Proceedings of Meetings on Acoustics - ICA 2013 Montreal

Moore et al.

Proceedings of Meetings on Acoustics Volume 19, 2013

http://acousticalsociety.org/

ICA 2013 Montreal Montreal, Canada 2 - 7 June 2013 Acoustical Oceanography Session 1aAO: Estuarine Acoustics 1aAO3. Acoustic measurements of the spatial distribution of suspended sediment at a site on the Lower Mekong River Stephanie A. Moore*, Guillaume Dramais, Philippe Dussouillez, Jérôme Le Coz, Colin Rennie and Benoît Camenen *Corresponding author's address: Civil Engineering, University of Ottawa, 161 Louis Pasteur St, OTTAWA, K1N6N5, Ontario, Canada, [email protected] The Mekong River spans thousands of kilometers, flows through six countries and its basin is one of the world's richest in terms of biodiversity. However, land-use changes, dredging of the river bed and the construction of dams are changing its sediment dynamics and morphology, with consequences such as increased bank erosion and reduction in sediment supply to floodplains. In order to monitor these changes, the current conditions must be well understood. Comprehensive measurements of the spatial distribution of sediment (both suspended and bed load) were made at three locations in different physiographic regions of the Lower Mekong at the end of the 2012 rainy season. Data are presented from the Luang Prabang site. Acoustic Doppler Current Profilers and a multi-frequency acoustic backscatter system, the AQUAscat, were used in combination with water sampling to provide high resolution measurements of concentration and grain size. The AQUAscat consisted of four monostatic transducers operating at 0.5, 1, 2.5 and 4 MHz. It was deployed horizontally at four across-stream positions and 4-6 depths per vertical; a 10 m profile was recorded at each point. Particle size and concentration are determined from the multifrequency attenuation data. This data set provides a baseline for future measurements. Published by the Acoustical Society of America through the American Institute of Physics

© 2013 Acoustical Society of America [DOI: 10.1121/1.4799128] Received 22 Jan 2013; published 2 Jun 2013 Proceedings of Meetings on Acoustics, Vol. 19, 005003 (2013)

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I NTRODUCTION The Mekong River Basin contains the world’s largest inland fisheries and is second only to the Amazon in terms of biodiversity. However, the river is being affected by anthropogenic activities, as the demand for electricity (hydropower), building supplies (sand extractions) and food (fishing) increases with Southeast Asia’s growing population. Land-use changes, sediment extractions and the construction of dams on the Mekong and its tributaries all impact the river’s sediment transport and morphology, with potential negative consequences such as increased bank erosion and a reduction in nutrient-rich sediments to the floodplains. In order to monitor these changes, the current conditions must be well understood. The four member countries of the Mekong River Commission (MRC), Cambodia, Lao PDR, Thailand and Viet Nam, regularly monitor flow rates and discharge at numerous stations. They also measure the concentration of suspended sediment at certain sites, but until recently sediment sampling consisted primarily of sporadic surface water samples (Walling, 2005). This is insufficient for sediment budget calculations and for the study of the river’s morphology since the sediment near the surface (the wash load) is composed of fine sediments (silt and clay) that are generally carried by the river but are not easily deposited, while it is mostly fine sands that contribute to bed material load. Flow-proportional depth integrated suspended sediment sampling is currently the standard practice at the MRC’s Discharge and Sediment Monitoring Programme’s sites (Koehnken, 2012). However, since the suspension of sand likely varies in the vertical, a significant issue for the Mekong River is to better characterise vertical concentration profiles of sand particles. Sediment fluxes and the spatial distribution of particle size and concentration were measured at three locations in different physiographic regions of the Lower Mekong at the end of the rainy season in September 2012. This project was a collaboration between CEREGE, Irstea, the University of Ottawa, the World Wildlife Fund (WWF), the MRC and its member countries. From upstream to downstream, the sites were Luang Prabang (Lao PDR), Khong Chiam (Thailand) and Kratie (Cambodia); they are indicated on the map of the Lower Mekong Basin in Figure 1. Since intensive water sampling is time consuming, acoustic instruments were used to provide remote measurements of both flow speed and sediment distribution with high spatial and temporal resolution. Water samples collected at distinct depths were analyzed for grain size and concentration in order to calibrate the acoustic data. In this paper we present a selection of the data collected at Luang Prabang with the AQUAscat 1000R, which is a multi-frequency, high-resolution acoustic backscatter system that was deployed in a horizontal configuration. These data can be used to determine the grain size and concentration of suspended sediment using either the backscattered intensity directly, or the attenuation values calculated from the intensity profiles. In this paper only the attenuation method will be presented.

Proceedings of Meetings on Acoustics, Vol. 19, 005003 (2013)

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F IGURE 1: Discharge and sediment monitoring stations of the Information and Knowledge Management Programme of the Mekong River Commission (MRC). The three sites indicated with arrows were studied in this project and the measurements presented in this paper are from Luang Prabang. Figure taken from Koehnken (2012).

T HEORY The acoustic backscatter systems that are used to monitor flow speeds and suspended sediment concentrations are typically composed of monostatic piezoelectric transducers that operate over a narrow bandwidth. The backscattered intensity from a suspension of particles, such as sediment suspended in a river, largely depends on (1) the concentration of particles and (2) the product of the size of the particles and the wavenumber of the incident wave, ka, where k is the incident wavenumber and a is the particle radius. For ka < 1, the backscatter increases with ka. The backscatter decreases with range from the instrument due to spherical spreading and to attenuation of the signal by the water and by the suspended sediment. Suspended silts and clays lead to frictional losses (viscous attenuation) around the particles, while sand-sized particles lead to losses due to scattering in directions other than 180◦ (Urick, 1948). In all cases the attenuation increases with increasing frequency. As an example, at 4 MHz, which is the highest frequency used in this study, the viscous attenuation per unit concentration from particles with radii less than 20 μm is greater than the attenuation due to scattering. Since the scattering and attenuation at a given frequency depend on a combination of particle size and concentration, without a priori information on these variables, data at multiple frequencies are required to determine both concentration and particle size. The theory that details how grain size and concentration can be obtained from multi-frequency backscatter data is detailed in many places (e.g. Crawford and Hay (1993), Thorne and Hanes (2002), Aquatec Subsea Ltd (2012)) and the multi-frequency attenuation inversion method is presented in Moore et al. (revised paper submitted 2012-12-15) and Moore (2012). The attenuation method consists in finding the particle size (typically the mean of the distribution) that minimizes the difference


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between concentration estimates at frequencies i and j. This difference is labeled i, j and is defined as i, j =

αs i

〈ζ i 〉

−

αs j

〈ζ j 〉

,

(1)

where αs is the measured attenuation due to suspended sediment and 〈ζ〉 is the theoretical suspended sediment attenuation function averaged over the particle size distribution. The two terms on the right side of Equation 1 are the concentration estimates at frequencies i and j, respectively. If it can be assumed that concentration and grain size are constant over two or more measurement ranges, then the simplest way to determine αs is to use the slope of the fluid-corrected intensity profile following the procedure outlined in Wright et al. (2010). The expression “fluid-corrected” indicates that the intensities have been corrected for spherical spreading and attenuation by water. If concentration and grain size are constant with range from the instrument and if there is insufficient sediment to cause attenuation, then the fluid corrected intensity profile should be a horizontal line. If there is detectable sediment attenuation, then the fluid-corrected intensity will decrease with range. Assuming a bimodal suspension (silt + sand), the suspension of silt may be assumed homogeneously distributed across the river section (horizontally and also vertically), but the suspension of sand may vary vertically (and also horizontally if there is a gradient due to varying water depth). Therefore, the multi-frequency backscatter system was deployed horizontally in order to simplify the interpretation of the data.

I NSTRUMENTATION An Acoustic Doppler Current Profiler (ADCP) constructed by Teledyne RD Instruments was used to record profiles of backscatter and flow speed, as well as water depth. By attaching an ADCP to a motor boat, the velocity of the water can be measured across an entire cross section as the boat traverses from one river bank to the other. The wetted area, calculated using the water level and bathymetry, is multiplied by the average velocity to obtain the discharge. ADCPs operating at 600 and 1200 kHz were used in this study. The vertical resolution of the profiles was 25 cm. The AQUAscat 1000R was used to provide data with higher spatial and temporal resolution. This system, which consisted of four monostatic piezoelectric transducers operating at 500 kHz, 1 MHz, 2.5 MHz and 4 MHz, a data logger and pressure and temperature sensors, was attached to a heavy frame, with the transducers facing horizontally (see Figure 2). The frame was lowered to specific depths using an electric winch and was kept at each position for 2 minutes. Four vertical profiles were collected across the section, with four to six sampling elevations per vertical. Backscatter profiles were acquired with a resolution of 4 cm; the profiles were between 2 and 10 m long in the spanwise direction, depending on the frequency of the transducer. With the pressure and temperature data recorded by the instrument, the intensity data could be corrected for losses due to spreading and attenuation by the water. A Van Dorn sampler was used to collect 2-L water samples at specific depths. Grain size distributions of the sediment in these samples were measured within a few days of sample collection using a laser diffraction particle sizer, the Laser In Situ Scattering and Transmissometry (LISST) Portable. This instrument measures the light that is diffracted by a suspension of particles and uses Mie theory to relate the scattering pattern to a grain size distribution, assuming that the particles are either spherical or randomly shaped. Its detectable range is 2.5 − 500 μm (equivalent spherical diameter).


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F IGURE 2: A photograph of the AQUAscat 1000R attached to the fish-shaped frame of a bedload sampler. The four transducers rested on the top of the mouth of the sampler and the body of the instrument and cables were taped to the top of the frame after this photo was taken.

M EASUREMENTS

AND

A NALYSIS

The velocity, bathymetry and backscattered intensity recorded with the 1200 kHz ADCP at Luang Prabang are depicted in Figure 3. From this figure it can be seen that the river was approximately 430 m wide with a maximum depth of just under 13 m. The backscatter has been corrected for range using the default equation of WinRiver II, which is the software used to visualize ADCP flow gauging data. Since the software only applies the correction at ranges in the far field (Teledyne RD Instruments, 2007), although it is unclear whether this is taken to be 2.1 or 4.2m, this may explain why the intensity values near the surface are higher than those at mid-depths. Higher flow speeds were observed on the left half of the river; regions of higher flow speeds corresponded with regions of higher backscatter, except near the bottom where the backscatter is higher. The bathymetry shown in Figure 3 corresponds to the average depth measured by each of the four beams of the ADCP, which is not necessarily equivalent to the depth directly below the instrument. Since the bed material was composed of sandy sediments, the waviness of the bottom suggests that there were sand dunes, which is consistent with our knowledge of the site. The increased fluid-corrected backscatter that is observed within a few meters of the bed in the left side of the river could therefore be interpreted as increased suspended sediment over the dunes. The vertical lines labeled V1-V4 in Figure 3 roughly indicate the across-stream positions at which the AQUAscat was deployed. Figures 4 and 5 show the fluid-corrected intensity profiles recorded with the AQUAscat at 1 MHz and 2.5 MHz, respectively. The scales of both the x and y axes differ for each subplot since the time at each location differed slightly and the data are only depicted at ranges at which the backscattered pressure exceeded each transducer’s noise level. The 1 MHz transducer profiled between 3 m and 9 m, while the 2.5 MHz transducer typically profiled less than 4 m. This suggests that more sediment at a given location led to a stronger backscatter at 1 MHz, resulting in an extended profile, but did not lead to attenuation of the 2.5 MHz signal below its noise level. It can be seen (Figures 4 and 5) that the fluid-corrected backscatter recorded by both transducers near the bed at V2, V3 and V4 was substantially greater than that near the surface. What is most striking about the near bed measurements is the variability in the backscattered intensity with both time and range. At 1 MHz, the fluid-corrected backscatter often increased with range, which indicates that the suspended sediment varied horizontally near the bed. This was not observed to the same extent at 2.5 MHz, partly because of the transducer’s limited range, but also because the increased sediment may have led to increased attenuation as opposed to increased backscatter. Since the backscatter recorded at vertical V1 by all four transducers of the AQUAscat and by the ADCP was less than that in the rest of the cross section, this could be interpreted as an observation of less or finer sediment towards the right bank than in the remainder of the cross


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F IGURE 3: Intensity (top panel) and velocity data (bottom panel) recorded with the 1200 kHz ADCP at Luang Prabang. Intensities have been corrected for range by the WinRiver II software. The left bank is on the left side of the figure. Vertical black lines indicate the across-stream locations of the AQUAscat measurements.

section. However, water samples collected throughout the section indicate a more or less homogeneous distribution of suspended sediment concentration and size (Dramais et al., 2012). The average, minimum and maximum concentrations were 155 mg/L, 118 mg/L and 174 mg/L, respectively. The median grain diameter of all samples ranged from 12 - 16 μm, but sand-sized particles were observed in some of the samples. The sample that contained the largest particles was collected near the bed. This is consistent with the observation of increased backscatter at 1 MHz near the bed. The lack of a clear trend of increased concentration in the physical samples collected near the bed may simply indicate that water samples were not collected as close to the bed as the backscatter data. This is likely considering the AQUAscat was installed on a heavy frame which kept the line vertical, while the water sampler was only attached to a rope.


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F IGURE 4: Fluid-corrected backscatter (in dB) recorded with the 1 MHz transducer of the AQUAscat at the alongstream positions depicted in Figure 3. The transducer was deployed horizontally.

F IGURE 5: Fluid-corrected backscatter (in dB) recorded with the 2500 kHz transducer of the AQUAscat at the alongstream positions depicted in Figure 3. The transducer was deployed horizontally.

Figure 6 is a plot of the time-averaged fluid-corrected intensity profiles recorded at the shallowest (a) and deepest (b) depths of V2. The decrease in fluid-corrected intensity with range at all frequencies in Figure 6a indicates that suspended sediment size and concentration did not vary over the profiled range at the surface. The attenuation values calculated from these


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profiles were 0.14, 0.29, 0.83 and 1.55 dB/m at 0.5, 1, 2.5 and 4 MHz, respectively. As expected, αs increased with increasing frequency. The time-averaged fluid-corrected intensity profiles measured near the bed (Figure 6b) indicate substantially higher attenuation at 2.5 and 4 MHz than near the surface (note the different scales on the y-axes). However, the 0.5 and 1 MHz profiles are not smooth due to the temporal and spatial variation in sediment transport that they detected. Ideally, these data should be averaged over smaller time and range scales in order to account for this variation, but the αs values calculated from Figure 6b were 0.48, 0.48, 1.72 and 4.27 dB/m at 0.5, 1, 2.5 and 4 MHz, respectively.

F IGURE 6: The average intensity profiles recorded at the surface (a) and the bottom (b) of the second vertical (V2), corrected for spherical spreading and attenuation due to water.

With the αs values calculated from Figure 6, Equation 1 can be used to determine an effective acoustic size of the suspended sediment. At each frequency, αs is then divided by the theoretical sediment attenuation function for that size to get an estimate of concentration. The particles were assumed to be monosized spheres for this purpose. Although this is a simple assumption, it allows us to determine whether the multi-frequency inversion method can provide reasonable estimates of concentration at this site. The size estimate from the data presented in Figure 6a is 0.4 μm, while it is 0.2 μm for Figure 6b. This is about an order of magnitude less than the median particle radius measured by laser grain sizer in all samples (6 μm). Nevertheless, the concentration estimates from the data at the three highest frequencies are not unreasonable. They are 222, 198 and 225 mg/L at the surface, and 737, 590 and 744 mg/L at the bottom. The acoustic estimates of concentration at the surface are similar to the average concentration measured in water samples (155 mg/L), while those determined at the bottom are more than three times the highest observed concentration. This may be realistic considering no water samples were collected right at the bed. The agreement of the acoustically-inferred concentration at the surface with the physical samples, despite the unrealistic estimate of grain size, is related to the fact that the sediment attenuation coefficient is not monotonic. Thins means that our estimate of size could be incorrect, while the theoretical sediment attenuation coefficient used to determine concentration could be correct. The sizing method therefore needs refining, but the concentration estimates obtained using only multi-frequency attenuation data appear realistic.

C ONCLUSIONS It was shown that the high resolution backscatter profiles recorded with a multi-frequency system (4 cm horizontal resolution) provided insight on the sediment transport near the bed in a way that neither the lower resolution ADCP data (25 cm vertical and 1-2 m horizontal resolution), nor physical sampling could. These data indicated that sediment transport near the bed happened in bursts. Grain size and concentration will be determined using both the


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attenuation and backscatter methods using different models of particle shape. These results will be compared to laboratory measurements made using a laser diffraction system and a scanning electron microscope.

A CKNOWLEDGEMENTS This work was part of a joint project between the World Wildlife Fund and the Mekong River Commission. The authors thank all those who were involved in the planning, execution, and financing of this project. In alphabetical order the organizations were CEREGE, Irstea, MRC, University of Ottawa and WWF. The measurements at Luang Prabang were made with the Department of Meteorology and Hydrology (DMH) Lao PDR team. We thank Marc Goichot (WWF), Hourt Khieu (MRC) and Khoun Phonh Khanmany (DMH) for coordinating the Luang Prabang measurements. The AQUAscat system used in this work was provided to Dr. Moore by Aquatec Group Ltd in the form of an equipment award.

R EFERENCES Aquatec Subsea Ltd (2012). “Application Note AN4, Inversion Technique”, AN4 Rev 1.0. Crawford, A. M. and Hay, A. E. (1993). “Determining suspended sand size and concentration from multifrequency acoustic backscatter”, Journal of the Acoustical Society of America 94, 3312–3324. Dramais, G., Dussouillez, P., and Moore, S. A. (2012). “Study of the hydro-sedimentary dynamics of the Lower Mekong River: 2012 Sept. 25 - Oct 6 field mission. draft report submitted to the World Wildlife Fund”, Technical Report. Koehnken, L. (2012). “IKMP Discharge and Sediment Monitoring Programme review, recommendations and data analysis. Part 2: data analysis of preliminary results”, Technical Report. Moore, S. A. (2012). “Monitoring flow and fluxes of suspended sediment in rivers using side-looking acoustic doppler current profilers”, Ph.D. thesis, Université de Grenoble. Moore, S. A., Le Coz, J., Hurther, D., and Paquier, A. (revised paper submitted 2012-12-15). “Using multi-frequency acoustic attenuation to monitor grain size and concentration of suspended sediment in rivers”, Journal of the Acoustical Society of America . Teledyne RD Instruments (2007). “Winriver II User’s Guide”, P. 47-48. Thorne, P. D. and Hanes, D. M. (2002). “A review of acoustic measurement of small-scale sediment processes”, Continental Shelf Research 22, 603–632. Urick, R. J. (1948). “The absorption of sound in suspensions of irregular particles”, Journal of the Acoustical Society of America 20, 283 – 289. Walling, D. (2005). “Evaluation and analysis of sediment data from the lower mekong river”, Technical Report, submitted to the Mekong River Commission. Wright, S. A., Topping, D. J., and Williams, C. A. (2010). “Discriminating silt-and-clay from suspended-sand in rivers using side-looking acoustic profilers”, in Proc. 2nd Joint Federal Interagency Sedimentation Conference, available here: http://acwi.gov/sos/pubs/2ndJFIC (date last viewed 2012-12-10).


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