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1 INTRODUCTION
Ports are a major component of the 21st century global
economy. Therefore, in a context of climate change,
assessing and predicting the sea level rise (SLR) have
become paramount in Port Engineering. Back in 2005,
there were 136 ports worldwide — thirteen of which
were among the planet’s twenty most important ones
[13]. However, not every port is affected in the same
way by SLR. Exposure to climate change effects
increases more drastically in developing countries
[13].
Continuous records of long tidal series on the
Brazilian coast are scarce, given its 8,500 km length
(Table 1) and Figure 1.
Table 1. Observed rates of sea level rise for several Brazilian
locations.
_______________________________________________
Location Rate Tidal Gage Reference
(mm·yr
-1
) Period
_______________________________________________
Santos (SP) 3.3 1940–2014 [14]
Equatorial Atlantic 4.0 Altimetry [8]
Belém (PA) 3.4 1948–1970 [4]
Fortaleza (CE) 0.3 1950–1970 [4]
Recife (PE) 5.6 1946–1988 [9]
Salvador (BA) 2.0 1950–2009 [11]
Canasvieiras (BA) 4.1 1950–1970 [4]
Rio de Janeiro (RJ) 3.6 1950–1970 [4]
Ubatuba (SP) 2.1 1954-1991 [2]
Santos (SP) 3.3 1940–2014 [2]
Cananeia (SP) 3.8 1957–1993 [2]
Imbituba (SC) 0.7 1950–1970 [4]
_______________________________________________
Behavior of Sea Level in the Period of 1980 to 2017 on
the
Port Area of Gulf of Maranhão, Brazil
P. Alfredini,
E. Arasaki & E. Fortner
Polytechnic School of S
ão Paulo University, São Paulo, Brazil
ABSTRACT: The Gulf of Maranhão, North Coast of Brazil, is one of the regions in the world with largest tidal
ranges. The Port Area of Maranhão, in São Marcos Bay, represents the second most important in Brazil. The port
facilities are naturally sheltered from swell, with nautical operations and maintenance dredging volumes
directly conditioned by macro-tides, which exceed the 6.0 m tidal range, and associated tidal currents, which
can reach 7.0 knots. In order to assess the behavior of sea levels in recent decades, in view of the influence of
climate changes on tides in various ports around the world, a period of two lunar nodal tidal cycles of 18.61
years, from 1980 to 2017, was investigated using unpublished data recorded in tide gauges. The trend pattern
obtained was analyzed statistically and, unlike many other port areas, a sensitive stability of the mean sea level
was noted. An important conclusion is about the reduction in HHW and increasing in LLW, leading to a
reduction in tidal ranges, in tidal currents and a significant reduction of the shear stress in the bottom, which
may increase the dredging rates in the port areas in the next decades, due to an increasing siltation.
http://www.transnav.eu
the
International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 15
Number 3
September 2021
DOI: 10.12716/1001.15.03.24
684
Figure 1. Site location of Brazilian main tidal stations
network (adapted from GLOSS).
It has long been suggested that the use of a period
of at least two lunar nodal tide cycles (encompassing
37.2 years) for proper sea level assessments is highly
recommended [10, 15]. As there is usually a strong
interest in extrapolating the trend detected with past
data, it is also important that the records extend to
dates as close to the present day. Only four tide
stations located on the Brazilian coast meet these
requirements, Gulf of Maranhão (MA), Ilha Fiscal (RJ),
Santos (SP) and Cananeia (SP).
The city of São Luís is the capital of the state of
Maranhão, in northeastern Brazil. It is in São Marcos
Bay (in Gulf of Maranhão), the largest Brazilian bay,
which is home to the second largest port complex in
Latin America, and one of the world's largest in terms
of cargo movement [3], therefore of great importance
for Brazilian exports. With a 4 m average tidal
amplitude, the region presents strong currents, which
can reach 7.0 knots. This incurs complex port
operations in São Marcos Bay [5], such as berth
window management, approach channel navigation
and overall vessel maneuverability, and ship loading.
Due to the region's increasing urbanization, its
economic importance for the Country and the safety
of its port operations, the present article had the
purpose to evaluate, with unpublished data, the
future behavior of sea level to be faced in São Marcos
Bay and surroundings. Lastly, such behavior was
classified according to international benchmark on the
subject and conclusions about possible consequences
regarding the modification of tidal currents were
drawn.
2 STUDY AREA LOCATION
São Marcos Bay is the largest Brazilian bay. It is
approximately 500 km long, 100 km wide and it is
bordered by the mainland (to the west), the city of São
Luís (east) and the mouth of the Mearim River
(south). Due to its natural great depth (which allows
receiving 23 m draft ships) and wave protection
aspect, this region's port potential is known at least
since 1612, when French invaders established the first
port of São Luís. Nowadays, several important ports
are set within São Marcos Bay area, among which are
the Itaqui Port and the Ponta da Madeira Terminal.
Figure 2 shows their location.
Figure 2. São Marcos Bay area in Gulf of Maranhão, State of
Maranhão, Brazil.
Itaqui Port began operations in 1974. It currently
handles general cargo, solid bulk, and liquid bulk in
the order of 20 million tons per year [7]. Ponta da
Madeira Terminal is a private port, and its operations
began in 1986. This terminal, with depths up to 25.0 m
(Chart Datum) allowing vessels up to 23.0 m of draft,
is specialized mainly in the export of iron ore and, in
the last decade, its throughput has increased from
96 million tons (2010) to 198 million tons (2018) –
therefore being the Brazilian port with the largest
cargo throughput [1].
3 MATERIAL AND METHODS
3.1 Sea level, tides, and available data
Despite its enormous propensity towards port
development, São Marcos Bay is affected by
astronomical, semi-diurnal tides usually ranging from
3.1 m (mean neap tidal range) to 5.4 m (mean spring
tidal range) — typical of macro tidal regions. Such
tidal amplitude results in strong currents, which pose
a real challenge to maritime operations in the region
[6]. Table 2 shows the main tidal aspects rendered
from harmonic analyses of Itaqui Port and Ponta da
Madeira Terminal tide gauges.
Table 2. Tide gauges, mean sea level, and mean tides in São
Marcos Bay area.
_______________________________________________
Ponta da Madeira Itaqui
Terminal Port
_______________________________________________
Location 02º 33,9' S 02º 34,6' S
44º 22,7' W 44º 22,2' W
Period 01/08/1991 – 01/01/1985 –
31/07/1993 31/03/1985
MSL, [Z
0] (cm) 328 343
MHWS / MHWN (cm) 597 / 482 625 / 502
MLWS / MLWN (cm) 59 / 172 61 / 184
Spring / Neap 538 / 310 564 / 318
Tidal Range (cm)
_______________________________________________
Source: [17].
Highest High Water (HHW), Mean Sea Level
(MSL) and Lowest Low Water (LLW) time series were
obtained for both port locations. Ponta da Madeira
Terminal data were promptly, digitally available and
covered a 30-year span, ranging from 1988 to 2017
(but missing 1989–90). Older data, from Itaqui Port,
were available for the years 1980 and 1984–86.
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However, they were not recorded in digital format,
which required a cumbersome transcription work.
Data generated from the Ponta da Madeira
Terminal tide gauge covers 28 years over a 30-year
span. Itaqui Port, on the other hand, has available data
for older years. Its tide gauge lies within only 1.2 NM
of the Ponta da Madeira Terminal tide gauge.
Together, these gauges begotten data for 32 years over
a 38-year span. This makes up for 84% data
completion over a period of two lunar nodal tidal
cycles.
Sea level behavior in São Marcos Bay will be
compared with British recommendations [18] (see in
Table 3).
Table 3. British recommended contingency allowances for
sea level rise rates (mm·yr
-1
) and cumulative SLR (m).
_______________________________________________
Time Period Low-Rate Moderate Rate High Rate
[Cum. SLR] [Cum. SLR] [Cum. SLR]
_______________________________________________
1900–2025 2.5 [0.09] 3.5 [0.12] 4.0 [0.14]
2025–2055 7.0 [0.30] 8.0 [0.36] 8.5 [0.40]
2055–2085 10.0 [0.60] 11.5 [0.71] 12.0 [0.75]
2085–2115 13.0 [0.99] 14.5 [1.14] 15.0 [1.21]
_______________________________________________
According to [6], the maximum tidal currents
(flood or ebb) in São Marcos Bay are proportional to
the tidal range elevated to exponent 0.67, based on
measurements made for the Ponta da Madeira
Terminal project.
3.2 Quality of the time series
Ponta da Madeira Sea Level Station is registered with
number 200 in GLOSS – Global Sea Level Observing
System of the Intergovernmental Oceanographic
Commission of UNESCO - to provide final quality
assessed high frequency data in near-real time and
metadata to the GLOSS Sea level station Monitoring
Facility (see Figure 1). It is also registered with
number 1736 in PSMSL – Permanent Service for Mean
Sea Level of The Global Sea Level Observing System.
In the Quality Assessment of Sea Level Data by the
University of Hawaii Sea Level Center/National
Oceanographic Data Center Joint Archive for Sea
Level, is registered as Contributor DHN – Diretoria de
Hidrografia e Navegação Banco de Dados
Oceanográficos of Brazilian Navy. Therefore, data
quality is audited by the Brazilian Maritime
Authority.
Data are analog, with instrument type float and
stilling well and digitalized interval spot hourly from
01 Jan 1988. The gaps over than 1 month occurred: 10
Feb 1988 - 21 May 1988; 09 Jun 1988 - 18 Nov 1989; 11
Dec 1989 - 05 Jan 1991;27 Feb 1991 - 01 Jul 1991.
4 RESULTS AND DISCUSSION
Figure 3 shows the historical series, the linear trend
lines, and the confidence intervals (95%) for MSL,
HHW and LLW for São Marcos Bay region. Itaqui
Port vertical datum was employed.
Albeit LLW level increased with a rate of 4.8
mm·yr
-1
throughout the period, both HHW and MSL
decreased with rates of -6.4 and -0.7 mm·yr
-1
,
respectively. According to the British criterion
depicted in Table 3, São Marcos Bay MSL fall within
the low-rate contingency allowance for SLR, while
HHW decrease rate and LLW increase rate lie in the
high-rate contingency allowance for SLR. Still,
considering the confidence intervals calculated for
each data series, no significant change in sea level in
São Marcos Bay region can be stated. Mean sea level
remains statistically constant between 1980 and 2017.
However, the mean tidal range decreased 1.116 cm·yr
-
1
, corresponding in a linear trend reduction of 1.12 m
from 1980 to 2080, which is equivalent to a reduction
of the maximum tidal currents of 11.55%
(proportional to the tidal range elevated to exponent
0.67) and of the shear stress in the bottom of the bay of
21.77% (shear stress is proportional to the square of
the velocity).
These are highly unanticipated results. From Table
1, it follows that for Belém (650 km westward from
Ponta da Madeira) and Fortaleza (700 km eastward
from Ponta da Madeira), the most nearby port cities
from São Luís (see Figure 1) with long tide series,
with SLR rates of 3.4 mm·yr
-1
and 0.3 mm·yr
-1
,
respectively. However, since these figures were
calculated from tide gauge data over a 20-years period
(i.e., comprising only one lunar nodal tide cycle), such
SLR rates may be overestimated [10, 15] and are from
old series ended in 1970. It should be mentioned that
the region is tectonically stable, as well as there are no
subsidence effects due to the extraction of
underground fluids (water, natural gas or oil).
A similar process, but of increasing tidal ranges,
off the coasts of the North Sea from Belgium to
Germany, was attributed [12, 19] to a shift of the
amphidromic points of harmonic constituent M2
(lunar semidiurnal) tide wave of North Atlantic (see
Figure 4). From 1950 to 2005 (three lunar nodal tide
cycle), the trend corresponded to 30.5 cm·century
-1
.
Just as there is a system of amphidromic points in the
North Sea, there is also one of them in the Caribbean
Sea (see Figure 4), and its shift may be causing the
trend, that is, the reduction of tidal ranges in Gulf of
Maranhão. There are a lot of consequences of an
accelerated reduction of the mean tidal range, e.g.,
changes in coastal morphology and sedimentation
problems, causing siltation of port nautical depths in
access channels, basins, and berths.
Figure 3. Historical series (1980–2017) and analyses for
annual tidal level variability in São Marcos Bay.
686
Figure 4. Cotidal and coamplitude (in cm) charts of
semidiurnal (M2) tide of the Atlantic Ocean, adapted from
[16].
5 CONCLUSION
In this article was evaluated the sea level behavior
(HHW, MSL and LLW) in São Marcos Bay from 1980
to 2017. Data covering two lunar nodal tide cycles,
and unpublished till now, were employed. Even so, it
cannot be stated that there is a statistically significant
SLR trend. This is an unexpected result, given what is
being observed in other port locations in Brazil and
around the world. It is suggested that this could be
due to a shift of the amphidromic point of harmonic
constituent M2 (lunar semidiurnal) tide wave since
there is one in the Caribbean Sea.
Otherwise, an important conclusion is that mean
tidal range decreased 1.116 cm·yr
-1
, corresponding in a
linear trend reduction of 1.12 m from 1980 to 2080,
which is equivalent to a reduction in the maximum
tidal currents of 11.55% and the shear stress in the
bottom of 21.77%. A lot of consequences from this
accelerated reduction in the mean tidal range can be
expected, e.g., changes in coastal morphology and
sedimentation problems, causing siltation of port
nautical depths in access channels, basins, and berths,
increasing the necessity of maintenance dredging.
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