Oka Estuary Research Articles

Holocene environmental evolution and relative sea-level change in the Oka estuary (Urdaibai Biosphere Reserve, northern Spain)

The Holocene environmental evolution in coastal areas, including deltas and estuaries, is vital to understanding coastal dynamics and how they may change in the future. Here, we studied the Holocene environmental evolution of the Oka estuary (Urdaibai Biosphere Reserve, northern Spain) from sedimentary cores that were 11–49 m long. We applied a multi-proxy approach of benthic foraminifera and grain size, and a temporal framework of radiocarbon dates to reconstruct paleoenvironments. The benthic foraminifera assemblages varied from near-marine (allochthonous and autochthonous hyaline species such as Lobatula lobatula, Rosalina irregularis, Ammonia tepida and Haynesina germanica) to brackish intertidal (autochthonous hyaline species such as A. tepida and H. germanica) to salt-marsh (autochthonous agglutinated species Trochammina inflata and Entzia macrescens) environments. Grain size analysis supported the foraminiferal assemblages and the sedimentary sequences were sand-dominated in near-marine intertidal environments and mud-dominated in the brackish intertidal and salt-marsh environments. The chronology was constrained by thirty four radiocarbon dates from plant macrofossils, marine shells and wood fragments. Our paleoenvironmental reconstructions were used to produce an updated local Holocene relative sea-level (RSL) record based on new data, revised reconstructions, and age recalibration of previous research. Marine limiting dates showed that RSL changed from above −20.4 ± 0.2 m at ∼9100 cal. yrs. BP to above −4.2 ± 0.2 m at ∼6700 cal. yrs. BP. In the late Holocene, sea-level index points (SLIPs) reconstructed the RSL position from −2.7 ± 1.2 m at ∼4900 cal. yrs. BP to −1.4 ± 1.2 m at ∼1500 cal. yrs. BP. Estimates of rates of RSL change from the Errors-in-Variables Integrated Gaussian Process (EIV-IGP) model show RSL gradually rose at 0.3 ± 2.1 mm yr−1 in the late Holocene. There was a progressive shallowing of the estuary as rates of RSL rise reduced during the Holocene. Between ∼9000 and 7000-6000 cal. yrs. BP pre-Holocene fluvial gravels (Lowstand Systems Tract) were replaced by both sandy marine and brackish intertidal sediments (Transgressive Systems Tract). After ∼7000-6000 cal. yrs. BP salt-marsh sediments deposited (Highstand Systems Tract). The sequence was overlain by anthropogenic deposits derived from agricultural, dredging and dumping activities since the 18th century (Anthropogenic Systems Tract). The Holocene environmental evolution and relative sea-level change of Oka is broadly consistent with previous studies in northern Spain, although it places the onset of the Highstand Systems Tract ∼2000 years earlier.

Modelling the morphological response of the Oka estuary (SE Bay of Biscay) to climate change

Adaptation in and around estuaries, to ensure the future resilience of both the natural environment and the communities that are located alongside is an increasingly pressing need. As part of an adaptation planning program for the Basque estuaries (northern Spain, south-eastern Bay of Biscay), a modelling system to simulate the morphological response of the Oka estuary to climate change has been developed. Existing knowledge was used as the basis to frame this pilot modelling examining both the short-term change (storm, seasonal, annual) and the longer-term changes (decades to eons). In this paper, we focus on the long-term response and consideration of the output from the process model is limited to the data that was extracted to help parameterize the long-term model. The latter was then used to examine the response of the estuary to the past changes over a historical time scale, where observations from foraminiferal analysis and saltmarsh response were used to establish the predictive validity of the model.Forcing the model with a linear rate of sea level rise representative of the 20th century, the estuary keeps pace with sea level rise in agreement with the observations. An 18.6-year periodic variability was identified over the saltmarshes, by including the effect of the nodal tide, with the last deposition maximum occurring in 2020.The effect of the opening of a saltmarsh that was drained at the beginning of the 18th century was investigated using a simplified schematization and showed sedimentation rates of up to one order of magnitude larger than in natural marsh, in general agreement with the rates obtained in previous studies.To predict the future response of the estuary, simulations with an exponential sea level rise were performed using scenarios obtained from the latest IPCC projections. The estuary fails to keep pace with sea level rise in any of the scenarios considered. The attendant increase in the hydroperiod over the saltmarshes is also likely impact to saltmarsh species productivity and distribution.Despite the simplifications inherent to the model, and the simplifications made in the representation of the estuary and forcing conditions, the methods presented here provide additional insights into long-term behaviour that complement information obtained from more detailed, but short-term, process models. The combined modelling approach allows adaptation measures to climate change to be considered over longer timescales.

Estimated footprint of shellfishing activities in Zostera noltei meadows in a northern Spain estuary: Lessons for management

Intertidal seagrasses often occupy areas of high ecological importance that are also targeted for human activities, such as shellfishing and leisure walking. These activities could affect the growth or survival of seagrass. The society at large lacks knowledge of the ecological relevance of seagrasses and their vulnerability, presenting an obstacle for their conservation. Here, the Oka estuary (Basque Country, N. Spain) is used as a case study to analyse the spatial extent of shellfishing activities in seagrass (Zostera noltei). This estuary holds 84% of the intertidal seagrass meadows present in the Basque Country and is visited during the year by professional and recreational shellfishers. Bait digging is legal year-round, and clam digging (by professionals only) from October to February. Human presence over 11 years averaged 17 people on the tidal flat in each daylight hours from August to October. Each professional shellfishing daily trip affected 7.4–21.6 m2 by fishing and 110–188 m2 by trampling in seagrass, but was substantially different between clam and bait (crustacean, worm) fishing. Scaling up this area to estimated annual number of shellfishers, 8–69% of the total seagrass area is trampled and 1–9% is dug per year. In comparison to a habitat suitability map for seagrass, 98% of trampling occurred within a very highly suitable area for seagrass presence (i.e., >90%). This pressure could adversely affect seagrass density and prevent the expansion of meadows into areas suitable for seagrass. Management of public impacts to seagrass in this estuary could occur through prohibition of some activities in seagrass and awareness campaigns among shellfishers and walkers.

Ethno-political situation in the Oka estuary region on the eve of the Mongol invasion

The article is devoted to the ethno-political situation in the region of the Oka estuary up to 1238. In 1221 Nizhny Novgorod was founded in this region. The forthcoming 800th anniversary of the city lends particular social and cultural relevance to the present study; these aspects of the history of Eastern Europe and North-Eastern Russia are considered here for the first time. The research is based on the study of chronicles and narratives from the 17th to 19th centuries, as well as of toponyms. The source corpus is represented here by the Lavrentyevskaya chronicle, the Ipatyevskaya chronicle, the Simeonovskaya chronicle, the Tolkovaya Paleya, as well as the “Slovo o pogibeli russkoi zemli” (“The word about the death of the Russian land”), in addition to results of archaeological and toponymic research. The author offers a novel interpretation of the Cheremis-Meshcher group in the region of the Oka estuary, by demonstrating that these Cheremis lived between the rivers Volga, Oka and Klyazma; these Cheremis has no direct relation to the Mari. The article presents arguments in favor of the hypothesis that before 1221 there had been no Slavic population in the Oka estuary. The assumption that Brodniks were living in this region cannot be confirmed. There are not enough representative sources to interpret the occurrence of the term Purgasova Rus ’ . The Mordvins lived far from the Oka estuary. Polovtsy (Cumans), however, were present in the region. This territory remained beyond the political influence zone of Volga Bolgaria. Accordingly, it must be accepted that Nizhny Novgorod was founded on an uninhabited place. The area around Nizhny Novgorod was a frontier zone where the interests of Volga Bolgaria clashed with those of the Vladimir Principality. The foundation of Nizhny Novgorod changed the nature of the conflict in this frontier. The Cheremis were blocked, and a military-diplomatic offensive was launched on the Mordvins. The Polovtsy (Cumans) acted as allies of the Vladimir Principality, and the latter was victorious in the Russian-Bolgarian war of 1223-1230. The reconstruction of the complex ethno-political situation in the region of the Oka estuary enables us to study the imperial practices of the princes of Vladimir before the Mongol invasion.

Morphodynamic evolution of Laida beach (Oka estuary, Urdaibai Biosphere Reserve, southeastern Bay of Biscay) in response to supratidal beach nourishment actions

Laida beach, located at the Oka estuary mouth (Urdaibai Biosphere Reserve) in the southeastern region of the Bay of Biscay, suffered the impact of a severe succession of storms during the first months of 2014. As a result of the erosion induced by these events, the beach lost its supratidal zone almost completely.The absence of a supratidal beach generated an impact on the recreational use of the beach during the summer 2014, and represented a potential impact for the coming summer 2015. Furthermore, it resulted in an overexposure and damage of adjacent infrastructures due to impinging strong waves. Therefore, the competent authorities, in coordination, decided to take action in order to nourish the supratidal zone of this beach.The solution adopted combined two different actions. The first one accomplished in spring of 2015, consisted in the mobilization of 44,800m3 of sand from an area of 35,200m2 equal to the 7% of the intertidal zone of Laida beach interpreted as the existing surface between the average low and high tidal limits, to the zone next to the eastern rocky beach contour. This action successfully resulted in an increase of the supratidal beach for the entire summer 2015 without negatively perturbing the morphological system. The second action was somewhat experimental and consisted in the mechanical plough of the previously existing intertidal low-amplitude ridges with the aim of increasing the sand transport toward the supratidal beach. Although this action did not lead to the increase of the supratidal beach, it seems to have resulted in an acceleration of the natural onshore migration of the bars.The objective of this contribution is to describe the morphodynamical response of the estuarine mouth after the performed actions with special emphasis on the evolution of extracted sites and the supratidal Laida beach area. The information here presented represents an innovative step in the understanding of the complex mechanisms driving the supratidal beach formation at the mouth of Oka estuary and by extension of the majority of the estuaries of the southeastern Bay of Biscay.

Effect of trampling and digging from shellfishing on <em>Zostera noltei</em> (Zosteraceae) intertidal seagrass beds

Seagrass beds are among the most valuable ecosystems in the world but they are also among the ones most affected by human activities, and they have decreased significantly in recent decades. In many areas, such as in the Basque Country (northern Spain), seagrass beds occupy areas that are also of interest for human activities such as recreation and shellfishing. They may therefore face a number of pressures that cause damage or irreversible states. Taking into account the limited distribution of seagrass beds in the Basque Country and the interest in their conservation, an eight-month field experiment focusing on the Zostera noltei growing season was carried out to evaluate the effect of shellfish gathering. We used generalized linear models to assess different intensities of trampling and digging, as the most important pressures of shellfishing applied to Zostera noltei beds. The results indicated that shoot density of Z. noltei was negatively altered by trampling treatments and positively affected (as a recovery) by digging treatments. This finding suggests that shellfishing adversely affects seagrass abundance and is potentially responsible for its low density in the Oka estuary. Our findings are important for management and should be taken into account in seagrass conservation and restoration programmes.

Sand-spit accumulations at the mouths of the eastern Cantabrian estuaries: The example of the Oka estuary (Urdaibai Biosphere Reserve)

The majority of the eastern Cantabrian estuaries occur on a coast with waves approaching dominantly from a NW direction. However, at their mouths sand-spit accumulations occur on their eastern margin. These sedimentary features have extended westwards, although the main littoral transport path on this coast is dominantly eastwards. The origin, morphology and development of these sand accumulations can be considered unusual, because they do not show the expected sedimentary response. Through a multidisciplinary approach (surface sediment analysis, morphodynamic, oceanographic and meteorological monitoring, and mathematical modelling) developed during the period May 2005–April 2006 in the Oka estuary, it has been possible to establish a genetic, hydrodynamic and morphodynamic sedimentary model for these estuaries. The main factors to determine the origin of these sedimentary features are wave intensity and direction, wave refraction and diffraction processes, spatial orientation and morphology of estuarine mouths, tidal range and the intensity and direction of tidal currents. On this coast, but elsewhere due to the influence of headlands, usually situated in the western margin of the estuarine mouths, wave diffraction and a variation in energy occur along the approaching waves. Consequently, wave set-up is forced in the eastern side of the estuarine mouths. Because of this induced wave pressure gradient an energy-compensation is needed and thus wave induced currents towards southwest originate. The action of the tidal currents is also important. In this paper a cyclical sedimentary transport pattern for the nearshore zone around the mouths of these estuaries is described. This process is taking place under tidal influence that induces wave set-up and breaking processes and reinforces also the sedimentary transport towards the estuarine inlet. The combination of all these hydrodynamic and sedimentary processes has produced these “anomalously-oriented” sedimentary features.

Relationships between polychlorinated biphenyls in molluscs, hydrological characteristics and human pressures, within Basque estuaries (northern Spain)

Interannual variability of polychlorinated biphenyls (PCBs), measured in the soft tissues of Mytilus galloprovincialis mussels and Crassostrea gigas oysters, collected from estuarine waters within the Basque Country (Bay of Biscay), are investigated. Samples were collected in the autumn, between 2002 and 2011. Sites located within the ports of Bilbao and Pasaia showed the highest PCBs concentrations in molluscs; the lowest were observed in the mouth of the Oka estuary, an area of low population and industrial activity. Congener profiles of PCBs in the tissues of molluscs reveal the predominance of hexachlorobiphenyls (CB153 and CB138). In addition, redundancy analysis has shown that residence time, river flow and a ‘pressure index’ explain 57% of the variability in the PCB congener concentrations (the higher the values of these variables, the higher the concentration). Finally, Σ7PCB median concentrations in molluscs and sediments, collected from nearby sampling sites, were found to be moderately correlated (r2=0.513, p<0.01).

An evaluation of the Rouse theory for sand transport in the Oka estuary, Spain

The Rouse profile has been traditionally used to represent the vertical distribution of suspended sand in a marine benthic boundary layer. Yet it is one of the biggest unknowns in estuarine morphodynamics, largely due to uncertainties of the ratio of the sediment fall velocity to bed friction on which the Rouse exponent (R=ws/βku⁎) is based. A field campaign was carried out at three different locations in the Oka estuary, northern Spain, in order to examine these uncertainties. Each location differed in grain size and flow condition thus offering a wide range of settings. The first survey was inside the estuary (wave sheltered, flood tide dominated and relatively broad estuary section), the second was at the distal ebb delta (ebb tide dominated and narrow estuary section), and the third was over the wave exposed proximal ebb delta (wave/flood tidal current combined flows and open sea). The aim of this study is to evaluate the applicability of the Rouse (1937) theory for the distribution of sand in suspension throughout a turbulent benthic boundary layer. A modified version of a Helley–Smith sampler was used to trap sand and measure the vertical distribution of sand in the water column. As well, a 1200 ADCP was used to measure flow velocity and backscatter together with an ADV (turbulence). The sand traps were found to have a sampling efficiency of 44%. The grain size at all stations was finer near the surface and coarser near the bed. The sand transport inside the estuary (Station 1) is inwards dominant. By contrast, the sand concentration during the ebb tide was ten times higher than during the flood tide at Station 2 and even higher at Station 3, which suggests that the sand transport over the ebb delta is seawards. The average Rouse parameters for Stations 1, 2, and 3 are 0.48±0.035, 0.78±0.23, and 0.46±0.06 respectively, which correspond to a coefficient of proportionality of the movability number, (χ) of 4 (Van Rijn, 1993). These differ from previous findings of Villatoro et al. (2010) and Amos et al. (2010b).

Projecting future distribution of the seagrass Zostera noltii under global warming and sea level rise

In future decades, coastal ecosystems are expected to be exposed to increased risk of experiencing adverse consequences related to climate change, exacerbated by human induced pressures. The seagrass Zostera noltii forms meadows mainly within the intertidal zone, leading it to be particularly vulnerable to seawater temperature increase and sea level rise (SLR). Considering the presently declining situation and the predicted scenarios of increasing seawater temperature and SLR by the end of the 21st century, we assessed the response of Z. noltii to climate change (i) accounting for changes in seawater temperature at its entire biogeographical range level; and (ii) under SLR scenarios at estuary level (Oka estuary, Basque Country, south-eastern Bay of Biscay). Objectives were addressed coupling habitat suitability models with climate change simulations. By the end of the 21st century, seawater temperature increase will trigger a northward distributional shift of 888km in the suitable habitat of the species, and a retreat of southernmost populations. The loss of southernmost populations due to climate change may imply future conservation problems. In contrast, SLR and derived changes in current velocities are expected to induce the landward migration of the species in the Oka estuary, increasing the available suitable intertidal areas (14–18%) to limits imposed by anthropogenic barriers. This modelling approach could lead to an advanced understanding of the species’ response to climate change effects; moreover, the information generated might support conservation actions towards the sites where the habitat would remain suitable for the species under climate change.

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.

Marine biological valuation mapping of the Basque continental shelf (Bay of Biscay), within the context of marine spatial planning

Marine Biological Valuation (BV) has increased in importance in recent years, due to the need to establish accurate maps of biodiversity value. However, there have been few exercises undertaken in Southern Europe, in putting a value on marine biodiversity whilst at the same time looking at several biological components. This paper presents the complete Biological Valuation Map (BVM) of the Basque continental shelf and estuaries, using the methodology developed for the Belgian Continental Shelf. It includes all available biological data (zooplankton, macroalgae, macrobenthos, demersal fish, seabirds and cetaceans), from 2003 to 2010. BVMs aim to compile all available biological and ecological information for a selected study area, allocating an integrated intrinsic biological value to the subzones within the study area. Here, the results highlight specific areas (such as Jaizkibel or Cap Breton Canyon), as having high or very high integrated BV, using all of the components. Furthermore, some biodiversity ‘hotspots’ have been identified, according to a specific ecosystem component (e.g. mid-parts of the Oka estuary, for macroalgae, and the Cap Breton Canyon, for cetaceans). Comparison with the results obtained from other European countries, and with previously high-importance delimited zones within the study area, showed similar spatial trends and patterns. Therefore, the objectives of this contribution are: (i) to analyse and establish a spatial ecological value map of the continental shelf of the Basque Country (southern Bay of Biscay), using present BV methods; (ii) to compare the results obtained to other European countries, and (iii) to explore the application of these results to the Marine Spatial Planning (MSP) and the European Marine Strategy Framework Directive (MSFD) requirements. This map can serve as a baseline for future MSP and can also be used for the determination of the environmental status, within the MSFD, for the qualitative descriptor 1 (biodiversity).

Modelling suitable estuarine habitats for Zostera noltii, using Ecological Niche Factor Analysis and Bathymetric LiDAR

Predicting species distribution and habitat suitability is of considerable use in supporting the implementation of environmental legislation, protection and conservation of marine waters and ecosystem-based management. As other seagrasses, Zostera noltii has declined worldwide, mainly due to human pressures, such as eutrophication and habitat loss. In the case of the Basque Country (northern Spain), the species is present only in 3 out of 12 estuaries. From the literature, it is known that at least 6 of these estuaries were formerly vegetated by this seagrass. Consequently, efforts to monitor and restore (potential) habitats have been enhanced. Therefore, we aim: (i) to determine the main environmental variables explaining Zostera noltii distribution, within the Basque estuaries based upon the Oka estuary; (ii) to model habitat suitability for this species, as a wider applicable management-decision tool for seagrass restoration; and (iii) to assess the applicability and predicted accuracy of the model by using internal and external validation methods. For this purpose, Ecological Niche Factor Analysis (ENFA) has been used to model habitat suitability, based upon topographical variables, obtained from bathymetric Light Detection And Ranging (LiDAR); sediment characteristics variables; and hydrodynamic variables. The results obtained from the ecological factors of the ENFA (Marginality: 1.00; Specialization: 2.59) indicate that the species habitat differs considerably from the mean environmental conditions over the study area; likewise, that the species is restrictive in the selection of the range of conditions within which it dwells. The main environmental variables relating to the species distribution, in order of importance, are: mean grain size; redox potential; intertidal height; sediment sorting; slope of intertidal flat; percentage of gravels; and percentage of organic matter content. The model has a high predicted accuracy (Boyce index: 0.92). Model validation using an independent dataset in the Bidasoa estuary has shown the applicability but also the limitations in extrapolating the habitat suitability model to select suitable transplantation areas in other estuaries with similar morphological and biogeographical characteristics. ENFA-technique, applied with an accurate selection of environmental predictors, could be a promising tool for predicting seagrass habitat suitability which could assist on seagrass conservation and restoration programs worldwide.

Capabilities of the bathymetric Hawk Eye LiDAR for coastal habitat mapping: A case study within a Basque estuary

The bathymetric LiDAR system is an airborne laser that detects sea bottom at high vertical and horizontal resolutions in shallow coastal waters. This study assesses the capabilities of the airborne bathymetric LiDAR sensor (Hawk Eye system) for coastal habitat mapping in the Oka estuary (within the Biosphere Reserve of Urdaibai, SE Bay of Biscay, northern Spain), where water conditions are moderately turbid. Three specific objectives were addressed: 1) to assess the data quality of the Hawk Eye LiDAR, both for terrestrial and subtidal zones, in terms of height measurement density, coverage, and vertical accuracy; 2) to compare bathymetric LiDAR with a ship-borne multibeam echosounder (MBES) for different bottom types and depth ranges; and 3) to test the discrimination potential of LiDAR height and reflectance information, together with multi-spectral imagery (three visible and near infrared bands), for the classification of 22 salt marsh and rocky shore habitats, covering supralittoral, intertidal and subtidal zones. The bathymetric LiDAR Hawk Eye data enabled the generation of a digital elevation model (DEM) of the Oka estuary, at 2 m of horizontal spatial resolution in the terrestrial zone (with a vertical accuracy of 0.15 m) and at 4 m within the subtidal, extending a water depth of 21 m. Data gaps occurred in 14.4% of the area surveyed with the LiDAR (13.69 km 2). Comparison of the LiDAR system and the MBES showed no significant mean difference in depth. However, the Root Mean Square error of the former was high (0.84 m), especially concentrated upon rocky (0.55–1.77 m) rather than in sediment bottoms (0.38–0.62 m). The potential of LiDAR topographic variables and reflectance alone for discriminating 15 intertidal and submerged habitats was low (with overall classification accuracy between 52.4 and 65.4%). In particular, reflectance retrieved for this case study has been found to be not particularly useful for classification purposes. The combination of the LiDAR-based DEM and derived topographical features with the near infrared and visible bands has permitted the mapping of 22 supralittoral, intertidal and subtidal habitats of the Oka estuary, with high overall classification accuracies of between 84.5% and 92.1%, using the maximum likelihood algorithm. The airborne bathymetric Hawk Eye LiDAR, although somewhat limited by water turbidity and wave breaking, provides unique height information obscured from topographic LiDAR and acoustic systems, together with an improvement of the habitat mapping reliability in the complex and dynamic coastal fringe.

The effects of dredging operations on the hydrodynamics of an ebb tidal delta: Oka Estuary, northern Spain

The hydrodynamics associated with the Mundaka Sandbar are described. The sandbar (ebb tidal delta) is responsible for the formation and the quality of a “world famous” surfing wave, at the mouth of the Oka Estuary in the Urdaibai Biosphere Reserve. The primary factor controlling the shape of the sandbar is the shoreline configuration. In turn the size of the sandbar is controlled by the volume of the tidal outflow and the wave energy. Periods of tidal dominance occur during summer (associated with sandbar accretion); with mixed energy conditions during winter and wave dominance under storms (sandbar erosion). The surfing wave lost its characteristics following, intensive dredging along the estuary, combined with beach nourishment, in May 2003. Between May 2005 and April 2006, the outer part of the estuary was monitored, to understand the processes controlling recent morphological changes over the area. Recent studies undertaken in the mouths of estuaries, together with the results of the monitoring are described; these are complemented by the numerical modelling of wave hydrodynamics under medium and extreme sea states. Analyses of the wave and current survey data, combined with the modelling outputs, have demonstrated that the changes were not irreversible. As such, the natural shape of the sandbar has recovered gradually, as has the quality of the Mundaka surfing wave.

Consequences of estuarine sand dredging and dumping on the Urdaibai Reserve of the Biosphere (Bay of Biscay): the case of the “Mundaka left wave”

In the spring of 2003, 240,000 m3 of sand were dredged from the main channel of the lower Oka estuary (Urdaibai Reserve of the Biosphere) and dumped on the southern area of Laida beach located at the estuary mouth. After few months, strong erosion of the deposited sediments occured as a consequence of the northwards migration of the final meander of the main estuarine channel before it reached the estuary inlet. At the same time, the estuarine inlet abandoned its original position near the rocky western margin and migrated eastwards cutting and eroding the sandy ebb delta (Mundaka sand bar). This caused a change in the way the incident wave broke as it reached the shallow water of the ebb tidal delta and beach and made surfing impossible under the same optimal conditions previously enjoyed due to the production of a wave that gradually broke from right to left as it reached shallow water, the so called “Mundaka left wave”. As a consequence, the Billabong Pro surf championship was cancelled in 2005. This paper describes the results obtained during a study of the outer estuary carried out during the period May 2005-April 2006 to characterize the processes operating on the most dynamic area of the estuary throughout the year and to evaluate the influence that the anthropogenic activities in the estuary had on the wave characteristics. No corrective measures were taken in the outer estuary during the study period and the environment recovered its normal morphological features naturally. This natural restoration it is hoped will be more stable, long lasting and reliable to ensure the continuing future of the “Mundaka left wave”.