Sediment suspension events in the inner surf and swash zone. Measurements in large-scale and high-energy wave conditions

Coastal Engineering 58 (2011) 657–670

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Coastal Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o a s t a l e n g

Sediment suspension events in the inner surf and swash zone. Measurements in large-scale and high-energy wave conditions José M. Alsina a,⁎, Iván Cáceres b a b

Dept. Idraulica, Strade, Ambiente e Chimica (ISAC), Università Politecnica delle Marche, 60131, Ancona, Italy Laboratorio de Ingeniería Marítima, Universidad Politécnica de Cataluña, 08034, Barcelona, Spain

a r t i c l e

i n f o

Article history: Received 26 February 2010 Received in revised form 28 February 2011 Accepted 2 March 2011 Available online 2 April 2011 Keywords: Swash zone Inner surf zone Sediment suspension Large-scale wave flume Bore turbulence Wave–swash interactions Long- and short-wave components

a b s t r a c t The present study presents a database of hydrodynamic properties and suspended sediment concentration collected within the inner surf and swash zones aiming to improve the current understanding of the sediment dynamics occurring within the beach area closest to the shoreline. Experimental measurements were conducted in a large-scale wave flume under high-energy wave conditions at three cross-shore locations, representing inner surf and swash zone conditions. 47 tests, each one comprising 500 wave trains with identical wave conditions were measured. Obtained hydrodynamic properties and suspended sediment concentrations were observed to be highly repeatable between successive tests despite the sediment suspension event-like pattern, the beach evolution between tests, and the apparent randomness of the sediment suspension phenomenon. The hydrodynamics close to the shoreline (inner surf and swash zone) is dominated by short incident broken waves and long-wave water level oscillations induced by wave grouping. The analyzed time series of measured water surface elevation, horizontal velocity, computed Turbulent Kinetic Energy (TKE), and sediment concentration revealed that the suspended sediment concentration in this coastal zone does not correlate strongly with either the incident bore height, or the short-wave horizontal velocity or the TKE; in other words high/low values of these variables do not always promote high/low values of sediment suspension. In contrast, the highest suspended sediment concentrations were observed to occur by the combined action of incident bores and the trough of long-period water level oscillations. This pattern was more apparent in the inner surf than in the swash zone. High suspended sediment concentrations were also observed to coincide with negative peaks in long-wave horizontal velocity modulation resulting in enhanced negative sediment transport rates and beach erosion close to the shoreline. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The beach area closest to the shoreline is a highly dynamic zone where a large part of the coastal sedimentary processes take place. It can be subdivided into inner surf and swash zone. The inner surf zone is the surf subsection closest to the shoreline and can be considered as the transition boundary between the wave breaking area and the swash zone. The swash zone is the limit between the aerial and submerged beach areas intermittently covered and exposed by wave action. A typical swash event consists of an uprush (when water climbs the beach face and reaches its maximum beach elevation, i.e. the maximum run-up) and a downrush (when the water recedes during the beach face to a minimum shoreline position, where it encounters the next arriving wave, i.e. the run-down). The inner surf and the swash zone form the last beach area where waves dissipate or reflect their ⁎ Corresponding author. Tel.: + 9 071 2204466; fax: + 9 071 2204528. E-mail addresses: [email protected] (J.M. Alsina), [email protected] (I. Cáceres). 0378-3839/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.coastaleng.2011.03.002

remaining energy after travelling from the open sea towards the shore (Brocchini and Baldock, 2008). They are characterized by strong unsteady flows, high turbulence levels, large sediment transport rates and rapid morphological changes. Although in random conditions it is difficult to distinguish between the inner surf zone and the lower swash area, the swash zone cannot be regarded as a mere extension of the surf zone, as swash and surf zones have different dynamic behaviors. The swash zone is dominated by a hydrodynamic forcing set at the inner surf boundary and, at the same time, wave beach-face and wave–swash interactions occur in the swash zone also forcing morphodynamic boundary conditions and thereby affecting surf zone dynamics. Understanding the existing links between the surf, inner surf and swash zones is necessary for understanding beach dynamics as a whole. Traditionally, a large part of the littoral sediment transport process has been reported as occurring in the swash zone, with higher concentrations of suspended sediment in the swash than in the surf zone (Osborne and Rooker, 1999).

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Inside and outside the surf zone, high sediment suspensions have been observed in an event-like manner at time scales of seconds (associated with the incident frequency of wind sea waves) and of minutes (associated with wave groups and long waves) (Nielsen, 1992). At the incident wave frequency, sediment suspension events have been observed as intermittent spikes not correlated with the orbital velocity (Smyth and Hay, 2003), suggesting a large influence of seabed-generated turbulence (Smyth and Hay, 2003). At a lowfrequency time scale, sediment suspension events have also been observed as characterized by higher concentrations than at the incident wave frequency band and associated with the passing of wave groups (Osborne and Greenwood, 1992). In the other hand, close to the shoreline, short waves decay while low-frequency energy amplifies and, eventually, reflects back seawards. Moreover, the beach slope configuration modulates the incident waves and strong wave–wave interactions occur at different time scales, leading to the generation and reflection of further low-frequency waves (Mase, 1995). Swash zone dynamics are, therefore, dominated by a combination of short broken waves and low-frequency waves, depending on wave climate and beach morphology. Few studies have measured sediment suspension close to the shoreline, comparatively with those conducted on surf zone or outer areas, consequently it is not yet clear whether the sediment in the swash zone goes into suspension as forced by either incident bores or low-frequency waves. However, the answer is probably case-specific and depends on Iribarren's number defined at the swash zone (Miche, 1951). Several authors have conducted sediment studies on gently sloping beaches in which wind and swell waves are fully dissipated and the swash motion is at infragravity frequencies, with wave uprush and backwash durations exceeding 10 s (Aagaard and Hughes, 2006; Masselink et al., 2005; Osborne and Rooker, 1999). These studies have reported the presence of large suspended sediment concentrations, generally one order of magnitude higher than that observed in the surf zone, and associated with long-period swash events. Similarly, several sediment dynamics studies conducted in non-saturated conditions have found high sediment suspension in the swash zone attributed to incident bores (Masselink and Russell, 2006). Miles et al. (2006) compared the sediment suspension in the inner surf and swash zones of an intermediate respect to a dissipative beach, and reported higher sediment concentrations (one order of magnitude larger) in the intermediate beach dominated by incident wave frequencies than in the dissipative beach where sediment suspension is dominated by infragravity frequency motions. Predictions of suspended sediment patterns or sediment transport rates close to the shoreline become more complicated than in outer areas. For example, different mechanisms have been proposed to explain the large amount of sediment suspension and higher sediment transport occurring during uprush than during the backwash and compared with the off-shoreward tendency displayed by swash velocities. One of the driving forces in predicting such sediment suspension is bed shear stress and associated boundary layer processes. Friction factors and bed shear stresses have been computed in the swash zone by fitting a log-law to the velocity profiles observed (Cox et al., 2000; O'Donoghue et al., 2010) or by direct measurements (Barnes et al., 2009; Conley and Griffin, 2004). In these studies, higher friction factors are typically found during the uprush than during the backwash. Barnes et al. (2009) performed time-dependent direct bed shear stress measurements at different locations in the swash zone. A maximum onshore peak was found during the initial stages of the uprush to be typically 2–4 times greater than the maximum shear stress during the backwash. Moreover, the maximum time-averaged shear stress was found to occur in the lower swash zone. These

differences in shear stress may be due to turbulence, which is enhanced during the uprush due to wave breaking or bore collapse turbulence contribution. Some authors have attributed the sediment suspension and transport rate patterns to be closely related to the boundary layer behavior in the swash zone. Masselink et al. (2005) related suspended sediment to bed shear stress in a representative long swash event on a dissipative beach. They suggested that the uprush is more efficient at suspending sediment due to differences in the boundary layer during uprush and backwash. However, sediment transport predictions using direct shear stress measurements (Barnes et al., 2009) have not shown any improvement over more traditional models and highlight the complexity of sediment dynamics in the swash zone. Turbulence enhancement in the nearshore due to wave breaking and bore collapsing has also been proposed as a mechanism inducing sediment suspension (Aagaard and Hughes, 2006). Higher turbulent kinetic energy has also been reported during uprush than during backwash (O'Donoghue et al., 2010; Petti and Longo, 2001), and this difference has been attributed to the advection of bore-related turbulence into the swash zone during the uprush (Alsina et al., 2009a; Longo et al., 2002) while during the backwash it is generated from the bed (O'Donoghue et al., 2010). Turbulent velocity fluctuations have been correlated with sediment suspension and have, therefore, been suggested both as a forcing for sediment suspension in the inner surf zone (Kobayashi and Tega, 2002), and as a cause for the differences between the uprush and backwash swash sediment transport (Aagaard and Hughes, 2006). It is the purpose of this study to provide a comprehensive data set of sediment suspension events occurring in the inner surf and swash zones and on the variables controlling sediment suspension. Water elevation, flow velocities and sediment concentration measurements were conducted, with precise instrumentation, in a large-scale wave flume under high-energy wave conditions at three cross-shore locations, representing inner surf and swash zone conditions. Correlation between suspended sediment concentrations, flow velocities and TKE at incident and long-wave frequency is also analyzed. Wave flume experimental configuration and data analysis methodology are presented in Section 2, and the obtained results and discussion in Sections 3 and 4 respectively. Finally, conclusions are given in Section 5. 2. Experimental setup 2.1. Large-scale wave flume (CIEM) The data presented in this work were obtained under the SANDS Project (Scaling and Analysis and New instrumentation for Dynamic bed testS), a Hydralab-III European project, focused on mobile bed experiments. In the framework of this project, a set of experiments was conducted in three wave flume facilities of varying sizes. The three facilities keep the same scale ratio for all dimensions and hydrodynamic properties, undistorted models, and are scaled to ensure similar Froude number (Hughes, 1993). The Large Wave Channel (GWK) of the Coastal Research Center (FZK) in Hannover is considered the prototype, as it is 300 m long, 7 m deep and 5 m wide, with a median sediment size (d50) of around 0.30 to 0.36 mm. The results herein analyzed refer to experiments performed at the Canal de Investigación y Experimentación Marítima (CIEM) at the Universidad Politécnica de Cataluña, Barcelona, which has a 1:1.9 scale ratio with respect to the GWK. Cáceres et al. (2008) presented a similitude analysis and scaling performance reporting good similitude in the morphological evolution between the CIEM and the GWK experiments, and showing no scale distortion effects. All the parameters and values herein presented correspond to morphodynamic data obtained in the CIEM flume.

J.M. Alsina, I. Cáceres / Coastal Engineering 58 (2011) 657–670

1 0.5 Resistive Gauges

h (m)

0 −0.5

ADVs & OBSs

−1 −1.5 −2 −2.5

−60

−50

−40

−30

−20

−10

0

X (m) Fig. 1. Wave flume layout with initial bathymetry, wave gauge, acoustic Doppler velocimeter (ADV) and optical backscatter sensor (OBS) locations for the Barcelona CIEM experiments.

The beach configuration is illustrated in Fig. 1, where the xcoordinate origin is at the initial shoreline with a still water level of 2.47 m, negative towards the wave paddle (offshore) and positive towards the beach face (onshore). Subsequently, all the cross-shore locations will be referenced using this coordinate system. The beach profile consists, from the wave paddle towards the shoreline, of an initial 30 m long plane section, followed by a small 1:13 slope section, and by a 20 m long plane bed and by a beach stretch with a 1:15 slope (see Fig. 1 for details). The beach at CIEM is made of commercial well-sorted sand with a d50 of 0.25 mm, a d10 of 0.154 mm and a d90 = 0.372 mm. The generated waves reproduce erosive conditions (Hs = 0.53 m and Tp = 4.14 s) and the time series correspond to a Jonswap spectrum (γ = 3.3). To avoid uncertainties with second-order generation and absorption, which depend on the kind of paddle in use, the time series was generated using the first-order approximation. 47 realizations of the same wave time series were performed during these experiments, each series comprising a total of 500 waves. The same hydrodynamic forcing conditions were simulated for the 47 tests, which can, thus, be regarded as statistically equivalent. Differences in measured hydrodynamics between tests were expected due to the resulting morphologic evolution and turbulence. 2.2. Instrumentation The experiments performed at the CIEM aimed to measure sediment transport and bottom evolution in the areas closer to the shoreline (inner surf and swash zone). Water surface elevation were measured with 14 resistive wave gauges placed along the wave flume at the following cross-shore locations: x = −61, −60.3, −59.3, −58.6, −51.1, −38.1, −32.6, −26.8, −20.9, −17.9, −15, −12, −9.1, and −7.4 m (refer to Fig. 1). Acoustic displacement sensors (ADSs) were placed close to the shoreline in order to obtain water surface elevations within the swash

659

zone. Systematic errors were identified during experiments due to wave steepness, echoes between sensors and/or malfunctioning devices and their data were discarded from the analysis. Resistive wave gauges and acoustic displacement sensors had a sampling frequency of 40 Hz. Simultaneously, water table fluctuations were measured with seven, 100 Hz buried resistive wave gauges, protected by a permeable plastic frame to avoid calibration problems due to the different conductivity between sand and water. These were placed at the following locations: x = −0.20, 1.3, 2.76, 4.27, 5.81, 7.3 and 8.5 m. When the wave action directly reaches these sensors during a swash event, they measure the free water surface elevation during the swash. The first gauge (x = −0.20 m) was located seaward of the shoreline from the beginning of the experiments to measure the free water surface elevation. As the shoreline eroded, the buried wave gauges closer to the shoreline were directly exposed to water level fluctuation and wave actions. As ADS sensors displayed malfunctioning, water surface analysis close to the shoreline is only given with reference to the buried resistive wave gauge data. Water flow velocities and sediment concentrations in the nearshore were measured with seven Acoustic Doppler Velocimeters (ADVs) and seven Optical Backscatter Sensors (OBSs), distributed over four crossshore locations along the inner surf and low swash zones. The location of the ADVs coincided with that of the OBSs in cross-shore positions and vertical elevations with respect to the bed, although they were not located in the same long-shore position (perpendicular to the wave flume main axis). The ADVs were placed close to one of the flume walls while the OBSs were located at the opposite wall with a separation of around 2 m (width wise) existing between the OBSs and the ADVs. This enabled velocity–concentration correlations at the same vertical and horizontal locations. Due to the progressive erosion occurring within the swash zone, the z-location of ADVs–OBSs was verified and corrected daily before the experiments started, in order to obtain the same vertical distance from the bed during the various tests. The vertical distance from the bed was set approximately at 3, 6 and 9 cm above the seabed level for three sensor arrays, at 4 and 9 cm for two sensor arrays and at 3 cm for single sensors (locations onshoreward of the shoreline). Although some variability due to bed level variations might be expected during each test, the vertical distance was also checked between tests, since many ADVs allow for a measurement of the distance from the sensor head to the seabed. The arrangement of ADVs and OBSs was modified as the beach face experienced erosion and the run-down position moved onshorewards. The location of the ADV–OBS instrument during the various experiment tests is summarized in Table 1, the x-coordinate origin being the shoreline position at the initial beach configuration. The ADV sampling frequency was 100 Hz, while sediment suspension was sampled at 40 Hz. The seabed evolution during the experiments was measured using a standard mechanical bottom profiler (Cáceres et al., 2008). Beach profiles were measured after tests 1, 3, 5, 7, 9, 12, 15, 18, 21, 25, 29, 35, 41 and 47.

Table 1 Acoustic Doppler velocimeter (ADV) and optical backscatter sensor (OBS) locations during the various experimental tests. Tests 1 to 8

Tests 9 to 41

Tests 42 to 47

X-location (m)

No. of probes

z-level (cm)

X-location

No. of probes

z-level (cm)

X-location (m)

No. of probes

z-level (cm)

− 4.36

3

~3 ~4 ~9

− 4.36

2

~4 ~9

− 2.63

2

~4 ~9

− 2.71

2

~4 ~9

− 2.71

2

~4 ~9

− 0.81

2

~4 ~9

− 0.8

1

~3

− 0.8

2

~4 ~9

0.94

2

~4 ~9

0.92

1

~3

1

~3

2.94

1

~3

0.92

660

J.M. Alsina, I. Cáceres / Coastal Engineering 58 (2011) 657–670

a

2.3. Data analysis techniques

b 106

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi fc +δf að fc Þ = 2∫ Sð f Þdf fc −δf

ð1Þ

2.4. Turbulent kinetic energy (TKE) computations Turbulent flow velocity fluctuations are a potential driver of sediment suspension. Previous studies in the swash zone have stressed the importance of turbulence, enhanced by wave breaking or collapsing bores close to the shoreline in inducing sediment transport (Aagaard and Hughes, 2006; Alsina et al., 2009a). TKE was calculated to analyze the influence of turbulent velocity fluctuations on sediment suspension close to the shoreline. TKE was computed by extracting the turbulent component following a Reynolds decomposition from the measured horizontal and vertical velocities,

Sww (m2/s2/Hz)

104

−1/3

102 −5/3

−1/3

101

−5/3

100

10−1

100 10−2 10−2 −2 10

10−1

100

f (Hz)

where δf is half the band width of the finite frequency band and S(f) is the spectral density of the water surface elevation. The ADV velocity data were processed and spike noise eliminated using the method developed by Goring and Nikora (2002). Lowquality data, where signal to noise ratio or signal amplitude was low, were discarded and cubic interpolation was performed. Each deployed OBS was calibrated using the glycerol technique developed by Butt et al. (2002). Incident energy due to controlled irregular wave conditions was monitored during the experiments, thus allowing test repeatability analysis. The long-wave component of horizontal velocity was obtained by filtering measured signals through 0.1 Hz cut-off frequency digital filters. The short-wave component was obtained by removing the long-wave component from the raw signal and subsequent noise filtering. Within the inner surf and swash zones, the water surface oscillates in the shape of incident bore-type waves running over a long-period water level oscillation. This area is intermittently emerged and submerged and this could potentially affect statistical analysis procedures (Hughes and Baldock, 2004). In this area, the long-period water level variation was computed as the envelope of the minimum water level elevations at each wave event, while the short brokentype (bore) wave heights were computed by removing computed long-period oscillation from the water surface signal. Low water levels below the sensor vertical elevation were easily identified by the water surface signal and by the signal to noise ratio and amplitude values of the ADVs. Data measured during emerging periods were discarded from the analysis.

  2 2 2 1 TKE = =2 u0 + v 0 + w 0

102

Suu (m2/s2/Hz)

The water surface signal was analyzed using spectral analysis techniques. The power spectrum was computed from Fourier transform using a modified Welch (averaged) periodogram, dividing the water surface signal into 8 segments with 50% overlapping; each section was windowed with a Hamming window technique. Signifpffiffiffiffiffiffiffi icant wave height was computed as 4 m0 , where m0 is the zero order spectral moment. The amplitude, a(fc), at different finite frequency bands centered at fc was computed using the following relationship (Baldock and Huntley, 2002):

103

101

102

10−3 −2 10

10−1

100

101

f (Hz)

Fig. 2. Example of computed power spectrum density of horizontal (left figure) and vertical (right figure) flow velocities at cross-shore location x = − 0.8 m, test number 10 and vertical elevations from the bed of 4 cm (grey line) and 9 cm (black line).

high-pass filter with a specified cut-off frequency to remove orbital velocity. Several methods have been proposed to select the cut-off frequency, they are based on the coherence between the velocity and water surface elevations (Thornton, 1979), the excess measured velocity variance relative to predicted orbital velocity variance (Rodriguez et al., 1999), or the spectral slope breaks (Smyth and Hay, 2003). The spectral slope break technique, as described in Smyth and Hay (2003) was chosen for the present study. Fig. 2 displays the computed power spectrum of horizontal and vertical velocity for test 10 at location x = − 0.8 m and two vertical elevations of 4 and 9 cm with respect to the bed level. Power spectrums obtained at different tests and locations were observed to display a similar pattern. The spectral slopes on the highfrequency side of the incident wave band for Suu are close to the expected value of − 1/3 for shallow water waves (Thornton, 1979) but flatter than the expected value for Sww. A slope break is apparent at around 1 Hz similarly to field studies (Foster et al., 2000). The spectral slopes for the inertial subrange for Suu and Sww are slightly flatter than the expected value of − 5/3. A flattening of the spectral slope also occurs in the high-frequency tail (10 Hz) and suggests a noise floor (Smyth and Hay, 2003). To analyze the turbulence, the velocity signals were band-pass filtered with a 1 Hz low limit cut-off frequency to remove most of the incident wave energy band and a 6 Hz high limit cut-off frequency to remove the high frequency noise. The slope breaks on Fig. 2 suggests that this range limits the inertial subrange. Similar cut-off frequencies have been used by Foster et al. (2000) and Smyth and Hay (2003) among others. The filter method selected was a digital filter with a Finite-Duration Impulse Response (FIR) and a magnitude response of around −100 dB. Since the low-limit cut-off frequency in the filtering process is higher than the incident frequency, most of the incident wave component was removed from the turbulent velocity. However, the filter method underestimates the amount of turbulence present as filtering removes any turbulent energy, which might occur at wave frequencies.

ð2Þ 3. Results

where u, v and w refer to the x, y and z components of velocity and the ′ indicates the turbulent oscillation component. Separation of orbital and turbulent velocity components using measurements from a single instrument in irregular and strongly nonlinear wave conditions involves several serious limitations (Longo et al., 2002). In such conditions, the method most frequently used is a

3.1. Hydrodynamic properties and overall bottom evolution Fig. 3 displays the erosive profile evolution for CIEM tests and the significant wave height across-shore distribution for the tests 1, 5, 10, 22 and 47.

J.M. Alsina, I. Cáceres / Coastal Engineering 58 (2011) 657–670

a

0.4

0.8

0.35

0.4

Power Spectral density (m2 s)

Hm0

0.6

0.2

b

0 1 0

h (m)

661

−1 −2 −60

−50

−40

−30

−20

−10

0

10

0.3 0.25 0.2 0.15 0.1 0.05

X (m) Fig. 3. Measured spectral significant wave height (top panel) and beach profile (low panel) at the initial experimental stages (solid black line), at test 5 (solid grey line), tests 10 (dashed black lines), 22 (dotted grey line) and final (dashed dotted black line).

During the first test series, waves broke over the constant-slope bed, eroding the beach face and generating an initial bar at the crossshore location where the significant wave height starts decaying. As the successive erosive tests continued, the bar and the breaking point translated seawards and the bar increased in volume. A secondary, rather small bar developed landwards of the primary bar at around test case 30. This behavior has been extensively reported (Hoefel and Elgar, 2003 among others), while the wave height transformation over bar profiles can be accurately predicted by state-of-the art numerical models (Alsina and Baldock, 2007). During the first test series (tests 1 to 4), the beach evolved very rapidly, and large amounts of sediment in suspension with apparent stochastic suspension patterns and high sediment transport rates were observed. After this initial rapid seabed evolution and bar development, suspended sediment concentrations decreased significantly and suspension patterns were more apparent. In the final experiment stages a quasi-equilibrium situation had already been reached when sandbar migration slowed down significantly. This time evolution of the seabed profile displayed a tendency similar to prototype morphodynamics, ensuring model similitude (Cáceres et al., 2008). Surf zone conditions may be characterized using the Iribarren parameter, ξ0, defined as β ξ0 = rffiffiffiffiffiffiffiffiffi .ffi H0

ð3Þ

L0

where H0 and L0 are the wave height and wavelength at the generation point (in this case) and β is the beach slope (1:15). Obtained Iribarren numbers were around 0.4, values characteristic of dissipative beaches where standing, low-frequency waves dominate shoreline motions because the short-wave energy is dissipated in a saturated surf zone. Wave breaking was visually identified as plunging type breaker most of the time, and incident bore influence on the shoreline was also shown to be important in addition to lowfrequency components. The power spectrum analysis for wave gauges at different crossshore locations during test 10 are displayed in Fig. 4. The corresponding profile location and wave height cross-shore distribution are illustrated in Fig. 3. At location x = −61 m (closest to the wave paddle), the incident frequency peak is evident but some energy is also present at low frequencies (0.04 Hz), corresponding to the incident bound and outgoing long waves. At the breaking location (x = − 12 m), significant energy dissipation at incident frequency due to the breaking processes can be observed as well as energy transfer to

0 0

0.2

0.4

0.6

0.8

1

f (Hz) Fig. 4. Computed water surface power spectral density for test 10 and at cross-shore locations x = − 61 (solid black line), − 12 (solid grey line) and − 7.4 m (dashed dotted black line).

higher frequencies (f N 0.4 Hz). This is also noticeable in the inner surf zone (x = −7.4 m). Additionally, energy transfer to subharmonics is apparent at these two locations. Energy peaks appear at low frequencies of around 0.02, 0.04, 0.06 and 0.08 Hz (T = 50–12 s) in these two cross-shore locations. Closer to the shoreline (x = −7.4 m) the low-frequency energy contribution is important, where most of the incident high-frequency waves are broken and the energy at low frequency is similar to the incident wave component. The cross-shore wave amplitude distribution was computed at different low frequencies in order to analyze the long-wave component structure. Fig. 5 shows measured and computed longwave amplitude cross-shore variation for test 10. Note that they are normalized with the incident wave height (Hrms0). Theoretical amplitude values are obtained by monochromatic long-wave numerical simulation at the selected frequencies f using a nonlinear shallow water equation (NLSWE) model (Li et al, 2001). At low frequencies in the range f = 0.04–0.05 Hz (Fig. 5b–c), the cross-shore amplitude increases during the shoaling of short wave groups displaying a non-linear cross-shore variability respect to the incident wave height from outside the surf zone to the breaking location (Fig. 5b and c at x = − 12 m). This behavior was consistent with the structure of an incident bound long wave associated with wave groups dissipated during the breaking process (Baldock, 2006; Baldock and Huntley, 2002). There is a space shift in the maximum and minimum computed and measured amplitudes, attributed to wave group forcing which is not accurately predicted by NLSWE models. At higher frequencies, between 0.06–0.07 Hz (Fig. 5d–e), the long-wave cross-shore structure seemed to be dominated by the combination of incident bound waves, breakpoint-generated outgoing free waves and reflected longwaves at the shoreline (Baldock and Huntley, 2002). The displayed long wave structure is translated into a low-frequency water surface and flow velocity modulation close to the shoreline, which has an important effect on sediment suspension patterns. 3.2. Sediment suspension This section presents the sediment suspension measurements conducted during tests 6, 10 and 12 at cross-shore locations x = −2.71, − 0.8 and 0.9 m. It aims to describe sediment suspension dynamic in the inner surf zone and swash area. These tests were selected because the initial test (test 1) started from a plane 1:15 beach slope, where the beach compaction might not

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J.M. Alsina, I. Cáceres / Coastal Engineering 58 (2011) 657–670

normalized amplitude

a

b 0.2

0.15

0.15

0.1

0.1

0.1 0.08 0.06 0.04

0.05

0

0.05

−60

−40

−20

0

0

d normalized amplitude

c

0.2

0.02

−60

−40

−20

0

0

e 0.1

0.1

0.08

0.08

0.08

0.06

0.06

0.06

0.04

0.04

0.04

0.02

0.02

0.02

−60

−40

−20

0

−40

−20

0

−60

−40

−20

0

f

0.1

0

−60

0

−60

−40

X (m)

−20

0

0

X (m)

X (m)

Fig. 5. Cross-shore distribution of the computed and measured long-wave amplitude normalized with the incident Hrms0 within different frequency finite bands: a) f = 0.024 Hz; b) f = 0.04 Hz; c) f = 0.05 Hz; d) f = 0.063 Hz; e) f = 0.07 Hz and f) f = 0.086 Hz. Starred lines correspond to measured data while solid lines give the monochromatic numerical simulation.

be representative of real beach conditions. After test 4, a sediment suspension pattern was more apparent, while the beach face continued to erode and the bar location moved seaward. At tests 6,

h (m)

a

10 and 12, the shoreline position was located at 0.65 m, 1.6 m and 2.0 m respectively. It was assumed that after test 12, the cross-shore location x = 0.9 m corresponds to the low swash area. At test 25 the

0.3

0.2

0.1

0

b

1.5

U (m/s)

1 0.5 0 -0.5 -1

c 200

SSC (g/l)

150 100 50 0 150

200

250

300

350

400

450

Time (s) Fig. 6. Time variation of incident bore surface elevation (a) horizontal short wave velocity (b) and suspended sediment concentration (c) for erosive tests 6 (solid black line), 10 (dashed dotted black line) and 12 (solid grey line). Water surface elevation is measured at x = − 0.20 m while velocity and sediment concentration are measured at x = − 0.8 m.

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shoreline is located at 2.5 m, thus, location x = 0.9 m is no longer assumed as the low swash area. Cross-shore location x = − 0.8 m corresponds to the inner surf zone after test 4 and up to test 47 approximately. The submergence ratios for cross-shore locations −0.8 and 0.9 m were approximately 96% and 84% of the measured time during test 6. During tests 10 to 12 the submergence ratio at these locations increased to 98% (x = − 0.8 m) and 87%. (x = 0.9 m). Aagaard and Hughes (2006) reported submergence ratios between 75–95% for lower swash conditions. Therefore, for the selected tests (6, 10 and 12), the cross-shore location x = −0.8 m is assumed to represent inner surf zone conditions while location x = 0.9 m represents low swash conditions. Visual inspection of the water surface elevation signal seemed to corroborate these assumptions. During the experiments, maximum run-up was visually identified between x = 8.5 m and x = 9.8 m, while minimum run-down was visually located at around −1 m during test 1 but moved onshore as the beach was eroding. Fig. 6 displays a time series of measured incident bore heights at cross-shore location x = −0.20 m, short wave horizontal velocity and suspended sediment concentrations measured at 4 cm from bed level, at location x = −0.8 m, for tests 6, 10 and 12 (a short period of time series is displayed). In this cross-shore location (inner surf zone), waves arrived as broken waves (turbulent bores), some collapsing close to this location. Bore collapsing is defined here as the transition of a bore-type wave to run-up on the beach surface (Yeh, 1991). Turbulent water movement is translated into a more organized water

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flow climbing the beach face. The occurrence of this transition depends on the bore height relative to the water elevation, which oscillates with a longer period than incident bores. The incident bore water surface was computed as explained in Section 2.3. There was a 0.6 m shift between cross-shore locations where water surface and velocities were measured. According to the computed averaged bore celerity (around 2 m/s), this resulted in a 0.4 s time lag between measurement (swash velocities at x-location − 0.8 m occur 0.4 s ahead of water level measured at x = − 0.20 m). The lack of velocity data existing in the figure is due to the sensor emergence time periods. Fig. 6 demonstrates test repeatability, despite the fact that bottom evolution occurred between test 6, 10 and 12 (morphologic evolution occurred under the attack of 2000 erosive waves between tests 6 and 10, and 1000 erosive waves between test 10 and 12). Incident bores are of the same height and occurred in the same time sequence. Horizontal velocities also show an identical pattern despite some high-frequency oscillations, which might be attributed to turbulence stochasticity or small bed evolutions. Similarly, the sediment concentration values were very similar in magnitude and timing, especially for the larger suspension events and similar tests (e.g. tests 10 and 12). Small differences are partly attributed to the randomness in the sediment suspension mechanism in relation to the hydrodynamics and bed evolution between tests. The same identical patterns were found throughout the whole time series. More information can be found in Alsina et al. (2009b). Sediment suspension repeatability

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decreases between tests conducted further apart, since the bed evolution is more pronounced. These results are encouraging with respect to experimental repeatability and shed light into the understanding of the sediment suspension processes. It is also evident from Fig. 6 that sediment suspension occurs in an event-like manner and that suspension events do not respond well to incident bore height or to horizontal velocity, i.e. the highest bores do not promote the highest concentration, and the sand appears not to react to the highest negative/positive horizontal velocities. This statement is further discussed below. Figs. 7 and 8 show water surface elevation, horizontal velocity, computed TKE values and suspended sediment concentration corresponding to the same time span measured during test 10 at two different locations within the inner surf zone. In Fig. 7, the ADVs and OBSs were located at −2.71 m, while in Fig. 8 the ADVs and OBSs were located at − 0.8 m. In both figures the water surface was measured at x = −0.20 m. There was a time-lag between water surface elevation time series and velocity and sediment suspension time series due to the space shift in sensor locations. This time shift is estimated to be 0.4 s at cross-shore location x = − 0.8 m and 1.1 s at cross-shore location x = −2.71 m. The incident and the long-wave component are obtained as explained in Section 2.3 and plotted in these figures for the water surface elevation and horizontal velocities. Sediment suspension events are noticeable again in an event-like manner at both locations with a

similar timing in many peaks of sediment suspension. A slightly larger number of sediment suspension events of higher concentrations were observed at cross-shore location x = − 0.8 m (Fig. 8) than at location x = − 2.71 (Fig. 7). Notice the different y-axis scaling for sediment concentration and TKE existing at Figs. 7 and 8. It is also apparent that sediment suspension events do not respond well to horizontal velocity, incident frequency, or the longer period component. This also happens with the TKE: sediment suspension does not correlate to the computed TKE at the time scale of short or long waves. However, Figs. 7 and 8 show that sediment suspension events occur in the combined presence of the long period water level oscillation trough and high incident bores (refer to events happening at time instants around 690, 736, 756, 800, 900, 950 s), corresponding to incident bores in shallow waters. These events are not always correlated with peaks of high horizontal short wave velocities. This pattern repeats during the entire series recorded at different tests. The inner surf zone during the present tests was characterized by incident waves propagating as turbulent bores over a longer period oscillating water level due to wave groups. A characteristic period of around 25 s is observed between significant low-water levels which correspond to the incident bound long wave (0.04 Hz) explained in Section 3.1. During the periods in which the water level is low (depression of long wave water level oscillations), the sandy bottom feels the effect of incident bores promoting sediment stirring. These

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incident bores in low water levels and high suspension surprisingly do not seem to correspond to high values of TKE when bores are expected to inject high turbulence values into the water column. Indeed, sediment suspension tends to occur at the bore front and at the sudden exchange between offshore and onshoreward velocities, while the highest turbulent kinetic energy (TKE) values for a single wave event occur just after the maximum onshore peak. Fig. 9 displays water surface elevation for cross-shore location x = 1.3 m, horizontal velocities, turbulent kinetic energy and suspended sediment concentration at the cross-shore location x = 0.9 m during test 10. This location corresponds to the low swash area. Bore collapsing and the run-down limit is variable due to the long-wave shoreline oscillation that makes some bores collapse close to this location. Wave–swash interaction takes place when one run-up overcomes the preceding one or one backwash encounters the following run-up (Hughes and Moseley, 2007). The sediment suspension pattern is influenced by this hydrodynamic pattern. Again incident and long wave components are displayed for horizontal velocity and water surface elevation. The space difference between water surface and velocity measurement locations is 0.4 m, resulting in a time lag of around 0.2 s. Missed values of sediment concentration, velocity and TKE occur when the sampling area was too shallow or emerged and the ADV and OBS

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sensors (located 3 cm from the bottom) could not measure. Sediment suspension occurred in an event-like manner and no evidence of strong correlation between incident short horizontal velocity, computed TKE and suspended sediment concentration was observed. The occurrence of high concentration events coincident with the combination of minimum water levels of the long wave oscillation and incident bores is also apparent in the low swash area. However, this tendency is less clear than at the inner surf zone, due to the fact that high concentration events are more frequent in this location than in the inner surf zone and can be better correlated to high incident bores. In this zone, incident bores occur over the emerged beachface or in very shallow waters most of the time (shallower than in the inner surf zone), and the bottom is more likely to “feel” the wave height impact, not only at minimum long wave water. Previous works (Baldock et al., 1997) have shown that when significant wave grouping is apparent at the shoreline, bore height modulation induces swash low frequency motion. During the longwave run-up and run-down, incident bores reach the preceding bore in a wave–swash interaction of type wave capture, i.e. the next arriving wave overcomes the preceding swash event (wave uprush or weak wave backwash interactions). On the other hand, at the depression of the long-wave water level oscillation, strong wave–backwash

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sensors). Moreover, this interaction is translated into a momentum exchange between the water mass receding during the backwash and the incoming mass of water with the following uprush (Erikson et al., 2005), which sometimes results in a smaller velocity peak (due to opposite direction momentum exchange). This contributes to reduce the correlation between horizontal velocity signal and suspended sediment concentration.

interaction are more likely to occur due to the interaction between the end of strong long-wave backwash and the next arriving wave, resulting in a hydraulic jump (Hughes and Moseley, 2007). The obtained data suggest this kind of wave–swash interaction to be efficient at setting sediment in suspension (see details at times 800, 900 s in Fig. 9), which has probably been mobilized during the preceding backwash (although the sediment mobilized during backwash might not be captured by OBS

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The moving averaging window is set to 5 s and the PMCC value is displayed in the figures. It is observed that the SSC–u2 correlation computed only during low water levels almost doubles the SSC–u2 correlation coefficients (Fig. 11c respect to Fig. 11a), although moderate correlation values were still observed. Nevertheless, the highest suspended sediment concentration values occur for low values of the long wave oscillation (Fig. 11b). Fig. 12 displays similar correlation plots but computed at the crossshore location x = − 0.8 m. The SSC vs u2 correlation computed during the trough of the long wave water level oscillation slightly improve the correlation. However, it is clearly observed (Fig. 12b) that the highest suspended sediment concentration values are confined to low values of the long wave oscillation. In the location corresponding to the swash zone (x = 0.9 m), the SSC–u2 correlation display a behavior similar to Fig. 12 with a PMCC-R value of 0.32 (figure not displayed). In this location, the SSC vs u2 correlation computed during the trough of the long wave water level oscillation does not improve the correlation values although, again, high values of SSC are found for low values of the long-wave water level oscillation. Correlation between SSC and horizontal velocity acceleration, and between SSC and vertical velocities have been also computed. They showed low correlation values in general, and the computed values were always below 0.2.

Detailed sediment suspension events are illustrated in Fig. 10 where water surface elevation, horizontal velocity and sediment concentration for cross-shore location x = −0.8 m (left pictures) and 0.9 m (right pictures) are shown for test 10. In these plots, total component and low-pass filtered signal of water surface elevation and horizontal velocity are depicted. Sediment suspension events during incident bores occurring in the trough of the long wave modulation, and the relative importance of long wave backwash–bore interaction are illustrated. Long wave horizontal velocity modulation indicates that most of the time high concentration coincides with negative velocities, resulting in shoreface erosion, this is consistent with visual observation during experiments of sediment clouds drifting seaward after suspension. The correlation between suspended sediment concentration, water surface elevation, different velocity parameters and turbulent kinetic energy has been computed. This correlation was calculated for moving averaged values in order to remove possible time lags between instruments, missing data and random effects which might affect it. The Pearson's product-moment correlation coefficient (PMCC) was computed. PMCC between two random variables X and Y is typically defined as the covariance of the two variables divided by the product of their standard deviations: RX;Y =

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PMCC oscillates between −1 and 1 indicating perfect negative/ positive linear dependence between variables and as it approaches to zero there is less of a relationship. PMCC values have been computed at every instrument location. The computed PMCC values indicate that no correlation existed between SSC and TKE values with Pearson coefficients (R) below 0.1 in the inner surf and swash areas for any test. It also suggests that very low correlations are found between SSC and incident bore height (R ≤ 0.1) and low correlations between SSC and absolute values of incident horizontal velocity (Rb 0.25). However, moderate correlations (R~ 0.3–0.4) were observed between SSC and the square of the short wave velocity (u2). Fig. 11 displays correlation plots for test 10 and cross-shore location x = −2.71 m between moving averaged values of: SSC and u2 (Fig. 11a), SSC and the long wave component of water surface elevation (Fig. 11b), and SSC and u2 values coincident with the trough of the long wave oscillation of the water level (Fig. 11c).

4. Discussion The results presented here are based on the analysis of water surface elevation, velocity and sediment concentration data obtained very close to the shoreline (inner surf and low swash zone) at a largescale mobile bed facility during highly energetic hydrodynamic conditions. Sediment suspension displays an event-like pattern in the three selected cross-shore locations, which do not correlate well with the measured horizontal velocity or computed TKE at the short or at the long wave scale. Interestingly, similar event-like patterns have been reported in suspended sediment measurement in the outer surf zone with no correlation with incident velocity or turbulence values but associated with the passage of wave groups (Nielsen, 1992). In the data presented here, the event-like pattern is repeated during successive identical time series, suggesting a deterministic suspension mechanism behind the apparent randomness.

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High sediment suspension occurs due to incident bore occurring during minimum values of the long water level oscillation. This correlation is better observed at the cross-shore locations identified as the inner surf zone than at the cross-shore location recognized to be the low swash area. This is attributed to the higher emergence ratio in the low swash location, which makes short wave bores impact in lower water levels rather than in the inner surf zone. When the water level decreases due to long period water level oscillations, the run-down cross-shore location reaches minimum values and long and strong backwash occurs (Nielsen, 2009), which encounters the next incident bore and induces suspension of the sediment previously moved during the backwash sheet flow phase. This mechanism is also associated with the presence of highconcentration events in the low swash area. Long-wave backwash is believed to be responsible for large beach erosion during storm conditions (Nielsen, 2009). Sheet flow sediment transport might occur underneath of the OBS vertical location (~4–5 cm) during the backwash and not being detected by sensors. Computed correlation coefficients support the observed pattern, showing poor correlations between SSC and incident bore height, SSC and TKE or SSC and the absolute values of incident and long wave components of the horizontal velocity. Moderate correlation values are found between SSC and the incident square horizontal velocity. This correlation increases when computed at the trough of the long wave water surface elevation at the inner surf zone, almost double at the cross-shore location x = − 2.71 and does not show any improvement in the swash zone. The bore impact sediment suspension mechanism at low water levels is not correlated with computed values of TKE, and show a moderate correlation with incident square velocity values. At minimum long wave water levels, strong bore–backwash interactions are more likely due to long-wave backwash and incident waves. These interactions result in momentum exchange (a thin fast water layer reversing, which meets a fast incident bore) responsible for relatively small velocity peaks which might partially explain the observed moderate correlation. The non-correspondence of high TKE values observed at this suspension event is still not clear. Broken waves inject turbulence and air into the water column, which has been postulated as a sediment suspension driver (Aagaard and Hughes, 2006). In the present data set, high values of TKE were usually found after the passage of the bore front, indicating that turbulence produced at the bore was advected or diffused downward (Cox and Shin, 2003; Deigaard, 1993), while sediment suspension tended to occur at the bore front or at the velocity inversion point from minimum negative velocity peaks to positive values. Moreover, computed TKE values were not sensitive to long wave oscillation like sediment suspension. Nielsen (1992) reports that broken wave turbulence has no effect in the sediment pickup rate and, hence, in the concentration very close to the bed except for extreme cases of a plunger hitting the bed, although turbulence can affect the vertical sediment mixing. Data in Figs. 7 and 8 and computed correlation at different bed level elevations equally showed poor correlations between sediment concentration and TKE. The authors believe that during an important part of the measuring period of time, the ADV sensors might be located outside the boundary layer. Therefore, it is expected that a larger scale turbulence than computed here (f ~ 1– 6 Hz) might display a better correlation to suspended sediment concentration since it will show the signal of suspended sediment trapped in larger eddies. Some authors have also suggested that there are other relevant mechanisms not related to turbulence inducing suspended sediment transport in the inner surf zone and swash zone. An example is the influence of dynamic pressures on the sea bottom (Suzuki et al., 2009). Nevertheless, it is acknowledged that the methodology used to compute TKE in the present work is rather crude and more sophisticated measurements are needed.

The arrival and collapse of short-wave bores on the shoreline and their influence in inducing sediment mobilization have been previously reported by different authors. The bore collapse at the shoreline and the transition from a turbulent bore type wave movement to a run-up type flow has been described by Yeh (1991). Hughes et al. (2007), on the other hand, suggested that incident bores are efficient at mobilizing sand when they get very close to the still water line and the relative ratio bore height to water depth is close to 1. Alsina et al. (2009a) measured sediment mobilization induced by solitary broken waves and maximum distances from the still shoreline in which the bottom feels the influence of the bore height. An approximate bore height to depth ratio of 1 was also found. Hsu and Raubenheimer (2006) suggested that this ratio might be higher under random waves due to wave–wave interactions and the subsequent enhanced turbulence. The present data demonstrated the definite influence of long wave nearshore water level fluctuations on the sediment suspension induced by incident bores, and indicated poor correlations between bore injected turbulence and sediment suspension. The long wave component also seemed to have an impact on the overall sediment transport process. The highest concentration occurring at the long wave trough coincided with the trough of the long wave modulation of horizontal velocity (negative velocities). To illustrate this, suspended sediment transport rates were computed from velocity–concentration time series. The velocity–concentration product was decomposed following a Reynolds decomposition of velocity and time and vertically integrated. The velocity was decomposed in a low (≤0.1 Hz) and a high (N0.1 Hz) frequency component as described in Section 2. The computed suspended sediment transport was calculated as: QS =

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where ulow is the low frequency (long wave) velocity component, uhigh the incident velocity component, C is the sediment concentration and T is the total time in the time series. The vertical integration is performed only up to the highest sensor and is quite rough since only one or two vertical locations are employed. Nevertheless, it provided good information on the transport magnitude for comparison between different cross-shore locations and different component contribution. It should be noted that only suspended transport occurring at vertical elevations higher than 4 cm from the bed were computed and bed load transport was dismissed (bed load here means every sediment transport occurring in a layer lower than 4 cm). Sediment transport occurring as bed load is believed to be an important part of the total sediment transport rate (Masselink et al., 2009) although its relative importance has not been properly quantified yet. Note also that velocity was decomposed in a low- and a highfrequency component and a mean velocity does not appear in Eq. (5)). This is because very close to the shoreline, emergence–submergence periods make it difficult to define a mean depth or velocity (as usually done in the surf zone) and, although mathematically a mean velocity can be computed (velocity at emerged time periods not considered in the computation), it is difficult to physically justify a mean velocity in this area that would be the result of asymmetries in the incident or low frequency velocity components. The obtained suspended sediment transport rates for test 10 are illustrated in Fig. 13. It is observed that higher sediment transport rates occur close to the shoreline. The long-wave component results in a negative suspended sediment transport (seaward). Although shortwave component of velocity (induced by incident bores) is positive, this signal is modulated by the long-wave component rising to predominantly negative velocities occurring at the time interval of high sediment concentration (see also Fig. 10). The obvious outcome

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is that suspended sediment is moved mostly offshore, promoting erosion close to the shoreline. Some authors have previously reported an erosive tendency with long wave dominance in the swash area (Miles et al., 2006). This is consistent with the present data, which show an important relation between beach erosion and sediment suspension induced by long wave components. The long-wave component influence in the sediment suspension and suspended transport is more evident in the inner surf zone than in the lower swash area, and it is expected that its impact diminishes in the upper part of the swash area where the incident bore influence is predominant. Computed total transport showed an inflexion from negative to positive sediment transport close to the shoreline. However, the analysis should be carefully interpreted due to the low resolution in vertical instrument spacing and also notice that bed load transport occurring during the backwash might modify this sediment transport distribution. A cross-shore location in the swash area at x ~ 6.5 m is found (see Fig. 3), in which the erosion observed in the inner surf and approximately two thirds of the seaward part of the swash locations turned into accretion and berm development. This has also been reported by Miles et al. (2006) who found an erosive behavior in the seaward half part of the swash but accretion in the shoreward half of the swash area. However, sediment suspension in locations of the shoreward half of the swash area was not measured in the present work. 5. Conclusion The present work provides new morphodynamic experimental data obtained on inner surf and swash zones at a large-scale facility operating under high energetic conditions. Identical erosive time series were generated during the experiment in 47 successive tests. The resulting beach evolution revealed shoreline erosion and the development of a bar during the initial tests, with successive seaward movement throughout the experiment. The results presented herein correspond to experiments scaled in relation to a prototype according to a Froude scale ratio of 1:1.9 (large scale). Overall, beach behavior and hydrodynamic properties correlated well with prototype measurements as reported in Cáceres et al. (2008). Hydrodynamic conditions in the area closest to the shoreline (inner surf, swash) are driven by the combination of broken incident waves and a long-wave oscillation of the water level. The oscillating long wave is dominated by the influence of wave groups.

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Hydrodynamic properties obtained during the successive tests were highly repeatable despite morphological evolution and morphodynamic feedback. The sediment suspension pattern occurs in an event-like manner but it is also highly repeatable between tests. The suspended sediment concentration in the inner surf zone and low swash area is significantly influenced by short broken waves and long wave structure. High concentration sediment suspension events were shown to occur in a combination of long wave trough and incident bores. This tendency was clearer in the inner surf zone than in the low swash area due to bore incidence with lower water levels in the swash than in the inner surf zone. High sediment concentration events showed low correlation with horizontal velocity at short wave frequency or at long wave component and with computed TKE values. Low to moderate correlation between suspended sediment concentration and square incident horizontal velocity was found, and this correlation improves when computed at the trough of the long wave water level oscillation. The sediment suspension mechanism is, therefore, explained by the incidence of short broken waves in periods of low water levels or emergence induced by long-wave water level oscillation. It is also suggested that the role of the wave–swash interaction takes place when the long period backwash meets the next arriving short bore inducing sediment suspension at the low swash area. Long wave impact on suspended sediment transport was also observed to be important because of horizontal velocity long wave modulation, which makes high sediment concentration events coincident with negative (seaward) velocity modulation, enhancing the sand moving from the low swash, inner surf zone towards offshore. The results presented herein are characteristic of a mild slope beach with energetic wave conditions, resulting in the Iribarren number of 0.4. Plunging breaker waves were apparent and shoreline dynamics were largely influenced by incident bores and a long wave oscillating water level closely related to incident wave groups. In such conditions, both the long wave shoreline oscillation and incident bores have been shown to influence sediment suspension, and both phenomena should be considered when detailing modeling of sediment dynamics in inner surf and swash zones. This situation might be different to low Iribarren number situations in which the long wave seems to dominate the shoreline dynamics, or high Iribarren numbers in which the shoreline might be dominated by incident bores. Acknowledgement The data presented here were measured within the framework of the European Hydralab-III SANDS Project (contract number: 022441) (RII3). First author support through a “Juan de la Cierva” Spanish Education Ministry fellowship is greatly appreciated. The authors wish to thank all those who have collaborated in the SANDS experiments, and especially Joaquim Sospedra. The comments of two anonymous reviewers have contributed to improve the paper significantly. Finally, inputs and comments from Peter Nielsen, Maurizio Brocchini and Carlos Gonzalez are much appreciated. References Aagaard, T., Hughes, M.G., 2006. Sediment suspension and turbulence in the swash zone of dissipative beaches. Marine Geology 228 (1–4), 117–135. Alsina, J.M., Baldock, T.E., 2007. Improved representation of breaking wave energy dissipation in parametric wave transformation models. Coastal Engineering 54 (10), 765–769. Alsina, J.M., Falchetti, S., Baldock, T.E., 2009a. Measurements and modelling of the advection of suspended sediment in the swash zone by solitary waves. Coastal Engineering 56 (5–6), 621–631. Alsina, J.M., Sánchez-Arcilla, A., Cáceres, I., 2009b. Suspended sediment fluxes in the inner surf and swash zones. Large scale data under erosive wave conditions. Coastal Dynamics'09, Tokyo, Japan. Baldock, T.E., 2006. Long wave generation by the shoaling and breaking of transient wave groups on a beach. Proceedings of the Royal Society of London, Series A 462, 1853–1876.

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