Morphodynamics of Fetch-Limited Beaches in Contrasting - e-Geo

Journal of Coastal Research
SI 56
183 - 187
ICS2009 (Proceedings)
Portugal
ISSN 0749-0258
Morphodynamics of Fetch-Limited Beaches in Contrasting
Environments
P. Freire†, Ó. Ferreira‡, R. Taborda∞, F. S. B. F. Oliveira†, A. R. Carrasco‡, A. Silva∞, C. Vargas†,
R. Capitão†, C. J. Fortes†, A. B. Coli§ and J. A. Santos†
†National Laboratory of Civil
Engineering, Lisbon
1700-066, Portugal
{pfreire, foliveira, cvargas,
rcapitao, jfortes, jasantos}@lnec.pt
‡FCMA/CIMA
Algarve University, Faro
8005-139, Portugal
{oferreir, azarcos}@ualg.pt
∞LATTEX,
Lisbon University, Lisbon
1749-016, Portugal
{rtaborda, amasilva}@fc.ul.pt
§CEPEMAR Meio ambiente
Vitória, Espírito Santo
29050-650, Brasil
[email protected]
ABSTRACT
FREIRE, P.; FERREIRA, Ó.; TABORDA, R.; OLIVEIRA, F. S. B. F.; CARRASCO, A. R.; SILVA, A.; VARGAS, C.;
CAPITÃO, R.; FORTES, C. J.; COLI, A. B. and SANTOS, J. A., 2009. Morphodynamics of Fetch-Limited Beaches in
Contrasting Environments. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal
Symposium), 183 – 187. Lisbon, Portugal, ISSN 0749-0258
Sandy beaches can be found in fetch-limited environments that are protected from ocean generated waves, as
estuaries, lagoons, and backbarriers, and where fetch characteristics allow local wind-generated waves to
develop and maintain a beach. The morphodynamics of these low-energy beaches present a peculiar behaviour
and general open-ocean models are inappropriate for their study. The main objective of this work is the
development of a fetch-limited beach classification scheme based on the relative importance of wave, tidal and
river flow forcing. This objective was pursue through the study of the evolutionary pattern of two fetch-limited
beaches located in two different Portuguese systems, Alfeite (in Tagus Estuary) and Ancão backbarrier (in Ria
Formosa barrier system). Results show that both beaches display a typical fetch-limited profile, with a narrow
reflective beach face and a wide dissipative tidal flat, and that profile shape exhibits small changes without a
seasonal pattern typical of ocean beaches. At Alfeite, profile changes are restricted to the beach face and result
from low-frequency extreme events (strong wind generated waves, Hs>0.5m). At Ancão, although major
morphologic changes are also related to the beach face, which is typical behaviour of wave dominated beaches,
sediment transport, mainly driven by tidal currents, extends through the entire profile. A tentative fetch-limited
beach classification, based on the relative importance of wave and tide effects, is presented.
ADDITIONAL INDEX WORDS: locally-generated waves, morphological evolution, beach classification
INTRODUCTION
Sandy beaches, resulting exclusively from local wind waves, can
be found in fluvial and in marine fetch-limited systems as large
dam reservoirs, estuaries, lagoons and backbarriers. Without
influence of ocean waves, these beaches occur where sediment is
available and fetch characteristics allow local wind-generated
waves to create and shape a beach (NORDSTROM, 1992).
In the last two decades, several studies were dedicated to beaches
in fetch-limited environments, mainly in estuaries and enclosed
bays. This scientific interest resulted from the increase of the
human pressure in these systems and the awareness of their
environmental value. The research undertaken pointed out the
differences between fetch-limited beaches and beaches affected by
ocean waves (NORDSTROM, 1977; NORDSTROM, 1980; NORDSTROM,
1992; NORDSTROM and JACKSON, 1992; JACKSON and NORDSTROM,
1992; JACKSON, 1995; JACKSON et al., 2002; TRAVERS, 2007): (1)
limited fetch generates small waves with short periods that promote
high ratios between tidal range and wave height in macro to
mesotidal systems; (2) physical processes as wave generation and
propagation and shoreline evolution are site specific dependent, by
factors as water depth, tidal currents, wind and human
interferences; (3) beach morphologic changes are mainly associated
to high-energy events with low-frequency and no seasonal/cyclical
evolutionary pattern is evident; and (4) beach morphodynamics
differs from the established open-ocean models.
Considering wave characteristics and their effect in beach
morphologic features and profile dynamics, fetch-limited beaches
can be considered as low-energy shorelines characterized by
(JACKSON and NORDSTROM, 1992; NORDSTROM, 1980; HEGGE et al.,
1996; NORDSTROM et al., 1996; JACKSON et al., 2002): non-storm
significant wave heights, Hs, smaller than 0.25 m, with Hs during
storm winds not exceeding 0.50 m; beach faces are narrow (e.g. less
than 20 m in microtidal environment), planar, usually without
backshore, limited seaward by an extensive sub-horizontal tidal
flat; morphologic features include inherited forms from previous
energy events. Fetch-limited differs from other low energy
environments that are sheltered from offshore wave energy, e.g.
situated on the landward of ocean islands and on windward of
estuarine basins, by the dominance of locally generated waves,
comparatively to ocean waves, which heights depend primary on
wind conditions and basin dimensions (JACKSON et al., 2002).
Besides the locally-generated waves, other forcing factors, as tidal
currents or river flow, can play an important role in determining
beach morphology and sedimentary characteristics. In this context,
the present paper aims to discuss a possible classification for
fetch-limited beaches based on the relative importance of waves,
Journal of Coastal Research, Special Issue 56, 2009
183
Morphodynamics of Fetch-Limited Beaches
Figure 1. Location of Alfeite beach in Tagus estuary and Ancão
backbarrier in Ria Formosa.
1.3 m during neap tides. The field site extends over ~150 m, is
located at the backbarrier, and therefore protected from all oceanic
direct influence and subject to a different wave and currents
regime.
The sandy beach is typically characterized by a low, narrow and
reflective morphology, exhibiting small cuspate forelands. The
steep beach face (D50=0.8 phi) and the low wave energy result in a
narrow surf and swash zones. A gently sloping tidal flat of medium
sand (D50=1.0 phi) mixed with fine sediments (silt and clay =
10%) is present, ending at a parallel sand spit (D50=0.9 phi) cut-off
by a small transverse secondary tidal channel (Figure 1). The
overall beach profile exhibits a convex-curvilinear shape.
Main forcing mechanisms acting at the backbarrier include tidal
currents and waves generated by local wind over a short fetch
distance. Waves are considerably small (Hmean~0.05 m, Tmean<1 s),
being on the order of few centimetres, with the exception of waves
generated by exceptional strong winds. No river influence is
observed at this study site.
tidal currents and river flow in beach characteristics and evolution.
METHODS
Two different fetch-limited beaches in Portugal (Alfeite and Ancão)
Beach cross-shore profiles were measured in both sites using a
were investigated through the medium-term monitoring of
total station, from April 2005 to July 2007 at Alfeite, and from
cross-shore profiles complemented with meteo-oceanographic data,
March 2006 to March 2008 at Ancão. The morphological mobility
and the observed evolution and morphodynamic characteristics are
of both sites was evaluated based on the maximum vertical
discussed.
variability in each cross-shore profile over successive surveys.
Mean beach face slope was also determined in order to attest the
STUDY SITES
sediment displacement through the upper beach. Surface sediment
samples were collected at the main morphological units, and the
classical grain-size parameters were obtained using the Moment
Alfeite beach
Alfeite beach is located in the inner bay of Tagus estuary, one of Method according to FRIEDMAN (1961, 1967).
The typical beach profile variability is confronted with the main
the largest estuaries in Europe (Figure 1). The estuary is
characterized by a complex morphology with a broad and shallow acting forcing mechanisms. Mean wave heights (locally-generated),
inner domain separated from the ocean by a narrow fault-controlled mean current flow and river influence were characterized in attempt
inlet channel. The inner estuary, protected from ocean waves to evaluate their relative dominance at both sites. At Alfeite beach,
incursion, extends northwards with wide mudflats and salt marshes the water column height was measured with resistive wave gauges
resulting from fluvial fine sediment input. Sandy beaches, (at a frequency of 25 Hz) and pressure transducers (at frequencies
developed along the southern margin, are affected by a low energy of 25 and 2 Hz), and the wave parameters were computed through
wave climate resulting from locally generated wind waves that classical spectral analysis based on the surface elevation
rework sediment from local sources (FREIRE and ANDRADE, 1999; measurements. The energy attenuation induced by depth was
FREIRE et al., 2007). The estuary presents a semi-diurnal mesotidal corrected based on Airy linear theory. Also the influence of the
regime with a mean tidal range in Lisbon of 3.2 m and 1.5 m, ferry-boats crossing the area of interest was removed from the
respectively, in spring and neap tides (PORTELA and NEVES, 1994). original data measurements. Local wind data was obtained with an
Typical tidal current speed is about 1.0 m.s-1, with maximum values anemometer in each campaign. The influence of the tide and the
at the inlet of 2.5 m.s-1 (MARETEC, 2001). The Tagus river is the Tagus river flow in the current velocity and water level at the beach
main source of fresh water into the estuary with c. 300 m3.s-1 of was estimated by numerical modeling (VARGAS et al., 2008) and
field observations. At Ancão beach, estimative of wave parameters
mean flow (http://snirh.pt/).
Due to the shoreline’s main alignment, WNW-ESE, the were performed at the beach face, for a wide range of wind
directional wind sectors that affect Alfeite beach, by wave conditions, based on visual observations, supported by a marked
generation, are NW and N, predominant during Spring and ruler, and video records. To attest the medium term influence of
Summer, each with 20% of frequency of occurrence; and NE and E, wave generation, wind data was obtained from the closest wind
predominant during Autumn and Winter, with 15% and 3% of station (Faro airport), between March 2006 and March 2008. Tidal
current data was recorded at Ancão beach, during spring tidal
frequency of occurrence, respectively (OLIVEIRA et al., 2009).
The beach profile at Alfeite includes two different cycles, in the two most important morphological units: at the beach
geomorphologic units separated by a residual gravel deposit: a face (with a bidirectional Electromagnetic Current Meter), and at
narrow and steep beach face with sediment median diameter, D50, the sand spit (with an Aquadopp Profiler).
of 0.2 phi, and a broad and sub-horizontal tidal flat limited offshore
by a tidal channel. Locally dominated by fine grained sediments
RESULTS
(between 2% and 60%), dissipative conditions prevail on the tidal
flat.
Alfeite beach
Alfeite beach is relatively stable in a medium-term analysis
Ancão beach
(months to years): maximum vertical variability of the beach profile
Ancão beach belongs to the Ria Formosa, a multi-inlet barrier is 0.30 m and beach face slope varies between 0.08 and 0.12
island system located in southern Portugal (Figure 1). Tides in the (Table 1). Episodically significant morphologic changes are
area are semi-diurnal, average ranges are 2.8 m for spring tides and
Journal of Coastal Research, Special Issue 56, 2009
184
Freire et al.
associated to strong wind generated waves (Hs>0.5 m, Table 1).
During these low-frequency extreme events sediment is removed
from the upper beach face, with an erosive scarp formation
(Figure 2), and deposited on the lower beach face without
significant changes in profile slope. Erosive morphologic features
can persist as relict landforms during non-storm conditions. The
different magnitude of the energy dissipated, per unit surface, in
both morphologic units restricts sediment transferences between
them. The active part of the profile is the steep beach face,
especially during the high tide. Long-term (years to decades) beach
evolution is mainly associated to human interferences in the system,
as land occupation and dredging activities (FREIRE et al., 2007).
Although sediment content presents high textural and
compositional spatial variability, it was not possible to relate grain
size statistics to different energetic events.
In normal conditions, tidal currents are not significant at the
beach; although, under extreme storm events, the average velocity
of the water column due to the tidal current during ebb, obtained by
numerical modelling, can reach 0.3 m.s-1 in the channel in front of
the beach (VARGAS et al., 2008). Numerical results also pointed out
that the river has low impact in either the currents or the water level
at Alfeite beach. Nevertheless, field observations show that when a
flooding event, normally associated to low pressure conditions,
occurs simultaneously with high tidal levels overtopping of the
beach high-berm can occur, promoting morphologic changes in the
upper foreshore and backshore areas (FREIRE, 2003). Strong SW
winds (also associated to low pressure events) can also be
also no visible response to strong events (e.g. storms, wind) during
the studied period. Nevertheless, small short-term volumetric
changes (between months) within profiles can be observed, being
related with changes in wind direction, by morphological
realignment to the prevailing wave induced characteristics. Most of
the wind occurrences for the study period are related with S-SW
wind conditions (mean 4 m.s-1). Local waves generated by S-SE
wind are frequently associated to small erosion at the beach face,
while transition to the SW domain is associated with accretion.
There is no evidence of net alongshore sediment transferences
within the study area, indicating that morphological changes are
mainly dependent on cross-shore exchanges at short-term scale.
This cross-shore exchange dot not imply exchange of sediment
between different morphologies (e.g. between beach face and tidal
flat), but a simple cross-shore readjustment within each
morphology limits (e.g. Figure 3). The evident absence of
sedimentary exchange between morphologies is mostly related with
Figure 3. Cross-shore transport evidence at the beach face,
Ancão; displacement of small sand waves.
Figure 2. Erosive morphologic features in Alfeite beach resulting
from storm wind generated waves.
responsible for important sediment mobility in the higher units of
the beach profile (FREIRE and ANDRADE, 2000).
Ancão beach
The overall volumetric changes at medium-scale are generally
very small (vertical variability and slope), with maximum vertical
displacement observed of 0.5 m (Table 1). The global evolution
points towards an overall stability (from months to years) with
several accretion/erosion changes for the studied period. There is no
seasonality or cyclical behaviour in the obtained results. There is
the predominant low energetic wave climate (Hmax=0.10 m,
Table 1).
Tidal currents assume particular importance when explaining
other magnitude variations, at a longer scale of analysis (long-term
scale). The beach proximity to the tidal channel (Figure 1-B), and
the inherent vulnerability to the tidal current dynamics
(speed/duration of ebb and flood tide) lead to other patterns of
sediment transport (long-term scale). Sediment exchange follows
the preferential current flow and respective fluctuations. For
instance, the occurrence of a differentiate transport (ebb dominated,
Table 1) caused by flow asymmetry, can lead to a dominant SE
longshore transport.
DISCUSSION
Both Alfeite and Ancão beaches are affected by locally generated
waves and protected from the influence of ocean waves. Their
morphologic features, a steep beach face and a gently sloping tidal
flat, are typical characteristics of fetch-limited beaches.
In a medium-term analysis, both beaches present
morphologic-sedimentary stability with no seasonal evolutionary
pattern typical of ocean beaches.
Journal of Coastal Research, Special Issue 56, 2009
185
Morphodynamics of Fetch-Limited Beaches
Table 1: Characteristics and morphodynamics constrains of the
study beaches (n. – negligible).
Alfeite
Ancão
estuary
backbarrier
Morphology
Beach location
Beach orientation
E-W
W-E
12 000 (NE)
4 000 (NW)
0.10, 0.12, 0.08
0.07, 0.07,0.06
0.30
0.50
Median diameter ()
0.20
0.40
Standard deviation ()
Silt-clay fraction
(<0.063mm) (%)
0.80
0.11
<1.00
0.03
NE
SW
0.11, 0.84
0.05, 0.10
n.
0.24
n.
none
Max. fetch (m)
Tanβ (aver., max.,
min.)
Beach max. vertical
variability (m.y-1)
Beach face
sediments
Forcing factors
Predominant wind
direction
Waves (m; Hmean,
Hmax)
Tidal currents at the
beach (m.s-1; max)
River influence in
water level
At Alfeite, the evolution is episodically forced by
high-energy/low-frequency events related with wave generation by
northerly strong winds. Cross-shore sediment transport between the
upper and the lower beach face, without significant change in
profile slope is the main source of morphologic changes. Sediment
transferences between the beach face and the tidal flat can be
considered negligible. Tidal currents do not affect beach stability,
i.e., do not cause sediment transferences within beach face, due to
the wide tidal flat that separates the beach from the tidal channel.
Also with no seasonality, the evolution of Ancão beach is driven
by generated waves, at short-term, and by tidal currents, at
long-term, with different magnitude changes. At short-term
(between months) changes are of low magnitude, and occur mainly
at the beach face as result of alterations in local wave conditions.
However, the associated volumetric variability is low, and the local
wave climate is just responsible for second order readjustments in
the beach profile. The induced cross-shore transport is restricted to
the morphology limits (e.g. displacement of small sand waves at the
beach face, Figure 3), without exchanges between the main
morphologic profile features. Significant changes and persistent
variations in the beach shoreline do not seem to have a direct
relation with wave conditions, and can only be seen, when
considering a larger scale of analysis (years to decades). The
overall time of beach response to waves is slow and continuous,
and the main morphological evolution is probably more related
with changes on the tidal currents regime (CARRASCO et al., 2008).
Beach profile is affected by current oscillations induced by
ebb/flood fluctuations, with possible stimulation of longshore
transport (mainly at the sand spit). Due to the difficulty of
interpretation, further analysis is needed to confirm beach
dependence on tidal currents. In particular, it is necessary to
quantify the magnitude of the transport and associated profile
response, and relate them to the tidal cycle fluctuations.
While at both systems river influence is negligible at Alfeite
beach, located in the inner estuary, waves are the main responsible
for beach evolution; at Ancão backbarrier, in addition to waves,
tidal currents play an important role in beach evolution.
Obtained results suggest the need of a simple beach classification
system that can be useful to characterize the beach dominant
forcing mechanism which, in turn, controls the beach
morphodynamic behaviour.
DAVIS and HAYES (1984) have approached this subject
quantifying the relative effect of wave and tidal forcing. However,
the influence of wave action was based on qualitative approach
that, even though can be useful for ocean beach systems, seems to
be inadequate in fetch-limited systems where wave heights are
considerable smaller. In the scope of this study a new classification
scheme is proposed, also based on the relative tide and wave
effects, but where both influences are quantified through a similar
physical quantity: tidal current at the ebb/flood peak and the
magnitude of maximum breaking wave near bottom orbital
velocity, respectively. Assuming the linear wave theory (KOMAR,
1998) the orbital velocity at breaking (Hb) is given by 0.5(.g.Hb)0.5,
which can be simplified to 1.4 Hb 0.5, since  is the ratio of wave
height to water depth (≈ 0.78) and g is the acceleration of gravity
(9.81 m.s-2). Considering that a system is a wave dominated
environment when wave orbital velocity excess tidal current, a
simple classification scheme can be developed (Figure 4). In this
classification both studied beaches fall within fetch-limited domain
(mean wave height < 0.25 m, as defined by JACKSON et al., 2002)
but showing different energy levels and differences in the relative
effect of wave and tide. Alfeite beach shows higher energy than
Ancão and in the latter the relative importance of wave effect is
smaller.
CONCLUSIONS
The morphodynamics of two fetch-limited beaches within
different
geomorphologic
settings
(Alfeite-estuary
and
Ancão-backbarrier) are compatible with the morphotypes of these
beaches presented in the literature. In a medium-term analysis
(months to years), both beaches present a relatively
morphologic-sedimentary stability and no seasonal evolutionary
pattern is observed in opposition to beaches affected by ocean
waves.
Site specific factors, related with the geomorphologic settings of
the study systems (e.g. fetch; distance between the beach and the
tidal channel), are responsible for the mechanisms that promote
beach evolution and how this evolution takes place. At Alfeite
beach, waves are the main driven factor of beach evolution
promoting cross-shore sediment transferences within the beach
face; beach stability is episodically interrupted by storm wave
action causing significant erosion in the upper part of the beach
face; non-storm waves slowly promote profile recuperation. At
Ancão, even if the waves promote short-term/low-magnitude
changes at the beach face, tidal currents seem to be responsible for
significant and persistent variations at the beach at a large time
frame.
A simple beach classification system for fetch-limited beaches,
based on the relative wave and tide effects is proposed. Studied
beaches present differences in energy levels and in the relative
importance of wave/tide effects. Further developments are needed
to achieve a better quantitative specification of the wave/tide
relative importance in beach morphodynamics and to improve and
confirm the beach classification suggested here. Future work will
include testing this classification in other fetch-limited systems.
Journal of Coastal Research, Special Issue 56, 2009
186
Freire et al.
1.40
tidal current (m.s-1)
1.20
tide dominated
vw=vc
1.00
mixed energy
0.80
0.60
vw=2vc
0.40
wave dominated
Ancão
0.20
0.00
0.00
fetch-limited beaches ocean beaches
Alfeite
0.10
0.20
0.30
0.40
0.50
wave height (m)
Figure 4. Classification of fetch-limited beaches based on wave
height and tidal current (vw-wave velocity; vc-current velocity).
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748-760.
This paper is a contribution from project BERNA - Beach
JACKSON, N., 1995. Wind and waves: Influence of local and Evolution in Areas of Restricted fetch: experimental and numerical
non-local waves on mesoscale beach behaviour in estuarine analysis
(POCTI/CTA/45431/2002),
supported
by
FCT
environments. Annals of the Assoc of American Geographers, (Foundation for Science and Technology).
85 (1), 21-37.
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