of the flood currents and generated a rapid channel infilling. Van Rijn (2004) stated that decreases of depth occur in zones where the transport capacity of currents is reduced and demonstrated that dredging activities can produce this effect. The need for more dredging in the straight channel after 1973e1978 supports this view. D’Agata and Mc Grath (2002) showed that dredging of a system in equilibrium provokes a rapid response by the estuarine system in order to restore the lost conditions. Also, these authors stated that natural morphologies are not fortuitous and therefore must be respected. In the same way, Morris (2007) demonstrated that dredging activities modify the pattern of filling of estuaries by moving them away from dynamic equilibrium. All dredgings were done to produce straight channels and interfered with the ebb/flood channel cells. After each dredging, due to the tendency of flood multi-channel cells to recover their original situation, the main ebb channel gradually migrated. However consecutive dredging did not permit the channel to regain
its original position. Also, dredging did not respect the original morphology of meanders and these were consequently abandoned. Because of the straight morphology of the dredged channel, the rate of sedimentation probably became higher. Without meanders the dissipation of energy during the ebb was lower and in the absence of evasive channels the main channel filled up rapidly during low and high tidal stages. Due to this response, further dredging was necessary. The continuous navigation problems in the lower estuary are an evidence of the sedimentary process described. Moreover, if the evolution of intertidal mudflats in the middle estuary from 1971 to 2005 is examined (Fig. 3), it is possible to see that the intertidal creek drainage net has been cutting down slowly. Probably, the continuous rectilinear dredging produced in that zone favored the erosion of the banks and beds of the creeks in order to equilibrate their profiles to the post-dredging situation. The dumping of 1973e1978 over the San Kristobal (204,000 m2) and Axpe (36,000 m2) salt marshes (Fig. 4) eliminated an important salt marsh area covered by protected biological species and reduced the estuarine flooding area. This reduction probably provoked the flooding of other estuarine areas or the erosion of the estuarine margins in order to accommodate the tidal prism. Moreover, due to the nature of the dumped sediment, the characteristics of the substrate were severely modified and the benthic organisms distribution was probably changed. The dumping of 1984 at San Antonio (50,000 m2) provoked similar consequences. An intertidal mudflat disappeared and the estuarine flooding surface was reduced again. Also, the repeated dumping of 1987, 1998/1999 and 2003 over an area adjacent to the shipyard (15,000 m2) eliminated an important salt marsh area. The dumpings of 1992 and 1995e1996 in Sandindere significantly modified the complex dynamics of the estuary mouth and contributed to the consolidation of that zone as a supratidal area (Fig. 7). These dumpings were done upon a lateral lobe of a flood channel which was forced to migrate towards the north. Later, dumped sediment was re-worked and transported consequently to the northwest by the action of main ebb channel currents. Hence, dumped sands were transported towards the ebb channel, which became narrower and provoked the avulsion of the flood channel and the creation of the chute morphologies (Van Straaten, 1954). Consequently, the Arketa meander was abandoned. The other dumping at Kanala, produced the formation of a supratidal sandy zone in what was previously a mixed intertidal area. Later, this area was rapidly colonized by vegetation and its edges were eroded. Sandy sediments were transported towards the middle estuary through the flood channels and altered the grain size characteristics of that estuarine zone. The progradation of these sediments landwards forced the ebb channel to modify its position and the curvature of the Kanala meander increased. Finally, once again the intertidal zone area was reduced. The dumping activities on the Laida beach in 1995, 2001 and 2003 contributed to the short-term restoration of its supratidal area but the mass of dumped sand was eroded rapidly by waves and tidal currents and re-introduced into the estuary across the tidal inlet and also towards the ebb-delta provoking an alteration in the pattern of the breaking waves (Monge-Ganuzas et al., 2008). Laida beach has continued to exhibit a morphological instability caused by wave energy variability generated by natural processes which modify the coastline and show a general erosive tendency in this location. Engineers tend to consider estuaries as long-term dynamic equilibrium environments when they plan stiff structures or dredging operations, i.e there is a balance between fluvial, tidal and meteorological inputs, the marine and fluvial sediment sources, and the estuarine capacity (the water volume between the estuarine basal surface and the water surface during spring tides is
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Fig. 7. Morphodynamic responses of the lower Oka estuary after the (a) 1995e1996 and (b) 2003 dredging and dumping activities.
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always constant). This implies that fluvial or coastal sediments do not accumulate inside the estuary except during short periods of time, and that much of the sediments which originated in the fluvial basin by-pass the estuary to contribute to sedimentation on the adjacent shelf. With the aim of examining this hypothesis, O’Connor (1987) studied the variability in capacity of various estuaries and discovered that capacity is controlled by several anthropogenic and natural interrelated factors. Some of them contribute to increase sedimentation and, hence, reduce the estuary capacity (negative factors). Other factors increase estuary capacity by sediment removal (positive factors). Following McDowell and O’Connor (1977), the negative factors are: sedimentary dynamics that infills the estuary with sediments, vegetation that traps sediment, interaction between waves and tidal currents that normally is prone to accumulate sediment inside the estuary, asymmetry of tidal currents that provokes sediment trapping if flood is dominant, density currents that provoke sediment accumulation within the estuary, ebb/flood channels and anthropogenic structures that also provoke sediment trapping. On the other side, positive factors are: dredging that increases the estuary capacity at least initially, port construction when is associated with dredging operations, important fluvial discharges that provoke estuarine surface erosion and sediment export, channel meandering that reduces flow velocity increasing sedimentation rate, and the introduction of pollutants which have an adverse effect on vegetation and consequently on sediment trapping. A dynamic equilibrium would occur if there was a balance between positive and negative factors. However, it is unlikely that there is a long-term equilibrium in a global sense if the observed changes in sedimentary systems through out time are taken into account (O’Connor, 1987). For instance, various authors (Inglis and Kestner, 1958; Kestner, 1961; Price and Kendrick, 1963; Wilkinson et al., 1973; Mersey Dock and Harbour Company, 1978) concluded that in the Mersey estuary (UK) the sedimentary balance during periods of 60e120 years was negative (loss of capacity). This would be a positive sedimentary balance, as sediment volume increases. It would be negative for estuary water capacity. In all these studies the observed variations were considered to be due to the natural introduction of sediment from the erosion of adjacent cliffs, beaches and the nearshore zone. Moreover, they stated that this estuary has not been in dynamic equilibrium, at least during the last 100 years. The reason for that imbalance is that this estuary, as the majority of estuaries, has been under anthropogenic pressures. Also, development of present estuaries started during the period of sea-level lowstand of the last glacial stage, when the fluvial channels eroded into the underlying rocks and carried large volumes of sediment onto the adjacent marine shelf. After melting of the Pleistocene ice and during the following eustatic sea-level rise sediment was transported into the estuaries due to wave, eolian and tidal action (O’Connor, 1987). However, the loss of capacity in each estuary will vary depending on its specific environmental and anthropogenic conditions. Other authors (Jonge de, 1983, 1992; Talke et al., 2009; Chernetsky et al., 2010) analyzed the relations between dredging activities, suspended matter concentrations, and the development of the tidal regime in the Ems estuary (NL). Between 1980 and 2005, successive deepening of the Ems estuary altered its tidal and sediment dynamics. The tidal range and the surface sediment concentration increased and the position of the turbidity zone shifted into the freshwater zone. The physical response to dredging of that estuary resulting in the trapping of sediment, thus a loss of capacity (negative consequence). Although there is insufficent bathymetric data on the development of the Oka estuary over a long time period, if anthropogenic activities are analyzed and compared with morphodynamic
variations it is possible to state that the modern Oka estuary is not in a dynamic equilibrium and that it has shown a natural tendency to infill, i.e, to lose its capacity as shown by historical and Quaternary geological data (Cearreta et al., 2005, 2006; Monge-Ganuzas et al., 2006). Also, it is clear that dredging and dumping operations have provoked a morphological and sedimentary imbalance of the ebb/flood channel cells and their adjacent intertidal zones which have modified their behavior, often with some important consequences (eg. the disappearance of the famous surfing wave in 2003e2005). 5. 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 channel situated to the south of Laida (i.e inland of) adapted to its existing morphology. Consequently, it has presented a variable morphology depending on the extent and location of the Laida beach. The sand-dumping upon the beach (1995e2003 period) increased its supratidal area temporarily but at the same time this sediment became available for transportation and was re-worked by waves and tidal currents and re-introduced into the estuary. In the absence of dredging and dumping operations (1957e1973 period), the lower Oka estuary had a four ebb-flood tidal multichannel system that showed little spatial variability. The consecutive dredging and dumping altered this natural channel distribution and modified the estuarine sedimentary dynamics. Dredged channels rapidly filled and provoked the necessity of new dredgings with an average periodicity of 5 years. Based on the consequences of dredging in other estuaries around the world and the process of capacity loss described by different authors, together with the response of the lower Oka estuary to the 2003 dredging it is possible to state that this estuary is currently unbalanced and shows a tendency to a gradual loss of capacity. Moreover, dredging and dumping operations have accelerated this loss of capacity, causing further imbalance in its estuarine dynamics. There is a human tendency to have a static vision of the coastal area. The case described shows that this does not conform to actual estuarine morphodynamics behavior. A lesson to be learned from this case study is that further deterioration of multi-channel systems, intertidal channels, intertidal flats, shoals and salt marshes should be avoided. This type of action requires a wellplanned strategy of future dredging and dumping operations if the kind of effects described here are to be prevented. 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 (ComplutenseUniversity, E) greatly improved the original manuscript with
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their comments and suggestions. It represents Contribution #6 of the Geo-Q Research Unit (Joaquín Gómez de Llarena Laboratory).
References Cearreta, A., Monge-Ganuzas, M., Iriarte E., 2004. Seguimiento y evaluación sedimentológica del proyecto de regeneración dunar en Laida (Reserva de la Biosfera de Urdaibai). Informe final (Octubre 2001eDiciembre 2004). Unpublished Report for the Environment and Land Use Department of the Basque Government. p. 42. Cearreta, A., Monge-Ganuzas, M., Irabien, M.J. and Iriarte, E., 2005. Herramientas geológicas para la interpretación medioambiental del yacimiento romano en Forua (Reserva de la Biosfera de Urdaibai). Unpublished Technical Report for the Culture Department of the Biscay Province Council. p. 31. Cearreta, A., Monge-Ganuzas, M., Iriarte, E., 2006. Análisis micropaleontológico (foraminíferos) y evolución ambiental holocena del estuario superior del Oka (área de Portuzarra, Gernika-Lumo). Illunzar 6, 57e68. Chernetsky, A.S., Schuttelaars, H.M., Talke, S.A., 2010. The effect of tidal asymmetry and temporal settling lag on sediment trapping in tidal estuaries. Ocean Dynamics 60, 1219e1241. Dalrymple, R.W., Zaitlin, B.A., Boyd, R.,1992. Estuarine facies models e conceptual basis and stratigraphic implications. Journal of Sedimentary Petrology 62, 1130e1146. Mersey Dock and Harbour Company Ltd, 1978. Quinquenal Survey of the Riser Mersey. Unpublished Technical report. p. 58. Dyer, K.R., 1973. Estuaries: A Physical Introduction. Wiley & Sons, London, p. 140. D’Agata, M., Mc Grath, J., 2002. The use of Currumbin Creek as a sand reserve: towards better dredging management?. In: Proceedings of the Littoral 2002 Conference. Porto, Portugal, pp. 145e149. Hayes, M.O., 1975. Morphology of sand accumulations in estuaries. In: Croning, L.E. (Ed.), Estuarine Research, 2. Academic Press, New York, pp. 3e22. Inglis, C.C., Kestner, F.J.T., 1958. The long-term effects of training walls, reclamation, and dredging on estuaries. Proceedings Institute of Civil Engineers 9, 193e216. International Association of Dredging Companies and Central Dredging Association (IADC/CEDA), 1998. Environmental Aspects of Dredging: Investigation, Interpretation and Impact. IADC, The Hague. Jeuken, M.C.J.L., Wang, Z.B., 2010. Impact of dredging and dumping on the stability of ebb-flood channel system. Coastal Engineering 57, 553e566. Jonge de, V.N., 1983. Relations between annual dredging activities, suspended matter concentrations, and the development of the tidal regime in the Ems estuary. Canadian Journal of Fisheries and Aquatic Sciences 40, 289e300. Jonge de, V.N., 1992. Tidal flow and residual flow in the Ems estuary. Estuarine, Coastal and Shelf Science 34, 1e22. Kestner, F.J.T., 1961. Short term changes in the distribution of fine sediment in estuaries. Proceedings of the Institute of Civil Engineers 19, 185e208. Lindeman, K., 1997. Comparative management of beach systems of Florida and the Antilles: applications using ecological assessment and decision support
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Procedures. In: Cambers, C. (Ed.), Managing Beach Resources in the Smaller Caribbean Islands. UNESCO, pp. 134e164. Coastal Region and Small Island Reports. McDowell, D.M., O’Connor, B.A., 1977. Hydraulic Behaviour of Estuaries. Mcmillan Press Ltd, London, p. 300. Monge-Ganuzas, M., Iriarte, E., Cearreta, A., 2006. Análisis sedimentario y evolución holocena del estuario superior del Oka en el área de Portuzarra (Gernika-Lumo, Bizkaia). Illunzar 6, 39e55. Monge-Ganuzas, M., Cearreta, A., Iriarte, E., 2008. Consequences of estuarine sand dredging and dumping on the Urdaibai Reserve of the Biosphere (Bay of Biscay): the case of the “Mundaka left wave”. Journal of Iberian Geology 34, 215e234. Morris, R., 2007. A nature conservation perspective of port-related dredging. Maritime Engineering 160, 19e23. O’Connor, B.A., 1987. Short and long term changes in estuary capacity. Journal of the Geological Society of London 144, 187e195. Price, W.A., Kendrick, M.P., 1963. Field and model investigation into the reasons for siltation in the Mersey estuary. Proceedings of the Institute of Civil Engineers 24, 473e518. Pritchard, D.W., 1952. Salinity distribution and circulation in the Chesapeake Bay estuarine system. Journal of Marine Research 11, 106e123. Pritchard, D.W., 1960. Lectures on Estuarine Oceanography. Chesapeake Bay Institute and Department of Oceanography. JohnHopkins University, p. 154. Robinson, A.H.W., 1960. Ebb-flood channel systems in sandy bays and estuaries. Geography 45, 183e199. Talke, S.A., de Swart, H.E., Schuttelaars, H.M., 2009. Feedback between residual circulation sand sediment distribution in highly turbid estuaries: an analytical model. Continental Shelf Research 29, 119e135. Toffolon, M., Crosato, A., 2007. Developing macro-scale indicators for estuarine morphology. The case of the Scheldt estuary. Journal of Coastal Research 23, 195e212. Van Rijn, L.C., 2004. Estuarine and coastal sedimentation problems. In: Proceedings of the 9th international Symposium on River Sedimentation, Yichang, China, pp. 156e177. Van Straaten, L.M.J.U., 1954. Composition and structure of recent marine sediments in the Neatherlands. Leidse Geologische Mededelingen 19, 1e108. Van Veen, J., 1936. Onderzoekingen in the Hoofden. SGravenhage, p. 252. Van Veen, J., van der Spek, A., Stive, M., Zitman, T., 2005. Ebb and flood channel systems in the Netherlands tidal waters. Journal of Coastal Research 21, 1107e1120. Wilkinson, H.R., de Boer, G., Thunder, A., 1973. A Cartographic Análisis of the Changing Bed of the Humber, vol. 1 and 2. Departament of Geography Reports, University of Hull. Winterwerp, J.C., Wang, Z.B., Stive, M.J.F., Arends, A., Jeuken, C., Kuijper, C., Toolen, P.M.C., 2001. A new morphological schematization of the western Scheldt estuary, The Netherlands. In: Proceedings of the 2nd IAHR Symposium on Rivers, Coastal and Estuarine Morphodynamics Symposium, Obihiro (Japan), pp. 525e533.
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Morphodynamic consequences of dredging and dumping activities along the lower Oka estuary (Urdaibai Biosphere Reserve, southeastern Bay of Biscay, Spain) M. Monge-Ganuzas a, *, A. Cearreta b, G. Evans c a Oficina Técnica de la Reserva de la Biosfera de Urdaibai, Dirección de Biodiversidad y Participación Ambiental del Gobierno Vasco, Carretera Gernika-Lumo s/n, P.K. 130, 48300 Gernika-Lumo, Spain b Departamento de Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Universidad del País Vasco/EHU, Apartado 644, 48080 Bilbao, Spain c Department of Ocean and Earth Sciences, National Oceanographic Centre, University of Southampton, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 19 February 2012
Dredging and dumping in the lower Oka estuary (southeastern Bay of Biscay) during 1973e2003 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 (1957e1973), 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. Ó 2012 Elsevier Ltd. All rights reserved.
1. 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, 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
* Corresponding author. E-mail addresses: [email protected] (M. Monge-Ganuzas), [email protected] (A. Cearreta), [email protected] (G. Evans). 0964-5691/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ocecoaman.2012.02.006
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 1e2 m/s (period MayeOctober). Occasionally within this period isolated peaks with velocities up to 6 m/s occur. On the other hand, the period NovembereApril 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).
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2. Materials and methods 2.1. Vertical aerial photographs and ortophotos
Fig. 1. Location of the Oka estuary and image of study area. Localities and morphological elements mentioned in the text are shown.
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 shortterm 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 1957e2005, determine the consequent morphological changes in the estuary and finally to establish morphodynamic criteria for suitable management of the latter.
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; 1982 flight scale 1:18000 and 1995 flight scale 1:18000; author: Biscay Province Council) and the photo-archive of the Environment Department of Basque Government (1992 flight scale 1:5000; 1996 flight scale 1:5000; 2001 (ortophoto) 0.5 m/píxel; 2002 (ortophoto) 0.25 m/ pixel and 2005 (ortophoto) 0.25 m/pixel; author: Basque Government). Due to scale variability of the photographs and ortophotos and with the aim of homogenizing the results, a common 1:20,000 scale was chosen for this study. The images corresponding to the 1957e1996 time period were not geo-referenced so they were first scanned (300 pixel/inch) and then geo-referenced using ArcMap GIS software. During the referencing process an RMS (Root Mean Square) lower than 20 m and an error higher than 30 m for each coordinate were deemed not to be acceptable. Later, photo interpretation and identification of different sedimentary environments throughout the temporal series was made using GIS software. Existing environmental units and their sedimentary structures were first identified in the field and compared with their air photo expression. As well, the minimum unit to map (100 m2) was chosen taking into account the work-scale and image quality. The sedimentary environments chosen were: tidal channels, intertidal sandy flats, intertidal mudflats, mixed intertidal flats, salt marshes; supratidal areas and zones of human occupation. The corresponding polygons for each sedimentary environment were delineated over each image. Later, the polygons were exported as shape files into Surfer 8 software and maps of sedimentary environments were produced for each group of historical images in order to illustrate their evolution throughout time. Also, the temporal variability of the ebb and flood channels and estuarine surface sedimentary structures (eg bedforms) were mapped and interpreted in relation to dredging and dumping operations. 2.2. Oblique aerial photographs In order to complete the analysis and to obtain additional information, historical oblique aerial photos obtained by individuals, private companies and public institutions were compiled and interpreted. 2.3. Public archives Likewise, historical and technical information was compiled from the local archives of the Ministry of Environment (1973e1993 period) and the Governing Board of the Urdaibai Biosphere Reserve (1994e2005 period). This information was used in order to describe the technical details of the dredging and dumping operations carried out during the last 50 years. 2.4. Bathymetric surveys In order to analyze the effects of the most recent dredging and dumping carried out in the lower Oka estuary different bathymetric
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surveys (before, immediately after, and 39 months after dredging) were carried out of the dredged zones. The first two were produced by the Murueta shipyard and were made available to the authors. The third was carried out using the bottom tracking tool of an Acustic Doppler Current Profiler (ADCP Winriver RDI 1200 with a Trimble GPS located on board a 4 m long vessel). These data were corrected using the nearby Bilbao tide-gauge record and bathymetric maps and volumetric calculations were produced using the Surfer 8 software. 3. Results 3.1. Period 1957e1972 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 later changes. 3.2. Period 1972e2005 3.2.1. Dredging 1973/1978 It was during this interval that the dredging and dumping operations were first undertaken in the estuary. A total of 223,000 m3 of sandy sediment was extracted from the main ebb channel of the estuary over an area 2800 m long, 40 m wide and to a depth of 2 m with reference to the Bilbao ordnance datum (Fig. 2). Extracted sandy material was dumped over the San Kristobal and Axpe salt marshes (Fig. 1). These were undergoing renaturalization after reclaimed agricultural land had been re-
Fig. 2. Evolution of sedimentary environments in the lower Oka estuary throughout the study period (1957e2005). Dredging periods are indicated.
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flooded by estuarine waters. After dredging and dumping, the meandering channels of Busturia and Kanala became inactive and turned into a single straight channel. The zones that were not dredged maintained their features as existed during the 1957e1972 period with the exception of the Laida beach which was intensively eroded. During the following years the morphology of this beach continued to vary (Fig. 3). 3.2.2. Dredging 1984/1987 In 1984 new dredging was undertaken and 125,000 m3 of sand were extracted from the main ebb channel in an area 1912 m long, 40 m wide and to a depth of 1 m below the Bilbao ordnance datum. The dredged material was dumped over the San Antonio salt marshes. Subsequently, in 1987 an unknown volume of muddy sediments was dredged opposite the Murueta shipyard and dumped over an adjacent salt marsh environment (Fig. 2). 3.2.3. Dredging 1992 In 1992 further dredging was undertaken, with 56,000 m3 of sand being extracted and dumped over the Kanala intertidal mudflat (68%) and the sandy intertidal flat situated to the north of Sandindere (32%) (Figs. 1 and 2). After the operations, the dredged
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zone became a straight channel that ran from the Murueta shipyard to Kanala. Small creeks draining into the dredged channel developed along its margin. The Kanala meander maintained its morphology but the Arketa meander was abandoned. Consequently, the main ebb channel avulsed towards the main flooddelta channel. Meanwhile, in the areas of dumping, the sandy sediment was re-worked by tidal currents. In front of San Antonio, the original flood-ebb tidal channel complex became deeper and wider after the dredging of the ebb channel located to the west. Consequently, the flood-ebb complex was destabilized. 3.2.4. Dredging 1995/1996 During this interval the zones previously dredged were dredged again and approximately 45,000 m3 of sand were dumped in areas of Kanala and Sandindere and, for the first time, also on the Laida beach (Figs. 1 and 2). After these operations a sandy supratidal accumulation appeared on the Laida beach, and the rebuilt Arketa meander was abandoned once again as the flow utilized the newly dredged channel. The area in the vicinity of Sandindere and Kanala became supratidal and the main ebb channel was constituted by two straight sections with an inflexion point in front of the Axpe dumping area. Chute structures (Van Straaten, 1954) developed in
Fig. 3. Dredging and dumping activities carried out in the lower Oka estuary throughout the study period (1957e2005).
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front of Kanala. The Kanala meander was also abandoned due to the cut induced by the action of the newly excavated channel. The edge of Axpe and San Kristobal dumping areas was re-worked by tidal currents and developed eroded margins with the eroded sediment being spread-out nearby. The edges of the Kanala dumping area were also partially eroded. These sediments were transported towards the inner estuary area and formed a sandy tongue over the mudflats further inland. Finally, in the mudflats the previously described network of creeks continued its development.
completely abandoned. The tidal inlet was reduced in width and the flood-delta showed a clear development. After dredging, the main ebb-tidal channel migrated towards the northwest in an attempt to regain its previous meandering configuration. These changes were faster around the Arketa meander than around the Kanala meander. The Kanala and Sandindere dumping areas formed new supratidal zones and were colonized by vegetation. Furthermore, the creek network draining into the straight dredged channel continued to develop in the mudflats.
3.2.5. Dredging 1998/1999 During this period two new dredgings were undertaken to maintain the channel width of 40 m and depth of 1 m below the Bilbao ordnance datum (Fig. 2). In 1998 the main ebb channel was dredged through out a length of 2010 m with the extraction of 50,600 m3of sand. In 1999 this zone was dredged once again and additional 42,000 m3 of sand were extracted. The muddy sediment was transferred away from the estuary, i.e out of the system, and the sandy sediment was dumped onto the Laida beach except for a small volume that was dumped over the San Antonio dumping area. After these dumpings, the supratidal area of the Laida beach increased markedly and the Arketa and Kanala meanders were
3.2.6. Dredging 2003 The last dredging, up to present, in the lower Oka estuary was carried out in 2003 along the main ebb channel in order to maintain the 40 m width and 1 m depth described (Fig. 2). A total of 243,000 m3 of the extracted sand was dumped onto the Laida beach and 44,000 m3 of muddy sediment were disposed elsewhere and removed from the estuarine system. After dumping, Laida beach gained a large supratidal area. The Kanala meander became active once again and the main ebb channel started migrating towards the north eroding the southern edge of the dumped accumulation. Also, the main ebb channel located on the western flanks of the estuary modified its position and moved eastwards (Fig. 4). These
Fig. 4. a 1e3: views of the lower Oka estuary after the 1973/1978 dredging. Observe salt marshes filled with sandy dredged sediment in ae1 and route of dredged channel in ae2 and ae3; b: view of the lower Oka estuary after the 1995/1996 dredging. Observe the dredged channel and cutted meanders; c: view of the estuary mouth after the 2003 dredging. Observe the accumulation of sediment over the mouth area that provoked the variation of the wave breaking.
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changes modified the quality of the famous surf waves in the area for more than 3 years (2003e2005) and provoked the suspension of the surf Billabong Pro championship and had an adverse economic impact on the surrounding area (Monge-Ganuzas et al., 2008). Due to the increase in the availability of sediment which could be transported into the estuary mouth, the flood tidal delta increased its length and prograded towards the east. Furthermore, inside the estuary the limit between the intertidal sandflats and mudflats moved approximately 200 m landwards, and the network of creeks over the mudflats continued developing. Fig. 5 illustrates the morphological response of the lower Oka estuary to the 2003 dredging and dumping. It shows the situation before dredging (A: March 2003) where the ebb main channel had a variable depth above the Bilbao ordnance datum between 1 and 2 m. After dredging (B: June 2003) the whole main ebb channel showed a depth of approximately 1 m above Bilbao ordnance datum. After 39 months (C: September 2006) the depth of the main ebb channel was dramatically reduced to between 0 and 1 m with reference to Bilbao datum. Zones adjacent to the main ebb channel gained significant accumulations of sand which were related to the lateral and front lobes of the flood channels that sometimes extend inland invading the previous main ebb channel. Therefore, in just over three years after the dredging operations were completed the lower Oka estuary had a reduced depth and had evolved towards a situation worse than existing before dredging commenced. 4. 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
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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 upper than in the lower estuary. These channel types are prone to be evasive due to tidal asymmetries (Robinson, 1960). In the lower Oka estuary such tidal asymmetries exist. During the last period of the ebb phase although the flood currents have started, ebb currents continue emptying the estuary for approximately 1 h (Monge-Ganuzas et al., 2008). Hence, the morphology of the Oka estuary under natural conditions (before 1973) displays a regular repetitive pattern that consists of mutually evasive meandering ebb channels and straight channels separated by intertidal shoals and linked by connecting channels (Fig. 6). This morphology has been referred to as a multi-channel system (Jeuken and Wang, 2010) and has been described in various estuaries around the world (Van Veen et al., 2005; Toffolon and Crosato, 2007). Human interferences on this ebb-flood tidal multi-channel system can produce important morphodynamic changes. Winterwerp et al. (2001) discussed the effect of human interference in the morphodynamics systems. They used the term cells (a chain of so-called macro-cells and meso-cells, based on morphological characteristics and numerically computed patterns of tide averaged sand transports. Each macro-cell consists of a main ebb channel and a main flood channel, displaying a characteristic morphologic behavior that is associated with net sediment exchanges between the macro-cells; Jeuken and Wang, 2010; page 553) to explain the process. These authors stated that the alteration of channel cells by dredging can provoke the complete degeneration of the affected cell and then, the alteration of all the cells in the estuary. Also, they showed that dumping of sediments in one of the cell channels increases the channel transport capacity. Hence, dumped sediments can be eroded and transported. On the contrary, when sand-dumping exceeds more than 10% of the transport
Fig. 5. Temporal evolution of the lower Oka estuary bathymetry. A: before the 2003 dredging; B: immediately after the 2003 dredging; C: 39 months after the 2003 dredging (A and B diagrams from Azti-Tecnalia).
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Fig. 6. Multi-channel cells identified in the lower Oka estuary under natural conditions in 1965 before the first historical dredging.
capacity of the channel, its hydraulic resistance increases considerably, current velocity decreases and therefore its transport capacity can lead to the cell degeneration. After the 1973e1978 dredging, the Busturia and Kanala meanders were abandoned. The Busturia meander remained inactive whilst the Kanala meander recovered during the 2003 dredging. The existence of the straight channel after dredging probably increased the transport capacity of the flood currents and generated a rapid channel infilling. Van Rijn (2004) stated that decreases of depth occur in zones where the transport capacity of currents is reduced and demonstrated that dredging activities can produce this effect. The need for more dredging in the straight channel after 1973e1978 supports this view. D’Agata and Mc Grath (2002) showed that dredging of a system in equilibrium provokes a rapid response by the estuarine system in order to restore the lost conditions. Also, these authors stated that natural morphologies are not fortuitous and therefore must be respected. In the same way, Morris (2007) demonstrated that dredging activities modify the pattern of filling of estuaries by moving them away from dynamic equilibrium. All dredgings were done to produce straight channels and interfered with the ebb/flood channel cells. After each dredging, due to the tendency of flood multi-channel cells to recover their original situation, the main ebb channel gradually migrated. However consecutive dredging did not permit the channel to regain
its original position. Also, dredging did not respect the original morphology of meanders and these were consequently abandoned. Because of the straight morphology of the dredged channel, the rate of sedimentation probably became higher. Without meanders the dissipation of energy during the ebb was lower and in the absence of evasive channels the main channel filled up rapidly during low and high tidal stages. Due to this response, further dredging was necessary. The continuous navigation problems in the lower estuary are an evidence of the sedimentary process described. Moreover, if the evolution of intertidal mudflats in the middle estuary from 1971 to 2005 is examined (Fig. 3), it is possible to see that the intertidal creek drainage net has been cutting down slowly. Probably, the continuous rectilinear dredging produced in that zone favored the erosion of the banks and beds of the creeks in order to equilibrate their profiles to the post-dredging situation. The dumping of 1973e1978 over the San Kristobal (204,000 m2) and Axpe (36,000 m2) salt marshes (Fig. 4) eliminated an important salt marsh area covered by protected biological species and reduced the estuarine flooding area. This reduction probably provoked the flooding of other estuarine areas or the erosion of the estuarine margins in order to accommodate the tidal prism. Moreover, due to the nature of the dumped sediment, the characteristics of the substrate were severely modified and the benthic organisms distribution was probably changed. The dumping of 1984 at San Antonio (50,000 m2) provoked similar consequences. An intertidal mudflat disappeared and the estuarine flooding surface was reduced again. Also, the repeated dumping of 1987, 1998/1999 and 2003 over an area adjacent to the shipyard (15,000 m2) eliminated an important salt marsh area. The dumpings of 1992 and 1995e1996 in Sandindere significantly modified the complex dynamics of the estuary mouth and contributed to the consolidation of that zone as a supratidal area (Fig. 7). These dumpings were done upon a lateral lobe of a flood channel which was forced to migrate towards the north. Later, dumped sediment was re-worked and transported consequently to the northwest by the action of main ebb channel currents. Hence, dumped sands were transported towards the ebb channel, which became narrower and provoked the avulsion of the flood channel and the creation of the chute morphologies (Van Straaten, 1954). Consequently, the Arketa meander was abandoned. The other dumping at Kanala, produced the formation of a supratidal sandy zone in what was previously a mixed intertidal area. Later, this area was rapidly colonized by vegetation and its edges were eroded. Sandy sediments were transported towards the middle estuary through the flood channels and altered the grain size characteristics of that estuarine zone. The progradation of these sediments landwards forced the ebb channel to modify its position and the curvature of the Kanala meander increased. Finally, once again the intertidal zone area was reduced. The dumping activities on the Laida beach in 1995, 2001 and 2003 contributed to the short-term restoration of its supratidal area but the mass of dumped sand was eroded rapidly by waves and tidal currents and re-introduced into the estuary across the tidal inlet and also towards the ebb-delta provoking an alteration in the pattern of the breaking waves (Monge-Ganuzas et al., 2008). Laida beach has continued to exhibit a morphological instability caused by wave energy variability generated by natural processes which modify the coastline and show a general erosive tendency in this location. Engineers tend to consider estuaries as long-term dynamic equilibrium environments when they plan stiff structures or dredging operations, i.e there is a balance between fluvial, tidal and meteorological inputs, the marine and fluvial sediment sources, and the estuarine capacity (the water volume between the estuarine basal surface and the water surface during spring tides is
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Fig. 7. Morphodynamic responses of the lower Oka estuary after the (a) 1995e1996 and (b) 2003 dredging and dumping activities.
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always constant). This implies that fluvial or coastal sediments do not accumulate inside the estuary except during short periods of time, and that much of the sediments which originated in the fluvial basin by-pass the estuary to contribute to sedimentation on the adjacent shelf. With the aim of examining this hypothesis, O’Connor (1987) studied the variability in capacity of various estuaries and discovered that capacity is controlled by several anthropogenic and natural interrelated factors. Some of them contribute to increase sedimentation and, hence, reduce the estuary capacity (negative factors). Other factors increase estuary capacity by sediment removal (positive factors). Following McDowell and O’Connor (1977), the negative factors are: sedimentary dynamics that infills the estuary with sediments, vegetation that traps sediment, interaction between waves and tidal currents that normally is prone to accumulate sediment inside the estuary, asymmetry of tidal currents that provokes sediment trapping if flood is dominant, density currents that provoke sediment accumulation within the estuary, ebb/flood channels and anthropogenic structures that also provoke sediment trapping. On the other side, positive factors are: dredging that increases the estuary capacity at least initially, port construction when is associated with dredging operations, important fluvial discharges that provoke estuarine surface erosion and sediment export, channel meandering that reduces flow velocity increasing sedimentation rate, and the introduction of pollutants which have an adverse effect on vegetation and consequently on sediment trapping. A dynamic equilibrium would occur if there was a balance between positive and negative factors. However, it is unlikely that there is a long-term equilibrium in a global sense if the observed changes in sedimentary systems through out time are taken into account (O’Connor, 1987). For instance, various authors (Inglis and Kestner, 1958; Kestner, 1961; Price and Kendrick, 1963; Wilkinson et al., 1973; Mersey Dock and Harbour Company, 1978) concluded that in the Mersey estuary (UK) the sedimentary balance during periods of 60e120 years was negative (loss of capacity). This would be a positive sedimentary balance, as sediment volume increases. It would be negative for estuary water capacity. In all these studies the observed variations were considered to be due to the natural introduction of sediment from the erosion of adjacent cliffs, beaches and the nearshore zone. Moreover, they stated that this estuary has not been in dynamic equilibrium, at least during the last 100 years. The reason for that imbalance is that this estuary, as the majority of estuaries, has been under anthropogenic pressures. Also, development of present estuaries started during the period of sea-level lowstand of the last glacial stage, when the fluvial channels eroded into the underlying rocks and carried large volumes of sediment onto the adjacent marine shelf. After melting of the Pleistocene ice and during the following eustatic sea-level rise sediment was transported into the estuaries due to wave, eolian and tidal action (O’Connor, 1987). However, the loss of capacity in each estuary will vary depending on its specific environmental and anthropogenic conditions. Other authors (Jonge de, 1983, 1992; Talke et al., 2009; Chernetsky et al., 2010) analyzed the relations between dredging activities, suspended matter concentrations, and the development of the tidal regime in the Ems estuary (NL). Between 1980 and 2005, successive deepening of the Ems estuary altered its tidal and sediment dynamics. The tidal range and the surface sediment concentration increased and the position of the turbidity zone shifted into the freshwater zone. The physical response to dredging of that estuary resulting in the trapping of sediment, thus a loss of capacity (negative consequence). Although there is insufficent bathymetric data on the development of the Oka estuary over a long time period, if anthropogenic activities are analyzed and compared with morphodynamic
variations it is possible to state that the modern Oka estuary is not in a dynamic equilibrium and that it has shown a natural tendency to infill, i.e, to lose its capacity as shown by historical and Quaternary geological data (Cearreta et al., 2005, 2006; Monge-Ganuzas et al., 2006). Also, it is clear that dredging and dumping operations have provoked a morphological and sedimentary imbalance of the ebb/flood channel cells and their adjacent intertidal zones which have modified their behavior, often with some important consequences (eg. the disappearance of the famous surfing wave in 2003e2005). 5. 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 channel situated to the south of Laida (i.e inland of) adapted to its existing morphology. Consequently, it has presented a variable morphology depending on the extent and location of the Laida beach. The sand-dumping upon the beach (1995e2003 period) increased its supratidal area temporarily but at the same time this sediment became available for transportation and was re-worked by waves and tidal currents and re-introduced into the estuary. In the absence of dredging and dumping operations (1957e1973 period), the lower Oka estuary had a four ebb-flood tidal multichannel system that showed little spatial variability. The consecutive dredging and dumping altered this natural channel distribution and modified the estuarine sedimentary dynamics. Dredged channels rapidly filled and provoked the necessity of new dredgings with an average periodicity of 5 years. Based on the consequences of dredging in other estuaries around the world and the process of capacity loss described by different authors, together with the response of the lower Oka estuary to the 2003 dredging it is possible to state that this estuary is currently unbalanced and shows a tendency to a gradual loss of capacity. Moreover, dredging and dumping operations have accelerated this loss of capacity, causing further imbalance in its estuarine dynamics. There is a human tendency to have a static vision of the coastal area. The case described shows that this does not conform to actual estuarine morphodynamics behavior. A lesson to be learned from this case study is that further deterioration of multi-channel systems, intertidal channels, intertidal flats, shoals and salt marshes should be avoided. This type of action requires a wellplanned strategy of future dredging and dumping operations if the kind of effects described here are to be prevented. 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 (ComplutenseUniversity, E) greatly improved the original manuscript with
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their comments and suggestions. It represents Contribution #6 of the Geo-Q Research Unit (Joaquín Gómez de Llarena Laboratory).
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