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A new technique for injecting fluorescent sand tracer in sediment transport experiments in a shallow water marine environment
Marine Geology, 33 (1979) M95--M98 M95 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
Letter Section A NEW T E C H N I Q U E F O R I N J E C T I N G F L U O R E S C E N T S A N D T R A C E R IN S E D I M E N T T R A N S P O R T E X P E R I M E N T S IN A S H A L L O W W A T E R MARINE ENVIRONMENT
B.J. LEES Institute o f Oceanographic Sciences (Taunton), Crossway, Taunton, Somerset (Great Britain) (Received May 21, 1979; revised and accepted August 3, 1979)
ABSTRACT
Lees, B.J., 1979. A new technique for injecting fluorescent sand tracer in sediment transport experiments in a shallow water marine environment. Mar. Geol., 33: M95--M98. A new method is described for the rapid injection of large quantities of fluorescent tracer sand onto the seabed in water up to 10 m or so deep. It has been used successfully in bedload sediment transport measurements, giving quantitative results comparable with those employing radioactive tracer. In particular the tracer was detected in significant quantities for more than half a year after injection.
INTRODUCTION Sand grains c o a t e d with a f l u o r e s c e n t d y e have b e e n used in s e d i m e n t t r a n s p o r t studies for over t w e n t y years. D i f f e r e n t t e c h n i q u e s have b e e n emp l o y e d to inject t h e t r a c e r o f f s h o r e , varying f r o m using a c h a m b e r i n c o r p o r a t ing a plunger which o p e n s o n striking t h e seabed (Jolliffe, 1 9 6 3 ) , placing t h e tracer in a plastic bag with an a u t o m a t i c knife blade device w h i c h o p e r a t e s o n reaching t h e seafloor ( V e r n o n , 1 9 6 6 ) , t o using w a t e r soluble bags (Stuiver and Purpura, 1968; Ingle and Gorsline, 1 9 7 3 ) . Ingle and Gorsline also m e n t i o n less elegant m e t h o d s such as using plastic bags t o be c u t o p e n b y divers, or divers using plastic pitchers to p o u r the t r a c e r o n t o t h e seabed surface. De Vries ( 1 9 7 3 ) describes a m e t h o d o f d e e p freezing the w e t t e d t r a c e r in tile f o r m and lowering the tiles to the seabed in a f r a m e . A s t u d y o f b e d l o a d t r a n s p o r t in t h e Sizewell-Dunwich Banks area o f f East Anglia in t h e s o u t h e r n N o r t h Sea r e q u i r e d t h e rapid injection o f 0.75 t o n n e o f f l u o r e s c e n t t r a c e r sand. T h e t e c h n i q u e s m e n t i o n e d above which were o p e r a t e d f r o m a ship c o u l d n o t readily c o p e with such a large q u a n t i t y and because o f p o o r s u b s u r f a c e visibility in t h e area diver d e p e n d e n t m e t h o d s were also to be avoided. A d i f f e r e n t t e c h n i q u e was devised and t h e results o f s u b s e q u e n t sampling over m o r e t h a n half a y e a r have s h o w n its usefulness. T h e t r a c e r used in t h e e x p e r i m e n t was a c o m m e r c i a l p r e p a r a t i o n p r o d u c e d by Feslente L i m i t e d , King's L y n n , G r e a t Britain. It is a washed and graded
M96
99.1% silica sand, of mean grain size 2.06 ¢ and standard deviation 0.51 ¢, chosen to match the sand on the seabed in the injection area. Feslente coat the sand with a red fluorescent dye which is fixed to the grains by means of a synthetic resin. EXPERIMENTAL PROCEDURE
The apparatus used is illustrated schematically in Fig.1. A slurry was made on deck in a water butt. The water supply rate was adjusted to keep the butt full, but not overflowing. The 0.75 tonne sand was added manually and the agitation produced by the water supply was adequate to keep the sand in suspension. A "Flygt" centrifugal pump forced the slurry out at the rate of 500 1 min -1, d o w n a 10-cm diameter vertical pipe lashed to the ship's rail, terminating 40 cm above the seabed (Fig.2). The pump in fact probably primed and regulated a siphon and such a system would be partially selfemptying. The pipe comprised standard and cut lengths of PVC pipe joined
~
FLUORESCENT TRACER
/•----•\1\ \
POWER CABLE-
FLEXIBLE HOSE AIRTIGHT CONNECTION
/
SHIP'S RAIL WATER BUTT TURBULENT MIXTURE OF TRACER SAND AND WATER
--VERTICAL PIPE TO SEABED
(2
HOSE TO PROVIDE NECESSARY WATER SUPPLY
"
0 I
0.5m I
I
I
I
J
APPROXIMATE SCALE
I
F LYGT" CENTRIFUGAL PUMP
DECK
Fig.1. Diagram to show apparatus for injection of fluorescent tracer slurry onto seabed.
~r
LASHINGS TO HOLD PIPE VERTICALLY AGAINST SHIP'S HULL
VERTICAL PIPE TERMINATING 4 0 c m s ABOVE SEABED,
0 -
-
Seabed
5
10 m I
SCALE
Fig.2. Diagram of RV "Edward Forbes" to show method of lashing pipe vertically against ship's hull.
M97 with O-rings supplemented with vinyl cement, while the hose and pipe join was effected by a rubber sleeve over the hose and inside the pipe. All the joins were reinforced with tape. This ensured t h a t no tracer escaped into the water column. The injection was carried out at night during high water slack in a depth of 7 m, two days after m a x i m u m spring tides. The sea was calm. The complete injection of the tracer sand t o o k only eight minutes, thus leaving some slack water time during which the material could settle. In the Sizewell-Dunwich area the water speed falls below 30 cm s -1 on average for t w e n t y minutes per half tidal cycle. Immediately after the injection the pipe was dismantled and the ship steamed due east for approximately 18 km. The deck area and anything which may have been in contact with the tracer were thoroughly hosed down and an ultraviolet lamp used to check that all fluorescent material had been removed. The ship then returned to the study area and the first post injection survey began. Previous experience and reference to the appropriate literature (Courtois and Monaco, 1968; Heathershaw and Carr, 1977) indicate that the mobile layer of sand on the seabed may be typically about 10 cm thick. In general calculation of sediment transport rates requires the total a m o u n t of tracer injected to be accounted for (de Vries, 1973). Consequently a grab rather than a sticky cards technique was used to sample the seabed in order to obtain information over the mobile layer. A Shipek grab was chosen for simplicity and reliability. It cuts a semi-cylindrical sample from the sediment with a m a x i m u m depth of 10 cm. Each sample from the post injection surveys was split into two sub-samples. One, of n o t more than 1 kg, was bagged and labelled for later laboratory analysis. The second sub-sample was spread out thinly and examined in the dark under ultraviolet light and the approximate number of fluorescent grains noted. This enabled the survey pattern to be modified as necessary. This procedure partially overcame the main disadvantage of fluorescent tracer compared with the radioactive kind, as with the latter the extent and intensity of the tracer cloud is measured in situ. Any remaining sample material was set aside for later disposal well away from the study area. Sampling positions were fixed using the Del Norte Trisponder which has a resolution of 1 m, and a range accuracy of + 3 m, and thus the tracer cloud could be delineated. With practice, and because of the close sampling interval, a sampling rate of thirty stations per hour was achieved. When the extent o f the tracer cloud had been established from the first post injection survey, box cores were taken in the area, using equipment similar in principle to a Reineck box corer. This was in order to measure the depth of burial of the fluorescent tracer. Subsamples were taken from the boxes to minimise disturbance effects, using 6-cm diameter plastic tubing. In the laboratory each grab sample was washed, sieved, dried and up to 120 g of the sand fraction separated and weighed. Smaller fractions were
M98
adequate for the samples with high tracer content. The fluorescent grains were then counted under ultraviolet light and concentrations calculated. This method offers an opportunity to discriminate for various fractions by sieving the samples and analysing these fractions separately (Komar, 1977). The results of the post injection surveys verify this procedure for examining the tracer plume. Calculations of the bedload transport rate have been made and the results will be published elsewhere. ACKNOWLEDGEMENTS
The project for which the technique was developed is financed by the Department of the Environment. The fact that the actual injection was made so smoothly and quickly was due entirely to teamwork, both in the early stages from the Institute of Oceanographic Sciences (Taunton) staff, and in the later one with additional constructive help from the master, officers and crew of the RV "Edward Forbes". The box corer used for depth of burial measurements was designed and operated by staff from the Ministry of Agriculture, Fisheries and Food (Lowestoft).
REFERENCES Courtois, G. and Monaco, A., 1969. Radioactive methods for the quantitative determination of coastal drift rate. Mar. Geol., 7: 183--206. Heathershaw, A.D. and Carr, A.P., 1977. Measurements of sediment transport rates using radioactive tracers. Coastal Sediments '77. American Society of Civil Engineers, Charleston, S.C., 20 pp. Ingle, J.C° Jr and Gorsline, D.S., 1973. Use of fluorescent tracers in the nearshore environment. Technical Reports Series No.145, International Atomic Energy Agency, Vienna. Jolliffe, I.P., 1963. A study of sand movements on the Lowestoft Sandbank using fluorescent tracers. Geogr. J., 129: 480--493. Komar, P.D., 1977. Selective longshore transport rates of different grain-size fractions within a beach. J. Sediment. Petrol., 47: 1444--1453. Stuiver, M. and Purpura, J.A., 1968. Application of fluorescent coated sand in littoral drift and inlet studies. Proceedings of 1 l t h Coastal Engineering Conference. Vernon, J.W., 1966. Shelf Sediment Transport System. Thesis, University of Southern California. Vries, M. de, 1973. Applicability of fluorescent tracers. Technical Reports Series No. 145, International Atomic Energy Agency, Vienna.
Letter Section A NEW T E C H N I Q U E F O R I N J E C T I N G F L U O R E S C E N T S A N D T R A C E R IN S E D I M E N T T R A N S P O R T E X P E R I M E N T S IN A S H A L L O W W A T E R MARINE ENVIRONMENT
B.J. LEES Institute o f Oceanographic Sciences (Taunton), Crossway, Taunton, Somerset (Great Britain) (Received May 21, 1979; revised and accepted August 3, 1979)
ABSTRACT
Lees, B.J., 1979. A new technique for injecting fluorescent sand tracer in sediment transport experiments in a shallow water marine environment. Mar. Geol., 33: M95--M98. A new method is described for the rapid injection of large quantities of fluorescent tracer sand onto the seabed in water up to 10 m or so deep. It has been used successfully in bedload sediment transport measurements, giving quantitative results comparable with those employing radioactive tracer. In particular the tracer was detected in significant quantities for more than half a year after injection.
INTRODUCTION Sand grains c o a t e d with a f l u o r e s c e n t d y e have b e e n used in s e d i m e n t t r a n s p o r t studies for over t w e n t y years. D i f f e r e n t t e c h n i q u e s have b e e n emp l o y e d to inject t h e t r a c e r o f f s h o r e , varying f r o m using a c h a m b e r i n c o r p o r a t ing a plunger which o p e n s o n striking t h e seabed (Jolliffe, 1 9 6 3 ) , placing t h e tracer in a plastic bag with an a u t o m a t i c knife blade device w h i c h o p e r a t e s o n reaching t h e seafloor ( V e r n o n , 1 9 6 6 ) , t o using w a t e r soluble bags (Stuiver and Purpura, 1968; Ingle and Gorsline, 1 9 7 3 ) . Ingle and Gorsline also m e n t i o n less elegant m e t h o d s such as using plastic bags t o be c u t o p e n b y divers, or divers using plastic pitchers to p o u r the t r a c e r o n t o t h e seabed surface. De Vries ( 1 9 7 3 ) describes a m e t h o d o f d e e p freezing the w e t t e d t r a c e r in tile f o r m and lowering the tiles to the seabed in a f r a m e . A s t u d y o f b e d l o a d t r a n s p o r t in t h e Sizewell-Dunwich Banks area o f f East Anglia in t h e s o u t h e r n N o r t h Sea r e q u i r e d t h e rapid injection o f 0.75 t o n n e o f f l u o r e s c e n t t r a c e r sand. T h e t e c h n i q u e s m e n t i o n e d above which were o p e r a t e d f r o m a ship c o u l d n o t readily c o p e with such a large q u a n t i t y and because o f p o o r s u b s u r f a c e visibility in t h e area diver d e p e n d e n t m e t h o d s were also to be avoided. A d i f f e r e n t t e c h n i q u e was devised and t h e results o f s u b s e q u e n t sampling over m o r e t h a n half a y e a r have s h o w n its usefulness. T h e t r a c e r used in t h e e x p e r i m e n t was a c o m m e r c i a l p r e p a r a t i o n p r o d u c e d by Feslente L i m i t e d , King's L y n n , G r e a t Britain. It is a washed and graded
M96
99.1% silica sand, of mean grain size 2.06 ¢ and standard deviation 0.51 ¢, chosen to match the sand on the seabed in the injection area. Feslente coat the sand with a red fluorescent dye which is fixed to the grains by means of a synthetic resin. EXPERIMENTAL PROCEDURE
The apparatus used is illustrated schematically in Fig.1. A slurry was made on deck in a water butt. The water supply rate was adjusted to keep the butt full, but not overflowing. The 0.75 tonne sand was added manually and the agitation produced by the water supply was adequate to keep the sand in suspension. A "Flygt" centrifugal pump forced the slurry out at the rate of 500 1 min -1, d o w n a 10-cm diameter vertical pipe lashed to the ship's rail, terminating 40 cm above the seabed (Fig.2). The pump in fact probably primed and regulated a siphon and such a system would be partially selfemptying. The pipe comprised standard and cut lengths of PVC pipe joined
~
FLUORESCENT TRACER
/•----•\1\ \
POWER CABLE-
FLEXIBLE HOSE AIRTIGHT CONNECTION
/
SHIP'S RAIL WATER BUTT TURBULENT MIXTURE OF TRACER SAND AND WATER
--VERTICAL PIPE TO SEABED
(2
HOSE TO PROVIDE NECESSARY WATER SUPPLY
"
0 I
0.5m I
I
I
I
J
APPROXIMATE SCALE
I
F LYGT" CENTRIFUGAL PUMP
DECK
Fig.1. Diagram to show apparatus for injection of fluorescent tracer slurry onto seabed.
~r
LASHINGS TO HOLD PIPE VERTICALLY AGAINST SHIP'S HULL
VERTICAL PIPE TERMINATING 4 0 c m s ABOVE SEABED,
0 -
-
Seabed
5
10 m I
SCALE
Fig.2. Diagram of RV "Edward Forbes" to show method of lashing pipe vertically against ship's hull.
M97 with O-rings supplemented with vinyl cement, while the hose and pipe join was effected by a rubber sleeve over the hose and inside the pipe. All the joins were reinforced with tape. This ensured t h a t no tracer escaped into the water column. The injection was carried out at night during high water slack in a depth of 7 m, two days after m a x i m u m spring tides. The sea was calm. The complete injection of the tracer sand t o o k only eight minutes, thus leaving some slack water time during which the material could settle. In the Sizewell-Dunwich area the water speed falls below 30 cm s -1 on average for t w e n t y minutes per half tidal cycle. Immediately after the injection the pipe was dismantled and the ship steamed due east for approximately 18 km. The deck area and anything which may have been in contact with the tracer were thoroughly hosed down and an ultraviolet lamp used to check that all fluorescent material had been removed. The ship then returned to the study area and the first post injection survey began. Previous experience and reference to the appropriate literature (Courtois and Monaco, 1968; Heathershaw and Carr, 1977) indicate that the mobile layer of sand on the seabed may be typically about 10 cm thick. In general calculation of sediment transport rates requires the total a m o u n t of tracer injected to be accounted for (de Vries, 1973). Consequently a grab rather than a sticky cards technique was used to sample the seabed in order to obtain information over the mobile layer. A Shipek grab was chosen for simplicity and reliability. It cuts a semi-cylindrical sample from the sediment with a m a x i m u m depth of 10 cm. Each sample from the post injection surveys was split into two sub-samples. One, of n o t more than 1 kg, was bagged and labelled for later laboratory analysis. The second sub-sample was spread out thinly and examined in the dark under ultraviolet light and the approximate number of fluorescent grains noted. This enabled the survey pattern to be modified as necessary. This procedure partially overcame the main disadvantage of fluorescent tracer compared with the radioactive kind, as with the latter the extent and intensity of the tracer cloud is measured in situ. Any remaining sample material was set aside for later disposal well away from the study area. Sampling positions were fixed using the Del Norte Trisponder which has a resolution of 1 m, and a range accuracy of + 3 m, and thus the tracer cloud could be delineated. With practice, and because of the close sampling interval, a sampling rate of thirty stations per hour was achieved. When the extent o f the tracer cloud had been established from the first post injection survey, box cores were taken in the area, using equipment similar in principle to a Reineck box corer. This was in order to measure the depth of burial of the fluorescent tracer. Subsamples were taken from the boxes to minimise disturbance effects, using 6-cm diameter plastic tubing. In the laboratory each grab sample was washed, sieved, dried and up to 120 g of the sand fraction separated and weighed. Smaller fractions were
M98
adequate for the samples with high tracer content. The fluorescent grains were then counted under ultraviolet light and concentrations calculated. This method offers an opportunity to discriminate for various fractions by sieving the samples and analysing these fractions separately (Komar, 1977). The results of the post injection surveys verify this procedure for examining the tracer plume. Calculations of the bedload transport rate have been made and the results will be published elsewhere. ACKNOWLEDGEMENTS
The project for which the technique was developed is financed by the Department of the Environment. The fact that the actual injection was made so smoothly and quickly was due entirely to teamwork, both in the early stages from the Institute of Oceanographic Sciences (Taunton) staff, and in the later one with additional constructive help from the master, officers and crew of the RV "Edward Forbes". The box corer used for depth of burial measurements was designed and operated by staff from the Ministry of Agriculture, Fisheries and Food (Lowestoft).
REFERENCES Courtois, G. and Monaco, A., 1969. Radioactive methods for the quantitative determination of coastal drift rate. Mar. Geol., 7: 183--206. Heathershaw, A.D. and Carr, A.P., 1977. Measurements of sediment transport rates using radioactive tracers. Coastal Sediments '77. American Society of Civil Engineers, Charleston, S.C., 20 pp. Ingle, J.C° Jr and Gorsline, D.S., 1973. Use of fluorescent tracers in the nearshore environment. Technical Reports Series No.145, International Atomic Energy Agency, Vienna. Jolliffe, I.P., 1963. A study of sand movements on the Lowestoft Sandbank using fluorescent tracers. Geogr. J., 129: 480--493. Komar, P.D., 1977. Selective longshore transport rates of different grain-size fractions within a beach. J. Sediment. Petrol., 47: 1444--1453. Stuiver, M. and Purpura, J.A., 1968. Application of fluorescent coated sand in littoral drift and inlet studies. Proceedings of 1 l t h Coastal Engineering Conference. Vernon, J.W., 1966. Shelf Sediment Transport System. Thesis, University of Southern California. Vries, M. de, 1973. Applicability of fluorescent tracers. Technical Reports Series No. 145, International Atomic Energy Agency, Vienna.