Climate Change Effects on navigation channel of Parana River - Guerrero JRBM 2013

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Effect of climate change on navigation channel
dredging of the Parana River
a
b
c
c
Massimo Guerrero , Mariano Re , LeanDro David Kazimierski , Ángel Nicolás Menéndez &
d
Rita Ugarelli
a
Hydraulic Laboratory, University of Bologna, Bologna, Italy
b
Hydraulics Laboratory, National Institute for Water, Ezeiza, Buenos Aires, Argentina
c
Hydraulics Laboratory, National Institute for Water, Ezeiza, Buenos Aires, Argentina
d
SINTEF, Oslo, Norway
Accepted author version posted online: 04 Jul 2013.Published online: 02 Sep 2013.
To cite this article: Massimo Guerrero, Mariano Re, LeanDro David Kazimierski, Ángel Nicolás Menéndez & Rita Ugarelli
(2013) Effect of climate change on navigation channel dredging of the Parana River, International Journal of River Basin
Management, 11:4, 439-448, DOI: 10.1080/15715124.2013.819005
To link to this article: http://dx.doi.org/10.1080/15715124.2013.819005
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Intl. J. River Basin Management Vol. 11, No. 4 (December 2013), pp. 439–448
# 2013 International Association for Hydro-Environment Engineering and Research
Research paper
Effect of climate change on navigation channel dredging of the Parana River
MASSIMO GUERRERO, Hydraulic Laboratory, University of Bologna, Bologna, Italy
Downloaded by [University of Newcastle, Australia] at 07:59 28 December 2014
MARIANO RE, Hydraulics Laboratory, National Institute for Water, Ezeiza, Buenos Aires, Argentina.
Email: mre@fi.uba.ar (Author for correspondence)
LEANDRO DAVID KAZIMIERSKI, Hydraulics Laboratory, National Institute for Water, Ezeiza, Buenos Aires,
Argentina
ÁNGEL NICOLÁS MENÉNDEZ, Hydraulics Laboratory, National Institute for Water, Ezeiza, Buenos Aires,
Argentina
RITA UGARELLI, SINTEF, Oslo, Norway
ABSTRACT
This paper presents an analysis of the effect of climate change on modifying the dredging cost to maintain the navigation channel at the actual capacity
of the Parana waterway (Argentina). The Parana2Paraguay Rivers system is one of the most important inner navigation waterways in the world, where
approximately 100 million tons of cargo are transported per year. Maintenance of the navigation channel requires continuous dredging by Hidrovı́a SA
(limited liability company), which is responsible for ensuring the minimum water depth for navigation. A failure event occurred during January 2012
when a bulk cargo carrier ran aground, interrupting fluvial trading for 10 days. Numerical models were applied to simulate hydro-sedimentation processes at the Lower Parana River to estimate dredging costs for a given flow discharge. The resulting function relates the sedimentation rate (i.e. the
dredging effort required to keep the present depth for vessel draft) to forcing hydrology conditions. This function and the statistical evaluation of climate
scenarios were used to calculate the probability of failure for navigation and the associated cost of channel maintenance. The most appropriate dredging
effort was estimated by detecting the minimum total cost (i.e. dredging plus failure) to varying the yearly average discharge and by analysing the sensitivity of the total cost to different degrees of economic impact.
Keywords: Parana River; climate change impact; navigation channel maintenance
1
Iguazu, approximately 1500 and 2000 km northwest and
northeast from Buenos Aires, respectively, at the border of
Argentina with Paraguay and Brazil (Figure 1(a) and 1(b)).
Fluvial trading in the downstream part of this system, from
Buenos Aires to Santa Fe, has increased continuously over
the last 10 years, passing from approximately 4100 to 5100
vessels of mostly bulk cargo, tanks and containers (Gómez
et al. 2012).
The Parana River is navigable for most of its route in Argentina, but low water-depth sections (‘pasos’ in Spanish) drastically
reduce the admitted vessel draft. Sediment deposition in the navigation channel therefore requires regular and systematic dredging to maintain the channel capacity. For the main portion of
the Parana waterway in Argentina, i.e. the Oceanic and Fluvial2
Maritime channels from Rio de la Plata at the ocean inlet to Santa
Introduction
Freight transportation in the world is performed primarily by
means of seas and large rivers because navigation is the
most inexpensive medium per unit weight. The Parana2
Paraguay navigation route is a relevant resource for the Mercosur and, in particular, for Argentina. In Argentina, 84% of
import2export goods are transported by navigation versus a
world average of 90%, and 50% of that trade is transported
by Parana navigation (Gardel 2008). This route is approximately 2500 km long, including long sections of the Parana
and Paraguay Rivers and three navigation channels of different
capacities: Oceanic for 10 m draft vessels, Fluvial2Maritime
for 7 m draft vessels and Barcacera for 3 m draft vessels. The
Parana2Paraguay waterway links the ocean to Asunción and
Received 18 February 2013. Accepted 20 June 2013.
ISSN 1571-5124 print/ISSN 1814-2060 online
http://dx.doi.org/10.1080/15715124.2013.819005
http://www.tandfonline.com
439
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Massimo Guerrero et al.
Figure 1 (a) La Plata Basin; (b) the Parana-Paraguay water way; and (c) the study section at Rosario with Paso Borghi.
Fe (Figure 1(b)), the sediment volume displaced for the year is
approximately 25 × 106 m3 (Garcı́a 2008).
Determining future efforts that must be expended in dredging
will provide important knowledge for the National Ports and
Waterways Authority and for the concession company Hidrovı́a
SA (limited liability company). Hidrovı́a SA has been in charge
of the maintenance of the aid system for navigation and for dredging from Santa Fe to the ocean since 1995. The cost effectiveness of freight transportation by means of the Parana waterway
depends mainly on the expected water depth and consequent
admitted vessel draft.
Climate change may affect navigation in rivers (Hawkes et al.
2010). Changes in rainfall over a watershed alter the occurrence
of extreme hydrological conditions and indirectly modify navigability. Climate may also force changes in river morphology by
modified erosion and sedimentation that, in turn, affect vessel
manoeuvres and the operational efficiency of navigation
structures.
Sung et al. (2006), De Wit et al. (2007) and Millerd (2011)
focused on water-level modification due to variations in hydrology that would eventually impact navigation. Bed-level change
effects on navigation channels were considered to a lesser
extent by Verhaar et al. (2010).
This paper presents a method to predict the cost to maintain
current admittance for vessel draft and the corresponding probability that the predicted dredging effort will be exceeded
given the probability distribution of the yearly average discharge.
In this context, navigation channel failure is defined as the
impossibility of maintaining actual admittance for oceanic
vessels (10 m draft) by means of dredging.
The method was applied to the Lower Parana River near
Rosario (Argentina). The low water-depth section, i.e. the Paso
Borghi, of that area may present considerable limitations to
fluvial trading by means of oceanic vessels because the largest
ports for agriculture and steel industry production are located
near Rosario. This part of the Lower Parana is representative
of the Fluvial2Maritime waterway from Santa Fe to the
Parana River bifurcation (Parana de las Palmas and Parana
Guazu) at the beginning of the lower delta, which is a navigation
channel 350 km in length.
The navigation channel morphodynamic dependence on
yearly average discharge at the Lower Parana River was simulated in a quasi-steady-state approach by our own developed
numerical models (HIDROBID II and AGRADA), as introduced
in Section 2 of this document. These models were developed and
already well tested on the Parana navigation channel (Menéndez
1990, 1994, Re et al. 2012). The simulation of the depth-averaged
flow field (i.e. 2D-H simulations), performed by HIDROBID II,
provided boundary conditions for the following 2D-V-morphodynamics runs. The AGRADA modelled the bidimensional
morphodynamics of the dredged trench within the vertical
plane aligned to the streamflow, i.e. a 2D-V scheme, for a given
yearly average discharge, with the bottom level being preliminarily dredged to current admittance, i.e. 10 m draft vessel.
Section 3 reports on the assessed functions that relate the
simulated sedimentation rate to the yearly average discharge.
The sedimentation rate is assumed to correspond to the dredging
effort to maintain the navigation channel. The statistical distributions of the twentieth century and of future discharge are
also presented in Section 3.
Given these functions and under the framework of ‘Risk
Assessment’ (ISO 2009, Ugarelli and Røstum 2012), the most
appropriate management approach, in terms of sustainable dredging efforts, is discussed in Section 4 with respect to the
Effect of climate change on navigation channel dredging
hydrology of the past century and to predicted future scenarios,
assuming the current unit cost of dredging and reliable costs of
failure per day.
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2
The morphodynamic modelling
Two of our own developed numerical models were applied with
the objective of simulating the average rate of sedimentation at
the navigation channel, which was assumed to correspond to
the dredging rate. To this end, the channel morphodynamics
was simulated with a quasi-steady-state approach, given the
yearly average discharge as the hydrological steady-state
condition.
The method is based on the reasonable assumption that, for
short-term modifications, the navigation channel is not
affected by changes in river morphology. Inter-decade morphodynamics was observed (Castro et al. 2007) and modelled
(Nones et al. 2012, 2013, Guerrero et al. 2013) at the Lower
Parana. Three-month bathymetry of the navigation channel is
regularly used to define the dredging plan for the following
period. This difference in time scales also corresponds to
different space scales, with the navigation channel width
being approximately 100 m, whereas the river cross-section
is several kilometres wide.
The overall river morphology was considered a fixed boundary, and a 25 km section, including the Paso Borghi and an
additional upstream stretch (Figure 1(c)), was hydraulically
simulated to provide boundary conditions for subsequent modelling of the navigation channel morphodynamics. These boundary
conditions include water levels, flow velocities and planimetric
misalignments between the navigation channel axis and the
streamflow. The selected method also implies geometrical oversimplifications of the simulated domain. For large river sections,
such as the 25 km Rosario reach, the depth-averaged scheme (i.e.
2D-H model) reliably represented the overall hydraulics, while
the development of the dredged trench was simulated within
the vertical plain (i.e. 2D-V model) aligned with the main streamflow (Figure 2).
441
The applied method consists of two steps: (1) a simulation of
boundary conditions on the regional scale (i.e. the 25 km section)
was obtained by means of the 2D-H HIDROBID II model
(Menéndez 1990) and (2) a 2D-V model AGRADA (Menéndez
1994, Re et al. 2012) was applied to simulate manmade trench
development at a given yearly average discharge, with the
bottom level being preliminarily dredged to current admittance,
i.e. 10 m draft vessel.
2.1
Method implementation and validation
Navigation channel maintenance across the Paso Borghi (Figure
1(c)) requires significant dredging activity. This stretch was
therefore selected as a relevant case study to characterize navigation channel dredging between Santa Fe and the Parana River
bifurcation, at the beginning of the lower delta, as well as the
consequent cost of maintenance. Given the actual depth of dredging, the Paso Borghi extends over a 431.3 – 438.5 km stretch of
the Parana navigation route, with the longitudinal coordinates
upstream rising, as reported in Figure 1(b).
The National Ports and Waterways Authority provided access
to the database of dredged volumes distributed along the navigation channel during the year April 2008 – April 2009. This data
set shows that the operations were concentrated in two subsections: the upper Paso Borghi (from 436.8 to 438.5 km) and the
lower Paso Borghi (from 431.3 to 433.9 km). Validation of the
method used for the upper Paso Borghi is reported in this paper.
To verify the numerical modelling, representative dredging
rates were assessed from dredged volumes and then compared
to simulated sedimentation rates. The dredging rate per year
was determined for 100 m segments of the dredged channel.
The upper Paso Borghi was finally grouped into three subsections that follow the operational dredging rate along the
channel, as reported in Figure 3. The time between dredging
operations was calculated for each segment, yielding a typical
period of approximately 60 days between dredging episodes.
The first step of the method gave the depth-averaged flow
field at the regional scale (i.e. the 25 km Rosario reach of
Figure 1(c)), which includes the Paso Borghi. The 2D-H (i.e.
Figure 2 The navigation channel stream flow misalignment within the river stretch (a and b) and the 2D-V AGRADA model scheme (b and c).
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442
Massimo Guerrero et al.
Figure 3 Schematic interpretation of dredging rates for the upper Paso
Borghi; the record of dredged volumes (dredging record) along the navigation channel is compared to the operated simplification (model
scheme) into simulated subsections (1– 3) corresponding to three
stream tubes.
vertically averaged) model was applied and calibrated, relying on
the bathymetric survey carried out in 2009 over the Rosario reach
(Guerrero and Lamberti 2013). The simulation domain covers
the active channel of the river from 455 km of the navigation
route to the Rosario2Victoria Bridge (430 km).
The measured flow discharge (of approximately 13,600 m3/s
on June 29 – July 3, 2009) and water level were imposed at
upstream and downstream boundaries, respectively, and a calibration was performed on the basis of the measured flow-velocity
profiles (by means of Doppler profilers) concurrent to the bathymetry survey (Guerrero et al. 2011, Guerrero and Lamberti
2013).
The second step was to apply the 2D-V hydromorphodynamic
model simulating sedimentation into the dredged trench. This
model calculates the sedimentation rate for the three subsections
of the Paso Borghi, accounting for suspended and bed load and
consequent morphological evolution. The hydrodynamic parameters and variables (i.e. specific flow discharge, water depth,
angle between the channel axis and the streamflow) were available from the first step.
The validation period (12 April 2008– 12 April 2009) for
sedimentation rate assessment was characterized by 14,700 m3/
s as average discharge. Stream tubes conveying 250 m3/s each
were defined on the basis of the 2D-H simulations. One stream
tube for each subsection of Figure 3 was selected, providing
the hydraulic parameters for the 2D-V simulations.
The available data from periodic bathymetric surveys of the
dredged channel during the validation period were applied to
represent the navigation channel shape in the 2D-V scheme,
which was finally modelled with a trapezoidal cross-section
with a 116 m wide bottom and 1-over-5 sloped sides, also in
agreement with the dredging project (Figure 2(b) and 2(c)).
Regarding the sedimentological parameters, different references were considered for implementation of the 2D-V morphodynamic model. The mean grain size and the porosity of
the river bed sediment were assumed to be equal to 260 mm
and 0.4, respectively, on the basis of the available data along
the navigation route (Menéndez 2002). The settling velocity
was 0.037 m/s according to the van Rijn formulation (1987).
Modelled profiles of suspended sediment concentration near
the dredged trench were consistent with the available profiles
measured in the Rosario area of the Lower Parana (Royal
Boskalis and Ballast Ham Dredging 2012). The resulting
mean concentrations are in agreement with the evidence from
the suspended sediment survey using backscatter profiling
(Guerrero et al. 2011). The suspended over bed-load ratio
was equal to approximately 7, consistent with field data
reported in Szupiany et al. (2010).
The morphodynamic model simulates sediment entrapment
by the dredged trench from the streamflow. This process gives
rise to trench migration and diffusion, ultimately resulting in a
flat river bed. The upstream bank advances because of sediment
settling, the downstream bank retreats because of erosion and the
gravitational force diffuses sediment particles sliding over the
banks.
Simulations that were performed spanned the time between
dredging operations, yielding the sedimentation rate per unit
length for each subsection to be compared to the dredging
rates, as recast in Figure 3. The sedimentation rate was assessed
by the difference between initial (i.e. just dredged) and final profiles of the simulated trench over the simulated time period (i.e.
one year). Table 1 reports the resultant rates of sedimentation for
the three subsections of the Paso Borghi and the corresponding
dredging rates. Given the applied oversimplifications and our
aim to support dredging management in light of climate
change, the agreement between these data appears to be sufficiently accurate. The applied method reliably simulated the
order of magnitude of the dredging rates that were applied to
reliably predict the future efforts required for dredging.
Table 1 Simulated rates of sedimentation and corresponding dredging rates within the validation period (12 April 2008–12 April 2009) at the upper
Paso Borghi.
No.
Subsections (km)
1
436.75–437.55
2
437.55–438.05
3
438.05–438.55
Average for the upper
P. Borghi
Specific discharge (m3/m/s)
Dredged volume (m3/m/day)
Simulated rate of sedimentation (m3/m/day)
12.4
11.1
11.9
–
0.14
0.30
0.41
0.28
0.26
0.64
0.41
0.44
Effect of climate change on navigation channel dredging
3
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3.1
Prediction of the need for dredging
Climate scenarios
Under the CLARIS-LPB research project (A Europe-South
America Network for Climate Change Assessment and Impact
Studies in La Plata Basin), the La Plata Basin surface hydrology
was represented using the variable infiltration capacity (VIC)
macro-scale semi-distributed model (Liang et al. 1994, 1996,
Nijssen et al. 1997). The VIC model simulates river runoff
from precipitation on a monthly scale. The details of the model
and its validation can be found in Su and Lettenmaier (2009),
Saurral (2010) and Montroull et al. (2012). The same project
also provided the resulting monthly discharges from the hydrological VIC model when forced with past climate and four regional
climate models (RCMs): PROMES (UCLM, Spain), covering a
continuous run from 1991 to 4CA (SMHI, Sweden), for 1981 –
2100; RegCM3 (USP, Brazil), with information for the periods
1981 – 2048 and 2071 – 2090; and LMDZ (IPSL, France), with
information for the periods 1991 – 2048 and 2071 – 2100. The
RCMs were forced with different global climate models. The
reliability of the scenarios that were produced was determined
on the basis of the ability of the selected RCMs to reproduce
the present climate features. More information on this dataset
can be obtained in Saurral et al. (2013).
The cumulative distributions of yearly averaged values of
monthly discharges from the VIC are reported in Figure 4 for
the four RCM data sets and for the past century. The same
figure also reports the histogram for the period 1995 – 2012,
which corresponds to the period of dredging activity by Hidrovı́a
SA, and the histogram of the entire future data set, which represents a reliable means for predicting future scenarios.
Since 1995, the most likely values of yearly average discharge
have been larger than 15,000 m3/s. The mean discharge of the
period 1995 – 2012 was approximately 17,000 m3/s, but most
Figure 4 Cumulative distributions of yearly average discharge corresponding to the past century, the 1995–2012 period, future scenarios
from four RCMs (PROMES, RCA, LMDZ and RegDM3) and the
mean prediction (scenarios mean).
443
of the values were near to the median value of 15,800 m3/s,
with only a few exceptions exceeding 18,000 m3/s. The
dredged volume from the ocean to Santa Fe was approximately
25 × 106 m3/year as an average over the same period (Garcı́a
2008), with 40% (10 × 106 m3/year) from Santa Fe to the
Parana River bifurcation, at the beginning of the lower delta,
and the rest (approximately 60%) from Parana de las Palmas
and Rio de la Plata dredging.
The values of 13,600 and 14,700 m3/s (corresponding to June
29– July 3, 2009 and the period April 2008 – April 2009, respectively) to calibrate the 2D-H and 2D-V models, respectively, are
consistent with the yearly average values resulting from VIC
equal to about 15,500 m3/s for 2008 and 2009. In fact, the
water level2discharge relation in Santa Fe gave discharge
reductions of 12% and 5% for the five days in 2009 and the
year from April 2008, respectively, with respect to the yearly
averages. These calibration values were derived from Acoustic
Doppler Current Profiler (ADCP) measurements in Rosario
(Guerrero and Lamberti 2013) and the 1D hydraulic models validated on the Parana River (Jaime and Ménendez 1997, Nones
et al. 2012, 2013).
By considering the entire twentieth century, the yearly average
discharge presents a larger variability, yielding 35% of the
observed values lower than 15,000 m3/s. These values were
grouped mostly in the mid-century (Guerrero et al. 2013). As
seen in Figure 4, a clear increase can be argued for the probability
of exceeding 15,000 m3/s passing from 50% of the twentieth
century to 90%, on average, over the four RCMs considered.
This occurrence, on average, will likely reduce the costs of navigation channel maintenance because of the corresponding increase
in average water depths and admittance for vessel draft.
3.2
Sedimentation rate functions
The validated method was applied to predict the need for future
dredging. To this end, and given the climate change in most
reliable scenarios, a sedimentation rate function of the yearly
average discharge was extrapolated within the interval
11,000– 20,000 m3/s, which also corresponds to the most probable interval for discharge over the last century (Figure 4).
River morphology modification may affect the navigation
channel on a larger time scale (Guerrero et al. 2013) than the
scale of the dredging operation timetable. The river morphology
was therefore assumed to be a fixed boundary; the long-term
effects of climate change on the navigation channel are beyond
the scope of this work.
The dredging rate to keep the present admittance for vessel
draft, i.e. 10 m draft, was simulated at different discharges, representing cases where the managing authority will observe hydrological modifications (i.e. the change in the yearly average water
level) and adjust the dredging reference level accordingly to
maintain the admittance.
Table 2 reports the simulated yearly average discharge and
corresponding sedimentation rate for the lower (431.3 – 433.9
444
Massimo Guerrero et al.
Table 2 Simulated average rates of sedimentation along the Paso Borghi.
Flow discharge
(m3/s)
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11,115
11,970
12,825
13,680
14,535
15,390
16,245
17,100
17,955
18,810
Lower Borghi
(m3/m/day)
Upper Borghi
(m3/m/day)
Weighted average
(m3/m/day)
0.76
0.93
1.05
1.27
1.26
1.25
1.26
1.24
1.17
0.99
0.44
0.52
0.57
0.63
0.60
0.56
0.57
0.54
0.52
0.51
0.61
0.74
0.82
0.97
0.95
0.92
0.93
0.91
0.86
0.76
km) and upper (436.8 –438.5 km) Paso Borghi and as a weighted
average for the entire Paso. The simulated discharges were
chosen on the basis of assessed dredging rates with the objective
of accurately representing the discharge-dredging rate function.
The highest sedimentation rates were observed within the interval of 13,500 –17,000 m3/s. Two opposing effects influence the
actual rate, giving rise to the discharge interval that most affects
the navigation channel: (1) the trappingefficiency of the dredged
trench increases at low discharge because the low water depth
requires a deeper trench and (2) sediment mobility is proportional
to flow discharge. Low mobility of sediment prevails at low discharge, whereas low trapping efficiency is dominant at high
water depth and discharge. In both cases, the dredging rate will
be limited to the resulting low rate of sedimentation.
Figure 5 Interpolation functions of simulated sedimentation rate and
probability distributions of the yearly average discharge.
4
Results and discussion
The simulated average rates of sedimentation along the Paso
Borghi (Table 2) were fitted using the parabolic equation or a
smoothing spline (Figure 5). The expected sedimentation rate
and the consequent cost of dredging were estimated for a variable
yearly average discharge, q. The direct consequence of interrupting navigation because of water depth that is too low, i.e. the
direct cost of failure, was roughly estimated on the basis of a relevant event that occurred in January 2012, when the Aristeas-P
bulk cargo of more than 30,000 tons ran aground 25 km upstream
of Rosario and interrupted fluvial navigation for 10 days (Gómez
et al. 2012). Summing the dredging and failure costs for varying
discharges yielded the most appropriate management approach
in terms of minimum total cost and the corresponding yearly
allowance for dredging.
The parabolic and spline interpolations, r(q), in Figure 5 were
assumed to represent the average dredging rate to ensure 10 m
draft along the 350 km navigation channel from Santa Fe to
the Parana River bifurcation at the beginning of the lower
delta. Given the reference period 1995 –2012, the yearly
average discharge was 17,000 m3/s, corresponding to approximately 0.91 m3/m/day, regardless of the applied interpolation,
and to a record high of 10 × 106 m3/year for the dredged
volume from Santa Fe to the Parana River bifurcation. These
values were used to scale the rate functions into the total
volume to be dredged per year.
The river morphology and sedimentation features downstream of the Parana bifurcation deviate significantly from the
studied reach. Additional pasos should therefore be modelled
to further extend the method applicability by means of a weighted
function of the sedimentation rate along the entire waterway.
The volume to be dredged was calculated to be within the
interval 7.0– 10.5 × 106 m3/year, regardless of the assumed
interpolation, but the maximum value corresponded to
14,200 m3/s and 15,400 m3/s for the spline and parabolic interpolations, respectively. This change is reflected in Figure 5,
where the applied interpolation functions are reported.
The volume to be dredged was multiplied by the actual cost
per dredged unit volume, equal to 4.48 US$/m3, resulting in a
total dredging cost, CD(q), which varied in the range of 29– 47
× 106 US$/year, regardless of the assumed interpolation.
Figure 5 also shows the normal distributions (within the range
11,000 – 20,000 m3/s) of the probability of yearly average
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Effect of climate change on navigation channel dredging
discharges, p(q), for the twentieth century and for the four RCMs
representing the future. The maximum sedimentation rate (and
volume to be dredged) almost corresponds to the mean discharge
of the twentieth century. Future discharges will likely increase
drastically, yielding a lower probability for the maximum
volume to be dredged. The discharge that has the greatest effect
on the river channel morphology, i.e. the effective discharge, corresponds to 15,000 m3/s on average over the twentieth century
(Guerrero et al. 2013). Based on this evidence, an interrelationship between the river and the navigation channel morphologies
can be hypothesized. At present, almost the same discharge
values appear to affect both morphodynamics processes the
most, notwithstanding an order of magnitude between the river
and the navigation channel space scales. This occurrence may
reflect a substantial equilibrium between hydrology conditions
and river channel morphology that mobilizes most of the bed sediment for discharge values close to the yearly average.
Climate change will drive different hydrology conditions,
with the yearly average discharge varying from 16,800 to
20,900 m3/s. The river morphology modifications will require
many decades to achieve a new equilibrium, and the navigation
channel morphodynamics will respond quickly. The river morphology was considered to be a fixed boundary, and the sedimentation rate functions did not depend on the varying hydrology. As
a result of long-term modifications, the sedimentation rate functions in Figure 5 may migrate slowly towards the future discharge distribution. Over the long term, the maximum rate may
be realigned to the modified mean of the yearly average
discharge.
The cost of failure per year, CF(q), was assumed to scale with
the probability of the expected sedimentation rate being lower
than the actual sedimentation rate. Past and future probabilities
of sedimentation rates that are lower than the actual sedimentation rate depend on the assessed normal distributions, p(q),
445
of the yearly average discharge and on the applied interpolation
functions, r(q), to simulated sedimentation rate values. Once the
expected yearly average discharge, q∗ , was fixed, the corresponding CF was estimated by multiplying the daily cost of
failure, c, for the total days characterized with a sedimentation
rate greater than the expected rate r(q∗ ).
CF|q=q∗ = 365 · c ·
r.r(q∗ )
p(q) · dq.
(1)
The total cost, CT(q), was finally estimated by summing the
total dredging cost and the cost of failure per year.
CT(q) = CD + CF.
(2)
Figure 6(a) and 6(b) reports the resulting total cost over the
corresponding yearly average discharge and dredging volume,
respectively, for the two applied interpolation functions, the
past and future normal distributions and the assumed cost of
failure per day.
The cost of failure per day was roughly estimated from the
Aristeas-P accident. On 18 January 2012 the Aristeas-P
oceanic cargo vessel ran aground at 390.4 km near Rosario
while sailing to the UK from San Lorenzo (445 km). This occurrence completely interrupted the waterway for 10 days, affecting
54 vessels that were forced to stand by fluvial docks, delaying
navigation plans and the delivery of goods. The Aristeas-P
blockade was particularly noticeable and rare because most
such events result in autonomous vessel manoeuvres that allow
for continued sailing and do not give rise to economic consequences. Gómez et al. (2012) investigated the direct costs of
the obstruction of navigation in the case of the 2012 major
Figure 6 Total cost function of yearly average discharge (a) and of dredging volume (b) for spline (S.) and parabolic (P.) fittings.
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446
Massimo Guerrero et al.
event that were incurred due to the increased docking and chartering times of oceanic Panamax vessels at the Parana waterway.
The docking fee in the Rosario port is equal to 0.14 US$/ton/day,
whereas the average chartering fee for Panamax vessels has fluctuated, varying from 30,000 to 10,000 US$/day over the last two
years, depending on specific agreements.
Assuming consequences for failure similar to the Aristeas-P,
the economic direct loss per day was roughly estimated by multiplying the total load capacity of 54 blocked vessels by the
Rosario docking fee, resulting in approximately 230,000 US$/
day, given that the average Panamax vessel load capacity is
equal to 30,000 tons. The increase in chartering time was
accounted for by 10,000 US$/day per individual vessel. The
direct economic loss was approximately 770,000 US$/day.
Given the variability of the chartering fee for Panamax vessels
and the uncertainty of failure consequences, a sensitivity analysis
was conducted on the resulting total cost by applying a unit cost
of failure varying from 0.25 × 106 to 2 × 106 US$/day. Figure
6(a) and 6(b) reports the sensitivity analysis results in terms of
total cost functions for expected yearly average discharge and
the corresponding volume to be dredged, respectively.
Regardless of the resulting total cost function, the most appropriate management approach is for the expected discharge corresponding to the maximum expected sedimentation rate and
volume to be dredged, i.e. r(q∗ ) ¼ max(r) in Eq. 1. In this
case, the total cost is minimum and equal to the maximum cost
of dredging (47 × 106 US$/year), with the integral in Eq. 1
being equal to zero, corresponding to zero probability of
failure. The corresponding discharge varies slightly relative to
the assumed function of sedimentation rate, finally resulting in
symmetric and deviated functions of the total cost in Figure
6(a) for parabolic and spline interpolations, respectively.
The failure cost per day and the probability distribution of the
yearly average discharge strongly affected the resulting cost
function, with the cost of dredging being of a lower order of magnitude compared to the cost of failure per year. Future climate
changes, on average, will halve the total cost, given the same
failure cost per day (Figure 6(b)).
The minimum total cost is well defined for increasing values
of the unit cost of failure, while it is smoothed in the case of lower
consequences of failure per day, especially when the spline
interpolation is applied (Figure 6(a)), reflecting a better balance
between dredging and failure costs. Passing from past to future
hydrology scenario introduces more sensitivity of the total cost
to the operational investment for dredging of the navigation
channel. For future hydrology and given the same daily cost of
failure, the plotted functions in Figure 6(b) approach the
minimum more slowly.
With different sensitivities, a reduced investment in navigation channel maintenance may produce relevant economic
losses rising to an order of magnitude higher than the predicted
maximum cost per year for dredging.
The analysis that was performed demonstrates that the failure
cost is dominant in fixing the total cost, and the most appropriate
strategy for navigation channel maintenance corresponds to the
maximum predicted effort for dredging (i.e. expected
maximum rate of sedimentation). This finding suggests that,
from the perspective of economic investment, the Parana waterway capacity may be increased. The maintenance of the actual
admittance (10 m draft) gave rise to a relatively low dredging
cost when compared to the resultant economical shortcomings
due to no admittance (i.e. failure costs). An increased waterway
capacity may also give rise to more serious consequences for
failure events because of the interruption of improved service.
The proposed method could therefore also be applied to investigate the actual influence of the waterway capacity enlargement
by comparing the additional cost of dredging to the increasing
consequences of failure.
5 Conclusions
This paper presented an analysis that was performed to investigate the effect of climate change on the dredging cost required
to maintain the navigation channel at actual capacity in the
Parana waterway.
The proposed method applied two of our own developed
numerical models to simulate the navigation channel morphodynamics and the consequent need of dredging per year, given the
expected yearly average discharge. The HIDROBID II model
assessed the depth-averaged velocity field for a case study of
the 25 km Rosario reach of the Lower Parana. The results from
this model provided the boundary conditions for the
AGRADA model, which simulated the dredged trench morphodynamics within the streamflow vertical section crossing the
low-depth area of the Paso Borghi.
To maintain the present admittance for vessel draft from Santa
Fe to the Parana River bifurcation at the beginning of the lower
delta, the variable cost was found to be in the range of 29 – 47 ×
106 US$/year for the yearly average discharge, with the
maximum roughly corresponding to the river effective discharge,
which was found to be equal to 15,000 m3/s.
Past-century and future distributions of the yearly average discharge were assessed under the CLARIS-LPB research project
using the semi-distributed hydrological model, VIC, which
was forced with past climate and four RCMs. These distributions
provided the failure probability of the waterway for a varying
yearly average discharge to assess the failure cost per year.
The most appropriate investment for dredging was estimated
by detecting the minimum total cost (i.e. dredging plus failure)
for a varying discharge.
Given the variability of the chartering fee for Panamax vessels
and the uncertainty of failure consequences, a sensitivity analysis
was also conducted on the resulting total cost by applying a unit
cost of failure that varied from 0.25 × 106 to 2 × 106 US$/day.
The unit cost was approximately 0.8 × 106 US$/day, based on
the documented economic losses, for the oceanic cargo accident
that occurred in 2012.
Downloaded by [University of Newcastle, Australia] at 07:59 28 December 2014
Effect of climate change on navigation channel dredging
The unit cost of failure and the probability distribution of the
yearly average discharge strongly affect the resulting cost function, with the cost of dredging being of a lower order of magnitude compared to the cost of failure per year. Future climate
change, on average, will halve the total cost, given the same
failure cost per day.
With different sensitivities, a reduced investment in maintenance of the navigation channel may produce relevant economic
losses that rise to an order of magnitude higher than the predicted
maximum cost per year of dredging.
The analysis that was performed demonstrates that the failure
cost is dominant in fixing the total cost, and the most appropriate
strategy for navigation channel maintenance resulted in the
maximum predicted effort for dredging.
The proposed method can also be applied to investigate the
actual convenience of the enlargement of the waterway capacity
by comparing the additional cost of dredging to the increasing
consequences of failure.
Acknowledgements
This research has received funding from the European Community’s Seventh Framework Programme (FP7/2007 – 2013) under
Grant Agreement No. 212492 (CLARIS LPB. A Europe-South
America Network for Climate Change Assessment and Impact
Studies in La Plata Basin).
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