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Livro de Resumos
© 4ª Conferência sobre Morfodinâmica Estuarina e Costeira | MEC2017,
Livro de Resumos, 2017, Porto - Portugal
SPATIAL RESPONSE OF THE CALDEIRA DE TRÓIA SALT MARSH
(SADO ESTUARY, PORTUGAL) TO SEA LEVEL RISE
Miguel Inácio(1), Ana Cunha(1,2), Mª. da Conceição Freitas(1,2), Carlos Antunes(2),
César Andrade(1,2) e V. Lopes(1,2)
(1) Departamento de Geologia, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal
miguel-inacio@outlook.com; agcunha@fc.ul.pt; cfreitas@fc.ul.pt; candrade@fc.ul.pt; vlopes@fc.ul.pt
(2) Instituto Dom Luiz, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal
cmantunes@fc.ul.pt; vplopes@fc.ul.pt
Abstract
The resilience of salt marshes to climate change and sea level rise mainly depends on the balance
between sedimentation rates and sea level rise rates. A model was developed to forecast how
Caldeira de Tróia salt marsh will respond to that global forcing factor. The results indicate that
the salt marsh can tolerate the projected rise in sea level for 2050. In 2100, a significant loss of
high marsh is observed, indicating a regression trend in marsh maturity.
Keywords: Sedimentation rates; Spatial modeling; Accretion; Morphodynamic; Global change.
1. Introduction and Study Area
The fate of intertidal environments – salt marshes and tidal flats - in the context of climate change
is a relevant issue from ecological and economic perspectives. Their vulnerability to sea level rise
(SLR) driven by climate change depends on different factors such as local geomorphology,
sediment supply, vegetation and rate of SLR. The northern sector of the Caldeira de Tróia
(C. Tróia (N)) (Sado estuary), sheltered by the tip of the Tróia sand spit, encompasses a salt marsh
covering ca. 4.1 ha that presents high vulnerability to sea level rise. This intertidal area has been
used as a pilot area to implement a methodology that allowed forecasting marsh responses to
that global forcing factor.
2. Materials and Methods
Four different parameters have been used to address the geomorphological evolution of C. Tróia
(N) under one SLR scenario: sedimentation rates in high/low marsh and tidal flat domains; high-
resolution spatial-elevation surveys; sea level rise rates adapted to the study area; and local tidal
data.
Cunha et al. (2017, this conference) used 210Pb isotopic activity in cored sediments to propose a
2.9 mm/yr sedimentation rate for the low marsh (LM). According to the literature, the
sedimentation rates will be higher in the tidal flat (TF) and lower or similar in the high marsh
(HM) domains; due to the lack of objective data, a 5.8 mm/yr value was assumed for the TF (i.e.
doubling the LM sedimentation rate) and 2.9 mm/yr was considered for the HM (similar to the
LM). A high resolution (2m) digital terrain model (DTM) was obtained from the LIDAR
(Silva et al., 2012) survey, validated with DGPS-RTK field data.
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The sea level rise scenario follows Antunes (2016) and considers an initial SLR rate of 2.1 mm/yr
in 2000 and an acceleration of 0.08 mm/yr2 from that time onwards. These parameters were
estimated using the daily mean sea-level data series of Cascais tide gauge (seasonal signal
removed, inverse barometric correction applied and relative vertical velocity of gauge
subsidence corrected). Based on the harmonic analysis of Setúbal-Tróia tide gauge 2005 data, the
tide model was estimated and used to build tidal tables for 2000 to 2016. Mean high water
springs MHWS, mean high water MHW and mean high water neaps MHWN were determined
for the same time span. MHWS (1.59m) was considered as the limiting elevation allowing for
high marsh accretion, MHW (1.25m) as the high/low salt marsh interface and MHWN (0.88m)
as the low salt marsh/tidal flat interface.
The model updates the elevation of each pixel of the DTM using Matlab R2016a and ArcGis 10.2
software for 2050 and 2100, considering these rates and tidal boundaries. In both projections, the
colonization of the surrounding areas by high marsh was allowed. The ratio between the area of
HM and LM was also calculated and the classification: HM/LM >1: mature; HM/LM = 1:
intermediate; HM/LM<1: young, was applied.
3. Results and Conclusion
Figure 1 presents the distribution of the three environments (HM, LM and TF) in C. Tróia (N) at
present, in 2050 and 2100. At present, a mature salt marsh (HM/LM = 2.8) fills and dominates
the marginal area. In both projections, the total marsh surface decreases, by 2% in 2050 and 60%
in 2100. In 2050 the maturity will diminish (HM/LM = 1.4), which, together with the global
decrease, shows a considerable regression of the whole marsh, in particular the high marsh. In
2100 the maturity is drastically lower (HM/LM = 0.5), showing inability of the high marsh to
keep pace with the rise in sea level (-82.4%), in contrast with the low marsh (-2.6%).
Figure 1. Distribution of the high marsh, low marsh and tidal flat areas in Caldeira de Tróia (N).
The C. de Tróia salt marsh shows a low resilience to future rise in sea level. In 2050 the system is
sustained by a growth of the low marsh. In 2100, a translation of the whole marsh occurs and
there are some high marsh zones replaced by low marsh. If the salt marsh surrounding area was
anthropically limited, its surface will almost disappear by that time.
References
Antunes, C., 2016. “Subida do Nível Médio do Mar em Cascais, Revisão da Taxa Actual”. Actas,
4.as Jornadas de Engenharia Hidrográfica, Instituto Hidrográfico. ISBN - 978-989-705-097-8
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Cunha, A., Inácio, M., Freitas, M.C., Silva, T.A., Andrade, C., Bao, R., Leira, M., Lopes, V., 2017.
“Characterizing the sediment deposition in Caldeira de Tróia salt marsh (Sado Estuary –
Portugal).” Resumos, 4ª Conferência sobre Morfodinâmica Estuarina e Costeira, Porto, 2017.
Silva, M., Patrício, P., Mariano, A., Morais, M., Valério, M., 2012. “Obtenção de Dados LiDAR
para as Zonas Costeiras de Portugal Continental”. Actas, 2.as Jornadas de Engenharia
Hidrográfica, Instituto Hidrográfico. ISBN – 978-989-705-035-0
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