The relative impacts of tidal Essay

Abstraction

The comparative impacts of tidal ( neap, spring ) and river discharge ( including a flood event ) coercing upon H2O and deposit circulation have been examined at the rock-bound Guadiana estuary. Near-bed and perpendicular profiles of current, salt, turbidness, plus surface suspended sediment concentrations ( SSC, at some Stationss merely ) , were collected at the lower and central/upper estuary during tidal and biweekly rhythms. In add-on, perpendicular salt and turbidness profiles were collected about high and low H2O along the estuary. Tidal dissymmetry produced faster currents on the wane than on the inundation, particularly at the oral cavity. This form of offshore current laterality was enhanced with increasing river flow, due to horizontal advection that was confined within the narrow estuarial channel. The fresh water inputs and, at a grade less, the tidal scope controlled the perpendicular commixture and stratification importance. Well-mixed ( spring ) and partially-stratified ( neap ) conditions alternated during periods of low river flows, with important intra-tidal fluctuations induced by tidal straining ( particularly at the partially-stratified estuary ) . Highly-stratified conditions developed with increasing river discharge. Intratidal variableness in the pynocline deepness and thickness resulted from current shear during the wane. A salt cuneus with tidal gesture was observed at the lower estuary during the flood event. Depending on the strength of disruptive commixture, the residuary H2O circulation was dominantly controlled either by tidal dissymmetry or gravitative circulation, The SSC was governed by cyclical local procedures ( resuspension, deposition, commixture, advection ) driven by the neap-spring fluctuations in tidal current speeds. More, intratidal variableness in stratification indicated the significance of tidal pumping at the partially- and highly-stratified estuary. The estuary turbidness upper limit ( ETM ) was enhanced with increasing current speeds, and displaced downstream during periods of high river discharge. During the inundation event, the ETM was expelled out of the estuary, and the SSC along the estuary was controlled by the sediment burden from the drainage basin. Under these highly-variable river flow conditions, our observations suggest that sand is exported to the nearshore over the long-run ( & gt ; old ages ) .

Keywords. Estuarine kineticss ; tidal dissymmetry ; salt stratification ; turbidness ; tidal straining ; Portugal.

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1. Introduction

Understanding hydrokineticss and sediment conveyance in estuaries is critical for the sustainable direction of these systems. In peculiar, in a context of increasing anthropogenetic force per unit area, cognition of the H2O and sediment circulation forms is progressively relevant for the care of pilotage channels, the destiny of atom edge contaminations, the ecology of benthal and oceanic communities and the morphological development of the environing coastline. The circulation of H2O in estuaries is governed by the denseness differences and the interaction between fresh and salt Waterss. Typically, a differentiation is made between the baroclinic ( density-driven, Hansen and Rattray, 1965 ) and barotropic ( pressure-driven, Li and O?Donnell, 1997 ) constituents of the flow. In estuaries with important tidal forcing, these force per unit area gradients may interact and bring forth internal tidal dissymmetries ( ebb-flood dissymmetries in denseness gradients ) ( Simpson et al. , 1990 ; Jay and Musiak, 1996 ) . Intratidal variableness, together with neap-spring fluctuations in denseness stratification, plays a important function in the development of residuary currents ( Sharples, et al. , 1994 ; Geyer et al. , 2000 ) . The comparative strengths of the barotropic and baroclinic circulations, and of the interactions between these factors, are governed by a combination of external forcing ( tides, river discharge and air current ) and internal commixture. Therefore, the conveyance of belongingss ( H2O, salt and deposit ) and its temporal variableness depends mostly on the external forcing ( e.g. Woodruff et al. , 2001 ; Restrepo and Kjerfve, 2002 ; Patchineelam and Kjerfve, 2004 ; Gallic et al. , 2008 ) .

Rock-bound ( bedrock-controlled ) estuaries consist of narrow and comparatively deep estuaries located on passive borders, with tidal prism that exceeds their mean fresh water discharge by at least one order of magnitude ( FitzGerald et al. , 2002 ) . They differ from other systems with narrow constricted morphology such as the rias of NW France, SW England or NW Spain ( Castaing and Guilcher, 1995 ) because of their comparatively big drainage basin. The Guadiana estuary is an illustration of rock-bound estuary located in a semi-arid environment ( southern Iberian Peninsula ) . This estuary has late been in the spotlight, in relation to the closing of the Alqueva dike ( February 2002 ) , 60 kilometer merely from the estuary caput, which forms the largest reservoir in Southern and Western Europe ( 4 150 hm3 storage capacity ) . Numerous concerns have been raised about possible negative downstream impacts of the dike, in peculiar upon the estuarine ecosystem ( Ch & A ; iacute ; charo et al. , 2001 ) and erosion/accretion forms ( Dias et al. , 2004 ) . However, the estuarine hydrokineticss before the dike closing has been ill described, doing it hard to place harmful dam-induced effects.

This survey analyses hydrographic informations collected at the Guadiana estuary under contrasted tidal and river discharge conditions. The aim of the paper is to analyze the comparative impacts of neap and spring tides and river discharge coercing upon H2O and deposit circulation within the estuary ( including the influence of inundation events ) . In add-on, the dataset was collected before closing of the Alqueva dike, and will function as a mention model of the pre-dam estuarine kineticss, for farther research about the impacts and direction of the dike.

2. Study Site Features

2.1. Geomorphology

The Guadiana River drainage basin is the 4th largest on the Iberian Peninsula, with a length of 810 kilometer and an country of 66 960 km2. The estuary ( Figure 1 ) lies at the southern boundary line between Spain and Portugal. And extends for about 80 kilometers, from its oral cavity to the weir of Moinho Department of State Canais ( where the tidal moving ridge is virtually dampened ; Silva et al. , 2000 ) . Along most of its class ( from the weir to the International Bridge ) , the estuary is confined in a deep and narrow vale, which was incised under strong tectonic control during the pre-Flandrian low base ( Oliveira, 1990 ) . Merely the outer range ( terminal 7 km seaward ) is embedded in soft deposit There, a wide vale was excavated and infilled during the Holocene, suiting extended salt marsh countries on both sides of the recess ( Boski et al. , 2002 ) . At present the extension of the salt marshes is much reduced, in relation with active deposit during the Holocene period ( Boski et al. , 2008 ) and intense transmutations due to strong anthropogenetic force per unit area ( e.g. urbanisation, agriculture, aquaculture ; M & A ; eacute ; nanteau et Al. 2005 ) ( Figure 1 ) . On the western border, the salt marsh country is sheltered by a littoral sand tongue and drains into the estuarial channel. The eastern border consists of barrier islands and tongues separated by broad salt marsh countries that drain chiefly to the sea through a tidal recess ( Carreras ) . Both recesss ( i.e. Carreras and the estuary oral cavity ) are stabilized by breakwaters built in the mid 70?s ( the eastern breakwater at the Guadiana oral cavity is submerged ) . Water circulation within the estuary is about entirely confined within the narrow channel which connects straight the river to the unfastened littoral zone. The estuarial channel is about 700 m-wide from km 0 to ~ km 6, and so narrows upstream, being 70 m at Mertola ( Silva et al. , 2000 ; Lobo et al. , 2004 ) . The deepness of the channel is by and large & lt ; 10 m ( referred to the mean H2O degree ) , with a average deepness about 5 m from the oral cavity to km 50, that slowly decreases upstream. However, of import fluctuations are observed locally, with maximum deepnesss ( of up to ~18 m ) by and large in forepart of brook ( Lobo et al. , 2004 ) . The estuary is prolonged offshore by a submersed delta ( Morales, 1997 ) , where fluvial deposit is assorted with the marine stuff geting from longshore sediment conveyance ( Figure 1 ) ( Gonzales et al. , 2004 ) .

From the analysis of surficial deposit in the estuarine channel, Morales ( 1993 ) distinguished three sectors along the Guadiana estuary ( upper, in-between and lower ) . The upper estuary consists chiefly of crushed rock and sand from the drainage basin. The in-between estuary is dominated by ill sorted deposit, with grain size runing from crushed rocks to clay and silt. The fraction of clay lessenings downstream and good sorted medium sand ( quartz, feldspar, bioclasts, plus lithic constituents of diverse beginning ) lies at the lower estuary ( Lobo et al. , 2004 ) . In add-on, some crushed rock, either assorted with sand or in little stray pockets, are besides observed at few ( and by and large deep ) locations of the lower estuary ( Dias et al. , 2001 ) . These ( lower, in-between, upper ) sectors are accordant with distinguishable ecohydrological features that were described by Ch & A ; iacute ; charo et Al. ( 2001 ) .

Suspended deposit is dominantly composed of phyllosilicates, represented chiefly by illite ( & gt ; 50 % ) , kaolinite and chlorite ( Machado et al. , 2007 ) . For Portela ( 2006 ) , the inputs of mulcts ( from the drainage basin ) to the estuary is about 0.5 – 1.5 106 t yr-1 ( period 1980 – 2000 ) , and merely a minor portion ( ~ 10 % ) is retained within the estuary. With decreased river discharge ( & lt ; 10 m3 s-1 ) , the axial suspended sediment concentration ( SSC ) ranges normally between less than 10 mg L-1 at the oral cavity, up to an estuarial upper limit of about 100 mg L-1 at the in-between estuary ( e.g. Wolanski et al. , 2006 ; Machado et al. , 2007 ) . Such maximal concentrations are typical of the estuary turbidness upper limit ( ETM ) of mesotidal estuaries ( e.g. the Tagus estuary ; Vale and Sundby, 1987 ) . Typically, the ETM is located at the forepart of the haline invasion ( salt & lt ; 2, measured utilizing the Practical Salinity Scale afterlife ) . Scarce information suggest that, under moderate flow conditions, the ETM oscillate between km 30 and km 50 with the tide, being centred near Alamo ( Figure 1 ) ( Silva et al. , 2003 ; Machado et al. , 2007 ) . Besides, comparatively long period of observations indicated that the haline invasion forepart, and therefore the ETM, was ne’er upstream of Alcoutim, including during drawn-out periods of low river overflow ( Rocha et al. , 2002 ) .

2.2. River outflow

Fresh H2O discharge values for the Guadiana estuary are recorded at Pulo do Lobo station, located 14 km upstream from Moinho Department of State Canais ( instead, records from Rocha da Gal & A ; eacute ; , few kilometers upriver from Pulo do Lobo, are used to make full some spreads in the information ) . Discharge measurings at this station are representative of 91 % of the full Guadiana drainage country.

The river inputs to the Guadiana estuary are extremely variable, at a seasonal and inter-annual graduated table. This form produces terrible drouths and episodic inundation in the river basin. For illustration, the monthly river discharge ranged from & lt ; 10 m3 s-1 to 4660 M3 s-1 for the period 1947 – 2001 ( Figure 2 ) , whilst maximal historical extremum discharges are estimated around 11 000 m3 s-1 in winter 1876 and, likely, 1603 ( Rocha and Correia, 1994 ; Ortega and Garz & A ; oacute ; n, 2009 ) . Figure 2 besides shows the episodic character of the inundations, with 80 % of the monthly discharges being & lt ; ~ 700 m3 s-1 ( and 50 % & lt ; 110 m3 s-1 ) . With the North Atlantic Oscillation ( NAO ) being the dominant manner of the winter clime variableness in the north Atlantic part, an reverse correlativity is observed between the ( one-year and wintertime ) Guadiana River discharge and the NAO index forms ( Dias et al. , 2004 ; Trigo et al. , 2004 ) .

From the 1950s ‘ onward, more than 100 big dikes were constructed in the river basin, chiefly for storage and irrigation intents. At the terminal of 2000, approximately 70 % of the drainage basin was controlled by dikes ( Alves et al. , 2001, cited in Portela, 2006 ) . Evapotranspiration, due to both the development of irrigation strategies and reservoir vaporization, has induced a important decrease in the mean river flow over the last decennaries ( Trigo et al. , 2004 ; Portela, 2006 ) . In add-on, flow ordinance has significantly reduced the frequence of inundations. Therefore, the ( opposite ) correlativity between river discharge and NAO index is poorer from the mid-60s ‘ ( Dias et al. , 2004 ) .

2.3. Wind, moving ridges and tide

North and south-westerly air currents prevail in the country, with monthly averaged velocities up to 5 m s-1 ( Rocha Faria et al. , 1981 ) . However, although no informations are available, northern or southern air currents are likely reinforced ( in both strength and frequence ) within the estuary, in relation with the ( narrow and profoundly incised ) morphology and ( north-south ) orientation of its vale. The consequence of air current has non been considered yet at the Guadiana estuary, although Plaza et Al. ( 2003 ) identified a 3.7 yearss sub-tidal frequence in the current signal at the lower estuary and at the shelf, which could be related to the air current government.

The offshore moving ridge clime, from a waverider buoy located offshore Faro ( c.a. 50 km due easts of the Guadiana estuary ) , is dominated by W-SW moving ridges ( 71 % of happenings ) and short period ( sea ) waves from the SE ( 23 % of happenings ; Costa et al. , 2001 ) . Wave energy is moderate, with yearly-averaged ( offshore ) important moving ridge tallness and peak period of 1 m and 8.2 s, severally ( Costa et al. , 2001 ) . Storm conditions ( i.e. offshore wave & gt ; 3 m in tallness ; Pessanha and Pires, 1981 ) correspond to less than 2 % of the offshore moving ridge clime government ( Costa et al. , 2001 ) . These conditions produce a strong long-shore current, from West to east. The ensuing ( due east ) longshore sediment conveyance has been estimated to be about 180 103 m3 yr-1, on the footing of sand accretion along the western breakwater of the estuary oral cavity ( Gonzales et al. , 2001 ) . The action of moving ridges is important at the submersed delta, but by and large neglected within the estuary, where tidal and riverine procedures dominate.

The tidal moving ridge propagates westward along the seashore. The signal is regular semidiurnal, with averaged tidal amplitude of about 2 m ( mesotidal ) . Diurnal inequality in tidal lift is low ( up to ~ 0.2 m ) . The chief harmonic components are O1, K1, M2 and S2, with a higher significance of the semidiurnal M2 constituent over the diurnal constituent K1 ( Silva et al. , 2000 ; Pinto, 2003 ) . Tidal currents on the shelf are of negligible magnitude, e.g. about 0.05 m s-1 off the Guadiana coastal country ( S & A ; aacute ; nchez et al. , 2006 ) . These currents are significantly amplified within the estuary ( Plaza et al. , 2003 ) . Top out spring current speeds at Vila Real de Santo Antonio ( VRSA, afterlife ) are typically about 1 m s-1 ( e.g. Fortunato et al. , 2002 ) . Relatively long tidal gage datasets are merely available at VRSA. These records indicate average tidal oscillations of 1.28 m at neap and 2.56 m at spring, and a maximum spring tidal scope of up to 3.44 m ( Instituto Hidrogr & A ; aacute ; fico, 1990, cited in Dias et al. , 2003 ) . The tidal moving ridge is stationary, with a important progressive constituent. Scarce information suggest that the slowdown between maximal H2O degree amplitude and peak currents is about 2 Hs throughout the full estuary ( e.g. Silva et al. , 2003 ) . The tidal prism is estimated around 40 106 and 20 106 M3 at spring and neap tide, severally ( Morales, 1993 ) . The chief constituent of the flow work stoppages by and large in the way of the estuarial channel ( Silva et al. , 2000 ) , except likely in the locality of channel meanders. Besides, giving the narrow morphology of the estuary, it is by and large assumed that there is no sidelong fluctuation of the flow.

3. Datas and methods

Current metre informations were collected over a tidal rhythm, at least, in February and September 2001 at assorted locations along the estuary ( see Table 1 ) . The instrumentality included: an Acoustic Doppler Current Profiler ( ADCP 600 kilohertz, RDI ) fitted with a force per unit area detector ; and, two electromagnetic current metres ( RCM9, Aanderaa ) with extra detectors to mensurate force per unit area, turbidness, temperature and conduction. The dataset covers utmost tidal ( neap, spring ) and river discharge conditions, including a inundation event ( see Table 1 ) .

The RCM9 provided ( individual point ) measurings of the flow, following two methods, i.e. at fixed station and bottom-mounted. At fixed station, perpendicular speed profiles were obtained from 5 min-averaged measurings ( at a 10 Hz trying rate ) every metre along the H2O column. The perpendicular profiles were collected every hr, during full tidal rhythms. For some of these studies, surface H2O samples were besides collected hourly utilizing a Niskin bottle. When bottom-mounted, the RCM9 provided 5 min-averaged informations, from measurings at ~ 0.7 m from the bed ( at a 2 Hz trying rate ) . In add-on, bottom-mounted ADCP informations were collected at 5 min sampling and averaging intervals, during two biweekly rhythms. However, merely a portion of this dataset is presented here, to characterize spring and neap tide conditions at the clip of the measurings at fixed Stationss ( Table 1 ) . The bin size of the ADCP measurings was set to 0.5 m, with the Centre of the first bin at 1.5 m from the underside.

In add-on, turbidness, force per unit area, conduction and temperature informations were collected with multi-sensor sondes ( Hidronaut ) during 4 studies, i.e. around low and high H2O at neap and spring tides ( Table 1 ) . For each of the studies, the informations were collected ( at the same tidal phase ) at several Stationss ( between 13 and 16 ) along the estuary. At each station, perpendicular profiles were obtained by take downing the sonde at a speed of about 1 thousand s-1. The river discharge was no more than 10 m3 s-1 during the studies.

The salt was computed internally by the instruments ( RCM9 and sondes ) based on the conduction, temperature and force per unit area readings. The conduction and turbidness detectors were calibrated in research lab following standard processs ( see Silva et al. , 1992 ) . For the turbidness, at least 5 formazine buffers, runing from 0.04 to 10 FTU ( Formazine Turbidity Units ) were used for standardization. The conduction detectors were calibrated based on the salt derived from the analysis ( in research lab ) of H2O samples from the site. At the haline forepart, high concentrations of suspended deposit ( due to the presence of the ETM ) affected the readings of the conduction. Therefore, salt values less than 2 were considered as nothing. Spikes in the informations were eliminated by filtrating and ocular review, following the process described by Instituto Hidrogr & A ; aacute ; fico ( 2000 ) . The force per unit area measurings at fixed Stationss were converted in H2O deepness ( referred to chart data point, CD, 2 m below the average sea degree ) utilizing the records from tidal gages installed temporarily nearby these Stationss.

4. Observations

4.1. Low river discharge

4.1.1. Vertical profiles ( tidal rhythms )

Spring tide

Well-mixed conditions were observed at spring tide, for river discharge & lt ; 10 M3 s-1 ( Figures 3 and 4 ) . At VRSA, extremum wane currents ( ~ 1 thousand s-1 ) were markedly faster than inundation 1s ( ~ 0.8 thousand s-1 ) , despite the comparatively short inundation stage ( ~ 6h ) . Downstream residuary currents along the H2O column at ISN confirmed the predomination of ebb currents at the lower estuary during this peculiar tidal rhythm ( Figure 5a ) . In item, the residuary speed profiles consisted of a bottom bed ( less than half the H2O deepness in tallness ) with changeless speeds ( & gt ; 0.1 m s-1 ) and an upper parabolic profile with maximal speeds ( & gt ; 0.2-0.3 m s-1 ) near the surface. At Odeleite, inundation and ebb currents were relatively more balanced and slower, with maximal speeds about 0.6 thousand s-1 at mid-depth ( Figure 4a ) . At both fixed Stationss, the salt and current speed contour lines were advected vertically upstream-downstream with the tide. In inside informations, nevertheless, tidal straining induced weak salt stratification during the wane at Odeleite, with somewhat lower salt values near the surface than at deepness ( Figure 4a, B ) . The ensuing density-driven circulation produced a ( 1-1h30 ) longer wane and inundation stage near the surface and near the underside, severally. Tidal straining was less marked at VRSA ( Figure 4d, vitamin E ) . At both Stationss, upper limit and lower limit in salt corresponded to slack Waterss, c.a. 1-2 H after high and low tide, severally.

The perpendicular SSC distribution was about homogenous, as indicated by sub-vertical turbidness contour lines along the estuary ( Figure 3 ) . The ETM was well-expressed, at the haline forepart and oscillated some 10 kilometers ( between Alamo and Odeleite ) , at least, with the tide ( Figure 3 ) . Surface values at the ETM were lower than at deepness. A 2nd zone of high turbidness with close surface upper limit was observed during the inundation ( Figure 3b, km 15-17 ) . This turbid zone was likely due to the eroding of soft muddy fringy Bankss at high H2O. At Odeleite, the turbidness and surface SSC increased during the wane, in relation with the ( downstream ) supplanting of the ETM ( Figure 4c ) . The subsequently reached the station at low H2O, as suggested by the low salt ( & lt ; 2 ) , maximal near bed turbidness, and surface SSC of approximately 100 milligrams L-1 ( i.e. the order of the expected concentration at the ETM during the dry season ) . During the undermentioned inundation, increased near-bed turbidness values indicated resuspension ( Figure 4c, 12-13h ) . Maximum in surface concentration ( ~140 mg L-1 ) occurred around extremum ( inundation ) currents, as resuspended deposit was quickly assorted along the H2O column. The concentrations dropped rapidly during the undermentioned slack wane, due to sediment subsiding. Turbidity degrees were much lower toward the oral cavity.

Neap tide

Partial stratification developed at neap tide, for river discharge & lt ; 10 M3 s-1 ( Figures 3 and 6 ) . The strongest perpendicular denseness gradients were observed between about km 10 and 15 ( Figure 3 ) , i.e. at a deep subdivision of the channel. Around low H2O, tidal straining induced horizontal layering of the isohaline and maximal bed-surface salt differences ( Figures 3 and 6 ) . Consequently, the estuary was better-mixed during the inundation than during the wane. Sub-parallel turbidness and salt contour lines near the surface indicate that tidal straining besides affected the distribution of mulcts at the surface beds, particularly at low tide ( Figure 3c, vitamin D ) . The turbidness, including at the ETM, was by and large much lower than for ( spring ) well-mixed conditions ( Figures 3, 4 and 6 ) . The tidal jaunt of the ETM was besides smaller at neap than at spring ( comparison for case the place of the ETM on Figure 3c at low neap tide, and on Figure 3a, 1h before low spring tide ) . At low H2O, the ETM was somewhat upstream of the haline forepart ( Figure 3c ) . This observation suggests that the SSC supplanting may show some slowdowns with regard to the salt conveyance. In understanding although the haline fresh water interface reached Odeleite at low H2O of the neap tidal rhythm, merely a little addition in the turbidness and surface SSC values was observed ( Figure 6 ) . Resuspended deposit during the inundation was restricted near the underside ( e.g. Figure 6c, 18-20h ) . Turbidity was low at VRSA during the full tidal rhythm.

The salt stratification was more marked at VRSA than at Odeleite. As such, the ebb stage at VRSA was markedly ( c.a. 3 H ) longer near the surface than near the bed. Minima and upper limit in salt corresponded to low and high H2O, severally ( Figure 6b, vitamin E ) . Maximal speeds ( & gt ; 0.8 and 0.6 thousand s-1 at VRSA and Odeleite, severally ) were observed near the surface and during the wane ( Figure 6a, vitamin D ) . Typically, residuary speeds, by and large & lt ; 0.1 m s-1, consisted of a two-layer flow, with an upstream flow in the bottom bed, and a downstream flow in the surface bed ( Figure 5b ) . Minimal and maximum turbidness ( and come up SSC ) were observed about high and low H2O, severally ( Figure 6c, degree Fahrenheit ) .

4.1.2. Near bed measurings ( biweekly rhythm )

At neap tide, near-bed currents were comparatively weak, approximately 0.4 thousand s-1 at upper limit at Alamo ( ~ 3 kilometer upstream from Odeleite ) and ISN ( nearby VRSA ) ( Figure 7 ) . Neap current speeds were by and large significantly higher on the inundation than on the wane. The turbidness signal was low at VRSA, proposing no or few sediment remobilization at the lower estuary. At Alamo, nearby the ETM, deposit is finer and turbidity events around slack wane indicate some episodic sediment deposition and resuspension, despite the low current magnitude.

Towards spring tide, wane and inundation near-bed currents were more balanced at Alamo ( ~ 0.6 thousand s-1, i.e. every bit high as the maximal currents observed at mid-depth at Odeleite – see Figure 4a ) . However, peak wane currents lasted significantly longer. At ISN, ebb currents were by and large higher than flood 1s with increasing tidal scope. On 18 September, nevertheless, inundation currents ( up to 0.8 m s-1 ) were higher than ebb 1s, face-to-face to the observations at VRSA along the H2O column ( Figure 4 ) . This suggests that baroclinic circulation reinforced ( opposed ) significantly landward ( seaward ) currents near the bed, during this tidal rhythm. Turbidity events were associated to top out currents and slack Waterss at both Stationss. At VRSA, these cloudy events likely correspond to the rapid resuspension of the advected ( all right ) deposit that was deposited on the flaxen bed during slack wane. Such procedure could explicate the upper limit in turbidness observed at an early phase of the inundation on Figure 4f ( 11h ) . At Alamo, likewise, turbidness extremums associated to top out inundation currents were much higher than those associated to top out ebb currents. These differences in turbidness suggest that more sediment deposited during ( the comparatively longer ) slack wane than slack inundations.

4.2. High river discharge

At VRSA, extremely graded conditions developed at neap tide for river discharge of 400 m3 s-1, ( Figure 8a, B ) . Flood currents were observed during most of the tide near the underside ( where the ebb stage lasted & lt ; 3h ) , but merely shortly near the surface. At deepness, inundation to ebb current reversal took topographic point when wane currents were close to top out values ( 1.2 m s-1 ) near the surface. Therefore, important current shear was observed along the H2O column during the wane. The current shear induced effectual perturbation and dislocation of the pynocline interface and, accordingly, a comparatively weaker stratification was observed about low H2O ( Figure 8a, B ) . During the inundation, the stratification was increasingly enhanced, and the pynocline was shallower. Maximum ( inundation ) currents ( & gt ; 0. 6 thousand s-1 ) were observed at mid deepness. These currents were similar in magnitude to those observed ( at neap ) during periods of low river flow ( Figure 6d ) . At ISN, residuary speeds were directed downstream, along the full H2O column, with comparatively changeless values ( & lt ; 0.1 m s-1 ) along ~ 70 % of the H2O column ( from the bed ) , but increasing drastically above ( up to ~ 0.35 thousand s-1 near the surface ) ( Figure 5c ) . The turbidness distribution at VRSA displayed important perpendicular stratification ( Figure 8b and degree Celsius ) . Minimum and upper limit in turbidness ( and come up SSC ) corresponded to slack inundation and wane, severally, as the suspended stuff concentration was basically controlled by advection. In add-on, the incline of the isocontours indicates that some deposit settled out during the period of slow ( near-bed ) wane currents ( Figure 8c, 8-10h ) . The newly deposited stuff was resuspended during the undermentioned inundation ( Figure 8c, 16-18h ) .

At Odeleite ( non shown ) , this tidal rhythm was characterized by fresh water and homogenous turbidness signal ( 100-120 FTU ) , proposing that the ETM ( if any ) was located farther downstream. In the absence of denseness gradients, current reversal was coincident along the H2O column. The ebb stage lasted 10 H, with peak currents ( 0.6 m s-1 ) at the surface that were twice faster than top out flood 1s.

During the inundation event, the fresh H2O discharge during the tidal rhythm ( ~ 90 Mm3 ) was more than twice the tidal prism estimated for spring tide and low river discharge conditions. Therefore, flood currents were up to ~ 0.2 thousand s-1, merely, at VRSA, despite spring tide ( Figure 9a ) . A salt cuneus developed on the inundation, due to cut down commixture and saltwater advection near the bed ( Figure 9b ) . Flood currents were observed during less than 4 H at the underside and less than 2 H at the surface. On the wane, peak currents reached 1.4 thousand s-1 near the surface. The salt cuneus displaced seaward and riverine features were observed, with a downstream flow of fresh water along the full H2O column. The turbidness at VRSA was homogenous along the H2O column during most of the tidal rhythm, with values & gt ; 500 FTU ( Figure 9c ) . However, the progress of the salt cuneus during the inundation was associated with a lessening in turbidness ( & lt ; 100 FTU at the terminal of the inundation ) , near the bed, due to the advection of ( less turbid ) sea H2O. In add-on, a little lessening in the turbidness degree was observed during the inundation along the full H2O column, due likely to the subsiding of mulcts in relation to weak ( inundation ) currents.

5. Discussion and decision

5.1. Water circulation

Tidal moving ridge

The distortion of the tidal moving ridge propagating along estuaries has a major influence on the current speeds distribution ( Dronkers, 1986 ) . The analysis of scarce informations presented by Morales ( 1993 ) and Silva et Al. ( 2003 ) indicates small fluctuation in tidal amplitude along the Guadiana estuary ( no more than 0.2 thousand every 10 kilometer ) . In item, the estuary is hypo-synchronous in its lower and in-between parts ( up to km 20-25, about ) , and hyper-synchronous upstream ( up to km 50, at least ) . This state of affairs suggests that, during a tidal rhythm, currents are the weakest at the passage between the center and upper estuary, i.e. where the tidal amplitude is dampened the most. In understanding, our observations indicated a decrease of current speeds between the lower estuary and Odeleite ( Figures 4, 6 ) or Alamo ( Figure 7 ) . Frictional fading, at the bed and across the entryway channel, may explicate this form, whilst tidal resonance may be responsible for the addition in tidal wave amplitude toward the estuary caput ( Dyer, 1997 ) . With moistening and so increasing of the tidal moving ridge propagating upstream, similar tidal scopes are by and large observed at the upper and lower estuary ( e.g. Morales, 1993 ; Silva et al. , 2003 ) .

The form of the tidal moving ridge is distorted when propagating upriver the Guadiana estuary, in relation with decreased bed clash and faster tidal wave speed about high H2O ( Dyer, 1997 ) . Hence, the inundation stage tends to be increasingly shorter than the ebb one toward the caput ( Silva et al. , 2000 ) . This consequence is more marked at spring tide ( e.g. Figure 7b – force per unit area ) . At neap, the inundation is clearly shorter in the upper estuary merely, whilst the state of affairs is more balanced downstream, with a longer flood stage locally ( Silva et al. , 2003 ) . Previous dataset has indicated that the differences between wane and inundation continuance averaged over the long-run ( months ) at the oral cavity are little, e.g. & lt ; +/- 5 min at VRSA ( Silva et al. , 2000 ; Dias et al. , 2001 ) . However, important variableness between back-to-back inundation and wane stages has been reported ( Dias et al. , 2001 ) , in understanding with our observations ( e.g. Figure 9 ) . Tidal variableness is explained by the discrepancy of harmonic components and of the meteoric and river discharge forcing ( e.g. Restrepo and Kjerfve, 2002 ) . The present observations over a biweekly rhythm indicated important variableness in near-bed currents speeds and continuance at ISN ( e.g. Figure 5a, 17-18 September ) , whilst the air current speed and river flow were kept low.. The current form was by and large more consistent at Alamo and characterised by comparatively longer peak wane stages ( Figure 5b ) .

Despite tidal variableness and dissymmetry, ebb currents are by and large reported to be faster when compared with inundation 1s, particularly at the lower estuary ( e.g. Morales, 1993 ; Pinto et al. , 2004 ; Lobo et al. , 2004 ) . Faster ebb currents were besides observed in this survey at fixed Stationss ( Figure 4a and vitamin D ; Figure 7a, vitamin D, Figure 9a, Figure 10a ) . This form seems to be temporally consistent. Ebb-dominance has been antecedently reported at similar narrow estuaries submitted to moo river discharge conditions, e.g. Douro estuary ( Portela, 2008 ) , Ria of Ferrol ( deCastro et al. , 2004 ) or deep channel of the Dee estuary ( Moore et Al, 2009 ) . The sweetening of ebb-currents at the Guadiana estuary when submitted to moo river flow may be related to the comparatively big hydraulic deepness of the estuarial channel, in comparing with the tidal amplitude, as described by Moore et Al. ( 2009 ) for the Dee estuary. More informations are required to analyze this peculiar point. With increasing river discharge, horizontal advection within the confined channel enhanced the tidal dissymmetry, with marked ebb-currents laterality ( e.g. Figures 8, 9 ) . During flood events, highly high ( low ) seaward ( landward ) currents were observed, being for illustration & gt ; 1.4 m s-1 at the Guadiana estuary for river flow of 2,000 m3 s-1 ( Figure 10 ) and & gt ; 4 m s-1 at the Douro estuary for river flow & gt ; 15,000 M3 s-1 ( Silva et al. , 2005 ) .

Vertical stratification and commixture

Distinct stratified conditions consequence in the weakening or strengthening of the gravitative circulation, which may do important alterations in the tidal dissymmetry ( Uncles, 2002 ) . The combined consequence of tides and fresh water inputs control the perpendicular commixture and stratification importance at the Guadiana estuary. By contrast to the extensively described estuaries of mid-latitude parts, the Guadiana estuary shows extreme graded conditions that range from well-mixed to highly-stratified. These utmost commixture conditions result from the extremely variable river flow government, together with the bottleneck of the flow within the narrow estuarial channel.

Our observations indicated that at spring tide and for low river discharge ( of up to 250 M3s s-1, at least, harmonizing to Ferreira et Al. 2003 ) , disruptive diffusion dominated, and the estuary was well-mixed, with unidirectional seaward residuary perpendicular profiles ( Figure 5a ) produced by the tidal dissymmetry. In add-on, tidal straining favoured freshwater advection, and enhanced seaward near surface residuary currents ( Simpson et al. , 1990 ) . However, flood currents at the oral cavity were enhanced near the bed ( compare Figures 4 and 7, 18 September 2001 ) . Therefore, it seems that gravitative circulation can impact significantly the barotropic flow, despite effectual turbulent blending throughout the H2O column.

With diminishing tidal scope and current speeds, blending at the bed was reduced, and stratification was increasingly enhanced. At neap, the estuary was partially stratified ( Oliveira et al. , 2006 ) , even with low fresh water inputs ( e.g. & lt ; 10 m3 s-1 ) . The partially stratified estuary tended to be better-mixed towards the caput ( e.g. Figure 3 ) , as antecedently observed by Ferreira et Al. ( 2003 ) , likely in relation to H2O deepness decreasing and therefore increasing clash at the bed. By contrast, the stratification may be enhanced locally at deep subdivisions of the channel ( e.g. km 10-15 ) . Tidal striving induced stronger stratification during the wane and the growing of a uniform ( assorted ) near bed bed during the inundation. Therefore, lower limit and maximal near-bed stratification occurred about high and low H2O, severally, as observed at the lower Tamar estuary ( Uncles, 2002 ) and Hudson estuary ( Nepf and Geyer, 1996 ) . The combined effects of tidal striving plus the ( jumping ) resistance and support of the barotropic and baroclinic gradients on the wane and inundation tides produced vigorous gravitative circulation, characterised by a two-layer residuary vertical profile ( Figure 5b ) . Therefore, there is an alternating constitution and dislocation of the two-layers residuary flow with spring and neap tides, as described for illustration at the river-dominated Columbia estuary ( Chawla et al. , 2008 ) . Baroclinic procedures moving at the partially-stratified estuary may besides change the perpendicular speed profiles of tidal currents ( Jay and Musiak, 1996 ; Friedrichs and Wright, 1997 ) . Barotropic force per unit area gradient is changeless with deepness and baroclinic force per unit area gradient increases towards the bed. These forces act in the same ( upstream ) way during the inundation, but speeds are reduced near the bed and near the surface, in relation to bed clash and fresh water residuary currents, severally. During the wane, the baroclinic and barotropic force per unit area gradients act in opposite way and near-surface currents are favoured by the induced residuary downstream circulation of fresh water. Hence, ebb speeds increased linearly toward the surface, and inundation speeds were by and large higher at mid-depth ( Figure 10a ) .

With increasing river flow, inundation currents ( and therefore blending ) were reduced, fresh H2O inputs reinforced ebb currents ( particularly near the surface ) , and stratification was enhanced. Highly graded conditions were observed, with a homogenous bed of salty H2O at the underside, for river discharge of about 400 m3 s-1. Intratidal fluctuations in stratification are reflected in the deepness and thickness of the pynocline ( Cudaback and Jay, 2000 ) . Our informations indicated that the pynocline grew thicker and turbulent commixture was more effectual on the wane, as observed at other highly-stratified estuaries ( e.g. Schettini et al. , 2006 ) , because of important current shear. With increasing turbulent commixture, density-driven landward flow developed near the bed, at mid-ebb. The form of the perpendicular speed profiles reflected the influence of gravitative circulation ( someway similar to the 1s of partly-stratified conditions ) , with speed profiles increasing linearly on the wane, and maximal speed at mid H2O deepness on the inundation ( Figure 10b ) . However, currents in the surface and bottom bed were oriented seaward and landward, severally, during most of the tide, in relation with horizontal advection. The strong laterality of inundation currents near the bed is opposite to the observation of unidirectional ebb-directed residuary speed profiles at ISN ( Figures 8, 10b ) . These differences in the observations might be due to sidelong fluctuations in stratification between wane and inundation at the oral cavity, under these ( high ) river discharge conditions. During a flood event, a salt cuneus with tidal gesture developed at the lower estuary. Horizontal advection of sea and fresh H2O controlled the H2O circulation under these conditions.

5.2. Sediment kineticss

ETM

For low river discharge conditions, turbidness measurings along the estuary suggest that the axial SSC varies significantly with the tidal scope, with high ( low ) values at spring ( neap ) tide ( Machado et al. , 2007 ) . The correspondence between salt and turbidness perpendicular stratifications showed the influence of turbulent commixture and tidal striving upon concentrations. In add-on, sediment resuspension was associated to periods of peak near-bed currents. These observations indicate that during periods of low river flow, concentrations of SPM at the lower and in-between estuary are mostly controlled by cyclical local procedures ( such as resuspension, deposition, blending and advection ) driven by the fluctuation of neap and spring tidal currents ( Mantovanelli et al. , 2004 ) . At neap ( and low river flow ) , horizontal advection governs the SPM concentration. The gravitative circulation at the lower estuary Acts of the Apostless to concentrate the all right deposit at the ETM, through an upstream flux of suspended stuff at deepness ( Gibbs et al. , 1989 ) . In add-on, important intratidal dissymmetries in stratification strength at neap tide evidenced the importance of tidal pumping for SPM conveyance ( Uncles, 2002 ) . Typically, a portion of the suspended stuff deposited at slacks, and resuspension with increasing tidal currents was limited. As such the ETM was non really chiseled. Deposition of mulcts is likely the dominant procedure at neap, as described for many other estuaries ( e.g. Grabemann et al. , 1997 ) . The influence of resuspension and perpendicular commixture in SPM concentrations increases together with the tidal scope and current speeds. The inundation currents are associated to the extremum in SSC, due to the resuspension of mulcts deposited during the comparatively long predating slack wane. At Odeleite, for illustration, the near-bed turbidness was higher on the inundation ( Figure 4, 12-14h ) than on the wane ( Figure 4, 6-8h ) , despite similar current magnitude during both tidal stages. For high river discharge conditions, important tidal dissymmetry in the pynocline deepness and thickness was observed ( Figure 9 ) . Scully and Friedrichs ( 2003 ) showed that a stronger and lower pynocline on the wane may move as a barrier that limits sediment suspension. This procedure may bring forth landward pumping of suspended deposit, even when the residuary flow is advected downstream ( Scully and Friedrichs, 2007 ) ( Figure 5c ) . For Ferreira et Al. ( 2003 ) , the ETM maintains its place ( near Alamo ) for river discharge up to 250 M3s s-1 at least. This suggests that the estuarine response to higher river discharge is buffered by the increasing stratification, as observed in the Gironde estuary ( Allen and Castaing, 1973 ) . By contrast, Grabemann et Al ( 1997 ) showed that the ETM in the Tamar and Weser estuaries exhibited big spacial fluctuations, in relation to fluctuations in the river escape. Our observations indicated that for a river discharge of 400 m3 s-1, the ETM was located at the lower estuary. In add-on, our informations suggested that the ETM was expelled out of the estuary during the flood event. In understanding, Cravo et Al. ( 2006 ) observed the formation of turbid and fresh H2O plumes on the inner shelf, during such periods of high river flows. For these utmost river flows, the turbidness along the estuary is unvarying and an order of magnitude higher than for low river flows. The SPM concentration within the estuary is so driven by the inputs of SPM burden from the drainage basin.

Sand conveyance

Bedload sand conveyance at estuaries is by and large controlled by the local laterality of wane or inundation currents ( Dronkers, 1986 ) . For low river flows, maximal near-bed currents at the lower Guadiana estuary ( about 1 m s-1 ) are by and large ebb-directed, as observed in the present survey. Gravitational circulation may heighten the inundation currents near -the bed ( e.g. Figure 8a ) , but speeds seems to be excessively weak to bring on important sand conveyance upstream. Ebb-dominance of near-bed current is enhanced during episodic inundations, in relation to freshwater advection in the narrow channel. For illustration, on the 10-11 February 2001, seaward currents with high speeds ( up to & gt ; 1.4 m s-1 ) during most of the tide indicated monolithic conveyance of sand downstream during this event ( Figure 9 ) . In understanding, long-run geomorphologic observations ( decennaries ) show that sand Bankss located off the oral cavity are episodically reworked ( and even disrupted ) during inundation events ( Gonzales et al. , 2001 ; Dias et al. , 2004 ) .

The form of offshore current predomination at the lower estuary suggests that sand is exported to the nearshore over the long-run ( old ages, decennary and more ) . In understanding, there is no grounds of sand infilling at the lower estuarine channel, such as record of care dredging. Furthermore, stone outcrops have been identified in the deep outer channel, on the footing of side-scan echo sounder imagination ( Dias et al. , 2001 ) . Asymmetric 2D and 3D big dunes ( i.e. 10-100 m in wavelength and 0.75-5 m in tallness ; Ashley, 1990 ) observed at the lower estuary during drawn-out periods of low river overflow, indicated besides net sand conveyance seaward ( Lobo et al. , 2004 ; Morales et al. , 2006 ) . Export of sand over the long-run has been antecedently described at the Douro estuary ( Portela, 2008 ) , located besides in a semi-arid environment. The kineticss of rock-bound estuaries have been nevertheless antecedently described about entirely in high latitude parts, e.g. in British Columbia ( Milliman, 1980 ) , Oregon ( Boggs and Jones, 1976 ) and New England ( FitzGerald et al. , 2002 ) . There, they coincide with major rivers which seasonal river discharge variableness is driven by glaciation-deglaciation procedures instead than the rain government. Similarly to the state of affairs at the Guadiana estuary, seaward-flowing currents rule throughout the estuaries, in relation with the narrow and confined channel morphology, during periods of important fresh water discharge ( spring spates ) . This mechanism supplies farinaceous deposit to the system from the upstream river and governs net seaward deposit conveyance within the estuary ( Fenster and FitzGerald, 1996 ) . Sand is exported to the nearshore at a annually to centennial graduated table ( FitzGerald et al. , 2000 ; Fenster et al. , 2001 ; Brother et al. , 2008 ) . This form of seaward deposit conveyance within rock-bound estuaries does non suit into the conceptual theoretical accounts of wave- and tide-dominated systems ( FitzGerald et al. , 2000 ; Fenster et al. , 2001 ; Brothers et al. , 2008 ) . The latter theoretical accounts consider estuaries as deposit traps ( by contrast to river deltas ) with landward deposit conveyance from the Marine environment into the estuary ( Dalrymple et al. , 1992 ; Boyd et al. , 1992 ) . Bedrock-controls in a context of high river discharge variableness are non considered by these theoretical accounts. Toward the development of comprehensive theoretical accounts of estuaries, farther research is needed to find the importance of high river discharge events in bedrock-controlled estuaries and to measure the significance of these events in the entire estuarial deposit budget. In peculiar, old surveies at rock-bound estuaries have proposed that a river discharge threshold may command the net seaward bedload conveyance ( Fenster et al. , 2001 ; Brothers et al. , 2008 ) . With the closing of the Alqueva dike, the regulated flow at the Guadiana estuary may seldom ( or ne’er ) exceed this threshold, with deductions for the structural development of the system, including the environing coastal country.

Recognitions

The writers thank the Lusitanian Water Resources Institute ( INAG ) for the river discharge informations. The trying studies were completed in the frame of the research undertakings EMERGE and SIRIA. E. Garel is funded by a Post-Doctoral FCT research grant ( SFRH / BPD / 34475 / 2006 ) . We are besides thankful to an anon. referee who has contributed significantly to better the quality of the original manuscript.

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