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Morphodynamic evolution of Laida beach (Oka estuary, Urdaibai Biosphere Reserve, southeastern Bay of Biscay) in response to supratidal beach nourishment actions
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Morphodynamic evolution of Laida beach (Oka estuary, Urdaibai Biosphere Reserve, southeastern Bay of Biscay) in response to supratidal beach nourishment actions M. Monge-Ganuzasa,⁎, J. Gainzab, P. Liriac, I. Epeldec, A. Uriartec, R. Garnierb, M. Gonzálezb, P. Nuñezb, C. Jaramillob, R. Medinab a Urdaibai Biosphere Reserve Service, Environment and Territorial Policy Department, Basque Government, Madariaga tower, San Bartolome auzoa, 48350 Busturia, Bizkaia, Spain b Environmental Hydraulics Institute (IHCantabria), Universidad de Cantabria, Isabel Torres 15, 39011 Santander, Spain c AZTI-Tecnalia, Marine Research Unit. Herrera Kaia, Portualdea z/g. 20110, Pasaia, (Gipuzkoa), Spain.
A R T I C L E I N F O
A B S T R A C T
Keywords: Morphodynamics Nourishment Urdaibai Biosphere Reserve Bay of Biscay Beach erosion
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 44,800 m3 of sand from an area of 35,200 m2 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.
1. Introduction The southeastern Bay of Biscay is bounded by a rugged coastline of steep rocky cliffs, breached by narrow inlets and cut by small northward flowing rivers (Pascual et al., 2004) (Fig. 1). Changes in sea level during the Holocene have produced a series of isolated estuarine inlets (Leorri et al., 2013). After the drowning of the previous fluvial valleys, sand from the adjacent platform which chokes the mouths of the estuaries has been driven landwards by waves and tidal currents to
⁎
produce at the lower estuary a sandy wedge of coastal and estuarine sediments (Cearreta and Monge-Ganuzas, 2013). The Oka estuary was declared, in 1984, an UNESCO Biosphere Reserve. Nonetheless, it is influenced strongly by human activities. As such, the balance between industrial, recreational and natural conservation activities is not always easy to establish. For example, the last artificial movements of sand (dredging and dumping) within the estuary, carried out in 2003, altered the natural sedimentary equilibrium at the ebb tidal delta and the associated sandbar (Monge-Ganuzas et al.,
*Corresponding author. E-mail address: [email protected] (M. Monge-Ganuzas).
http://dx.doi.org/10.1016/j.seares.2017.06.003 Received 4 November 2016; Received in revised form 7 April 2017; Accepted 6 June 2017 1385-1101/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Monge-Ganuzas, M., Journal of Sea Research (2017), http://dx.doi.org/10.1016/j.seares.2017.06.003
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Fig. 1. Location of the study zone - Laida beach - in the context of the southeastern Bay of Biscay and location of the instruments deployed or used for the study.
extraction trenches, in order to conclude if the actions helped the beach to recover.
2008; Liria et al., 2009). Laida beach situated at the east of Oka estuary inlet has a variable morphology, depending upon the season (Monge-Ganuzas et al., 2004). As in many beaches around the world, this feature loses sand to the nearshore zone in winter, regaining it during summer (Masselink et al., 2006). In winter, it has a low-tide terrace which passes into a steep beach-face and a narrow berm capped by dunes. In summer, as sand is regained, a series of beach-ridges and runnels (King and Williams, 1949; Houser and Ellis, 2013), or low-amplitude ridges (King, 1972), develop on the low-tide terrace seaward of the beach face and berm. The described morphodynamic cycle guarantees in some way the existence of a variable in shape and area supratidal zone at Laida beach. Even though sometimes this supratidal zone disappears completely under natural conditions – i.e. before 1995 –, the driving forces of the sedimentary system are able after various sedimentary cycles to recover naturally the beach shape and extension. Monge-Ganuzas et al., 2013 analysis showed that the supratidal Laida beach area has varied throughout time. The sand-dumping upon the beach - 1995/2003 time 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. Storm successions occurred between January–March 2014 and beat dramatically against Laida beach. Consequently, its supratidal zone disappeared progressively (Burgoa, 2015, Fig. 7). This fact was cause for concern among beach users so authorities with competences (Council of Ibarrangelua, Deputy of Biscay, Basque autonomous Government and Spanish Ministry) all together agreed to take action after the advice of Liria et al., 2015. Accordingly, two different alternatives were carried out in order to solve the erosion problem: (1) the intertidal zone of Laida beach was nourished with sand excavated near the inlet of Oka estuary from May 11th, 2015 to June 4th, 2015. Several trenches of 1 m depth and separated from each other were accomplished in order to obtain sand (Fig. 2); (2) the most eastern part of the intertidal bar of Laida beach was ploughed in order to accelerate its onshore migration. The ploughing was carried out with a tractor during 22 low tides from July to September 2015. The aim of this study is to analyze how the actions affected the beach (supratidal and intertidal bar) and to follow the evolution of the
2. Methodology A multi-approach study has been carried out: (1) to monitor and evaluate the success of the performed actions in order to increase the supratidal zone of Laida beach; (2) to analyze and describe the morphodynamical response of this littoral sedimentary system.
2.1. Bathymetry and topography data In order to get volumetric (tri-dimensional) information of the morphological evolution of the system two bathymetric/topographic surveys along the full estuary were performed before extraction and dumping - May 2015 - and after the sedimentary environment response occurred - October 2015. These bathymetries and topographies were used to analyze the evolution of the supratidal zone of Laida beach and to model the morphodynamical evolution of the study area after the sand mobilization. Additionally, in order to monitor the intertidal bar migration, six cross-shore transects (P1-P6) separated 150 m from each other were defined (Fig. 3) and measured every 15 days from July to September 2015 with a GPS-RTK. In this study only three profiles will be analyzed (P1, P2 and P3). Further description of the other profiles is presented in Gainza et al. (2017). The basic equipment was a tripod, a GPS receiver Trimble R6 and a Radio System Trimble PDL 450 while the mobile equipment based on a monopod, a receiver GPS Trimble R6, a survey controller Trimble Tsc2. The following bathymetric and topographic data were used in the modelling: (1) 942 nautical chart from the Spanish Marine Hydrographic Institute to characterize offshore bathymetry; (2) from 100 m depth to the coast – resolution: 1 m – (Basque government, 09/ 28/2016); (3) Lower estuary bathymetry and topography (LIDAR and BATHYLIDAR): resolution: 1 m (Basque government, 09/28/2016); (4) detailed bathymetries and topographies of the estuarine mouth - May and October 2015, resolution: 1 m - (MAGRAMA, 2015). 2
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Fig. 2. A: Oblique photograph taken from the east during the extraction. At the bottom of the photograph it is possible to observe one of the extraction sites and at the left corner the deposition site over the intertidal zone of Laida beach – see the lorry -; B: Location of extraction sites in black and the deposition zone – downwards white triangle – on a TIMEX image taken immediately after the extraction works finished (06/06/2015). The upwards white triangle is located in the place where the excavator of A photograph is located. The plough was performed around the zone where the white triangle is situated over an existing low-amplitude ridge (intertidal sand bar).
Continuous data were recorded during three tidal cycles. These data were used to understand the processes that took place after the ploughing in Gainza et al. (2017). In this study, these data helped to calibrate the model at the intertidal zone. A summary of the hydrodynamic data used in this study is presented in Table 1.
2.2. Hydrodynamic data Hydrodynamic data from the Spanish Port instrument network located outside the study area were analyzed and a series of instruments were deployed during the survey period in the entrance channel and in the intertidal zone. These data were used to understand the local dynamics and for numerical model input, validation and calibration. Tidal levels and offshore wave data were analyzed and used as input for the model: Bilbao-Bizkaia buoy (Spanish Ports, 28/09/2016) for waves; (2) Bilbao tide gauge for sea levels (Spanish Ports, 28/09/2016) (Fig. 1). Later, the specific records from these sources relative to the study period were also analyzed. In addition, during the period between the 8th of August and the 6th of October two tide gauges and a current profiler were deployed in order to have detailed record of the hydrodynamics to calibrate de model. The tide gauges measured the pressure every 5 min and were deployed in Bermeo and Sukarrieta (Fig. 1). A Nortek Aquadopp Acoustic Doppler Profiler (ADCP) was deployed at the estuarine inlet and it measured tidal and wave induced currents every 5 min. These data were used to calibrate and validate the model. Finally, over the period of 12th–14th of August 2015 two Valeport current meter (ECM), Infinity current meter and three Aquatec pressure sensor (PS) gauges were installed at three locations (Station 1, Station 2 and Station 3) in order to monitor the intertidal sand bars (Fig. 3)
2.3. KOSTASystem littoral video-monitoring system The zone was monitored by KOSTASystem littoral video-monitoring system (KOSTASystem). This systems allowed us to observe and describe the morphodynamic response of the monitored coastal system under different hydrodynamic situations, and, precisely, to perform bidimensional (surface) analysis of the morphological changes. The application of video measurement methods to littoral areas is based on photogrammetric techniques, such as rectification or restitution helped with GCPs (Ground Control Points). The system allows obtaining planar orthorectified images of the whole study area by five (5) georeferenced video camera installed in the surroundings of the study area since 2007 (UTM ETRS89: 524576, 4805358). For the purpose of this study the images used were TIMEX images. These images are the result of the integration of images captured during a time interval representative in terms of wave action; and as well, short enough to be able to consider a uniform averaged tidal level (in this
Fig. 3. A: On the left there is the location of 6 cross-shore transects (P1–P6) carried out at Laida beach. B: In the central image it can see the location of the stations 1, 2 and 3 deployed and of the plough (right) and control (left) zones over the intertidal sand bar. C: On the right there is a detail of one of the stations deployed.
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Table 1 Location, time period and objectives of each of the deployments carried out. Instrument
Location (UTM ETRS89)
Data period
Objective of the deployment
Bilbao buoy Bilbao tide gauge Bermeo tide gauge Sukarrieta tide gauge Estuarine inlet current profiler Current meter & pressure sensor gauge
495967; 4831890 495971; 4804861 522821; 4807276 524951; 4804707 524849; 4804990 On the beach profiles (Fig. 3A)
1st January–30th October 2015
Model input
8th August–6th October 2015
Model calibration
12th–14th August 2015
Three different meshes were used to downscale the offshore wave climate from Bilbao-Vizcaya buoy to Laida beach: (1) a general one with 38 × 50 calculation elements with a horizontal resolution of 1000 m and; (2) a detailed one defined by 276 × 152 calculation elements with a horizontal resolution variable from 10 m (at Laida beach) to 340 m at the outer part; (3) a more detailed one defined by 417 × 152 calculation elements with a horizontal resolution variable of 10 m over the intertidal sand bars, 40 m at the inner estuarine part and 340 m at the outer estuarine part. For the morphodynamical simulation an averaged extraction depth of 0.70 m was assumed. With the aim of determining the precision of the simulated variables in relation with recorded real data, the Skill index (s) (Willmott, 1981; Ma et al., 2011) was used:
case 600 snap-shot images taken every 2 s during 20-minute periods). During June–October 2015, contour morphological variations of the supratidal zone of Laida beach were monitored by representative orthorectified TIMEX images every fifteen (15) days under average high tide conditions, while monitoring of excavated zones and low-amplitude ridges development was carried out by the same method under average low tide conditions and combined with obtained topographic and bathymetric data. For the selection of the representative images the reconstruction of the sea level was done using the main tidal astronomic components obtained from Pasaia tidal gauge (Fig. 1). 2.4. Numerical modelling Finally, the performed numerical modelling allowed us to identify accurately the main general hydrodynamic factors that provoke the observed morphodynamic response at the whole area of study. In order to evaluate the hydrodynamic and morphodynamic behavior of Laida beach and get volumetric information of the extraction evolution Delft3D numerical model (Roelvink and van Banning, 1994; WL/Delft Hydraulics, 2011) was applied. Data recorded from the 12th to the 14th of August 2015 were simulated and used to calibrate the hydrodynamic parameters of the model. Moreover, measured sea-level and induced currents during the 2-month instrument deployment were used to validate the outputs of the model. Hydrodynamic model was implemented with the following simulation period - GMT0 -: 10/08/2015, 12:30–14/08/2015, 12:00 and morphodynamic model (M) with this other: 29/05/2015, 00:00–01/ 10/2015, 00:00. The spin-up time used to stabilize the model was 36 and 48 h, respectively. Initial conditions consist of hourly tidal data from Bilbao gauge and hourly wave data from Bilbao-Vizcaya buoy (Spanish Ports, 28/09/ 2016). For the calibration of the hydrodynamics it is necessary to determine the roughness coefficient - C - in a variable spatial and temporal way according to the site depth using the Colebrook formula:
wherexobs are instrumental data, xmod are the results of the model and n the length of both data series. Regarding the sediment transport, Van Rijn (1993) and Soulsby (1997) formulas were tested. After studying the erosion/sedimentation patterns of Laida beach it was decided to apply the formula of Van Rijn (1993). According to the results of several authors (Grunnet et al., 2004; Mann et al., 2006; Van Rijn et al., 2007; Walstra et al., 2007; Briere et al., 2011) the default parameter values were used except for the wave-related sediment transport factors (0.05 was chosen) and for the transverse bed gradient factor for bedload transport (30 was chosen). During the study period 40 sediment samples were weekly recollected for grain size analysis in eight (8) different locations of the intertidal zone of Laida beach situated along the profiles P1 and P3 showed in Fig. 8. According to the grain size results the chosen value for the median grain size (D50) was 300 μm.
12H ⎤ C = 18 log10 ⎡ ⎢ ks ⎦ ⎥ ⎣
In this section the data presented in the previous part will be used to: (1) analyze the wave climate at the area during the study period; and (2) to describe the evolution of the supratidal beach, intertidal bar and the extraction trenches.
n
s=1−
(|x mod − x obs | + |x obs − x obs |)2
(3)
3. Results
(1)
where, (H) is the depth and (ks) is the equivalent roughness of Nikuradse. The typical value for the latter is 0.1–0.2 m. Eddy viscosity (ε), which depends on the cell size, was introduced by the following expression:
ε = k⋅Δx⋅u
∑1 |x mod − x obs |2 n ∑1
3.1. Hydrodynamic data during the monitoring period 3.1.1. 1. Data obtained throughout the monitoring period The data recorded by Bilbao-Bizkaia buoy and Bilbao tidal gauge from May 2015 to November 2015 and the 2-day instrument deployment - 12-14 August 2015 - are presented in Fig. 4 A and B respectively. After the analysis of the annual wave data, it is observed that the data collected from May 2015 to November 2015 (Fig. 4A) meet the expected pattern for the study area. In autumn (September–December) the waves at deep waters are relatively high corresponding to swell type waves - Hs: mean 4.5 m; maximum 8 m; minimum 1 m; Tp: mean 12 s; maximum 22 s; minimum 10 s. From April to July the waves are normally smaller corresponding to sea type waves – Hs: mean: 1 m; maximum 2 m; minimum: 0.5 m; Tp: mean 6 s maximum 9 s; minimum: 4 s.
(2)
Where, (k) is a constant which value varies between 0.05 y 0.15; Δx is cell size and u the characteristic velocity of the study area (0.5 m/s). Finally, wave propagation calibration was made through the calibration of the bottom friction coefficient - Cb. In order to choose the adequate values for those coefficients and to allow the model work properly, a sensibility analysis was carried out: 1. several simulations were carried out using different values for the mentioned parameters; 2. the obtained results were statistically compared with the recorded real data: sea-level, induced currents and waves - Hs and Tp. 4
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Fig. 4. A: Data obtained by Bilbao-Bizkaia buoy from May to November 2015. Wave height: Hs; Peak period: Tp: wave direction (Dir). Data obtained by Bilbao tidal gauge are shown at the bottom. Those data include astronomic and meteorological tides relative to the local hydrographic zero – Bilbao -; B: Data obtained by Bilbao-Bizkaia buoy during the study period – 12-14 August 2015. Data obtained by Bilbao tidal gauge are shown at the bottom.
compared with the velocity data provided by the model. As can be easily seen the modelled data and the real data fit very well for all the stations regarding to the tidal curves and moderately well in relation to wave Hs and Tp. Some disparity is observed regarding the velocity registered at the stations and the modelled due to the reasons previously stated. Fig. 7 shows the results relative to the averaged currents and averaged sediment transport over the low-amplitude ridges for each of the three tidal cycles studied. All cycles present a southwards net sediment transport that is one order of magnitude higher under storm conditions. Thus, during the summer period characterized by calms and moderate storms, the low-amplitude ridges progressively advance toward Laida beach supratidal zone.
The approaching direction of the swell type waves is mainly 300° with some punctual variations relative to sea type waves. Tidal level as usual for this zone of the world varied from 0 m to 4.5 m. As for the 2-day instrument deployment (Fig. 4B), two different behaviors were observed: a calm period during the first two tidal cycles– mean Hs: 1 m; mean Tp: 7 s – and a summer storm during the third tidal - mean Hs: 3.5 m; mean Tp: 10 s. The approaching direction of the waves was mainly northwest - N300E. The tidal level varied from 0.7 m to 4 m approximately and the tidal range slightly increased during the deployment period. The data collected by the Nortek Aquadopp ADCP deployed from the 4th August to the 6th October 2015 (see Fig. 1 for location), are shown in Fig. 5. The tidal range varied as usual for this part of the world. Its highest values were registered during September because of the equinoctial tidal wave. A strong variation of current velocity and direction was observed along each tidal cycle as expected from its location. Variations of the maximum current speed from 75 to 250 cm/s were reached around mid-tide. However, current velocities registered during the ebb were slightly stronger that the ones registered during the flood (see rose diagram of Fig. 5). Current directions were N and NNE for the ebb and SSE for the flood. Low values and a higher directional dispersion were registered during high and low tide phases.
3.2. Morphodynamics 3.2.1. Supratidalbeach The topographic data of three cross-shore profiles throughout the monitoring period are displayed in Fig. 8. The effects of the dredging and dumping can be clearly observed in those profiles. As it can be observed, P1 and P2 profile series were located close to the eastern cliffs while P3 profile was situated at the central part of Laida beach. Profile series recorded the morphological changes of the intertidal and supratidal beach during the study period (May–October 2015). The sand scraping and dumping was carried out from the 11th of May to the 4th of June. As it can be seen in May the supratidal zone in P1 (P1: x < 5400 m) was flat and its level was around z = 3 m. From May to July the amount of sand at the supratidal zone increased, lifting the level to z = 4.5 m. This amount of sand corresponds to the sand volume dumped that migrated southward (inshore) by wave action. From July to October a shoreward movement of the profile can be observed. It
3.1.2. Validation of the model Delft 3D model was used to simulate the hydrodynamics at Oka estuary and to get volumetric information of the evolution of the extraction. The model was calibrated and validated by using instrumental data. Fig. 6 contrasts the results relative to sea-level, currents and waves given by the model with the real data recorded by the performed deployments. Moreover, the numeric results obtained by tidal gauges and velocities recorded by the Aquadopp ADCP instrument deployed are 5
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Fig. 5. Data obtained by the Nortek Aquadop ADCP deployed during the study period: 04 August/06 October 2015. Tidal data include astronomic and meteorological tides relative to the local hydrographic zero – Bilbao. At the bottom can observe the sub surface current rose diagram corresponding to the cell 8 m above the location level of the instrument that was 10.9 m below the Mean Sea Water Level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
supratidal zone is observed, although during July already a sand accumulation along the still intertidal profile begins to form. The sand accumulated at the supratidal zone in P2 comes from P1 by the longshore currents. Later, in October, the supratidal accumulation migrates to the south west.
happens due to the fact that longshore currents (from east to west) transport the sand previously accumulated in P1 to the west. This longshore current can be observed in Fig. 7, where the vertically average current is modelled with Delft 3D. In P2 it is in August when the increase of the amount of sand at the
Fig. 6. Contrast of the data – sea level and velocity magnitude - supplied by the Delft3D model with the data provided by Bermeo and Sukarrieta tidal gauges and the Aquadopp ADCP - see Fig. 1 for location - during the study period 13-14/08/2015. The velocity magnitude obtained with the model is wave and depth averaged. The velocity magnitude of the ADCP corresponds to a depth of 0.5 m.
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decline of 0.5 m seen in the profile of October. Video images from May to September 2015 show the evolution of the dry beach (Fig. 9). In May the dry beach was nearly non-existent whereas in September a considerable area of beach can be observed. Notice that in June the sand was dumped at the bottom of the eastern cliffs and that during the next months this dumped sand was progressively transported across the intertidal Laida beach toward the southwest until it was welded to the dry beach generating a hooked sand-spit by September 2015. In order to quantify the dry beach volume gained due to the sand dumping, a further analysis of the topography data was carried out. The volume of sand of Laida beach above mean high water springs (Z > 3 m) and above mean high water neaps (Z > 4 m) were calculated from topo-bathymetric data. Fig. 10 shows the difference of sand volume above mean high water springs, before the sand dumping (black; May) and after the sand was redistributed (white; October). In Table 2 the sand volumes of the dry beach calculated during spring tides (Z > 4 m) and during neap tides (Z > 3 m) are shown. After dredging and dumping, Laida beach gained 30,400 m3 of sand (difference between May and October with neap tides), which is the 70% of the sand that was dumped.
Fig. 7. Depth averaged current velocity and direction – arrows – over the intertidal sand bars of Laida beach provided by the model for wave storm conditions – studied third tidal cycle.
3.2.2. Ploughedintertidalbarandcontrolzonebehaviors The intertidal bar of Laida beach was split in two at the moment the study was carried out (Fig. 11). In order to be able to observe the
In the profile series P3 are not observed significant variations along the zone situated at x < 5400 m, except the overall around level
Fig. 8. Morphological evolution of Laida beach intertidal and supratidal zone during the study period based on the topographic profiles carried out. The location of the represented profiles can be observed in Fig. 3. P1 and P2 profile series are relative to the ploughed lowamplitude ridge while P3 profile represents the undisturbed one. Vertical axis: bed level, Z (m). Horizontal axis: cross-shore distance (Northing, m), increasing offshore.
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Fig. 9. Monthly morphological evolution of the supratidal Laida beach area over TIMEX images during the study period.
Fig. 10. Left: Surface of the supratidal zone of Laida beach during spring tides (Z > 4 m). In black: May 2015; in white: October 2015. Right: Bed level variation in the supratidal area between October 2015 and May 2015. Positive values mean accretion.
that the accretionary sequence of Laida beach is characterized by a uniform inshore migration of the intertidal bar. However, after ploughing the most eastern part of the intertidal bar its migration pattern changed by accelerating the migration of the ploughed zone. In the plough zone this migration was approximately 3 times faster comparing to the control zone (advancement of 95 m vs 35 m). Moreover, in Fig. 11 a sand tongue situated close to the cliff can be seen. This sand comes from the ploughed area but in this specific area the effect of the contour enhanced its migration, reaching 300 m.
Table 2 Increase of the sand volume at the supratidal zone of Laida beach during the monitoring period at neap and spring tides.
May 2015 October 2015 Difference
Volume (neap tides, m3), Z > 3m
Volume (spring tides, m3), Z > 4m
18.200 48.600 30.400
1.300 13.500 12.200
morphodynamic response of this intertidal bar under the ploughing made with the aim of accelerating its onshore migration - and to make comparison with the bar behavior under natural conditions, the eastern zone of the bar was ploughed while its western zone was not altered (control zone, Fig. 3). P1 and P2 profile series (Fig. 8) were performed over the ploughed area while P3 profiles covered the natural bar. The intertidal bar in all profiles is located at (5700 < x < 5800). The profiles series show that in all profiles series there is an onshore movement of the intertidal bar. However, this shoreward movement is greater in P1 and P2 than in P3. Thus, it can be stated that the intertidal bar did not migrate uniformly. The eastern part (ploughed) moved faster than the western part (unaltered). If P1 and P2 profile series are compared it can be observed that the intertidal bar migrates about 40 m further inshore in P1 than in P2. Interestingly, previous studies (Monge-Ganuzas et al., 2008) showed
3.2.3. Extractionzones The surface evolution of the extraction zones was studied by KOSTASystem TIMEX images (Fig. 12). The contours of the extracted zones were digitized on the TIMEX images every month and the surface evolution of their areas was calculated. The extracted areas showed a progressive and homogeneous surface decreasing evolution (Table 3). By the end of October it was no longer possible to recognize the traces of the extraction areas as they had been completely filled. The volumetric evolution of the extraction areas (Fig. 13) over the 4 months of monitoring (June–October) was obtained by the model Delft 3D (Roelvink and van Banning, 1994; WL/Delft Hydraulics, 2011). This figure shows the percentage of recovered volume of the affected areas during the 140 days that the recovery process lasted. It was observed that 30% of the extracted area was filled again in one month, in 3 months the 90% of the extracted volume was recovered again and by the 4th month the extracted area was completely 8
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Fig. 11. Morphodynamic response of the ploughed low-amplitude ridge – right side – during the monitoring period – 17/07-30/09/ 2015 - compared with the undisturbed control zone – left side - provided by TIMEX images. The bottom plot represents the onshore migration of the ridges between these two dates in the control zone (35 m), in the plough zone (95 m), and along the cliff (300 m).
Fig. 12. Monthly morphological evolution of the extraction zones (in black) over TIMEX images during the study period.
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The fact of having opted for the method of choosing different relatively small extraction areas is considered adequate as by this method negative impacts in other sedimentary features of the lower estuary (i.e. Mundaka sandbar, in other words, proximal ebb tidal delta) has been avoided. Furthermore, the factthat the extracted and dumped sand volume (44,800 m3) has not been higher than the volume of sediment that is normally moved - estimated by Monge-Ganuzas et al. (2008) in around 55,000 m3 through the intertidal beach during the summer period is considered proper as well. However, there is not enough information to state that the response of the system will be similar if the action is repeated in the future. Thus, further work should be done about this topic. Moreover, there is no information about the cumulative effects (Council of Environmental Quality, 1997) that the repetitive extraction/dumping actions can cause in this zone of the estuary. Special attention should be paid over the estuarine morphodynamics and hydrodynamics, the beach steepness, the cycles of destruction/ formation of Mundaka sand bar, the possible instability of existing navigation channels and the supratidal zone stability, among others. Consequently, a strong piece of scientific information about the characteristics of the sedimentary system and its short- medium- and longterm response to human actions should be available in order to give reliable advice about further extraction/dumping actions at Laida beach. This fact implies that, as the uncertainties are still considerable, more research should be implemented. In this way, the authors consider indispensable the definition of a multiyear morphodynamic monitoring planning and a well-planned strategy for at least, the lower Oka estuary –better the whole estuary - in order to be able to provide reliable Science based criteria for future decisions. This statement is reinforced by the unquestionable importance of Oka estuary as provides important ecosystem services for humans (Turner and Schaafsma, 2015).
Table 3 Surface evolution of the extraction areas during the study period. Month
Surface of the extraction areas (m2)
June July August September October
35,200 26,300 19,600 13,600 2800
Fig. 13. Evolution of the accumulated sediment volume at the extraction sites for the June to October 2015 time period calculated by the applied model.
recovered.
5. Conclusions 4. Discussion From the monitoring carried out it can be stated that the extraction method used did not provoke any irreversible damage over extraction sites and their surrounding areas as the extraction sites recovered its original situation in a time period of 140 days. The performed action – extraction and dumping - generated an increase of the supratidal zone of Laida beach by summer 2015. From the 44,800 m3 of sand that were dumped, the 70% was transported and accumulated at the supratidal zone (z > 3 m) of Laida beach. By summer 2015, there were 23,000 m3 of dry beach. Therefore this action was successful as the final purpose was reached. However, there are several doubts about the future morphodynamic natural response of the sedimentary system. There is also uncertainty about the cumulative effects that the used method could cause over the sedimentary system, in case it is decided to arrange it again in the future. Ploughing increased low-amplitude ridge inshore advance. However further study is necessary to conclude if the onshore migration of the intertidal bar can be accelerated by ploughing it. Furthermore, more field experiments in different type of beaches are required in order to study how the plough works in other situations and discard any possible negative affection over the behavior of the sedimentary system. As the wave, tidal, meteorological and wind action and their effects in the sedimentary environments of littoral of the Bay of Biscay are random in the small-scale – hourly, daily, weekly or monthly scale – and the data records provided by offshore buoys or the amount of available data from specific surveys is still scarce further work should also be done about this topic. In this way, the authors consider indispensable the definition of a multiyear morphodynamic monitoring planning and a well-planned strategy for at least, the lower Oka estuary –better the whole estuary - in order to be able to provide validated and reliable Science based data. This statement is reinforced by the unquestionable importance of Oka estuary as provides important ecosystem services for humans. This statement could be extrapolated to the majority of the sedimentary environments of the Bay of Biscay.
The results suggest that ploughing the intertidal bar and dumping extracted sand helped Laida beach to recover its dry beach by the summer of 2015. The decision to dump the extracted sand in this intertidal zone instead of dumping directly over the previous supratidal beach is considered appropriate:(1) natural processes redistributed dumped sand creating naturally structured sedimentary features much more robust against erosion than a simple dumping of unconsolidated sand over the shoreline that has probably been eroded more quickly; (2) the morphology of the sedimentary structure naturally generated after the dumping and the sand transport resulted in a much more natural landscape. This fact is especially interesting as the area is considered as a natural protected area. As it was stated by Monge-Ganuzas et al. (2015) and confirmed by the modelling performed in the present study, the general sediment transport pattern at intertidal Laida beach during summer consist on a southwestward growth of sand ridges, where the coastal contour morphology, the wave-induced currents and the local tidal currents can be considered as the main drivers of this sediment transport. Due to the influence of the headland located on the western margin of the estuarine mouth, waves approaching from NW diffract and results in a differential of energy, i.e. a wave height gradient along the beach (higher waves in the east and lower waves in the center of the bay). Consequently, this wave height gradient induces longshore current (from east to west). Moreover, it is possible to state that the use of quantitative techniques - topographic surveys - combined with quantitative/qualitative techniques –TIMEX images - and as well with modelling, reveals itself as an effective multi-approach method for morphodynamic monitoring of littoral processes. What is more, the performed numerical modelling allowed us to understand the main general hydrodynamic factors that provoke the observed morphodynamic behavior. 10
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dredging and dumping activities along the lower Oka estuary (Urdaibai Biosphere Reserve, southeastern Bay of Biscay, Spain). Ocean Coast. Manag. 77, 40–49. Monge-Ganuzas, M., Cearreta, A., Iriarte, E., 2008. Consequences of estuarine sand dredging and dumping on the Urdaibai Biosphere Reserve (Bay of Biscay): the case of the “Mundaka left wave”. J. Iber. Geol. 34, 215–234. Monge-Ganuzas, M., Iriarte, E., Cearreta, A., Arana, X., 2004. Sustainable management in the Urdaibai Reserve of the biosphere (Southern Bay of Biscay). Coastal dune regeneration. In: Green, D.R. (Ed.), Delivering Sustainable Coast: Connecting Science and Policy. Proceedings of the VII International Symposium LITTORAL. Aberdeen (U.K.) 20–22 September Vol. 2. Cambridge Publications, pp. 725–726. Roelvink, J.A., van Banning, G.K.F.M., 1994. Design and development of Delft3D and application to coastal morphodynamics. In: Verwey, A., Minns, A.W., Babovic, V. (Eds.), Proceedings of HYDROINFORMATICS 1994. Balkema, Rotterdam, pp. 451–456. Pascual, A., Cearreta, A., Rodriguez-Lázaro, J., Uriarte, A., 2004. Geology and paleoceanography. In: Borja, A., Collins, M.B. (Eds.), Oceanography and Marine Environment of the Basque Country. Elsevier Oceanography Series Vol. 70. pp. 53–70. Soulsby, R., 1997. Dynamics of Marine Sands. A Manual for Practical Applications. Thomas Telford Publications, Oxford (U.K.), pp. 249. Turner, R.K., Schaafsma, M., 2015. Coastal zones ecosystem services. In: From Science to Values and Decision Making. Springer International Publishing, Switzeland, pp. 239. Van Rijn, L.C., 1993. Principles of Sediment Transport in Rivers, Estuaries and Coastal Seas. Aqua Publications, The Netherlands, pp. 673. Van Rijn, L.C., Walstra, D.J.R., van Ormondt, M., 2007. Unified view of sediment transport by currents and waves. IV: application of morphodynamic model. J. Hydraul. Eng. 133, 776–793. Walstra, D.J.R., Van Rijn, L.C., Van Ormondt, M., Brière, C., Talmon, A.M., 2007. The effects of bed slope and wave skewness on sediment transport and morphology. In: Kraus, Nicholas C., Rosati, Julie Dean (Eds.), roceedings of COASTAL SEDIMENTS’07 ASCE New Orleans, USA, pp. 1–14 (May 13-17). Willmott, C.J., 1981. On the validation of models. Phys. Geogr. 2, 184–194. WL/Delft Hydraulics, 2011. Delft3D-FLOW, simulation of multidimensional hydrodynamic flows and transport phenomena, including sediments. In: User Manual. 2011 Delft.
Acknowledgements The authors acknowledge Puertos del Estado, Spain, for providing the climate data and as well the support of the Spanish “Ministerio de Economia y Competitividad” under Grant BIA2014-59643-R. ReferencesArticles Briere, C., Giardino, A., van der Werf, J., 2011. Morphological modeling of bar dynamics with Delft3D: the quest for optimal free parameter settings using an automatic calibration technique. Proc. Coast. Eng. Conf. 1 (32), 60. Burgoa, N., 2015. Evolución morfológica de la playa de Laida en el Estuario del Oka. ((unpublished) M.Sc. Thesis) Port and Coastal Engineering, University of Cantabriapp. 68 (plus Appendices). Cearreta, A., Monge-Ganuzas, M., 2013. Paleoenvironmental evolution of the Oka estuary (Urdaibai Biosphere Reserve, Basque coast): response to Holocene sea level rise. GeoTemas 14, 163–166. Gainza, J., Garnier, R., Nuñez, P., Jaramillo, C., González, M., Medina, R., Liria, P., Epelde, I., Uriarte, A., Monge-Ganuzas, M., 2017. Accelerating the beach recovery by ploughing the intertidal bar: Field experiment and numerical simulations. Coast. Eng (in press). Grunnet, N.M., Walstra, D.J.R., Ruessink, B.G., 2004. Process-based modelling of a shoreface nourishment. Coast. Eng. 51 (7), 581–607. Houser, C., Ellis, J., 2013. Beach and dune interaction. In: Shroder, J., Sherman, D.J. (Eds.), Treatise on Geomorphology, Coastal Geomorphology. Vol. 10. Academic Press, San Diego, CA, pp. 267–288. King, C.A.M., 1972. Beaches and Coasts, second ed. Edward Arnold, London, pp. 580. King, C.A.M., Williams, W.W., 1949. The formation and movement of sand bars by wave action. Geogr. J. 113, 70–85. Mann, D.W., Day, P.C.M., Benedet, P.L., 2006. South Palm Beach-Lantana Shore Protection Study, Delft3D Model Calibration. Leorri, E., Cearreta, A., García-Artola, A., Irabien, M.J., Blake, W.H., 2013. Relative sea level rise in the Basque coast (N Spain): different environmental consequences on the coastal area. Ocean Coast. Manag. 77, 3–13. Liria, P., Garel, E., Uriarte, A., 2009. The effects of dredging operations on the hydrodynamics of an ebb tidal delta: Oka Estuary, northern Spain. Cont. Shelf Res. 29 (16), 1983–1994. Ma, G., Shi, F., Liu, S., Qi, D., 2011. Hydrodynamic modeling of Changjiang estuary; model skill assessment and large scale structure impacts. Appl. Ocean Res. 33, 69–78. MAGRAMA, 2015. Real Decreto-Ley 2/2015, de 6 marzo, por el que se adoptan medidas urgentes para reparar los daños causados por las inundaciones y otros efectos de los temporales de lluvia, nieve y viento acaecidos en los meses de enero, febrero y marzo de 2015. Masselink, G., Kroon, A., Davidson-Arnott, R.G.D., 2006. Morphodynamics of intertidal bars in wave-dominated coastal settings-a review. Geomorphology 73, 33–49. Monge-Ganuzas, M., Evans, G., Cearreta, A., 2015. Sand-spit accumulations at the mouths of the eastern Cantabrian estuaries: the example of the Oka estuary (Urdaibai Biosphere Reserve). Quat. Int. 364, 206–216. Monge-Ganuzas, M., Cearreta, A., Evans, G., 2013. Morphodynamic consequences of
Restricted documents Council of Environmental Quality, 1997. Considering Cumulative Effects under the National Environmental Policy Act. (Unpublished report) pp. 64. Liria, P., Epelde, I., González, M., Uriarte, A., 2015. Alternativas para la regeneración de la playa seca de Laida de cara al verano de 2015. Elaborado por AZTI-Tecnalia para Diputación Foral de Bizkaia. Departamento de Medio Ambientepp. 31.
Web pages Basque government 09/28/2016. http://www.geo.euskadi.net. Spanish Ports 09/28/2016. www.puertos.es.
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Morphodynamic evolution of Laida beach (Oka estuary, Urdaibai Biosphere Reserve, southeastern Bay of Biscay) in response to supratidal beach nourishment actions M. Monge-Ganuzasa,⁎, J. Gainzab, P. Liriac, I. Epeldec, A. Uriartec, R. Garnierb, M. Gonzálezb, P. Nuñezb, C. Jaramillob, R. Medinab a Urdaibai Biosphere Reserve Service, Environment and Territorial Policy Department, Basque Government, Madariaga tower, San Bartolome auzoa, 48350 Busturia, Bizkaia, Spain b Environmental Hydraulics Institute (IHCantabria), Universidad de Cantabria, Isabel Torres 15, 39011 Santander, Spain c AZTI-Tecnalia, Marine Research Unit. Herrera Kaia, Portualdea z/g. 20110, Pasaia, (Gipuzkoa), Spain.
A R T I C L E I N F O
A B S T R A C T
Keywords: Morphodynamics Nourishment Urdaibai Biosphere Reserve Bay of Biscay Beach erosion
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 44,800 m3 of sand from an area of 35,200 m2 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.
1. Introduction The southeastern Bay of Biscay is bounded by a rugged coastline of steep rocky cliffs, breached by narrow inlets and cut by small northward flowing rivers (Pascual et al., 2004) (Fig. 1). Changes in sea level during the Holocene have produced a series of isolated estuarine inlets (Leorri et al., 2013). After the drowning of the previous fluvial valleys, sand from the adjacent platform which chokes the mouths of the estuaries has been driven landwards by waves and tidal currents to
⁎
produce at the lower estuary a sandy wedge of coastal and estuarine sediments (Cearreta and Monge-Ganuzas, 2013). The Oka estuary was declared, in 1984, an UNESCO Biosphere Reserve. Nonetheless, it is influenced strongly by human activities. As such, the balance between industrial, recreational and natural conservation activities is not always easy to establish. For example, the last artificial movements of sand (dredging and dumping) within the estuary, carried out in 2003, altered the natural sedimentary equilibrium at the ebb tidal delta and the associated sandbar (Monge-Ganuzas et al.,
*Corresponding author. E-mail address: [email protected] (M. Monge-Ganuzas).
http://dx.doi.org/10.1016/j.seares.2017.06.003 Received 4 November 2016; Received in revised form 7 April 2017; Accepted 6 June 2017 1385-1101/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Monge-Ganuzas, M., Journal of Sea Research (2017), http://dx.doi.org/10.1016/j.seares.2017.06.003
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Fig. 1. Location of the study zone - Laida beach - in the context of the southeastern Bay of Biscay and location of the instruments deployed or used for the study.
extraction trenches, in order to conclude if the actions helped the beach to recover.
2008; Liria et al., 2009). Laida beach situated at the east of Oka estuary inlet has a variable morphology, depending upon the season (Monge-Ganuzas et al., 2004). As in many beaches around the world, this feature loses sand to the nearshore zone in winter, regaining it during summer (Masselink et al., 2006). In winter, it has a low-tide terrace which passes into a steep beach-face and a narrow berm capped by dunes. In summer, as sand is regained, a series of beach-ridges and runnels (King and Williams, 1949; Houser and Ellis, 2013), or low-amplitude ridges (King, 1972), develop on the low-tide terrace seaward of the beach face and berm. The described morphodynamic cycle guarantees in some way the existence of a variable in shape and area supratidal zone at Laida beach. Even though sometimes this supratidal zone disappears completely under natural conditions – i.e. before 1995 –, the driving forces of the sedimentary system are able after various sedimentary cycles to recover naturally the beach shape and extension. Monge-Ganuzas et al., 2013 analysis showed that the supratidal Laida beach area has varied throughout time. The sand-dumping upon the beach - 1995/2003 time 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. Storm successions occurred between January–March 2014 and beat dramatically against Laida beach. Consequently, its supratidal zone disappeared progressively (Burgoa, 2015, Fig. 7). This fact was cause for concern among beach users so authorities with competences (Council of Ibarrangelua, Deputy of Biscay, Basque autonomous Government and Spanish Ministry) all together agreed to take action after the advice of Liria et al., 2015. Accordingly, two different alternatives were carried out in order to solve the erosion problem: (1) the intertidal zone of Laida beach was nourished with sand excavated near the inlet of Oka estuary from May 11th, 2015 to June 4th, 2015. Several trenches of 1 m depth and separated from each other were accomplished in order to obtain sand (Fig. 2); (2) the most eastern part of the intertidal bar of Laida beach was ploughed in order to accelerate its onshore migration. The ploughing was carried out with a tractor during 22 low tides from July to September 2015. The aim of this study is to analyze how the actions affected the beach (supratidal and intertidal bar) and to follow the evolution of the
2. Methodology A multi-approach study has been carried out: (1) to monitor and evaluate the success of the performed actions in order to increase the supratidal zone of Laida beach; (2) to analyze and describe the morphodynamical response of this littoral sedimentary system.
2.1. Bathymetry and topography data In order to get volumetric (tri-dimensional) information of the morphological evolution of the system two bathymetric/topographic surveys along the full estuary were performed before extraction and dumping - May 2015 - and after the sedimentary environment response occurred - October 2015. These bathymetries and topographies were used to analyze the evolution of the supratidal zone of Laida beach and to model the morphodynamical evolution of the study area after the sand mobilization. Additionally, in order to monitor the intertidal bar migration, six cross-shore transects (P1-P6) separated 150 m from each other were defined (Fig. 3) and measured every 15 days from July to September 2015 with a GPS-RTK. In this study only three profiles will be analyzed (P1, P2 and P3). Further description of the other profiles is presented in Gainza et al. (2017). The basic equipment was a tripod, a GPS receiver Trimble R6 and a Radio System Trimble PDL 450 while the mobile equipment based on a monopod, a receiver GPS Trimble R6, a survey controller Trimble Tsc2. The following bathymetric and topographic data were used in the modelling: (1) 942 nautical chart from the Spanish Marine Hydrographic Institute to characterize offshore bathymetry; (2) from 100 m depth to the coast – resolution: 1 m – (Basque government, 09/ 28/2016); (3) Lower estuary bathymetry and topography (LIDAR and BATHYLIDAR): resolution: 1 m (Basque government, 09/28/2016); (4) detailed bathymetries and topographies of the estuarine mouth - May and October 2015, resolution: 1 m - (MAGRAMA, 2015). 2
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Fig. 2. A: Oblique photograph taken from the east during the extraction. At the bottom of the photograph it is possible to observe one of the extraction sites and at the left corner the deposition site over the intertidal zone of Laida beach – see the lorry -; B: Location of extraction sites in black and the deposition zone – downwards white triangle – on a TIMEX image taken immediately after the extraction works finished (06/06/2015). The upwards white triangle is located in the place where the excavator of A photograph is located. The plough was performed around the zone where the white triangle is situated over an existing low-amplitude ridge (intertidal sand bar).
Continuous data were recorded during three tidal cycles. These data were used to understand the processes that took place after the ploughing in Gainza et al. (2017). In this study, these data helped to calibrate the model at the intertidal zone. A summary of the hydrodynamic data used in this study is presented in Table 1.
2.2. Hydrodynamic data Hydrodynamic data from the Spanish Port instrument network located outside the study area were analyzed and a series of instruments were deployed during the survey period in the entrance channel and in the intertidal zone. These data were used to understand the local dynamics and for numerical model input, validation and calibration. Tidal levels and offshore wave data were analyzed and used as input for the model: Bilbao-Bizkaia buoy (Spanish Ports, 28/09/2016) for waves; (2) Bilbao tide gauge for sea levels (Spanish Ports, 28/09/2016) (Fig. 1). Later, the specific records from these sources relative to the study period were also analyzed. In addition, during the period between the 8th of August and the 6th of October two tide gauges and a current profiler were deployed in order to have detailed record of the hydrodynamics to calibrate de model. The tide gauges measured the pressure every 5 min and were deployed in Bermeo and Sukarrieta (Fig. 1). A Nortek Aquadopp Acoustic Doppler Profiler (ADCP) was deployed at the estuarine inlet and it measured tidal and wave induced currents every 5 min. These data were used to calibrate and validate the model. Finally, over the period of 12th–14th of August 2015 two Valeport current meter (ECM), Infinity current meter and three Aquatec pressure sensor (PS) gauges were installed at three locations (Station 1, Station 2 and Station 3) in order to monitor the intertidal sand bars (Fig. 3)
2.3. KOSTASystem littoral video-monitoring system The zone was monitored by KOSTASystem littoral video-monitoring system (KOSTASystem). This systems allowed us to observe and describe the morphodynamic response of the monitored coastal system under different hydrodynamic situations, and, precisely, to perform bidimensional (surface) analysis of the morphological changes. The application of video measurement methods to littoral areas is based on photogrammetric techniques, such as rectification or restitution helped with GCPs (Ground Control Points). The system allows obtaining planar orthorectified images of the whole study area by five (5) georeferenced video camera installed in the surroundings of the study area since 2007 (UTM ETRS89: 524576, 4805358). For the purpose of this study the images used were TIMEX images. These images are the result of the integration of images captured during a time interval representative in terms of wave action; and as well, short enough to be able to consider a uniform averaged tidal level (in this
Fig. 3. A: On the left there is the location of 6 cross-shore transects (P1–P6) carried out at Laida beach. B: In the central image it can see the location of the stations 1, 2 and 3 deployed and of the plough (right) and control (left) zones over the intertidal sand bar. C: On the right there is a detail of one of the stations deployed.
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Table 1 Location, time period and objectives of each of the deployments carried out. Instrument
Location (UTM ETRS89)
Data period
Objective of the deployment
Bilbao buoy Bilbao tide gauge Bermeo tide gauge Sukarrieta tide gauge Estuarine inlet current profiler Current meter & pressure sensor gauge
495967; 4831890 495971; 4804861 522821; 4807276 524951; 4804707 524849; 4804990 On the beach profiles (Fig. 3A)
1st January–30th October 2015
Model input
8th August–6th October 2015
Model calibration
12th–14th August 2015
Three different meshes were used to downscale the offshore wave climate from Bilbao-Vizcaya buoy to Laida beach: (1) a general one with 38 × 50 calculation elements with a horizontal resolution of 1000 m and; (2) a detailed one defined by 276 × 152 calculation elements with a horizontal resolution variable from 10 m (at Laida beach) to 340 m at the outer part; (3) a more detailed one defined by 417 × 152 calculation elements with a horizontal resolution variable of 10 m over the intertidal sand bars, 40 m at the inner estuarine part and 340 m at the outer estuarine part. For the morphodynamical simulation an averaged extraction depth of 0.70 m was assumed. With the aim of determining the precision of the simulated variables in relation with recorded real data, the Skill index (s) (Willmott, 1981; Ma et al., 2011) was used:
case 600 snap-shot images taken every 2 s during 20-minute periods). During June–October 2015, contour morphological variations of the supratidal zone of Laida beach were monitored by representative orthorectified TIMEX images every fifteen (15) days under average high tide conditions, while monitoring of excavated zones and low-amplitude ridges development was carried out by the same method under average low tide conditions and combined with obtained topographic and bathymetric data. For the selection of the representative images the reconstruction of the sea level was done using the main tidal astronomic components obtained from Pasaia tidal gauge (Fig. 1). 2.4. Numerical modelling Finally, the performed numerical modelling allowed us to identify accurately the main general hydrodynamic factors that provoke the observed morphodynamic response at the whole area of study. In order to evaluate the hydrodynamic and morphodynamic behavior of Laida beach and get volumetric information of the extraction evolution Delft3D numerical model (Roelvink and van Banning, 1994; WL/Delft Hydraulics, 2011) was applied. Data recorded from the 12th to the 14th of August 2015 were simulated and used to calibrate the hydrodynamic parameters of the model. Moreover, measured sea-level and induced currents during the 2-month instrument deployment were used to validate the outputs of the model. Hydrodynamic model was implemented with the following simulation period - GMT0 -: 10/08/2015, 12:30–14/08/2015, 12:00 and morphodynamic model (M) with this other: 29/05/2015, 00:00–01/ 10/2015, 00:00. The spin-up time used to stabilize the model was 36 and 48 h, respectively. Initial conditions consist of hourly tidal data from Bilbao gauge and hourly wave data from Bilbao-Vizcaya buoy (Spanish Ports, 28/09/ 2016). For the calibration of the hydrodynamics it is necessary to determine the roughness coefficient - C - in a variable spatial and temporal way according to the site depth using the Colebrook formula:
wherexobs are instrumental data, xmod are the results of the model and n the length of both data series. Regarding the sediment transport, Van Rijn (1993) and Soulsby (1997) formulas were tested. After studying the erosion/sedimentation patterns of Laida beach it was decided to apply the formula of Van Rijn (1993). According to the results of several authors (Grunnet et al., 2004; Mann et al., 2006; Van Rijn et al., 2007; Walstra et al., 2007; Briere et al., 2011) the default parameter values were used except for the wave-related sediment transport factors (0.05 was chosen) and for the transverse bed gradient factor for bedload transport (30 was chosen). During the study period 40 sediment samples were weekly recollected for grain size analysis in eight (8) different locations of the intertidal zone of Laida beach situated along the profiles P1 and P3 showed in Fig. 8. According to the grain size results the chosen value for the median grain size (D50) was 300 μm.
12H ⎤ C = 18 log10 ⎡ ⎢ ks ⎦ ⎥ ⎣
In this section the data presented in the previous part will be used to: (1) analyze the wave climate at the area during the study period; and (2) to describe the evolution of the supratidal beach, intertidal bar and the extraction trenches.
n
s=1−
(|x mod − x obs | + |x obs − x obs |)2
(3)
3. Results
(1)
where, (H) is the depth and (ks) is the equivalent roughness of Nikuradse. The typical value for the latter is 0.1–0.2 m. Eddy viscosity (ε), which depends on the cell size, was introduced by the following expression:
ε = k⋅Δx⋅u
∑1 |x mod − x obs |2 n ∑1
3.1. Hydrodynamic data during the monitoring period 3.1.1. 1. Data obtained throughout the monitoring period The data recorded by Bilbao-Bizkaia buoy and Bilbao tidal gauge from May 2015 to November 2015 and the 2-day instrument deployment - 12-14 August 2015 - are presented in Fig. 4 A and B respectively. After the analysis of the annual wave data, it is observed that the data collected from May 2015 to November 2015 (Fig. 4A) meet the expected pattern for the study area. In autumn (September–December) the waves at deep waters are relatively high corresponding to swell type waves - Hs: mean 4.5 m; maximum 8 m; minimum 1 m; Tp: mean 12 s; maximum 22 s; minimum 10 s. From April to July the waves are normally smaller corresponding to sea type waves – Hs: mean: 1 m; maximum 2 m; minimum: 0.5 m; Tp: mean 6 s maximum 9 s; minimum: 4 s.
(2)
Where, (k) is a constant which value varies between 0.05 y 0.15; Δx is cell size and u the characteristic velocity of the study area (0.5 m/s). Finally, wave propagation calibration was made through the calibration of the bottom friction coefficient - Cb. In order to choose the adequate values for those coefficients and to allow the model work properly, a sensibility analysis was carried out: 1. several simulations were carried out using different values for the mentioned parameters; 2. the obtained results were statistically compared with the recorded real data: sea-level, induced currents and waves - Hs and Tp. 4
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Fig. 4. A: Data obtained by Bilbao-Bizkaia buoy from May to November 2015. Wave height: Hs; Peak period: Tp: wave direction (Dir). Data obtained by Bilbao tidal gauge are shown at the bottom. Those data include astronomic and meteorological tides relative to the local hydrographic zero – Bilbao -; B: Data obtained by Bilbao-Bizkaia buoy during the study period – 12-14 August 2015. Data obtained by Bilbao tidal gauge are shown at the bottom.
compared with the velocity data provided by the model. As can be easily seen the modelled data and the real data fit very well for all the stations regarding to the tidal curves and moderately well in relation to wave Hs and Tp. Some disparity is observed regarding the velocity registered at the stations and the modelled due to the reasons previously stated. Fig. 7 shows the results relative to the averaged currents and averaged sediment transport over the low-amplitude ridges for each of the three tidal cycles studied. All cycles present a southwards net sediment transport that is one order of magnitude higher under storm conditions. Thus, during the summer period characterized by calms and moderate storms, the low-amplitude ridges progressively advance toward Laida beach supratidal zone.
The approaching direction of the swell type waves is mainly 300° with some punctual variations relative to sea type waves. Tidal level as usual for this zone of the world varied from 0 m to 4.5 m. As for the 2-day instrument deployment (Fig. 4B), two different behaviors were observed: a calm period during the first two tidal cycles– mean Hs: 1 m; mean Tp: 7 s – and a summer storm during the third tidal - mean Hs: 3.5 m; mean Tp: 10 s. The approaching direction of the waves was mainly northwest - N300E. The tidal level varied from 0.7 m to 4 m approximately and the tidal range slightly increased during the deployment period. The data collected by the Nortek Aquadopp ADCP deployed from the 4th August to the 6th October 2015 (see Fig. 1 for location), are shown in Fig. 5. The tidal range varied as usual for this part of the world. Its highest values were registered during September because of the equinoctial tidal wave. A strong variation of current velocity and direction was observed along each tidal cycle as expected from its location. Variations of the maximum current speed from 75 to 250 cm/s were reached around mid-tide. However, current velocities registered during the ebb were slightly stronger that the ones registered during the flood (see rose diagram of Fig. 5). Current directions were N and NNE for the ebb and SSE for the flood. Low values and a higher directional dispersion were registered during high and low tide phases.
3.2. Morphodynamics 3.2.1. Supratidalbeach The topographic data of three cross-shore profiles throughout the monitoring period are displayed in Fig. 8. The effects of the dredging and dumping can be clearly observed in those profiles. As it can be observed, P1 and P2 profile series were located close to the eastern cliffs while P3 profile was situated at the central part of Laida beach. Profile series recorded the morphological changes of the intertidal and supratidal beach during the study period (May–October 2015). The sand scraping and dumping was carried out from the 11th of May to the 4th of June. As it can be seen in May the supratidal zone in P1 (P1: x < 5400 m) was flat and its level was around z = 3 m. From May to July the amount of sand at the supratidal zone increased, lifting the level to z = 4.5 m. This amount of sand corresponds to the sand volume dumped that migrated southward (inshore) by wave action. From July to October a shoreward movement of the profile can be observed. It
3.1.2. Validation of the model Delft 3D model was used to simulate the hydrodynamics at Oka estuary and to get volumetric information of the evolution of the extraction. The model was calibrated and validated by using instrumental data. Fig. 6 contrasts the results relative to sea-level, currents and waves given by the model with the real data recorded by the performed deployments. Moreover, the numeric results obtained by tidal gauges and velocities recorded by the Aquadopp ADCP instrument deployed are 5
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Fig. 5. Data obtained by the Nortek Aquadop ADCP deployed during the study period: 04 August/06 October 2015. Tidal data include astronomic and meteorological tides relative to the local hydrographic zero – Bilbao. At the bottom can observe the sub surface current rose diagram corresponding to the cell 8 m above the location level of the instrument that was 10.9 m below the Mean Sea Water Level. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
supratidal zone is observed, although during July already a sand accumulation along the still intertidal profile begins to form. The sand accumulated at the supratidal zone in P2 comes from P1 by the longshore currents. Later, in October, the supratidal accumulation migrates to the south west.
happens due to the fact that longshore currents (from east to west) transport the sand previously accumulated in P1 to the west. This longshore current can be observed in Fig. 7, where the vertically average current is modelled with Delft 3D. In P2 it is in August when the increase of the amount of sand at the
Fig. 6. Contrast of the data – sea level and velocity magnitude - supplied by the Delft3D model with the data provided by Bermeo and Sukarrieta tidal gauges and the Aquadopp ADCP - see Fig. 1 for location - during the study period 13-14/08/2015. The velocity magnitude obtained with the model is wave and depth averaged. The velocity magnitude of the ADCP corresponds to a depth of 0.5 m.
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decline of 0.5 m seen in the profile of October. Video images from May to September 2015 show the evolution of the dry beach (Fig. 9). In May the dry beach was nearly non-existent whereas in September a considerable area of beach can be observed. Notice that in June the sand was dumped at the bottom of the eastern cliffs and that during the next months this dumped sand was progressively transported across the intertidal Laida beach toward the southwest until it was welded to the dry beach generating a hooked sand-spit by September 2015. In order to quantify the dry beach volume gained due to the sand dumping, a further analysis of the topography data was carried out. The volume of sand of Laida beach above mean high water springs (Z > 3 m) and above mean high water neaps (Z > 4 m) were calculated from topo-bathymetric data. Fig. 10 shows the difference of sand volume above mean high water springs, before the sand dumping (black; May) and after the sand was redistributed (white; October). In Table 2 the sand volumes of the dry beach calculated during spring tides (Z > 4 m) and during neap tides (Z > 3 m) are shown. After dredging and dumping, Laida beach gained 30,400 m3 of sand (difference between May and October with neap tides), which is the 70% of the sand that was dumped.
Fig. 7. Depth averaged current velocity and direction – arrows – over the intertidal sand bars of Laida beach provided by the model for wave storm conditions – studied third tidal cycle.
3.2.2. Ploughedintertidalbarandcontrolzonebehaviors The intertidal bar of Laida beach was split in two at the moment the study was carried out (Fig. 11). In order to be able to observe the
In the profile series P3 are not observed significant variations along the zone situated at x < 5400 m, except the overall around level
Fig. 8. Morphological evolution of Laida beach intertidal and supratidal zone during the study period based on the topographic profiles carried out. The location of the represented profiles can be observed in Fig. 3. P1 and P2 profile series are relative to the ploughed lowamplitude ridge while P3 profile represents the undisturbed one. Vertical axis: bed level, Z (m). Horizontal axis: cross-shore distance (Northing, m), increasing offshore.
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Fig. 9. Monthly morphological evolution of the supratidal Laida beach area over TIMEX images during the study period.
Fig. 10. Left: Surface of the supratidal zone of Laida beach during spring tides (Z > 4 m). In black: May 2015; in white: October 2015. Right: Bed level variation in the supratidal area between October 2015 and May 2015. Positive values mean accretion.
that the accretionary sequence of Laida beach is characterized by a uniform inshore migration of the intertidal bar. However, after ploughing the most eastern part of the intertidal bar its migration pattern changed by accelerating the migration of the ploughed zone. In the plough zone this migration was approximately 3 times faster comparing to the control zone (advancement of 95 m vs 35 m). Moreover, in Fig. 11 a sand tongue situated close to the cliff can be seen. This sand comes from the ploughed area but in this specific area the effect of the contour enhanced its migration, reaching 300 m.
Table 2 Increase of the sand volume at the supratidal zone of Laida beach during the monitoring period at neap and spring tides.
May 2015 October 2015 Difference
Volume (neap tides, m3), Z > 3m
Volume (spring tides, m3), Z > 4m
18.200 48.600 30.400
1.300 13.500 12.200
morphodynamic response of this intertidal bar under the ploughing made with the aim of accelerating its onshore migration - and to make comparison with the bar behavior under natural conditions, the eastern zone of the bar was ploughed while its western zone was not altered (control zone, Fig. 3). P1 and P2 profile series (Fig. 8) were performed over the ploughed area while P3 profiles covered the natural bar. The intertidal bar in all profiles is located at (5700 < x < 5800). The profiles series show that in all profiles series there is an onshore movement of the intertidal bar. However, this shoreward movement is greater in P1 and P2 than in P3. Thus, it can be stated that the intertidal bar did not migrate uniformly. The eastern part (ploughed) moved faster than the western part (unaltered). If P1 and P2 profile series are compared it can be observed that the intertidal bar migrates about 40 m further inshore in P1 than in P2. Interestingly, previous studies (Monge-Ganuzas et al., 2008) showed
3.2.3. Extractionzones The surface evolution of the extraction zones was studied by KOSTASystem TIMEX images (Fig. 12). The contours of the extracted zones were digitized on the TIMEX images every month and the surface evolution of their areas was calculated. The extracted areas showed a progressive and homogeneous surface decreasing evolution (Table 3). By the end of October it was no longer possible to recognize the traces of the extraction areas as they had been completely filled. The volumetric evolution of the extraction areas (Fig. 13) over the 4 months of monitoring (June–October) was obtained by the model Delft 3D (Roelvink and van Banning, 1994; WL/Delft Hydraulics, 2011). This figure shows the percentage of recovered volume of the affected areas during the 140 days that the recovery process lasted. It was observed that 30% of the extracted area was filled again in one month, in 3 months the 90% of the extracted volume was recovered again and by the 4th month the extracted area was completely 8
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Fig. 11. Morphodynamic response of the ploughed low-amplitude ridge – right side – during the monitoring period – 17/07-30/09/ 2015 - compared with the undisturbed control zone – left side - provided by TIMEX images. The bottom plot represents the onshore migration of the ridges between these two dates in the control zone (35 m), in the plough zone (95 m), and along the cliff (300 m).
Fig. 12. Monthly morphological evolution of the extraction zones (in black) over TIMEX images during the study period.
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The fact of having opted for the method of choosing different relatively small extraction areas is considered adequate as by this method negative impacts in other sedimentary features of the lower estuary (i.e. Mundaka sandbar, in other words, proximal ebb tidal delta) has been avoided. Furthermore, the factthat the extracted and dumped sand volume (44,800 m3) has not been higher than the volume of sediment that is normally moved - estimated by Monge-Ganuzas et al. (2008) in around 55,000 m3 through the intertidal beach during the summer period is considered proper as well. However, there is not enough information to state that the response of the system will be similar if the action is repeated in the future. Thus, further work should be done about this topic. Moreover, there is no information about the cumulative effects (Council of Environmental Quality, 1997) that the repetitive extraction/dumping actions can cause in this zone of the estuary. Special attention should be paid over the estuarine morphodynamics and hydrodynamics, the beach steepness, the cycles of destruction/ formation of Mundaka sand bar, the possible instability of existing navigation channels and the supratidal zone stability, among others. Consequently, a strong piece of scientific information about the characteristics of the sedimentary system and its short- medium- and longterm response to human actions should be available in order to give reliable advice about further extraction/dumping actions at Laida beach. This fact implies that, as the uncertainties are still considerable, more research should be implemented. In this way, the authors consider indispensable the definition of a multiyear morphodynamic monitoring planning and a well-planned strategy for at least, the lower Oka estuary –better the whole estuary - in order to be able to provide reliable Science based criteria for future decisions. This statement is reinforced by the unquestionable importance of Oka estuary as provides important ecosystem services for humans (Turner and Schaafsma, 2015).
Table 3 Surface evolution of the extraction areas during the study period. Month
Surface of the extraction areas (m2)
June July August September October
35,200 26,300 19,600 13,600 2800
Fig. 13. Evolution of the accumulated sediment volume at the extraction sites for the June to October 2015 time period calculated by the applied model.
recovered.
5. Conclusions 4. Discussion From the monitoring carried out it can be stated that the extraction method used did not provoke any irreversible damage over extraction sites and their surrounding areas as the extraction sites recovered its original situation in a time period of 140 days. The performed action – extraction and dumping - generated an increase of the supratidal zone of Laida beach by summer 2015. From the 44,800 m3 of sand that were dumped, the 70% was transported and accumulated at the supratidal zone (z > 3 m) of Laida beach. By summer 2015, there were 23,000 m3 of dry beach. Therefore this action was successful as the final purpose was reached. However, there are several doubts about the future morphodynamic natural response of the sedimentary system. There is also uncertainty about the cumulative effects that the used method could cause over the sedimentary system, in case it is decided to arrange it again in the future. Ploughing increased low-amplitude ridge inshore advance. However further study is necessary to conclude if the onshore migration of the intertidal bar can be accelerated by ploughing it. Furthermore, more field experiments in different type of beaches are required in order to study how the plough works in other situations and discard any possible negative affection over the behavior of the sedimentary system. As the wave, tidal, meteorological and wind action and their effects in the sedimentary environments of littoral of the Bay of Biscay are random in the small-scale – hourly, daily, weekly or monthly scale – and the data records provided by offshore buoys or the amount of available data from specific surveys is still scarce further work should also be done about this topic. In this way, the authors consider indispensable the definition of a multiyear morphodynamic monitoring planning and a well-planned strategy for at least, the lower Oka estuary –better the whole estuary - in order to be able to provide validated and reliable Science based data. This statement is reinforced by the unquestionable importance of Oka estuary as provides important ecosystem services for humans. This statement could be extrapolated to the majority of the sedimentary environments of the Bay of Biscay.
The results suggest that ploughing the intertidal bar and dumping extracted sand helped Laida beach to recover its dry beach by the summer of 2015. The decision to dump the extracted sand in this intertidal zone instead of dumping directly over the previous supratidal beach is considered appropriate:(1) natural processes redistributed dumped sand creating naturally structured sedimentary features much more robust against erosion than a simple dumping of unconsolidated sand over the shoreline that has probably been eroded more quickly; (2) the morphology of the sedimentary structure naturally generated after the dumping and the sand transport resulted in a much more natural landscape. This fact is especially interesting as the area is considered as a natural protected area. As it was stated by Monge-Ganuzas et al. (2015) and confirmed by the modelling performed in the present study, the general sediment transport pattern at intertidal Laida beach during summer consist on a southwestward growth of sand ridges, where the coastal contour morphology, the wave-induced currents and the local tidal currents can be considered as the main drivers of this sediment transport. Due to the influence of the headland located on the western margin of the estuarine mouth, waves approaching from NW diffract and results in a differential of energy, i.e. a wave height gradient along the beach (higher waves in the east and lower waves in the center of the bay). Consequently, this wave height gradient induces longshore current (from east to west). Moreover, it is possible to state that the use of quantitative techniques - topographic surveys - combined with quantitative/qualitative techniques –TIMEX images - and as well with modelling, reveals itself as an effective multi-approach method for morphodynamic monitoring of littoral processes. What is more, the performed numerical modelling allowed us to understand the main general hydrodynamic factors that provoke the observed morphodynamic behavior. 10
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dredging and dumping activities along the lower Oka estuary (Urdaibai Biosphere Reserve, southeastern Bay of Biscay, Spain). Ocean Coast. Manag. 77, 40–49. Monge-Ganuzas, M., Cearreta, A., Iriarte, E., 2008. Consequences of estuarine sand dredging and dumping on the Urdaibai Biosphere Reserve (Bay of Biscay): the case of the “Mundaka left wave”. J. Iber. Geol. 34, 215–234. Monge-Ganuzas, M., Iriarte, E., Cearreta, A., Arana, X., 2004. Sustainable management in the Urdaibai Reserve of the biosphere (Southern Bay of Biscay). Coastal dune regeneration. In: Green, D.R. (Ed.), Delivering Sustainable Coast: Connecting Science and Policy. Proceedings of the VII International Symposium LITTORAL. Aberdeen (U.K.) 20–22 September Vol. 2. Cambridge Publications, pp. 725–726. Roelvink, J.A., van Banning, G.K.F.M., 1994. Design and development of Delft3D and application to coastal morphodynamics. In: Verwey, A., Minns, A.W., Babovic, V. (Eds.), Proceedings of HYDROINFORMATICS 1994. Balkema, Rotterdam, pp. 451–456. Pascual, A., Cearreta, A., Rodriguez-Lázaro, J., Uriarte, A., 2004. Geology and paleoceanography. In: Borja, A., Collins, M.B. (Eds.), Oceanography and Marine Environment of the Basque Country. Elsevier Oceanography Series Vol. 70. pp. 53–70. Soulsby, R., 1997. Dynamics of Marine Sands. A Manual for Practical Applications. Thomas Telford Publications, Oxford (U.K.), pp. 249. Turner, R.K., Schaafsma, M., 2015. Coastal zones ecosystem services. In: From Science to Values and Decision Making. Springer International Publishing, Switzeland, pp. 239. Van Rijn, L.C., 1993. Principles of Sediment Transport in Rivers, Estuaries and Coastal Seas. Aqua Publications, The Netherlands, pp. 673. Van Rijn, L.C., Walstra, D.J.R., van Ormondt, M., 2007. Unified view of sediment transport by currents and waves. IV: application of morphodynamic model. J. Hydraul. Eng. 133, 776–793. Walstra, D.J.R., Van Rijn, L.C., Van Ormondt, M., Brière, C., Talmon, A.M., 2007. The effects of bed slope and wave skewness on sediment transport and morphology. In: Kraus, Nicholas C., Rosati, Julie Dean (Eds.), roceedings of COASTAL SEDIMENTS’07 ASCE New Orleans, USA, pp. 1–14 (May 13-17). Willmott, C.J., 1981. On the validation of models. Phys. Geogr. 2, 184–194. WL/Delft Hydraulics, 2011. Delft3D-FLOW, simulation of multidimensional hydrodynamic flows and transport phenomena, including sediments. In: User Manual. 2011 Delft.
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Web pages Basque government 09/28/2016. http://www.geo.euskadi.net. Spanish Ports 09/28/2016. www.puertos.es.
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