Morphosedimentary multiyear changes on a barred coast (Gulf of Lions, Mediterranean Sea, France)

MARINE QEOLOQY INTERNATIONAL JOURNAL OF MARINE GEOLOGY. GEOCHEMISTRY AND GEOPHYSICS

ELSEVIER

Marine Geology 122 (1994) 47-62

Morphosedimentary multiyear changes on a barred coast (Gulf of Lions, Mediterranean Sea, France) J. Paul Barusseau a, Metodiu Radulescu b, Cyr Descamps a, Emile Akouango Alain Gerbe b

a

a Laboratoire de Skdimentologie et Gkochimie marines, Universitk de Perpignan, 52 Avenue de Villeneuve, 66860 Perpignan, France b Institute o f Regional Management and Environment, I.A.R.E., Agropolis, 34397 Montpellier, France Received 16 June 1993; revision accepted 7 September 1994

Abstract

An overall estimate of the relationship between the changes in the nearshore morphology, the evolution of the sedimentary budget, the beach morphodynamics and the variation of grain-size characteristics along the beach barrier of Thau Lagoon, Languedoc, France, was attempted by synthesizing the data from different surveys: a basic survey of two separate grids of 11 closely-spaced (50 m) transverse profiles during 1989-1991, a long-baseline (11 km) survey of 11 beach profiles during 1988-1991 and an earlier, long-run (1984-1990) survey of 6 widely-spaced bathymetric profiles covering discretely the whole stretch of the nearshore zone. Time intervals between profile monitorings within each type of survey differed, but many measurements during the recent 1989 1991 period were coincident. The stable two-bar system that was generally developed all over this nearshore zone has been gradually changing at the northeastern end, south of the town of S6te, since 1989, under the effect of a series of new defense structures upstream in the dominant N E - S W wave-induced longshore current, with a tendency toward a single-bar profile. The average nearshore slope was steeper (0.91%) in the S6te close-survey area than in the southwestern area (0.83%). During the same period, acceleration in beach erosion has been observed at the northeast, in contrast with a stable dynamic equilibrium over the southwestern beach and nearshore profiles. Over a longer period (1984-1990), scouring of the nearshore sand-prism down to the 10-m isobath at the northeastern end between the extreme bathymetric profiles, could be estimated at about 37,000 m 3, while the overall sedimentary budget was positive. Bar migration and morphologic changes in the study areas were not related to any seasonal cycle in the wave climate, but were more significant when this was more energetic, as during 1989. An overall attenuation in nearshore morphodynamics was observed during the following two years, although some rapid local changes between consecutive measurements could be detected. Temporal variation of grain-size characteristics were therefore small and/or not significant. Conversely, on the subaerial beach, alongshore differentiation in mean diameter and in sorting and skewness parameters is consistent with the evolution of the erosion-accumulation processes following a long-period wave-like sand-transfer from northeast to southwest.

0025-3227/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0025-3227(94)00102-2

48

J.P. Barusseau et al./Marine Geology 122 (1994) 47 62

1. Introduction

The nearshore zone of the sandy coast of the Gulf of Lions is characterized by a rather stable system of quasi-continuous shore-parallel waveformed sand bars. Their shape, number seawards and degree of stability is variable along the coast, following its local exposure to the dominant waves, the proximity of headlands, the slope and grainsize equilibrium, the volume of sediment in movement and the presence of sea defense structures. In this paper, based on the data from multiyear surveys of a beac~barrier nearshore zone, the spatial and temporal morphologic changes in the bar-system are analysed in connection with the corresponding changes in the beach dimensions, in grain-size characteristics of the sand and in the evolution of the nearshore sedimentary budget. A global approach to the coastal morphodynamics by interpolation between discrete, instantaneous, equilibrium forms--averaging the effects of waveinduced processes of quite different magnitude and frequency--attempts to compensate for the lack of technologically difficult measurements of sediment transport under storm conditions (Greenwood and Davidson-Arnott, 1979).

2. Study area and methods

2.1. Physical setting The beach barrier (lido) of Thau Lagoon stretches about 13 km in a nearly straight NE SW direction, along with a coastal highroad from the town of S6te to Marseillan Beach (Fig. 1). The sea coastal zone of this beach barrier, starting from the dune ridge down to about 10 m water depth, has been the object of a regular topobathymetric and sedimentologic survey carried out since the spring of 1989 within three different, partially coincident series of transversal profiles (Fig. 2): (a) a long-baseline series of bathymetric profiles numbered 13 to 17 from southwest to northeast, surveyed since an earlier date (1984) by the Maritime Service at long intervals (1-2 years); (b) a long-baseline series of 11 topographic

•••

lhau

IG°l~ uLm.n_. ~.'~ 50

km

/ l / ~

RudR eU i. f

I ~

HerautlR~.~ ui . ~$olllan LO Cap d'Rode

S.te

Plage

Port-LaNouuel M ! ! I ~ EDITERRANEAN SEA .. >o

AgRI yl v~. Perpignan Tech

~ . ~ $t Cyprlen

Rlu.~.

?~ "~'~:~

ap

SPAIN

km

ANCE

Bear

Fig. 1. Location of the study area.

beach profiles seasonally surveyed since 1989 (some of them earlier); and (c) two short-baseline series of 11 close topobathymetric profiles (spaced at 50 m) each, initially monitored at monthly intervals: one at the northeast, near S6te, 1 km from the sea defensive structures bordering the town beach; another nearer the Marseillan Beach, located midway along the beach barrier. These will be simply referred to as 'S6te' and 'Marseillan' close-survey areas. A semidiurnal microtidal regime is characteristic of the Mediterranean Sea. In the Gulf of Lions, the amplitudes of this mixed tide oscillate between 4 and 26 cm for periods ranging between !0 h and 14 h 50 rain, as measured south of Saint-Cyprien. These small tidal variations in sea level are negligible on the scale of our study, in the conditions of much larger wind-generated variations.

49

J.P. Barusseau et al./Marine Geology 122 (1994) 47 62

/

Thau lagoon

5m

/

Jrltrj,J ,llJ+rljl

10m

13

//

/ /

"\ 14

15 15b

16

17

Close survey profiles Beach prordes Long-base bathymetric profiles K39

MEDITERRANEAN SEA

Kilometric scale along the road

Fig. 2. Location of profiles in the study area. Strong winds blow from two main directions: from the northwest (land winds, more frequent) and southeast (sea winds), which is about a shorenormal axis. Speed peaks are generally higher for land winds and they are dependent on wind-storm duration, but it is the sea (onshore) winds which raise the highest waves. The significant wave height is directly related to the speed and duration of the wind. The wave climate is characterized by a large interannual variability which is marked, in the decade of the 1980s, by the irregular occurrence of some less frequent events: a very violent seastorm in 1982, several strong storms in 1984 and 1986 and some notable storms in 1987 and 1988. Earlier statistical analysis of a 426-day record of wave height gave significant-height values of 0.6 m (20%), 1.2 m (10%) and 1.85 m for 5% of the waves (Catalogue S6dimentologique des C6tes de France, 1984). Predominant wave-generated longshore current

and corresponding net longshore sediment transport are towards the southwest, with a decreasing gradient, because of a gradual change in the angle between the coastline and the prevailing southeastern incident waves in this direction. 2.2. Field and laboratory experimental methods

The long-run bathymetric profiles (a) as separately reported by the Maritime Service were simply superposed on one graph for each of the 6 cross sections. Then, the variation in unitary volume ( D V ) of sediment was measured graphically as the difference in surface described by each of the successive profile curves, the horizontal 10 m-depth line and the vertical of the base station on shore, in square metres for a distance 1 m wide alongshore. The 11 topographic profiles spaced about 1 km apart (b) were monitored by using a conventional level and rod survey. Elevation ranging was taken

50

j.P. Barusseauet al./Marine Geology 122 (1994) 47-62

from markers on the border of the coastal road, linked to the General French Datum (N.G.F.), down to the beachface in water depths between 0.5 and 1.5 m, depending on the wave climate. Profiles were reported directly on one and the same graph for each range line in order to obtain an envelope of 2-D variation and possibly detect a sequential evolution. On both sides of the profiles P2 (S6te) and P7 (Marseillan) of this series, shorter baselines (500 m) were established and marked on the border of the road for two grids of 11 dense profiles each (c). They were usually monitored either at monthly intervals or every two months, depending on the occurrence of high-energy sea state after the surveys. A Geodimeter-140 electronic tachymeter fitted with an electro-optical range finder was used for both the subae~ial beech and the nearshore zone. Each time the apparatus was placed directly over a marked base station and the range line was found by turning a predetermined angle from the baseline. Depth measurements were taken by using a Fuso fathometer aboard a Zodiac boat with an accuracy of :~5 cm; in fact, taking into account the frequent cases of slightly rough sea surface during the surveys, the real accuracy of sounding could be generally estimated at about + 15 cm. Synchronization of tick marks placed on the fathometer chart paper with records of the distance from the base station to the boat maintaining a constant speed was assured by keeping the boat driver on line by V H F walkie-talkie communication with the shore observer. In order to reduce all the bathymetric profiles to the N. G.F., the sea water-level was measured at the beginning of each hydrographic survey by siting the elevation of the beachface at the base of the uprush through the theodolite. As this operation is rather tricky to perform, particularly when breaking waves exceed 0.3 m in height, records from the S6te harbour self-recording sea-level gauge were more often made use of, knowing the exact time of each profile monitoring. Topo-bathymetric data records were processed and profiles were plotted using software able to retrieve cross-section variation and to drive maps and block diagrams of the two close-survey areas.

Only major elevation differences ( > 3 0 c m ) were considered in further analysis. Sediment sampling locations were chosen following the morphological differentiation within the bar system and the depth gradient of the sea 'glacis'. Crests of bars and bottoms of interbar troughs, as well as certain isobaths ( - 5 , - 1 0 m), were sampled by divers, in order to obtain a broader description of the nearshore seabed (bioturbation intensity, bedforms, sediment heterogeneity). Surveys from 1989 to 1992 provided 6 series of samples, under post-winter (spring surveys) and post-summer (autumn surveys) conditions. Sediment samples were taken along 3 cross sections (the two extreme and the central ones) of both close-survey areas. Taking into account its greater morphological diversity, nearly 150 samplings have been conducted in the Marseillan area, as against a little more than a hundred in the Sbte area. An example of a sampling location is given in Fig. 3. The collected sediments were examined with a stress on grain-size analysis--as they were exclusively s a n d y - - a n d the textural characteristics of the different types of sand were determined. The grain-size diagram was entirely explored for computation of textural indices when size distribution was unimodal, but only the main population (80-95% of total sediment) was regarded (as developed to 100%) in case of a bimodal distribution. Each sampled sediment was defined by: a position index: median diameter (in ~bunits); a sorting index: -

oi

=

(Qs4 - Q16)/4 + (Q95 - Q5)/6.6 a skewness index:

SKt = [(Q16 + Q84- 2Qso)/2(Q84 Q16)] + [(Qs + Q95 - 2 Q5o)/2 (Q95 - Q5 )] a kurtosis index: KG = (Q95 Qs)/2.44(Q75- Q25) The values taken by these indices for each morphobathymetric feature in the 3 sampled cross sections allowed us to define for each area a standard sediment characteristic of the respective feature under post-summer (PS) c o n d i t i o n s - - o n the basis

J.P. Barusseau et al.,/Marine Geology 122 (1994) 47 62

51

+4

I M1-8

Sealevel

.~ MI-6 MI-7 \\

M1-3 ~% - MI ~ ~

-12

Distance from baseline

(m)

1200 Fig. 3. Locations of sediment sampling in a cross section.

of the September 1989 sampling--and another standard sediment under post-winter (PW) conditions, on the basis of the May 1990 and June 1991 samplings. In both areas, PS and PW mean parameters were compared in order to check out the hypothesis of seasonal grain-size evolution. December 1991 samples were compared to both PS and PW standard sediments.

3. Results

3.1. Nearshore morphodynamics The coastal zone off the lido of Thau Lagoon presents a simple gradient of isobaths originated from coastal grading processes. Two shore-parallel wave-formed sand bars generally developed in the nearshore zone: an inner bar with slight crescentic modulations at a largely variable distance of 100 to 250 m from the baseline (50-150 m from

shoreline) and an outer bar at 350-450 m from the baseline (250-400 m from shoreline), even farther (450-500 m) in the southwestern profiles. A tendency towards a single-bar profile has been observed in some northeastern profiles since 1989. A third, ephemeral bar of irregular cuspal shape migrated in the southwestern profiles across the shallow zone separating the inner bar from the foreshore, to which it often attached during smooth swells following periods of wave set up. From both long-baseline (a) and short-baseline (c) series of profiles results that most of the morphological changes in the seabed took place within the inner and outer bar system. Over a longer period (a), the two main bars, as they had been positioned in 1984, were replaced by interbar troughs in 1990, with a maximum variation of 4 m in bed elevation. On a smaller scale, the overall wave-energy distribution is influenced by these changes in the nearshore sand-bars, which may induce a

J.P. Barusseau et al./Marine Geology 122 (1994) 47 62

52

topographic feedback effect (Greenwood and Davidson-Arnott, 1975) complicating the local refraction pattern and modifying the longshore periodicity of rip-cell patterns and sediment movement. A discrete summary of the change in sediment volume between the upper beach and the 10-m isobath is given in Table 1. The volumes assume a 1 m-swath alongshore. Despite the long time interval between these earlier surveys, one may notice that as a rule, accretion was found smaller and sediment loss larger in the northeastern than in the southwestern sections, except for the shorter, more recent 1989 1990 interval. Consequently, from 1984 through 1990, the sedimentary budget is in deficit at the northeast, but it becomes gradually accumulative towards the southwest. This cannot be simply explained by the annual rate of the N E - S W longshore sediment transport assessed theoretically at about 40,000 m 3 with a gradual decrease to 3000 m 3 towards the southwest ( S O G R E A H , 1985). In fact, by integrating the profile changes alongshore, sediment loss during the 6-year survey was found to be about 37,000 m 3 over a stretch of 1 km near S6te, while nearshore build up southwest of profile 16 exceeds 670,000 m a in volume. Nevertheless, one should bear in mind that this overall budget contains some great fluctuations, as actually shown by intermediate surveys. The nearshore zone was found entirely eroded during the two 1989 (May and October) monitorings in comparison with the bed elevations measured in 1984

and 1986; the total sediment loss over the 1986 1989 period was on the order of 4,000,000 m 3, but this quite considerable scour proves to have been completely recovered within a rather short interval, between October 1989 and June 1990. Even if the erosion of the emergent b e a c h - - o n the order of 70,000 m 3 during about the same above mentioned 6-year period as estimated by alongshore integration of unitary volume variation in the series of topographic (b) profiles--is considered as one of the sources for nearshore build up, it gives a small amount of sand in comparison with the still growing influx of nearly 550,000 m 3 towards the southwest of the surveyed zone. However, as compared to the volumes moved during shorter, intermediate periods (-4,000,000 m 3 in 1986 1989; +2,500,000 m 3 in 1989-1990), this apparent influx may be included in a loweramplitude long-term fluctuation corresponding to a natural stage in the sequence of adjustments to the dynamic equilibrium over a wider stretch of the coastal zone. 3.2. Short-term morphologic changes in the two close-survey areas The envelope of variation in the profiles monitored at short intervals (c) over the S6te area from May 1989 through December 1991 is shown in Fig. 4. The mean profile derived from this may be considered as a morphologic standard of the site, around which wave-induced temporary changes occurred, tending to a dynamic equilibrium profile.

Table 1 Variation in volume of sediment in the nearshore profiles between S6te and Marseillan

Years Profile 17 16 15 14 13

V 1984

V 1986

AV A84/86

V 1989

AV A86/89

V 1990

AV A89/90

AV A84/90

4165 4268 4492

4410 4535 4750 4780 4882

+245 +267 +258

3795 3900 4060 4515 4608

-615 -635 -690 -265 -274

4078 4285 4600 4690 4745

+ + + + +

- 87 + 17 + 108

4555

+ 327

283 385 540 175 137

+ 190

V= unitary volume (ma/m) of sediment in a cross section between the baseline vertical and the 10 m-depth line. A V=temporary difference in unitary volume of sediment in one cross-section.

J.P. Barusseau et al./Marine Geology 122 (1994) 47-62

53

Upper limit

/

°10

Lower limit Distance from baseline (m) I

-15 4~

8~

12~

Fig. 4. Envelope of depth variation in the profiles monitored over the S6te close-survey area from May 1989 through December 1991.

The inner bar appeared as rather indistinct and transient in its position during the survey period• Its low crest (0.1-0.3 m above the bottom) migrates within 100 m from shoreline; the bed slope becomes generally gentler seawards from the crest (Fig. 4). The bar height has rarely exceeded 0.5 m (in some profiles, in May and September 1989 and in May 1990). It often behaved as an ephemeral sand ridge near the shoreline. During 1990, while the outer bar was moving shorewards, the inner bar merged in the beach face. The outer bar was better distinguished and permanent all over the area. It formed an asymmetric ridge of about 1 m in height at a distance varying within a range of 190 250 m from the shoreline. As averaged over the mean profile, its steeper slope dips shorewards by 1.45%, while its gentler one declines seawards by 0.92%, joining a very constant inner-shelf gradient of 0.75% off the 5-m isobath. The interbar trough was averaged 3.3 m in depth and 175 m from the shoreline throughout the survey period. Average slope from the shoreline down to this t r o u g h - - w i t h o u t taking into account the transient inner b a r - - i s rather steep (1.88%). The shape and position of the outer bar changed without relation to any seasonal cycle, but rather as a function of the yearly wave regime. All the 1989 soundings showed a well defined bar in all profiles (1--1.5 m in height above the trough),

progressively more distant seawards from northeast to southwest throughout the year. Conversely, its elevation was small from February through December 1990, while it was approaching the shore• In 1991, its relief though at first less pronounced passed rapidly to steeper gradients between April and June and finally, the bar became stable at 220-230 m offshore. At the southwestern end of the area, in profiles S10 and S11, the outer bar was prominent also in 1990, while it moved 130 140 m o n s h o r e - - t h e same as in profile 16 of the long-run series (a), some 100 m farther to the southwest. This evolution continued in 1991, when the outer bar moved into an inner bar position at the same time as a new outer bar was developing in about its former place, between 250 and 300 m from the shoreline• A similar, less extended process was observed in profile $5 from August through December 1990. It is to be mentioned that a levelling of both nearshore bars since 1984 and more recently is observed; a tendency toward a single-bar profile have been characteristic for the cross section 16 of the long-run series (a), located near the southwestern end of the S6te close-survey area (Fig. 2). Fading of the inner bar seems to propagate southwestwards, as only one distant bar has been found in the cross section 15 b, lately surveyed since 1989. The envelope of change in bed elevation and the mean profile of the Marseillan close-survey area

J.P. Barusseau et al./Marine Geology 122 (1994) 4 7-62

54

are represented in Fig. 5. It shows a more complex morphologic structure, though the wave climate conditions are apparently identical. The standard profile is characterized by two permanent, generally well-defined nearshore bars, the crests of which are indicated by CI and CO. A smaller sedimentary feature (CTI) rather discontinuous and transiem--was migrating within the surf zone during the survey period. The outer interbar trough (TO), much deeper (exceeding 5 m) than in the S6te area, was frequently rippled with some local, irregular, depositional features without continuity alongshore (Fig. 6). One could notice such humps on the bottom of the outer trough in profiles 14 and 15 of the long-run series (a), situated on both northeastern and southwestern sides of this closesurvey area, during the 1984 and 1986 bathymetric measurements. The depths at the crest of the outer bar (CO) were greater (about 4 m) than in the S&e area (about 3 m), but the bar is always much farther off and the unitary volume of sediment in the sections within the 10-m isobath is conspicuously greater in the Marseillan area; this is consistent with the evolution of the long-run bathymetric profiles (a) between the two close-survey areas. As related to the 1989 bar-system development, shoreward movement of the inner bar occurred in

CTI

TI

CI

1990 and 1991, while its height oscillated frequently all over the area throughout the survey period. The outer bar decreased in height during the winter 1990 1991 and further on. It moved shorewards in no more than 4 of the 11 profiles, being more often stable. The interbar trough (TO) increased in depth in profiles M3 and M7 and a smoothening tendency was observed in its irregular bed-features. This trough reached a maximum width of 180 m in February 1990. As the wave climate was generally more energetic in 1989, the bar patterns were more prominent in both the S6te and Marseillan survey areas. Sequential bathymetric maps from different monitorings and the derived maps of bed-elevation variation reveal an overall attenuation in nearshore morphodynamics during the following years, although some rapid changes could be detected between consecutive surveys. The spatial distribution of bed-elevation change during the 1989 1990 winter period is shown in Figs. 7 and 8. Infilling of the interbar trough concomitant with bar recession can be noticed in the S6te area during this season. Erosion of both inner and outer bars and accretion in the outer interbar trough are even more evident in the Marseillan area between October 1989 and February 1990. Nevertheless, bars and troughs

CO

TO

Upper limit

,/

-5

O.

--

CO : Crest of outer bar

--

CI

-10

: Crest of inner bar

Lower limit

CTI : Crest of transient inner bar TO

: Outer interbar trough

TI

: Inner interbar trough

.~

I

Distance from baseline (m)

)

-15

0

400

800

1200

Fig. 5. E n v e l o p e o f depth v a r i a t i o n in the profiles m o n i t o r e d over the M a r s e i l l a n close-survey area from M a y 1989 t h r o u g h D e c e m b e r 1991.

J.P. Barusseau et al./Marine Geology 122 (1994) 47 -62

55

TI

CO

I r

•q

-10 m

250 450

~ 650

, s.;.~.; ~ "", 7.

850 1050 m Fig. 6. Block diagram of the nearshore zone in the Marseillan close-survey area, May 1989.

were well conserved over the profiles throughout the period. Fitting the bed-elevation data over the survey period shows that the nearshore zone of S6te is tending to a single lower-bar equilibrium profile of the type y = ax b, where x = distance from baseline; and y = bed elevation, both in meters (Bruun, 1954; Qu61ennec, 1984). The mean values of

parameters computed for this area would be: a = - 0 . 1 0 ; b =0.55. I f assuming the same function, two standard mean profiles could characterize the nearshore zone of Marseillan: one for the seaward slope of the outer bar and farther offshore, with a = - 0 . 1 0 ; b=0.60; another for the inner bar and the surf zone, with a = - 0 . 1 5 ; b=0.65.

56

J.P. Barusseau et al./Marine Geology 122 (1994) 47-62

T

NE =~ l

8

Erosion

Accretion

500

I00

300

500

700

Distance from baseline (m)

Fig. 7. Bed elevation change over the Sate close-survey area between September 1989 and February 1990.

An a=f(b) diagram (Qualennec, 1984) suggests a relative equilibrium in the Sate area, but with a significant erosion tendency, when considering the evolution toward a single-bar profile. The Marseillan area is proved to be in equilibrium in the outer bar zone, with a trend to accrete onshore (Fig. 9).

3.3. Beach morphodynamics The evolution of the subaerial beach shows a stronger differentiation in the erosion-accretion sequences between the northeastern and the southwestern sections. As the grid of topographic profiles (b) covers almost the whole stretch of the beach barrier (Fig. 10), a long-period shift of the shoreline, with an erosional trend could be observed in the northeastern part during the survey period, whereas in the southwestern part, the beach

breadth fluctuated widely with a rather depositional trend. The values of the differences in distance of the shoreline from the baseline marked along the profile lines between the dates of measurements in Fig. 10--are particularly high for a beach no more than 20 m to about 80 m wide from northeast to southwest. The envelope of change in lower-beach elevation exceeds 1.5 m to the northeast. Recession-accretion sequences did not follow a seasonal rhythm. There were periods of continuous accretion (e.g., between October 21, 1988 and October 3, 1989 from P4 to P9) or conversely, rapid and high beach width variation over a very short time interval (e.g,. all over the beach between October 1989 and March 1990). Over a longer period (1984 1991), it seems that an erosion-accumulation longshore sediment-wave was migrating following the resultant beach-

J.P. Barusseau et aL,/Marine Geology 122 (1994) 47-62

57

I | II $

1

$

/I 1 i

$

interbar trough

$

NE inner bar [,." crest

t I

t

i

P []

outer bar crest

/

I

I

Erosion O

.

v

Accretion

......

500 100

70O

500

300

Distance from baseline (m)

Fig. 8. Bed elevation change over the Marseillan close-survey area between October 1989 and February 1990.

0,0

DOMINANTLY

S

Mi []

-0,1

B

ERODING I,U I-i,Ll

Mo

DOMINANTLY

-0,2

£

,,(

RCCRETING m

-0,3

Sate

-0,4

O,

:

S

Mereeillen

-

Mereelllan

-

internal outer

part

slope

:

:

Mi

= ¢ u~I

Mo

!

!

i

0,2

0,4

0,6

i

0,8

1,0

PARAMETER b

Fig. 9. a =f(b) diagram of standard mean profile equations and corresponding evolution trends in the nearshore zone.

sediment drift from northeast to southwest. The sand removed from P1 to P3 had a tendency to settle in section P4 more recently; farther on,

erosion in P6-P7 provided material for an even more significant foreshore-accretion tendency culminating in P9 and, maybe, so farther. Fulcrum

J.P. Barusseau et al./Marine Geology 122 (1994) 47 62

58

21-06-91

15-02-91 01-10-90 22-05-90 2~0~90 07-~-89 021049 18-05-89

21-10-88

07-04-88 PI

P2

P3

P4

P5

P6

P7

P8

P9

NE

P10

Pll

SW

Fig. 10. Changes in beach width (m) in the long-baseline topographic profiles during the survey period.

points of this wave-like sediment transfer pattern are supposed to migrate alongshore within the sections P3-P4, P4-P5, P6 P7 and around P l l , depending on wave climate. Total sediment budget could be calculated on the basis of the negative and positive changes in beach width and height integrated through time and alongshore between the extreme profiles. A loss of about 70,000 m 3 of sand has thus been estimated for 1984-1991. 3.4. Sediment characteristics

No significant evolution could be observed in the grain-size characteristics over the nearshore zone during the survey period. Rather irregular variations with time were only detected in skewness and kurtosis parameters, which seemed more responsive to changes in wave-climate. Nevertheless, greater variability in the median diameter of grains was found on the foreshore slope, especially at S6te, where a decrease in the sorting and a progressive increase in the negative skewness of the distribution curve can be related to a morphologic degradation of the upper profiles from 1989 to 1991. High standard deviation (ai>0.7) and negative

skewness values ( S K I < - 0 . 1 5 ) have been also averaged on the northeastern subaerial beach within the sections P1-P5 of the long-baseline series (b) for the same time period. These show a coherent spatial distribution, passing gradually to a l < 0.4 and positive skewness values towards the southwest, as the mean grain size decreases from nearly 0.5 to less than 0.3 mm of diameter in the same direction (Radulescu et al., 1992). In the nearshore zone, the grain-size distribution did not much vary in the post-winter (PW) and post-summer (PS) standard sediments, except for the toe of the foreshore slope (Fig. 11 ). However, one may remark that the mean diameter of the PW sediment is somewhat smaller offshore and greater onshore from the bar, whereas finer sediment is to be found in the trough and on the foreshore slope under PS conditions.

4. Discussion

No model universally valid for the formation and movement of nearshore bars is yet available, in spite of a great number of studies on the subject. However, the permanent character of an outer bar

J.P. Barusseau et al./'Marine Geology 122 (1994) 4 7-62

59

-10 m

-5 m

%

%

I

0

r

'

I

Trough

2

Bar crest

%

% !

/! 0

+

10

Foreshore

r~

D (lam)

1000

%

10

D (~m)

1000

Fig. 1 I. Grain size distribution at characteristic points along the nearshore mean profile of the S6te close-survey area under postwinter (solid line) and post-summer (dashed line) regimes.

all along the Sbte-Marseillan beach barrier and the presence of a second, more fluctuating inner b a r - - o f t e n vanishing northeasterly--suggest that these are equilibrium forms adapted to a hydrodynamic instability on a dissipative, wave-graded nearshore slope. Following the earlier classification of bars by Greenwood and Davidson-Arnott (1979) in 6 groups (I VI) according to size, shape, number seaward, location, wave energy, wave breaking type and degree of stability, these nearshore bars belong essentially to group V I - - a s they are rather high and stable, asymmetric landward in profile, in general more than one in number, more spaced and higher offshore, sinuous to crescentic and developing under a wave climate dominated by

moderate spilling breakers. Towards the northeast, the bars seem to pass progressively--in space and with t i m e - - t o group V, as they are lower than the dominant waves and decreasing in number, rather straight, unstable and developing under a higher wave energy on a steeper nearshore slope (Figs. 4 and 5). The transient character of the low inner bar in the Sbte close-survey area could have some relationship to a more or less diffuse seaward sediment flux across its crest. Some aerial photographs show traces of rip currents in the zone of transition from group V to group VI bars, between profiles 16 and 15b of the long-run series (a). This is consistent with the apparent scour of the coastal sand-prism in the northeastern profiles as compared to its

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J.P. Barusseau et al./Marine Geology 122 (1994) 47-62

build up in the southwestern ones. On the subaerial beach, transition from erosion to stability in the same N E - S W direction follows the change in the nearshore bar system, which is more protective for the shore in group VI.

4.1. Morphologic and sedimentary evolution factors A certain polymorphism of the nearshore b a r s - going through straight, crescentic, or even transverse inner bars (Long and Ross, 1989) reflects the multitude of factors implied in their evolution. Slope of the nearshore profile is the main factor determining the number of bars in a cross section (Evans, 1940; Komar, 1976; Svensen, 1984). The average nearshore slopes of 0.91% in the S6te close-survey area and 0.83%, in the Marseillan area are consistent with the tendency toward a single-bar profile in the steeper, former one and the stable two-bar system in the latter. Joining some earlier observations in a southern area of the Gulf of Lions (Brissaud et al., 1989; Barusseau et al., 1991), the correlation between slope and number of bars is thus again verified. Bed slope and more or less rapid shoreward change in the H/d ratio (wave-height/water-depth) determine the type of wave-breaking. Both slope and predominant wave-height (90% of waves less than 1.2 m high) in the study area are favourable to spilling breakers and onshore-offshore convergent sediment flow beneath. Consequently, group VI bars are predominant, except for the northeastern stretch, where plunging breakers are more frequent during high-energy periods after the recent southwestward extension of the stone-block revetment of |he S6te shorefront and a subsequent bed-scouring in the adjacent inshore zone. Edge waves are apparently present as a factor of bar formation in the surveyed zone. Crescentic, rhythmic alongshore forms are usually observed at the shoreline, especially towards Marseillan. The effect of edge waves has been already detected in the Gulf of Lions (Barusseau and Saint-Guily, 1981) and has been described for a southern crescentic bar-system (Barusseau et al., 1991). Infragravity waves could not be observed directly in the S6te and Marseillan survey areas,

while rip current distribution is considered by some authors to be closely related to these long period waves (Bowen, 1969; Mei and Liu, 1977). The occurrence of rip currents may be presumed in the outer bar at S~te and in the inner bar at Marseillan, as the longshore profiles show rhythmic bar-crest lowerings which could be traces of rip channels. Some transverse bed-forms (megaripples) migrating in the interbar troughs, where the greatest part of the shore-parallel movements are supposed to be concentrated, could be generated by local decelerations in the longshore currents. Other depositional features appearing on the bottom of the outer interbar trough in the southern nearshore zone seem rather to represent some temporary stages of interbar shoreward sediment movement. On a larger scale, at the shoreline, accretion in some sections appears in direct proportion to erosion in the neighbouring sections, as if erosiveaccumulative sand-waves were resulting from a variation in the potential charge of a dominant longshore sediment flux (Fig. 10). Wave energy is an essential factor of bar-system development. Distinction is usually made between the effects of fine weather swells and storm waves (Hayes, 1972; Winant et al., 1975; Short, 1979; Sallenger et al., 1985; Wright et al., 1979; Fox and Davis, 1976). Rapid changes in bed morphology correspond to sudden changes in sea state. From our observations, it results that the greatest variations occur after storms of lower than annual frequency. During the period of study, the wave energy of the 1988 1989 winter had a greater effect than the wave energy developed throughout the following three years. The wind effect has been outlined by many authors (Niedorora et al., 1985; Davidson and McDonalds, 1980; Sherman and Greenwood, 1985). It is certain that the shore-normal direction of the most frequent and strongest wind storms in this region of the Gulf of Lions (southeast and northwest) favours an onshore offshore sand movement. Migration of bars occurs during storm periods, when field work is unfeasible. On the other hand, wave-climate data were insufficient for defining and a f o r t i o r i quantifying the complex waveinduced coastal processes which occurred between

J.P. Barusseau et al./Marine Geology 122 (1994) 47-62

surveys. Therefore, the global volumetric results presented here attempt to compensate for the lack of direct measurements of sediment transport. The sedimentary budget of the littoral prism down to the 10-m isobath between profiles 13 and 17 (a) shows total loss during stormy years and total accretion during the relatively calm periods succeeding then (Table 1). It suggests that sediment transfer was temporarily returned from southwest to northeast and even from the deeper offshore landwards during periods of southwestern swell, which are rather frequent in the region. Morphologic changes and textural variation of sediments in the nearshore zone have rarely been examined together at this multiyear time scale. Temporal variations in grain-size distribution were small during the survey period; after 1989, only some slight changes in median diameter and in sorting, skewness and kurtosis indices were recorded. Although seasonal variations have been frequently reported by different authors (Dubois, 1989; Liu and Zarillo, 1989), the results of the 1989 1991 surveys in this nearshore zone led us rather to give a greater attention to the effect of exceptional storms, of pluriannual return period, on the significant sedimentary dynamics. So, by the end of the high-energy 1988-1989 period, which was marked by the most important morphologic changes, the sedimentary material was characterized by a very good sorting, a low skewness and a mesokurtic character of the distribution curve. Further on, through a longer, less energetic period, a slow evolution of the nearshore features was accompanied by a somewhat continuous degradation in grain-size distribution (less sorting, negative skewness parameters and leptokurtic tendency of the curves).

5. Conclusions

Results of a multiyear survey of morphologic and sedimentologic evolution of the shore and nearshore zones of the beach barrier from S6te to Marseillan show that the beach and the sand bars are in dynamic equilibrium with the storm waves and the wave-induced sediment transport during

61

high-energy periods. Their morphologic changes are small or not significant during periods when storms are not frequent, regardless of season (as was the case after 1989) but the nearshore sedimentary budget can recover rapidly in such periods. Maps of bed-elevation change over the closesurvey areas show that: - Formation of distinct bars is accompanied by higher scale sediment movements implying permanent nearshore readjustments. - Temporary steep slopes within the bar system favour sand entrainment. - Significant changes in morphology (e.g., passage from well differentiated forms to less pronounced bar patterns, or vice versa) are rapid (several weeks). Spatial, alongshore variation in coastal processes is introduced by a natural headland and by the defensive structures of S6te at the northeast of the study area. As a combined groin effect under dominant southeastern wave action, the shore is eroding south of the town, where the inner bar is transient with a tendency toward a single-bar nearshore profile. A decreasing gradient of a gradually saturated longshore sediment transport towards the southwest is followed by a transition to a stable, rather crescentic two-bar system, at the same time as the beach is widening, the sand is becoming finer and the long-term sedimentary budget of the nearshore zone accumulative in this direction. Wide alongshore periodicity of the erosion-accumulation processes at the shoreline suggests a pulsating movement in the longshore currents. Finally, there is no homology between the outer bars of the two separate close-survey areas of Marseillan and Sbte. Free-type rip currents may occur in the sector of transition between the two different groups of bars, while rip-cell circulation is supposed to govern the sedimentary dynamics towards the southwest of the nearshore zone. Sediment exchange with the adjacent nearshore and with the deeper offshore zones is considerable. During storm periods, the sand loss is greater, but during relatively calm post-storm periods the compensation is smaller in the northeastern sections, which are initially most exposed to the predomi-

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J.P. Barusseau et al./Marine Geology 122 (1994) 47 62

nant storms. Consequently, the equilibrium of the coastal zone is changing here with time, adapting gradually to a steeper beach and upper nearshore slopes and to coarser sediment. Conversely, in the southwestern part of the study area, the beach and the nearshore bars are stable, developing in dynamic equilibrium with the wave climate variation.

Acknowledgements This paper presents in a condensed form a part of a larger study on the management of the Languedoc-Roussillon coast supported by the Council of the L.-R. Region and the French Ministry of the Environment. Thanks go to J. Krantz and B. Bickelhaupt who kindly helped us in correcting the English.

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Dubois, R.N., 1989. Seasonal variation of mid-foreshore sediments at a Delaware beach. Sediment. Geol., 61:37 47. Evans, O.F., 1940. The low and ball of the eastem shore of Lake Michigan. J. Geol., 48:476 511. Fox, W.T. and Davis, R.A., 1976. Weather patterns and coastal processes. SEPM Spec. Publ., 24:1 23. Greenwood, B. and Davidson-Arnott, R.G.D., 1975. Marine bars and nearshore sedimentary processes, Kouchibouguac Bay, New Brunswick. In: J. Hails and A. Carr (Editors), Nearshore Sediment. Dynamics and Sedimentation. Wiley, London, pp. 123-150. Greenwood, B. and Davidson-Arnott, R.G.D., 1979. Sedimentation and equilibrium in wave-formed bars. A review and case study. Can. J. Earth Sci., 16:312 332. Hayes, M.O., 1972. Forms of the sediment accumulation in the beach zone. In: R.E. Meyers (Editor), Waves on Beaches and Resulting Sediment Transport. Academic Press, New York, pp. 297 356. Komar, P.D., 1976. Beach Processes and Sedimentation. Prentice Hall, Englewood Cliffs, N J, 429 pp. Liu, J.T. and Zarillo, G.A., 1989. Distribution of grain-sizes across a transgressive shoreface. Mar. Geol., 87: 121-136. Long, B. and Ross, N., 1989. Revue et classification des barres d'avant-c6te. Bull. Soc. Natl. Elf Aquitaine Prod., 13: 175 187. Mei, C.C. and Liu, P.L.F., 1977. Effects of topography on the circulation in and near the surf zone linear theory. Estuarine Coastal Mar. Sci., 5: 25-37. Niedoroda, A.W., Swift, D.J.P. and Hopkins, T.S., 1985. The shoreface. In: R.A. Davis Jr. (Editor), Coastal Sedimentary Environments. 8:533 624. Qu61enec, R.E., 1984. Int6r6t de l'analyse des profils de plage pour l'&ude et la dynamique s6dimentaire littorale. Cas du littoral du delta du Nil. 18th Journ6es Hydraulique. (Marseille.) L'Hydraulique et la Maltrise du Littoral, 6 pp. Sallenger, A.H., Holman, R.A. and Birke-Meyer, W.A., 1985. Storm-induced response of a nearshore-bar system. Mar. Geol., 64: 237-257. Sherman, D.J. and Greenwood, B., 1985. Wind shear and shore-parallel flows in the surf zone. In: D.L. Forbes (Editor), C. R. Conf. Canadienne Littoral 1985. pp. 53 65. Short, A.D., 1979. Three dimensional beach stage model. J. Geol., 81:42 64. SOGREAH, 1985. Protection de la plage de S6te. Etude diagnostic. Rep., 45 00 85R2, 34 pp. (Unpubl.) Svensen, I.A., 1984. Mass flux and undertow in a surf zone. Coastal Eng., 8: 347-365. Winant, C.D., Inman, D.L. and Nordstrom C.E., 1975. Description of seasonal beach changes using empirical eigenfunctions. J. Geophys. Res., 80:1979 1986. Wright, L.D., Chappel, J., Thom, B.G., Bradshaw, M.P. and Cowel, P., 1979. Morphodynamics of reflective and dissipative beach and inshore systems: Southeastern Australia. Mar. Geol., 32:105 140.