magdalena river, colombia

Transcription

E-proceedings of the 36th IAHR World Congress
28 June – 3 July, 2015, The Hague, the
Netherlands
SEDIMENT TRANSPORT AND GEOMORPHOLOGICAL ADJUSTMENT IN A HIGH DISCHARGE
TROPICAL DELTA (MAGDALENA RIVER, COLOMBIA): INSIGHTS OF A PERIOD OF INTENSE
CHANGE AND HUMAN INTERVENTION (1990-2010)
(1)
(2)
(2),
(3)
(1)
JUAN C. RESTREPO , KERSTIN SCHROTTKE , CAMILLE TRAINI ANDRÉS OREJARENA , JUAN C. ORTIZ , ALDEMAR
(1)
(1)
(3)
HIGGINS , LUIS OTERO ,& LEONARDO MARRIAGA
(1)
(2)
Grupo de Física Aplicada: Océano y Atmósfera, Departamento de Física, Universidad del Norte, Barranquilla, Colombia
[email protected]
Institute of Geosciences, JRG Sea-Level Rise and Coastal Erosion – Christian Albrechts University of Kiel, Kiel, Germany
[email protected]
(3)
Centro de Investigaciones Oceanográficas e Hidrográficas – Dirección General Marítima, Cartagena, Colombia
[email protected]
ABSTRACT
The Magdalena River in northwestern South America provides the largest supply of freshwater (205.1 km 3 yr-1) and the
greatest amount of suspended sediment (142.6 x106 t yr-1) to the Caribbean Sea. The river mouth has been intervened
since 1930 to allow shipping into the port of Barranquilla. In the last two decades, the watershed also has undergone
severe human interventions. Furthermore, in 2000, a shift in its hydrological patterns appeared. The response of the
Magdalena delta to these stressors remains unclear. Therefore, data of streamflow, suspended sediment and riverbed
dynamics from 1990 to 2000 were analysed in this study to estimate changes in the suspended sediment transport
regime and related processes as well as erosional/depositional patterns in critical zones of the delta. It can be shown,
that the streamflow increased at a higher rate than suspended sediment transport, promoting changes in the sediment
transport regime between the 1990s and the 2000s. These changes led to erosion of the mouth/frontal bar and outlet
zones and modified the erosional/accretionary balance in the prodelta, in the early 2000s. Erosional/sedimentary cycles
were controlled by the magnitude of fluvial discharges and river bed scouring in the river outlet, whilst effluent diffusion
and sediment dispersion were dominants in the delta front. High freshwater discharges, as buoyancy inputs, promoted
the transfer of sediments from the river channel to the outer prodelta through the upper layers of the water column. The
total sediment accumulation in the delta corresponded to <5% of the annual mean SSL of the Magdalena River. In
general, the morphology of the delta remained relatively stable; it experienced a slow progradational state. Higher
sedimentation rates (≤1430 mm yr-1) appeared in the deeper zones.
Keywords: sediment regime, deltaic processes, morphological changes, delta evolution, Magdalena River
1.
INTRODUCTION
Boundary conditions and forcing factors such as the sediment supply, freshwater discharges, coastal energy,
accommodation space and differences in density between fluvial, estuarine and marine waters control the dynamics and
architecture of deltaic zones (Syvitski and Saito, 2007). Changes in any of these factors will influence the
geomorphological, hydrodynamic and sedimentological balance of these systems (Wang, Hassan and Xiaoping, 2006).
Thus, changes in freshwater discharge and sediment loads can lead to significant changes in estuarine processes and
geo-morphological evolution of river deltas and adjacent coastal shelf environments (e.g., Fan and Huang, 2005; Gao et
al, 2011; Le et al., 2007; Wang et al., 2010; Wang et al, 2006; Yang et al., 2003). Consequently, there is an increasing
focus on exploring how deltaic systems evolve and respond to significant changes in river inputs, especially due to the
recent dominant influence of anthropogenic interventions over natural effects in controlling these changes (e.g., Lane,
2004; Milliman et al., 2008; Syvitski and Kettner, 2011; Syvitski and Milliman, 2007; Syvitski and Saito, 2007; Wang et
al., 2010).
Variations in catchment conditions have been associated with significant changes in freshwater discharge and sediment
transport, in turn leading to disruption of the sedimentary processes in the delta (e.g., Chen et al., 2001; Chen et al.,
2005; Gao et al., 2011; Yang et al., 2002; Zhang et al., 2008). For example, the Aswan dam (Egypt) traps more than
90% of the sediment load that was formerly delivered to the Mediterranean Sea by the Nile River, causing increasing
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erosion rates (>10 m yr ) at two active tributaries (Fanos, 1995; Stanley and Warney, 1998). There are other
documented examples of the adjustments and responses of deltas to changes in river inputs. They include variations in
sediment dispersal patterns, shifts in turbidity maximum zones, changes in the sediment retention index in subaqueous
deltas, formation and migration of shoals and bars, and modifications of subsidence/sinking processes (e.g., Fan and
Huang, 2005; Gao et al., 2011; Lane, 2004; Li et al., 1998; Maillet et al., 2006; Wang et al., 2010; Xu, 2002; Yang et al.,
2003; Zhang et al., 2008). However, the response mechanisms of sedimentary processes in deltas to significant
changes in the corresponding catchment areas are not fully understood (e.g., Gao et al., 2011; Syvitski and Saito, 2007;
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E-proceedings of the 36th IAHR World Congress,
28 June – 3 July, 2015, The Hague, the Netherlands
Wang et al., 2010). This is especially the case in rivers characterised by various severe anthropogenic interventions and,
thus, by significant environmental changes (Syvitski and Saito, 2007).
Figure 1. (a) Location of the Magdalena River in northwestern South America; (b) detailed view of the river mouth site showing the main
engineering structures (1-7) and the mooring sites for the field survey.
The Magdalena River, in northwestern South America (Figure 1), provides an example of a river basin-delta-coastal zone
interaction in the context of environmental change and recent human intervention. With a drainage basin of 2.57 x10 5
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km , the Magdalena River delivers 26% of the total fresh water discharge (205 km yr ) and 86% of the total sediment
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load (144 x10 t yr ) to the Caribbean Sea (Restrepo et al., 2014; Restrepo and Kjerfve, 2000). Thus, it is considered to
be one of the world’s rivers showing the highest sediment and freshwater discharges to the ocean (e.g., Milliman, et al.,
1995; Milliman and Meade, 1983; Restrepo and Kjerfve, 2000). There is currently ample evidence of changes taking
place in the Magdalena River basin. Restrepo et al. (2014) detected a shift in the hydrological patterns of the Magdalena
River from 1998-2002. This shift was characterised by higher hydrologic variability, a steady increase in the mean
streamflow, and strengthening of a quasi-decadal oscillatory signal, leading to major floods in 1999 and 2010-2011. The
Magdalena River basin also experiences dramatic land surface disturbances, such as forest clearing, arable land
expansion, increased reservoir construction, and mining exploitation. Thus, the current fluvial sediment delivery from the
main tributaries significantly deviates from long-term natural rates (Restrepo and Syvitski, 2006). It is doubtful that
earlier estimates of the sediment discharge rates of the Magdalena River (i.e., Restrepo and Kjerfve, 2000) based on
data from 1975 to 1995 include recent natural and human-induced changes. Thus, it is unclear how sediment transport
processes and the subaqueous delta architecture respond to changes in fluvial inputs. Changes in sediment regime
patterns must be evaluated to obtain a better understanding of their role in sediment transport and morphological
adjustment processes at river outlets (Gao et al., 2011; Syvitski and Saito, 2007; Yang et al., 2003).
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In this study, we put forth the hypothesis that the combined effects of changes in anthropogenic interventions and the
shift in the hydrological patterns of the Magdalena River basin are likely to alter the freshwater discharge and sediment
transport regimes, thus promoting changes in sediment accretion/erosion rates and growth patterns in the mouth and
subaqueous delta. The aims of this work were (i) to quantify suspended sediment transport rates and detect changes in
the sediment transport regime occurring within the last two decades; (ii) to analyse the recent morphological evolution;
and (iii) to evaluate the relative importance of the variability of freshwater discharge to the short-to-mid-term
morphological evolution of the Magdalena River delta. A comprehensive understanding of the direct link between
changes in the drainage basins as well as the responses of estuarine and coastal systems could strengthen coastal zone
management strategies and planning to address the effects of climate change.
2.
PHYSICAL SETTING OF THE MAGDALENA RIVER DELTA
The Magdalena River forms a 1690 km 2 delta (Figure 1) located in the active margin formed by the collision of the South
American, Caribbean, and Nazca plates (Duque-Caro, 1980). The Magdalena’s continental shelf is narrow due to deltaic
progradation. It exhibits a ~0.1° slope gradient and displays a width of between 2 and 26 km (Estrada et al, 2005). The
main delta mouth has gone through several southwest – northeast migrations (from Bahía de Cartagena to Ciénaga
Grande de Santa Marta) associated with major adjustments of the tectonic blocks of the San Jacinto and Sinú fold belts
(Duque-Caro, 1980). The Magdalena River delta currently presents a main discharge channel (the so-called Bocas de
Ceniza), another major distributary discharging into Cartagena Bay (Canal del Dique), and a complex network of minor
connections with the Ciénaga Grande de Santa Marta (Figure 1). The delta exhibits a microtidal regime, which is mixed
but primarily diurnal, ranging between 0.64 m and 0.48 m during spring and neap tides, respectively. During most of the
year (December - July), the delta wave system is dominated (96%) by the presence of swells from the northeast, which
display a significant wave height (Hs) of 2.2±1.1 m and a peak period (Tp) of 6.7±2.3 s. The delta experiences high
energy wave conditions between January and March caused by cold fronts and from June to November associated with
storm surges and hurricanes. Maximum Hs values of 4.0-5.0 m have been reported in deep waters during extreme wave
events (Ortiz, 2012; Ortiz et al., 2012, 2013).
Figure 2. Schematic representation of the Magdalena River delta between 1894 and 1961, according to historical maps and charts from (a)
1894 – 1911, (b) 1924, (c) 1936, and (d) 1961. Modified and adapted from Borda et al. (1973).
The main discharge channel has undergone major physical changes following a variety of engineering activities during
the last century (Figure 2). Before 1924, the river was a wave-dominated type delta with cuspate or lobate forms. The
number and position of coastal landforms changed continuously, forming different connections between the river
channel and the coastal lagoon systems (Figure 2a). To prevent siltation processes at the river mouth and to promote
commercial navigation to the port of Barranquilla, a series of engineering structures were planned along the main
channel. The main goals were to channelise the outlet, reduce the river mouth section, increase the current flow, and
strengthen the sediment transport capacity (Alvarado, 2008). Thus, from 1936 onwards, the delta has been dominated
by a single discharge channel, which is isolated from the coastal lagoon systems and forced into a straight line (northnorthwest orientated) by two jetties (Figure 2b). Despite these interventions, siltation processes occurred in 1942 and
1945. In addition, the engineering structures affected the erosion/accretion balance along the front of the delta, leading
to a significant retreat of the coastline and the expansion of unconsolidated areas along the western and eastern coasts,
respectively (Alvarado, 2008). From 1936–1961, the net estimated coastal retreat on the west coast was ~3.0 km. By
1961, the river mouth was just 512 m wide and 9.15 m deep (Figure 2c). Between 1994 and 1995, a training wall (1.2
km in length) was attached to the eastern margin to close a secondary channel and concentrate the flow along the main
channel. Four additional contraction groynes (0.07 km – 0.29 km in length) were built further north to reduce the river
section, increase current flow, and displace the deep channel toward the western margin (Alvarado, 2008) (Figure 1b).
Finally, between 2008 and 2009, two outer contraction groynes of 0.67 km and 0.23 km were attached to the northern
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section of the eastern jetty to further reduce the mouth section of the delta. Currently, the mouth exhibits a width of 430
m and a minimum depth of 9.15 m in the deep channel, and the western and eastern jetties extend for 7.4 km and 1.4
km, respectively (Figure 1b).
The shift of the Intertropical Convergence Zone (ITCZ) defines two wet seasons in the Magdalena drainage basin. The
first season extends from April to June, when the ITCZ migrates from south to north. The second, stronger, season lasts
from September to November, when the ITCZ shifts southward. The seasonal distribution shows high discharges of
freshwater (9237 of m 3 s-1) and suspended sediment (690 x103 t d-1) in November. The lowest mean values are
experienced in March, when there are freshwater and suspended sediment loads of 3685 m 3 s-1 and 146 x103 t d-1,
respectively (Restrepo et al., 2014; Restrepo and Kjerfve, 2000). At interannual scales, major anomalies in hydrological
patterns have been linked to both phases of the El Niño - Southern Oscillation (ENSO). The ENSO warm phase (El
Niño) promotes an increase in the mean air temperature, a decrease in soil moisture and the vegetation index, and a
decrease in rainfall rates. Anomalies during the ENSO cold phase (La Niña) generate abundant and intense rainfall
(Poveda et al., 2001). The mean annual discharges during El Niño and La Niña years are 5512 and 8747 m 3 s-1,
respectively. The mean daily sediment loads amount to 256 x103 t d-1 during El Niño years and 511 x103 t d-1 during La
Niña years (Restrepo and Kjerfve, 2000).
The Magdalena River drainage basin experienced deforestation rates of 234 x 103 ha yr-1 between 1970 and 1990,
implying the conversion of ~4.8 x 106 ha of forests into agricultural and pasture lands (Restrepo and Syvitski, 2006).
Between 1980 and 2000, mining in the drainage basin rose steadily up to 2% of the national GDP. For example, coal
exploitation (i.e., open pit mining) increased from 4 x 106 t yr-1 in 1980 to 20 x 106 t yr-1 in 2000. Furthermore, within the
drainage basin, there are 39 dam reservoirs with a total water storage capacity of 8.2 x 109 m 3 (IDEAM, 2001). Restrepo
and Syvitski (2006) indicated that these changes have modified the natural hydrology of the main tributaries of the
Magdalena River, leading to changes in its freshwater discharge and variability of sediment transport.
3.
METHODS
3.1 Analyses of data on streamflow and suspended sediment load
The data on the monthly (1941, 1972 to 2010) streamflow and suspended sediment load (SSL) measured at the
hydrologic station in Calamar (Figure 1) that were used in this study were provided by IDEAM (Instituto de Hidrología,
Meteorología y Estudios Ambientales de Colombia). Simultaneous measurements of water stage, streamflow, and
sediment concentration were done on several occasions by the IDEAM during high, intermediate, and low river discharge
conditions at this gauging station. Rating curves for water stage-streamflow and streamflow-suspended sediment
transport were obtained using these data. The daily stage readings for the whole record were converted to streamflow
via the established rating curve, and then into suspended sediment transport using the sediment rating curve. According
to the measuring techniques and procedures for estimating the respective rating curves, the inaccuracy of the
streamflow and SSL values is estimated as low (<7%) (IDEAM, 2013). The Calamar station is located close (~91 km) to
the mouth of the watershed. A valuable integrated signal between the gains and losses of the continental water cycle
(i.e., precipitation, evapotranspiration, runoff) can therefore be gained from these data (e.g., Labat, 2010; Milliman et al.,
2008).
The time series analysis performed in this study included plots of annually averaged data (e.g., Wang et al., 2010). The
Mann-Kendall Test (MKT) was conducted to detect and evaluate the significance of monotonic trends in selected periods
(e.g., Yue et al., 2002). The MKT is a non-parametric, rank-based statistical test, in which no trend in the time series is
considered as a null hypothesis. A standardised variable (Z) was calculated for verification. The null hypothesis was
rejected for a significance level α if Z > Z(1- α/2), where Z(1- α/2) is the standard value of a normal distribution with a
probability of α/2 (Yue et al., 2002). The MKT is considered one of the most robust techniques available to identify and
estimate linear trends in hydrological data (e.g., Milliman et al., 2008; Yue et al., 2002; Zhang et al., 2008). Continuous
wavelet transform analyses (Morlet Wavelet Spectrum) were applied to estimate the periodicities and patterns of
variability and to distinguish temporal oscillations in the time series, identifying the intermittency of each time-scale
process (e.g., Labat, 2010). Generalised local base functions (i.e., mother wavelets) were used for the continuous
wavelet transform (CWT). They were stretched and translated in terms of the frequency and time resolution (Torrence
and Compo, 1998). In addition, the global wavelet spectrum and the cross-correlation wavelet (XWT) between
streamflow and SSL were also calculated. The advantage of the global wavelet spectrum consists of its efficient
estimation of the characteristic scales of the long-term processes, allowing the determination of the variance signal
distribution between the different scales. The XWT was performed on the 1972-2010 interval to highlight the scale
dependant relationship between these signals, exposing regions with high common power and revealing information
about phase relationship (e.g. Labat, 2010; Torrence and Compo, 1998). The CWT was applied on monthly
deseasonalised time series. Defining T as the total length of the hydrological record, the cut-off frequency (T/2) and edge
effects (T/2√2) limited the statistical significance of the signal processes identified from the time series analysis. Thus, to
identify major significant fluctuations, the 95% confidence levels for contours and edge effects area were calculated
following the Torrence and Compo (1998) methodology.
3.2 Processing and Analysis of Bathymetric Data
The bathymetric geo-referred data from 1994 to 2012 used in this study were collected by the National Hydrographic
Service (Servicio Hidrográfico Nacional – Centro de Investigaciones Oceanográficas e Hidrográficas). The accuracy of
the provided bathymetric data was typically 0.1 m in the vertical direction and 1.0 m in the horizontal direction. As
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successive surveys do not cover the same area, three zone of interest were defined: (1) the outlet, (2) the mouth/frontal
bar, and (3) the delta front, to adjust the boundaries and ensure concordance within the surveyed zone. Each
bathymetric dataset was used to generate digital elevation models (DEMs) via triangulation interpolation (Triangular
Irregular Network -TIN). This method provides high accuracy relative to the density of the source data (Maillet et al.
2006). Each triangulated network was converted to a raster with a cell size, ranging from 2.5 m to 10.0 m (Table 1). This
procedure was carried out using geographic information software (ARC Gis Version 10®). Net erosion/accretion areas
and the corresponding volumetric gains/losses in the delta were determined by superposing successive soundings
surveyed during the same hydrological season (Table 1). By characterising the bulk properties of the sediment, the
volumetric loss/gain can be converted into gravimetric sediment masses. These estimates represent only the apparent
sedimentation because processes of consolidation/compaction and changing porosities within sediment deposits are not
considered. This approach has been successfully employed in various estuarine systems (e.g., Rowan et al., 1995;
Lane, 2004; Maillet et al., 2006).
Table 1. Location and dates (month-year) of the soundings whose data were used in this study and the cell sizes of the interpolated
bathymetric data (raster format).
Area
Cell Size (m)
Outlet
8.5
(1) 06-2000, (2) 05-2004, (3) 07-2011
Mouth/frontal bar
2.5
(1) 08-1994, (2) 06-2000, (3) 05-2004, (4) 07-2011
Delta front
4.
Soundings (mm-yyyy)
10.0
(1) 08-1994, (2) 06-2000, (3) 04-2008, (4) 04-2010, (5) 06-2012
RESULTS
4.1 Magnitude and variability of the streamflow and the suspended sediment load (SSL)
From 1941 to 2010, the mean annual streamflow of the Magdalena River was 6501 ± 1370 m 3 s-1, which is equivalent to
205 km 3 yr-1. During this period, the annual streamflow experienced an increasing trend, which was significant at the
95% confidence level. This trend was particularly pronounced within the last 20 years, during which the Sen’s slope
estimates rose to 297.7 m 3 s-1 yr-1 and 493.7 m 3 s-1 yr-1. Between the 1990s and the 2000s, the mean annual streamflow
increased 12.6%, from 6565 m 3 s-1 to 7391 m 3 s-1 (Table 2 and Figure 3a).
Table 2. Streamflow and suspended sediment load - annual mean and results of the Mann-Kendall and Sen’s slope tests for selected periods.
Streamflow
Annual
3
mean (x10
3 -1
m s )
Period
Suspended sediment load
Mann-Kendall Test
Test Z
p Value
Annual mean
6
-1
(x10 t yr )
Sen’s slope
3
-1
Mann-Kendall Test
Test Z
p Value
Sen’s slope
6
-1
(x10 t yr )
-1
(m s yr )
1941/1972 - 2010*
6.5 ± 1.4
2.02
p < 0.05
17.26
142.0 ± 48
0.99
n.s.
0.84
1972 – 1990
6.5 ± 0.9
0.42
n.s.
17.26
141.2 ± 48
1.89
p < 0.10
3.80
1990 – 2000
6.5 ± 1.7
1.40
n.s.
297.72
151.4 ± 46
0.62
n.s.
1.10
2000 - 2010
7.4 ± 1.7
2.18
p < 0.05
493.69
137.9 ± 54
2.02
p < 0.05
14.29
Note. *Streamflow from 1941 to 2010; *Suspended sediment load from 1972 to 2010; n.s.= no statistical significance.
The continuous wavelet spectrum of the Magdalena’s streamflow highlighted a 6-month component that was visible from
approximately 1974-1995 and 2000-2010. The annual signal appeared as a quasi-non-intermittent process of highest
magnitude from 1972 to 1990, 1994 to 2001, and 2007 to 2010 (Figure 4a). At an interannual scale, the streamflow of
the Magdalena River exhibited a 3-4 year process between 1979 and 1984 and a 4-7 year oscillation between the
intervals 1972-1981 and 1989-2003. The 4-7 year signal exhibited a maximum power between 1994 and 2002. A quasidecadal oscillation (8-12 year) appeared in 1990 and extended to 2010; this signal was particularly strong between 1998
and 2010. The wavelet spectrum also reflected a period of intense activity from 1998 to 2002, characterised by
superimposed oscillations of 0.5-1, 3-4, 4-7, and 9-12 years. A strong quasi-biennial oscillation of the Magdalena River
streamflow arose in 2009 (Figure 4a). This fluctuation superimposed with an 8-12 year process and coincided with a
period of severe floods (Figure 3). The annual band appeared as the main oscillatory component, while the 4-7-year
band emerged as a second-order source of variability (Figure 4c).
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Between 1972 and 2010, the Magdalena River transported 5.5 x10 t of SSL, averaging 142.0 ± 48.6 x10 t yr . During
this period, the annual mean showed a slight increasing trend, though it was not statistically significant (Table 2 and
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Figure 3a). The average annual SSL decreased from 154.4 x10 t yr in the 1990s to 137.9 x10 t yr in the 2000s,
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which represents a drop of 8.9%. This pattern reverted around 2006, when values >150 x 10 t yr led to an upward
trend during the 2000 – 2010 period, which was significant at the 95% confidence level (Table 2 and Figure 3a).
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The continuous wavelet spectrum of the Magdalena’s SSL highlighted a 6-month process, showing maximum power in
1974-1976, 1986-1992, 1993-1995, 1999, and 2007-2010 periods (Figure 4b). The annual signal exhibited its maximum
power from 1974 to 1976, 1986 to 1992, 1994 to 2000, and 2007 to 2010 (Figure 4b). At an interannual scale, the SSL
exhibited a quasi-biannual process between 1987-2000 and 2009-2010. In addition, a 4-5 year oscillation from 19972002 and a 5-7 year signal from 1983 to 2000 showing peak power from 1985 to 1996 was detected. A quasi-decadal
oscillation (8-12 year) appeared in 1985 and extended to 2010. The wavelet spectrum also highlighted periods of intense
activity around 1988-1990 and 1995-1998, characterised by superimposed oscillations of 0.5-1, 2-3, 5-7, and 8-12
years, as well as between 2009-2010, with superimposed oscillations of 1 year, 2-3 years, and 8-12 years (Figure 4b).
These periods coincided with a high SSL (Figure 3). The annual band of the SSL appeared to be the main oscillatory
component, whereas the 5-7-year band emerged as a second-order source of hydrological variability (Figure 4d).
Figure 3. Streamflow and suspended sediment load of the Magdalena River: annual mean (thin line with circles), long-term trend (thick gray
line) and short-term trend (thick black line) for selected periods.
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Figure 4. Magdalena River - Continuous wavelet transform (CWT) spectrum for (a) streamflow and (b) SSL. Global wavelet spectrum for (c)
streamflow and (d) SSL. (e) Cross-wavelet transform (XWT) of the streamflow and SSL. The red colours in the wavelet spectra correspond to
high values of the transform coefficients (power). The thick black contour delimits the 95% confidence level against AR(1) red noise, and the
cone of influence where edge effects are not negligible is shown as a shaded contour. The arrows reflect the relative phase relationship (inphase pointing right, anti-phase pointing left, streamflow leading SSL by 90° pointing straight down).
Oscillations greater than one year were not statistically significant for both, streamflow and SSL, so it must be
interpreted carefully (Figure 4c and 4d). These information, however, were considered useful because (1) the CWT
isolates signals hidden in noise, (2) are within the range defined by cutoff frequency and edge effects, and (3) the zero
padding technique might reduce the true power of lower frequencies. More data is needed to test the true statistical
significance of these oscillations. The XWT revealed a significant common power in the ~0.5 year band from 1974-1983,
1985-1992, 1993-1995, 1999-2005, and 2006-2010. The highest scale dependence was observed in the annual band,
except in the 2001-2007 period, where both time series exhibited low powers. The latter indicates low scales of variability
during this period. The XWT showed that streamflow and SSL are in-phase in all the sectors with significant common
power. Outside the areas with significant power the phase relationship is also predominantly in-phase, indicating phaselocked (Figure 4e).
4.2 Patterns of erosion and sedimentation in the river mouth
From 1994 to 2000, large sedimentation zones, extending over 0.24 km2 (82% of the total subarea evaluated),
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containing 0.61 x10 m of accumulated sediments (Table 3), appeared in the mouth/frontal bar sector of the Magdalena
River (Figure 5b). This accumulation was especially apparent in the southern part of the mouth, which experienced
sediment accumulation of greater than 2.3 m (0.38 m yr -1) (Figure 5b). This period of intense sedimentation was
followed by an interval of erosion from 2000 to 2004, not only in the mouth/frontal bar but also in the main outlet (Figure
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5). In this period, the erosional areas covered 0.19 km (65% of the total subarea evaluated) and 1.61 km (69% of the
total subarea evaluated) in the mouth/frontal bar and main outlet sectors, respectively. The total volumetric loss
amounted to 0.24 x106 m 3 in the frontal bar and 2.16 x106 m 3 in the main outlet (Table 3). During this period, the main
outlet experienced sediment deficits of >1.1 m (0.27 m yr-1). In contrast, patchy areas of sedimentation were identified in
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the lateral segments of the main outlet, showing an accumulation rate of 0.35 m yr (Figure 5). Between 2004 and 2011,
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the depositional areas in the main outlet and mouth/frontal bar increased in number and size (75% total coverage),
presenting sedimentation volumes of 4.15 x106 m 3 and 0.67 x106 m 3, respectively (Table 3, Figure 5). In contrast,
elongated erosion patches were identified in lateral segments of the main outlet, especially along the western jetty
(Figure 5).
Table 3. Accretion and erosion balances (area and volume) in different zones of the Magdalena River delta between 1994 and 2012.
2
Zone/ Bathymetric data
Comparison periods
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Area (km )
Accretion
3
Volume (x10 m )
Erosion
Accretion
Erosion
Outlet
2000 (June) – 2004 (May)
0.706
1.610
1.888
2.160
2004 (May) – 2011 (July)
1.743
0.572
4.151
1.676
Mouth/Frontal bar
1994 (August) – 2000 (June)
0.243
0.052
0.609
0.054
2000 (June) – 2004 (May)
0.101
0.194
0.196
0.243
2004 (May) – 2011 (July)
0.224
0.071
0.671
0.189
Delta Front (I)
1994 (August) – 2000 (June)
3.221
3.267
6.331
5.003
2000 (June) – 2008 (April)
2.738
3.750
21.361
15.970
Delta Front (II)
1994 (August) – 2000 (June)
1.575
1.622
2.427
2.343
2000 (June) – 2008 (April)
2.044
1.154
19.503
6.627
2010 (April) – 2012 (June)
1.712
1.485
9.991
4.840
4.3 Patterns of erosion and sedimentation in the delta front
Between 1994 and 2000, erosional and depositional areas were balanced in the delta front (Table 3). Erosion of up to
4.0 m (0.66 m yr-1) dominated at the northern (11°08’00N, 74°51’30 W) and eastern (11°08’00N, 74°51’30 W) seaward
margins of the delta front (Figure 6). Patchy areas of erosion were also identified in the central zone of the delta front,
especially toward the western margin (11°07’30N, 74°51’45 W) (Figure 10a). In contrast, the sector closest to the river
mouth (11°07’00N, 74°51’30 W) and the southwestern seaward margin (11°07’00N, 74°51’45 W) experienced sediment
accumulation >1.3 m (0.21 m yr-1) (Figure 6a). From 2000 to 2008, the erosional/accretional balance changed. Although
erosional areas covered 58% of the total subarea evaluated, total sediment accumulation of 21.4 x10 6 m 3 prevailed, in
comparison with 15.9 x106 m 3 of lost sediment (Table 3). Overall, erosional processes predominantly appeared in the
shallower zones of the eastern margin (i.e., eastern shoal). Erosion patches of ~4.0 m (0.50 m yr -1) grew or merged over
the eastern shoal and northern seaward margin of the delta front. Significant sedimentary processes were observed in
the deepest zones, corresponding to the path to the Magdalena Canyon, with an accumulation >4.5 m (0.56 m yr -1)
being recorded, promoting the progressive infilling of this pathway (Figure 6). Between 2010 and 2012, sedimentary
processes became consolidated over the western margin, especially in the pathway to the Magdalena Canyon and
-1
adjacent areas, which experienced sediment accumulation of >4.0 m (2.0 m yr ) (Figure 6b). In this period, the
2
depositional areas covered 1.71 km (54% of the total subarea evaluated), and the total volumetric gain amounted to
9.99 x106 m 3 (Table 3). Erosion patches were also typical of the northern and southern seaward margins of this sector
(Figure 6b).
8
Figure 5. Comparison of the accretion (positive values) and erosion (negative values) volumes in the Magdalena River delta for different years at
the (a) outlet and (b) mouth/frontal bar.
5.
DISCUSSION
5.1 Changes in the sediment transport regime
Under natural conditions, there is a strong positive correlation between the SSL and streamflow (Walling and Fang,
2003; Yang et al., 2003; Yan et al., 2002). Spectral wavelet analysis of the discharge of the Magdalena River revealed
that the SSL and streamflow exhibited almost the same modes of variability, where the annual and 4-7-year bands were
the first and second sources of hydrologic variability, respectively (Figure 4). These periodic oscillations reflect a
naturally induced variability, mainly driven by climate shifts (i.e., Labat, 2010; Li et al., 1998; Zhang et al., 2008). In the
Magdalena River, these periodic oscillations were linked to the annual shift of the ITZC and the ENSO phenomenon (47-year band) (Restrepo et al., 2014). However, we hypothesize that the magnitude and phase of these oscillations
appears to be modulated by the responses of the SSL and streamflow to large-scale anthropogenic impacts. The
conversion of forest into agricultural and extensive grazing zones, retention in dam reservoirs and mining are the main
human activities altering the runoff and sediment supply in drainage basins (e.g., Syvitski and Kettner, 2011; Syvitski
and Milliman, 2007; Walling and Fang, 2003). The Magdalena River drainage basin has experienced major human
interventions in the last 40 years (IDEAM, 2001; Restrepo and Syvitski, 2006). The peak of these interventions appeared
in the 1990s, when almost 69% of the Andean forest and 30% of the Caribbean lowland forest, which covered the
Magdalena drainage basin, were cleared as a result of deforestation rates as high as 2.4% per drainage basin area
(Etter et al., 2006; IDEAM, 2001). Most of the dam reservoirs began to operate at full capacity in the 1990s, and four
9
additional dam reservoirs were built in this period, resulting in a total water storage capacity of 7.7 x109 m3 by the end
of the 1990s, which represents ~93% of the current water storage capacity in the drainage basin. In addition, open pit
mining of coal and gold increased from 8 x 106 t yr-1 to 20 x 106 t yr-1 and from 29 t yr-1 to 36 t yr-1 between 1990 and
2000, respectively (IDEAM, 2001). Hence, the combined effects of these anthropogenic interventions and the high
oscillatory activity experienced in the 1990s led to high annual mean SSLs (except in 1992 and 1997) and to an increase
in the Sen’s slope of streamflow (Figure 3 and Table 2). There is an indication that the level of human intervention
declined, or at least remained steady in the decade of 2000-2010. Deforestation hot spots migrated from the Magdalena
River drainage basin to the southern region of Colombia, close to the Ecuadorian border, and to the western Pacific
coast (Etter et al., 2006). The contribution of mining in the Magdalena River drainage basin to GDP remained steady at
2-3% (SIMCO, 2012). Additionally, only two new dam reservoirs were built in third-order tributaries of the Magdalena
River (i.e., Porce River) (EPM, 2013). Therefore, it is reasonable to assume that the dramatic reduction of the annual
mean SSL observed in the early 2000s (< 120 x 106 t yr-1), was induced by the low annual mean streamflow (< 6600
m3 s-1) (Figure 3), as is indicated by the powers and the phase relationship (i.e. phase-locked) of the CWT and XWT
analyzes, respectively (Figure 4). The low streamflow was, in turn, the result of climate-driven shifts (Restrepo et al.,
2014) and presumably, to a lesser degree, of human intervention. The high oscillatory activity experienced in the late
2000s (Figure 4) induced the significant increasing trends of both streamflow and SSL during the 2000-2010 period.
However, the rate of increase, measured as the ratio of Sen’s slope to the annual mean, was higher for the SSL than for
streamflow (Table 2).
Figure 6. Comparison of the accretion (positive values) and erosion (negative values) volumes in the Magdalena River delta for different years at
the (a) delta front (I) and (b) delta front (II).
10
Although the streamflow and SSL exhibited almost the same modes of variability (Figure 4), they showed significant
differences in their rates of change (i.e., mean values, significant trends) (Figure 3 and Table 2). Consequently, it can be
assumed that one of the most important features of the sediment transport regime of the Magdalena River is the lack of
proportionality in the variability of the streamflow and SSL. The changes in the power coefficients of the dominant scales
(i.e. semi-annual, annual and 3-7 year bands) in the XWT analysis (Figure 4e) also suggest a lack of proportionality in
the variability of streamflow and SST. This lack of proportionality reflects significant changes in the suspended sediment
concentration (SSC) in the Magdalena River. An increase in the SSC was registered in the period from 1990-2000, while
the period from 2000-2010 was characterised by a decrease in the SSC. A t-test indicated that there is a statistically
significant difference between the means of the SSC during these time intervals at the 95% confidence level (CSS 1990-2000
= 0.73±0.04 kg m -3, CSS2000-2010 = 0.65±0.01 kg m -3; t = 9.20, p = 7.0x10-6). In addition to the differences in the response
mechanisms related to human interventions, the sediment retention in lagoons might also contribute to this lack of
proportionality. The change in the river sediment load depends on the balance between the yield and retention of
sediments in the catchment, including reservoirs, lakes and lagoons (Syvitski and Kettner, 2011; Walling and Fang,
2003). The lower basin of the Magdalena River is characterised by gentle slopes (< 5°) and a large floodplain (12144
km 2) that houses a complex network of lagoons and swamps, extending over 2596 km 2 (IDEAM, 2001). In addition, the
movement of tectonic blocks defines a broad area of differential subsidence in the lower basin that allows progressive
infilling with fluvial sediments. van der Hammen (1986) estimated sedimentation rates between 2.1 mm yr -1 and 3.0 mm
yr-1, implying sediment deposition of ca. 10.7 x 106 t yr-1 (5.0 x 106 m 3 yr-1) in the lower basin of the Magdalena River.
5.2 Magnitude, causes, and implications of morphological changes in the delta
The standard error of bathymetric data with the 0.1 m accuracy in surveys from 4 to 8 years apart range between 25.0
mm yr-1 to 12.5 mm yr-1 (e.g. Yang et al., 2003). The accuracy and standard error of the applied bathymetric data were
below the range of the calculated rates of accretion/erosion (Figure 5 and 6), and the data were therefore considered to
be useful for obtaining volumetric estimates. The net sedimentary balance between the accretional and erosional
volumes (Table 3) indicated that, overall, sedimentation dominated in the Magdalena River delta during the surveyed
period, apart from the river outlet, which suffered from erosion between 2000 and 2004. Average rates of sedimentation
and erosion, ranging between 37 and 822 mm yr-1 (Figure 5 and 6), as well as high sedimentation rates from specific
sites (2000 mm yr-1), were of a comparable magnitude with the rates reported from deltas experiencing severe sediment
turnover as a result of human interventions or large floods (e.g., Lane, 2004; Maillet et al., 2006; Wang et al., 2006;
Yang et al., 2002, 2003).
Thus far, the morphology of the Magdalena River delta has been seen as the result of the supply rate, the capacity of
marine processes for resuspension and transport and the accommodation space for sediments. New data on the
magnitude of the sediment flux due to freshwater discharge (Table 2 and Figure 3) strengthens the hypothesis that the
river itself is the main source of sediment. This is in accord with statements made by other authors (e.g., Ercilla et al.,
2002; Estrada et al., 2005; Klingebiel and Vernette, 1979; Kolla and Buffler, 1985). The subaqueous portion of the delta
appeared to be sensitive to changes in the sediment transport regime, although the morphological response was not
uniform. For instance, erosion became the dominant process at the mouth/frontal bar between 2000 and 2004, with a
shift from a mean sedimentation rate of 320.6 mm yr -1 in 1994-2000 to a mean erosion rate of 39.9 mm yr-1 in 20002004 being observed (Table 3 and Figure 9). At the same time, the riverine SSL decreased by 30% (Figure 3).
6
-1
6
-1
Conversely, the riverine SSL increased from 112 x 10 t yr in 2000-2004 to 154 x 10 t yr in 2004-2011 (Figure 3),
corresponding to mean sedimentation rates of 232.5 mm yr -1 and 152.7 mm yr-1 in the mouth/frontal bar and outlet,
respectively (Table 3 and Figure 5). The amount of annual sediment deposition in the river outlet between 2004-2011
6
3
-1
(0.59 x 10 m yr ) was almost equal to the annual volume of dredged sediment in the river channel (from the mouth to
6
3
-1
20 km upstream) between 2001 and 2012, which was estimated to be 0.81 x 10 m yr (Cormagdalena, 2013). In the
delta front, changes in the SSL were followed by changes in the magnitude of sedimentation rates (Table 3 and Figure
6). The 30% reduction of the SSL between 1994-2000 and 2000-2008 coincided with an increase in sedimentation rates
-1
at the delta front, from 36.7 to 103.8 mm yr . This increase was particularly large in the westernmost part of the delta
-1
front (i.e., delta front II), where the sedimentation rates changed from 5.1 mm yr (within the uncertainty range) in 1994-1
2000 to 503.2 mm yr in 2000-2008 (Table 3). In last this area, the sedimentation rates increased even more during the
period from 2010-2012 (821.9 mm yr-1), coinciding with a period of severe floods with SSLs as high as 223 x 106 t yr-1.
These response patterns may reflect the spatial differences in hydrodynamic and sediment transport processes. In the
river outlet, erosional/sedimentary cycles are controlled by the magnitude of fluvial discharges and river bed scouring.
During low discharges, as seen for the period between 2000-2004, the scouring of the river bed presumably was
enhanced due to the channel configuration (i.e., the narrow channel and jetty revetment), but the removed sediments
were not replenished by fluvial inputs. A large proportion of the sediments deposited within the subaqueous delta front
was affected by weak tidal currents and decreased river discharge, which in turn led to a relatively weak buoyant layer.
As the particle size increases, this process might induce siltation of the river mouth. During high freshwater discharges,
sand was deposited in the river outlet. In addition, an enhanced buoyant plume promoted effluent diffusion, while
cohesion and turbulent shear stress might induce flocculation and settlement in the delta front (e.g. Chen et al., 2006;
Yang et al., 2002). Given the flow conditions and particle size composition, most of the sediment was transferred to the
prodelta and continental shelf (Restrepo, 2014). For instance, considering a dry bulk density of 2.65 g cm -3 and a mean
porosity of 80%, the overall net sediment accumulation in the mouth/frontal bar zone during the 1994-2011 period was
6
-1
6
-1
0.12 x 10 t yr , while that at the delta front was 2.53 x 10 t yr during the period of 1994-2008. Thus, the total
sediment accumulation in the delta (including dredging) corresponded to <5% of the annual mean SSL of the Magdalena
River (Qs = 137 – 144 x 106 t yr-1). In the river mouth, manmade structures also limited primary responses to changes in
sediment transport, such as the formation/abandonment distributaries due to levee breakthroughs or infilling, lateral
11
shifting of the active channel, morphological changes in cross-sections, or crevasse planes (e.g., Syvitski and Saito,
2007; Wang et al, 2006). The erosion rates observed in the eastern shoal, mainly between 2000 and 2008 (Figure 6),
can be explained by wave dynamics rather than by changes in the sediment supply. When the sediment deposits
reached their maximum thickness on top of the eastern delta front, they were remobilised by swell waves in the
shallower zones (10 to 20 m depth). Given the significant high waves and periods of predominant swells (northnortheast) (Ortiz et al., 2012, 2013), the southwestward along-shore transport of sediment is enhanced during low river
discharge conditions. New re-settlement may be favoured in the westernmost part of the delta front due to a decrease in
wave-induced currents as a result of the increase of depth (> 20 m). Changes in storminess may therefore contribute to
changes in the rate of the infilling of the pathway to Magdalena Canyon and should consequently be analysed
thoroughly.
6.
CONCLUSIONS
Changes in freshwater discharge and the SSL were particularly pronounced within the last 20 years. Although the
spectral wavelet analysis revealed that the two parameters presented nearly the same modes of variability (the annual
band and the 4-7-year band), their respective responses to large-scale anthropogenic impacts appear to modulate the
magnitude and phase of these oscillations. Consequently, there was a lack of proportionality in the variability of the
streamflow and SSL in the Magdalena River, which in turn led to significant changes in the sediment transport regime
between the 1990s and the 2000s.
The subaqueous part of the delta appeared to be sensitive to changes in the sediment transport regime. The changes in
this regime led to erosion of the mouth/frontal bar and outlet zones in the early 2000s, while they modified the
erosional/sedimentary balance observed during the 1990s in the prodelta. The average rates of sedimentation and
erosion (37 - 822 mm yr-1) were of a comparable magnitude with rates reported from deltas experiencing severe
sediment turnover as a result of human interventions or large floods. Progressive infilling of the path to Magdalena
canyon was also identified. The total sediment accumulation in the delta corresponded to <5% of the annual mean SSL
of the Magdalena River. These response patterns reflect the spatial differences in hydro-sedimentary processes. In the
river outlet, erosional/sedimentary cycles are controlled by the magnitude of fluvial discharges and river bed scouring. In
the delta front, effluent diffusion and sediment dispersion depend upon the relative importance of outflow buoyancy,
turbulent diffusion, and turbulent friction.
ACKNOWLEDGMENTS
This research was funded by the Research and Projects Office (DIDI) at Universidad del Norte. The following institutions
provided valuable historic data: Instituto de Hidrología, Meteorología y Estudios Ambientales (IDEAM, Colombia), and
Centro de Investigaciones Oceanográficas e Hidrográficas (CIOH – DIMAR). The support from a Universidad del Norte
Fellowship and CEMarin Fellowship (Colombian Center of Excellence for Marine Science Research sponsored by the
German Academic Exchange Service - DAAD) awarded to one of the authors (J.C. Restrepo) is gratefully acknowledged.
Trabajo sometido en Journal of Coastal Research.
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14

magdalena river, colombia

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