Continental Shelf Research 41 (2012) 61–76

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Research papers

A detailed, event-by-event analysis of suspended sediment concentrationin the swash zone

Ivan Caceres a,n, Jose M. Alsina b

a Laboratori d’Enginyeria Marıtima, Universitat Polit�ecnica de Catalunya, 08034 Barcelona, Spainb Department of Idraulica, Strade, Ambiente e Chimica (ISAC), Universit �a Politecnica delle Marche, 60131 Ancona, Italy

a r t i c l e i n f o

Article history:

Received 7 October 2011

Received in revised form

14 March 2012

Accepted 9 April 2012Available online 16 April 2012

Keywords:

Swash

Sediment transport

Suspended sediment concentration

Large-scale experiments

Wave-swash interactions

43/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.csr.2012.04.004

esponding author. Tel.: þ34 934010936.

ail addresses: i.caceres@upc.edu (I. Caceres),

univpm.it (J.M. Alsina).

a b s t r a c t

This work presents a detailed analysis of suspended sediment concentration measured in the inner surf

and swash zone under large-scale wave flume conditions. Controlled hydrodynamic conditions tested

in the laboratory corresponded to highly energetic (erosive) conditions. Hydrodynamic, bottom

evolution and suspended sediment concentration (SSC) data were acquired at several locations within

the inner surf and the swash zone. A limited number of hydrodynamic type of events have been

identified to induce important suspended sediment concentrations. These hydrodynamic conditions

include incident broken wave, wave capture, weak and strong wave backwash interactions or pure

backwash events. The acquired data showed the variability of the suspended sediment concentration

pattern depending on the cross-shore location from which measurements were taken. This suspension

mechanism also varied depending on the time series characteristic. There was a clear predominance of

incident wave/bore as a forcing term in the surf zone, whilst the SSC measured in the swash zone

showed that wave-swash interaction controls the most significant events. Finally, the importance of a

wave-by-wave analysis is highlighted in this study. It was observed that despite the fact that most of

the time series induce profile accretion in the mid and upper swash area, only one or two swash events

are able to erode extensive parts of the beach and change the final output of the entire time record from

accretion to erosion.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The swash zone is that part of a beach alternately covered andexposed by up-rush and backwash, and includes areas that arenearly always under submerged conditions (95% of the total timeseries) and areas that only receive one or two swash eventswithin a wave time series (around 1% of the total time series).Various classifications of the swash zone’s sub-areas can be foundin the literature (Aagard and Hughes, 2006; Hughes and Moseley,2007). Aagard and Hughes (2006) identify three areas in theswash zone (lower, mid and upper swash) depending on theirsubmergence ratio. If the equipment is submerged between 75and 95% of the measuring time, it said to be located in the lowerswash, if the immersion period is of between 40 and 75% of thetotal time the instrument is located in the mid swash; andimmersion periods of between 1 and 40% of the total correspondto a location in the upper swash. Hughes and Moseley (2007), onthe other hand, divided the swash into outer and inner swash

ll rights reserved.

sub-regions according to the swash dynamics in each sub-area.The outer swash is the region where wave-swash interactionoccurs and, thus, the number of water depth maxima (waves) isgreater than the number of swash events, where a swash event isthe submerged period at a certain position between two dry bedperiods. The inner swash is defined as the region where wave-swash interactions are entirely absent and, thus, there are onlypure swash events, either run-up or run-down, without anyinteractions with previous or further swash events.

In both classifications, the most dynamic area is locatedaround and behind the shoreline (low and mid swash or outerswash depending on the classification used) where wave-swashinteractions occur.

Wave-swash interactions (Hughes and Moseley, 2007) andswash–swash interactions (Brocchini and Baldock, 2008; Puleoet al., 2007) are different terms for the same phenomena, namely,the interaction between an incident wave (in the form of a brokenwave or run-up) and the swash event (run-up or backwash) ofpreceding waves. In this paper, the terms wave-swash (general)and wave-backwash (interactions with opposite directions, i.e.advancing waves and receding backwash) will be used.

In the inner surf and the swash zone, sediment transport showsdifferent behavior depending on the zonation. Butt et al. (2002a)

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–7662

reported bed erosion in the seaward half part of the swash butaccretion in the shoreward half part of the swash (gently slopingbeach, where d50¼0.25 mm in macrotidal conditions). Similarly,Alsina and Caceres (2011) have shown that there is a highererosive tendency in the inner surf than in the swash zone due tothe higher influence of long wave water level oscillations insediment suspension in the inner surf zone than in the swashzone where sediment suspension is more influenced by incidentbores. Hughes and Moseley (2007) had previously suggested thatdifferent forcing terms control the sediment transport at differentlocations. The degree of influence of the different suspendedsediment concentration forcing terms on the surf and swash zoneis yet to be resolved due to the large number of terms that interactand affect sediment transport in really fast changing conditions.There are several parameters (wave height, wave period, beachslope, sediment size, sediment fall velocity, tide presence, barpresence, etc.) that can alter sediment transport and their combi-nation gives rise to a large number of scenarios. Below is a listof different ‘‘forcing’’ phenomena considered by state-of-the-artliterature as sediment transport controlling terms in the surf andswash zone:

�

Wave breaking and bore collapsing. Puleo et al. (2000), Jacksonet al. (2004) and Aagard and Hughes (2006) stated that bore-related turbulence can be injected into the swash zone at borecollapse and can be advected by the swash front with theturbulent face in contact with the bed. This turbulence willdepend on the bore height and local depth (Flick and George,1990). � Backwash. Horn and Mason (1994) concluded that bed-load is

the dominant mode of transport during the backwash, whilstButt and Russell (1999) identified that long infragravity back-washes can exceed the threshold for sediment suspension, andincrease the offshore tendency for transport in the swash.

� Sediment advection. Jackson et al. (2004) and Alsina et al.

(2009) reported that advection is a major factor in landwardtransport by wave up-rush once there has been sedimententrainment by the bore collapse.

� Infiltration/ex-filtration effects are expected to have a con-

siderable influence on gravel beaches. However, its degree ofinfluence is not yet clear and depends on the groundwaterelevation, beach material permeability and the degree of soilsaturation (Turner and Nielsen, 1997; Elfrink and Baldock,2002; Baldock and Nielsen, 2010).

� Fluid velocity asymmetries. Roelvink and Stive (1989), and

Marino-Tapia et al. (2007) reported that this phenomenoncontrols onshore sediment transport outside the surf zone andit has been argued that differences between the uprush andbackwash phases (velocity asymmetries) are important tothe net sediment transport in the swash zone (Houser andBarret, 2010)

� Long waves and wave groups alter the sediment transport

patterns along the surf and swash zone. Baldock et al. (2011)presented data where the presence of wave groups promoteshigher erosion values than monochromatic waves or mono-chromatic waves perturbed with free long waves each waveconditions with the same energetic content.

� Acceleration asymmetry in the cross-shore flows has an ongoing

debate over its relevance when contributing to the sedimenttransport. Hanes and Huntley (1986) considered its effect for thenet sediment flux. When considered by Puleo et al. (2003) in anenergetic model for sediment transport there was an importantimproving of previous predictions. Whilst Baldock and Hughes(2006) and Pedrozo-Acuna et al. (2011) refuted the existence ofonshore-directed fluid accelerations in the swash zone reportinga predominant offshore fluid acceleration.

The run-up is characterized by decelerating flows, whereas theflow during run-down gradually accelerates until it reaches itsmaximum value in the final stage of the run-down. Sedimenttransport in the swash zone occurs by means of two differenttransport modes: bed-load and suspended load. Both modes arecontrolled by the physical processes described above, as well asby the various wave-swash interactions that occur within theswash zone. Using a set of measurements taken in the innersurf and low swash, Alsina and Caceres (2011) showed how highsediment concentration events presented low correlation withincident horizontal velocity and computed TKE values in generalconditions. Moreover, they showed that sediment suspensionevents of high concentration occur in a combination of long wavetrough and incident bores.

Kemp (1975) initially proposed that wave–swash interactionsmight be significant in determining the net direction of sedimenttransport and profile change. Wave–swash interactions are con-trolled by the slope of the beach face, the period of the arrivingshort waves, the presence of long waves or wave groups and thebore height. Different combinations of these phenomena willdetermine the existence of just pure wave motions involvingincident wave bores and significant backwash retreats, as mea-sured by Miles et al. (2006), or the presence of wave–swashinteractions that can have an important influence on sedimenttransport in the swash zone. Holland and Pulleo (2001) showedthat the presence or lack of swash collisions could describewhether foreshore accretes or erodes. On foreshore slopes whereswash excursion times are of a longer duration than the incidentwave period, steepening is expected to occur (Erikson et al.,2005). In contrast, on beaches where swash events are of ashorter duration than the incoming bores, erosion is expected tooccur and the foreshore will be flattened. Brocchini and Baldock(2008) presented the parameter Tˇ ¼ Ts=T to quantify the degree ofwave–swash interaction, where T is the incident wave period andTs is the natural swash period. Small values of Tˇ correspond to nointeraction and values of Tˇ greater than or equal to 1 is associatedto an important number of interactions occurring in the swashzone. Ts is function of bore height, speed at the mean water levelshoreline, the beach slope and gravitational acceleration (Baldockand Holmes, 1999). As will be seen in the following sections, thepresence of wave–swash interactions increases or even controlsthe amount of suspended sediment, which will boost the sus-pended sediment transport governed by the resulting current ateach interaction.

Three different wave–swash interactions are considered hereand presented in Fig. 1, following a review of previous studies(Erikson et al., 2005; Hughes and Moseley, 2007) and the analysisof data presented in Sections three and four.

1.

Wave capture. The second wave captures the previous oneduring both up-rush stages.

2.

Weak wave–backwash interactions, in which an incomingwave advances across the front of an existing swash lens asit recedes down the beach as backwash. The receding back-wash has a limited amount of energy and the incoming up-rush overrun results in an onshore flow.

3.

Strong wave–backwash interaction, similar to the previous onebut with a stronger backwash than the incoming up-rush,which results in a stationary bore that has been compared to ahydraulic jump by several authors (Puleo et al., 2000; Elfrinkand Baldock, 2002; Hughes and Moseley, 2007). The resultingflow is offshore directed.

In the presence of wave–swash interactions (wave capture,strong or weak wave–backwash interactions), the SSC peakscan occur during the up-rush phase (wave capture) or during

Table 1Equipment distribution along the surf and swash zone during the SCESE experi-

ments, the values in brackets and in gray indicate vertical elevation from the

bottom; more than one value means that different acquisition devices were

deployed at the same cross-shore location and at different vertical elevations.

Cross-shore position (distance from the bottom)

ADV �7.41 (0.2, 0.15), �5.6 (0.11, 0.11), �2.69 (0.04, 0.04)

and �0.41 (0.04, 0.04) m

OBS �7.41 (0.03, 0.07), �2.69 (0.03, 0.07), �0.41 (0.03, 0.07),

0.58 (0.03) and 1.92 (0.03) m

PPT �7.32, �5.53, �0.49, �0.41, �0.33 and �0.25 m

ADS �25.25, �11.17, �8.17, �2.76, �1.64, �0.54, 0.45,

1.82, 3.46, 4.3 and 5.32 m

Fig. 1. Wave-swash interactions: (a) wave capture, (b) weak wave–backwash

interaction and (c) strong wave–backwash interaction. The length of the arrows is

proportional to the energy of each flux.

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–76 63

the receding backwash phase (strong or weak wave–backwash inter-actions). The final direction of sediment flux will be the result ofthe balance between the energies of the incoming and recedingwater masses (usually seaward for strong wave–backwash interac-tions, and shoreward for wave capture and weak wave–backwashinteractions).

Reliable sediment transport measurements are difficult to obtainin the swash zone. Bed-load is usually measured using sedimenttraps, while suspended load is measured using pump samplers oroptical/acoustic backscatter sensors. The main problem of thesemeasurements lies in the use of sediment traps that tend to under-estimate bed-load sediment transport (Masselink et al., 2009), whilstsampler pumps or backscatter sensors measure average time andvolume, respectively. An accurate method for measuring total trans-port is by means of beach face surveys, which do not have enoughspace and time resolution to allow an accurate study of the transportmechanisms in the surf and swash zone. Other methods used tomeasure bed level changes in the alternatively emerged-submergedbeach-face include ultrasonic sensors or buried pore pressure sensorswith continuous time measurements but scarce spatial resolution.Therefore, sediment transport is biased by the lack of good accuratemeasurements in space and time that, as will be seen throughoutthis paper, lead to local conclusions that do not give a completepicture of sediment transport.

This work focuses on suspended sediment concentration (SSC)events, measured in the surf and swash zone in order to determinethe degree of influence of the various parameters that control thefinal erosion/accretion pattern. A data sets of large-scale experimentsis presented and analyzed. Wave height, velocity and suspendedsediment concentration data are evaluated in order to determine the

forcing mechanism that controls sediment suspension events. Theexperiments’ layout is presented in Section 2. Section 3 analyzes theprofile evolution, the SSC events terms in space and time, andthe role that they play within the erosion/accretion pattern.Section 4 is devoted to discussion while, finally, Section 5 sum-marizes the main conclusions of the analysis.

2. Experimental set-up and instrumentation

To conduct this research, the SCESE (Suspended ConcentrationEvents in Swash Experiments) data set acquired in the Canald’Investigacio i Experimentacio Marıtima (CIEM) flume of Barcelonawas used. This large research facility, which is located in theUniversitat Polit�ecnica de Catalunya, is 100 m long, 3 m wide and5 m deep. The experiment presented here had a beach profileconfiguration that from the wave paddle towards the shorelineconsisted of an initial 30 m long flat concrete section, followed by a1/13 slope, a flat 20 m long sandy bed and a final 1/15 slope beachconfiguration. The beach consisted of commercial well-sorted sandwith a medium sediment size (d50) of 0.25 mm, with a narrow grainsize distribution (d10¼0.154 and d90¼0.372 mm) and a measuredsettling velocity of 0.034 m/s.

The distribution of equipment is presented in Table 1 andFig. 2, where the x-coordinate origin is at the initial shoreline witha still water level of 2.5 m, negative towards the wave paddle(offshore) and positive towards the beach face (onshore). Thewave height at different points of the profile were measured bymeans of resistive wave gauges in the deeper part of the flume,and Pore Pressure Transducers (PPTs) and Acoustic DisplacementSensors (ADSs) in the surf and swash zone. The velocity field wasmapped by means of Acoustic Doppler Velocimeters (ADVs), whileseveral Optical Backscatter Sensors (OBSs) were used to recoverthe suspended sediment concentration. Most of the equipmentwas deployed close to the shoreline in order to obtain suspendedsediment concentration, velocity, bore heights and swash thick-ness in the inner surf and swash zone.

Considerable efforts were made to co-locate the ADVs, OBSs andADSs so that a good spatial resolution could be obtained during theexperiments to study the phenomena that control SSC events. Mostof the ADV locations coincided with the OBSs on the cross-shorelocations along the inner surf and swash zones (Table 1 and Fig. 2).However, they did not coincide in the long shore direction (perpen-dicular to the wave flume walls). The ADVs were usually locatedclose to one of the flume walls, whilst the OBSs were located in thesame cross-shore location and vertical elevation with respect to thebed level but close to the opposite wall (with a distance betweenboth measuring equipment of 2.3 m). The OBS and ADV verticalelevations with respect to the sandy bed were checked at thebeginning of each test to be at 3 and 7 cm from the bottom.

ADV velocity data were acquired with Nortek’s Vectrinovelocimeter, the data were processed and spike noise eliminated

−25 −20 −15 −10 −5 0 5

−1

0

1

Dep

th (m

)

Distance (m)

−0.5

0.5

−1.5

Fig. 2. CIEM configuration for the SCESE experiments with the initial profile (solid

black line) and the profile after 8 erosive tests (dashed red line). The marks show the

position for ADVs (solid black pentagram), OBSs (empty red circles), PPT (solid black

squares) and ADSs (empty blue squares). (For interpretation of the references to color

in this figure legend, the reader is referred to the web version of this article.)

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–7664

using the method developed by Goring and Nikora (2002). Lowquality data, where signal to noise ratio or signal amplitude wasbelow 75 for signal amplitude and 15 dB for signal to noise ratio,were discarded and cubic interpolation was performed. The qualityof velocity gathered data was very high and just a low proportion, anaverage value o1%, of the information acquired was dismissed. TheOBS deployed are OBS-3 from D & A Instrument Company, each OBSdeployed was calibrated using CIEM sand samples and the glyceroltechnique developed by Butt et al. (2002b). The worst regressioncoefficient (R2) of the linear regression done during this calibrationprocess was 0.98 with 13 different points. Despite the air bubbleseffect on optical backscatter sensors ongoing discussion (Puleo et al.,2006), air bubbles have not been considered in the present data setand just the spikes clearly produced by non physical effects havebeen eliminated (sudden peaks in the intensity with short durationusually related to air bubbles). It must be considered that during thepresented experiments fresh water was used, as previously reportedthe flume sediment has a narrow grain size distribution with lowamount of fine particles and no mud, and during the experiments nofoam was appreciable in the swash zone. These three factors reducethe amount of false positives in the OBS signals. On the other hand aslight scour, not able to distort the important and repeated SSCmeasured events, was observed around some of the OBS deployedduring the performed experiments.

Following the example of Turner et al. (2008), ADSs were usedhere to recover information from the swash events (height of theswash lens, bottom evolution and run-up velocities). According tothe manufacturer’s specifications, the accuracy of the ADS (Micro-sonic micþ130) is 0.18 mm. However, as stated by Turner et al.(2008), there is a significant scattering of the reflected energy whenwork is conducted on sandy bottoms. The calibration performed inthe laboratory presented an accuracy of 72 mm, based on theworking distance (around 1 m) and the electronic noise inherent inthe data-logging system.

The instruments location within the inner surf and swash zonewas estimated following the submergence ratio criteria proposedby Aagard and Hughes (2006). The submergence time relative tothe duration of each time series is obtained independently fromADS and ADV measurements.

The ADS is processed to get the emerged times when there isno signal change for more than 2 s as explained above. The ADV

signal provides an estimation of submergence due to the signalquality drop off in emergence periods (signal to noise ratio lowerthan 15 and signal amplitude lower than 75). A comparison forco-located ADV–ADS pairs showed discrepancies in the submer-gence ratios lower than 4%, thus ensuring a proper distribution ofthe probes throughout the surf and swash locations in the profile.

The wave time series tested in the laboratory corresponded toerosive conditions, based on the criteria used by Dean (1973),Dalrymple (1992), and Kraus et al. (1991). The irregular time seriesfollowed a Jonswap spectrum (gamma¼3.3) distribution withHs¼0.48 m and Tp¼4.4 s. These wave conditions were close to theSANDS wave conditions (Sanchez-Arcilla et al., 2011), same spec-trum characteristics with Hs¼0.53 m and Tp¼4.14 s, and wereselected so close in order to assure the same good similitude inthe morphological evolution previously obtained between the CIEMand prototype, as reported in the SANDS experiments (Sanchez-Arcilla et al., 2011). The same erosive time series, containing 500waves, was consecutively repeated 8 times while acquiring themorphological bed change for 6 different bed profiles. The bottomprofile information was acquired with a mechanical profiler, a wheelon a pivoting arm mounted on a platform that moves at constantvelocity along the flume while recording the X and Z position. Themechanical profiler accuracy is 1 cm. The different bottom profileswhere acquired at the beginning of the experiments (initial profile)and at the end of the 1st, 2nd, 4th, 6th and 8th test.

3. Results

Dissipative conditions dominated the experiment, with anIribarren number (Iribarren and Nogales, 1949) of 0.5 during thefirst eight time series of the data set. These values are character-istic of beaches where standing low frequency waves dominateshoreline motions, whilst short-wave energy is dissipated in asaturated surf zone. Wave breaking was visually identified as aplunging type of breaker most of the time, and the incident boreinfluence on the shoreline was also shown to be considerable inaddition to low-frequency components.

The power spectrum analysis for the data gathered on waveheight was performed and presented in Fig. 3. At locations closeto the wave paddle, the main energy is clearly at the incidentfrequency, whilst there is also some energy at low frequencies(0.02–0.06 Hz) corresponding to the incident bound and outgoinglong wave. After the bar, x¼�7.3 m, there is an incidentfrequency energy dissipation due to wave breaking, whilst anincrease in energy occurs at around 0.02–0.08 Hz.

The profile evolution during the measured tests showed aconsistent erosive pattern highly repeatable from one test to another(Alsina and Caceres, 2011). Tests 4 and 8, of the SCESE experiments,have been the most used tests to compare the different parametersstudied in the forthcoming sections. The selection of these two testsanswered to the aim to compare time series and events alongdifferent bottom evolution stages at the middle (once the highsuspended sediment transport induced by the artificial 1/15 initialslope is not interfering with the OBS event measurements) and endof the performed experiments. The suspended sediment concentra-tion in both tests has a clearer event-like pattern allowing the studyof the phenomena that control the SSC events while the shorelineretreat in between both tests is 0.92 m.

3.1. Beach dynamics and surf/swash zonation

During the first time series, waves broke over the constant-slope bed, eroding the beach face and generating an initial bar atthe cross-shore location where the significant wave height startsdecaying. There is an initial fast evolution of the seabed with high

−15 −10 −5 0 5

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

0.8

Dep

th (m

)

Initial profile500 waves1000 waves2000 waves3000 waves4000 waves

−0.8

Distance (m)

Fig. 4. Bar, shoreline and berm evolution during the energetic wave conditions (Hs 0.48 m and Tp 4.4 s) reproduced in the SCESE experiments. The bottom profile depicted

were acquired at the beginning of the experiment (black line) and after the 1st (blue line), 2nd (red line), 4th (green line), 6th (cyan line) and 8th (violet line) test

corresponding to the number of waves done within the legend figure. (For interpretation of the references to color in this figure legend, the reader is referred to the web

version of this article.)

Fig. 3. Computed water surface power spectral density for test 8 of the SCESE experiments and at cross-shore locations x¼�65 m (solid black line), �23 m (solid gray

line) and �7.3 m (dashed dotted black line).

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–76 65

suspended sediment concentrations and considerable sedimentfluxes. This induces a fast growth of the bar, whose crest,promotes wave breaking over it allowing the smaller waves toreach the swash zone unaltered. The bar crest elevation hencecontrols the height of the bores arriving to the shoreline andconsequently the bore-induced sediment suspension. This filteringeffect together with the more ‘‘stable’’ beach profile reduces thesuspended sediment concentrations after the initial tests (1–4),thus showing apparent suspension patterns that are repeatable insuccessive time series (Alsina and Caceres, 2011).

Fig. 2 presents the major features of the SCESE experiments: amain bar around 12 m from the shoreline, a second bar shorewardof the first one, considerable shoreline erosion, and a berm forma-tion at the end of the active beach face. Fig. 4 shows the shorelineand bottom evolution during the erosive tests performed for theSCESE experiments. The final bottom profile has been comparedwith the morphological features previously obtained within the

SANDS project. The bottom evolution comparison of the twoexperiments is quite good and consistent with the similarity of thereproduced waves. The shoreline evolved from the initial position atx¼0 m with the same 1/15 initial slope, towards x¼1.32 m inSANDS and 1.02 m in SCESE at the end of the first eighth comparabletests. The bar peak ended at �12.6 m in SANDS and �12 m inSCESE, whilst the height of the bar measured at this position fromthe initial profile was 0.26 and 0.33 m, respectively. The maindifferences between the two experiments were the result of thelack of sediment compaction in the beach face prior to the initialround of SANDS experiments and, as stated above, due to the fewdifferences in the sea states reproduced in the two experiments. Thiscomparison and the previous SANDS results allow us to use theSCESE data set as a mobile bed experiment free of scale errors.

During the presented experiments the incident energy dissipa-tion increases with the formation of the bar, which suggest a barmorphological-control on hydrodynamics. The shoreline retreats

Table 2Swash location following Aagard and Hughes (2006) submergence ratio distribution.

Test X location Submergence

ratio (%)

Swash location

4 x¼�0.41 m 90 Lower swash

x¼0.45 m 89 Lower swash

x¼1.82 m 55 Mid swash

x¼3.46 m 19 Upper swash

x¼4.3 m 5 Upper swash

x¼5.32 m 3 Upper swash

8 x¼�0.41 m 96 Inner surf zone

x¼0.45 m 96 Inner surf zone

x¼1.82 m 73 Mid swash

x¼3.46 m 33 Upper swash

x¼4.3 m 10 Upper swash

x¼5.32 m 4 Upper swash

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–7666

from x¼0.11 m to x¼1.03 m between tests 41 and 81 of SCESEexperiments. Simultaneously, the bar moved from x¼�10 m tox¼�12 m and its height from 0.18 m to 0.33 m, measured on theoriginal 1/15 slope. The bar growth and offshore displacementproduces an increase in the energy dissipation induced by wavebreaking. There is a clear filter effect of the bar on incident waveheights, as seen in the spectral analysis in Fig. 3, whereby only thesmaller waves are allowed to go undistorted through the bar.

Finally, a berm formation was observed during the SCESE andSANDS experiments at around x¼7 m cross-shore location. Thenumber of events that exceeded the x¼5.32 m location during theSCESE experiments is around 17 and 31 for tests 4 and 8, respec-tively. The berm formed during the SCESE experiments had amaximum height of 3–4 cm around 7 m from the shoreline. Thesediment lost from the shoreline active profile (from x¼0 untilx¼5.32 m) was 1.2 m3, whilst the berm gain during these tests was0.15 m3. These figures allow us to assume that the sedimenttransport beyond our last measuring position (x¼5.32 m) is negli-gible and will not significantly affect the computations of suspendedload and total transport done within forthcoming section.

An analysis of the deployed equipment within the inner surfzone or swash sub-areas is presented next, because the place andtime at which wave–swash interactions occur play a major role inthe final sediment transport pattern. Following the submergenceratio criteria (Aagard and Hughes, 2006) most of the ADVs wereplaced in the inner surf and only the last one (x¼�0.41 m) was inthe lower swash during the first tests, whilst the OBSs werelocated in the surf, low and mid swash (Table 2).

In accordance with Hughes and Moseley (2007), the outer swashis where the wave–swash interactions occur, whilst the inner swashis defined by the lack of these interactions and only pure swashevents take place (the number of wave depth maxima is equal to thenumber of swash events). The cross-shore location that divides theinner and outer swash was done by looking at the time signals of thefour probes in the swash zone for the different tests and experi-ments. Visual inspections revealed that this division point (outer/inner swash) is between x¼1.8 m and 3.5 m in the SCESE experi-ments. The limit for the wave swash interaction area is identifiedusing two probes, one probe located at 1.8 m in the outer swashshows wave–swash interactions and the second probe located at3.5 m is in the inner swash and is absent of interactions. In thepresent experiments the maximum run-up position was observed ataround x¼8 m (visual inspection) while the average run-down waslocated at around x¼�1.5 m during tests 4–8.

3.2. Analysis of suspended sediment concentration events

As stated in the Introduction, the mechanism that inducessuspended sediment concentration may vary considerably from

the inner surf zone to the various swash zone sub-areas. Thesedifferences will be studied in this section, and the analysis of themajor sediment suspension events in the inner surf and swashzone in terms of suspended sediment concentration and eventduration will be presented. These events will be then related todifferent swash hydrodynamic conditions.

The ADSs deployed closer to the shoreline (x¼�0.53, 0.45, 1.82,3.46, 4.3, 5.3 m) will provide the run-up characteristics (thickness,velocity and maximum run-up distance) of each individual event.The combination of OBSs and ADVs deployed at x¼�0.41 m wasused to study the velocities and sediment fluxes at one point closethe initial shoreline. This made possible to link the SSC eventsstudied and their hydrodynamic conditions with seaward/shore-ward suspended sediment transport at the initial shoreline position.

The measured time series of suspended sediment concentrationshows an event-like pattern, i.e. relevant suspended concentrationappears only at selected timing and not every incident wave inducessediment suspension as will be shown next. This behavior (event-like suspended sediment concentration) has been reported both infield conditions (Kularatne and Pattiaratchi, 2008; Smyth and Hay,2003) and laboratory (Alsina and Caceres, 2011). The event-like SSCmeasured pattern are repeatable at different time series withidentical timing and concentration values. Henceforth, a sedimentsuspension event is defined as a peak of important sedimentconcentration (larger values than average and always over thethreshold of 40 g/l) than can be characterized by its repeatabilityat different tests and by the repeatability of the forcing hydrody-namics. The different hydrodynamic conditions that were observedand related to major SSC events are as follows:

1.

Incident wave/bore. Fig. 5a presents the arrival of an incomingbore in the upper panel (t¼696 s) when the wave height atx¼�0.54 and 0.45 m (black and blue line respectively) measurethe arriving bore which induces a peak of suspended sediment(in the mid panel) at x¼�0.41 and 0.58 consecutively as thebore goes through. The incident wave/bore close to the shorelineinduces a rapid velocity increase with positive velocities (shore-ward) and values that easily go over 1 m/s (lower panel). Thesewave/bore arrivals are more effective in stirring the sedimentwhen they occur in the trough of a long wave (Alsina andCaceres, 2011). These events are easily identified from the watersurface signal since the same incident bore clearly appears atdifferent x locations without interactions with previous waves orswash events. Sediment advection is often associated to this typeof event when the suspended sand is trapped and carried withinthe turbulent bore (Alsina et al., 2009). In Fig. 5a, time¼696 and697 s, it is observed that the same bore produces sedimentsuspension at two successive cross-shore locations (x¼�0.41and 0.58 m) with a time lag between both peaks similar to thetime needed by the bore to travel that spacing. The concentrationmeasured in the more shoreward location (x¼0.58 m andt¼697 s) is assumed to be partly due to advection from the firstpeak but also due to the forcing by the incident bore. Separatingone influence from the other (advection from bore-forcing) isdifficult with this kind of measurements. Previous measurementsin laboratory studies have reported that up to 25% of pre-suspended sediment reaches one half of the run-up distance(Alsina et al., 2009). Another important peak is observed ataround t¼705 s, associated to another type of event (strongwave backwash interaction) that will be later explained.

2.

Wave capture. Fig. 5b presents a group of waves in which asecond and third wave catches-up with the previous ones andcreates a high run-up event able to ride the entire swash zone.The overrunning waves (at x¼�0.54 m with black line, thefirst bore can be seen at t¼670 s, the second bore arrives att¼673 and the third one at t¼675 s in the upper panel)

690 695 700 705−0.1

0

0.1

0.2

0.3

0.4H

(m)

690 695 700 7050

50

100

150

200

SS

C (g

/l)

690 695 700 705−2

−1.5−1

−0.50

0.51

1.5

U (m

/s)

670 675 680 685−0.1

0

0.1

0.2

0.3

0.4

670 675 680 6850

50

100

150

200

670 675 680 685−2

−1.5−1

−0.50

0.51

1.5

−0.54m 0.45m 1.82m 3.46m

−0.41m 0.58m

−0.41m

Time (s) Time (s)

Fig. 5. Time variation in incident bore elevation (upper panel), suspended sediment concentration (mid panel) and horizontal short wave velocity (lower panel); (a) shows

the wave bore collapsing situation, whilst (b) shows the wave capture case. The legend shows the cross-shore location at which the equipment was deployed.

(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–76 67

usually induce a set of velocity oscillations (the velocities canrange, in the group of waves, from positive velocities to higherpositive values or from a state of no movement to positivevelocities and back). A suspended sediment event is linked tothis oscillatory movement; in the mid panel a peak of sus-pended sediment at the beginning of the waves train (t¼670 s)linked to the arrival of the wave/bore in the through of a longwave (Incident wave/bore conditions), but the most importantevent occurs at the peak of the third wave (t¼675 s). Advec-tion may be associated to this type of event in a similar waythan explained above for the incident wave/bore case, as canbe observed in Fig. 5b. After a set of overrunning waves, thereis normally considerable backwash with high and sustainednegative velocities, induced by the pilling-up that occursduring the successive arrival of bores.

3.

Strong wave–backwash interaction (Fig. 6a). After a major run-up event there is an momentum exchange between the recedingbackwash and the incoming up-rush. The velocity during thisinteraction comes from negative values (seaward directed)pushed by the backwash and resisted by the arriving bore, whichdecreases in intensity or even reaches values close to 0 m/s fora while until the bore energy is depleted and the remainingbackwash energy sweeps away the suspended sediment. Despitethe energy of the arriving bore, the resultant flow velocity isoffshore directed. As shown in Fig. 6a, the bore does notpropagate shoreward and the wave height attenuation is con-siderably higher than it should be without the receding backwash(water surface elevation peaks at x¼�0.54 m around t¼1575 s,

flattens at x¼0.45 m, but is negligible at x¼1.82 m and does notappear at x¼3.46 m).

4.

Weak wave–backwash interaction (Fig. 6b). This is a variation ofthe case presented above in which the incoming wave is moreenergetic than the receding backwash. This case usually occurs atthe very end of the backwash process, and this interaction resultsin positive velocities (onshore flow) during the SSC event. Asshown in Fig. 6b, the arriving wave at x¼�0.54 m produces ahigh peak at the same location (t¼1487 s) and positive velocities(shoreward), induced by the energy of the incoming wave beinghigher than the receding backwash.

5.

Backwash. In this case, a suspended sediment event is con-trolled by the seaward current that results from the recedingswash on the beach face. Negative (seaward) sediment advec-tion is also associated with this type of event.

The analysis presented here was conducted by selecting the 15most significant suspended sediment events, in terms of sedimentconcentration magnitude, within each time series at each OBSlocation. One event is considered as the time span during whichthe sediment suspension measured by one OBS goes over a certainthreshold (usually 40 g/l). The recovered events were sorted con-sidering its maximum concentration and then compared to theevents at other OBS during the same time span. This methodologyaim is to characterize the different events producing sedimentsuspension. When an event is measured simultaneously at differ-ent instrument locations, it is just selected the location that recordsthe higher concentration peak (Figs. 5a and b).

Fig. 6. Time variation in incident bore elevation (upper panel), suspended sediment concentration (mid panel) and horizontal short wave velocity (lower panel); (a) shows

a strong wave backwash interaction, whilst and (b) shows the weak wave backwash interaction.

Table 3Event characterization during the SCESE experiments along the surf and swash zone.

Test x¼�2.69 m (%) x¼�0.41 m (%) x¼0.58 m (%) x¼1.92 m (%)

Inner surf zone Low swash Mid Swash Mid Swash

Others 0.0 0.0 0.0 0.0

4

Incident wave/bore 60.0 33.3 20.00 13.3

Wave capture 13.3 6.6 0.0 0.0

Strong wave–backwash 20.0 6.6 26.6 33.3

Weak wave–backwash 6.6 46.6 46.6 53.3

Backwash 0.0 6.6 6.60 0.0

Inner surf zone Inner surf zone Inner surf zone Mid Swash

8

Others 13.3 0.0 0.0 0.0

Incident wave/bore 73.3 60.0 33.3 26.6

Wave capture 0.0 20.0 0.0 0.0

Strong wave–backwash 0.0 0.0 13.3 46.6

Weak wave–backwash 13.3 20.0 33.3 26.6

Backwash 0.0 0.0 20.0 0.0%

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–7668

Table 3 presents the analysis performed on the 15 mostsignificant events in the time series at each OBS location. TheSSC events and their magnitude were higher at the beginning ofthe experiment with an artificial slope of 1/15 than in test 8,when the bar had already been formed and was filtering thebigger waves by inducing them to break on top of it. The tabledata show that incident wave/bore events control the SSC eventsin the surf zone (the percentage of events decreases while going

from the surf (60%) towards the swash (26–13%)), whilst the lowand mid swash zone is mainly controlled by the wave–swashinteractions (72 and 86% at x¼0.52 and 1.92 m respectively whenconsidering weak and strong wave–backwash interactions for test4 and 46 and 72% for test 8). From the tests measured, it seemsthat the weak wave–backwash interaction is the most frequentforcing term to induce high SSC events, whilst strong wave–backwash interactions generate the SSC events with bigger

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–76 69

concentrations. However, advection which is considered in thebox of others (Table 3), is rarely measured as a forcing term due tothe difficulties to differentiate the role of advection from incidentbore inducing suspended sediment concentration, and the factthat just the first (in time) and bigger (in magnitude) events willbe considered within the presented study.

As presented in the data acquired, backwash plays a minor rolein inducing major SSC events at the specified sensor elevation(although backwash can be highly significant in inducing bed-load sediment transport (Horn and Mason, 1994)).

Table 4 presents the average values of suspended sedimentconcentrations over the 15 most significant events in tests 4 and 8.The events with a bigger SSC occur in the swash zone, as reported inthe literature by Beach and Sternberg (1991), and Osborne andRooker (1999). The data in Table 4 show the presence of maximumSSC landward on the shoreline at each time step. These data supportthe data presented where maximum SSC events are linked to strong(mainly) and weak (secondly) wave–backwash interactions. Theseinteractions occur in the low and mid swash areas.

3.3. Bottom evolution during the most significant swash events

The deployment of several ADSs allows to compare the bottomevolution at different swash zone positions (x¼1.82, 3.46, 4.3 and5.3 m in Fig. 7 for test 4) during an entire time series. In the fourpanels of Fig. 7, the blue circles show the time measurements atwhich bottom measurements remain constant for more than 2 s(emerged periods without water over the pinpoint locations and,

Table 4Mean suspended sediment concentration measured along the swash and surf zone

during SCESE experiments.

Test 4 (g/ l) Test 8 (g/ l)

x¼1.92 m 147 154

x¼0.58 m 151 122

x¼�0.41 m 130 81

x¼�2.69 m 101 76

x¼�7.41 m 114 97

0 200 400 600 800 1

−0.01

0

0.01

h (m

)

AWG at 1.82 m

0 200 400 600 800 1

−0.01

0

0.01

h (m

)

AWG at 3.46 m

0 200 400 600 800 1

−0.01

0

0.01

h (m

)

AWG at 4.3 m

0 200 400 600 800 1

−0.01

0

0.01

h (m

)

AWG at 5.32 m

Ti

Fig. 7. Bottom evolution at cross-shore locations of 1.82, 3.46, 4.3 and 5.3 m are shown fr

test 4 of the SCESE experiments. Blue empty circles represent the times at which the AD

study the morphodynamic behavior. (For interpretation of the references to color in th

therefore, reliable measurements for still bed level positions). Thefirst panel corresponds to the ADS at x¼1.82 m in the mid swash,whilst the other panels show the measurements for the ADS inthe upper swash zone (Table 2). At locations x¼1.82, 3.46 and4.3 m (first, second and third panels in Fig. 7) there is clearerosion when comparing the beginning and end of the time series,whilst at x¼5.32 m (fourth panel) there is accretion at the end ofthe time series, which coincides with the berm formation mea-sured during this experiment. Despite the total erosion behaviorin the three upper panels, there is a clear difference between themeasurements at location x¼1.82 m and the measurements atx¼3.46 and 4.3 m. In the location closer to the shoreline, thereare various accretive/erosive cycles along the time series with afinal erosion result. Locations x¼3.46 and 4.3 m present a clearconstant accretion pattern interrupted by an almost unique eventthat controls the final erosional behavior of the wave time seriesreproduced. Similar behavior to that reported here can be seen fortests 5, 6, 7 and 8, in which there is a general accretion patternwith a reduced number of erosional events. After test 4 and whilewaves kept eroding the shoreline, the alternating erosion/accre-tion pattern seen at x¼1.82 m became less clear due to the majorreduction of 2 s span periods without water.

The event occurring at around t¼1730 s is the event that inducesbigger erosional changes within the emerged beach face at every test.The hydrodynamics and sediment concentration details measuredfor this event are shown in Figs. 8 and 9. This event occurred duringthe most significant run-up of the total time series (the swash heightduring this event presents values of around 14 and 9 cm for x¼3.46and 4.3 m, respectively, which are double the values of the heightmeasured in other swash events at these positions). Fig. 8 presentsthe time variation through tests 4–8 at each event around time1730 s. In this figure, the repeatability of the measured data duringthe different tests ensures low randomness of the physical para-meters measured, namely, water surface elevation (upper panel), SSCmeasurements (middle panel) and velocity (lower panel). As dis-cussed by Alsina and Caceres (2011), in the SANDS data set theincident bores are of the same height and occur in the same timesequence, horizontal velocities show an identical pattern despite thehigh frequency oscillation, which could be attributed to turbulence

000 1200 1400 1600 1800 2000

000 1200 1400 1600 1800 2000

000 1200 1400 1600 1800 2000

000 1200 1400 1600 1800 2000me (s)

om the upper to the lower panel, respectively. Measurements were acquired during

S at each location is measuring the bottom; the pink line is a linear interpolation to

is figure legend, the reader is referred to the web version of this article.)

1710 1715 1720 1725 1730 1735 1740 1745−0.1

00.10.20.30.4

H (m

)

1710 1715 1720 1725 1730 1735 1740 17450

50

100

150

200

SS

C (g

/l)

1710 1715 1720 1725 1730 1735 1740 1745−1.5

−1−0.5

00.5

11.5

U (m

/s)

−0.54 m0.45 m1.82 m3.46 m

−0.41 m0.58 m1.92 m

−0.41 m

Time (s)

Fig. 8. Time variation for incident bore surface elevation (upper panel), suspended sediment concentration (mid panel) and cross-shore velocity (lower panel) for tests 3, 4,

5, 6, 7 and 8. The legend of each panel shows the cross-shore location at which the equipment was deployed.

1710 1715 1720 1725 1730 1735 1740

0

0.2

0.4

H (m

)

1710 1715 1720 1725 1730 1735 17400

50

100

150

200

SS

C (g

/l)

1710 1715 1720 1725 1730 1735 1740

−1

0

1

Vel

ocity

(m/s

)

1710 1715 1720 1725 1730 1735 1740−100

−50

0

50

100

Time (s)

flux

(Kg/

m2s

)

−0.54 m 0.45 m 1.82 m 3.46 m 4.3 m 5.32 m

−0.41 m 0.58 m 1.92 m

−0.41 m

Cross−shore U −0.41 mVertical W −0.41 m

Fig. 9. Time variation in incident bore elevation (a), suspended sediment concentration (b), cross-shore and vertical velocities (c) and the suspended sediment flux (d) of

the most erosive event measured during test 4. The legend of each panel shows the cross-shore location at which the equipment was deployed.

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–7670

Table 5Erosion/deposition pattern in m at the emerged beach face locations, during

the time span of the most significant event measured (t¼1716 s to t¼1740 s) for

tests 3, 4, 5, 6, 7 and 8.

Test 3 Test 4 Test 5 Test 6 Test 7 Test 8

x¼1.82 0.004 0.001 0.006 0.010 0.001 0.004

x¼3.45 �0.009 �0.011 0.001 �0.002 �0.007 �0.003

x¼4.3 �0.013 �0.009 �0.007 �0.006 �0.003 �0.004

x¼5.3 0.001 0.002 �0.001 �0.004 �0.003 �0.001

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–76 71

stochasticity or to the underlying smooth but constant evolution ofthe bottom profile. However, the most significant results are relatedto the fact that sediment suspension values are very similar inmagnitude and timing, especially for the bigger suspension events(i.e. the red line corresponding to OBS at x¼1.92 m). Similar patternsare found throughout the whole of the time series.

Fig. 9 presents the same event as Fig. 8, but only test 4 is plottedfor the purposes of clarity. Panel (a) shows the ADS measurementsat different cross-shore locations (in colors). Starting at time 1718 s,the different arriving waves overrun the previous ones by inducing awave capture event until the maximum water surface elevation(t¼1724 s) induces the maximum run-up event of the time series(6 cm swash height at x¼5.3 m, t¼1727 s). From that time, there isa decrease in the recorded swash height, whilst at positions closer tothe shoreline further incoming waves arrive and interact with thereceding backwash (1730 and 1735 s), until the event disappears ataround a time equal to 1740 s. Panel (b) shows the suspendedsediment concentration data presenting two important suspendedsediment concentration events measured at x¼1.92 m; one corre-sponds to the wave/bore arrival (t¼1724 s), and the second to thestrong wave–backwash interaction occurring at around t¼1731 s(both at OBS deployed at x¼1.92 m). Although of lesser relevance,other SSC events at the OBS located at x¼�0.41 and 0.58 m alsoappear in the image. The two peaks corresponding to the OBS atx¼0.58 m at t¼1719 s and t¼1737 s are the ones with the highestmagnitude, and correlate to an incident wave/bore in the trough of along wave and a weak wave–backwash interaction, respectively.Furthermore, both OBSs (x¼�0.41 and 0.58 m) present disperseconcentrations (ranging from between 20 and 60 g/l) linked to theevents occurring at x¼1.92 m described above (incident wave/boreand strong wave–backwash interaction). These sediment concentra-tions are due to advection and diffusion of the suspended sediment.In panel (c), the cross-shore and vertical velocities are presented atx¼�0.41 m. The main feature of the measured velocity during thistime span is the long backwash measured for more than 10 s. Thislong backwash has a peak current intensity of �1.3 m/s at the end ofthe backwash event (this velocity is measured at x¼�0.41 m wherethe last ADV was deployed). Despite the apparent correlation of thevertical velocity fluctuations measured at t¼1713 or 1719 s with SSCpeaks at this position, these fluctuations shown here do not appearto be directly correlated with the SSC events. Alsina and Caceres(2011) presented low correlation values between both terms (ver-tical velocity and SSC) in the SANDS data set and the measurementspresented show how some vertical oscillations appear around SSCpeaks but that the phenomena is not reciprocal and SSC peaks do notappear around vertical velocity oscillations. Finally, panel (d) pre-sents a rough estimation of the suspended sediment flux behavior bycomputing this value considering the velocity and SSC productmeasured at both ADV and OBS collocated equipment.

During the strong wave–backwash interactions measured(around 1731 s, in Fig. 9), the momentum exchange between theretrieving backwash and the incoming up-rush appears to slowdown the wave/bore or even halt it. At such at time, the borebecomes quasi-stationary, thus putting large amounts of sedimentin suspension. Once the bore collapses, there is an offshore directedflow which advects the suspended sediment (this advection can beseen at x¼0.58 m at t¼1734 s), resulting in a significant process oferosion. In the presented case, Fig. 9 shows the second arriving waveat x¼�0.54 and 0.45 m at around a time equal to 1730 s, but it isalso seen at position x¼1.82 m and t¼1732 s with a significantlylower amount of energy (which does not correlate with standardwave height dissipation induced by bore breaking in an interaction-free beach face run-up), and perfectly correlates with the strongwave–backwash energy loss and SSC pick-up expected from astationary hydraulic jump. It is important to highlight the behaviorof the ADV velocities measured during this event, as despite the

fact that the energy linked to the arriving wave/bore was taken ataround time 1730 s, the velocity measured was unable to reachpositive values (landward) due to the significant backwash inducedby the previous wave. As presented in the previous section, theenergy exchange of the receding backwash and the incoming wavehas been highlighted due to their effectiveness in producing highSSC events during the data analysis of swash zone OBSs. This samephysical velocity and wave height behavior can be observed in thestrong wave backwash interaction presented in Fig. 6a.

When considering the effect of this event at cross-shorelocations x¼3.46 and 4.3 m (Fig. 9), it must be highlighted thatneither of these locations have wave–swash interactions duringthis event, as both locations are in the inner swash zone, whichmeans that only pure run-up or backwash occur at this point ofthe profile. Several swash events, run-up events with water layerthickness over 1 cm, were measured at these positions (81 and 29events for x¼3.46 and x¼4.3 m respectively) during test 4, butthe swash event at t¼1730 s is by far the highest run-up event(14 and 9 cm swash height measured at 3.46 and 4.3 m, respec-tively). The run-up measured at the two locations was able tomobilize and remove a large amount of sediment (Fig. 7). Thishigh run-up event could contribute to the amount of the SSCmeasured at position x¼1.92 m a few seconds later (t¼1733 s). Itis expected that the SSC peak event, at around 150 g/l, measuredin the strong wave–backwash interaction at t¼1733 s andx¼1.92 m was induced by the suspension of sediment, by theturbulent bore created during the strong wave–backwash inter-action, previously mobilized during the backwash phase. Once thesediment is put into suspension, it appears in the visual range ofthe OBS, which is at 4 cm from the bottom.

The total sediment transport measured during the maximumSSC event presented above, in time ranges of between 1716 s and1740 s, can be evaluated considering the bottom changes pre-sented in Table 5. The data reported present the bottom changesmeasured with the ADS at positions x¼1.82, 3.45, 4.3 and 5.3 m ofthe emerged beach face. These beach face emerged values werelinearly interpolated (from x¼1.82 to x¼5.3 m) and space integratedto find the sand volume within this control area at each time step.The sand volumes changes, namely, the beginning and end of anevent or an entire time series, were computed to study the gain/lossof sediment during the time lag studied (it is considered thatsediment loss at the end of the beach profile is negligible, as discussedin Section 3.2). Based on the output of the emerged beach integrationand the sand density (1565 Kg/m3 including porosity), the sedimentloss was found to be (all values calculated are negative) �30.5, �32,�0.7, �3, �19 and �7.3 Kg/m in the selected event betweent¼1716 and 1740 s, and for tests 3, 4, 5, 6, 7 and 8, respectively.

When this analysis was conducted for the entire time series, thetotal erosion measured was �16.5, �40.9, �65, �53.8, �53.5 and�23.3 Kg/m for tests 3, 4, 5, 6, 7 and 8, respectively. As presented inFig. 7, a comparison of the two sets of numbers clearly shows thatjust one event is able to move almost the same amount of sedimenttransported during one time series (as previously reported byBlenkinsopp et al., 2011). From Fig. 7 and the two computednumbers previously presented (sediment transport event and total

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–7672

sediment transport), there is a clear erosion control by just a limitednumber of events. During the analysis performed, our time serieswas limited to 500 waves, repeated iteratively, which reduces/limitsthe studied events and, therefore, includes this ‘‘unique’’ high energyevent able to move this large amount of sediment. Despite thelimitation imposed by the short time series, and the fact that themost significant event (t¼1730 s) predominates all other measure-ments, it must be stressed that other strong wave–backwashesinteractions were measured during the studied time series withsignificant SSC, as reported in Table 3, and they also cause erosion.

A different approach was also taken by analyzing the velocity andsuspended sediment concentration measured time series to obtainsuspended sediment transport rates. Using the information gathered

600 620 640 660−0.2

−0.1

0

0.1

0.2

0.3

0.4

H (m

)

Ti

640 645 650 6550

50

100

150

200

SS

C (g

/l)

640 645 650 655−2

−1.5

−1

−0.5

0

0.5

1

1.5

U (m

/s)

640 645 650 655−10

−5

0

5

10

Flux

(Kg/

m)

Time (s)

Fig. 10. The upper panel presents the time variation in incident bore elevation in black and

The second, third and fourth panels show the SSC, cross-shore velocity and suspended sedi

cross-shore location x¼�0.4 m during test 6. (For interpretation of the references to color

with the OBS, ADV and ADS deployed around x¼�0.41 m, a directestimation of the suspended sediment flux going through thatposition was obtained. The measuring equipment was located attwo different positions x¼�0.41 m for SSC and velocity, andx¼�0.54 m for the wave height information, but the time-lagbetween the two positions (lower than 0.1 s) was negligible and,therefore, the three pieces of measuring equipment are consideredhere as co-located. The sediment fluxes calculated during the eventthat lasted from t¼1716 s to 1740 s are equal to 1.6, �2, �11, �5.3,�10 and �6.9 Kg/m for tests 3, 4, 5, 6, 7 and 8, respectively. Thesediment flux is obtained after q¼

RðSSC � U � DÞdt, where SSC is

the suspended sediment concentration, U is the velocity and D is thewater depth plus free surface elevation.

680 700 720 740 760me (s)

705 710 715 7200

50

100

150

200

705 710 715 720−2

−1.5

−1

−0.5

0

0.5

1

1.5

705 710 715 720−10

−5

0

5

10

Time (s)

long wave filtered signals in red; the arrows present the underneath panel study cases.

ment flux calculated for each case, respectively. All measurements were performed at

in this figure legend, the reader is referred to the web version of this article.)

200 250 300 350 400 450 500 550 600 650

−0.2−0.1

00.10.20.3

H (m

)

200 250 300 350 400 450 500 550 600 6500

100

200

300

Time (s)

SS

C (g

/l)

−0.54 m

−0.41 m

Fig. 11. In the upper panel the incident (black) and long wave component (red)

are depicted for water surface elevation measured at cross-shore location

x¼�0.54 m. In the lower panel the SSC at x¼�0.41 m is presented. Both time

series correspond to test 4, and the significant suspended sediment concentration

events found during this time span are pointed by red arrows (upper panel) and

red circles (lower panel). (For interpretation of the references to color in this figure

legend, the reader is referred to the web version of this article.)

I. Caceres, J.M. Alsina / Continental Shelf Research 41 (2012) 61–76 73

There is a clear discrepancy between the total sedimenttransport and the suspended sediment transport data presented,but it must be considered that two assumptions were made inorder to obtain the suspended sediment transport. Firstly, that thewater velocity and sediment concentration was constant through-out the water column (the velocity and suspended sedimentconcentration considered are obtained at a single position at3–4 cm from the bottom); and, secondly, that all transport wasdone by suspended load. The suspended sediment concentrationhomogeneity should not be far from reality. The water depth atthat measurement point ranged from 6 to 10 cm in tests 3–8 in stillwater conditions, and despite some logical, minor discrepanciesbetween the measurements at different heights, the data gatheredwith OBSs at different heights showed vertical sediment homoge-neity distribution for similar low water depth locations. As reportedby Masselink et al. (2009), the second hypothesis seems to befurther from reality as bed-load plays a major role in the swash zoneand this transported sediment is far from being negligible.

Despite the limitations described above of the second methodto evaluate the sediment flux, it was used to qualitatively assessthe importance and direction of the suspended sediment trans-port. When this sediment flux information was evaluated for anentire time series, there was a clear tendency for strong wave–backwash interactions and backwash events to produce negativesediment transport (erosion), as can be seen in the left-handpanels of Fig. 10. Positive sediment transport (accretion) is usuallyinduced by impulsive wave/bore controlled events (short eventsusually formed by one or two waves and arriving in the trough ofa long wave as presented in the right-hand panel of Fig. 10), wavecapture events and weak wave–backwash interactions.

4. Discussion

By comparing the SCESE data with previous SSC data gatheredfrom the literature, the difference in the SSC pattern in the variousstudy sites can be seen. Osborne and Rooker (1997) presented aconcentration diagram per measured wave, where an initial SSCpeak is measured early in the up-rush and another event at theend of the backwash phase. These data were collected in a highenergy dissipative beach (d50 of 0.15 mm) with clear individualevents controlling the shoreline morphodynamics. Similar resultshave been reported by Butt et al. (2004) and Miles et al. (2006),who mainly considered pure swash events to analyze field dataacquired under wave periods of 7 s and 12.5 s that induce SSCevents, mainly due to incident wave/bore and under high boreturbulences. The data presented here show concentration eventslinked to the arrival of bores (like those presented by Butt et al.,2004; Miles et al., 2006) and mostly associated with onshorewardsuspended sediment transport, as well as wave–swash interac-tions. The presented work support the idea of Butt and Russell(2005), who suggested that strong wave–backwash interactionsplay an important role in seaward sediment transport in the surfand swash zone. The research cited above was conducted underimportant tide conditions (mesotidal and macrotidal) linked withhighly energetic sea states in a saturated surf zone. Neverthelessboth data sets present some similarities, there is a clear effect ofwave–swash interactions that stir up the sediment and thepresent data shows a detailed analysis of such influence.

The data obtained in the surf and swash zone during the SCESEexperiments and presented here highlight the different mechanismsthat induce sediment suspension depending on the location ofequipment in the surf and swash zone. The frequency distributionof the most significant SSC events in terms of concentrationmagnitude and duration is shown in Table 3. The most frequentmechanism inducing high SSC events are the wave/bore incident in

the inner surf zone and the weak and strong wave–backwashinteractions in the outer swash zone. Alsina and Caceres (2011)stated that short waves in the trough of long waves produceseaward sediment transport. In their analysis, they included shortbores in the trough of a long wave and both kinds of wave–backwash interactions (strong and weak) that usually occur in thetrough of a long wave. Fig. 11 shows the distribution of the mostsignificant events captured by the OBS located at x¼�0.41 mduring test 4. All captured events, at this time span of Fig. 11, areshort bores arriving in the through of the long wave but withimportantly different hydrodynamic conditions (4 incident wavebores, 2 strong and 4 weak wave backwash interactions), whichdepending on the resultant velocity field will produce a suspendedsediment flux shoreward or seaward directed. Short waves in thetrough of long waves or wave capture events usually produceshoreward sediment transport. Single wave/bore incidents have aclear shoreward sediment flux transport; wave capture eventsusually have a shoreward sediment flux transport, due to theoverlap in time between maximum SSC and shoreward velocity,which produces greater suspended sediment transport than thatmeasured during the following backwash. The events induced bywave–swash interactions are mainly controlled by the wave period,beach slope and wave groupiness, and their sediment balance wasmeasured as shoreward under weak wave–backwash interactionsbut seaward under strong wave–backwash interaction. Strongwave–backwash interaction is a frequent event in the swash andis also associated with the trough of wave groups. It has beendemonstrated (Alsina and Caceres, 2011) that the coincidence of SSCevents in the trough of a long wave enhances seaward directedsuspended sediment transport. Therefore, it is likely that theseinteractions play a significant role in eroding the inner surf zoneand outer swash areas. Moreover, bed-load transport during thelong backwash associated with these interactions may also promotethe erosion found in these areas in the beach profiles measured. Itmust be highlighted that bed-load transport occurring underneathOBS sensors on a vertical elevation (around 4 cm) was not measuredand could give rise to considerable differences.

Although advection was not explicitly considered as a forcingterm in the analysis presented, it must be pointed out that this is

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due to the methodology used to detect the most significantevents in each time series. The 15 cases studied account for 3%of the total time in each time series. Despite this low percentage,the most relevant beach face morphological changes occurduring these events and in some tests these episodes are ableto influence the erosion/accretion pattern of the entire timeseries. Despite the methodology used and the link betweenwave–swash events and SSC peaks, the data obtained clearlyshow the advection of suspended sediment as ‘‘disperse clouds’’(with measured concentrations of around 30–40 g/l that movefrom one OBS to another). Although far from the concentrationpeaks measured of around 150 and 200 g/l, these clouds ofsediment play a role that can be fundamental in the finalaccretion/erosion behavior.

The data presented also emphasize the wide gaps in themethodology used to recover the various modes of sedimenttransport in the swash zone. The complete absence of bed-loadinformation hinders part of our understanding about the nature ofthe processes that control sediment transport in the surf, butmainly in the swash zone. This assumption is further corrobo-rated by the major difference found when the total sedimenttransport and the suspended sediment transport are comparedwithin the previously presented data. The total sediment trans-port measurements performed correspond to �16.5, �40.9, �65,�53.8, �53.5 and �23.3 kg/m for tests 3–8 respectively whenconsidering the entire time series, while the suspended loadmeasured in these same tests was 190, 147, �39, 44, �65 and4.9 kg/m for tests 3–8, respectively. Despite the fact that both setsof figures have their drawbacks (total transport was calculated byinterpolating the 5 points under the ADSs from x¼1.8 m tox¼5.3 m without considering the distance from shoreline to1.8 and by performing a linear interpolation between each point;suspended transport was calculated by measuring the sedimentflux through the shoreline by using the water surface elevation atthis location, and the velocity and SSC at 4 cm from the bottom),the differences between the two data sets are significant enoughto acknowledge that a piece of far from negligible information isbeing lost. Masselink et al. (2009) reported that sediment trapsseem to underestimate the total load sediment transport as theytrap the suspended load and part of the bed-load component ofthe transport, but until it is somehow possible to recover this bed-load fraction, it is probably a good option to use some sedimenttraps to better explain the role of the different sediment transportforcing terms in the surf and swash zone.

Another important point presented in the data analyzed refers tothe fact that isolated events are able to influence importantly thegeneral erosion/accretion pattern within an entire time series.Several authors (Shepard, 1950; Bascom, 1954; Russell, 1993) havereported that few storms tend to erode great amounts of coastlinewhilst most of the time the predominant wave conditions tend toinduce a shoreline or beach accretion. In the data set presented here,despite the erosive wave conditions reproduced (based on Dean,1973; Dalrymple, 1992; Kraus et al., 1991 formulations), most of theADSs along the different time series give an accretive trend, whilstonly a few waves reported coastal erosion that are able to revert thetrend to a final erosion.

Recently, Brocchini and Baldock (2008), Baldock et al. (2005) andTurner et al. (2008) have presented data of large bed-level changesinduced by single events. Blenkinsopp et al. (2011),who performed their measurements for a 1/15 Atlantic beach,Hs¼2.8 m, Tp¼12.8 s and d50¼0.4 mm, presented events able tomove up to 1 cm sand thickness, the same order of magnitude as thenet morphological change at a point on the beach face over acomplete tide. In the SCESE data set isolated events, pure swashmotion without wave–swash interactions, are found at x locations of3.46 and 4.3 m (inner swash). Each of the events measured at

location x¼4.3 m during the four initial tests, 38 on average, usuallyhave swash heights below 5 cm and tend to create sedimentaccretion below 3 mm. The first time series, starting from the 1/15initial slope, is the one that present a more important sedimentaccretion, up to 2.3 cm during the first 1700 s at 3.46 and 4.3 m,until the most erosive event occurs at around t¼1730 s with asediment loss of 1.3 cm. During this initial time series, most of whatwas accreted during the initial 1700 s was eroded during one event,but the final outcome at x¼3.46 and 4.3 was still positive (smallaccretion). The next time series present smaller accretions duringthe initial 1700 s, but similar erosion during this high erosive event(even a little higher in forthcoming time series) and, therefore, theyproduce final erosion within the profile at these locations during theentire time series. This single or ‘‘unique’’ erosive event, plotted inFig. 9, presents the largest run-up with swash thickness averagevalues of 16 and 11 mm at x¼3.46 m and 4.3 m, respectively. Theaverage velocity calculated at the beginning of the bore front of themost erosive event (t¼1726 s) is around 1.5 m/s with a standarddeviation of 0.3. These velocities were calculated considering thearrival of the swash bore between locations 3.46, 4.3 and 5.3 m forthe 8 erosive tests conducted.

From the various wave gauges deployed, the swash velocity canbe determined by calculating the volume changes between thedifferent ADSs (Hughes and Baldock, 2004; Houser and Barrett,2010). The precision of the computed swash velocities heavilydepends on the accuracy of the ADSs used to measure isolatedswash events, the distance between the deployed equipment and theswash velocities. The ADSs’ accuracy (2 mm) and their arrangementin the lab experiments (spatial distance of 1 and 1.5 m between twoconsecutive ADS) did not allow us to perform the analysis to thesame standard as the authors cited above, but the results obtainedgive us a good idea of the maximum run-up velocity, the duration ofthe run-up and run-down phase, and the maximum negative velocityduring backwash. Following Houser and Barrett (2010), who basedtheir analysis on different conditions (d50E0.39 mm, settling velo-cityE0.17 m/s and beach slope 1/6.6, which allowed a wave-by-wave analysis of swash motion with limited swash interaction orinfra-gravity wave dominance), the various isolated swash eventsrecovered here (at cross-shore locations 3.46, 4.3 and 5.32 m) areanalyzed. In the swash events studied, there is a common asymmetryat locations x¼�0.54, 0.45 and 1.82 m, whilst the swash is moresymmetrical at the inner swash locations (x¼3.46, 4.3 and 5.32 m).The shape of the run-up and run-down were analyzed for differenttests when their isolated swash events were considered, from whichit was seen that symmetry or the lack of it does not seem to controlthe erosion/accretion pattern. As presented by Houser and Barrett(2010), accretion processes occur once there is a run-up preceded byan SSC peak. The concentration peaks measured were not the mostsignificant of the time series (an average of 90 g/l at cross-shorelocation x¼1.92 m, whilst the average mean concentration of the 15most significant events is 145 g/l). The velocities of these accretiveevents are mostly slower than 1 m/s and the run-up events seem toend close to x¼4.3 m without reaching the ADS at x¼5.32 m. Underthese conditions, namely, slow run-up velocities with a previousconcentration peak at the nearest x location, there is usually anaccretion process at location x¼4.3 m. However, the erosion eventswere not properly characterized during these tests (between one andfive events are responsible for the erosion pattern) and, therefore, thelack of enough study samples prevents us from properly analyzingthese cases.

5. Conclusions

This work presents a new set of large-scale laboratory experi-ments where standard wave height measurements were performed

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along a flume, in addition to which velocity, suspended sedimentconcentration, the thickness of the swash lens and bottom evolutionmeasurements were acquired in the surf and swash zones. The datahere presented are free of scale effects and present as key points therepeatability of the time series, the absence of tide and, therefore,accurate locations of the measuring probes in the surf and swashzone areas. The amount and quality of the data gathered (mainlythe SSC events and hydrodynamic characterization) must also behighlighted.

The data presented show a clear zonation in the surf and swashzone of the forcing terms that induce the SSC events. Incident wave/bore events control the SSC events in the surf zone, whilst the outerswash zone is mainly controlled by the wave–swash interactions.Weak wave–backwash interaction is the most frequent forcing termto induce high SSC events, whilst strong wave–backwash interac-tions generate the SSC events with higher concentrations. There is aclear tendency for strong wave–backwash interactions and back-wash events to produce negative suspended sediment transport(erosion). Whilst incident wave/bore events, wave capture eventsand weak wave–backwash interactions produce positive suspendedsediment transport (accretion).

Finally, it is important to highlight the role that unique events areable to play within a time series. After studying the bottomevolution of the emerged beach face and the suspended sedimenttransport fluxes along the shoreline, there is a clear predominance ofsignificant or unique erosive events that are able to control theaccretion/erosion pattern of the entire time series. In the data setpresented, despite the erosive wave conditions reproduced, a con-tinuous accretion measurement of the upper part of the swash zonewas measured, whilst only a low number of waves reported coastalerosion. Despite the few erosive events measured, at the end of eachtime series there is clear and continuous erosion of the shoreline.

Acknowledgments

Part of the data presented here were measured within theframework of the European Hydralab-III SANDS Project (contractnumber: 022441) (RII3). Second author’s support through a MarieCurie IEF fellowship from the EU is greatly appreciated. Theauthors wish to thank all those who have collaborated in theSANDS and SCESE experiments, and especially Joaquim Sospedrafor his patience and kindness.

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