- Email: [email protected]
A new tracer technique for measuring bedload in natural channels
CATENA
Vol. 9, 77-80
Braunschweig 1982
A NEW TRACER TECHNIQUE FOR MEASURING BEDLOAD IN NATURAL CHANNELS
P. Ergenzinger & J. Conrady, Berlin
SUMMARY With a new tracer technique using permanent magnets measurements of coarse material bedload transport were made in a fiumara in Calabrien (South Italy). The movement of magnetic cobbles was determined by recording disturbances of the magnetic field at a measuring site. ZUSAMMENFASSUNG Mit Hilfe tier neuen Magnet-tracer-Technik wurden Messungen zum Grobgeschiebetransport in Kalabrien (Sfiditalien) durchgefiihrt. Die Bewegung von Magnetschottern wurtie am Mel3ort als St/Srsignal von einem station/iren Magnetfeld erfal3t. The investigations were financed by the "Deutsche Forschungsgemeinschaft", Bonn.
It is extremely difficult to determine the current amount of solid material transport in mountainous or arid region with a high amount of coarse material bedload. While the measurement of the concentration of suspended material is relatively easy, the rate of bedload can be determined only under very favourable conditions. The coarser the material the less suitable are the existing measuring devices and the well-known bedload transport functions. In order to determine the present rate of erosion of the Buonamico basin above Lago di Costantino (a fiumara in the Aspromonte Mountains of Calabria/South Italy) we tried to measure the water budget, suspended load and bedload during the winter months of 1978/79 and 1979/80. The only reliable technique for the determination of the present rate of bedload transport in the Aspromonte is the geodetic determination of the growth of the Buonamico delta at Lago di Costantino. The lake was formed in January 1973 on the upper side of a new landslide. Since then the development of the delta was regularly measured until 1980. We planned to use radioactive tracers to determine the flow and transport conditions during individual floods. However, the authorities in Reggio/Calabria were not prepared to issue the necessary permit. Instead the magnetic tracer technique was developed for the determination of cobble transport. The basic idea of the magnetic tracing of cobbles is simple. Whenever a permanent magnetic cobble passes the magnetic field of a fixed coil at a certain velocity an electronic
78
ERGENZINGER & CONRADY
signal is generated. On January 9th, 1980 a first field test was made in oder to try out this new technique on the Buonamico delta just above Lago di Costantino. The aim was to determine the hydraulic conditions at the start of cobble movement. Since there is no natural magnetic material in the Buonamico, we prepared 100 finegrained granitic cobbles with permanent bar magnets. The average weight of the cobbles was 243 g and their average size was as follows:
s
Length (ram)
Width (mm)
Thickness (mm)
72 9
55 8
38 7
The bar magnets (diameter: 6 mm, length: 25 mm) were inserted into holes drilled through the cobbles along the axis of thickness. Each magnet weighed 5.6 g and replaced about 2.9 g of rock material. The remaining cavity was filled with adhesive material. The weight ofthe cobbles was increased by approximately 3.1 g to about 246 g, i.e. an increase of about 1.3%. The optimal position of the magnet in the centre of gravity was not always attained. The equipment for the detection of the moving magnetic cobbles is a simple iron-detector circuit (Fig. 1). The magnetic field is achieved by a coil with 250 m m diameter holding 5000 m of copper wire (Cu 0.3). With these dimensions it was possible to detect the bar magnet in a circular field of 0.8 m diameter at a distance of 0.7 m. At the Buonamico it was sufficient to screw four coils on to aluminium girders. The aluminium frame was positioned 0.5 m above water level across the channel. The signals created as magnetic disturbances by the rolling and jumping cobbles passing beneath the coils were amplified and registered by two recorders.
I ~ M A G N E T
/','.~:~.--~',,-, ,
# o
,,
\',', \
// I# |! |
,,,
!
Iv L
I I ~ t I I s 0 O I I
I
,_,_,.,
I mV •
•
t
•
,
"
] sj # •
,,' ~
,,.,," ~
L 1 ....
LN
L:5000 .0=25 Cu =0.3
Fig. 1: Circuitdiagram ofthe measuring system
~
ECORDER
MEASURING BEDLOAD: A NEW TRACER TECHNIQUE
79
At the time of the test the channel of the Buonamico was 5 m wide and eroded below the delta surface to a depth of approximately I m. The average velocity was only slightly higher than 1 ms -~. Twenty magnetic cobbles were placed on the channel bottom 20 m above the measuring site at 1103a.m. and additional 80 cobbles at 1240 p.m. Owing to some snow thawing, the flow conditions varied during the test. While the water level rose only 1 cm from 25 to 26 cm the turbidity rose from 50 to close to 500 NTU units (compare Fig. 2). During the first test series only 5 magnetic cobbles were recorded, whereas we gained 46 signals during the afternoon. The recorded movements of magnetic cobbles indicated that there was a weak transport of cobbles even during this stage with relatively low velocities. Number
of tracer pebbles
registered
p e r 1/2
hour NTU UNITS "500
Number of n' registered pebbles
%..
I
i
\
i !
"1
t I ! ! !
I I ! !
i
I ! ! ! I I I
400
I ! t t t
300
t I 20
/
t I
/"
200
I
/' /
~suspended •
I I 10-
--6cm
load
f
/"
sS
'
100
"%%%
_ 5 0 5cm
1100
--7,
1130
1. In;u! of magnetic pebbles~ (n = 20)
120°
123° 1 13oo ! 2. Input: (n = 80)
1330
14°0
Fig. 2: The transport rate of magnetic pebbles at Buonamico (9. I. 1980)
143o
I
15oo
['
1
lauge
Time
80
E R G E N Z I N G E R & CONRADY
The bedload functions listed by GRAF (1971,156-158) were tested with the Buonamico data of January 9th, 1980 (slope: 0.040, depth: 0.25 m, width: 5 m, average velocity: 1.03 ms -I, s • 2.65, 1 : 1,p: 999.7 kg m-3). The results are listed in the following table: Bedload transport rate (kg m-ts-l):
d50:
d90:
7.9 m m 50.1 m m
Meyer-Peter
Schocklitsch
Kalinske
Einstein
0.001 -
0.005 -
-
0.008 0.001
The magnetic cobbles were only slightly bigger than d90. The results of the calculations are not encouraging. Given coarse material and steep slopes, bedload cannot be calculated adequately by means of the known bedload functions. In order to determine the relationship between the flow conditions and the sediment transport of coarse material more measurements are essential. The magnetic cobble technique provides good possibilities especially in fluvial basins with a certain amount of cobbles and boulders with natural magnetism.
REFERENCES
G ..RAF,W.H. (1971): Hydraulics of Sediment transport, New York. MUHLHOFER, L. (1933): Untersuchungen tiber Schwebstoff- und Geschiebefiihrung des Inns n~ichst Kirchbichl. Wasserwirtschaft, H. 1-6.
Anschrift der Autoren: Peter Ergenzinger und Jochen Conrady Freie Universi~t Berlin, Institut f'tirPhysische Geographie Grunewaldstr. 35, D-1000 Berlin 41
Vol. 9, 77-80
Braunschweig 1982
A NEW TRACER TECHNIQUE FOR MEASURING BEDLOAD IN NATURAL CHANNELS
P. Ergenzinger & J. Conrady, Berlin
SUMMARY With a new tracer technique using permanent magnets measurements of coarse material bedload transport were made in a fiumara in Calabrien (South Italy). The movement of magnetic cobbles was determined by recording disturbances of the magnetic field at a measuring site. ZUSAMMENFASSUNG Mit Hilfe tier neuen Magnet-tracer-Technik wurden Messungen zum Grobgeschiebetransport in Kalabrien (Sfiditalien) durchgefiihrt. Die Bewegung von Magnetschottern wurtie am Mel3ort als St/Srsignal von einem station/iren Magnetfeld erfal3t. The investigations were financed by the "Deutsche Forschungsgemeinschaft", Bonn.
It is extremely difficult to determine the current amount of solid material transport in mountainous or arid region with a high amount of coarse material bedload. While the measurement of the concentration of suspended material is relatively easy, the rate of bedload can be determined only under very favourable conditions. The coarser the material the less suitable are the existing measuring devices and the well-known bedload transport functions. In order to determine the present rate of erosion of the Buonamico basin above Lago di Costantino (a fiumara in the Aspromonte Mountains of Calabria/South Italy) we tried to measure the water budget, suspended load and bedload during the winter months of 1978/79 and 1979/80. The only reliable technique for the determination of the present rate of bedload transport in the Aspromonte is the geodetic determination of the growth of the Buonamico delta at Lago di Costantino. The lake was formed in January 1973 on the upper side of a new landslide. Since then the development of the delta was regularly measured until 1980. We planned to use radioactive tracers to determine the flow and transport conditions during individual floods. However, the authorities in Reggio/Calabria were not prepared to issue the necessary permit. Instead the magnetic tracer technique was developed for the determination of cobble transport. The basic idea of the magnetic tracing of cobbles is simple. Whenever a permanent magnetic cobble passes the magnetic field of a fixed coil at a certain velocity an electronic
78
ERGENZINGER & CONRADY
signal is generated. On January 9th, 1980 a first field test was made in oder to try out this new technique on the Buonamico delta just above Lago di Costantino. The aim was to determine the hydraulic conditions at the start of cobble movement. Since there is no natural magnetic material in the Buonamico, we prepared 100 finegrained granitic cobbles with permanent bar magnets. The average weight of the cobbles was 243 g and their average size was as follows:
s
Length (ram)
Width (mm)
Thickness (mm)
72 9
55 8
38 7
The bar magnets (diameter: 6 mm, length: 25 mm) were inserted into holes drilled through the cobbles along the axis of thickness. Each magnet weighed 5.6 g and replaced about 2.9 g of rock material. The remaining cavity was filled with adhesive material. The weight ofthe cobbles was increased by approximately 3.1 g to about 246 g, i.e. an increase of about 1.3%. The optimal position of the magnet in the centre of gravity was not always attained. The equipment for the detection of the moving magnetic cobbles is a simple iron-detector circuit (Fig. 1). The magnetic field is achieved by a coil with 250 m m diameter holding 5000 m of copper wire (Cu 0.3). With these dimensions it was possible to detect the bar magnet in a circular field of 0.8 m diameter at a distance of 0.7 m. At the Buonamico it was sufficient to screw four coils on to aluminium girders. The aluminium frame was positioned 0.5 m above water level across the channel. The signals created as magnetic disturbances by the rolling and jumping cobbles passing beneath the coils were amplified and registered by two recorders.
I ~ M A G N E T
/','.~:~.--~',,-, ,
# o
,,
\',', \
// I# |! |
,,,
!
Iv L
I I ~ t I I s 0 O I I
I
,_,_,.,
I mV •
•
t
•
,
"
] sj # •
,,' ~
,,.,," ~
L 1 ....
LN
L:5000 .0=25 Cu =0.3
Fig. 1: Circuitdiagram ofthe measuring system
~
ECORDER
MEASURING BEDLOAD: A NEW TRACER TECHNIQUE
79
At the time of the test the channel of the Buonamico was 5 m wide and eroded below the delta surface to a depth of approximately I m. The average velocity was only slightly higher than 1 ms -~. Twenty magnetic cobbles were placed on the channel bottom 20 m above the measuring site at 1103a.m. and additional 80 cobbles at 1240 p.m. Owing to some snow thawing, the flow conditions varied during the test. While the water level rose only 1 cm from 25 to 26 cm the turbidity rose from 50 to close to 500 NTU units (compare Fig. 2). During the first test series only 5 magnetic cobbles were recorded, whereas we gained 46 signals during the afternoon. The recorded movements of magnetic cobbles indicated that there was a weak transport of cobbles even during this stage with relatively low velocities. Number
of tracer pebbles
registered
p e r 1/2
hour NTU UNITS "500
Number of n' registered pebbles
%..
I
i
\
i !
"1
t I ! ! !
I I ! !
i
I ! ! ! I I I
400
I ! t t t
300
t I 20
/
t I
/"
200
I
/' /
~suspended •
I I 10-
--6cm
load
f
/"
sS
'
100
"%%%
_ 5 0 5cm
1100
--7,
1130
1. In;u! of magnetic pebbles~ (n = 20)
120°
123° 1 13oo ! 2. Input: (n = 80)
1330
14°0
Fig. 2: The transport rate of magnetic pebbles at Buonamico (9. I. 1980)
143o
I
15oo
['
1
lauge
Time
80
E R G E N Z I N G E R & CONRADY
The bedload functions listed by GRAF (1971,156-158) were tested with the Buonamico data of January 9th, 1980 (slope: 0.040, depth: 0.25 m, width: 5 m, average velocity: 1.03 ms -I, s • 2.65, 1 : 1,p: 999.7 kg m-3). The results are listed in the following table: Bedload transport rate (kg m-ts-l):
d50:
d90:
7.9 m m 50.1 m m
Meyer-Peter
Schocklitsch
Kalinske
Einstein
0.001 -
0.005 -
-
0.008 0.001
The magnetic cobbles were only slightly bigger than d90. The results of the calculations are not encouraging. Given coarse material and steep slopes, bedload cannot be calculated adequately by means of the known bedload functions. In order to determine the relationship between the flow conditions and the sediment transport of coarse material more measurements are essential. The magnetic cobble technique provides good possibilities especially in fluvial basins with a certain amount of cobbles and boulders with natural magnetism.
REFERENCES
G ..RAF,W.H. (1971): Hydraulics of Sediment transport, New York. MUHLHOFER, L. (1933): Untersuchungen tiber Schwebstoff- und Geschiebefiihrung des Inns n~ichst Kirchbichl. Wasserwirtschaft, H. 1-6.
Anschrift der Autoren: Peter Ergenzinger und Jochen Conrady Freie Universi~t Berlin, Institut f'tirPhysische Geographie Grunewaldstr. 35, D-1000 Berlin 41