Natural Recovery of a Mixed Sand and Gravel Beach after a ...miguelos/documentos/Bramatoetal2011.pdf · 0 5 7–9 s; 0.19% occurrence per year; duration 36 h), followed by mild to

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Natural Recovery of a Mixed Sand and Gravel Beach after aSequence of a Short Duration Storm and Moderate Sea States

Simona Bramato, Miguel Ortega-Sanchez, Christian Mans, and Miguel A. Losada

Grupo de Investigacion de Dinamica de Flujos AmbientalesCentro Andaluz de Medio AmbienteUniversidad de GranadaAvenida del Mediterraneo s/n18006 [email protected]

ABSTRACT

BRAMATO, S.; ORTEGA-SANCHEZ, M.; MANS, C., and LOSADA, M.A., 2012. Natural recovery of a mixed sand andgravel beach after a sequence of a short duration storm and moderate sea states. Journal of Coastal Research, 28(1), 89–101. West Palm Beach (Florida), ISSN 0749-0208.

Using time-averaged images collected by a high-resolution camera, this paper examines the natural recovery of a mixedsand and gravel beach (Carchuna, Spain) after the passing of a westerly storm (significant wave height, H0 . 3 m;significant wave period, T0 5 7–9 s; 0.19% occurrence per year; duration 36 h), followed by mild to low-energy (H0 , 1 m,T0 5 4–6 s, 33.78%) and moderate-energy (H0 5 1–3 m, T0 5 6–9 s, 15.80%) sea conditions. The response of the beachafter two storms approaching from the east (0.28%) is also examined. The analysis focuses on one of the beach horn-embayment cells characterized by a bimodal sediment distribution, which is representative of the beach coastline.

The most significant morphological changes occurred after the westerly storm episode and consisted of a shorelineerosion in the order of 5 m/d with a reduction in the beach slope and the cusp horn cross-shore dimension. In conditions oferosion, the sand on the coastline was rapidly transported cross-shore, exposing the underlying gravel sediments whichwere dragged a short distance seaward to create a longshore bar. The bar subsequently acted as a natural barrier, whichhelped to protect the beach from further erosion.

During the mild to low-energy and moderate sea states which followed, the beach rapidly recovered and the coastlinemorphology showed a visible seaward advancement of the horn with an increase in the width of the cross-shore section incomparison to the initial prestorm profile. Through conditions of accumulation, the sand was resuspended and mobilizedonshore, creating a layer of sand along the shoreline and rapidly recuperating the beach to its original state.

Morphologic evolution of Carchuna is further investigated through several parameterizations on the forcing conditionand in consideration of sediment distribution and beach bathymetry. The results provide insight into the interrelationbetween the sand and gravel of a mixed beach, showing that bimodal beaches respond differently to commonly acceptedempirical sandy-beach relationships.

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ADDITIONAL INDEX WORDS: Sediment transport, beach profile, beach erosion and accretion, coastline.

INTRODUCTION

The shoreface is a transition zone between the beach and the

inner shelf, a complex, nonlinear dynamic element that

continuously approaches equilibrium states through the

persistence of morphodynamic stages that vary with changes

in the forcing conditions: wave, tide, wind, current, and their

combination (Backstrom et al., 2008; Baquerizo and Losada,

2008). The coastal-system response to forcing conditions is

through sediment transport; its patterns are controlled by

variables internal and/or external to the system, by forcing

factors, and by boundary conditions (Coco and Murray, 2007;

Klein, Nicholls, and Thomalla, 2003; Komar, 1976).

The coastal morphological response to external forcing

develops primarily in the temporal scale of the ‘‘storm event’’

(Baquerizo and Losada, 2008; Short, 1999), which includes the

time between the initiation of the forcing and the first

morphological response (reaction time) and the period in

which the coastline reacts to these perturbations by recuper-

ating to its original state (relaxation time). Reaction and

relaxation periods are not necessarily consecutive in time but

depend on the progression of events and a broad variety of

additional parameters (i.e., morphodynamic characteristics,

hydrodynamic processes related to the reflection and dissipa-

tion of incident energy, beach slope, etc.; see Avila, 2007).

Coastal orientation, shoreface morphology, and storm pa-

rameters induce spatial variation in storm response (Back-

strom et al., 2008). Sediment size distribution is of particular

importance, because coarse-sediment beaches are known to

respond faster than beaches with fine sand (Ivamy and Kench,

2006; Losada, Desire, and Merino, 1987). However, the

response of a mixed sand and gravel beach is not clearly

defined (Buscombe and Masselink, 2006; Lopez de San Roman-

Blanco et al., 2006), as most studies have focused on the

processes attributed to beaches with fine sand (Jennings and

DOI: 10.2112/JCOASTRES-D-10-00019.1 received 5 February 2010;accepted in revision 18 May 2010.Published Pre-print online 27 August 2010.’ Coastal Education & Research Foundation 2012

Journal of Coastal Research 28 1 89–101 West Palm Beach, Florida January 2012

Shulmeister, 2002). Presently, there is a growing interest in

properly defining the morphological processes of a mixed beach

due to the increased use of coarse sediments in the artificial

recuperation of eroded beaches, as they are characterized by a

higher hydraulic roughness and provide a better defense to the

forcing processes induced during storm events (Kirk, 1992).

Although comparatively rare on a global scale (Kirk, 1980;

Mason and Coates, 2001), mixed beaches are common along the

southeastern Spanish coastline (i.e., Motril coast). This

coastline is coming under increasing human pressure, and

recent beach erosion is requiring artificial nourishment to

protect human resources (Hanson et al., 2002). Beach replen-

ishments are frequently made with fine-sand deposits that are

quickly eroded during subsequent storm events, slowly

returning landward under calm conditions (Wright and Short,

1984).

In order to properly study the shoreline evolution of a mixed

beach, it is necessary to obtain high-quality data (temporal and

spatial resolution) under different forcing conditions. Although

they are advanced, contemporary models are still incapable of

predicting the interaction between coastal geology and marine/

atmospheric processes (Backstrom et al., 2008). Video-imaging

techniques have been an important tool for morphological

studies (Holman and Stanley, 2007), particularly for studying

bathymetry and sandbars (Aarninkhof et al., 2003; Plant et al.,

1999; Van Enckevort and Ruessink, 2001), but they have

seldom been applied to shoreline and beach face responses to

storm events.

This paper analyzes the natural recovery of Carchuna

Beach, a mixed sand and gravel beach located on the Motril

coast, to one westerly and two easterly storm events. Through

images collected from a high resolution camera, the erosion

and recovery processes of the beach are determined in the

temporal scale of the event. Sediment mobility is analyzed from

current measurements captured during a 7-day field survey.

Morphologic evolution is investigated through several param-

eterizations of the forcing (using offshore wave forcing and

nearshore conditions simulated with the Simulating Waves

Nearshore [SWAN] model), sediment distribution, and beach

bathymetry. Although the analyzed processes are short term,

the principal aim of this paper is to illustrate that the

commonly accepted management techniques for pure sand or

pure gravel beaches may not be applicable for mixed sand and

gravel beaches.

FIELD SITE DESCRIPTION: CARCHUNA BEACH

Carchuna is a 4-km-long reflective beach (Figure 1a; Wright

and Short, 1984) located on the southeastern coast of Spain

(Figure 1b). The coastline is characterized by a series of large-

scale cuspate features with an alongshore spacing in the order

of hundreds of meters, bounded by a series of seaward-

extending horns (H1–H6, Figure 1a). These horns have a

cross-shore length that varies with the wind and wave forcing

conditions; however, previous studies of this morphology show

that their alongshore spacing remains constant (Ortega-

Sanchez, Losada, and Baquerizo, 2003). The beach is bounded

to the west by Cape Sacratif (H1), elevated 200 meters above

the mean water-surface level and to the east by the promontory

‘‘Punta del llano’’ (H6).

Carchuna can be classified as a mixed sand and gravel beach

(Jennings and Shulmeister, 2002) both at cross-shore and at

depth. The sediment composition varies from fine sand to very

coarse elements with diameters ranging between 0.1 mm and

50 mm (Ortega-Sanchez, Losada, and Baquerizo, 2003). Based

on sediment samples recovered from the foreshore at H2

(Figure 1a), the sediment distribution presents visible vertical

gradation (Figure 2), with a bimodal distribution between

sand-size material (diameter ,2 mm) on the surface and

gravel-size material (diameter .2 mm) at the lower levels

according to the Wenthworth scale (Reeve, Chadwick, and

Fleming, 2004).

The beach cross-shore profile (Figure 1c) indicates the

presence of a scarp located at the upper swash limit, which

appears to vary with the nearshore hydrodynamic regime

(Buscombe and Masselink, 2006; Masselink and Hughes,

2003). The beach slope varies along the length of the

embayment, ranging from 0.04 to 0.3 due to the alongshore

variation in the sediment grain size and distribution, where the

steeper sloping zones are correlated with higher concentrations

of gravel sediments.

The regional atmospheric climate is dominated by the

passing of extratropical Atlantic and Mediterranean storms

(Quevedo et al., 2008). South Atlantic and south Mediterranean

storms generate wind waves under limited fetch conditions

(approximately 300 km) with average wind speeds, U10, of 18–

22 m/s. The wave directionality can be summarized as E

(31.66%), ESE (16.01%), SW (8.83%), WSW (30.11%), and W

(6.70%). Mild to low wave-energy conditions (H0 , 1 m) are

dominant (71.88%), whereas moderate-energy sea states

(1 m , H0 , 3 m) and storm conditions (H0 . 3 m) are less

frequent (27.65% and 0.48%, respectively). The zone experi-

ences an astronomic semidiurnal tide with an average tidal

range of 0.5 m.

METHODOLOGY

Video Images

The morphological evolution of Carchuna Beach has been

captured with an ARGUS coastal monitoring system, installed

on Cape Sacratif (H1, Figure 1a) by the Grupo de Investigacion

de Dinamica de Flujos Ambientales (University of Granada).

The station comprises three video cameras, each collecting

images during the first 10 minutes of every daylight hour,

obtained by averaging instantaneous frames collected with a

sample frequency of 2 Hz. The morphological changes were

analyzed from a rectified plan view of video images for the

westerly storm and an oblique plan view for the two easterly

storms.

The camera location and the position of a number of control

points were surveyed relative to a known benchmark (Quevedo

et al., 2008). Using a photogrammetric transformation and

following the technique described by Holland and Holman

(1997), it was then possible to obtain a geo-referenced

digitalization of the morphodynamic characteristics with a

resolution in the order of a single pixel. For the present study,

90 Bramato et al.

Journal of Coastal Research, Vol. 28, No. 1, 2012

this corresponds to a spatial resolution of 1.2 m and 2.6 m in the

cross-shore and alongshore directions, respectively, in the area

surrounding H2. The position of the shoreline within each time-

averaged video image is determined from color differences

between the wet and dry beach (Aarninkhof et al., 2003), giving

the instantaneous waterline and the maximum runup limit.

Following Boak and Turner (2005), uncertainties in the

shoreline displacement associated with the storm surge are

determined from barometric pressure and wind shear varia-

tions. Values of the wind and wave setup during the study

period were calculated according to Dgwind 5 (twind / rgh0)DX

(Bowden, 1983) and Dgwave ~ 0:18g0:5H0:50 maxT0 tanb (Holman

and Sallenger, 1985), where DX is the wave fetch from the

center of the low-pressure system to the coast. The depth of

the base level is represented by h0 5 L0 / 4, a function of the

deepwater wave length, L0. The tangential wind stress twind is

obtained from twind 5 raCDVwind, where Vwind is the wind

speed, ra is the air density, and CD is the drag coefficient whose

value varies with wind speed. The value r is the sea water

density, and g is the gravity acceleration. Deep water

maximum daily wave height and period are represented

by H0max and T0, respectively, and tan b 5 0.1 is the mean

slope. Sea level rise induced by barometric setup is calculated

according to Dgbarometric 5 (DPa / rg) Dgbarometric ~ DPa=rgð Þ(Benavente et al., 2006), where DPa represents atmospheric

pressure variation.

Forcing Conditions and Wave Propagation

Deepwater wave and wind characteristics were obtained

from point WANA2020013 provided by the Puertos del Estado,

Spain (Figure 1b). The data set from this point has contained

daily forecast wave data (waves model [WAM]) and wind data

(high-resolution limited-area model, or HIRLAM) since 1996.

Figure 1. Study area of Carchuna Beach. (a) Bathymetry and grid locations used in the numerical simulation, with the two grid nodes N1 and N2; V and AW

correspond to the location of the current meter and wave probe, respectively, and dx and dy are the longshore and cross-shore grid spacings. (b) General

location of N2. (c) Beach cross-shore profile in N2.

Natural Recovery of a Mixed Sand and Gravel Beach 91


This information was used to identify the storm events

according to the classification introduced by Ortega-Sanchez,

Losada, and Baquerizo (2003).

Astronomic and meteorologic tidal components were ob-

tained by applying a low-pass digital filter to the data set

captured at Motril Harbor, 5 km west of Carchuna (data was

also provided by Puertos del Estado). The filter eliminates

energetic components with a period less than 1 hour.

The beach has experienced storm events on four separate

occasions during the 4 years in which the high resolution

camera has been operational (January 2004–March 2008). On

two occasions, the storms were swell waves originating from

the east (January 28 and December 19, 2007). The other two

storm events originated from the west, lasting from March 20,

2008, to March 27, 2008 and from April 29, 2004, to May 6,

2004, both followed by mild to low-energy and moderate-energy

sea-state conditions.

A submarine canyon lies to the west of Carchuna beach

(Figure 1a). To account for its effect on the wave-propagation

patterns, the identified storm events were propagated from the

deepwater WANA location to the nearshore using the SWAN

model (Booij, Ris, and Holthuijsen, 1999; Ris, Holthuijsen, and

Booij, 1999). The model domain for Carchuna consists of three

quadrangular grids (Figure 1a), the first covering the entire

Carchuna region (8 km 3 4 km), the second nested grid (6 km 3

2 km) covering the beach, and the third nested grid (2 km 3

1.5 km) covering the section of the beach selected for the pre-

sent study. The frequency discretization used 21 frequencies

ranging from 0.04 to 0.4 Hz with a logarithmic distribution. The

directional discretization covered a range of 360u with

increments of 5u. For further analysis, simulated data were

extracted at two grid nodes offshore from H2 (N1, Figure 1a)

and in the middle of the embayment between H2 and H3 (N2,

Figure 1a).

The model was validated with data captured during a 7-day

field survey (March 7–14, 2008) consisting of simultaneous

atmospheric and hydrodynamic measurements during moder-

ate westerly sea-state conditions (Ortega-Sanchez et al., 2008).

The measurement campaign included six wave and current

probes, positioned just outside the embayment between H3 and

H4, five of which were deployed in an alongshore array at a

depth of 25 m, with the remaining probe at a depth of 28 m to

define the study-area hydrodynamics. The significant wave

height at position AW (Figure 1a) during the field survey was

obtained from data captured by a Nortek acoustic wave and

current meter (Nortek AS, Rud, Norway), positioned 350 m

seaward of the mean shoreline position and mounted pointing

vertically upward 0.55 m above the seabed on a metal tripod.

The mean water depth at this location is approximately 8 m.

The sensor sampled the water-surface elevation, pressure, and

orbital velocities with a sample rate of 1 Hz for 1024 seconds at

the beginning of every hour.

Figure 3 presents the wave height measured at position AW

during the field survey and the equivalent wave height

propagated with the SWAN model at the same location. The

model is shown to match well with the measured wave heights.

Figure 2. Observed stratification of the sediment distribution: (a) superficial sand-size sediment and (b) lower-level gravel-size sediment.

92 Bramato et al.


Sediment Mobility

To verify that the moderate wave-energy conditions created

the appropriate stress for initiating sediment motion, the bed

shear stress and Shields parameter were estimated from

current measurements captured by a Nortek acoustic Doppler

velocimeter (Nortek AS) and pressure gauge mounted on a

metal tripod pointing vertically downward. The sensor mea-

sured the three velocity components of the water current at a

fixed height of 0.5 m above the bed at location V (Figure 1a)

during the 1-week survey (March 7–14, 2008). The sensor

sampled the pressure and the cross-shore and alongshore

current at 16 Hz for 1024 seconds at the beginning of every

hour. Considering that bedload sediment transport is the

dominant mode for moderate velocity flows on mixed sand and

gravel beaches (Jennings and Shulmeister, 2002), the volume

of bedload sediment transport is generally related to the shear

stress near the bed and the median grain sizeD50. Assuming a

flat bed, the initiation of sediment transport is estimated by

comparing the Shields parameter with the critical value. The

Shields parameter is given by the general expression hs 5 ts/

((rs 2 r)gD50), where ts is the wave shear stress and rs the

sediment density. Various explicit forms have been proposed

in the literature to estimate this critical value. In the

present paper the expression of Soulsby (1997) is used,

given as hcr 5 0.3 / (1 + 1.2D�) + 0.055[1 2 exp(20.02D�)]hcr ~ 0:3= 1z1:2D�ð Þz0:055 1{exp {0:02D�ð Þ½ � where D� is

the nondimensional grain diameter.

The wave shear stress is obtained from the wave shear

velocity, calculated using u�2w ~ (1:39=2)|(A=z0){0:52U2m

(Soulsby, 1997), where A 5 UmT / 2p represents the semiorbi-

tal excursion, T is the wave period, and z0 is the roughness

length for hydraulically rough flow. The maximum orbital

current velocity for irregular waves is determined from

Um 5 2.8su (Aagaard and Greenwood, 1994; Masselink and

Pattiaratchi, 1998), where su is the standard deviation of the

cross-shore current.

The Shields parameter analysis was compared with the

result obtained following the energetic theory of nonbreaking

irregular waves (Losada, Desire, and Merino, 1987), in which

the bottom-motion rate coefficient is introduced as a 5 A3 /

D50E, where E 5 [(rs 2 r)g / rw2]2 and w is the angular

frequency.

Dimensionless Analysis

Dimensionless analysis was used to quantify the erosion and

recovery of the beach as a function of the magnitude of the

storm event, sediment distribution, and beach bathymetry.

The parameters were evaluated using both the deepwater

WANA information and nearshore propagated-wave data

(SWAN results at N1 and N2). The first parameter is the

dimensionless fall velocity (H /vsT), where vs represents the

sediment fall velocity calculated as (Soulsby, 1997)

vs ~n

D

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi10:362z1:049D�3� �q

{10:36

� �,

which is a function of the nondimensional grain diameter, D�,and the cinematic viscosity, n. With respect to D, according to

the classification introduced by Pontee, Pye, and Blott (2004), a

mixed sand and gravel beach can be characterized by

D50 5 2 mm. To better characterize the behavior of a mixed-

sediment beach and the tendency of the sand to mobilize with

respect to gravel, a second value of Df 5 0.35 mm (associated

with the first peak in the bimodal sediment distribution shown

in Figure 2) is also represented.

The dimensional fall velocity parameter Hs /vsT and the

relative tidal range parameter RTR 5 TR / Hs are also used to

predict the morphodynamic state of the beach (Jackson,

Cooper, and del Rıo, 2005; Masselink and Short, 1993), where

Hs, T, and TR are the modal wave height, the most frequent

wave periods, and the mean spring tidal range, respectively.

The variation of the deepwater wave steepness and the

deepwater bed slope parameters are, respectively, the left and

right terms of the equation of Sunamura and Horikawa (1974):

H0 / L0 5 C(tan b)20.27(D / L0)0.67, where C 5 18 is an empirical

constant and tan b 5 0.046 is the average nearshore bottom

slope to a water depth of 20 m. Although this equation is

originally defined for deepwater waves, the wave steepness is

also determined at the nearshore for comparison (N2).

Finally, the Iribarren number is defined by the relation

Ir ~ tanb=ffiffiffiffiffiffiffiffiffiffiH=L

p(Iribarren and Nogales, 1954), presented in

both deep water (WANA) and the nearshore (N2). The

Iribarren number is a useful classification of beach state and

wave breaker type for gravel beaches (Austin and Buscombe,

2008; Austin and Masselink, 2006).

The onshore migration of inner bars was studied following

the relation (Sunamura and Takeda, 1984)

5D=(Hb)maxv(Hb)max=gT2maxv20D=(Hb)max,

where (Hb)max is the maximum value of the daily wave breaker

height, approximated from (Hb)max 5 (H0)max(tan b)0.2((H0)max /

(L0)max)20.25, and Tmax is the wave period. The average speed of

onshore bar migration is a function of the wave breaker height,

(Hb)max, and grain size, D50 and Df.

RESULTS AND DISCUSSION

General Beach Evolution under Storm Conditions

The beach evolution due to a westerly storm followed by mild

to low-energy and moderate-energy sea states, which occurred

in March 2008, will be presented in detail. The event is

Figure 3. Comparison between offshore WANA input wave data (grey

line), propagated wave data measured at position AW (discontinuous line),

and SWAN wave model results at the location of AW (black line).



characterized by (1) the spectral significant wave height, H0; (2)

the peak spectral period, T0; and (3) the mean wave direction, h,

reported at 3-hour intervals, as illustrated in Figure 4. The

beach experienced a westerly storm event (H0 . 3 m, T0 of

approximately 7–9 s; labeled as S in Figure 4) between two

moderate-energy events from the east and west (H0 . 1 m, T0

of approximately 6–9 s; labeled as ME and Mw in Figure 4). The

start of the monitoring period (March 20) corresponded with

the spring tidal range followed by its descending phase. The

offshore wave sea states propagated by the SWAN model at

positions N1 and N2 are also included in Figure 4. As shown,

wave height reduces as it propagates to the nearshore, and

refraction reduces the obliquity of the wave near the coast;

however, the angle of incidence of the resulting nearshore

waves is still significant and capable of producing longshore

sediment transport. Wave heights are greatest at the center of

the embayment rather than at the horn, as previously

predicted by Ortega-Sanchez, Losada, and Baquerizo (2003),

producing an alongshore variation of the wave height that may

induce nearshore circulation and sediment transport patterns.

Figure 5 presents, through a sequence of rectified time-

exposure images, the morphology of the study area throughout

the duration of the storm (Storm S). The images represent the

beach shoreline at the daylight hour corresponding to low tide.

Initially, the beach had an accreted form with a traditional

berm profile, a stratified sediment composition, and a steep

beach face (Figure 5a). The passing of moderate swell waves

(ME) induced a low sediment-transport rate, but no significant

changes at horn H2 are observed. This initial state is indicated

as BE (berm) in the figures. On March 22, storm ‘‘S’’ eroded the

beach, particularly around horn H2; the beach face became

flatter and eventually assumed the eroded profile represented

in Figure 4, indicated as ER (eroded). A significant volume of

sand was transported seaward to form a longshore storm bar

(Figure 5d), exposing the gravel on the beach face; these may

interact with each other to produce still finer material (Curtiss,

Osborne, and Horner-Devine, 2009; Warrick et al., 2009). The

larger sediments are transported a limited distance, forming a

natural protective barrier. The new mild-slope profile promotes

wave breaking over the newly formed storm bar, resulting in

less turbulence and less capacity to transport sediments,

protecting the beach from further erosion. One of the physical

mechanisms that can differentiate sandy beaches from mixed

sand and gravel beaches is the armoring effect (Mason and

Coates, 2001). In a similar process to river armoring, the storm

conditions are capable of moving the sand offshore while the

gravel may remain, thereby increasing the stability of the

beach and reducing the recuperation time in comparison to

sandy beaches.

The storm was followed by 2 days of mild to low-energy

conditions. Although an eroded profile usually requires weeks

or months to recuperate (Curtiss, Osborne, and Horner-Devine,

2009), Carchuna presents variations in its state after less than

24 hours, transforming from its eroded state to a more

intermediate state, indicated as IN (intermediate). This implies

that during the milder conditions, the sand-size sediments are

transported onshore.

On March 26 the moderate energetic event (Mw) arrived from

the west. Instead of causing further erosion, the recuperation of

the beach to its original berm configuration was accelerated

with the sand moving onshore, contributing to a further

advancement of the beach shoreline, particularly around H2

(Figure 5h). The beach face rapidly recuperated to the previous

stratified sediment distribution.

Figure 4. Time evolution of (a) significant wave height, (b) significant

wave period, and (c) mean wave direction for WANA (black line), N1 (gray

discontinuous line), and N2 (black discontinuous line). Upper subfigures

provide sketches of observed beach state cross-shore profiles.

94 Bramato et al.


The morphological characteristics observed at Carchuna

were identified as correlating well with the hydrodynamic

behavior presented in Ortega-Sanchez, Losada, and Baquerizo

(2003). It was predicted that under a westerly storm, waves

break uniformly outside the embayment, eroding the zone

surrounding the horn and depositing the sediment in the

central part, as observed in Figure 5d. During westerly

moderate conditions, waves propagate inside the embayment

before breaking with a nonuniform distribution of the wave

energy along the shoreline, producing a circulatory system and

Figure 5. Time evolution of the beach shoreline behavior using rectified plan view images, from March 20, 2008, to March 27, 2008 (a–h, respectively), where

the continuous and discontinuous black lines represent the original and the modified shoreline. The discontinuous line in (c) represents the highest

water level.



sediment transport from the center of the embayment toward

the horns, resulting in their cross-shore advancement (Fig-

ure 5h). With respect to the circulatory system associated with

easterly moderate conditions, sediments move from the center

toward the western side of the horn. This is less effective than

moderate conditions approaching from the west, leads to no

significant variations in the horn cross-shore dimension, as

observed in the camera images after the moderate sea state ME

(Figure 5b).

A summary of the time evolution of the beach shoreline is

represented in Figure 6, which presents the overlapped

shorelines for the 7 days. Although not presented here, the

same beach behavior was observed during another similar

storm sequence: a westerly storm followed by mild to low-

energy and moderate-energy westerly events that occurred in

May 2004 (Bramato, 2008).

Table 1 presents the time evolution of the offshore maximum

daily energy flux per unit width (Fmax) and the mean daily

shoreline displacement, where the time period is defined as the

time between the afternoon low tide event and the null value

corresponds to the original shoreline location (March 20). The

maximum water elevation that occurred during Storm S is also

indicated, along with the rate of beach erosion and recovery (t)

and the wave setup (Dgwave) and wind setup (Dgwind). It can be

concluded that the incident energy associated with the storm

was capable of mobilizing a large volume of sediments from the

berm, reducing the beach cross-shore section. The low-energy

conditions which followed induced a total recovery of the beach

with an onshore mobilization of the sediments, resulting in a

wider beach section. The moderate conditions (MW) imply that,

after a regression of the coastline, the beach recovered to a

nonbarred berm profile. Storm S induced a mean regression of

the shoreline location of approximately 5 m/d followed by a

rapid recovery of the beach during the proceeding calm

conditions. The mean daily recuperation of the coastline was

approximately 2 m/d. The cross-shore errors in the shoreline

displacement determined from wind shear variations and wave

setup range between a maximum value of 5.3 m during the

storm and a minimum value of 1.4 m during low-energy

conditions. Atmospheric pressure variations of the order of

1 mbar are noted to occur between the African and European

coasts, producing a negligible sea level displacement.

Figure 7 presents the response of the beach to the two

easterly storm events that occurred in 2007. These storms are

characterized by a significant offshore wave height and peak

period of H0 5 3.6 m and T0 5 8.4 s (Storm S1) and H0 5 3m

and T0 5 7.4 s (Storm S2). The figure shows the sediment

erosion in the system embayment cell and a net transport

toward the western end of the embayment (H2).

Backstrom et al. (2008) recorded the shoreface morphody-

namics for the same storm (S1) at a beach approximately 60 km

to the west of Carchuna Beach. The authors found that the

morphological processes associated with the storm varied with

shoreline orientation, with differences in the nearshore and

shoreface. Sediment disposition was concentrated in the zone

where the shoreline changed orientation up to a water depth of

40 m, similar to observations in the present study around the

horn (H2). The morphological characteristics at Carchuna for

the observed easterly storms correlate well with the predictive

theoretical model of Ortega-Sanchez, Losada, and Baquerizo

(2003).

Morphological Evolution by Parameterization ofthe Forcing

The beach evolution during moderate and mild sea-state

conditions and the existence of sediment transport at a depth of

5 m was estimated using the Shields parameter (Soulsby, 1997)

Figure 6. Time evolution of the beach coastline behavior in scaled plan view (m) derived from Figure 5.

96 Bramato et al.


and the bottom-motion rate parameter (Losada, Desire, and

Merino, 1987); these were calculated from current meter

measurements collected 1 m from the bed (location V,

Figure 1a). Figure 8 (upper graph) shows that the Shields

parameter is consistently greater than the critical value

(hcr 5 0.0385), confirming that sediment transport is not

limited to the zone close to the coastline but can also be

observed at the instrument location (5-m depth), although it

never exceeds a value of 0.8, which is the limit for sheet flow

over a flat bed.

Figure 8 also represents, in the lower graph, the bottom-

motion rate parameter analysis. Comparisons show good

agreement with the Shields parameter. However, the a values

also provide additional information related to the limit of (1)

incipient motion (a 5 0.061), (2) general motion, and (3) the

initiation of ripples (a 5 0.28). The values derived at Carchuna

never exceed a 5 9.34, which corresponds to the disappearance

of ripples.

Changes in beach morphology of a mixed sand and gravel

beach are directly related to the exchange of sediments

between the berm and the bar. Recent laboratory and field

studies show that dimensionless fall-velocity values larger

than 2.0 promote offshore sediment transport and a bar-type

profile (Masselink and Hughes, 2003), while values less than

2.0 tend to promote an onshore sediment transport and a berm

profile. Figure 9a presents the dimensionless fall-velocity

results for deepwater and nearshore wave conditions calculat-

ed according to the SWAN model, repeated for D50 5 2 mm and

Table 1. Time evolution of beach conditions from March 20, 2008, to March 27, 2008. Fmax 5 maximum daily energy flux per unit width, Ds 5 daily coastline

displacement referenced to the March 20 coastline location, DDs 5 daily coastline displacement referenced to the previous daily location, t 5 rate of beach

erosion and recovery (negative and positive values correspond to beach erosion and recovery, respectively), Dgwave 5 wave setup, and Dgwind 5 wind setup.

Day Fmax (W/m) Ds (m) DDs (m) t 5 DDs / Fmax (m2/W) Dgwave (m) Dgwind (m)

March 20–21 19,646 20.67 20.67 23.41 3 10205 0.61 0.04

March 21–22 95,270 220.36* — — 0.87 0.18

March 22–23 100,957 25.59 24.92 24.87 3 10205 0.88 0.18

March 23–24 2574 10.74 16.33 0.0063 0.20 0.14

March 24–25 1922 12.08 1.34 0.00069 0.18 0.10

March 25–26 18,146 2.24 29.84 20.00054 0.47 0.19

March 26–27 25,147 3.36 1.12 4.44 3 10205 0.55 0.15

* Maximum storm water displacement.

Figure 7. Time evolution of the beach morphology taken by the high resolution camera (a) before the passing of Storm S1 on January 26, 2007; (b) after the

passing of Storm S1 on January 30, 2007, where the continuous line indicates the original shoreline taken from image (a); (c) before the passing of Storm S2 on

December 17, 2007; and (d) after the passing of Storm S2 on December 22, 2007, where the discontinuous line indicates the original shoreline taken from

image (c).



Df 5 0.35 mm. A dimensionless fall-velocity limit of 2.0 is

indicated in the figure to define the boundary between erosive

and accumulative conditions. The figure suggests both the sand

(Df 5 0.35 mm) and gravel (D50 5 2.0 mm) sediments react

with a berm-bar behavior. The sand exceeded the limiting

value of 2.0 throughout almost the entire period of the study,

suggesting the sand remains in a state of suspension and

transition, promoting offshore sediment transport and the

formation of a bar-type profile. Comparison with the high-

resolution camera images presented in Figure 5 show that the

bar profile corresponds with the period when the larger

sediments were mobilized during Storm S and the dimension-

less fall velocity calculated with D50 exceeded the limit of 2.0.

The results highlight the major difference in the behavior of

sandy beaches and mixed sand and gravel beaches: under the

same conditions a sandy beach would theoretically present a

constant bar profile.

The dimensionless fall velocity and the relative tidal range

parameters provide a tool for predicting the beach state of

Figure 8. Shields parameter (upper graph) formulated according to

Soulsby (1997) and bottom-motion rate parameter (lower graph) following

Losada, Desire, and Merino (1987).

Figure 9. Time evolution of (a) the deepwater dimensionless fall velocity

for D50 (black line) and Df (grey line) and the nearshore dimensionless fall

velocity for D50 (black cross point) and Df (grey cross point); (b) the

deepwater wave steepness (black dotted line) and nearshore wave

steepness (gray dotted line along with deepwater slope parameter (solid

line) for D50 (black line) and Df (gray line); and (c) the Iribarren parameter

calculated for offshore forcing (black line) and nearshore forcing (gray line).

98 Bramato et al.


Carchuna. Assuming a modal wave height Hs 5 0.6 m and the

mean spring tidal range TR 5 0.6, the relative tidal range is in

the order of 1. For the characteristic beach grain size

(D50 < 2 mm) and a wave period range corresponding to the

modal wave height (4 s , T , 7 s), the dimensionless fall

velocity is determined to be between 0.83 and 0.48. Thus, the

predicted beach state is type reflective (Masselink and Short,

1993; Jackson, Cooper, and del Rıo, 2005).

Waves are typically defined as steep for H / L . 0.02, leading

to a net offshore transport of sediments and the formation of a

bar-type profile. Figure 9b shows that the wave steepness

exceeded this limit consistently after the passing of Storm S

and the following moderate conditions (MW); however, the bar-

type profile is visible only after the passing of the storm

(Figure 5d). This indicates that the wave steepness limit,

H / L . 0.02, on a mixed sand and gravel beach may not be an

appropriate criterion for defining the onshore-offshore net

sediment transport tendency and the shoreline behavior

during a sequence of events; rather the wave steepness limit

should be evaluated in combination with relationships that

consider sediment composition and beach bathymetry.

The accretion-erosion relationship derived by Sunamura and

Horikawa (1974) states that beach erosion takes place when

the deepwater wave steepness is greater than the deepwater

bed slope (presented in Figure 9b) and vice versa for beach

recovery conditions. The figure indicates with a gray bar the

interval in which shoreline erosion of the larger sediments

(D50 5 2 mm) takes place, reaffirming a similar sediment

behavior obtained with the dimensionless fall velocity limit

described in Figure 9a. However, Figure 9b suggests the sand

(Df 5 0.35 mm) is eroded during the storm and moderate

energetic events, predicting that the beach recuperation is

limited to the mild to low-energy sea state. The camera images

captured during the study period correlate well with this

behavior.

The Iribarren number considers the interaction between

wave height, wave period, and beach slope (Figure 9c). The

parameter correlates well with observed breaking-wave char-

acteristics, with values ranging between 0.5 and 1.0. This

range indicates that the waves predominantly break as

plunging and, due to the high beach slope, near to the shore.

Generally, when waves break as plunging waves, a large

amount of the sand is elevated toward the water surface at the

plunging point and injected horizontally along the surface,

moving sediments onshore. As the value of the Iribarren

parameter decreases and the breaker type tends toward

spilling values, near-vertical vortices associated to the breaker

type start to appear, facilitating the suspension of sand and its

transport offshore (Aagaard and Greenwood, 1994; Nadaoka,

Hino, and Koyano, 1989).

Figure 10a identifies the theoretical migration direction of

the longshore bar (onshore, offshore, or no migration) for the

grain size D50, in function with the maximum daily breaking

wave height and wave period. The figure suggests the storm

bar never promotes offshore migration and sediments do not

exit the nearshore zone. During the storm event (S), the values

tend toward but never exceed the limit of offshore movement,

confirming the erosive state of the beach. During the moderate

event (MW), the relation tends toward the no-migration limit,

confirming the recuperation stage of the beach. Figure 10b

Figure 10. Identification of onshore, offshore, and no migration bar condition for (a) D50 5 2 mm and (b) Df 5 0.35 mm, with conditions recorded during the

week in function with the maximum daily breaking wave height and wave period. Moderate (M), storm (S), and calm (C) conditions are indicated from the

first through the seventh day.



identifies the theoretical migration direction of the longshore

bar (onshore, offshore, or no migration) for the sand

(Df 5 0.35 mm), which is shown to be in an almost constant

state of offshore migration. Again, this highlights the most

significant difference in sandy beaches, which under the same

conditions would present a constant bar profile with offshore

migration, consequently increasing the distance of the offshore

sand transport and an expansion of the beach recuperation

time.

CONCLUSIONS

This work examines the natural recovery of a mixed sand and

gravel beach (Carchuna, Spain) to storm events using high

resolution camera images. During a sequence of events

recorded in March 2008, the storm created westerly swell

waves (H0 . 3 m, T0 5 7–9 s) that broke uniformly alongshore

and outside the embayment (Ortega-Sanchez, Losada, and

Baquerizo, 2003), eroding the beach with a reduction in the

cross-shore dimension of the cusps’ horns and the formation of

a longshore storm bar within the embayment. The rate of

erosion in terms of the mean daily coastline displacement is

,5 m/d. The incident energy flux at this stage is capable of

mobilizing the sand and gravel from the original berm,

generating an eroded bar profile. Field analysis of current

meter measurements indicate, under moderate westerly sea

state conditions, that sediment 2 mm in diameter can be

mobilized up to a depth of 5 m.

The beach recovered from the eroded state during the mild to

low-energy (H0 , 1 m, T0 5 4–6 s, west incidence) and

moderate-energy (H0 5 1–2 m, T0 5 6–9 s, west incidence)

sea states which followed, with a net accretion in terms of the

mean daily coastline displacement of ,2 m/d. The beach

appeared to recuperate, but before being able to stabilize itself,

a second moderate event passed across the region. Instead of

causing further erosion, the beach responded by accelerating

its recuperation with a further advancement of the embayment

horn (H2).

This is in contrast to easterly storm events, during which

waves erode the sand along the length of the horn-embayment

cell, which is deposited at the western end of the embayment.

The cross-shore advancement and reduction of the cuspate

features also confirm previous studies by Ortega-Sanchez,

Losada, and Baquerizo (2003).

This study provides evidence for formulating conceptual

models of the complex behavior of a mixed sand and gravel

beach after a succession of storm events, suggesting that this

data can be integrated into future coastal-management

projects. During accumulation, the sand-size material remains

in suspension for longer periods of time, creating an upper

layer of sand that is deposited above the gravel. During erosion,

the overlying sand is rapidly transported offshore, exposing the

gravel size-sediments. The gravel is dragged a limited distance,

forming a natural barrier that protects the beach from further

erosion.

Analysis of various characteristic dimensionless parameters

suggests the accretion-erosion relationship derived by Suna-

mura and Horikawa (1974) is the most appropriate relation-

ship for estimating sediment behavior of a mixed sand and

gravel beach. Studies are ongoing to better understand this

relationship, including the capture of high resolution images at

Trafalgar Beach (Cadiz). Trafalgar Beach is a sandy beach

located nearby, enabling the comparison of the responses of

beaches with different sediment composition under similar

storm conditions.

ACKNOWLEDGMENTS

This work was funded in part by the Spanish Ministry of

Science and Education (Project BORRASCAS CTM2005-

06583) and Junta de Andalucıa (Projects P05-RNM-968,

P06-RNM-1573, and TEP-4630). The authors acknowledge,

with thanks, David Navidad, Elena Quevedo, Alejandro

Lopez, Francisca Martinez (Consejo Superior de Investiga-

ciones Cientıficas), and three anonymous reviewers for

valuable comments and suggestions.

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