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Continental Shelf Research 28 (2008) 505–515

Seiche-forced resuspension events in a Mediterranean harbour

A. Jordia,�, G. Basterretxeab, B. Casasb, S. Anglesc, E. Garcesc

aMarine Sciences Research Center, State University of New York, USAbIMEDEA (UIB-CSIC), Illes Balears, Spain

cInstitut de Ciencies del Mar (CMIMA-CSIC), Barcelona, Spain

Received 5 July 2007; received in revised form 29 August 2007; accepted 30 October 2007

Available online 17 November 2007

Abstract

The variability of suspended sediments in a Mediterranean harbour was studied using high-frequency moored instruments. Data

analysis revealed that the fundamental mode of oscillation or seiche is the main contribution to the sea level variability and currents. Yet

a direct relation between the seiche and suspended sediment was not clear. To study the sediment dynamics, a coupled hydrodynamic-

sediment one-dimensional vertical model for sand and non-cohesive mud was implemented. Results showed good agreement between

most of the observed and simulated resuspension events. Indeed, the vertical extension and the duration of these events were

satisfactorily simulated for mud class. Model simulations were used to determine the time delay between suspended sediment and

currents, which improved the correlation between the seiche and suspended sediment to some degree. Scarce events of sediment advection

from outside waters and fresh water discharges from land-based sources were identified. It is suggested that phytoplankton species

detected in the harbour were responsible for discrepancies between observations and simulations.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Sediment dynamics; Sea level; Seiches; Coastal currents; Modelling; Harbours; Mediterranean Sea

1. Introduction

A good understanding of sediment dynamics in coastalmarine ecosystems is a topic of key relevance for coastalmanagement. The transport and fate of sediments play amajor role in marine ecology, water quality and morpho-logical evolution of shallow areas. Suspended sedimentmay prevent sunlight penetration into the water affectingsubsurface biological habitat and primary production.Additionally, sediments discharged from the continent intothe marine environment tend to supply pollutants andorganic matter for the coastal marine system. These effectsare particularly intense in enclosed and semi-enclosedenvironments such as lakes, harbours and embaymentswhere advective water renewal is restricted.

Various hydrodynamic processes determine sedimentresuspension, transport and deposition into the marine

system. Waves and currents generated by high energyevents (i.e. storm) are often considered as a predominantdriving force for sediment resuspension (Ogston et al.,2000). River flooding and tidal currents are also majorprocesses driving sedimentation on river- and tide-domi-nated continental shelves, respectively (Ribbe and Hollo-way, 2001; Fan et al., 2004). The vertical sedimenttransport is controlled by sediment settling, verticaldiffusivity generated by the turbulence and vertical motionof the water particles. Conversely, the advection is the mainmechanism that transports horizontally suspended sedi-ment away from resuspension areas and other inputsources such as rivers (Wang and Pinardi, 2002).When considering resuspension in semi-enclosed bodies

of water such as estuaries or harbours, it is necessary toexamine the possible effects of other hydrodynamicprocesses. In particular, seiches have been proposed aspossible mechanisms for sediment reworking and transport(Pritchard and Hogg, 2003b), specially where tides are notnoteworthy. To our knowledge there are no previousstudies that demonstrate seiche-dominated sediment trans-port in harbours, although it has been observed in lakes

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0278-4343/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.csr.2007.10.009

�Corresponding author.

E-mail addresses: ajordi@notes.cc.sunysb.edu (A. Jordi),

vieagbo@uib.es (G. Basterretxea), benjamin.casas@uib.es (B. Casas),

sangles@icm.csic.es (S. Angles), esther@icm.csic.es (E. Garces).

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(Bloesch, 1995; Shteinman et al., 1997). Seiches are naturalmodes of oscillations at long periods in the range of one toseveral minutes. External factors such as strong winds orlong atmospheric waves can excite natural modes byresonance and cause intense currents and sedimentresuspension. Although all semi-enclosed basins experiencecertain degree of oscillatory movements, some harboursexperiment considerable seiches (e.g., Monserrat et al.,1991).

The objective of this study is to describe the seichingmotion as the principal mechanisms that contribute to thesediment resuspension and control the variability of thesuspended sediment particles within the water column in aMediterranean harbour (Arenys de Mar). In order toachieve this goal, we first determine the role of seiche in theharbour dynamics using time series of sedimentologicaland hydrodynamic parameters. Next, a sediment transportequation is incorporated into a one-dimensional verticalnumerical model based on Mellor–Yamada level 2turbulence closure scheme (Chen et al., 1988) to providemore insight into the physical processes that determineresuspension, transport and deposition of suspendedmatter.

2. Material and methods

2.1. Study site

Arenys de Mar is a small harbour located on coast ofCatalunya, in the northwestern Mediterranean Sea (Fig. 1).

It comprises a winding elongated basin of about 1200mlength and two small sub-basins, one near the entrance andanother near the northeastern corner. Averaged harbourdepth is 4m, reaching depths of up to 6m near theentrance. The harbour is protected from offshore seas andgeneral winds by straight sidewalls. Forcing by tides isalmost negligible in the Mediterranean, with a spring tidalrange of less than 0.25m (Tsimplis et al., 1995). Theharbour receives occasional fresh water discharges fromland-based sources in heavy-rain episodes.

2.2. Field observations

A bottom mounted ADCP and a SBE26 sea levelrecorder were deployed in Arenys de Mar harbour from29th October to 21th November 2005 at locations shown inFig. 1. Vertical profiles of current velocities and acousticsignals were measured at 0.5m depth intervals with thebottom mounted ADCP (1MHz Nortek Aquadopp) set tomeasure at 60 s intervals. Sea pressure data were obtainedevery 30 s with the SBE26 sea level recorder. The originalpressure time series was adjusted to sea level with atmo-spheric pressure obtained at a nearby station (scale factor1 cmmb�1).Suspended sediment concentration was derived from the

acoustic signal recorded by the ADCP. Once the signal wasconverted to decibels, sediment concentration was esti-mated using a random phase acoustic backscatter model asdescribed by Thorne and Hanes (2002)

10 logðcÞ ¼ Bþ K0, (1)

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2

68

12

13

17

21

23

24

25

26

ADCP

SBE26

0 100 200 m

5°W 0° 5°E 10°E 15°E

36°N

38°N

40°N

42°N

44°N

Arenys

de Mar

Mediterranean Sea

Fig. 1. Map of the Arenys de Mar harbour and location of instruments (ADCP & SBE26) and bottom granulometric samples.

A. Jordi et al. / Continental Shelf Research 28 (2008) 505–515506

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where c the suspended sediment concentration (in10�3 kgm�3), B is the corrected signal or echo intensity(dB) and K0 an empirical constant (dB), which is usuallydetermined by calibration against water samples. In thiscase, a value of K0 ¼ 36 dB was selected, which are withinranges of those used by other researchers (e.g., Souza et al.,2004; Ferre et al., 2005).

During 28th October 2005 CTD profiles and sedimentsamples were obtained at 11 sites in order to obtaininformation about hydrology and sediment granulometry(see Fig. 1). Samples were washed, had the organic matterremoved and oven dried in the laboratory a few hours aftercore subsampling. Sediment grain-size was analysed using asettling tube (sand fraction) and a Sedigraph 5100 (mudfraction).

2.3. Model description

We used a coupled hydrodynamic-sediment one-dimen-sional vertical model in order to analyse the resuspensionevents and the variability of the suspended sediment withinthe water column. Horizontal advection is assumed to below for most of the time since harbours are characterizedby reduced advective exchanges. As we will see, this is asevere assumption for the seiche. However, we note thatthe contribution to sediment transport arising from theseiche may be negligible over the time scale of a seiche. Thegoverning equations are

qu

qt� fv ¼ �

1

rqp

qxþ

qqz

Km

qu

qz

� �, (2)

qv

qtþ fu ¼ �

1

rqp

qyþ

qqz

Km

qv

qz

� �, (3)

qT

qt¼

qqz

Kh

qT

qz

� �, (4)

qS

qt¼

qqz

Kh

qS

qz

� �, (5)

qci

qt¼

qqz

Kh

qci

qz

� �� ws;i

qci

qz, (6)

where t is time (s), x, y and z the space coordinates (m), u

and v the horizontal components of current velocityðms�1Þ, r the water density ðkgm�3Þ, T the temperature(1C), S the salinity (psu), ci the suspended sedimentconcentration ðkgm�3Þ for each sediment size class i, p

the pressure (Pa), f the Coriolis parameter (s�1), Km and Kh

the vertical eddy viscosity and diffusivity coefficientsðm2 s�1Þ, respectively, and ws;i the settling velocity ðms�1Þfor each sediment size class i. Both viscosity and diffusivitycoefficients are computed using the Mellor–Yamada level 2turbulence closure scheme, which allows coefficients to beas small as 10�5 m2 s�1. A detailed description of thehydrodynamic model was given by Chen et al. (1988).

Several assumptions were made to describe the sedimentdynamics. We first treated all sediment fractions as non-cohesive and the sediment mixture as a Newtonian fluid.Furthermore, we assumed that suspended sediment con-centrations were small and had no effects on the waterdensity (Wang and Pinardi, 2002). Finally, we only tookinto account suspended-load transport and the seabedprovided an indefinite source for the resuspended load.Eq. (6) is solved separately for each sediment size class.

Independent solvers are used for the handling of verticaldiffusion and vertical settling. The vertical diffusionscheme is similar to the equations for temperature andsalinity (Eqs. (4) and (5)) in the hydrodynamic model(Chen et al., 1988). Following Blaas et al. (2007), thevertical settling scheme includes a piece-wise parabolicmethod (Colella and Woodward, 1984) and a weightedessentially non-oscillatory scheme (Liu et al., 1994). Thesettling velocities for sediment particles are computedaccording to Stokes law

ws;i ¼gd2

i

18nrs

r� 1

� �, (7)

where di is the sediment grain diameter (m) for each size-class, n the molecular kinematic viscosity ðm2 s�1Þ and rs

the sediment grain density ð2650 kgm�3Þ.At the sea surface, sediment fluxes and point sources or

sinks are absent. At the sea bottom, the net sediment fluxðkgm�2 s�1Þ is the sum of the deposition flux (�wsci) andthe resuspension flux (Ri) at the seabed for each size class.The resuspension flux for each size class is computed usingthe formula

Ri ¼ R0f i

tb

tc

� 1

� �for tb4tc, (8)

where R0 is an empirical entrainment rate ðkgm�2 s�1Þ, f i

the volumetric fraction of sediment of class i, tb the bottomshear stress ðkgm�1 s�2Þ and tc the critical shear stressðkgm�1 s�2Þ. The entrainment rate may depend uponproperties such as sediment type, composition, density,and thickness (Ribbe and Holloway, 2001). Although thereis considerable uncertainty associated with the entrainmentrate, we selected a constant value of 10�4 kgm�2 s�1, whichranged between typical literature values (e.g., Clarke andElliott, 1998; Blaas et al., 2007; Stanev et al., 2007).The bottom shear stress is computed through the

relationship

tb ¼ rCdðu2 þ v2Þz¼za

, (9)

where

Cd ¼k

lnððH þ zaÞ=z0Þ. (10)

Cd is the drag coefficient, k the Von Karman constant, H

the total water depth (m), za the reference height above thebed (m), and z0 the bottom roughness (set to 10�2 m).

ARTICLE IN PRESSA. Jordi et al. / Continental Shelf Research 28 (2008) 505–515 507

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The critical shear stress is determined using the Shieldsfunction

tc ¼ r0:1g rs � r

� �n

r

� �2=3. (11)

The resuspension flux (Ri) is zero for tb smaller than tc.

3. Results

3.1. Data analysis

The sea level spectral content was estimated using aHamming window of 16 384 points with half windowoverlapping (Fig. 2). The presented spectra were addition-ally smoothed so that the resulting spectra averages anumber of adjacent frequencies increasing with frequency(Emery and Thomson, 1997). Major peaks were observedat tidal frequencies (24 and 12 h) and at approximately12.8min. This latter period is fully consistent with thefundamental mode of oscillation (seiche) for the geometryof Arenys de Mar harbour (L ¼ 1200m, H ¼ 4m)computed following Gomis et al. (1993), who analyticallysolved the linearized long-wave equations in a non-rotatingocean considering a flat-bottom rectangular inlet of long-itude L and depth H to obtain the modes of sea leveloscillation (seiche).

To find out which is the major forcing (seiche or tide) forArenys de Mar harbour, we explored the possibility thatcurrents may have a comparable evolution with theseiching. The energy corresponding to the seiche wascomputed by integrating sea level spectra between 11 and15min (the seiche band) at 3 h intervals. Fig. 3 shows acomparison between the time evolution of sea level energy

at the seiching band, and measured current velocities andsuspended sediment concentrations by the ADCP. Cur-rents were generally barotropic and highly correlated to theseiching motion. Correlation coefficients between seichingenergy and currents were around 0.9 at all vertical levels,which suggests that seiching was the main forcing.As expected, suspended sediments were mostly concen-

trated at deeper levels, but episodical enhancementsreaching the shallowest levels were observed. Despite thatsome peaks appeared to be correlated with major seichingevents, some enhancements occurred with negligible seicheoscillations. Particularly the events on November 1st and9th were not correlated with high levels of energy at theseiche band. Correlation coefficients between seiche andobserved suspended sediments were up to 0.5 at deeperlevels and decreased linearly to very low values (o0:1) atupper levels. We did not expect high correlation valuesbetween sediment and seiche (or currents) because of thedelayed response of sediment suspension to changes in thevelocity. The processes of scour and settling lag to explainthe time-dependent response of sediment suspension toflow velocity have been discussed by different authors (e.g.van Straaten and Kuenen, 1958; Postma, 1961; Pritchardand Hogg, 2003a). The scour lag is caused by the finite timefor the current speed to erode sediment and mix it upward.The settling lag is measured by the finite time which it takessediment to settle out of suspension.The bottom sediments in the harbour are composed

predominantly of very poorly sorted slightly gravelly sandymud with a mean particle size of 25:5� 10�6 m (Table 1).Most samples present trimodal size distributions combin-ing fine sand, very fine sand and silt, although in somecases the larger modes are not well differentiated (notshown). Fine sediments (silt and clay fractions) average68% of the sediments and present higher values towardsthe inner part of the basins (St 23, 13 and 17). Sands aremost abundant at the entrance (St 21) and along the N–Saxis (St 2, 26, 24 and 25). Coarse materials, mainly gravel,always represent less that 1% of the sample. Particle sizedistribution shows that, in contrast with the lower sortingcoefficient of the nearby coastal sediments (St 21,s ¼ 3:8� 10�6 m), very poorly sorted sediments (s between5� 10�6 and 9� 10�6 m) are observed within the harbour.Lower values (s�5� 10�6 m) correspond to the samplingpoints along the N–S axis (St 2, 26, 24 and 25) whichsuggests an exposure to enhanced water motion beforesedimentation in this area.

3.2. Model simulations

The model was integrated over the period from 29thOctober to 21th November 2005 with a time step of 1 s. Avertical grid of 30 uniform levels was used to simulate thewater column at the ADCP location (Fig. 1) with a depthof 6m. We considered two sediment classes, one being mudand the other sand with grain sizes and volumetricfractions obtained from mean sampled values (Table 1).

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Fig. 2. Sea level spectral density at the SBE26 location. Periods of the

major peaks are indicated.

A. Jordi et al. / Continental Shelf Research 28 (2008) 505–515508

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The initial profiles of temperature and salinity wereobtained from mean values of CTD measurements. Bothprofiles display a uniform distribution in the water columnaround 19:2 �C and 37.9 psu. The initial velocity fields andsuspended sediment concentration profiles for both classeswere set to zero.

The model was driven by sea level variation due toseiching motion (Zs). The sea level measured at SBE26 wasfiltered between 11 and 15min (the seiche band) in order to

obtain the sea level variation due to the seiche (Zs). Thevalues for each time step were obtained by interpolatingthe filtered time series data and were incorporated into themodel through the horizontal pressure gradient terms inEqs. (2) and (3). We assumed that the pressure gradientswere caused by a Kelvin barotropic wave, i.e.

�1

rqp

qx;qp

qy

� �¼

qZs

qt; f Zs

� �. (12)

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En

erg

y0

1

2

3

De

pth

(m

)

0

2

4

6 0

2

4

6

8

De

pth

(m

)

30 01 03 05 07 09 11 13 15 17 19

0

2

4

6 10-9

10-7

10-5

10-3

November 2005

Fig. 3. Time evolution of (a) spectral energy ðm2 sÞ at the seiching band (12.8min) at the SBE26 location computed at 3 h intervals, (b) 3 h averaged

currents ð10�2 m s�1Þ at the ADCP location, and (c) 3 h averaged suspended sediment concentration ðkgm�3Þ at the ADCP location. Horizontal black lines

represent the zone measured by the ADCP.

Table 1

Grain size (d) and volumetric fraction (f) of sediment classes and sorting coefficient (s) as sampled in 28th October 2005

Station Gravel Sand Fines s (10�6 m)

d (10�3 m) f (%) d (10�6 m) f (%) d (10�6 m) f (%)

2 42 0.2 147.6 45.2 17.3 54.6 5.1

6 42 0.6 177.6 26.3 14.0 73.0 6.3

8 42 0.8 202.7 27.6 10.2 71.7 8.6

12 – 0.0 178.3 46.1 13.6 53.9 6.1

13 42 0.4 161.3 13.0 10.0 86.6 5.8

17 42 0.5 114.0 16.9 13.7 82.6 5.8

21 – 0.0 114.7 55.5 21.6 44.5 3.8

23 42 0.3 280.6 25.5 10.0 74.2 9.1

24 42 0.4 133.8 36.7 17.1 63.0 5.4

25 42 0.6 128.6 37.5 18.0 61.9 5.4

26 – 0.0 125.1 18.3 14.9 81.7 5.5

Mean – 0.3 160.4 31.7 14.6 68.0 –

A. Jordi et al. / Continental Shelf Research 28 (2008) 505–515 509

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Although points where sea level was measured (SBE26)and where the model was applied (ADCP) are not thesame, this is a reasonably approach since they are close andboth inside the harbour. The main effect of limiting theresonant response in a harbour is due to seaward radiationof energy from the harbour mouth (Liu et al., 2003). TheCoriolis acceleration was included in the model because itmay be important when considering long periods (i.e.tides), but it could be justifiably ignored in the case ofseiche periods which are much shorter than the localinertial period. Neither wind forcing nor flux of heat or saltwas considered.

The evolution of simulated currents and turbulence overtime is illustrated in Fig. 4. Current variability was highlycorrelated to seiching motion, in good agreement withobservations. Correlation coefficients between observedand simulated currents were greater than 0.9. During mostof the time the entire water column showed very weakvertical velocity gradients. Only deeper levels experiencedrelatively strong vertical gradients since currents almostvanished near the bottom due to friction. Eddy diffusivitywas also correlated to the seiche, but high levels ofturbulence were restricted to deeper levels, where verticalvelocity gradients were greater.

Fig. 5 shows time evolution of suspended sedimentconcentration for both simulated particle classes. Ingeneral resuspension occurred during events of enhancedseiching motion, although there were large differences inresuspension efficiency of the mud and sand fractions. Themud was easily resuspended by the currents reachingdepths of around 3m. During the major peak onNovember 14th and 15th, the suspended sediment arrivedup to 2m. In contrast, sand spent much shorter time insuspension because of its higher settling velocity. Sand

suspended concentrations were limited to the first meterabove bottom.It is interesting to compare the simulated suspended

sediment with observations. We centred on the simulatedmud concentration because sand did not reach themeasured levels by the ADCP. Most of the simulatedpeaks coincided in time and vertical extension withobserved peaks. Discrepancies appear on the duration ofthose peaks. Correlation coefficients between observed andsimulated sediment concentrations were about 0.77 atdeeper levels (below 3.5m).

3.3. Sensitivity analysis

The sediment resuspension and deposition can beaffected by various parameters. We examined the effectsof bottom friction and settling velocity. We focused thesensitivity analysis on the suspended mud concentrationsince sand is hardly resuspended.To analyse the effects of bottom friction, the bottom

roughness was increased to 5� 10�2 m and reduced to 5�10�3 m (Fig. 6). The time variability of mud sediment wassimilar to previous simulation for both cases. However, theeffect of increasing (decreasing) bottom friction was anincrease (decrease) of resuspension events with higher(lower) sediment concentrations. Correlation coefficientsbetween observed and simulated sediment concentrationswere at deeper levels around 0.69 for increased bottomroughness and 0.79 for the reduced case. The increase onthe correlation in the second case should be attributed toreduced duration of resuspension events, rather to thedecrease in the number of these events.Effects of bottom friction on suspended sediments can be

interpreted in terms of resuspension and turbulence. First,

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De

pth

(m

)

0

2

4

6 0

2

4

6

8

De

pth

(m

)

30 01 03 05 07 09 11 13 15 17 19

0

2

4

6 0

1

2

3

November 2005

Fig. 4. Time evolution of simulated 3 h averaged (a) currents ð10�2 m s�1Þ and (b) eddy diffusivity ð10�4 m2 s�1Þ. Horizontal black lines represent the zone

measured by the ADCP.

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varying bottom roughness modifies near bottom currentsand drag coefficient, which play opposite effect on nearbottom stress and therefore on the resuspension flux. Inour simulations, the effect on drag coefficient dominated.Additionally, the bottom boundary layer also depends onbottom friction. The vertical extent of the bottomboundary layer was larger for an increased friction,enhancing vertical velocity gradients and turbulence inthat zone. In consequence, suspended sediment reachedupper levels than the case with reduced bottom roughness.

The settling velocity was determined using the Stokeslaw (Eq. (7)), however, higher sediment concentrationsmay form larger aggregates which in turn have largersettling velocities (Stanev et al., 2007). To evaluate thiseffect, as a first approximation the settling velocity wasdoubled and reduced by a half (Fig. 7). As expected, thesuspended mud concentration was higher and the durationof resuspension events was longer for reduced settlingvelocity, since the vertical velocity controls the time that asediment particle spends in the water column. At deeper

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De

pth

(m

)

0

2

4

6

November 2005

De

pth

(m

)

30 01 03 05 07 09 11 13 15 17 19

0

2

4

6 10-9

10-7

10-5

10-3

10-9

10-7

10-5

10-3

Fig. 6. Time evolution of simulated 3 h averaged suspended sediment concentration (kgm�3) for mud class varying the bottom roughness: (a) z0 ¼

5� 10�2 m and (b) z0 ¼ 5� 10�3 m. Horizontal black lines represent the zone measured by the ADCP.

De

pth

(m

)

0

2

4

6

November 2005

De

pth

(m

)

30 01 03 05 07 09 11 13 15 17 19

0

2

4

6 10-9

10-7

10-5

10-3

10-9

10-7

10-5

10-3

Fig. 5. Time evolution of simulated 3 h averaged suspended sediment concentration ðkgm�3Þ for (a) mud class and (b) sand class. Horizontal black lines

represent the zone measured by the ADCP.

A. Jordi et al. / Continental Shelf Research 28 (2008) 505–515 511

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levels, correlation coefficient between observed and simu-lated sediments were up to 0.83 for doubled settlingvelocity and 0.61 for the reduced velocity. This indicatesthat the formulation of settling velocity should beimproved for future applications.

3.4. Response time

To understand the different behaviour of the sedimentclasses, as well as the poor correlation between theobserved velocities and concentrations, we explored thesediment response time. Fig. 8 shows a comparisonbetween current speed and suspended sediments duringthe major seiche event (November 14th). The association ofpeaks of current speed (above a threshold) and delayedpeaks of suspended sediments was apparent. For bothsimulated sediment classes, the resuspension took placerapidly and sediments were mixed throughout the watercolumn. However, muddy sediments did not immediatelyfall out of suspension after the current peak and suspensionalso depended on antecedent peaks. Mud took significantlylonger to settle than predicted using its settling velocity(�10�4 m s�1), indicating that the turbulence generated bythe seiche kept the sediment suspended long after thecurrent dropped below the threshold required to resuspendthe sediment. Conversely, sand fall out of suspensionalmost immediately after the current peak, suggesting thathigher turbulence levels are necessary to keep the sand insuspension.

Following Stansby and Awang (1998), we defined thesedimentary response time scale as the time for suspensionrate to become 99% of the deposition rate. Vertical profilesof response time scales for mud and sand illustrate that thetime scale increased with the elevation above the bottom

(Fig. 9). As expected, response time scales were muchlonger for fine sediment (mud) than for sand, especially farfrom the bottom. Moreover, a considerable variability wasobserved in response time scales probably due to turbu-lence produced by the seiche. It is also noteworthy tomention that response time scale was not sensitive tovariations of bottom roughness (not shown). In the case ofsettling velocity, the sensitivity of response time scale wasinversely proportional to changes in settling velocity formuddy sediments, whereas no dependency was observedfor coarse sediment. Stansby and Awang (1998) predictedthis behaviour for fine and coarse sediments solvinganalytically and numerically an idealised situation of abed of sediment in a steady current with initially clearwater.We return now to the problem of estimating correlation

between seiche and sediment. We expected that incorpor-ating the response time scale (obtained from the modellingresults) improved the correlation between observed seicheand suspended sediment concentrations. Although thecorrelation did not increase substantially at deeper level(4.75m), it was roughly constant (0.5) until a depth of3.5m (the depth reached by the simulated mud sediment).This result represents an improvement respect to thecorrelation computed without the delayed time. However,a correlation coefficient of 0.5 should indicate that a part ofthe observed suspended sediment concentrations were notcaused by the seiche activity.

4. Discussion and conclusion

In this study, we have described the seiche in Arenys deMar harbour as the principal mechanism that controls thesediment resuspension and the variability of the suspended

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De

pth

(m

)

0

2

4

6

November 2005

De

pth

(m

)

30 01 03 05 07 09 11 13 15 17 19

0

2

4

6 10-9

10-7

10-5

10-3

10-9

10-7

10-5

10-3

Fig. 7. Time evolution of simulated 3 h averaged suspended sediment concentration ðkgm�3Þ for mud class varying the settling velocities for sediment

particles: (a) double and (b) half. Horizontal black lines represent the zone measured by the ADCP.

A. Jordi et al. / Continental Shelf Research 28 (2008) 505–515512

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sediment using both observations and model simulations.The agreement between observed and simulated currentsand their correlation with the seiche gave us confidence inthe model performance. However, there were somedifferences between the trend in observed and simulatedsuspended sediment concentrations. Some of these dis-crepancies might be explained with the sedimentaryresponse time and using an improved formulation ofsettling velocity. Other observed peaks of sediment were

not related to the seiche motion and, consequently, notreproduced by the model. The main point of discussion isfocussed on understanding these peaks.First, observed sediment concentrations displayed high

levels at the entire water column on November 1st and 9th,not reproduced by the numerical model. This suggests thatother processes which are not accounted for in the modelmay occur during these events. For example, the sharptemperature drop on November 1st measured by the

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Fig. 9. Vertical structure of mean response time scale for simulated mud and sand sediments. The thick solid line corresponds to the basic case, the thin

solid line to the half settling velocity and the dotted line to double settling velocity. The gray shaded area represents the variability of response time scale

for the basic case.

Cu

rre

nt

0

10

20

Mu

d

10-6

10-4

10-2

Sa

nd

19:15 19:30 19:45 20:00 20:15 20:30 20:4510-9

10-6

10-3

14th November 2005

Fig. 8. Time evolution of simulated currents ð10�2 m s�1Þ at 0.01m above the bottom, and suspended sediment concentrations ðkgm�3Þ for mud and sand

classes at 0.01, 0.03, 0.05, 0.07, 0.09, 1.10, 1.30 and 1.50m above the bottom (lines from top to bottom, respectively) for 14th November 2005 during 19:15

and 20:45. Vertical gray lines indicate the position of current peaks.

A. Jordi et al. / Continental Shelf Research 28 (2008) 505–515 513

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ADCP (Fig. 10a) suggests intensification of the exchangewith offshore waters and, presumably, entrance of sedi-ment. During November 1st, an intense storm with wavesabove 3.5m of significant height was registered in the area(data from Puertos del Estado). Moreover, considering desediment characteristics in the nearby coast and the factthat sands are most abundant at the bottom sedimentssampled at the entrance and along the N–S axis, it is clearthat occasionally water and sediment entrances shouldexist. On the other hand, the increase of sediments at upperlevels on November 9th was not related to water cooling,but coincided with an episode of heavy rain, as shown inFig. 10b.

Second, there were other peaks generally with lowconcentrations not captured by the model simulationsand not correlated with the seiche. Since the sea levelspectrum showed a contribution of tides in the sea levelvariability, we hypothesized that tidal generated currentswere the responsible for those peaks. The model was forcedby tidally induced sea level variations. Currents were smalland no resuspension was simulated (not shown). Althoughthe model (one-dimensional) cannot produce non-linearand/or bathymetric effects on tidal dynamics that maygenerated stronger tidal currents, the high correlation(40:9) between observed and simulated currents (forced bythe seiche) demonstrates that these effects are notsignificant in Arenys de Mar. This agrees with otherobservational and numerical studies of resupension in theMediterranean coastal areas (e.g. Guillen et al., 2006; Puiget al., 2007; Wang et al., 2007). Therefore, we discardedtide as an effective mechanism for resuspension in Arenysde Mar harbour.

Another possibility is that observed suspended sedimentconcentration was affected by the presence of biologicalparticles. Indeed, blooms of phytoplankton species arerecurrent in Arenys de Mar harbour from the last decadeand are frequently detected year-round at low concentra-tions (Vila et al., 2005; Garces and Vila, 2007). Speciesabundance usually displays a typical diel cycle of verticallymigrating organisms with maximum abundance in the

afternoon. This behaviour would fit with our observations.Furthermore, the dominant species in the bloom isAlexandrium minutum, which has a spherical form with adiameter of 20mm, similar to the sediment detected in theharbour.On the other hand, A. minutum develops a dormant

resting stage in response to physical, chemical andbiological conditions. Resting cysts remain buried in thesediment and survive for several years (Garces et al., 2004).However, knowledge on vertical cyst fluxes and excystmentprocesses which allow further outbreaks of A. minutum isnot well understood. These cysts with smaller density thansediment grains would be resuspended easily by nearbottom currents. However, more observations arerequired to support cyst resuspension or biological activeparticles affecting measured suspended sediment concen-trations. Furthermore, the role of seiching in the harbourand the associated resuspension should be investigatedin relation with blooms of phytoplankton species in Arenysde Mar.In summary, study of time series data collected in Arenys

de Mar emphasizes the key role of the fundamental modeof oscillation or seiche on suspended sediment dynamics inthe harbour. A coupled hydrodynamic-sediment one-dimensional vertical model was found to adequatelyreproduce the main resuspension events caused by theseiche and to quantify the time delay of the suspendedsediment in relation to seiche. Moreover, the analysis of thedifferences between observed and simulated sedimentconcentrations suggests that other processes such as highoffshore seas or heavy-rain episodes contribute sporadi-cally to the suspended sediment variability.

Acknowledgements

This study was financed by the EU funded ResearchProject SEED (GOCE-CT-2005-003875). A. Jordi workwas sustained by a Postdoctoral grant and E. Garces by aRamon y Cajal contract, both from the Spanish Ministryof Science and Education.

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Fig. 10. Time evolution of (a) sea water temperature at the ADCP location and (b) daily accumulated rain at a nearby weather location.

A. Jordi et al. / Continental Shelf Research 28 (2008) 505–515514

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