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Journal of Coastal Research, Special Issue No. 70, 2014
Study of suspended sediment dynamics in a temperate coastal lagoon: Ria de Aveiro (Portugal) 604
Study of suspended sediment dynamics in a temperate coastal lagoon:
Ria de Aveiro (Portugal)
Sandra Plecha†, Ana Picado†, Pedro Chambel-Leitão‡, João M. Dias†, Nuno Vaz†
†NMEC, CESAM University of Aveiro
Aveiro, Portugal
{sandraplecha, ana.picado, joao.dias, nuno.vaz}@ua.pt
‡ MARETEC Technical University of Lisbon
Lisbon, Portugal
INTRODUCTION Tidal dominated coastal systems, such as estuaries and lagoons,
have experienced over the time, changes in geomorphology
(especially erosion), which may be attributed to sea level rise
(SLR), variations in the amount of sediment deposited on the coast
and to anthropogenic degradation (Santos and Miranda, 2006).
These changes could increase marginal inundation risk, salinity
intrusion on adjacent land, marginal and bottom erosion, as well as
restricting the navigation in shallow channels. Moreover, SLR
may also contribute to changes in estuarine patterns of dissolved
and non-dissolved matter, such as fine-grained cohesive
sediments.
The importance of cohesive sediments in coastal regions has
been widely recognized, playing an important role in the
functioning of healthy ecosystems, since their transport,
propagation and concentration influence the development of flora
and fauna in estuaries and lagoons. Additionally, changes in sea
level could induce changes in the spatial distribution of cohesive
sediments and influence ecosystem biogeochemistry.
The variation in composition and spatial variability in
suspended sediment dynamics render them difficult to study in the
field. This is further hindered by complex interactions between
tide and freshwater discharge. One way to overcome difficulties
and study spatial and temporal variability of suspended sediments
is to use high resolution numerical models.
In this study a numerical model is used to evaluate cohesive
sediment dynamics in Ria de Aveiro (Figure 1), under both
present mean sea level and a future sea level rise (SLR) scenario
(+0.42 m).
Several studies have been undertaken on the effect of SLR in
Ria de Aveiro. For example, Lopes et al. (2001) analyzed
numerically the influence of tides and river inputs on suspended
sediment concentration (SSC) in Ria de Aveiro lagoon,
emphasizing Laranjo Bay from a bio-geophysical point of view.
Also, Dias et al. (2003), Lopes et al. (2006) and Abrantes et al.
(2006) used numerical models to investigate the spatial and
temporal variability of SSC in this lagoon. Those authors
concluded that tidal, wind- and river-induced residual currents
contribute to the net export of water and sediments seaward,
which is consistent with the ebb dominant nature of the system.
In this study, a high resolution numerical model is used to
investigate the impact of SLR in the spatial and temporal patterns
of suspended sediment in the Ria de Aveiro.
STUDY AREA The Ria de Aveiro lagoon (Figure 1), is located in a temperate
climate area on the northwest coast of Portugal. Between the
Vouga river basin and the Atlantic Ocean, this lagoon is highly
influenced by marine and fluvial interaction. The lagoon is
characterized by a complex geometry comprising several channels
and intertidal areas, and is connected to the Atlantic Ocean
through a narrow channel of 1.3 km length, 350 m wide and more
than 30 m deep.
The Ria de Aveiro is a mesotidal system (with an average range
at the inlet of 2.0 m) with a tidal semidiurnal pattern (Dias et al.,
2000). The tidal prism of the lagoon is approximately 139.7×106
m3 for maximum spring tide and 65.8×106 m3 for minimum neap
tide (Picado et al., 2010), with the water flux through the inlet
ranging between 6.26×103 m3s-1 for spring tide and 2.95×103 m3s-1
for neap tide (Lopes et al., 2013).
ABSTRACT
Plecha, S., Picado, A., Chambel-Leitão, P., Dias, J.M., Vaz, N., 2014. Study of suspended sediment dynamics in a
temperate coastal lagoon: Ria de Aveiro (Portugal). In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th
International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 604-
609, ISSN 0749-0208.
Suspended sediment concentrations are simulated at Ria de Aveiro, a lagoon located in a temperate climate area in the
northwest of Portugal. The fine-grained suspended sediment concentration is analyzed using the numerical model
MOHID (www.mohid.com) and spatial maps of instantaneous and maximum concentration and also temporal
variability at specified locations are analyzed in order to characterize the influence of the tide and sea level conditions in
the suspended sediment concentrations within the lagoon. The highest suspended sediment concentrations were found in
upstream areas during ebb conditions due to the river’s proximity, while the minimum concentrations were observed
near the inlet due to the presence of marine water poor in suspended sediments. When a 0.42 m sea-level rise (an
estimate of conditions in 2100) is modelled, a decrease in suspended sediment concentration is observed for the overall
study area, as result of an increase in the tidal prism.
ADDITIONAL INDEX WORDS: Suspended sediment, sea level rise, sediments modeling, MOHID.
www.JCRonline.org
____________________
DOI: 10.2112/SI70-102.1 received 29 November 2013; accepted 21
February 2014. © Coastal Education & Research Foundation 2014
© Coastal Education & Research Foundation 2013
www.cerf-jcr.org
Journal of Coastal Research, Special Issue No. 70, 2014
Study of cohesive sediment dynamics in a temperate coastal lagoon: Ria de Aveiro (Portugal) 605
The lagoon receives freshwater from several rivers: the Antuã,
Cáster, Gonde, Boco and Vouga rivers and Valas de Mira (see
Figure 1). However the major fluvial input comes from the Vouga
River which contributes 2/3 of the freshwater entering the lagoon,
with an annual average flow of approximately 70 m3s-1.
The residual circulation is determined essentially by flood/ebb
asymmetry, with higher velocities on ebb than on flood. As the
lower lagoon is ebb-dominant, it shows a trend to export
sediments to the ocean (Oliveira et al., 2006, Plecha et al., 2012).
The bottom granulometry of Ria de Aveiro channels is highly
variable, being characterized by a combination of medium to fine
sands with a variable content of finer particles (silt and clay),
which increases with distance from the lagoon mouth. The inner
zone of intertidal flats is mainly composed of medium to fine
sand, silty clay and sandy clay (Freitas et al., 2005).
METHODS The cohesive sediment concentration was analyzed using the
numerical model MOHID (www.mohid.com) which encompasses
a 2D advection-diffusion equation coupled to the hydrodynamic
model.
Numerical model The numerical model used in this work is the two-dimensional
baroclinic model MOHID (www.mohid.com), developed by the
Instituto Superior Técnico (Universidade Técnica de Lisboa)
(Martins et al., 2001). A finite volume discretization approach has
been implemented (Martins et al., 2001), allowing the use of a
generic vertical coordinate. MOHID has been applied to several
different coastal and estuarine areas and has shown its ability to
simulate complex features of the flows (Ruiz-Villarreal et al.,
2002, Vaz et al., 2005, 2009, Saraiva et al., 2007, Mateus et al.,
2012). The hydrodynamic application described in this study is
based on previous implementations for Ria de Aveiro (Vaz et al.,
2009, 2011).
The cohesive sediment transport model considers an Eulerian
approach to compute fine sediment concentration in the water
column. A 3D advection-diffusion equation integrated in MOHID
is used in which the particle settling velocity is represented by the
vertical advection (Cancino and Neves, 1994 a, b; 1995a, b). A
differential control volume is used in computations considering
the sediment accumulation rate and the sediment fluxes in and
through the volume, respectively.
The governing equation can be written in the conservative form
as:
z
C
zy
C
yx
C
x
z
CWw
y
uC
x
uC
t
C
zyx
s )(
(1)
Where C is the suspended sediment concentration, t is time, x, y
are the horizontal co-ordinates, z is the vertical co-ordinate, εx, εy
and εz are the sediment mass diffusion coefficients, WS the
sediment fall velocity and u, v, w the flow velocity components in
x, y, z directions.
Numerical Setup Numerical simulations are performed considering a numerical
grid with spatial resolution of 100 m in order to minimize
computational costs and a time step of 5 s.
The freshwater inputs from 6 main rivers are considered as
landward boundary conditions. Each river inflow was predicted
through the SWAT numerical model (Arnold et al., 1998), which
is a river basin scale model developed to quantify the impact of
land management practices in large, complex watersheds.
The cohesive sediment transport model is initialized with a
constant sediment concentration of 0.01 mgL-1 for the whole
domain. The suspended sediment concentration fluctuation is
simulated by imposing a specific suspended sediment
concentration for each river discharge. Due to the lack of in-situ
data for cohesive sediment concentration associated to each river
inflow, reference values between 8 and 10 mgL-1 were used as
boundary conditions. For sediment deposition a settling velocity
of 0.00001 ms-1 is used.
The initial simulation period was between 1st December 2009
and 15th January 2010, integrating a spin-up period of 30 days, in
order to encompass two times the maximum Ria de Aveiro
residence time. The numerical results analyzed and presented here
relate to the final 15 days of simulation.
Several numerical simulations were performed in order to
characterize the cohesive sediment concentration under two sea
level scenarios: current mean sea level (MSL) and a predicted rise
of 0.42 m (SLR) to 2100 (Lopes et al., 2011). Then, the sediment
concentration patterns were analyzed along the tidal cycle and also
in the spring-neap cycle, in order to evaluate differences due to
tidal propagation. Spatial maps of instantaneous and maximum concentrations as
well as the temporal variability of sediments concentrations at
specified locations (along the main channels of the lagoon) were
analyzed in order to characterize the influence of the tide and sea
Figure 1. Location of Ria de Aveiro, identifying its main
channels and the stations used in the study.
Journal of Coastal Research, Special Issue No. 70, 2014
606 Plecha et al.
level conditions in the cohesive sediment concentrations
distribution within the lagoon. The future RSL condition assumes
no change in bed levels.
RESULTS AND DISCUSSION
Cohesive sediment dynamics Simulations show that cohesive sediment dynamics in the Ria
de Aveiro are characterized by a fortnightly pattern, related to the
spring and neap tide, as illustrated in Figure 2 for five stations
located within the lagoon. These five stations (Figure 1) were
chosen in order to encompass distinct areas of the lagoon. The
Barra station is located near the inlet and is the most downstream
station, being under oceanic influence. The remaining four stations
are located in the main channels of the Ria de Aveiro. The
Espinheiro station is located in the central area of the lagoon under
the influence of the major freshwater source: the Vouga River.
The Torreira station is located in the S. Jacinto channel and is
influenced by two rivers: the Cáster and Gonde. The Mira and
Ílhavo channels are represented by the Costa Nova and Vista
Alegre stations under the influence of Valas de Mira and Boco
rivers, respectively. An additional river (Antuã) located in Laranjo
Bay region (see Figure 1) was also considered in simulations.
The results depicted in Figure 2 show that the rivers are the
main cohesive sediment source, which is evidenced by the high
concentrations found at the Espinheiro station located close to the
Vouga river mouth. Additionally, higher concentrations are
observed during ebb than flood conditions, since the main
cohesive sediment source is the river discharge. The wide range in
cohesive sediment concentration observed, between 0.01 mgL-1 at
Barra and 10 mgL-1 at Espinheiro, shows the influence of marine
and fluvial water in the lagoon.
The range of cohesive sediment concentration in upstream
stations (Figure 2) is higher during spring tide than on neap tides,
reflecting the higher intrusion of marine water (low concentration)
that dilutes the fluvial waters (high concentration). The riverine
influence on sediment concentration is also evident for neap tide
periods when minimum cohesive sediment concentrations are
higher than 2.5 mgL-1.
For the SLR scenario, a decrease in cohesive sediment
concentration is observed, induced by an increase of the lagoon
tidal prism (Lopes et al., 2013) and consequent higher intrusion of
marine water with low cohesive sediment concentration.
A concentration decrease is predicted at the Torreira station,
located in the S. Jacinto channel, with an average decrease of 0.60
mgL-1. This reduction is caused by the importance of this channel
in the water distribution through the lagoon, since 33% of the total
volumetric flux of the lagoon crosses this channel.
The concentration in Espinheiro and Vista Alegre stations
presents a decrease of about 0.50 mgL-1 (on average), since the
tidal prism, due to the SLR at these channels increase at a higher
rate (25%) than for the remaining channels (14%) (Lopes et al.,
2013).
Maximum suspended sediment concentration The spatial distribution of maximum suspended sediment
concentration was computed for 15 days and is illustrated in
Figure 3, for a spring/neap tide period and for the MSL and SLR
scenarios.
The highest sediment concentrations are observed at the
upstream regions of S. Jacinto channel, Laranjo Bay and Mira and
Ílhavo channels, justified by the weak influence of the tide when
compared with the river influence. In addition the cohesive
sediment concentration maximums occur at spring tide (Figures
3a-c), since the stronger ebb currents at this tidal stage produce a
downstream advection of water with high suspended sediment
concentration.
For the Ria de Aveiro northern area, high concentrations are
observed in spring tide (Figures 3a-c), with values ranging
between 4 and 8 mgL-1, while in neap tide period (Figures 3d-f) it
ranges between 3.5 and 8 mgL-1. For the central region of Ria de
Aveiro, which encloses the Espinheiro channel, the Vouga river
influence is noticeable, with maximum cohesive sediment
concentrations of 10 mgL-1. At Ílhavo and Mira channels, the
maximum cohesive sediment concentrations are observed at
spring tide, with values of 8.2 mgL-1, while in neap tide they are
10% smaller.
For the SLR scenario, the maximum cohesive sediment
concentrations are illustrated in Figure 3b,e. To better analyze the
changes induced by the SLR, the differences between MSL and
SLR concentrations were computed and are illustrated in Figures
3c and f. It is found that the SLR induces a generalized decrease in
the maximum concentrations, in particular for spring tide (Figure
3a-c) due to an increase in seawater volume entering the lagoon.
On the other hand, a slightly increase in maximum concentration
is observed in Mira channel. Figure 2. Water level (m) at Barra and suspended sediment
concentration (mgL-1) for five stations within Ria de Aveiro, for a 15 day period.
Journal of Coastal Research, Special Issue No. 70, 2014
Study of cohesive sediment dynamics in a temperate coastal lagoon: Ria de Aveiro (Portugal) 607
Suspended sediment concentration – tidal cycle
dependence The suspended sediment concentration dependence on the tidal
stage is illustrated in Figure 4 for four conditions: flood (a,e), high
tide (b,f), ebb (c,g) and low tide (d,h), for spring (a-d) and neap (e-
h) tide periods, respectively.
The spatial distribution of suspended sediment concentrations
reveals a spatial gradient. In the upstream regions high suspended
sediment concentrations are found, close to those in the inflowing
rivers which are cohesive sediment sources. Elsewhere, in
downstream regions, the influence of sea water is evidenced by
Figure 3. Maximum cohesive sediment concentration (mgL-1) in Ria de Aveiro for spring (a, b, c) and neap (d, e, f) tide periods, under
contemporary mean sea level (a, d) and a sea level rise of 0.42 m (b, e). Differences between the two sea levels are shown in (c) and (f).
Journal of Coastal Research, Special Issue No. 70, 2014
608 Plecha et al.
the observed patterns of low concentrations (2 mgL-1). Similarly to
what was observed in Figure 3, the suspended sediment
concentration is higher on spring than neap tides.
The highest concentrations are observed in the central area of
the lagoon at low tide, with values ranging from 3 mgL-1 at the
inlet to 8 mgL-1 at the middle of the channels. During flood and at
high tide the influence of sea water is higher during spring tide
than neap tide, reaching 12 km and 10 km in the S. Jacinto and
Espinheiro channels, respectively. At high tide, the suspended
sediment concentration in the central area of the lagoon is
approximately 0.5 mgL-1 lower on spring tide than during neap
tide.
The suspended sediment concentration along the tidal cycle
(Figure 4d,h) is higher at low tide in the central area of the lagoon,
with values of 6-10 mgL-1 and 7-10 mgL-1 for spring and neap tide
periods, respectively. At the upstream regions of S. Jacinto, Mira
and Ílhavo channels, the suspended sediment concentrations are
smaller, ranging between 3 and 8 mgL-1.
Furthermore, at spring tide the sea water intrusion is higher than
during neap tide, due to high tidal prism, and consequently the
sediment concentration is lower.
In the inlet region, the suspended sediment concentration is on
average 30% higher in spring tide than in neap tide. This is
evident during flood and high tide, due to the high propagation of
low suspended sediment concentration marine water. This lower
concentration during neap tide is due to the weak tidal currents
that transport less sea water volume into the lagoon (small
sediment concentration), turning the river flow the main
suspended sediment source in the lagoon. The regions close to the
river mouths also have higher concentrations during spring tide
due to the intense ebb currents that re-suspend the sediments,
enhanced by the river inflow velocity.
Figure 4. Cohesive sediment concentration (mgL-1) in a tidal cycle for a spring (a-e) and neap (e-h) tide periods at flood (a,e), high tide
(b,f), ebb (c,g) and low tide (d,h).
Journal of Coastal Research, Special Issue No. 70, 2014
Study of cohesive sediment dynamics in a temperate coastal lagoon: Ria de Aveiro (Portugal) 609
In the Mira and Ílhavo channels, and due to their smaller
extension and inlet proximity, the sediment concentration is
similar for both spring and neap tide, being slightly higher (10%)
in spring than in neap tide, as a result of the strongest tidal and
fluvial influence on the water flow, with higher sediment
concentration in the upstream-downstream direction.
On the other hand, in S. Jacinto and Espinheiro channels, the
sediment concentrations depend strongly on the tidal period.
CONCLUSION The suspended sediment dynamics in the Ria de Aveiro have a
fortnightly pattern related to spring and neap tide and sediment
concentration is highly dependent on the concentrations imposed
at each river boundary.
The maximum sediment concentrations were found at the
upstream areas during ebb conditions, due to the combined effect
of tide and river discharge. The minimum suspended sediment
concentrations were observed near the inlet under flood
conditions, revealing the effect of seawater propagation into the
lagoon. The highest suspended sediment concentrations were
found at the central area of the lagoon (Espinheiro channel), which
connects the main freshwater source (Vouga River) to the ocean.
When considering a sea level rise as consequence of climate
change, a decrease in the cohesive sediment concentration is
predicted for the overall study area, due to the higher intrusion of
marine water scarce of sediments.
The cohesive sediment model developed in this study
constitutes a promising tool to study the local sediment dynamics
under different forcing conditions. Building on this work it is
planned to implement a biogeochemical model that uses the
results of the transport model to compute the dynamics of
Chlorophyll in Ria de Aveiro, evaluating the limiting factor for
phytoplankton growth. The model described here constitutes the
first step toward the development of a predictive biogeochemical
model for the Ria de Aveiro lagoon.
ACKNOWLEDGEMENT This work has been partly supported by FCT and by European
Union (COMPETE, QREN, FEDER) in the frame of the research
project PTDC/AAC-AMB/121191/2010 - BioChangeR:
Biogeochemical Processes induced by Climate and Anthropogenic
Circulation Changes - The Case Study of Ria de Aveiro. The
European Commission, under the 7th Framework Programme,
also supported this study through the collaborative research
project LAGOONS (contract n°283157). The second author has
been supported by the Fundação para a Ciência e Tecnologia
through the doctoral grants SFRH/BD/79920/2011. The last
author is supported by the Ciência 2008 program.
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