Flow Measurement and Instrumentation 41 (2015) 10–17
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Comparison of real time suspended sediment transport measurements in river environment by LISST instruments in stationary and moving operation mode Stefan Haun a,n,1, Nils Rüther a, Sándor Baranya b, Massimo Guerrero c a
Department of Hydraulic and Environmental Engineering, The Norwegian University of Science and Technology, Trondheim, Norway Department of Hydraulic and Water Resources Engineering, Budapest University of Technology and Economics, Budapest, Hungary c Hydraulic Laboratory, DICAM Department, Bologna University, Bologna, Italy b
art ic l e i nf o
a b s t r a c t
Article history: Received 30 June 2014 Received in revised form 7 September 2014 Accepted 23 October 2014 Available online 31 October 2014
Suspended sediment and particle size measurements were in this study performed with two instruments based on laser diffraction. The measurements were conducted in the river Danube during a 1-year flood, which enabled measurements in a large scale and under conditions with high sediment load. The aim of this study was to compare LISST-SL stationary measurements with LISST-SL moving measurements to gain knowledge for extensive field surveys of large areas. The moving measurements were performed in four different depths along a transect and compared with stationary measurements in five verticals in the same depths. The results showed that the measured suspended sediment concentrations are smaller for the moving measurements compared to the stationary measurements. In addition thus the d50 was smaller for the moving measurements, which is an indicator for the phenomena of nonisokinetic sampling. A comparison of the pump speed of the two operation modes proofed that during LISST-SL moving measurements a non-isokinetic sampling effect occurred. Hence, a suction effect happened, resulting in an underestimation of particles 463 mm. Beside the LISST-SL simultaneously a LISST-STX was used during the field survey to investigate the behavior and the results of the instrument under conditions where it is originally not designed for. However, the predominant suspended sediment concentrations in the river influenced the measurements and thus multiple scattering occurred. As a consequence the measurements of the suspended sediment concentrations showed lower values for the measured suspended sediment concentrations compared to the LISST-SL stationary measurements. It could also be seen that due to the high amount of fine particles, which were transported during the flood event, a re-scattering of light occurred also in case of measurements where the value for the optical transmission was with 0.38 above the standardized used limit of 0.3. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Flood event Laser diffraction method LISST Particle size distribution River Danube Suspended sediment transport
1. Introduction An increasing demand on reliable data regarding the sediment transport is given in order to get a better understanding of the sediment transport mechanisms in rivers. At the moment representative data of suspended sediment transport is not available for a large number of rivers. The main reason for this fact is that the collection of suspended sediment data is still a challenging task in river engineering. In general direct and indirect sampling methods are widely used, where direct sampling methods can be subdivided into taking physical samples instantaneously (bottle/bag samples,
n
Corresponding author. Tel.: þ 4773594771. E-mail address:
[email protected] (S. Haun). 1 Postal address: S.P. Andersensvei 5, 7491 Trondheim, Norway.
http://dx.doi.org/10.1016/j.flowmeasinst.2014.10.009 0955-5986/& 2014 Elsevier Ltd. All rights reserved.
also collected by a pump sampler) or a time integrated sampling (point or depth integrated sampling). Nevertheless, in all cases a subsequent laboratory analysis is necessary. In addition an isokinetic sampling has to be ensured to avoid a deviation in the suspended sediment concentration (SSC) and in the particle size distribution (PSD) due to a differing momentum between the water and the sediments. In the last decades indirect sampling methods have been improved and developed, which are based on turbidity (bulk optics), acoustic backscatter, laser diffraction, digital optical imaging and on pressure differences [12]. Advances in technologies can especially be observed in optical and acoustic techniques. However, each method has to be suitable for the in-situ conditions of the studied river reach to give reliable and accurate results. The big advantages of using e.g. the laser diffraction method is that (a) no site specific calibration is necessary and that (b) next to the SSC also the PSD distribution can be evaluated directly with the device. The disadvantage is that with
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the existing instruments only point measurements are possible and these are in the case of using an anchored vessel time consuming. The measurements in this study were carried out by using a LISST-SL and a LISST-STX device, both based on laser-diffraction. The instruments measure simultaneously optical transmission, particle size distribution, suspended sediment concentration, depth and temperature [4]. The LISST-SL device measures also the flow velocity by a built in velocity sensor (pitot tube), which is the input for the pump speed and necessary for an isokinetic and correct sampling. Both instruments are designed to be used in a stationary operation mode e.g. from a bridge or an anchored vessel. For the evaluation of a river reach moving measurements would have the big advantage that a more detailed spatial pattern of the SSC and the PSD can be achieved within the same time. However, mainly laboratory studies and field surveys conducted, in estuaries and in marine environment, can be found where the LISST-100 or the LISST-STX were applied. Studies performed in laboratory flumes, rivers, lakes and reservoirs are limited and conducted by Refs. [8,19,25,27,23,16,14]. For the LISSTSL even less field surveys are published compared to the LISST-100 [2,5,15]. However, all above mentioned studies have the similarity that the LISST devices were used in a stationary operation mode. The objective of the study was to get better knowledge of the devices, especially the feasibility of using the LISST-SL device in a moving operation mode. In this paper detailed suspended sediment measurements along a transect in the Hungarian part of the river Danube are analyzed and presented to verify the deviation between results from moving measurements compared to stationary measurements. The measurements were performed during a one year-flood event from a vessel in moving and quasistationary mode. The in-situ conditions and the performed field measurements proofed to be a unique possibility for a comparison of the two modes. In addition a LISST-STX device was simultaneously used during the survey, which give additional data and enables a comparison with a device which is based on the same measurement principle, but designed in a different way. Where the LISST-SL is designed in a streamlined way (Cd value o0.2) which makes it so suitable for a use in rivers (for flow velocities in the range between 0 and 3 m/s), the LISST-STX is in contrast originally not built for such kind of measurements, due to the shape of the head of the instrument [21,22]. The LISSTSTX, is a special form of the LISST-100 series, but can be used similar to a LISST-100, taking into account the in-situ conditions (www.sequoiasci.com/article/using-the-lisst-stx-as-a-lisst100x). An overview about the measurement principle of the LISST devices and a short overview about the limitations of both instruments is given in Section 2. Results of the LISST-SL measurements, stationary and moving, are then presented and discussed in Section 3, followed by the results of the LISST-STX measurements, again for stationary and for moving measurements. The main findings are finally summarized at the end of the paper (Section 4).
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to the receiving lens at a particular angle. The light structure is in consequence printed on a silicon multi-ring photo-detector, located behind the receiving lens ([26,1,21,22]). The photodetector represents 32 small forward angles with logarithmically increasing radii. The rings cover a range from 2.1 to 381 mm for the LISST-SL and a range from 1.2 to 250 mm for the LISST-STX. The scattering intensity is recorded by the instrument, also known as volume scattering function, where due to the use of the mathematically inversion, the scattering intensity is used to convert the data into a PSD. A detailed description on the subject of the inversion process was presented by Hirleman [18]. In addition a photo-diode is placed behind a centered small hole in the detector, which makes it possible that the optical transmission of the laser beam is measured. The range of suspended particle concentrations, which can be measured by the instrument, is a function of the particle size (d) and the path length (l) of the collimated laser beam [4]. However, both LISST devices should in practice only be used for an optical transmission in the range between 0.3 and 0.98. In the case of an optical transmission 40.98 the sample volume contains too less particles, which means the sampling volume is too clear for a reliable result. On the opposite side values lower than 0.3 are influenced by multiple scattering effects, means the sample volume is too turbid. Multiple scattering refers to a rescattering of scattered light, which can be related to the optical transmission. Due to the strong dependency on the grain size distribution it is almost impossible to specify a sharp line for the SSC in the water, where the device is still suitable. It has to be noted that the laser diffraction based instruments do not measure the suspended sediment concentration directly, but measure volume concentration (VC). In order to get mass concentration the VC has to be multiplied by the particle density. In this study a constant value of 2650 kg/m3 was applied. Regarding measurable SSC ranges, in a previous conducted study by Traykovski et al. [26] a SSC, depending on the geometric mean grain size, of 150 mg/l for 5–25 mm particles and of about 500 mg/l for 25–65 mm particles was successful measured by a LISST-100, which is similar to the LISST-STX. This is about a value of 12.9d. It is possible to increase the limit of suspended concentration for the LISST-STX by using an additional lens to reduce the path length (50 mm in original). The LISST-SL is suitable for higher concentrations due to the shorter path length of only 3.2 mm. A measurable value for the SSC of up to 13,500 mg/l can be found on the manufacturer webpage. The laser diffraction method delivers the PSD equivalent to spheres sizes, where shape effects of grains are included in processing of the results in an empirical way (ISO standard natural particles; [3]). However, the mineral composition (shape) can still influence the results in a way which cannot be neglected and have therefore to be proved, e.g. if a high quantity of mica is part of the sediment mixture [10]. 2.2. Field site, measurement methodology
2. Instruments and methods 2.1. The LISST instruments The LISST-SL as well as the LISST-STX instrument use the technique of laser diffraction (Mie theory) to obtain the particle size-distribution (PSD) as well as the suspended sediment concentration (SSC). The laser diffraction method is an optical method and was invented in the 1970s (ISO standard: 13320-1; [4]). LISST is an abbreviation for Laser In-Situ Scattering and Transmissometry. The measurement principle is that a collimated laser beam is send through a sample volume, with a predefined path length of 3.2 mm for the LISST-SL and 50 mm for the LISST-STX. Particles, which cross the collimated laser beam, forward the scattered-light
The field measurements were conducted in the river Danube close to the city Esztergom, Hungary, at May 10th, 2013. The river Danube can be characterized in this section (middle Danube) as free-flowing with a mean flow of 2000 m3/s and a mean slope of 0.05–0.1 per mill [6]. The average width in this section is about 500 m and the average depth about 6 m. The bed material is dominated by mainly gravel and sand-gravel [7]. The average suspended sediment concentration is around 30 mg/l [24]. The measurements were performed in the falling limb of an approximately 1-year flood event (flow discharge about 3290 m3/s). The transect chosen for the comparison is about 380 m long and lays about 300 m downstream of the bridge Maria Valeria, connecting Hungary and Slovakia. The coordinates of the start- and end point of the transect are N471470 54.0″, E181430 40.9″ (left bank) and
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N471470 52.4″, E181430 58.9″ (right bank), respectively. The measurements were conducted from a small vessel for the stationary as well as for the moving measurements. For the fixed boat measurements five verticals were chosen along the transect (Fig. 1a and b). The measurements were conducted in each vertical in depths of 0.5, 1.5, 3.0 and 4.0 m for a period of 1 min. Time averaged characteristics of the suspended sediment were generated such as SSC and PSD. To ensure that a period of 1 min is sufficient enough, compared to e.g. 2 min used by Melis et al. [19], also longer periods (up to 5 min) were tested during the first day of the measurement campaign (May 07th, 2013). For e.g. vertical 2, depth 4 m the measurement period was extended to 2 min and showed a deviation smaller 1% for the SSC as well as for the PSD compared to the 1 min measurement. An evaluation of the data for a period of 30 s showed a deviation of about 2.5% for the SSC and about 4.4% for the PSD for the same point. In addition, a noise test was conducted to show the reliability of the measurement devices. The results showed values for the noise of the instruments smaller than 0.5 mg/l, which proved that the instruments worked properly and which leads to the conclusion that the noise can be neglected for the evaluation of this measurement series (only between 0.13 and 0.23% of the measured SSC). First the stationary measurements were conducted, starting in a depth of 0.5 m and from the right bank going to the left bank, and then the same procedure was repeated in a depth of 1.5, 3.0 and 4.0 m. Afterwards the moving measurements were conducted, starting again in a depth of 0.5 m. The moving measurements were conducted with a boat speed of 1.15 m/s, where the transect was crossed two times in each depth to exclude systematic errors. The two crossings along the transect were averaged for further comparisons. During post processing of the moving data the measurements were correlated to the locations of the stationary measurement points. The whole measurements cycle took about 103 min. As mentioned above, next to the LISST-SL also a LISST-STX was mounted on the moving vessel in a distance of
Fig. 1. (a) Location of the field site and transect [11]; (b) sketch of the chosen transect with the 20 sampling points for the stationary measurements. The moving measurements were conducted in the same depths (0.5, 1.5, 3.0 and 4.0 m) and repeated twice, starting from each bank once.
Fig. 2. Evaluated SSC and d50 of the LISST-SL stationary measurements.
about 1.5 m. Both devices were lowered by winches equipped with a depth meter into the relevant depths. All measurements were conducted simultaneously to enable an exact comparison of the results of the different devices.
3. Results and discussion 3.1. Results and comparison of the LISST-SL measurements 3.1.1. LISST-SL stationary measurements The results of the LISST-SL stationary measurements in the five verticals and in four different depths are presented in Fig. 2. The analysis shows that the SSC increases from the left bank (vertical 1) to the right bank (vertical 5) in the range between 87.2 mg/l (4.0 m depth) and 105.5 mg/l (1.5 m depth). In contrast to the increase in the SSC, the d50 decreases along the transect in the range between 4.1 mm (1.5 m depth) and 8.3 mm (4.0 m depth). The deviation between the concentration in 0.5 m depth and 4.0 m depth is in the range between 6.6 mg/l in vertical 4 and 21.6 mg/l in vertical 2. The concentrations were in all five verticals higher close to the surface (0.5 m) than close to the river bed (4.0 m). However, it has to be mentioned that the error bound of the measurement, due to the use of time averaged characteristics, is in about the same range as the measured deviation of the SSC over the depth (7.3–18.5 mg/l), with a slight tendency to higher error bounds for lower concentrations (vertical 1 and 2). The d50 was coarser for all five verticals close to the surface (0.5 m) compared to the measured values in a depth of 4 m (between 4.2 mm and 8.2 mm in vertical 4 and vertical 3, respectively). This slight decrease in the d50 over the depth is larger than the calculated standard deviation of the d50 which is in the range between 1.4 mm and 3.2 mm. The temperature was for all measurements in the range of 15.1 and 15.4 1C. The optical transmission was for the LISST-SL stationary measurements in the range of 0.92 and 0.96 and so within the limitation of the instrument. 3.1.2. LISST-SL moving measurements The results of the LISST-SL moving measurements (Fig. 3) show along the transect in overall the same pattern as it was observed from the stationary measurements. The differences in the SSC between vertical 1 and vertical 5 are in the range between 78.3 mg/l (3.0 m depth) and 105.2 mg/l (1.5 m depth) and so compared to the stationary measurements slightly smaller. Again coarser particles were measured at the left bank of the transect. The deviation in the d50 was between 1.9 mm in 1.5 m depth and 7.4 mm in 4.0 m depth comparing the left and the right bank. For the moving operation mode all measured SSC values are higher in a depth of 4 m compared to the values at the surface (0.5 m). The deviation between the measured SSC in 0.5 m depth and 4.0 m
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depth is in the range between 6.1 mg/l (vertical 3) and 40.8 mg/l (vertical 1) and thus larger than in the stationary measurement. Like in the stationary measurements a decrease in the d50 over the depth was measured in all five verticals (between 1.5 mm in vertical 2 and 6.8 mm in vertical 5). The temperature was for all measurements almost constant and in the range of 15.1 and 15.5 1C. The optical transmission was for the LISST-SL moving measurements in the range of 0.92 and 0.97 and so within the limitation of the instrument. Fig. 4 shows the measured concentrations along the transect by the LISST-SL device in moving operation mode with the related
Fig. 3. Evaluated SSC and d50 of the LISST-SL moving measurements.
Fig. 4. SSC as result of the moving LISST-SL measurements, with the 5 verticals were the point measurements were conducted. Each moving measurement was repeated twice (1 and 2) and averaged for the final evaluation.
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verticals where the stationary measurements were performed. The figure illustrates the trend that the SSC increased from the left to the right bank. It can be seen that the lowest concentrations in the transect as well as the highest magnitude was between vertical 1 and the left bank and between vertical 5 and the right bank, respectively. Close to the left bank a small increase of the SSC and close to the right bank a decrease of the SSC was measured. These effects may be explained by the fact that the vessel decreased the speed at the end of the transect. Apparently, the LISST-SL device started as consequence to oscillate due to the change in the drag force. However, this phenomenon was not further analyzed in the field. The results of the moving measurements enhance once more the importance of the cross section calibration when the suspended sediment load for the entire cross section is to be studied based on single sediment measurements [12]. By comparing the results of the measurements from the stationary and moving measurements the increasing trend in the SSC from the left to the right side can clearly be seen. However, the moving measurements showed in general lower values for the SSC compared to the stationary measurements (Fig. 5a). The deviation in the SSC is in the range between 4.1% (vertical 5, depth 4.0 m) and 24.5% (vertical 1, depth 0.5 m). A specific trend that particular measurements (position in the transect or measured depth) have a smaller or larger deviation cannot be seen from the results. The evaluation of the d50 showed a coarser value for all stationary measurements compared to the moving measurements (between 0.7% difference in vertical 3, 4 m depth and 23.9% in vertical 3, 3 m depth; Fig. 5b). The difference in the d50 between the moving and the stationary measurements was in all five verticals along the transect smallest in a depth of 4 m (marked in Fig. 5b by an ellipse), where a similar trend can not be observed for the evaluated SSC. A further subdivision of the LISST-SL data showed that the differences in the SSC and also in the PSD can be clearly traced back to the fraction of fine sand particles. In Fig. 6 the percentage of the measured sand particles (particles 463 mm) as percentage of the total SSC are compared. In almost all measurements (except vertical 1, 3.0 and 4.0 m depth and vertical 3, 4.0 m depth) the percentage of particles 463 mm is smaller for the moving measurements compared to the stationary measurements, which leads to the conclusion that during the moving measurements too little sand particles were measured, which as consequence influences the total SSC. In the above mentioned three points where the percentage of sand particles is higher for the moving measurements the deviation in the SSC is below 10% and thus lower than in the other points.
Fig. 5. (a) Comparison of the measured SSC as result of the stationary and the moving measurements with the LISST-SL in 5 verticals and in 4 different depths along the transect; (b) comparison of the measured d50 as result of the stationary and the moving LISST-SL measurements, in the same verticals and depths.
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The differences in the measured SSC and especially in the PSD could be explained with a suction effect during sampling. This suction effect leads to a separation of the coarser and finer material. The drag force of this suction leads to an underestimation of the coarse material and to a reduced measured SSC. This effect is illustrated in Fig. 7. Here Edwards and Glysson [9] showed that once the flow velocity Vwater is lower than the pump velocity Vpump, the SSC is not measured iso-kinetically and therefore object to underestimations. An evaluation and comparison of the recorded velocities during the measurements showed that the pitot tube of the LISST-SL measured higher flow velocities for the moving measurements compared to the stationary measurements. The pump speed of the LISST-SL, which adapts to the flow velocities, showed flow velocities in the range between 1.29 m/s and 1.64 m/s for the stationary measurements and in the range between 1.68 m/s and 2.26 m/s for the moving measurements (Fig. 8a). This indicates that a nonisokinetic sampling was performed during the moving measurements. In addition the percentages of the differences between the pump speed for the moving and the stationary measurements were plotted against the percentage of the difference between the SSC for
the moving and the stationary measurements (Fig. 8b). The plot indicates a correlation between the difference in the measured SSC and in the difference of the pump speed. A linear trend line shows that for a velocity difference of 10% a difference in then SSC of about 8.8% has to be assumed, followed by a further successive increase in the concentration difference (50% velocity difference lead to a difference in the SSC of 17.1%). The difference in the pump speed is a result of the moving velocity of the vessel, which was 1.15 m/s during the moving measurements. The measured flow velocity during the moving measurement is hence a resultant out of the flow velocity of the river and the moving velocity of the vessel. During post processing of the data also other possible factors, which could have influenced the moving measurements were investigated, e.g. different depths between the stationary and the moving measurements or a shift in the coordinates which would result in different locations for the measured points for the two operation modes. However, these errors can be excluded. Also the optical transmission was controlled and very similar values between 0.92 and 0.97 for moving and stationary LISST-SL measurements were found, which will not lead to multiple scattering. The measured data of the LISST devices were in addition sent to the manufacturer of the instrument to confirm the functionality of the device and the quality of the measurements. Next to the calibration settings especially settings which are related to an accurate isokinetic sampling were checked, e.g. the length of the pump operation and the pump voltage, which can be related to the velocities measured by the pitot tube. From the feedback can be concluded that the LISST-SL stationary measurements are reliable, accurate and from a high quality. It can therefore be assumed that no instrument specific error occurs and that no uncertainties are introduced by the instrument into the results. 3.2. Results and comparison of the LISST-STX measurements
Fig. 6. Percentage of coarse particles ( 463 mm) for stationary and moving LISST-SL measurements along the transect.
Fig. 7. Scheme of a non-isokinetic sampling (based on [9]).
3.2.1. LISST-STX stationary measurements The results of the LISST-STX stationary measurements in the five verticals and in the four different measured depths are presented in Fig. 9. The SSC increases also for the LISST-STX measurements from the left bank to the right bank, in the range of 93.3 mg/l (0.5 m depth) and 107.4 mg/l (3.0 m depth). Again coarser particles were measured at the left bank. The difference in the d50 was between 7.1 mm in 1.5 m depth and 9.8 mm in 0.5 m depth. Almost no change in the SSC over the depth occurs during the measurements. The deviation between the SSC in 0.5 m depth and 4.0 m depth is in the range between 0.3 mg/l in vertical 2 and 13.1 mg/l in vertical 5. In all five verticals the SSC was slightly
Fig. 8. (a) Comparison of the pump speed of the LISST-SL stationary and moving measurements; (b) difference in the pump speed of the LISST-SL compared the difference in the concentrations between the stationary and the moving measurements.
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Fig. 9. Evaluated SSC and d50 of the LISST-STX stationary measurements.
higher close to the bed (4.0 m depth). Similarly to the LISST-SL measurements, it has also here to be mentioned that the error bound of the measurement is in about the same range as the measured deviation of the SSC over the depth (4.3–9.7 mg/l), without a tendency to higher error bounds for a certain vertical. The d50 shows in all five verticals about the same value over the depth. However, the slight increase in the d50 over depth which can be seen in the verticals 2 (0.9 mm), 3 (0.5 mm) and 5 (1.2 mm) are below the calculated standard deviation, which is in the range between 1.3 mm and 2.5 mm. In vertical 4 the opposite trend could be observed, where the d50 is about 0.4 mm larger at the surface compared to the measurement in 4 m depth. This vertical showed the highest calculated standard deviation with a value of up to 3.6 mm in 4 m depth. The temperature was for all measurements almost constant and in the range of 14.9 and 15.1 1C. The optical transmission was for the LISST-STX stationary measurements close to the limit of the instrument (0.3) and beneath. The measurements in vertical 1 (0.36–0.37) and vertical 2 (0.33) showed values where the instrument should theoretically work accurate, where the optical transmission in vertical 3 (0.28–0.29), vertical 4 (0.26–0.27) and vertical 5 (0.23–0.24) are outside the limitation of the instrument.
3.2.2. LISST-STX moving measurements The results of the LISST-STX moving measurements (Fig. 10) show a similar pattern compared to the LISST-STX stationary measurements. The differences in the SSC between vertical 1 and vertical 5 are in the range between 98.6 mg/l (3.0 m depth) and 110.7 mg/l (4.0 m depth) and so slightly higher compared to the stationary measurements. However, a decreasing trend of the d50 along the transect, as it can be seen from the stationary measurements, could not be seen in all depths anymore. In a depth of 0.5 m (0.1 mm), 3.0 m (3.9 mm) and 4.0 m (7.5 mm) a decrease happens along the transect, where in a depth of 1.5 m an increase (1.7 mm) was observed. The change in the SSC over the depth (0.5 m depth compared to 4.0 m depth) is quite small and in the range between 0.1 mg/l (vertical 2) and 14.4 mg/l (vertical 4), where a higher SSC is measured mainly in 0.5 m depth. Only in vertical 5, a higher concentration was measured in the lower depth (3.0 mg/l difference). There is also no clear trend visible by comparing the d50 in 0.5 m depth and in 4.0 m depth. Where in verticals 1 (4.1 mm), 3 (2.1 mm) and 4 (3.6 mm) the d50 increases over depth, a decrease can be seen in the verticals 2 and 5 (1.7 mm and 3.3 mm, respectively). The temperature was for all measurements almost constant and in the range of 15.0 and 15.4 1C. The optical transmission was for the LISST-STX moving measurements again close to the limit of the instrument (0.3) and beneath. The measurements in vertical 1 (0.35–0.38) and vertical 2 (0.30–0.35) showed values where the
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Fig. 10. Evaluated SSC and d50 of the LISST-STX moving measurements.
Fig. 11. SSC as result of the moving LISST-STX measurements, with the 5 verticals were the point measurements were conducted. Each moving measurement was repeated twice (1 and 2) and averaged for the final evaluation.
instrument should theoretically work accurate, where vertical 3 (0.26–0.30), vertical 4 (0.23–0.26) and vertical 5 (0.22–0.24) are outside the limitation of the instrument. Fig. 11 illustrates the measured SSC for the moving operation mode of the LISST-STX with the related verticals, where the stationary measurements have been performed. Again, an increase of the SSC along the transect can be seen from the left to the right bank. By comparing Figs. 4 and 11 it can be seen that the results of the LISST-STX showed a far smoother pattern along the transect and a smaller magnitude in the concentrations compared the LISST-SL measurements. Also a comparable but smaller increase in the concentrations close to the left bank and a decrease of the SSC close to the right bank is shown in Fig. 11. The lowest concentrations in the transect was measured between vertical 1 and the left bank, where the highest concentration was measured almost in vertical 5. The results for both operation modes of the LISST-STX showed only a small shift in the concentrations between the stationary and the moving measurements (between 2.6 and 9%, in vertical 4, 0.5 m depth and vertical 2, 1.5 m depth, respectively; Fig. 12a), with a slightly but not significant higher deviation for the lower measured concentrations (vertical 1 and 2). The measurements showed for almost all point measurements (15 out of 20) a coarser d50 (Fig. 12b) for the stationary measurements compared to the moving measurements (between 6.4%, vertical 5, 0.5 m depth and 18.9% in vertical 2, 1.5 m depth). Due to the design of the head of the instrument, the LISST-STX is made for small flow velocities. In addition, it is not able to collect measurements iso-kinetically due to the absence of an internal suction pump with variable speed. The change in the orientation of the LISSTSTX head between the stationary and the moving measurement may explain the difference in the PSD and in consequence also the relative difference in the SSC. However, the difference in the SSC between the
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Fig. 12. (a) Comparison of the measured SSC as result of the stationary and the moving measurements with the LISST-STX in 5 verticals and in 4 different depths along the transect; (b) comparison of the measured d50 as result of the stationary and the moving LISST-STX measurements, in the same points and depths.
LISST-STX measurements (stationary and the moving) and the results provided by the LISST-SL stationary measurements (Fig. 2), which are the basis for a comparison, cannot be explained by the orientation of the head. These differences can be traced back to the optical transmission and are thus a consequence of the phenomena of multiple scattering. Multiple scattering refers to a re-scattering of scattered light, and starts for values of the optical transmission under 0.3 [26]. This limit was exceeded for the LISST-STX measurements in the verticals 3–5, with a minimum value for the optical transmission in vertical 5 (0.22; highest SSC). However, multiple scattering effects influenced also the results in verticals 1 and 2, where the values for optical transmission where above the given limit (e.g. 0.38 for vertical 1, depth 1.5 m, moving operation mode). The reason why multiple scattering also occurs for values above 0.3 was in this case the majority of fine material, which was transported during the flood event [26]. Theoretical studies showed that multiple scattering effects result in a bias toward to small particles [1]. The data showed a 2.4–2.5 times higher clay and fine silt content in the LISST-STX stationary measurements, compared to the LISST-SL stationary measurements for vertical 5, in the depths of 4 m and 0.5 m, respectively. This effect agrees quite well with the description of multiple scattering [1]. Unfortunately, the deviation in the SSC and the PSD due to multiple scattering effects cannot be set in direct relation to the optical transmission. It could be seen from previously conducted studies with the LISST, e.g. Haun et al. [15] and Haun and Lizano [17], that if the majority of the occurring particles in the water body is in the range of clay and silt, the results of the measurements show smoother pattern compared to cases when e.g. fine sand particles are present. The occurring bias toward to small particles due to multiple scattering effects may increase this effect and lead to a fairly smooth distribution as it can be seen from Fig. 11. In general, an increase of the SSC and d50 towards to the river bed is expected in accordance to the concentration profile by Rouse [20]. However, the results of the LISST measurements show an almost uniform distribution of the SSC as well as the d50 over the depth. This may be a consequence of the occurring flood event and the high amount of fine sediments which were transported during the surveys. For the investigated transect in this study the lowermost measurements were performed in a depth of 4 m, so that the LISST devices could be used in moving mode without a possible contact with the river bed. Hence, no near bed velocity measurements are available and so an evaluation of the shear velocity (un), which would be necessary to evaluate a Rouse number, is not straightforward. However, additional investigations were conducted further downstream of the river by means of LISST and ADCP measurements the days before. During this measurements next to the SSC also the flow velocities were assessed, similar to the measurements conducted by Baranya and
Józsa [7] and Guerrero et al. [13] for the rivers Danube and Paraná, respectively. The evaluation of the data from May 7th 2014 showed values for un in the range of 0.088 to 0.117 m/s, which were further used to evaluate the Rouse number in different points. From the results it could be seen that the Rouse number is smaller than 0.8 for the measured particles sizes in the range between 2.1 to 181 mm (about 98% of the measured sediments are in this range). This is a clear indicator for wash load as mode of transport. The particles in the range between 213 to 381 mm showed a Rouse number 40.8 and o1.2 and are transported in accordance to Rouse as suspended sediments.
4. Conclusions In this study a LISST-SL and a LISST-STX device were used to measure the SSC and the PSD in the river Danube during a 1-year flood. The aim of the study was to compare the results of stationary and moving measurements to see if the devices can be used for extensive field survey in the future. The moving measurements were carried out along a specified transect in depths of 0.5, 1.5, 3.0 and 4.0 m. In addition, stationary measurements in five verticals at the above mentioned depths were conducted. The analysis showed that the LISST-SL stationary measurements gave a higher value for the SSC compared to the moving measurements (a difference of up to 24.5% was evaluated). The results showed in addition that the PSD (d50) is coarser for the stationary operation mode than for the moving ones (a difference of up to 23.9% was evaluated). These differences in the results can be explained by comparing the flow velocities measured by the pitot tube during the stationary and the moving measurements. While during the stationary measurements the pump speed is regulated in accordance to the measured flow velocity of the river, for the moving measurements the pump speed is adjusted to the measured resultant velocity of the river and the boat. As consequence, this leads to a non-isokinetic sampling. A difference of up to 56.9% in the pump speed was found by comparing the two different operation modes. A further comparison between the stationary and the moving measurements showed that a difference of 10% in the pump speed caused a difference in the SSC of about 8.8%. An almost linear trend could be observed, where the maximum measured velocity difference of 56.9% leads to a difference in the SSC of 18.6%. However, it may also be assumed that these results are site specific with a strong dependency on the predominant conditions in the river during the flood event (PSD and SSC). The LISST-STX instrument was used simultaneously to the LISST-SL. Only a slight shift in the results of the SSC measurements can be observed by comparing the two operation modes of the LISST-STX (a difference of up to 9.0% was evaluated). However, significantly lower SSC were measured for the moving as
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well as for the stationary measurements compared with the LISST-SL stationary measurements (a difference of up to 17.5% was found for the stationary and 23.9% for the moving measurements). The analysis of the data showed that the optical transmission for the LISST-STX measurements (stationary and moving) was in the range between 0.22 and 0.38, where from existing literature can be seen that the critical value for multiple scattering is 0.3. The measurements in three verticals showed lower values and were thus directly influenced by the effect of a re-scattering of scattered light. As result a bias toward the fine particles could be proofed. However, an interesting finding was that also in the other two verticals, where values of up to 0.38 were recorded for the optical transmission, multiple scattering effects were detected. This effect is a result of the in-situ conditions (SSC and PSD) during the flood event where a high amount of fine grain sizes were transported in the river. For the LISST-SL multiple scattering did not proof to be a problem due to the shorter path length of the laser (3.2 mm compared to 50 mm for the LISST-STX). From the field survey can be seen that the SSC as well as the PSD changed significantly along the chosen transect. This proofs once more the necessity of a cross sectional calibration, when evaluating suspended sediment data of a natural river reach. This study showed that the development of instruments and methods, which increase the resolution from only single point measurements to e.g. a contour plot of the transect, is an essential measure in river engineering.
Acknowledgements The authors gratefully acknowledge the support given by Sequoia Scientific, Inc., especially Yogi Agrawal for sharing his knowledge and for his input to this paper. We acknowledge also the funding of Sándor Baranya from the János Bolyai fellowship of the Hungarian Academy of Sciences. A further thank goes to the technical staff of the Department of Hydraulic and Water Resources Engineering, Budapest University of Technology and Economics for the support during the field measurements. References [1] Agrawal YC, Pottsmith HC. Instruments for particle size and settling velocity observations in sediment transport. Mar Geol 2000;168:89–114. [2] Agrawal, YC, Pottsmith, HC. The isokinetic streamlined suspended-sediment profiling LISST-SL—status and field results. In: Proceedings of the eighth federal interagency sedimentation conference (8thFISC). Reno, USA; 2006. [3] Agrawal YC, Whitmire A, Mikkelsen OA, Pottsmith HC. Light scattering by random shaped particles and consequences on measuring suspended sediments by laser diffraction. J Geophys Res 2008:113. [4] Agrawal, YC, Mikkelsen, OA, Pottsmith, HC. Sediment monitoring technology for turbine erosion and reservoir siltation applications. In: Proceedings HYDRO 2011 conference. Prague, Czech Republic; 2011. [5] Agrawal, YC, Mikkelsen, OA, Pottsmith, HC. Grain size distribution and sediment flux structure in a river profile, measured with a LISST-SL instrument. Report,
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