Coastal Hydrodynamics and Longshore Transport of Sand on Cassino Beach and on Mar Grosso Beach,...

Coastal Hydrodynamics and Longshore Transport ofSand on Cassino Beach and on Mar Grosso Beach,Southern Brazil

Jose A. S. Fontoura†, Luiz E. Almeida‡, Lauro J. Calliari§, Augusto Muniz Cavalcanti††,Osmar Moller, Jr.††, Marco Antonio Rigola Romeu†, and Bruno Ramos Christofaro†

†Universidade Federal do Rio GrandeEscola de EngenhariaLaboratorio de Engenharia CosteiraAv. Italia s/n, Km 8, Campus CarreirosCEP 9620-900, Rio Grande, RS, [email protected]

‡Universidade Federal do Rio GrandeInstituto de OceanografiaLaboratorio de Oceanografia GeologicaAv. Italia, Campus CarreirosCEP 96201-900, Rio Grande, RS, Brazil

§Universidade Federal do Rio Grandedo Sul

Instituto de Pesquisas Hidraulicas (IPH)Av. Bento Goncalves s/n, Campus do ValeCEP 91501–970, Porto Alegre, RS, Brazil

††Universidade Federal do Rio GrandeInstituto de OceanografiaLaboratorio de Oceanografia FısicaAv. Italia, Campus CarreirosCEP 96201-900, Rio Grande, RS, Brazil

ABSTRACT

Fontoura, J.A.S.; Almeida, L.E.; Calliari, L.J.; Cavalcanti, A.M.; Moller, O., Jr.; Romeu, M.A.R., and Christofaro, B.R.,2013. Coastal hydrodynamics and longshore transport of sand on Cassino Beach and on Mar Grosso Beach, southernBrazil. Journal of Coastal Research, 29(4), 855–869. Coconut Creek (Florida), ISSN 0749-0208.

The hydrodynamic and morphodynamic parameters of a region in southern Brazil were determined by collectingsediments with stream traps (bedload and suspended load) in the surfzone and capturing images, studying topography,and researching sedimentology with the help of a movie camera, a total station, and laboratory work. Data on deep-waterwaves were garnered with a CPTEC/INPE model. Field work was carried out in 50 field trips during a 1-year period; 42out of 50 trips focused exclusively on hydrodynamic and morphodynamic data, whereas eight of the trips were also usedfor collecting sediments. In addition, data on wind direction and velocity and on the longshore currents were alsocollected. All data were obtained in normal atmospheric conditions. Field work was carried out in six cross-shore profilesalong Cassino Beach and Mar Grosso Beach (three profiles on each side of the estuary of Patos Lagoon).

ADDITIONAL INDEX WORDS: Longshore transport, littoral drift, coastal sediment transport.

INTRODUCTIONThe study of sediment transport in the surfzone is not a new

issue, not for coastal engineers nor for oceanographers. The

importance of sediment transport studies in the surfzone is

equal to the difficulties and limitations posed by the critical

conditions in the surfzone. Such problems are not only limited

to the poor equipment resources that have been developed to

study it but also to the difficulties related to adequate

representation of the existing phenomena through mathemat-

ical approximation. Estimates of the quantity of transported

sediment are important for coastal management processes and

for civil construction development. However, in Brazil, few

well-instrumented experiments have been performed in either

the natural environments or in laboratories; instead, studies

have primarily compared the sediment transport in regions

away from the surfzone. Therefore, it seems that more

investment is needed to increase the knowledge focused on

this important area.

Coastal balance is usually dynamic. Even though known

beaches and coastlines show the same morphology for long

periods, it does not mean that the sediments rest on the bottom

throughout that time. On the contrary, the morphology is

maintained by the constant entrance and exit of equal amounts

of sediments into and out of the system during a certain period.

This is the correct interpretation of the dynamic balance of a

coastline.

To disturb that balance, a new disposition must be presented

to the forces that are involved in this process, so that the former

status can be changed. Such a change usually happens because

of natural phenomena—the result of an intrinsic modification

that occurs in nature—or because of human interference, such

as coastal constructions, which alter the free passage of

sediments. Erosion and accretion of sediments along the

coastlines are phenomena that result from that disturbance

of forces. Therefore, coastal engineers must recognize their

causes and manage them accordingly.

DOI: 10.2112/JCOASTRES-D-11-00236.1 received 29 December2011; accepted in revision 6 May 2012.Published Pre-print online 18 December 2012.� Coastal Education & Research Foundation 2013

Journal of Coastal Research 29 4 855–869 Coconut Creek, Florida July 2013

The first attempts to measure wave climate and to quantify

longshore transport scientifically on the coast of Rio Grande do

Sul (RS) state were carried out by Vitor Freire Motta (1963) and

Erasmo Pitombeira (1975), respectively. However, some tech-

nological limitations prevented the latter researcher from

determining net transport values that were consistent with

the geomorphological signs. Regarding wave climate, the

author had some difficulties determining the angles of

incidence, but, even so, the measurements were close to

current data collected by a directional waverider. Recent data

from directional waveriders (Coli, 2000; Strauch, 1996) enabled

the wave data to be reviewed and updated, mainly in the Rio

Grande region.

The aim of this research was to cooperate with the previous

studies on the behavior of coastal hydraulics between the

breakline and the maximum run-up point to find the true

littoral drift of Cassino beach, Mar Grosso beach, and the area

around the east and west jetties in the Barra do Rio Grande.

The results are based on current data of the waves, currents,

and winds.

Hydrodynamics and the transport were analyzed, both in

normal conditions and in extreme events. The effects of wind

intensity and direction on longshore currents and littoral drift

were also studied.

DESCRIPTION OF THE AREA UNDER STUDYThe study area for this research was the mouth of the lagoon

system at Patos-Mirim and its adjacent oceanic beaches. The

central spot was located at the mouth of Barra do Rio Grande

(3289030 00 S, 5285030 00 W). To the south, the study area was

limited by Cassino beach (32814056 00 S, 52890036 00 W), about 10

km from the root of the west jetty of Barra do Rio Grande. To

the north, it was limited by Mar Grosso beach (3283028 00 S,

51855048 00 W), about 10 km from the root of the east jetty in

Barra do Rio Grande.

The coastline in this region is oriented by a NE–SW axis,

whereas the central axis of the mouth of the Barra do Rio

Grande has a general NW–SE orientation; the angle is about

738 of the general coastal alignment. The physical limits of the

processes under study were the surf and swash zones up to 2 m

deep because the collection equipment could not be operated at

greater depths.

Taking into account the studies carried out by the Danish

Hydraulic Institute (DHI) with the computer model Mike-21

(DHI Water Environment Health) for the Environmental

Impact Study–Environmental Impact Report (EIA-RIMA),

regarding the enlargement of the jetties (FURG, 1999), the

coastline was subdivided into two conceptual zones: one of them

was influenced by the jetties, and the other one was outside

that influence. The direct influence of the construction (shadow

zone) was considered important up to 3 km away to both the

south and the north.

Six profiles were planned for this research: three of them

were located on Cassino beach, whereas the other three profiles

were located on Mar Grosso beach. Two of them were in the

influence zone, two were on its limit, and two were completely

outside its influence. On Cassino beach, these profiles were

distributed as follows: Profile-1 (P1) was located in front of the

Universidade Federal do Rio Grande (FURG) Aquaculture

Marine Station (EMA), 10 km to the south of the west jetty

(32812036 00 S, 52810048 00 W); profile-2 (P2) was located in front of

the terminal, 3 km to the south of the west jetty (32810014 00 S,

5287044 00 W); and profile-3 (P3) was located beside the root of the

west jetty, 300 m to its south (3289040 00 S, 52860 W). On Mar

Grosso beach, the profiles were distributed as follows: Profile-4

(P4) was located beside the root of the east jetty, 300 m to its

north (3288027 00 S, 5284030 00 W); profile-5 (P5) was located 3 km

to the north of the east jetty (3287013 00 S, 5283018 00 W); and

profile-6 (P6) was located in front of Caramujo restaurant, 10

km to the north of the east jetty (3282020 00 S, 51859040 00 W), as

shown in Figure 1.

MATERIALS AND METHODSThe analysis of available options led to the use of portable

traps as the best solution for measuring solid transport. This

method has long been used by geologists to collect sediments

from rivers (Kraus, 1987). Kraus (1987) also used this

equipment to collect coastal sediments, mainly in the surfzone.

The measurement equipment consisted of a metallic struc-

ture and a set of capture nets (Figures 2 and 3). The structure

was made of carbon steel and abrasion-resistant welding to

avoid risks to the operators and damage to the nets. The nets

were fixed individually to metallic frames that worked as the

capture mouths. These trap mouths were fixed to the metallic

structure by disposable plastic strings. The metallic structure

enabled the installation of 10 simultaneous nets, which were

Figure 1. Location of the study area and the positions of the profiles.

Journal of Coastal Research, Vol. 29, No. 4, 2013

856 Fontoura et al.

numbered 1 to 10 in a vertical, ascendant sequence. Nets 1–6

were 60 cm long. Nets 7 and 8 were 70 cm long, whereas nets 9

and 10 were 110 cm long. These are the minimum measure-

ments to be used in the surfzone because lower values can lead

to the loss of sediment because of flow return. The nets were

made of polyamide screen, 100% nylon, M-250 (0.063 3 0.063

mm), and the diameter of the tread was 0.62 mm. The screen

was 1 m wide, and any length can be bought. The mesh

coincides with the inferior limit of the size of noncohesive

sediments (very fine sand). Further details about the equip-

ment and the application of the method can be found in

Fontoura (2004).

Collection must follow a standard procedure, common to all

experiments. First, the bathymetric profile of the beach is

determined by a Nikon total station. The profile is plotted, and

the points where sediment will be collected are selected and

signaled by colorful buoys. The choice of the collection spots is

based on the local bottom characteristics, which can influence

sediment dynamic behavior. In the case of Cassino Beach, the

banks and troughs were chosen. Afterward, portable traps

were installed at each point. Every set was positioned with its

mouth opposed to the direction of the longshore current. The

feet of the structure were totally buried, so that the first trap

mouth touches the seabed. The researcher must stand behind

the metallic structure, between the structure itself and the

beach to avoid altering the natural alongshore transport of the

flow and to minimize the risk of being thrown against the

structure by the waves. When currents are weak, the traps will

flap and tangle up around the structure legs. However, when

velocities are above 0.30 m/s, the traps will stay fully in the

horizontal position, enabling the collection, which is limited by

the depth of the water and the height of the waves. Such

measurements will rarely go beyond the depth of the second

bank: the higher the waves, the smaller the possibility of

working in the deeper zones.

The first trap collected bedload sediments; the others

collected suspended loads. After 5 minutes of collection, an

intense, erosive process begins, which affects the reliability of

the measurements, starting at the bottom. To avoid collecting

unreliable data, time per station must be from 3 to 5 minutes. A

new set of traps was installed on the structure so that a new

collection could begin. Details of the operation are shown in

Figures 4 and 5. This method enabled determination of the

Figure 3. Metallic mouth and net.

Figure 2. Front view of the trap.


Longshore Transport on Southern Brazilian Coast 857

mean transport rate, the vertical behavior of the transport, and

the vertical distribution of the transported sediments mea-

sured in the surfzone, as well as the identification of the

characteristics of the sediment. The flow that passes between

two consecutive traps was calculated as shown in Equation (2),

according to the methods of Wang, Kraus, and Davis (1998).

The beach profiles were obtained with a Nikon DTM330 total

station and an adjustable buoy equipped with a reflecting

prism. The water depth (hb) at the breakpoint was taken

directly from the transverse profile, representing the distance

between the bottom and the waterline. The mean position of

the waterline was defined according to the mean position

between maximum and minimum swash recorded on the face of

the beach during field work.

The estimated mean width in the surfzone (Lb) was based on

visual information collected by aerial photographic sources, not

necessarily on field trips. In normal atmospheric conditions,

data of georeferenced aerial photographs were collected with a

digital ADAR-1000 system (Fontoura and Hartmann, 2001).

Sometimes, when the width of the surfzone was narrow, the

mean could be measured with the total station. In practice, the

width of the surfzone was defined as the horizontal distance

between the high waterline and the position of the farthest

breakline toward the ocean.

The beach slope in the surfzone was obtained directly from

the transverse profile. The tangent of the angle of the slope

(tanb) was determined by the relation between the water depth

at the breakpoint (hb) and the distance between it and the

humid line of the beach face (tanb).

Longshore currents were determined with a drifting buoy, a

chronometer, and a tape measure. Three measurements were

carried out for each profile, and their averages were deter-

mined afterward. The buoys were polyethylene terephthalate

(PET) bottles filled with a little sand, so that only their necks

stood above the surface of the water; that ensured that the wind

on the surface would not affect the results. Because the depth of

the water in this place was not above 2 m, the velocity of the

current was considered homogeneous along the entire water

column.

The intensity and the instantaneous direction of the local

winds were collected with a portable anemometer and a

compass. Daily mean values were taken from the FURG

database.

The characteristics of the waves in deep waters were

collected by simulations of the model WAVEWATCH III

(Tolman, 1999). They were carried out by Instituto Nacional

de Pesquisas Espaciais (INPE) for this place and research

period. The height and the period of waves in the breakpoint

were determined by continuous filming of the wave fields and

future laboratory analysis. The wave fields were filmed with a

Panasonic PV-908 VHS (zoom: 123), which was firmly fixed on

the beach with a strong tripod to avoid unexpected movements

caused by the wind. A rod with colorful 50-cm segments

(preferably white and red stripes) was positioned on the

surfzone, at the farthest spot reachable on foot (usually, the

second bank). The automatic mode of the movie camera was set,

and the filming frame was fixed with the rod at its center. The

movie camera was on for 10 minutes, enough time to acquire at

least 30 recordings of usable waves. The positioning of the

movie camera must enable images that clearly show the wave

trough and the wave crest spots on the rod. The angles of the

incidence of the waves in the breakpoint were obtained with a

compass and were originally determined in relation to the

north.

Minimum sets of 30 waves were used for determining the

mean value of the wave height in the breakpoint. Mean values

(rms) were obtained with Equation (1):

Hrmsb ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

N

XNi¼1

H2bðiÞ

vuut ð1Þ

where Hb(i) is the height of the break of wave i, and N is the

number of measurements.

Figure 4. Collection operation. Figure 5. Preparation and labeling.


858 Fontoura et al.

The flow of sediments that passes through each profile

(Figure 6) was calculated with Equations (2), (3), and (4),

according to Wang, Kraus, and Davis (1998). The sediments

that pass through the space between two consecutive traps are

estimated with Equation (2):

DFi ¼Fiþ1

ziþ1þ Fi�1

zi�1

2

!Dziðkg=min=m2Þ ð2Þ

Total flow, Ij, which passes through set j is calculated with

Equation (3):

Ij ¼Xn

i¼1

Fi þXn�1

l¼1

DFiðkg=min=m2Þ ð3Þ

Several sets are installed along a profile (see Figure 7). The

total longshore-transport rate in the profile was given by

Equation (4):

QTotal¼XKk¼1

Ij þ Ijþ1

2

� �Akðkg=min=m2Þ ð4Þ

RESULTSFifty field trips to carry out measurements were distributed

in the six profiles during the four seasons of the year. Forty-two

of 50 trips focused exclusively on hydrodynamic and morpho-

dynamic measurements, whereas eight trips were used for

carrying out those measurements and collecting sand.

Results of the characteristics of the waves at three different

depths are shown and compared below. These findings show

how much the behavior of the waves regarding coastal

hydrodynamics is influenced by local bathymetry. The long

continental platform and the almost parallel distribution of the

isobathymetry on the coastline (Pimenta, 1999) promote the

constant refraction/diffraction of the waves that come to the

beach; the ones that are in the extremities of the spectrum (NE

and SW) move to more central positions, close to the SE. Figure

8 shows the height, period, and direction of waves modeled for

deep waters and measured on the 15-m contour and in the

breakpoint, respectively. They are shown from left to right.

Figure 9 shows the effects of the bottom action (refraction) on

the direction of the propagation of those waves, from deep

water to the breakpoint.

The modeled heights and periods of waves in deep water were

compared with data collected by a nondirectional waverider

installed at a point very close to the region where the model was

run. The results of this comparison suggest strong correlation

between the estimated values and the measured ones. Figure

10 shows the correlation among heights.

Longshore currents and the longshore component of local

winds showed a strong correlation; convergence was much

higher than expected for measurements between them and the

direction of the incidence of waves. The study showed a

convergence rate between currents and winds in 86.5% of the

cases under observation, whereas only 64% of the cases showed

convergence between longshore currents and wave direction. It

is worth mentioning that there was an 82% convergence

between the wind and the waves.

Figure 11 shows the diagram of the convergence between the

wind and the current, whereas Figure 12 shows the distribu-

tion of the intensities of longshore currents, according to the

coastal orientation. In 62% of the cases, NE–SW currents were

observed, 34% recorded SW–NE currents, and only 4% had no

currents to record.

The collection of sediments was carried out according to the

same scheme shown in Figure 7. The only exception was P5,

where the conditions of the bathymetry enabled only three

collection stations. Table 1 shows the data on the eight field

trips when measurements of sediment transport were taken,

i.e., the mean values of the wave heights, periods, and velocities

of the longitudinal current, as well as the total values of the

sediment transport in the profile. Table 2 shows detailed data

on the sediment transport recorded in each profile (P1–P6) on

their respective collection dates. Likewise, Figures 13–18 show

the diagrams of the distribution of sediment transport and of

longitudinal transport rates for each profile plotted on the local

bathymetry on the collection day.

DISCUSSIONThe distribution of waves in the surfzone (Figures 8 and 9)

shows that most directions are SE and S, the result of an

intense refraction/diffraction process caused by the occurrence

of isobathymetry parallel to the coast and a long and shallow

Figure 6. Scheme of the flow in the traps and spacing among them.

Figure 7. Location of sets (Tj), local depth (hj), and area between two

consecutive sets (Ak).



platform (Pimenta, 1999). It is worth mentioning that different

methodologies were used to collect data on waves, shown in the

previously mentioned Figures. Data were modeled in deep

water, measured by a waverider in intermediate waters, and

captured by video images in the surfzone.

When results are compared, deep water shows waves from all

quadrants, except the ones that head to the continent (W and

NW). The highest occurrence was from the NE (35%), followed

by the S quadrant (23%), whereas the lowest occurrence was

from the SE (5%). Studies on the behavior of these waves in 15–

18 m (Coli, 2000; Motta, 1963; Strauch, 1996) show that

directions are diminished in the E, SE, S, and SW. The SE was

the most representative sector (60%), and the SW was the least

important one (2%). Finally, in the surfzone, with depths

around 2 m, an important concentration of directions in the SE

sector (78%) could be observed. The remaining ones were

distributed in the south (10%) and east (2%).

Regarding depths between 15 and 18 m, it is interesting to

compare the Motta (1963) records on Tramandaı beach, located

about 300 km north of Cassino Beach, with values measured by

Strauch (1996) in Rio Grande. Similar behavior was observed

in both distributions, despite the spatial, temporal, and

technological distance that separates these two events. Motta

(1967) found that 68% of the waves came from the north of the

beach profile in Tramandaı and just 32% came from the south.

That may partly explain the net transport to the SW, found by

Pitombeira (1975), against all existing morphological signs.

Strauch (1996) recorded that 48% of the waves came from the

north of the beach profile in Rio Grande and 52% from the

south; those results agree with the net transport to the NE and,

at the same time, shows its low value, which agrees with the

observations made throughout this study. Another important

aspect of this comparison is that the central axis of the Motta

(1963) measurements tends more to the north than it does in

Figure 8. Characteristics of waves in (a) deep water (INPE, 2004), (b) 15-m contour (Coli, 2000), and (c) breakpoint.


860 Fontoura et al.

Strauch (1996), which may be explained by the general

positions of the coastlines in Tramandaı (248 N) and Rio

Grande (458 N), with a difference of about 218. In general,

records made by Motta and Strauch are very similar. As waves

move from deeper waters to 15–18 m bathymetry, NE and SW

directions gradually disappear, and there is a slight concen-

tration of these waves in the SE and E sectors. Because there is

continuous displacement toward the coastline and consequent

entrance in the break zone (at about 2-m contour), the E

direction totally disappears and moves SE in 35% of the

occurrences, besides partial migration of S waves, which also

move SE.

Therefore, the broad spectrum of directions modeled in deep

water is restricted to the SE and S directions in the surfzone,

whereas the most meaningful direction—which was NE in deep

water in 35% of the occurrences—is replaced by the SE

direction in the breakpoint in 78% of the occurrences. These

observations show that almost all waves that come to the

beach, mainly the ones under normal atmospheric conditions,

have a low angle of incidence with the beach profile in the

coastline.

Figure 12 shows that longshore currents to the SW have a

mean velocity of about 0.40 m/s and that no currents above 0.80

m/s were recorded in that direction. On the other hand,

although currents to the NE mostly have values below 0.40 m/s,

at least 2% have velocities greater than 0.80%, thus showing

little predominance of intensities to NE.

These observations show that the most intense values to the

NE may be associated with winds from the south quadrant,

mainly in autumn and winter. These winds, mainly from the

SW, originate intense currents that can reach values greater

than 1.20 m/s. The main characteristic of winds from the south

quadrant is their intensity, whereas the primary characteristic

of winds from the north quadrant is their duration. This kind of

convergent relation between the current and the wind is

similar to the behavior that Lanfredi and Framinan (1986)

observed on Union beach in the south of Argentina. There, the

agreement between the general direction of the coastline and

most-dominant winds also favored the convergence between

the direction of the longshore component of the wind and the

direction of longshore currents.

The data in Table 1 show the hydrodynamic conditions (apart

from extreme events) in the region recorded throughout a year,

whereas the data in Table 2 show that all the profiles studied

had low values of littoral drift. Compared with the high values

in the Pitombeira (1975) and Alfredini (1999) findings on the

coast of RS, a strong discrepancy is noticeable. However, those

authors focused on establishing the general values of littoral

drift on the coast of RS; they considered neither the

hydrodynamic particularities resulting from the jetties in the

mouth of the Patos Lagoon nor the influence of the large

Figure 9. Distribution of waves according to their approximate direction in

deep water (black), at 15-m depth (dashed line), and at the surf line (light

gray), respectively.

Figure 10. Correlation between the height of measured and modeled waves.

Figure 11. Convergence between currents and wind.



construction on the hydraulic behavior at this spot along the

coast. Therefore, it is reasonable to accept that the Pitombeira

(1975) and Alfredini (1999) findings may be confirmed at some

spots along the coast, primarily at particular hot spots where

high erosive rates have been observed (Calliari et al., 1998;

Pimenta, 1999). The values found by those authors cannot be

generalized to all regions with an open coast because, even far

from the jetties, preliminary data collected by Lisniowski

(2006), Perotto (2007), and Albuquerque et al. (2008) have

shown very low transport values on several beach stretches

from Cassino to Chui. A detailed discussion about the behavior

of sediment deposits that have accumulated close to the jetties

since their construction is mandatory in order to clarify

whether the low values that were found have been consistent

or just represent temporary conditions.

This analysis of the behavior of the coastline south of the

west jetty, which aimed at explaining the low levels of littoral

drift around the jetty construction in the light of historical data,

was based on the nautical charts of the Barra do Rio Grande.

Those charts were generated annually from 1883 to 1956 and

were published by the 188Distrito do Departamento Nacional

de Portos, Rios e Canais (DEPREC, 1959).

The first hypothesis to explain the low levels of littoral drift

comes from the nautical charts: About 2,300,000 m3 of sand was

deposited from 1907 to 1915 at this spot on the coast, for an

average of 287,500 m3/y. That was doubtlessly the period in

which the highest variation in the position of this coastline was

recorded (Figure 21). An analysis of nautical charts published

before 1911—when the construction of the west jetty started—

shows that there was a cyclical process forming a sandy bank

600 m from the coastline and subsequent migration and

accumulation on the beach during a 5-year period, according

to Toldo and Dillenburg (2001). The high quantity of sediments

found at this spot during that period may be the result of the

retention of littoral drift because of the jetty associated with

part of the sediments from that sandy bank (previously

mentioned), which had already formed in that place since

1909. These nautical charts also show that no significant

alterations occurred between 1915 and 1947, except for small

fluctuations, which were sometimes crescive and sometimes

erosive. Lelis (2003) studied metric aerial photographs taken

Figure 12. Distribution of the intensities of the longshore current in the (a) NE–SW and (b) SW–NE directions.

Table 1. Waves in the surfzone, longshore current, and littoral drift recorded on the date of collection.

Profile Date

Wave Dataa

Hb (m) Tb (s) LC (m/s) Q (m3/h)

Profile-1 July 1, 2003 0.57 14 0.14 NE–SW 1.92

Profile-2 September 19, 2002 0.59 11.2 0.41 NE–SW 2.60

January 16, 2003 0.15 3 0.31 NE–SW 0.23

February 20, 2003 0.78 9.4 0.1 SW–NE 25.1

Profile-3 May 02, 2003 0.72 8.5 0.28 SW–NE 4.24

Profile-4 September 5, 2003 0.97 11.6 0.36 SW–NE 43.33

Profile-5 January 17, 2003 0.68 7.0 0.37 NE–SW 2.54

Profile-6 February 21, 2003 1.26 8.6 0.70 SW–NE 31.74

aAbbreviations: Hb ¼waves at the breakpoint, Tb¼ period at the breakpoint, LC ¼ longshore current, Q ¼ solid flow.


862 Fontoura et al.

Table 2. Sediment collected for each profile (P1–P6) with the corresponding Figure number.

Profile No. Date Hours Trap (t)

Collection StationCorresponding

FigureE4 (g) E3 (g) E2 (g) E1 (g)

1 July 1, 2003 1000–1700 No. (t) Cta (min) 5 5 5 5 13

t10 — — — —

t9 — — — —

t8 — — — —

t7 — 0.93 0.11 0.34

t6 0.43 4.17 0.41 0.15

t5 0.82 5.56 1.21 1.37

t4 11.71 12.30 1.77 0.89

t3 67.40 19.29 3.49 0.85

t2 142.17 33.40 3.76 1.22

t1 45.13 48.56 6.99 34.44

2 July 19, 2002 0800–1800 4 4 4 4 14

t10 — — 0.33 0.02

t9 — — 1.13 0.05

t8 — 1.00 2.06 0.02

t7 — 2.00 2.91 —

t6 — 2.41 3.12 0.05

t5 3.68 2.48 5.30 0.11

t4 17.20 4.92 5.74 0.15

t3 57.07 6.81 6.21 0.62

t2 65.06 9.55 7.30 1.98

t1 27.30 14.42 8.30 5.79

January 16, 2003 0800–1630 5 5 5 5 15

t10 — — — 0.26

t9 — — — 0.67

t8 — — 1.23 0.17

t7 — — 1.28 0.09

t6 — 2.31 1.43 0.16

t5 1.18 2.66 2.18 0.52

t4 1.30 2.72 2.50 1.00

t3 1.70 3.60 2.68 1.20

t2 3.36 2.60 4.14 0.95

t1 15.54 19.01 6.6 0.90

February 20, 2003 0800–1600 5 4.34 5 4.38 16

t10 — — 0.12 0.26

t9 — — 0.23 1.29

t8 — 0.30 1.26 3.08

t7 — 5.74 2.10 2.38

t6 — 27.81 3.07 2.06

t5 0.12 117.02 5.41 8.29

t4 203 249.36 1.78 7.10

t3 38.57 615.09 3.79 2.90

t2 121.59 527.90 33.04 2.55

t1 177.17 656.69 17.88 16.24

3 May 2, 2003 1300–1800 3 3 3 3 17

t10 — — — 0.23

t9 — — 0.12 1.78

t8 — — 0.90 4.10

t7 — — 3.75 3.55

t6 — 0.79 17.24 1.99

t5 0.37 6.20 20.24 7.03

t4 5.97 10.95 10.21 7.17

t3 7.48 6.45 20.79 12.77

t2 9.03 21.89 30.00 23.14

t1 47.02 43.58 37.03 15.25

4 September 5, 2003 0800–1700 4 4 4.2 4.1 18

t10 — — — 0.16

t9 — — — 2.93

t8 — — — 13.89

t7 — 0.23 14.17 50.61

t6 1.09 3.40 35.86 20.77

t5 8.47 18.92 143.06 352.44

t4 14.68 27.07 210.06 409.06

t3 31.40 29.63 261.31 424.13

t2 38.17 33.93 292.88 436.12

t1 54.74 75.95 374.64 576.35



from 1947 to 2000 and noticed that there were no significant

alterations to the coastline at this spot during this period

either. These findings suggest that the intense crescive process

occurred at a specific point that coincided with the first 4 years

after the large jetty construction was begun.

The same behavior can be observed in the position of the

bathymetric contours (at the base and on the ocean side of the

west jetty) recorded in the nautical charts. The positions of the

current isolines are the same as they were in 1915, showing

that the intense crescive process also stopped that year in the

subaqueous environment. From then on, few alterations have

occurred, even though 100 years have elapsed. In the

subaqueous environment, between 1907 and 1915, it is

estimated that about 1,200,000 m3 of sediment were added in

the 8 years, an average of 151,500 m 3/y. Therefore, it can be

inferred that during this period, 3,500,000 m3 were aggregated

at this spot on the coast, for an average rate of 562,500 m3/y.

From then on, this spot along the coast has been relatively

balanced, according to Lelis (2003).

This balance is dynamic, with the sediment equilibrium

tending toward zero, rather than a static balance generated by

the absence of sediment transport. This balance is thought to be

the result of the interaction between the net longshore

transport to the NE and the net transverse transport to the

sea. Therefore, because there is no evidence of strong

transverse transport to the sea in this region, it is implicit

Table 2. Continued.

Profile No. Date Hours Trap (t)

Collection StationCorresponding

FigureE4 (g) E3 (g) E2 (g) E1 (g)

5 January 17, 2003 0800–1700 1.5 3.23 2.04 — 19

t10 (1.35 m) — 0.098 0.25 —

t9 (1.20 m) — 0.07 0.67 —

t8 (1.05 m) — 0.14 0.30 —

t7 (0.90 m) — 0.104 0.39 —

t6 (0.75 m) — 0.083 0.55 —

t5 (0.60 m) 0.78 0.026 0.44 —

t4 (0.45 m) 17.31 0.34 0.35 —

t3 (0.30 m) 25.25 0.18 0.41 —

t2 (0.15 m) 26.58 0.20 0.83 —

t1 (0.045m) 68.10 0.58 0.85 —

6 February 21, 2003 0800–1630 4 2.48 2.57 2.32 20

t10 — — — 0.22

t9 — — — 0.91

t8 — — 1.29 3.60

t7 — 3.95 13.51 15.76

t6 6.08 21.35 52.03 37.57

t5 0.59 168.13 210.93 88.78

t4 41.63 289.70 354.70 65.44

t3 100.52 376.29 445.82 64.51

t2 168.52 333.14 490.34 40.88

t1 287.01 460.25 565.86 75.52

aCt ¼ collection time.

Figure 13. Lateral and vertical distribution of the longshore transport rate

of sand measured in profile-1 during a field trip on July 1, 2003.


of sand measured in profile-2 during a field trip on July 19, 2002.


864 Fontoura et al.

that a null or very low balance demands that the net longshore

transport to the NE be low. According to Marques (2009),

longshore currents generated by NE winds generate transport

toward the SE and that there is an anticyclonic vortices that

remains in the region of the west jetty. In this somewhat

protected area, close to the foot of the jetty, sediments are held

in a recirculation zone that favors deposition and makes it

difficult for sediments to resuspend and move to the south.

Another conclusion that arises from the nautical charts, even

though the opposite of the previous hypothesis, is that the low

longshore transport suggests the existence of crescive trans-

verse transport, at least in the first years after the construction.

Nautical charts before 1913 show that the sandy bars that

closed the mouth stretched parallel to the beach and far beyond

them (south and north). When the jetties were built, they cut

these bars; so, the ocean side had a shallower profile than it has

today. For instance, the position the 5-m contour occupies today

was occupied by a 2-m contour in 1913, and so forth. Therefore,

it can be inferred that a meaningful part of the sediments

added to the base and the coast of the west jetty was the result

of crescive transverse transport, carrying sediment that was

available in the shallow areas to the beach. Therefore, those

shallow points were deepened because the sediment removed

was not replaced. It is worth mentioning that a similar bottom

profile occurs at the base and the coast side of the east jetty,

where the shallow stretch is longer. The nautical chart from

1913 (and previous ones) also shows a long terrace, which

might have been part of the existing bar.

Regarding the hydrodynamics and sediment transport along

the east jetty, however, the erosive process close to the foot of

the jetty is clear because of the lack of sand coming from the

SW. According to Long and Paim (1987), the erosive process

was observed from 1875 to 1962, when these authors studied


of sand measured in profile-2 during a field trip on January 16, 2003.


of sand measured in profile-2 during a field trip on February 20, 2003.


of sand measured in profile-3 during a field trip on May 2, 2003.


of sand measured in profile-4 during a field trip on September 5, 2003.



this site and found a constant regression of the coastline. The 5

m/y erosion calculated by the authors may not have been

stronger during that period because of the bidirectional

characteristic of the littoral drift at this spot, which may

occasionally supply sediments to the foot of the east jetty

whenever the drift moves from NE to SW; thus, the complete

erosion of the foot of the construction has been avoided.

Regarding the west jetty, both the first and second

hypotheses show that the current longshore transport is very

low. The former suggest that there is intense transport, at least

during the first 2 years after the beginning of the construction

and the following stabilization in low levels. The latter suggests

low longshore transport throughout. Therefore, based on this

discussion, it may be said that low longshore transport values

in this region nowadays are not circumstantial; they represent

coastal behavior associated with the construction of the jetties.

Therefore, the following question must be answered: Why is the

transport so low? That low, local transport can be seen in the

results shown in Table 2 and in Figures 13 to 20.

The answer may be found in the low values of the significant

height of the waves in the breakpoint (Hbr) in normal

atmospheric conditions and, mainly, in the low average value

of their angles of incidence in the surfzone. The refraction and

diffraction that resulted from the construction also seem to

have had an important role in this process, mainly at the points

that are closer to the foot of the jetty, in profiles 3 and 4. The

influence of the low angles of incidence in the longshore

transport in the region can be better understood if we consider

that the mean approximation angle in the breakline was about

6.58 and that, in normal atmospheric conditions, more than

70% of the angles in the surfzone were less than 38, whereas the

remaining 30% were no greater than 88. Such low values

suggest important reductions in the value of the radiation

stress (Longuet-Higgins, 1972) that penetrates the surfzone,

thus limiting the transport and the mobilization of the

sediment.

Another important issue is the high quantity of the fluid mud

from the continental plume of the Patos lagoon. It covers the

sandy bottom of the beach between the�4 contour and the�10

m contour, from the area close to the west jetty to Querencia

(Calliari and Fachin, 1993), as shown in Figures 22 and 23. It

suggests that this muddy mass may be responsible for both the

partial cooling of the waves (mainly in storms)—because it

removes a meaningful part of the energy that could be available





Figure 21. Area with sediment deposited adjacent to the foot of the west

jetty and the coastlines in 1907 and 1915 (the darker the area, the higher the

deposit rate).


866 Fontoura et al.

in the surfzone for sediment mobilization and transport—and

the formation of a protective shield of part of the bottom

sediment, thus preventing its mobilization and consequent

feeding of the surfzone.

Finally, the quantity of sediments transported close to the

seabed (sediments collected by the net located between the

seabed and 0.10 m above the seabed, i.e., first net), represent,

on average, 18% of the total value of sediments measured at

this site. Because Komar (1978) and Wang, Kraus, and Davis

(1998) suggest that seabed transport refers to all sediment

transported in the layer from the bottom up to 0.10 m, it may be

said that, in practice, the seabed transport is all the sediment

collected by the first net.

CONCLUSIONThe data collected and analyzed throughout this study

suggest that, during the period under observation, there was

convergence between the direction of the longshore component

of the wind on the coast and the direction of the longshore

current 86.5% of the time. These indices suggest that the

direction of the longshore current and the littoral drift were

strongly affected by the direction and intensity of the longshore

component of the wind (Figure 11). Changes in the wind

direction caused immediate reaction to the direction of the

current and the littoral drift: Both immediately followed the

wind direction. The only exceptions were when the wind was

very low or absent; in those cases, the current became almost

Figure 22. Bank of fluid mud in front of Cassino beach (enclosed by the white line).



the sole determiner of the wave direction in the surfzone. The

percentage (13.5%) that corresponds to the divergence occurred

only because of the inertial movement of the current after a

sudden change in wind direction, but those situations were

rare.

Observations strongly suggest that the longshore transport

is bidirectional with a low net result toward the NE, even in

normal atmospheric conditions, which implies that the net

transport might be magnified to the NE when coastal storms

come from the south quadrant. That increase in the transport

may be mainly associated with the growth of the hydrodynam-

ics in the surfzone, as well as with the increase of the water

level on the coast, leading to coastal processes closer to the

frontal dunes, the natural stocks of sediment. However, it is

worth mentioning that the methodology used in this study (the

streamer trap) is not adequate for collecting data during

storms. Therefore, results refer to normal atmospheric condi-

tions.

Another conclusion is that the longshore transport in this

region is very low, mainly in comparison with spots along the

coast where bathymetric conditions favor waves associated

with intense erosive processes. Data suggest that the reduction

of the local transport rates may be associated with the low

values of the angles of incidence in the surfzone, the short mean

height of the waves in the breakpoint due to the attenuating

action of the fluid mud on the forebeach, and the partial

freezing of the sediment stocks on the forebeach due to the

muddy mass.

Finally, 18% of the entire transport happened because of the

bedload in a layer about 10 cm above the bottom. The

remaining 82% occurred because of suspension on the water

column.

LITERATURE CITEDAlbuquerque, M.G.; Fontoura, J.A.S.; Calliari, L.J.; Serpa, C.G., and

Falcao, T.O., 2008. Caracterizacao do fluxo sedimentar na zona desurfe de praias de micro e meso mares—aplicacao a praia do

Cassino (RS) e praia do Futuro (CE). In: Anais do III Seminario eWorkshop em Engenharia Oceanica (SEMENGO’2008). RioGrande, RS, Brasil: Universidade Fedral do Rio Grande (FURG),9p.

Alfredini, P., 1999. The longshore transport evaluation used as a toolfor the Brazilian coastline littoral processes knowledge. In: Anaisdo VII Congresso Brasileiro da Associacao Brasileira de Estudos doQuaternario. Sao Paulo, Brasil: ABEQUA, 13p.

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