2 t- {;- . (; U I , ~6h

Reprinted from

SEDIMENT ARYGEOLOGY

.Sedimentary Geology 101 (1996) 55-67

.

Short- and medium-term grain size changes in deltaic beaches(Ebro Delta, NW Mediterranean)

Jorge Guillen, Albeit Palanques

. lnstituto de Ciencias del Mar (C.S.I.C.), Plaza del Mar, 08039 Barcelona, Spain

Received 26 October 1993; revised version accepted 23 February 1995

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ELSEVIER

SedimentaryGeology

Sedimentary Geology 101 (1996) 55-67

Short- and medium-term grain size changes in deltaic beaches(Ebro Delta, NW Mediterranean)

Jorge Guillen, Albert Palanques

(. lnstituto de Ciencias del Mar (C.S.l.c.), Plaza del Mar, OS039 Barcelona, Spain

Received 26 October 1993; revised version accepted 23 February 1995

Abstract

The beach sediment of the Ebro Delta shows significant temporal grain size changes unrelated with seasonalprocesses. The evolution of the grain size during the 1988- I99 I period shows coarsening and fining variations, with ageneral coarsening trend which ranges from 2.10 to 1.96 phi (about 8, 10-3 mm/year). This low-magnitude grainsize change is considered significant because it is indicated by both mean values from the data set (textural zones)and individualized sampling points. The coarsening trend of the beach sediment in the Ebro Delta is mainly causedby the sedimentary deficit affecting the Ebro coast. The finer sand fractions of the sediment arc progressivelywinnowed from the initial beach deposit without being replaced by new river sediment supplies. Superimposed onthe general coarsening trend there exist grain size variations that may be related with processes of a shorter periodsuch as changes in the sediment supplied from the river into the nearshore zone, variations in the wave energyaffecting the beach, and man-induced actions, such as beach nourishment. The mean grain size of beach sedimentsampled 20 years ago indicates that the coarsening gradient measured during the study period cannot beextrapolated to longer periods of time (decades). This work shows that processes of a different temporal scale but asimilar magnitude must be integrated in order to explain the evolution of sediment grain size in deltaic beaches.

. 1. Introduction

The grain size of beach sediment shows spatial(longshore and cross-shore) and temporalchanges. The longshore changes are mainly re-lated to variations in the incident wave energyalong the beach, the selective sorting of sedimentduring transport or changes in the amount andsize of sediment supplied to the beach (McCave,1978; Bryant, 1982; McLaren and Bowles, 1985;Stone et aI., 1992). The sediment grain size distri-bution in the cross-shore profile is usually relatedto the "equilibrium profile" concept, which indi-cates that a particular grain size tends "to move

0037-0738/96/$15.00 @ 1996 Elsevier Science B.V. All rights reserved

SSDI 0037-0738(95)00021-6

across the profile to a position in which it is inequilibrium with the wave and flows acting onthat sediment grain" (Horn, 1992). Temporalchanges of grain size in beach sediment havereceived comparatively less attention than long-shore and cross-shore changes, probably becausetemporal variations have a lower magnitude andare more difficult to recognize (Stubblefield etaI., 1977; Davis, 1985; Liu and Zarillo, 1989).

Studies of the temporal evolution of beachsediment grain size have focused on seasonalvariations in the beach sediment textural parame-ters related to the seasonal wave climate affectingthe beach (Prakash and Prithviraj, 1988; Dubois,

56 1. Guillen, A. Palanques /SedimentGl}' Geology 101 (1996) 55-67

1989; Stone et aI., 1992). However, if the seasonalwave climate were the single controlling factor oftemporal grain size evolution, the beach sedimentwould show a steady grain size trend over time.Inter-annual grain size variations in deltaicbeaches must be caused by other factors, such asthe variations in the amount and texture of sedi-ment supplied to the beach. The sediment can besupplied from rivers, from offshore, from erodedareas of the coast or from anthropogenic beachnourishment. An increase in sediment supplyleads to a net deposition of sediment, whereas adecrease produces an erosional trend in the beachand the eroded sediment is transported to accre-tional areas. An accretional trend in the beach isgenerally associated with the fining of sediment,whereas an erosional trend produces a coarsersediment (McLaren, 1981; Prakash and Prithviraj,1988; Dubois, 1989). Thus, besides the seasonalcomponents associated with annual wave climatechanges, the temporal evolution of beach sedi-ment grain size over .several years can be ex-pected to include longer period oscillations causedby variations in the sediment supply and otherrandom events.

The temporal evolution of the sediment grainsize on the Ebro Delta coast was studied duringthe period 1988-1991. The Ebro Delta shorelineis 50 km long; it is a microtidal, storm-dominatedcoast that has developed from the sand sedimentsupplied through the Ebro River (Fig. 1). Themean wave climate affecting the Ebro coast is oflow energy (H = 1 m and T = 3.5 s), though dur-ing storms wave heights can be up to 6 m (Flos,1985; Gracia et aI., 1989). Strong storms usuallyoccur during the October-February period,whereas for the remaining months they are un-usual. The sand sediment extends in the EbroDelta from the backshore to about 11 m depth.The low amounts of sediment supplied from theEbro River cause a sediment deficit in thenearshore zone and an erosional trend along theshoreline (Guillen and Palanques, 1992; Guillen,1992).

The main objectives of this paper are: (1) tostudy the grain size evolution of beach sedimentin an erosion-dominated deltaic coast, (2) to de-termine the control factors of th'is evolution, and

(3) to evaluate the possibility of using the tempo-ral changes in beach sediment size as one mea-sure of erosion and accretion processes in thecoast.

10 cm wi(sediment Jranges froconsiderecthe beachthe sampl2. Methods

Eight sampling surveys were carried out duringthe period 1988-1991 on the Ebro Delta coast(June and October 1988, March and July 1989,March and July 1990, March and July 1991). Oneach survey 31 pre-fixed profiles spaced 1-2 kmapart along the Ebro Delta shoreline were sam-pled, and on each profile a sample was taken ofthe dune, swash, trough and bar crest (Fig. 1).Profiles located in the Ebro River mouth (P-26 toP-29) are not considered. The total number ofsamples analyzed in this study was about 1,100.Location of sampling profiles was indicated bypermanent poles situated along the beach. Ineach profile, standard topographic procedureswere used in order to det,ermine the exact posi-tion of samples. Sediment was collected by drag-ging on the bottom a plastic bag inserted in anoval metallic frame. The mouth of this frame is

.

.40°40'

I 6km I

0°40' 0°50'

Fig. 1. Study area and locations of sampled profiles along theEbro Delta coast.

Fig. 2. Longshofldifferentiated texiof profiles ill Fig.

OCTOBER 1988

Yvv'iJ'vl

JULY 1989

~~JULY1990

"-'\~~..

2.4 JULY19912.2

\1:2.0

~1.8

1.60 4 8 12 1620 24283236

PROFILES

1. Guilten,A. Palanques / Sedimentary Geology 101 (1996) 55-67

the tempo-, one mea-,ses in the

10 cm wide and the vertical penetration in thesediment is about 2-4 cm. The weight of samplesranges from 100 to 200 g. Each sample can beconsidered the result of all the processes acting inthe beach during a certain period of time beforethe sampling. It is assumed that samples are

! out duringDelta coast

July 1989,y 1991). On

:ed 1-2 km: were sam-las taken ofest (Fig. 1).-uth (P-26 to

number ofabout 1,100.Indicated by: beach. In

procedures: exact posi-ted by drag-serted in anthis frame is

JUNE 1988

!:: ~ 0/<t~1.9

(8 1.7

MARCH 1989.-2.3:2Q.~2.1 ~ .

.:E1.9

J - \..I y r

"-V\

1.7

MARCH 19902.3.-:c

$2.1~~1.9

1.7

2.3

~J~..Jv

MARCH 1991

'"

~25 CAPE

24 TORTOSA2 UDA ISLAND

2/

'~

. -i::: vJ\ V

1.7

1EACH

2.2 GENERAL TREND

::; ~: -

~~ 1.9: ; :,

I ,II

Ij

I' "I

1.8 : I : II :111: IV; : V ,

0 4 8 12 1620 24 283236PROFILES

R

, 6km I

iO'

57

representative of daily or weekly sedimentaryprocesses, and that the effect of processes operat-ing on scales of hours immediately before thesampling have a subordinate influence in thegrain size. This assumption is achieved becausethe sampling was carried out during non-storm

profiles along the Fig. 2. Longshore variation in the sediment grain size in each sampling survey and the resulting general trend, indicating thedifferentiated textural zones (I = Fangar, II = Trabucador Bar, III = Eucaliptos Beach, W = Buda Island, V = Fangar) (Iocationsof profiles in Fig. 1).

1.7 D=2.12 . 0.00521

2.3ZONEIII

-:c~2.1

z<w::e

1.9"' -

1.7 D=2.03 - 0.00411

2.3ZONE V

..-..:c

2.1.!;z<w::e

1.9 (

1.7 D=2.13 - 0.00391

co co 0) 0) a aT"" T'""co co 00 co

(j)O'J 0'>

CJ)

J: J: J:LU

() () ()

f- c: ~c: ~a:~Z <I: <I:;;) () ;;) ;;) <I: ;;)""")

0 ::!: J ::!: ....,::!: ..,

58

f'

J. Guillen, A. Palanques / Sedime11lary Geology 101 (J996) 55-67

conditions, the volume of sediment sampled isrelatively high and at least 4 samples are used inthe characterization of the sediment grain size ateach point of the profile.

The grain size distribution of sediment wasdetermined by settling tube, according to thetechniques explained in Giro and Maldonado(1985), and textural parameters were computed

2.3ZONE I

........:c

..e: 2.1z<w::e

1.9

by the moments method (Friedman, 1967). Fur-thermore, 21 beach samples taken in 1970 alongthe Ebro Delta beachface were re-analyzed inorder to obtain a medium-term approach to thegrain size evolution.

The changes in sediment grain size were foundto be of low magnitude along beaches of the EbroDelta during the study period. The magnitude of

ZONE II

.D=1.98 - 0.00461

ZONE IV

D=2.0 1 - 0.00431

GENERAL TREND .--

-- --

D-2.05 - 0.0041

COCO(J)cnco 00 co co

J:

wl-~>-z()<I: ...J

;So::!: ;S

0 aO'J 0')J:

~ ~« ;;)::!: ..,

.- .....O'J 0)

J:()

c: >-« J::!: ;S

Fig. 3. Temporal evolution from June 1988 to July 1991 of the beachface sediment grain size in the Ebro Delta, showing the

resulting coarsening gradient in the five textural zones and the general gradient (D = grain diameter in phi units, t = time inmonths).

Fig. 4. Temporal l

Delta, showing thl

0000 0) 0) 00 yo-"'" 0000 0') 0') 00 ,.... ,... coco 0') 0') 00 ,....'f'""0000 00 OJ 0)0') C) (j)

«lea co ex:> 0'>0') 0)0'>00000000 Cj)0'> C')C')

J: J: J: J: J: J: J: J: J:

LUf- a1 >- a1~ a1 ~\!i!f-a1 ~a1~ a1~\!i!f-a1~

u ua:~ a:~Zu «....J «::> « ::> ::>u «::> «::> «::>::>u«::> «::> «::>;;;0::;:;;; ::;:..., ::;:...,...,0::;:..., ::;:..., ::;:...,...,0::;:..., ::;:..., ::;:...,

J. Guillen, A. Palanques / Sedimentmy Geology 101 (1996) 55-67 59

1967). Fur-1970 along

malyzed in)ach to the

.

en 2'

~2'3 3

2.24

2.35

~V.~.

"'" "'" -- ~

~..,~,~b

~ 2'2

bd~'

b2

7 2'

~2

8 2'

~2

9z .

~ '"-,

"-

"'.

"" ".

'""'.

/."'.

a.

1.8

'- ".

1.8

.

1.

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'1." "

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~2.1

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10' 2.111 2.112

2.113

~f~k~1.L.'</~'30

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b2

15 2'

~2. 16 2'

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,

.

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~'

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U

~',.,~o

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2'

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.

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.

.

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1.8 1. 1. 1.

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63

33

~ Y ......

.....

.

-~ ""'" --, .

1.9 1. 1. 1.

were foundof the Ebroagnitude of

iI

(8I

enI-Z::J:Ea.

2'2

D4 2'

~3

35 2'

b3

36 ~~~

~ ~~",

"

u u~-

. wf- a::>- a: >-""""

.

"""

.

5(,) «:5 «:5

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.'.

, ...,0 ::;:..., ::;:...,""""

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1.8 1.9 1.

y-T"""0')(7)

J:Ua:~«::>::;:...,

elta, showing theunits, t = time in Fig. 4. Temporal evolution from June 1988 to July 1991 of the beachface sediment grain size in the sampling profiles of the Ebro

Delta, showing the coarsening trend in all of them (locations of profiles in Fig. 1).

Mean Deviation Skewness

June 1988 2.10 0.44 - 2.31October 1988 2.05 0.31 -2.55March 1989 2.01 0.37 - 1.78July 1989 1.96 0.34 -1.61March 1990 1.94 0.38 - 1.00July 1990 1.91 0.34 -0.68March 1991 1.93 0.34 -0.66July 1991 1.96 0.34 -0.41

Mean values from all the samples taken in each survey (meanand standard deviation in phi units).

June 1988 2.07 2.02 2.14 2.17October 1988 2.07 2.00 2.09 2.01March 1989 1.93 1.99 2.12 2.03July 1989 1.93 1.91 2.06 1.96March 1990 1.89 1.88 2.01 1.95July 1990 1.82 1.83 1.99 1.97March 1991 1.86 1.93 2.02 1.97July 1991 1.88 1.86 2.03 2.07

co OJ()) Q)

0 g cr;co co 00 coQ)

0-,:r: J: J:

w U U UZ f- a:: >- a:: ~a:~;; u <{

:5 « :J «~0 ::;; -, ::;;

""?

::;;

60 J. Guillen, A. Palanques / Sedimentary Geology 101 (]996) 55-67

the grain size variation in individual samples wassimilar to the expected variability caused by sam-pling methods and grain size determination tech-niques. Therefore, highly reliable data are ob-tained by using the mean values of samples. Itwas necessary to consider the mean value ofgroups of samples (profiles or surveys) for theinterpretation of the results. The average mean-size of the four samples taken at each profile (M,the "mean sediment-size profile") is used here asthe characteristic sediment size at each site of thebeach. The longshore distribution of sedimentwas computed as the mean value of all the sam-ples corresponding to the textural zone and tothe survey.

3. Sediment-size changes

The spatial distribution of the sediment grainsize along the Ebro Delta beaches maintained thesame general patterns and trends during the1988-1991 survey period (Fig. 2): (1) the finersediment is located in the La Banya, EucaliptosBeach and Fangar zones, and (2) the coarsersediment is found in the Trabucador Bar andBuda Island. The alongshore grain size sedimentdistribution allows us to define five textural zonesalong the Ebro coast: La Banya (zone I), Trabu-cador Bar (zone 11), Eucaliptos Beach (zone III),Buda Island (zone IV) and Fangar (zone V) (Fig.2). Longshorc gradients of the mean grain sizerange from 1. 10-3 to 10. 1O~3 mm/km, being

Table IChanges in textural parameters of beach sediment on theEbro Delta coast

Table 2Temporal evolution of mean grain size of the sediment indune, swash, trough and bar zones (values in phi units)

Dune Swash Trough Bar

of sedimctextural zgradientsshows a ~tween MaSIze was c,caliptos B(zones II,

minimum from the river mouth towards the Fan-gar and maximum from Cape Tortosa towardsthe south.

The mean sediment grain size was 1.99 phiduring this period of time. However, the tempo-ral variation of the grain size shows a generalcoarsening trend from a mean size of 2.10 phi inJune 1988 to 1.96 phi in July 1991. The coarsen-ing evolution is accompanied by a better sorting(from 0.44 to 0.34 phi) and a less negative skew-ness (from - 2.31 to - 0.41) of the beach sedi-ment (Table 1). This temporal coarsening trend

.

2.50

2.30

:2 2.10.eo

z<Cw::E 1.90

l ; . tI-' t . . :. ' h, I .. . : 'i -': .2't-. . I.

I

1 (2' - 'I, -J --~.!. -I: ! 1 - J . (31 .--.t

1 1I: :

.

1.70

1.50

Fig. 5. Temporal evolution from June 1988 to July 1991 of the

beachface sediment grain size in all the profiles showing thespatial variability of the sediment and the resulting temporal

trends (1: r2 = 0.23; 2: r2 = 0.36; 3: r2 = 0.(3)

Fig. 6. Temporal

the profile and tl

seasonal oscillati<,

0; ;;;:I:U

~.J a:«~~:::;

le sediment in,hi units)

h Bar

2.172.012.031.961.951.971.972.07

:rds the Fan-tosa towards

,vas 1.99 phi., the tempo-ws a general)f 2.10 phi inThe coarsen-1etter sortingcgative skew-.; beach sedi-rsening trend

: ., .. I- --1--1- i;1J .--.

I.

to July 1991 of theofiles showing theresulting temporal03)

J. Guillen, A, Palanques / Sedimentary Geology ]01 (1996) 55-67

of sediment grain size is observed in the fivetextural zones, although some different texturalgradients may occur (Fig. 3). The Banya (zone I)shows a strong coarsening gradient, except be-tween March and July 1991, when the sedimentsize was considerably finer. The Trabucador, Eu-caliptos Beach and Buda Island textural zones(zones II, III and IV) show a progressive coarsen-

0.1ZONE I

0.0

ieII

-:2.9: 0.00u..is

61

ing trend from June 1988 to March 1990 and afining trend between July 1990 and 1991. Thecoarsening trend has a low gradient in the Fangar(zone V), where the sediment is finer betweenMarch and July 1991 (Fig. 3). These gradientsrepresent a rate of grain size change that rangesfrom 7, 10-3 mm/year in the Fangar to 11 . 10-3

mm/year in the Banya. The coarsening pattern

ZONE II

ZONE IV

/\~J..

GENERAL TREND

m./

co <XI en 0')ex) co co coJ:

UJf-~>-Z U «..J;;;

0 ::;;;;;

0 a

'" '"J:~ ~« ::>:;; ...,

, ,....(j) Cj)

J:Ua:: ~« ::>:;; ...,

-0.05

-0.10

0.10ZONE III

Fig. 6. Temporal evolution from June 1988 to July 1991 of the sediment grain size difference (Din between the sediment size ofthe profile and the estimated size from the grain size variation gradient. Although the general coarsening trend is suppressed,seasonal oscillations are unobserved (location textural zones in Fig. 2).

0.05

I

I

i

1.

.-.:2

.9: 0.00 -u..is

-0.05~.

-0.10

0.10ZONE V

0.05..-.

~;;:- -0.00is

-0.05

-0.10OJ 00 cn 0'>00 co CD co

J:UJ

f- ~ >-Z u « :5;;;0 ::;; ...,

a 0en 0')J:

~ ~« ::>::;; ...,

'" '"J:Ucr: >-« -J::;; ;;;

62 J. Guillen, A. Palanques / SedimentalY Geology 101 (J996) 55-67

can also be observed in the temporal grain sizeevolution of the individualized sampling profiles(Fig. 4) and in each of the different morphologi-cal areas of these profiles that were sampled(Table 2): dune, swash, trough and bar. The gen-eral coarsening trend of beach sediment grainsize along the Ebro Delta beaches during the1988-1991 period estimated from the mean valueof each survey can be obtained from the resultinglinear relationship

D = 2.05 - 0.004t

where D is the mean diameter of the sediment inphi units and t is the time in months. Thisrelation implies that the sediment shows a coars-ening gradient of about 8.5' 10-3 mm/year dur-ing the study period. The coefficient of determi-nation (r2) of this relation is 0.65. Better correla-tion is obtained if two phases are differentiatedin the general coarsening trend of sediment: (1)from June 1988 to July 1990 (r2 = 0.95), when thesediment is progressively coarser (17' 10-3 mm/year), and (2) from July 1990 to July 1991 (r2 =0.94), when the sediment is gradually finer (-7 .to-3 mm/year) (Fig. 3). These two phases ofsediment evolution can also be identified in mostof the individual sampling profiles (Fig. 4).

The coefficients of determination have also.been calculated for the individualized profile datain order to determine the significance level of thegeneral gradient (Fig. 5). The value of r2 is 0.23when the general coarsening trend is considered.This value increases if the June 1988-July 1990period is considered separately (r2 = 0.36), butduring the July 1990-July 1991 period the statisti-cal significance of the fining gradient is null (r2 =0.03). These moderate correlations between thegeneral trend and individualized profiles are dueto the fact that the spatial variability of the sedi-ment grain size along the Ebro coast is moreimportant than the temporal one (Fig. 5). Thecorrelation with the general coarsening trend im-proves considerably if the textural zones are ana-lyzed independently and the spatial variability ofthe sediment is reduced (r2 = 0.32 to 0.39, exceptin the Fangar zone where r2 = 0.23).

These data clearly show the existence of ageneral coarsening trend in the Ebro coast during

the study period and indicates that the detectedgrain size changes have periods longer than 1year (Fig. 3). In order to check the existence ofseasonal or shorter-term variations in the beachsediment grain size, the sediment size difference(Dit) between the mean sediment size of theprofile (M) and the sediment size (D) estimatedfrom the equation of the temporal sediment sizevariation in each textural zone (Dif = D - M)was evaluated (Fig. 6). Thus, the general coarsen-ing gradient of the sediment is suppressed and aresidual grain size value is obtained: a Dif posi-tive value implies a deviation to finer sedimentand a Dif negative value indicates a deviation tocoarser sediment. Fig. 6 shows the sediment sizedeviation of the five textural zones and the meangrain size. Deviation to finer sediment, or a nega-tive value of Dif, occurred during the June-Oc-tober 1988 and July 1991 surveys and deviation tocoarser sediment (positive Dif) occurred the re-mainder of the time (Fig. 6). The analysis of theresidual grain size values shows more clearly theexistence of a coarsening and a fining trend dur-ing the study period, with some disturbances intextural zones I and V (Fig. 6). However, a sea-sonal change in the grain size of sediment wasnot observed.

4. Discussion

On the Ebro Delta coast the sediment grainsize is affected by changes of a different temporalscale: (1) a general medium-term coarsening trendrelated to processes with a temporal scale higherthan the study period (3 years), and (2) modifica-tions superimposed on the general coarseningtrend caused by processes of a lower temporalscale.

4.I. General medium-term coarsening trend

The textural variations observed in the sedi-ment of the Ebro Delta (coarser, better sortedand more positively skewed) are characteristic oflag deposits (McLaren and Bowles, 1985), whichindicates that the present beach sediment is a lagdeposit of the pre-existing beach sediment. The

.

coarseningduring a pchad domin:has been pment andfractions 0charge of tby the tralduring the1990; Guill,of sand sedithe Ebro Lriod was 101992). Thenegative seethe delta arshoreline ((Arcilla, 199:nowing of tl~

The relatening evolut:sedimelJt buof the Ebrothe grain sizeality with theSlve processcstronger coalmaximum encoarsening gJrium periods.grain size var

. ~ 20.§.I-Z 10W:Ew~ 0

~5a -10C

~::i -20WIX0J: -30en 0 5

(I .

Fig. 7. Shoreline (

coast during thebetween profiles 2(

le detectedger than 1:xistence oft the beach~ differencesize of the») estimated

diment sizeif=D -M)ral coarsen-cssed and aa Dif posi-

cr sedimentdeviation to:diment sizeld the meant, or a nega-te June-Oc-deviation to

lrred the re-lalysis of thee clearly theIg trend dur-turbances invever, a sea-ediment was

diment grain'ent temporalrsening trendi scale higher(2) modifica-

11 coarseningwer temporal

~ trend

I in the sedi-better sorted

,aracteristic of, 1985), whichEment is a lagsediment. The

J. Guillen, A. Palanques / Sedimentary Geology fOf (1996) 55-67

!-I!

coarsening trend of the beach sediment occurredduring a period of time when erosional processeshad dominated on the coast (Fig. 7). The erosionhas been produced by the shortage of river sedi-ment and the winnowing of the finer sedimentfractions on the beachface. The sediment dis-charge of the river has been drastically reducedby the trapping effect of the dams constructedduring the last few decades (Palanques et aI.,1990; Guillen and Palanques, 1992). The amountof sand sediment supplied from the Ebro River tothe Ebro Delta coast during the 1988-1991 pe-riod was lower than 1. 105 tons/year (Guillen,1992). The shortage of sand sediment caused anegative sediment budget in the external coast ofthe delta and a dominant erosional trend in theshoreline (Guillen, 1992; Jimenez and Sanchez-Arcilla, 1993), which favoured the selective win-nowing of the finer fractions.

The relationship observed between the coars-ening evolution of the grain size and the negativesediment budget in the external nearshore zoneof the Ebro Delta suggests that the gradient ofthe grain size change may have some proportion-ality with the intensity of the accretional or ero-sive processes in the beach. In this situation, thestronger coarsening gradients would occur duringmaximum erosion periods and fining or weakercoarsening gradients during accretion or equilib-rium periods. However, the use of the temporalgrain size variation rates as a measure of erosion

63

10

, -.....2:'20§.I-Z 10W:a:w

~ 0oJD..

!!2 -100WZ:J -20Wa:0J: -30(/)

30 35

'->-1I-Z

~ -10wU«...JCL

5 -20

wZ::iwa: -300J:en

0 8.

!\VV1V\V..

\i

15 20

PROFILES

Fig. 7. Shoreline displacement trends along the Ebro DeltaCoast during the study period. The river mouth is locatedbetween profiles 26 and 30 (location profiles in Fig. 1).

5 10 25

. .. . . .

-400 20 40 60

GRAIN SIZE CHANGE (microns)

Fig. 8. Comparison between the shoreline displacement andthe sediment grain size variation at each profile from June1988 to March 1991. The plot shows that the erosion andaccretion processes are not proportional to the grain sizevariation when individual profiles are considered.

or accretion processes in the coast is limited to ageneral qualitative trend. The comparison be-tween the sediment grain size variation at eachprofile during the 1988-1991 period and theshoreline displacement rate in the profile duringthe same period of time shows a bad correlation(Fig. 8). The coarsening of the beach sedimentwas of 28, 28, 32, 32 and 11. 10-3 mm fromtextural zones I to Y, respectively, during the1988-1991 survey period (Fig. 4). These temporalgrain size variations show that the coarseningtrend of sediment in the Ebro Delta beaches is ageneral process affecting the entire delta coast:the coarsening trend occurs both in erosional (i.e.Trabucador Bar) and in accretional (i.e. La Banya)zones (Fig. 7). On sandy deltaic coasts such as theEbro Delta where the longshore transport ofsediment is dominant (Jimenez and Sanchez-Arcilla, 1993), the gradual winnowing of finergrain sizes in erosional zones produces the coars-ening trend of sediment in these areas, but it alsoproduces a coarsening trend in the sediment grainsize of accretional zones located downdrift, be-cause the sediment supplied from erosion zones

0

co 00 m CJ>co co co coI

UJ()

~Z t- c:::J

()« ::>-, 0 ::;;

""":I

. g 1.2.

t-:I: 1.0CjW:I:w 0.8>

~t- 0.6z0:(()

0.4u:ZCjin 0.2

(() (X)

co co

UJI-2: ~()0

Fig. 11. Monthly,

Delta during the

nents of the wave

64 J. Guillen. A. Palanques /SedimentalY Geology 101 (J996) 55-67

800is progressively coarser (Guillen and Jimenez,1995).

The erosional trend along the Ebro Delta coasthas occurred during the last decades(Maldonado,1972; Califs et aI., 1988). However, a medium-termcoarsening rate of the beachface sediment hasnot been estimated. If the coarsening rate hadbeen a constant, the rate estimated by the 1988-1991 survey period (8' 10-3 mm/year) could beextrapolated and the grain size of the beachsediment two decades ago would be 3.5 phi. How-ever, some samples taken in 1970 along the Ebrobeachface indicate that the mean grain size sedi-ment was about 2.15 phi. Although the number of1970 samples is comparatively low, it is obviousthat the coarsening rate during the last twodecades estimated from the 1970 samples (about1. 10-3 mm/year) is lower than the gradientdetermined for the 1988-1991 period (Fig. 9).This change in the coarsening gradient can bepartially explained by the different intensity ofthe erosive processes affecting the coast duringthe two periods considered. Mean erosion rateswere about 7 m/year during the 1988/1991 pe-riod and 4 m/year during the 1970-1988 period(Califs et aI., 1988; Guillen, 1992).

4.2. Shorter-term grain size modifica,tions

The shorter-term grain size variations canmainly be associated with shorter-term variationsin the intensity of erosive or accretional processesaffecting the coast. Three main factors can mod-ify the intensity of these processes: (1) changes inthe amount of the river sand supply, (2) variationsin the wave energy reaching the beach, and (3)

2.1:2S:~ 2.0w:!:

.1970. 1988-91 TREND, _~1"rR"'"

(8x10-3 mm/yr)-.

C:~!D{1.S)(10'J

~J'- ft1ft1/Yr ):

1.91970 1975 1980 1985

YEARS

Fig. 9. Measured sediment grain size in 1970 and during the1988-1991 period, and resulting coarsening trend.

1990

u;-;;;- 600.§.wC!c::«:c: 400()C/)Q

c::w~ 2003:

0 00'>

(j)

I()

a: ~« ::>:E -,

0'> ;;;I(,)

c: >-<t: J::;; ;;

Fig. 10. Monthly data of water discharge of the Ebro Riverbetween May 1988 and July 1991 showing the low waterdischarges during most of the study period.

man-induced action on the beach. Changes in thegrain size of the sand sediment supplied to thecoast are not expected because the only sedimentsource (the Ebro River) supplies homogeneoussand (Guillen and Jimenez, 1995).

(1) Short-term changes in the amount of sandsediment supplied from the Ebro River are rc-lated to water discharge variations (Fig. 10). Sandtransport in the lower Ebro River only occurswhen the water discharge is higher than 400 m3/s(Guillen et aI., 1992). Thus, during the studyperiod the Ebro River could only supply signifi-cant amounts of sand in June 1988, and in theperiod April to May 1991. These periods of highwater discharge correspond with periods of com-paratively finer sediment on the beach face (Fig.6): the mixing of the new sand river inputs andthe former (strongly winnowed) coarser sedimentresults in a finer sediment on the beachface. Thehigher water discharge of the Ebro River inApril-May 1991 allowed sand sediment suppliedfrom the river to reach the coast. As a conse-quence, finer sediments were detected in July1991 in the Fangar area and in some profiles ofthe Buda Island located close to the river mouth(Figs. 3 and 5).

.

(2) Theshows seasmediate laThe sumrr,period, anthigh-energ;served graition to thi:grain size cmore relatwaves thanbetween stting the stUtperiod bet\long. TheDelta coastTable 3. Thtthan 4 m iJThe comparbefore andMarch 1991along the colar to, or finIn the Eucal Isize was scartrend of thethe Trabuca(La Banya th(storms. In sp

:§:1.2

I-J: 1.0.C)WJ:

0.8w:it

~0.6I-Z«u 0.4u:ZC)(ij 0.2

00 0000 00

I.!.JI-Z;; U0

0 a ~0;(1'J Q'J

J: J:U U wa: ~cr: z<: ~c(~::; ::;

m ;;;:r:~ ~~ ~

the Ebro River~ the low water

'hanges in thepplied to theonly sedimenthomogeneous

nount of sandRiver are re-Fig. 10). Sand

'r only occursthan 400 m3/sing the studysupply signifi-)8, and in theJeriods of higheriods of com-Jeachface (Fig.ler inputs andarser sedimentJeachface. The3.bro River iniment supplied,t. As a conse-~tected in Julyome profiles of:he river mouth

1. Guillen. A. Palanques / Sedimentary Geology 101 (1996) 55-67

.

(2) The wave climate affecting the Ebro Deltashows seasonal variations characteristic of inter-mediate latitude areas (Jimenez, 1995) (Fig. 11).The summer corresponds with the low-energyperiod, and during the autumn and the winterhigh-energy conditions occur. However, the ob-served grain size changes do not show any rela-tion to this seasonal regime. The wave-inducedgrain size changes in the Ebro Delta are probablymore related to the action of sporadic stormwaves than to mean wave conditions. The relationbetween storm waves and grain size changes dur-ing the study is not completely clear because theperiod between the sampling campaigns is toolong. The major storms occurred in the EbroDelta coast during the study period are shown inTable 3. The significant wave height is only higherthan 4 m in October 1990 and February 1991.The comparison between the sediment grain sizebefore and after these storms (July 1990 andMarch 1991) shows that the resulting depositalong the coast displayed a mean grain size simi-lar to, or finer than, the pre-existing one (Fig. 3).In the Eucaliptos Beach and Fangar the sedimentsize was scarcely modified by the storms. A finingtrend of the sediment grain size was observed inthe Trabucador Bar and Buda Island, whereas inLa Banya the sediment was still coarser after thestorms. In spite of these variable responses, it is

65

Table 3Major storms occurred in the Ebro Delta area during thestudy period

Date

01-09-1988

03-12-1988

25-02-1989

13-04-198908-10-1990

02-02-1991

19-02-199124-03-1991

Hs (m)

3.D3.D3.63.64.54.33.43.0

~

Hs = significant wave height (data from Jimenez. 1994).

evident that the general coarsening trend of sedi-ment was interrupted by these storms. Althoughit is usually considered that storm waves producea coarsening of the beach sediment, examples ofsediment fining caused by severe storms have alsobeen reported (Winkelmolen and Veenstra, 1980).The sediment fining related to the strongest stormWaves is caused by the remobilization of all thefractions of previous sediment during the storm.The new sediment would reflect the characteris-tics of the previously underlying sedimentary body(Dubois, 1989), or it is a poorly sorted depositcomposed of all the grain size fractions availableon the coast (Winkelmolen and Veenstra, 1980).

(3) Human action related to beach nourish-ment also alters the general grain size trend. Anartificial dune was constructed on the area of theTrabucador Bar affected by overwashing duringthe storm in October 1990 (profile 11). The dunewas built up with sand slightly finer than thebeach sediment mined from the AIfacs Bay dur-ing the November 1990-February 1991 period.The fining of the beach sediment was detected inMarch }991 in profiles 10 and 11, where the dunewas constructed, and finer sediments were alsodetected towards the south in July 1991 (profiles6, 7, 8 and 9) (Fig. 4). The distribution of thesefiner sediments was caused by the progressiveerosion of the dune and the transport of sedi-ment towards the southwest by littoral drift.

The 1988-1991 grain size evolution shows aninitial coarsening phase and a second fining phase(Fig. 5). The coarsening phase (June 1988-July

~0)001:Ua:«::;

~;;

Fig. 11. Monthly data of significant wave height in the EbroDelta during the study period showing the seasonal compo-nents of the wave climate (data from Jimenez, 1994).

66

r

1. Guillen, A. Palanques / Sedimentary Geology 101 (1996) 55-67

1990) can be explained by the selective winnow-ing of the finer fractions of the beach sediment.The selective winnowing produces a better sort-ing and a less negative skewness in the sediment.The coarsening of sediment has been significantfrom La Banya to Buda Island since October1988. In Fangar the sediment became coarserlater, in July 1989 (Fig. 3). The action of the threefactors analyzed was different along the texturalzones. The Fangar zone was especially sensitiveto the sediment inputs from the Ebro River be-cause most of them accumulate in this zone. Thedune construction affected first the TrabucadorBar and later the Banya zones, whereas stormaction had an irregular impact along the coast.These factors produced a sediment fining andtheir action was mainly concentrated at the endof the study period (Fig. 6).

S. Summary and conclusions

The temporal evolution of the beachface sedi-ment grain size is difficult to investigate becausetemporal variations have a lower magnitude thanspatial grain size changes. Observations about thetemporal evolution can also be influenced by thenoise produced from sampling techniques. Oneway to reduce these problems is to work with ahigh number of samples and to consider texturalzones with a homogeneous sand content. Themain factors causing temporal grain size varia-tions on deltaic beaches are the sediment dis-charge and the wave regime. These factors havedifferent temporal scales of action, from a singlestorm or a river flood to a medium-term (de-cades) erosional trend in the shoreline. The ob-served textural changes result from the combinedaction of all the processes. .

The beach sediment of the Ebro Delta is af-fected by temporal grain size changes unrelatedto seasonal variation. This sediment was progres-sively coarser, better sorted and less negativelyskewed during the study period (1988-1991). Thecoarsening in the beach sediment is mainly causedby the negative sediment budget in the nearshorezone and the resulting erosion processes on thebeach. The erosional processes produce a pro-

gressive winnowing of the finer fractions and acoarsening trend of the beachface sediment.

The coarsening gradient of this period (8. 10-3mm/year) cannot be extrapolated to a medium-term temporal scale because it is higher than thatwhich occurred during recent decades. Erosionalprocesses and the resulting coarsening trend wereespecially intense during the 1988-1991 periodbecause of the low amounts of sand suppliedfrom the Ebro River. The sediment coarsening isgeneral, affecting both erosional and accretionalareas due to the longshore transport pattern ofthe Ebro Delta. It can be observed that twodifferent shorter-term oscillations (coarsening andfining periods) are superimposed on the generalcoarsening trend. These shorter-term oscillationsmay be related to changes in the amount ofsediment supplied from the Ebro River, modifi-cations induced by storms and anthropogenic ac-tions. All these processes cause grain size varia-tions of a similar magnitude and their identifica-tion is complex.

The study of beach sediments from the EbroDelta shows that the temporal evolution of thesediment size on deltaic beaches may be con-trolled by processes not related to a seasonaltemporal scale. Furthermore, the inter-annualevolution of the beach sediment grain size can beused to understand the dominant controlling pro-cesses of the sediment dynamics on the coast.

Reference~

8

Bryant, E.,reflectivetralia. J. :

Califs, L.M..Collado,propostaEstudi ~.Politecni(

Davis, R.A..and R.L.mentatior379-449.

Dubois, R.N.ments at ;

Flos, J., 1985Key EnviOxford. p:

Friedman, G.rameters .beach and

Giro, S. andpor metodgraph. Act

Gracia, V., C.Ana)isis y Idel Ebro: (

pp.Guillen, J., 1(

ambientes ,

Thesis, UniGuillen, J. am

shore variaDelta coast

Guillen, J. antihydrodynanlated by da579.

Horn, D.P., 19equilibriumMar. Geol..

Acknowledgements .We thank J.I. Dfaz and B. Alonso for the

critical review of the preliminary manuscript. J.M.Anguita drew the figures. We would like to thankto K.A.W. Crook, P. McLaren, D. Swift and ananonymous referee whose suggestions and com-ments have helped to improve the final paper.This work was supported by the Direcci6 Generalde Ports i Costes (Autonomous Government) un-der a collaboration agreement between the Con-sejo Superior de Investigaciones Cientfficas(Marine Science Institute) and the UniversitatPolitecnica de Catalunya (Marine EngineeringLaboratory ).

:tions and adiment."iod (8. 10-3) a medium-ler than that;s. Erosionalg trend were1991 periodnd supplied:oarsening isI accretionalt pattern of~d that two1rsening andthe general

I oscillationsamount of

iver, modifi-')pogenic ac-n size varia-ir identifica-

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