2005 International Nuclear Atlantic Conference - INAC 2005
Santos, SP, Brazil, August 28 to September 2, 2005
ASSOCIAÇÃO BRASILEIRA DEENERGIA NUCLEAR - ABEN
ISBN: 85-99141-01-5
THE USE OF NUCLEAR TECHNIQUES FOR THE QUANTIFICATION
OF SEDIMENT TRANSPORT AND BULK DENSITY OF DEPOSITS –
APPLICATION TO DREDGING AND DUMPING OPERATIONS
OPTIMISATION AND ENVIRONMENTAL IMPACT STUDIES
Jefferson V. Bandeira 1 , Lécio H. Salim1 , Pedro E. Aun1
1
Centro de Desenvolvimento da Tecnologia Nuclear (CDTN / CNEN- MG)
Serviço de Meio Ambiente e Técnicas Nucleares - TR-1
Rua Prof. Mário Werneck s/n
Cidade Universitária – Pampulha – CP 941
30123-970 Belo Horizonte, MG
[email protected]; [email protected]; [email protected]
ABSTRACT
Nuclear techniques are used, since the 1950’s and 1960’s, for studying the transport, in water environment
(rivers, estuaries, bays and open coast) [1, 2], of sandy bottom sediment and fine sediment (silt and clay) in
suspension, through the labelling of sediments with γ emitters radioactive tracers of a suitable half-life. The
labelling is followed by injection and subsequent in situ detection, by means of scintillation detectors sliding on
the bottom or transported at suitable depths, by positioned boats.
These kinds of studies associated with the knowledge of the hydrodynamic agents, e.g.: river, tidal, wind and
wave currents, are powerful tools for the understanding and quantification of the sediment transport and also for
its response to human interventions. Some of these interventions, are: construction of dams and training walls in
rivers; dredging of reservoirs, access channels and harbours and dumping of the dredged material, in water
environment.
A recent improvement for the utilisation of radioactive tracers in labelling fine sediment was the development
of the methodology for the use of 99mTc [3, 4, 5, 6, 7], broadly applied in Nuclear Medicine [11].
Another important application of nuclear techniques is the use of sealed sources of γ emitters radioisotopes (e.g.
241
Am; 137 Cs) for measuring vertical density profiles of fine sediments in reservoirs, access channels, turning
basins and berthing areas in harbours, and in the well of trailing suction hopper dredgers (TSHD) [1, 14, 19].
The use of these techniques contributes for the optimisation of the dredging works and allows the evaluation of
dumping sites and also the physical environmental impacts of the dumping. Furthermore, they allow calibrating
mathematical models for different aspects of sediment movement.
Some examples of applications of these techniques are shown for harbours in Santos-SP; São Luiz-MA and for
the Pampulha reservoir in Belo Horizonte-MG, and the watercourses downstream.
1. INTRODUCTION: NUCLEAR TECHNIQUES IN SEDIMENTOLOGY
1.1. Artificial Tracers
When dealing with various aspects of the behaviour of sediments in water environment,
tracers are usually a quite pragmatic approach to many engineering and environmental
questions. They provide responses for the long-term interaction between the sediment and the
flow processes that act over it, since they are able to integrate all the actions suffered by the
sediment during the observation period.
Since most applications are concerned with a well-defined site, it would be a very lucky
circumstance to find some natural tracer that would provide information about the sediment
behaviour at the area of interest. More generalised utilisation of tracers only became possible
with the advent of artificial tracers in the 1950's and 1960's. From this time on, sediment
labelled with material having radioactive or fluorescent properties (named tracers), have been
used to study sediment movement, provided they were able to accurate matching the
properties of the natural sediment. Also activatable tracers (activated in a nuclear reactor after
the sediment sampling) were considered.
Radioactive tracers (γ-emitters) of a suitable half-life are used in sediment movement studies
in water environment (rivers, estuaries, bays and open coast) [1]. The steps for these studies
are labelling of the sediment; injection; detection and analysis of the results.
The radioactive tracers have the advantage of versatility: they can be used for gravel, sand,
and fine sediment (silt and clay) transport studies. Moreover, they enable direct in situ
detection of the material, which means that the scanning of the tracer cloud is always done
with open eyes, a great help in planning the detection strategy. The in situ detection is
performed by means of scintillation detectors (towed by positioned boats – Lagrangean
detection), sliding on the bottom or transported at suitable depths, for sand bottom or for fine
sediment in suspension transport studies, respectively [1, 2]. For narrow watercourses, the
detectors could be hanged in bridges or poles (Eulerian detection) [3].
In relation to fluorescent (suitable only for labelling sand) and activatable tracers, the
problem is the blind detection, performed by sampling and subsequent analysis in laboratory,
which means that no feedback is being received when performing the field detection works.
During the sediment labelling with radioactive tracers, one has to pursue mass labelling, in
order to allow reliable quantitative results in the detection, that is: the counts must be
proportional to the sediment mass and not to the sediment surface.
In this way, for sand studies, ground glass (same density as sand) in the suitable grain-size
distribution and doped with an activatable element (e.g. 191 Ir, 197Au, 50 Cr, 45 Sc) to be
irradiated in a nuclear reactor previously to the injection, is almost universally used. The
resultant radioactive elements have the following half- lifes (T1/2 ) and the main γ radiation
energy: 192 Ir (T1/2 = 74.4 days, γ = 0.32-0.47-0.30-0.31-0.61MeV); 198 Au (T1/2 = 2.7 days, γ =
0.41MeV); 51 Cr (T1/2 = 27.8 days, γ = 0.32MeV) and 46 Sc (T1/2 = 84 days, γ = 0.891.12MeV). The mass of ground glass for each injection and the activity utilised does not
surpass, respectively, 1kg and 2Ci (74GBq), taking into account the statistical aspects of the
in situ detection [4] and the radiological safety standards [1, 5].
For silt and clay studies, the labelling is performed in the natural sediment (generally
electronegative) through its capacity to adsorb cations of radioactive tracers (e.g. 198 Au, 51 Cr,
46
Sc) [6]. This aspect makes necessary the irradiation of the chosen element for each
experiment. A few grams of the chosen radioactive tracer are necessary to label a
considerable mass of fine sediment. The radioactive is the unique tracer suitable for reliable
labelling of fine sediments, considering they allow mass labelling and the same behaviour of
the labelled and not labelled sediment, in hydrodynamic conditions [6, 10].
The time scale of the experiment determines the type of radioisotope to be chosen: short halflife isotopes for short time scales (e.g. study of fine sediment transport in suspension) and the
opposite for large scales (e.g. slow bottom sand sediment transport).
INAC 2005, Santos, SP, Brazil.
1.1.2. Development of 99m Tc as a tracer for fine sediment in suspension
A recent improvement for the utilisation of radioactive tracers in labelling fine sediment was
the development of the methodology for the use of 99m Tc (technetium meta-stable) [3, 7, 8, 9,
10], broadly applied in Nuclear Medicine [11].
The 99m Tc has the following properties: (T½ = 6.02 hours; γ = 0.14MeV), and it comes from
the parent nuclide 99 Mo (T½ = 66 hours; β = 1.2MeV; γ = 0.74; 0.18....MeV) through a metastable transition [11, 12]. Technetium generators, known as “milk generators” or “cows”, are
then supplied to Nuclear Medicine laboratories. These generators, due to the convenient halflife of the 99 Mo, make possible to obtain 99m Tc for one week or more, by its elution through
an ion-exchange column, using 6mL of a solution of NaCl (9mg/mL) passing through the
column. After each elution, the generator continues to produce the 99m Tc and in 24 hours, due
to its growth rate and considering the radioactive decay of the 99 Mo, it is possible to extract
about 88.5% of the activity previously extracted. The low γ radiation energy and the
portability aspect are two of the reasons for its broad use in Nuclear Medicine.
The 99m Tc is eluted as an anion TcO 4 - (pertechniate). In this form it is injected in the veins of
patients, in Nuclear Medicine applications.
It should be worthy if 99m Tc could also be used for labelling fine sediment and applied to
dynamic sedimentology studies, because of its characteristics, such as:
1. Short half-life adequate for short time scales of the studies of fine sediment in suspension;
2. Low γ energy (0.14MeV), implying moderate shielding and ease of manipulation in field
conditions;
3. Portability of the tracer when compared with 198 Au, for example, obtained by irradiation
in nuclear reactor for each experiment;
4. Availability even in places where there are no nuclear facilities, due to its widespread
application in Nuclear Medicine, making also possible its use in dynamic
sedimentological applications that can be performed daily, for more than one week, in
field studies, using only one portable generator.
As the 99m Tc is eluted as an anion, in order to be adsorbed by the fine sediment (generally
electronegative), studies were performed [7, 8] to transform it in an electropositive form.
Several reductors were tested for this purpose. It was selected and used the SnCl2 .2H2 O. The
reduction of Tc(VII) with Sn(II) is produced according to the following equations [13]:
3 Sn (II) + 2 Tc (VII) = 3 Sn (IV) + 2 Tc (IV)
(1)
In aqueous solution, a colloidal compound is formed after reduction.
TcO2+ + 2 H2 O = TcO(OH)2 + 2H+
(2)
Most probably this is the chemical specie which is adsorbed in sediments after reduction of
the pertechniate.
The labelling of the fine sediment with this colloid was studied considering the effect of: a)
mass of the reducing agent; b) pH; c) concentration of sediment; d) contact time; e)
INAC 2005, Santos, SP, Brazil.
possibility of release as a function of water turbulence [7, 8].
Only the labelling of the fine sediment with a radioactive tracer is not enough for its correct
application in field studies. It is necessary to prove first that the hydrodynamic behaviour of
the labelled and the not labelled sediment are the same. In this way, the settling behaviour, in
still water, of both sediments were compared, through sedimentation tests relating the
cumulative % of mass in function of the fall velocity of the particles [9, 10]. The results
obtained were very good. Afterwards, the tracer was used for studying the physical behaviour
(advection, dispersion, sedimentation rate) and the corresponding environmental impacts
(increase of concentration and possibility of deposition) of the fine sediment dredged in the
accreted Pampulha reservoir and dumped in the watercourses downstream [3].
It is expected that the 99m Tc, with its new use for labelling fine sediment, can be applied for
studies in estuarine and coastal environments, as the ones already performed using 198 Au [1],
some of them being briefly presented in Chapter 3 (Case Studies) of this paper.
1.2. Nuclear Gauges for the Determination of Bulk Density of Fine Deposited Sediment
The absorption (photoelectric effect) and scattering (Compton effect) of electromagnetic
radiation (X or γ) emitted by an artificial radioactive source are a function of the
concentration or the bulk density of the mixture sediment-water. In this way it is possible to
construct nuclear gauges based on these principles, provided that the system is calibrated for
known concentrations. The gauges use sealed sources and scintillation detectors. The gauges
based on the photoelectric effect and the Compton effect are, respectively, the transmission
and the backscattering gauges (figures 1a and 1b). They are normally equipped with depth
sensors and inclinometers for the underwater unit and are used as point measuring systems
being lowered on a suspension cable to sink into the soft bed by gravity. In this way, bulk
density profiles between 1.0 and 1.5 ton/m3 may be determined.
Figure 1a. Transmission gauge
After Salim et al. [14]
INAC 2005, Santos, SP, Brazil.
Figure 1b. Backscattering gauge
After Meyer et al. [15]
2. PRACTICAL USES OF ARTIFICIAL TRACERS AND NUCLEAR GAUGES
2.1. Sand Bottom Sediment Transport
Bed-load transport studies present different effectiveness in function of the environment in
which they are performed. Sand bottom sediment transport studies applying tracers are
important if the information provided is worthwhile. In the case of rivers, bed-load is only a
fraction of the sediment transported by the river, most of which (~90%) occurs in suspension.
Nevertheless, in some cases, such as the artificial deepening (e.g. dredging) or construction of
training walls with navigation purposes, the bed-load transport knowledge is very useful. In
the cases of estuaries and open sea this knowledge is important:
1. For monitoring sedimentation, by sand, in existing dredged areas (e.g. access channels) or
predicting the maintenance in future ones;
2. For design harbour inlet by-passing schemes, location and dredging of sand traps;
3. For the construction of coastal structures, such as: groins, jetties, detached breakwaters
and sea walls and for artificial beach nourishment. The associated sedimentological
studies related to the response (accretion or erosion) of the shoreline to the coastal
structures and the artificial beach nourishment require, respectively, a knowledge of the
littoral-drift and the cross-shore sand transport in the nearshore region;
4. For the temporary excavation below natural depths for the burial of pipelines and cables;
5. For the reclamation works in estuaries and the influence of the loss of tidal volume on the
sedimentary regime.
A detailed description of the methodology applied for the study of sandy bottom sediment
transport, applying radioactive tracers, can be seen in [2, 16 and 17].
2.2. Suspended Transport of Fine Sediment
The study of the dynamics of suspended sediment with tracers is of capital interest in
problems of civil engineering, mostly for the definition and optimisation of dumping sites for
dredged material (the nearest place environmentally suitable for the dumping) and in water
pollution problems. In such cases, it has to be known how a given pollutant adsorbed to fine
sediment or the sediment itself is dispersed.
The most important results obtained from such experiments are:
1. The path and drift caused by currents;
2. The mean velocity of transport;
3. The turbulent dispersion coefficients that characterise the dispersion of the sediment by
the water body;
4. The dilution of the sediment (maximum concentration in function of time);
5. The sedimentation rate;
6. The area over which the sediment is deposited.
The methodology for the study of fine sediment in suspension applying radioactive tracers
can be seen in [2, 17 and 18].
Results obtained for the bottom and suspended transport allows calibrating mathematical
models for different aspects of sediment movement and known hydrodynamic conditions.
INAC 2005, Santos, SP, Brazil.
2.3. Nuclear Gauges for Measuring Bulk Density
The nuclear gauges can be employed for measuring the bulk density in vertical profiles of
fine sediments deposited in:
1. Reservoirs, generally near the dam, when studying sedimentological balance of the
system lake catchment basin-reservoir;
2. Access channels, turning basins and berthing areas of harbours, in estuarine and coastal
regions, and in the well of dredgers, being these measurements related to the optimisation
of dredging works and environmental studies.
In harbours submitted to heavy fine sediment siltation, such as Rotterdam and Zeebrugge, in
Europe, and in some harbours in South America and Indonesia it exists, in certain parts, a
lower density layer above the more consolidated bed sediment, named fluid mud, which can
attain several metres. The top interface between the fluid mud layer and the water above is
detected by the echosounder of 210kHz frequency, normally employed for surveying nautical
depths. It was demonstrated, by ship manoeuvring and laboratory studies, that the fluid mud
is not an obstacle to navigation, provided its bulk density is equal to 1.2ton/m3 or less (Figure
2). In this way, since the late seventies, radioactive gauges have been operated at spot
locations in Rotterdam waterway, in conjunction with echo sounding surveys, allowing to
produce nautical charts showing the 1.2ton/m3 density depth contours. They are used for
navigation (addition of the loose silt layer to the keel clearance) and for improving dredging
efficiency (positioning of the dredger suction tube below the loose silt layer, when dredging).
Figure 2. Scheme of keel clearance. After Caillot et al [20]
A transmission gauge (Figure 1a) was used in Brazil, in the access channel to Alumar harbour
and in its turning basin and berthing area (Figure 6), between 1983 and 1994, and also in the
well of a trailing suction hopper dredger (TSHD), with good technical and economical results
[14, 19]. This last use allows evaluating the load efficiency of a particular dredging practice
(Figure 3). Loading curves of the dredger, according to the percentage of sand dredged
(related to the total amount of sand, silt and clay), are drawn. For a fixed travel and dumping
time (same dredged region and same dumping site), and for each loading curve, there is an
optimum time for dredging with overflow (t3 - t2), in order to get the optimum dredging
INAC 2005, Santos, SP, Brazil.
cycle time (tangent to the specific loading curve). Suspended sediment studies labelling the
full load of TSHD or barges (see case studies in Chapter 3) could promote the optimisation of
the dumping sites with a consequent reduction of the travel and dumping time (Figure 3),
shortening, even more, the optimum dredging cycle and increasing the dredging efficiency.
Figure 3. Dredging cycle for a trailing suction hopper dredger. After De Heer [21]
3. CASE STUDIES
3.1. Santos Bay, Santos, SP - Brazil [1, 22]
The shape of Santos Bay, in São Paulo State, Brazil, 60km far from São Paulo city, is roughly
quadrangular, limited at E and W by two rocky points, Ponta Grossa and Ponta de Itaipu
(Figure 4), with an area of about 36km2 . Two estuaries, Santos at E, and São Vicente at W
open into the bay, the contribution of Santos estuary to the overall flow in the bay being more
important. The most important Brazilian port facilities are installed at Santos estuary.
Adequate depths are kept at the harbour and access channel through permanent maintenance
dredging. Due to the importance of Santos harbour, several sedimentological studies were
performed at Santos Bay and surroundings, from 1973 to 1985, using a combination of
hydraulic measurements, physical movable bed models and radioactive tracer studies. Bottom
sediments in the bay cover all the range between fine sand and clay (Figure 5). The material
dredged in the harbour is mainly silt and clay, including also fine sand in the access channel.
INAC 2005, Santos, SP, Brazil.
Studies performed in 1973/74 [23] had the objective of evaluating several dumping sites to
reduce the distance from the dredging region to the disposal areas. In the first study it was
determined, by labelling the full load of barges (600m3 capacity) with 198Au, that the
disposal area near Ponta de Itaipu, at W, was inadequate, since the material could return, by
the action of hydrodynamic circulation, to the bay-estuary system (Figure 4). A new
alternative site outside the bay and at E was chosen, near Ilha da Moela. Its use, implemented
immediately after the studies, resulted in a sensible reduction on maintenance dredging costs.
Figure 4. Key map, Santos Bay and estuary. After Aun & Bandeira [1]
In 1985, another dumping site, also eastward to the bay entrance, near Ponta da Munduba
rocky point was defined (Figure 4), with the objective of reducing, even more, the
transportation distances [24]. Again, the technique used was the labelling, with 198 Au, of the
dredged material transported by barges. The new dumping site is being used since 1986.
Hydraulic measurements and tracer studies performed in 1980/81 [25], using ground glass
labelled with 192Ir, were intended to define the behaviour of the bottom fine sandy sediment
that is found in some regions of the bay, on behalf of physical movable bed model studies.
Six injections were performed in different regions of Santos Bay, three in the winter of 1980
and three in the summer of 1981 (Figure 5). Before the injections in summer, the bottom was
surveyed in order to detect the remaining of the radioactive clouds from the winter
experiments. It was still possible to find activities of more than twice the background, in the
INAC 2005, Santos, SP, Brazil.
regions inside the dotted lines, having origin in PI1 WINTER and PI3 WINTER. In this way,
the summer tracer injections were made outside these regions.
Figure 5. Santos Bay: tracer injection points (PI). Clouds near PI’s (Injection Points)
are related to the last complete detection performed. After Aun & Bandeira [1]
Both, in summer and winter, bottom sediment movement has an onshore resultant, being
more important in winter conditions. Transportation rates were quantified in both regimes
(Table 1). Waves, associated with tidal currents inside the bay, are the main transport agents,
the direction of movement being compatible with the prevailing wave incidence direction.
It was also determined that, when the depth of 7m is attained, bottom material under
movement tends to inflect towards Santos estuary entrance. This is indicated by the two sets
of dotted lines (having origin in the injection performed at 03/Aug./80 in PI1 WINTER ),
obtained in the detections dated 07/Feb./81 and 08/Apr./81 (Figure 5). It is also possible to
observe, from the successive positions of these dotted lines, the movement of the bottom
sediment towards the coast and also a smaller shift towards W of the western portion of the
lines limiting an activity equal to twice the natural bottom sand background.
The current circulation pattern inside the bay, mainly tidal influenced, was determined by
intensive field measurements at three levels (1.0m below water surface, mid depth and 1.0m
above the bottom) employing manned and recording currentmeters [26]. They demonstrated
that, for the eastern part of the bay, where the injections PI2 WINTER and PI2 SUMMER
were performed, the currents are always, in flood and ebb tide, and for the three levels,
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directed to N, flowing towards the interior of the bay. This is due to a counterclockwise
circulation inside the bay, during ebb flow.
Table 1. Main results obtained with sandy bottom sediment transport studies
in Santos Bay using ground glass labelled with 192Ir
Injection
Points
(Figure 5)
Time
interval
considered
Depth related
to
hydrographic
Datum (m)
Bottom
sediment
transport
rate
Azimuth of
the sediment
transport
cloud axis
(kg/m/day)
PI1 WINTER
PI2 WINTER
PI3 WINTER
PI1 SUMMER
PI1 SUMMER
PI2 SUMMER
PI2 SUMMER
PI3 SUMMER
03/08/80 to
22/09/80
08/08/80 to
18/09/80
06/08/80 to
19/09/80
09/02/81 to
08/04/81
08/04/81 to
20/06/81
21/02/81 to
07/04/81
07/04/81 to
19/06/81
19/02/81 to
02/04/81
7.9
270
342°
12.3
350
335°
10.0
200
14°
7.6
10
350°
7.6
20
350°
10.9
40
339°
10.9
40
339°
9.3
10
349°
As a consequence, in spite of the greater depths relatively to the other regions, the bottom
sediment transport rate for the eastern region is also higher, both in winter and summer
regimes (Table 1). This could be the reason for not finding remaining activities in the region
of PI2 WINTER radioactive injection, before the injection performed in the summer season.
From this Case Study it is possible to evaluate how powerful radioactive tracers can be in
answering bottom and suspended sediment movement questions, provided a good knowledge
of the hydrodynamic conditions of the site under study is available.
3.2. Alumar harbour, São Luis, MA - Brazil [1, 22]
Alumar harbour (Figure 6) has been built to serve a huge aluminium factory near São Luis
City, capital of Maranhão State, in the north of Brazil. The access channel in Estreito dos
Coqueiros, linking the terminal to São Marcos Bay, the turning basin and the berthing area
were dredged in a region between two islands: Taua Mirim and São Luis. The currents in the
region are mainly tidal influenced and can reach values above 2.0m/s during spring tides. The
tide is semidiurnal presenting spring tidal amplitude of about 7.0m. The natural sediment
encountered on the bottom of São Marcos Bay is mainly sand.
INAC 2005, Santos, SP, Brazil.
Figure 6. Key map, Alumar harbour. After Aun & Bandeira [1]
Due to the high natural suspended sediment concentration (about 600mg/L), being silt and
clay its main constituent, a strong sedimentation occurs in the artificially enlarged and
deepened dredged regions. Thus, the dredged material is mainly silt and clay, except when
certain parts of the access channel, nearer to São Marcos Bay are dredged. Adequate natural
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depths are kept at this bay due to the high tidal currents, being this region the natural
dumping site for the disposal of the dredged material.
In order to improve dredging works, various studies employing nuclear techniques together
with hydraulic measurements, were performed. Labelling full charges of a trailing suction
hopper dredger and dumping the material in São Marcos Bay showed that the conditions for
the dispersion of the dredged material, dumped into the bay, were very favourable [27]. Very
little of the fine material discharged by the dredger remained on the sandy bottom at the site
of disposal, being resuspended by the high-speed currents. Besides that, the concentration of
the discharged particulates remaining in suspension soon reached the concentration level
naturally present in the bay. Consequently, the chosen disposal area could be changed to a
site closer to the entrance of Estreito dos Coqueiros, care being taken to avoid disposal
shortly before or after tide reversal, while currents are not strong enough.
Other studies were made at the harbour region [28] in an attempt to implant some sort of
agitation dredging in this area where the sediment deposition is quite important. In this case,
the fate of the dredged spoil depends on the position of the discharge site and the stage of the
tide. During flood tide the suspended material is carried either to the stretch of the Estreito
dos Coqueiros south of the harbour or to Rio dos Cachorros, which opens in the harbour
region (Figure 6), and most of it settles down to the bottom. When the dumping is performed
during ebb tide, the position of the discharge site makes a great difference. Tests were made
near the western and the eastern extremity of the harbour terminal. The trajectories of the
sediment are quite different in both cases, but the material tends to move to sites of
preferential sedimentation.
When discharge is done through a pipeline from a hydraulic suction dredger operating near
the berth, a considerable part of the dredged spoil sinks directly to the bottom due to the
influence of the initial downwards momentum and negative buoyancy of the slurry jet. If the
discharge is made parallel and just below to the surface water, the slurry remains in
suspension for a long time (figures 7 and 8). Since the tidal stage is of utmost importance for
the behaviour of the dredged spoil which remains in suspension after the disposal, much can
be gained in terms of abatement of sedimentation in the berthing area and in the turning
basin, by matching the discharge regime to the tidal stages.
Six suspended sediment injections of fine material labelled with 198Au were performed in
different points of the Estreito dos Coqueiros and the harbour turning basin (Figure 9). The
labelled material was discharged into the jet of a suction dredger (injections 3 and 4) or
directly, parallel and just below the water surface (injections 1, 2, 5 and 6). Due to the small
depths in some parts of the studied region, two scintillation detectors were placed at one
metre below the water surface, in a rigid pole fixed to the detection boat. It was possible to
compute the advection velocity u, the longitudinal DL and transversal DT dispersion
coefficients, the dilution, the sedimentation rate SR, the necessary time for the sedimentation
of half of the cloud T1/2 and L1/2 = u.T1/2 , for the clouds of all the experiments. Table 2
presents the results of some of these parameters, where the dispersion values are calculated
for the T1/2 instant.
INAC 2005, Santos, SP, Brazil.
Figure 7. Sketch of the set-up for tracer injection in the dredger jet.
After Moreira & Bandeira [28]
Figure 8. Behaviour of fine sediment dumped in the ebb phase.
After Moreira & Bandeira [28]
INAC 2005, Santos, SP, Brazil.
Figure 9. Injection points of suspended sediment labelled with
After Moreira & Bandeira [28]
INAC 2005, Santos, SP, Brazil.
198
Au.
Table 2. Main results of suspended sediment transport studies in Alumar Harbour
region using mud labelled with 198 Au
Injection Gauge
number
1
2
3
4
5
6
1
2
1
2
1
2
1
1
2
1
Tidal
ampl (m)
and
stage
6.6
ebb
5.8
flood/ ebb
3.9
ebb
3.2
flood
3.4
ebb
5.4
ebb/flood
Average
Advect.
velocity
(m/s)
0.75
0.22-0.63
0.32
0.38
0.55
Longitudinal
dispersion
DL (m2 /s)
2.15
2.76
0.88
0.19
0.10
3.27
Transversal Sedimendispersion tation rate
DT (m2 /s)
SR
(g/ton/s)
0.23
431
0.18
361
0.28
532
0.30
306
0.11
2289
0.10
2257
233
1.10
2.89
0.08-0.28
1.12
3.43
0.53
449
371
917
L1/2
(m)
896
1069
290
449
94
95
1123
794
961
68
It can be inferred from Table 2 and Figure 9 that the higher sedimentation rates correspond to
injections (3 and 6) being performed in situation of ebb flow, in the sheltered region of the
turning basin, relatively to the ebb flow coming from the southern reach of Estreito dos
Coqueiros. For this situation, in this region, there is the composition of the ebb flows coming
from Rio dos Cachorros and from the southern reach of Estreito dos Coqueiros, producing a
gyre of the cloud from an E-W movement to a S-N one, in a kind of vortex, facilitating the
sedimentation. Furthermore, the third injection, resulting in the higher sedimentation rate
(Table 2), was performed into the jet of the suction dredger operating in the berthing area.
Current measurements made during the rainy season [29] showed the existence of a null-drift
point in the lower water layer in the vicinity of the harbour, at spring tides, which strongly
enhances sedimentation. This is unlikely to occur during dry seasons since fresh-water
contribution to the system is very low at that time.
In-situ density measurements, using a nuclear transmission gauge developed in Brazil (Figure
1a), allowed an evaluation of the thickness of the fluid mud layer up to the bulk density of
1.3ton/m3 . The variations in this thickness clearly showed a preferential sedimentation in the
turning basin and a still stronger tendency to deposition in the berthing area, halfway from its
centre, towards its eastern extremity. These measurements were performed, in a monthly
basis, from 1983 to 1994, to control depth variations and to optimise dredging operations at
Alumar harbour [14, 19].
3.3. Pampulha reservoir, Belo Horizonte, MG - Brazil [3]
The Pampulha reservoir, in Belo Horizonte, Brazil, is in an accelerated process of decrease of
its liquid volume and water surface, due to an accretion of the order of 400,000m3 /year,
mainly caused by fine sediment [3]. As the reservoir is located in an urban area, there is no
suitable place to dump the dredged material on land, on a permanent basis. In this way, one
reliable possibility is the implementation of the transposition, by dredging (e.g. using a
INAC 2005, Santos, SP, Brazil.
hydraulic suction dredger), of the sediments to the rivers downstream the dam, their natural
way in the absence of the dam. This problem was the main motivation for the pioneer studies
to labelling the fine sediment with 99m Tc [7, 8, 9 and 10].
Field experiments, with simultaneous and instantaneous injections of sediment and water,
labelled, respectively, with 99m Tc and Rhodamine WT, were performed, in dry season, to
measure the hydrotransport capability in a stretch of 25km, since Pampulha creek until Rio
das Velhas, inclusive (Figure 10).
Figure 10. Key map of the studied region. Pampulha Hydrographic Basin and Velhas
River. After Bandeira [3]
A recent mathematical model was applied and calibrated to the data obtained and, through
convolution, the sediment dumping using the hydraulic suction dredger system was simulated
(Figure 11), calculating also, the physical environmental impacts: increase of sediment
concentration and its decrease by subsequent dilution and the possibility of deposition.
Through the measurement of physical-chemical parameters of the water, the possibility of
desorption of the metals adsorbed in the sediment to be dredged, was evaluated. It was
concluded that, from the studied aspects, there is no impediment for the dumping of the
dredged fine sediment in the watercourses downstream Pampulha reservoir.
INAC 2005, Santos, SP, Brazil.
D1
1,00
D3
0,90
D4
0,80
D5
0,70
D6
0,60
FOZ
0,50
D7
D8
0,40
D9
0,30
DP
0,20
Concentration attenuation
0,10
DPRS
DJEQ
0,00
0
100
200
300
400
500
600
700
Time interval (10 min.)
Figure 11. Temporal variation of the concentration downstream the Pampulha
reservoir, in function of the dredging dumping lasting 9 hours, in D1 (Figure 10).
After Bandeira [3]
4. CONCLUSIONS
The use of artificial tracers and nuclear gauges contributes for the optimisation of the
dredging works and allows the evaluation of the dumping sites and also the physical
environmental impacts of the dumping. Furthermore, artificial tracers allow calibrating
mathematical models for different aspects of sediment movement.
The nuclear gauges for measuring bulk densities can be used in various applications, such as:
• In reservoirs, allowing the improvement of the sedimentological balance of the lake
catchment/basin-reservoir system;
• In access channels, turning basins and berthing areas in harbours, allowing the addition of
the fluid mud layer, up to the bulk density of 1.2ton/m3 , to the keel clearance of the ships,
reducing the amount of sediment to be dredged;
• In the well of trailing suction hopper dredgers, allowing the evaluation of the load
efficiency of a particular dredging practice.
It is expected that the 99m Tc, broadly used in Nuclear Medicine could also be applied for
studies in continental, estuarine and coastal environments, such as those mentioned in
Chapter 3. This is due to its new use for labelling fine sediment [3] and its advantages in
relation to the use of 198 Au, as mentioned in the Item 1.1.2. Besides that, the concentration of
the 99m Tc activity (Bq/mL of water) is of the order of 10-7 of the concentrations used in
medical applications (Bq/mL of blood). The reason is that, for medical applications, the
detector is placed outside the patient while for environmental use the detector is placed inside
the water, in a “4π Geometry”, and this allows an enormous gain in counting efficiency.
INAC 2005, Santos, SP, Brazil.
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INAC 2005, Santos, SP, Brazil.