BEAST—A portable device for quantification of erosion in natural intact sediment cores

Methods in Oceanography 5 (2013) 39–55

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BEAST—A portable device for quantification of erosion in natural intact sediment cores J. Grant a , T.R. Walker a,∗ , P.S. Hill a , D.G. Lintern b a

Dalhousie University, Department of Oceanography, 1355 Oxford Street, Halifax, Nova Scotia, B3H 4J1, Canada b Geological Survey of Canada, 9860 West Saanich Road, P.O. Box 6000, Sidney, British Columbia, V8L 4B2, Canada

highlights • • • • •

We report a new low cost, portable device for quantification of sediment erosion. Piston oscillation rates are linearly related to shear velocity on the bottom of the device. Predicted erosion thresholds from a validated model correspond to measurements made in the device. Real-time turbidity and digital imaging was employed to quantify sediment erosion. We quantified sediment erosion threshold, erosion rate, erosion sequences and size of resuspended particles.

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Article history: Received 29 July 2012 Received in revised form 17 December 2012 Accepted 4 March 2013 Available online 12 April 2013

abstract A portable Particle Erosion Simulator (PES) device, also referred to as the BEAST (Benthic Environmental Assessment Sediment Tool) (Walker et al., 2008) has been re-designed for quantifying erosion in natural intact sediment cores. The BEAST was reconfigured from an older design (Tsai and Lick, 1986), which had uncalibrated flow characteristics and was limited to viewing resuspension. In addition to calibrating friction velocity at the sediment–water interface, we employ a combination of real-time turbidity monitoring (via measurement of % transmission which decreases proportionally to suspended solid concentration) to quantify erosion threshold and calculate erosion rate, as well as digital imaging to document sequences of erosion and particle size response of resuspended material. The BEAST consists of a clear acrylic PlexiglasTM core liner with a perforated disc oscillating vertically in a piston motion. Performance of the device was

∗ Correspondence to: Dillon Consulting Limited, 137 Chain Lake Drive, Halifax, Nova Scotia, B3S 1B3, Canada. Tel.: +1 902 450 4000; fax: +1 902 450 2008. E-mail addresses: [email protected] (J. Grant), [email protected] (T.R. Walker), [email protected] (P.S. Hill), [email protected] (D.G. Lintern). 2211-1220/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mio.2013.03.001

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calibrated by (a) comparing predicted to observed friction velocity as a function of motor speed, (b) using a hot film anemometer in the chamber to measure shear velocity, (c) verifying the applicability of anemometric calibration by relating the power of the grid stroke to stress dissipation, and (d) comparing measured critical stress of foundry sand to predictions from a validated model. Measurements indicate the friction velocity is uniform over >50% of the radial distance from the core center. Bottom stress is highly sensitive to the final height of piston down-stroke, a variable that can be altered to control the range of friction velocities. A plot of piston motor RPM vs. predicted u∗ was identical to the regression fit through the observed data. We verified that the proportionality between power input and thermistor heat dissipation corresponds to the scaling of u∗ and RPM, consistent with our calibration using the stress sensor. An example of an erosion sequence is demonstrated from a field core obtained in the Beaufort Sea in which two erosion stages were clearly indicated in the combined results from measurements of % transmission (to determine turbidity), particle size, and erosion rate. Our studies confirm that the BEAST has predictable flow characteristics expected from first principles, and that applied shear stress causes erosion in a way quantitatively similar to horizontal shear. In addition, the predicted erosion threshold of sand-sized particles corresponds to within 3%–18% of measured values made using the device. These multiple sources of BEAST validation demonstrate its practical capability to provide quantitative field measurements of transport parameters from intact marine sediments if applied in a similar manner, and further contribute to predictive capability in modeling of benthic–pelagic coupling. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The quantification of sediment transport in coastal systems is essential in the understanding of coastal erosion as well as fluxes of particulate organic and inorganic matter and associated contaminants (Walker et al., 2008). Determination of sediment erosion thresholds by currents and waves is an important step in predicting sediment resuspension (Tsai and Lick, 1986; Lintern et al., 2011). Changes in sediment stability and susceptibility to erosion depend on biotic processes and sediment properties (Maa et al., 1998; Vignaga, 2012). Initial resuspension of bottom sediments is affected by many factors, including biostabilization, porosity, organic content, grain size, bioturbation and other sediment characteristics (Grant and Gust, 1987; Paterson, 1989; Dade et al., 1990; Davis, 1993; Grant and Daborn, 1994; Grant and Emerson, 1995; Amos et al., 1997; Sutherland et al., 1998; Tolhurst et al., 1999, 2002; De Deckere et al., 2001). Dynamics of particles in the water column are also affected by physical processes such as turbulent shear, differential settling velocities, sediment concentration, microbial polymers, salinity, and particle encounter frequency (Dyer, 1989; Eisma et al., 1991; ten Brinke, 1994). Loosely associated particles deposited on the sediment surface (fluff layer) cycle between the sediment and water column with continuous aggregation and disaggregation (Kuhrts et al., 2006). Incorporation of biotic and abiotic influences on sediment erosion requires experimental methods to determine which controlling variables can be isolated and documented (see Vignaga, 2012 for a comprehensive review). Interest in this approach has resulted in a wealth of literature on sediment entrainment using various tools including oscillating grids (Tsai and Lick, 1986; Lavelle and Davis, 1987; Davis, 1993); instruments for measuring shear strength in situ (ISIS) erosion devices (Williamson and Ockenden, 1996; Lintern et al., 2002); rotating vanes (Sills et al., 2004);

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cohesive strength meters (CSM) erosion devices (Paterson, 1989; Tolhurst et al., 1999, 2000; Watts et al., 2003); annular flumes (Amos et al., 1992; Widdows et al., 1998, 2007); portable benthic flumes (Black and Cramp, 1995; Aberle et al., 2004), recirculating channels (Grant et al., 1993, 1997), and other stirred chambers (Law et al., 2007). The use of erosion devices to quantify sediment transport involves many compromises, most concerning realistic hydrodynamics and a subsequent ability to extrapolate results back to natural conditions in the field. Many of these approaches thus attempt to work with samples subject to minimal disturbance (Maa et al., 2007). In situ flumes with more bed coverage maximize the area of undisturbed sediments, while allowing larger features such as ripples or burrows to be included. They have the disadvantage of increased cost, reduced portability and necessity of vessel deployment. All devices have calibration issues related to chamber geometry, secondary flow, wall effects, and spatial distribution of shear stress. Disadvantages include lack of calibration data in the case of piston erosion devices (Tsai and Lick, 1986), minimal portability, e.g. flumes (Grant et al., 1997; Righetti and Lucarelli, 2007) or prohibitive construction and deployment cost. Few approaches combine portability with the ability to utilize natural intact cores for experimental purposes. Among erosion devices, those involving core samplers have consistency with the scale and methodology other forms of benthic sampling. Both stirred and vertically oscillating devices fit this category, and we have chosen the latter to avoid some of the strong radial effects and their lack of portability associated with stirring. Piston-based grid devices have been around for some time, but suffer from lack of quantitative calibration (Davis, 1993), or if calibrated, the devices have been operated in a very different way than presented here. For example, Gust and Müller (1997) presented a range of erosion characteristics of a vertically oscillating grid, otherwise known as a Particle Erosion Simulator (PES) device, using hot film sensors, but they oscillated the grid an order of magnitude faster (300–850 RPM) and at a greater distance from the sediment bed (5 cm). Gust and Müller (1997) also performed their tests using abiotic Kaolinite and erosion time intervals were performed at 10 and 20 min. Because shear stress or the related shear velocity provides a better characterization of erosive force than free stream velocity, some aspect of shear is desirable in evaluating critical erosion threshold. Free stream velocity is often extrapolated to the bed in an assumed logarithmic profile and less frequently shear stress is measured directly. The geometry and flow generation of piston-based devices make these calibration measurements difficult. Beyond studies by Gust and Müller (1997) and Thomsen and Gust (2000), there appears to be very few direct fluid measurements made in any of the previous iterations of grid devices. Instead, they have been qualitatively calibrated using sand of known diameter and its entrainment threshold (e.g., Kaolinite). We propose the BEAST as an ideal solution to empirical measurements of natural sediment entrainment, since it is easy to build, inexpensive, uses intact cores, and is capable of quantifying several types of erosion features in field applications. Furthermore, the protocols described in this paper have been applied to cores across a wide variety of sediment types from Arctic, Pacific and Atlantic coastal field sites, and observed erosion sequences and behavior of the BEAST have been reproducible and predictable. However, until now the lack of calibration has seriously limited the application of a potentially useful tool to studies of sediment resuspension. Working with the basics of previous designs (Tsai and Lick, 1986), the BEAST was built as an inexpensive, robust, portable field instrument for quantifying sediment transport processes in a variety of sediment types and habitats. In previous iterations, shear stress within the design of Tsai and Lick’s device (Tsai and Lick, 1986), first referred to as the PES by Davis (1993), was estimated indirectly. A calibrated flume was used to compare resuspended sediment concentrations to applied stress. Similar concentrations achieved in the BEAST were assumed to occur at identical stresses, but it was not specified how the differing dilution volumes were normalized. In order to make more quantitative use of the BEAST, we undertook flow-based calibration for a range of typical operating conditions. Moreover, observed values were compared to calculated values derived from fluid mechanics. Although calibration is central to our improvement of the device, we also incorporate quantitative assessment of erosion threshold via turbidity (by employing a % transmission probe which decreases proportionally to suspended solid concentration), erosion rate, and particle size response to erosion via image analysis.

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An immediate concern is that the flow within the erosion chamber is potentially different from that of horizontal tidal flow in the natural environment. Modeling of the flow within the chamber has proved complex in part due to time-dependence of the piston velocity. Flow visualization via particle imaging velocimetry has been useful in other small scale studies (e.g. Fujisawa and Takizawa, 2003), but exceeds the scope of our study. We thus pose several questions which help focus the relevant characteristics of the erosion chamber. 1. Is the force imposed by the grid predictable from a description of fluid motion in the chamber? 2. How does horizontal shear imposed in nature or flumes relate to the vertical action of the piston, and how does this affect calibration of the device, including application of stress sensor calibration? 3. How does measured shear velocity change with motor speed, and how does it correspond to calculated values? 4. How does the proximity of the piston to the core bottom affect the magnitude of imposed stress? 5. What is the radial distribution of shear velocity in the core? 6. Does the erosion of sand measured with the BEAST correspond to theoretical values for a given grain size? In the following study, calibration results are provided, and flow considerations discussed in the application of the BEAST to erosion experiments. We also present an example of a field application using the erosion sequence of an intact non-cohesive sediment core from the Beaufort Sea by Walker et al. (2008), and combine these observations with results of an embedded % transmission sensor (to derive turbidity) and particle size measurements using videography. These calibrations and measurements serve to solidify oscillating grid devices as practical field-ready tools to quantify multiple aspects of sediment resuspension and transport for marine sediments. 2. Materials and methods 2.1. Design and instrumentation of the BEAST The structure of the BEAST (Fig. 1a–f) consists of a motor and controller box which is mounted on a lightweight portable aluminum frame. The motor arm is connected to a drive rod which has a perforated PVC grid mounted at the end. The grid performs oscillations within intact sediment cores, generating shear velocity on the sediment bed. A clear acrylic PlexiglasTM core liner of 11 cm ID is used with approximately 20 cm of water over the sediment bed, equivalent to a volume of 2 L. An o-ringed PVC base is inserted in the core liner bottom after a core is obtained. Dimensions and details of the design can be found in Tsai and Lick (1986), and a schematic of the BEAST used herein is shown in Fig. 2 with specific dimensions for replication of the device. The mechanical aspects of the erosion device were slightly changed, but the dimensions of the erosion chamber and oscillating disc were true to the original. The approximate hardware cost for a single unit that includes all materials plus the motor, controller and stand, but not the sensors, was < $1000 USD. The perforated disc (10.9 cm diameter, 1.2 cm length) includes 24 holes (each 1.4 cm diameter) spaced evenly in a grid pattern (Fig. 1e). The distance between the core wall and the oscillating disc remained constant throughout the calibration experiments and field applications, as previous studies have revealed differences in erosion characteristics even with subtle differences in the distance between the wall and disc (Gust and Müller, 1997). Therefore, we assumed that the pressure gradient characteristics would remain consistent. This was confirmed via reproducibility of experiments which were performed in triplicate. The grid has a fixed excursion of 2.54 cm and with the drive rod fully extended, the grid stops 2 cm above the bed. In the calibration experiments, lengthening the drive rod allowed the full extent of the grid transit above the base plate to be varied. The oscillating disc was powered by a DC motor (Leeson permanent magnet DC gear-motor, model: CM34D25NZ59B, 90 V, 2 A, 0.25 H.P.) and the speed of the motor was regulated by an analog controller box (Leeson speedmaster motor control, 0–100 RPM) (Fig. 1c). The grid oscillation frequency could be varied using the controller box in the range 14–100 RPM, corresponding to vertical speeds of the disc at mid-stroke of 7–40 cm s−1 .

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Fig. 1. (a) The BEAST without a sediment core or transmission probe (turbidity in the water column was determined via % transmission which decreases proportionally to suspended sediment concentration). The motor, speed controller box and guide plates are all mounted onto a custom made aluminum frame. (b) The BEAST performing an erosion sequence on an intact sediment core. (c) Details of the motor and speed controller box. (d) Details of the oscillating grid and mounted transmission probe 2 cm above an intact sediment core. (e) Details of the oscillating grid showing the arrangement of drilled holes in the PVC plate. (f) Details of the base plate showing the location of the flush mounted shear stress probe (dots), which varied from 0 cm (center), 1.5 cm (middle), and 3.0 cm (outer) from the center of the core.

Fig. 2. A schematic of the BEAST modified from Tsai and Lick (1986) showing dimensions for replication of the device. Note that stand dimensions are not given and these may vary depending on field applications but will not alter the pre-calibration values for the BEAST assuming core dimensions remain the same.

The onset of sediment erosion was detected via % transmission (to derive turbidity) and videography. Turbidity was monitored to quantify erosion rate via % transmission with a fiber-optic spectrophotometer (Brinkmann PC 800 colorimeter, 670 nm) mounted on the piston rod above the

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J. Grant et al. / Methods in Oceanography 5 (2013) 39–55 Table 1 Grain size characteristics of foundry sand used in BEAST erosion trials. Initial sieve class refers to the original screening of sediments. Subsequent sieve class is secondary screening based on the given sieve sizes. Initial sieving (µm)

Subsequent sieve class (µm)

Percent (%) weight

Median category (µm)

>1000

>2000

0.2 52.6 11.4 31.7 4.1

1250

1000–2000 710–1000 500–710 <500 500–1000

710–1000 600–710 500–600

1.4 11.3 87.4

550

250–500

300–500 250–300

80.7 19.3

400

125–250

180–250 125–180

74.1 25.9

215

grid. The probe was zeroed with filtered seawater and calibrated with a concentration series of suspended particulate matter (SPM) from field study sites that were made into slurries of varying concentrations and turbidity. At each % transmission measurement a sub-sample of slurry was filtered through pre-weighed cellulose acetate filters (0.45 µm, Sartorius, Göttingen, Germany), drying at 105 °C, and weighing again to determine the SPM concentration, which gave tight linear responses over a range of concentrations. Erosion rates (quantity of material eroded), expressed as g sediment m−2 s−1 were calculated for each motor interval based on regression analysis of the sediment % transmission (to derive turbidity) and SPM calibration curve (Walker et al., 2008). 2.2. Calibration with sand transport Foundry sand was pre-sieved into narrower grain size categories as specified in Table 1. In the laboratory, sand was placed into cores and eroded with the same procedure as for field cores. Erosion thresholds were determined visually, using criteria for sand established in previous flume experiments by Grant et al. (1986). Motor speeds were noted for the movement of several grains, and increased incrementally in steps of 2 RPM until a more generalized mobilization of the bed occurred, usually 2 steps from the initial grain movement. Critical erosion threshold was taken as the motor speed between these two points. Theoretical values for critical sand threshold were obtained by applying the model of Wiberg and Smith (1987), which uses as input drag coefficient, bed roughness, grain geometry, particle angle of repose, and bed slope. Using spherical geometry for the foundry sand, and default values for other variables, we calculated critical u∗ from this model and plotted it versus median grain diameter for comparison to values observed from the BEAST experiments. 2.3. Sensor calibration A quartz hot film thermistor (TSI probe 1150AA, 4 mm diameter) was used to measure shear velocity at different positions on the PVC base of the core liner. We calibrated the anemometer using differential pressure in a recirculating flume (50 cm channel width, 732 cm length; see Roegner et al., 1995) as in our previous studies (Muschenheim et al., 1986; Grant et al., 1993; Grant and Daborn, 1994). A factory-calibrated differential pressure sensor (Digi-Key, 0-6 PSI, accuracy <0.5%), with a linear relationship against voltage output (r = 0.995) was inserted into the working section of the flume (500 cm from the entrance), with the ports of the differential pressure sensor flush-mounted and separated by 148 cm in the stream-wise direction. The shear probe was flush-mounted 15 cm further downstream from the second pressure port. Voltage input of the thermistor was regulated by its power supply (TSI constant temperature linearized anemometer, model 1054B). Impeller-driven flow of the flume motor was varied via a controller box, and the voltage response of the thermistor and the differential pressure sensor were recorded at various flow speeds with a Vernier data logger using LoggerPro. Voltage output of the thermistor was tightly correlated with differential pressure

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(r = 0.96, n = 6) providing a direct measure of shear velocity, expressed as friction velocity u∗ (cm s−1 ) u∗ = (τ /ρ)0.5

(1)

where: τ = shear stress (dyne cm−2 ) and ρ = seawater density (g cm−3 ). The calibrated shear stress probe was then used in the BEAST measurements which varied (a) the radial distance of the probe from the center of the base, and (b) the height of the vertical extent of disc travel above the base. In the first experiment, the probe was flush-mounted at different positions in the base plate of the core (Fig. 1f), including the center (0 cm), and then moving radially outwards to 1.5 cm and 3 cm, the maximum practical distance. Drilled holes were plugged between successive experiments. In the second experiment, the effect of piston height on calibration experiments was performed with the oscillating grid stopping vertically at positions of: 1.0, 1.5, 2.0 and 2.5 cm above the surface of the base plate. In this experiment the stress measurements were made in the middle of the base plate (i.e., 1.5 cm from the center as indicated in Fig. 1f). In all cases, the chamber was filled with 2 L of filtered natural seawater at 20 °C and a salinity of 30h. Tolhurst et al. (2000) suggests that erosion rates from devices such as the oscillating grid can only be compared to other devices if the shear velocity increments are of similar magnitude and duration. During our calibration and subsequent field deployments all experimental increments were 2 min. Each calibration variable was measured in triplicate trials. During experimental trials with the BEAST we compared various time intervals to assess and derive our own erosion parameters. Using a wide variety of sediment types from Arctic, Pacific and Atlantic coastal regions, we found little or no discernible change in observed erosion parameters between 2 and 10 min intervals. However, the greatest variation existed when we compared time intervals of 1 and 2 min. Therefore, we chose to change the oscillation speed every 2 min and have documented our calibrations accordingly (as it applies specifically to the BEAST). For practical reasons the 2 min intervals also allowed for erosion sequences to be performed on multiple cores, which is an important consideration under various field scenarios (e.g., working on tidal mudflats or onboard research vessels transiting between sampling stations). 2.4. Erosion in a sediment core from the Beaufort sea An example of an erosion experiment is presented based on a natural intact sediment subcore obtained from a box core on the ‘CCCS Amundsen’ in the Beaufort Sea (September 30 2003, Site Number, CA06; 255 m depth; Location, 70° 37.808′ N, 127° 15.168′ W near Cape Bathurst; (Walker et al., 2008)). The acrylic PlexiglasTM core liner of the BEAST was inserted carefully into the box core to 15 cm sediment depth, and then sealed on the bottom with a PVC insert. The core was gently filled with approximately 2 L of seawater using a floating plastic disc overlying the sediment surface to eliminate disturbance of the bed. The core was stored in the dark in a water bath (10 °C) to equilibrate before the erosion trial performed at between 10–15 °C. The plunger disc was then inserted into the liner, and oscillation imposed for 2 min intervals in a speed series from 14–40 RPM. Erosion in the core was detected by turbidity using the optical sensor (% transmission) which had a tight linear relationship with SPM concentration measured by filtration and gravimetry (r 2 = 0.91, n = 6). Erosion rate (g m−2 min−1 ) was calculated as change in SPM concentration divided by the time interval and area of the core. 3. Results 3.1. Shear measurements in the erosion chamber The BEAST’s oscillating grid moves vertically in a piston motion with an up and down-stroke producing wide variation in the time series of shear velocity output during calibration (Fig. 3). For example, at an oscillation speed of 20 RPM the variation of shear velocity (u∗ ) ranges from 1.1 up to 2.0 cm s−1 . Because the logging interval is longer than the stroke period, the time series is aliased

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Fig. 3. Measured time series of shear velocity (u∗ ) showing the nature of variability of shear stress during increasing motor speeds of the oscillating grid, with the grid mounted 2 cm above the base plate. The flush mounted shear stress probe was positioned 1.5 cm (middle) from the center of the core. The solid line is a regression through the time series (r = 0.89), whereas the dashed line is the calculated theoretical value of shear velocity (via Eq. (2)) as a function of motor speed assuming that CD = 1.08, disc area A =69.9 cm2 , and disc travel speed U =1.17–3.33 cm s−1 (based on RPM).

with respect to periodicity, but it captures the full range of values in the chamber. Identical timeseries variation exists for the different piston heights tested, e.g. 1.0, 1.5 and 2.5 cm above base (data not shown). Despite the variation, the oscillation speed was strongly related with u∗ (Fig. 3; r = 0.89) for the 2 cm piston height. In addition, the trend of this curve was linear, indicating that increases in RPM during erosion experiments were additive. Other studies have observed similar time dependent variation in shear velocity with the vertical oscillation of the disc (Gust and Müller, 1997). 3.2. Predicted shear velocity In order to examine the behavior of the BEAST expected from piston flow, we describe the force exerted by a disc moving through a fluid as: F = 0.5 CD ρ AU 2

(2)

Where: CD = drag coefficient of a disc facing the flow, ρ = seawater density, A = area of the disc, and U = free stream velocity. For a smooth cylinder with the blunt end facing the flow, CD = 0.8 for Re in our range (Hoerner, 1965). Perforations add skin drag to the cylinder face, and laboratory studies suggest that an increase of 35% to CD = 1.08 is reasonable (Wolter, 2005). This consideration of CD suggests that the frontal area of the disc in Eq. (2) include the area of the holes. Calculating F /A in Eq. (2) as shear stress (τ ) and expressing it as friction velocity indicates that the predicted theoretical value of shear velocity based on the forces exerted by the piston on a downstroke comprise u∗ ranging from 0.86–2.45 cm s−1 (Fig. 3, dashed line). The measured u∗ for the BEAST at these RPM’s (with the maximum down-stroke 2 cm above core bottom), was in the same range (0.97–2.50 cm s−1 ; Fig. 3, solid line indicates a regression through the variation in time series) and a plot of RPM vs. u∗ predicted from Eq. (2) indicates that this line is approximately similar to the regression through the observed data when compared (Fig. 3). This scatter plot of measured time series of shear velocity (u∗ ) shows the nature of variability of shear stress. For example, at an oscillation speed of 20 RPM the variation of shear velocity (u∗ ) ranges from 1.1 up to 2.0 cm s−1 . This result provides

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confidence that the exertion of stress to the bed from the piston action is at least partially understood on the basis of simple fluid mechanics. However, this proportionality depends on the assumption that the force generated by the oscillating grid at steady state is balanced by the shear stress acting on the bottom and walls of the BEAST chamber and that other factors may change this balance (e.g., irregular surface of sediment bed, presence of biotic and abiotic features etc.). Similarly, others (Tsai and Lick, 1986) have shown a linear relationship between oscillation speed and stress, but as shown in Gust and Müller (1997) (in their Figs. 5 and 16); the linear relationship is less obvious due to logarithmic scale used for shear stress. Nonetheless, both of these previous studies provide evidence that this type of device has significant fluctuations in applied stress for a given oscillation speed (Tsai and Lick, 1986; Gust and Müller, 1997). However, in our experiments we do acknowledge that the relationship between RPM and u∗ at a maximum stopping height of 2 cm is much more linear than at other stopping heights above the core (discussed below), and that these other relationships more closely resemble a fractional power form which would be closer to the linear relationship between RPM and stress found by Gust and Müller (1997). Considering uncertainty in the drag coefficient, the proportionality between power input and kinetic energy dissipation produces a scaling between u∗ and RPM that is consistent with our calibration using the stress sensor observed in the measured relationship between u∗ and RPM (Fig. 3). We next consider the effects of height above core bottom and radial distribution of stress in the core. 3.3. General observations within the BEAST core chamber As the disc moves down within the core, a plug of water is advected to the bottom. We envision that water impacting the sediment surface is horizontally sheared traveling radially outward toward the core walls where it is then diverted upward toward the perforations in the disc. The least perforated central part of the disc where the rod is mounted is most opaque to the flow, and observations of sand transport in the chamber indicate that transport is initiated in the center of the disc. This was confirmed by visual observations and digital imaging. The anticipated fluid flow will disrupt a wall boundary layer due to shear generated by the plug of water moving ahead of the disc due to the complex flow of the piston stroke (Schlichting, 1979). In the lower chamber, the progress of the fluid is immediately impeded by the core bottom, and there are eddies from the disc that move outwardly from the core bottom and dissipate to the sides of the core and up the walls. As the piston reverses, any such boundary on the walls below the disc would be greatly diminished, as the entire lower chamber is fully turbulent and chaotic (Denny, 1988). Observations of particles in the chamber certainly suggest a fully mixed lower volume. 3.4. Radial distribution of stress The empirical measurements of bottom shear velocity at distances of 0 cm (center), 1.5 cm (middle), and 3.0 cm (outer) from the center of the core indicated a slight decay of shear radially outward (Fig. 4). There was however, no significant difference in u∗ as a function of distance from center (ANCOVA, p = 0.18), partially because the variation in piston stroke causes a relatively wide distribution of stress at each time point. Radial distributions at varying distances from the center of the grid in a similar device were tested by Gust and Müller (1997), but although there appeared to be differences in u∗ (shear velocity) as a function of distance from center, they were not statistically compared, presumably due to lack of replicates. There is a further 2.5 cm of outward radius in which measurements could not be made. However, the ratio between u∗ at 0 and 3 cm provides some indication of the extent of decay near the wall. If the top of the wall boundary layer is at most 1 cm, extrapolation of the decay from the core center as a linear decline to the this point suggests that u∗ (shear velocity) will decrease by up to 29% as it approaches the wall. This relatively small change compared to the temporal variation in u∗ indicates that the shear stress (dyne cm2 ) would decrease by approximately 50% as the wall is approached, which is a desirable property in an erosion device.

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Fig. 4. Mean shear velocity values (u∗ ) recorded at different radial positions in the base plate (•, center; ⃝, middle;H, outer), as a function of oscillating grid speed (RPM) and experimental duration (2 cm vertical grid travel). Plotted values are means ± standard error (n = 3) and were not significantly different, p = 0.18 (ANCOVA).

3.5. Effect of piston height Although we have selected 2 cm as the minimal height above bottom for travel of the perforated disc, information on the effects of this variable is useful in either accommodating more volume in the chamber or achieving higher shear velocity (piston dead center closer to the bed). The stress measurements were made in the middle of the base plate (i.e., 1.5 cm from the center) during these calibration experiments. These results indicate that piston position has a strong influence on shear velocity (Fig. 5). At smaller distances (1.0 and 1.5 cm), the stress is much higher at all RPM than for greater distances. Moreover, it becomes asymptotic at higher RPM suggesting that the capacity of the device to generate flow in this reduced chamber volume is limited. Most of the dissipation around an oscillating grid occurs very close to the grid, falling off as a high power of distance away from the grid (Brumley and Jirka, 1987). Thus, only a fraction remains to be dissipated at the walls and bottom of the core. This fraction will be higher the closer the grid is to the bottom of the core, producing the high sensitivity to grid distance in our calibration experiments that tested different piston heights, as shown in Fig. 5. In our previous work with microbial mats (Walker and Grant, 2009), we found that at small distances above the sediment surface, there was an increased risk that the disc could contact pieces of microbial mat, tube, or other biotic surface features that could be loosened by erosion. Other studies using similar devices chose to oscillate discs 5 cm above the sediment surface (Tsai and Lick, 1986; Davis, 1993; Lavelle and Davis, 1987), but due to the configuration of the BEAST, the RPM values required to generate observable erosion would be impractical. Therefore, we chose a fixed distance of 2 cm between the oscillating disc and sediment surface rather than 5 cm traditionally used by others. Results from our previous studies (Walker et al., 2008; Walker and Grant, 2009), confirm our choice of 2 cm as a more suitable distance for defining minimal chamber volume.

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Fig. 5. Mean values of base plate shear velocity (u∗ ) with dead center down-stroke at different heights above the plate (• , 1.0 cm; ⃝, 1.5 cm; H, 2.0 cm; 1, 2.5 cm), as a function of oscillating grid speed (RPM) at erosion increments of 2 min. Plotted values are means ± standard error (n = 3).

For consistency, natural cores exhibiting relatively smooth level surfaces were chosen for BEAST erosion assessments. Cores with irregular or sloped sediment surfaces were discounted from calibration and erosion experiments, as this device is extremely sensitive to variation in the height between the plunger and sediment surface as shown in Fig. 5. 3.6. Calibration with foundry sand Pre-sieving allowed sorting of the foundry sand into 4 size classes, although there was better sorting of grain sizes <1000 µm than in the coarser fraction (Table 1). For the latter category, grain size was dominated by the 1000–2000 µm fractions, but with substantial weight in the 500–710 µm bin. In the three smaller sieve classes, sediment was clearly divisible into narrow bins with 74%–87% in single sieve classes. Our visual observation of sediment movement thus reflects a range of grain sizes, but with a median chosen simply as the mid-point of the span of sieve sizes of the dominant fraction. Based on Wiberg and Smith (1987) we projected a predicted curve of u∗crit versus grain size for comparison to the results of BEAST experiments. From our experience of the variation of shear velocity ranges at a given oscillation speed (Fig. 3), we believe that initiation of motion is responding to maximum stress values, so our observed values use the top 5% of stresses (using the 95% confidence interval) at a given RPM to define the appropriate u∗crit for that RPM. The outcome of erosion trials indicate a reasonable agreement between observed and predicted u∗crit , with a 3%–18% difference between measured points and the theoretical line (Fig. 6). Critical erosion shear velocity is affected by grain hiding or protrusion on a poorly sorted bed (Wiberg and Smith, 1987). Within the grain sizes shown, there was a range of size classes that would create a potential source of variation. These effects were not considered in the calculations here (Fig. 6) because sieving produced relatively well sorted sediments. The general agreement between modeled critical erosion shear velocity and those measured with the BEAST indicates that the method for estimating shear velocity in this device is reasonable. 3.7. Application to a natural sediment core Prior to the onset of erosion in the Beaufort Sea core, sampled from a station near Cape Bathurst on the Arctic shelf break (255 m depth), the water in the core barrel remained clear, and there was

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Fig. 6. Comparison of critical friction velocity (u∗crit ) measured within the BEAST erosion chamber (H) using 95% confidence interval of stress for a given RPM to that predicted from grain size based on a linear fit to Wiberg and Smith (1987) (solid line).

little change in the % transmission value (Fig. 7). Sediments at this site were non-cohesive consisting of >90% silt–clay (<63 µm) with low-density and high pore-water contents. With increasing shear velocity applied to the sediment surface, small aggregates (<2 mm) began to lift from the bed. At the third motor speed increment (u∗ = 1.2 cm s−1 ), water column turbidity increased and resuspension of a loose unconsolidated surficial layer (top mm of surface) occurred initiating the first phase of erosion (Amos et al., 1992). Several particle size classes are represented, between 0.2 and 1 mm diameter (data not shown). The first phase of erosion may be defined by subsurface shear strength for a given sediment horizon, yielding to applied shear stress, and for each particle type and/or size, there may be a corresponding erosion threshold (27) as detailed in our previous work with the BEAST (Walker et al., 2008; Walker and Grant, 2009). During this phase of the experiment there is a steady increase in turbidity and erosion rate (Fig. 7) up to a shear velocity u∗ = 1.4 cm s−1 after which there is a precipitous increase in turbidity. At u∗ = ∼1.5 cm s−1 there is a boundary we designate as the second phase of erosion, due to the marked change in erosion behavior. From a visual standpoint, there appears to be failure of the bed, viewed as resuspension of the upper 1 cm of the sediment surface. However, this level of stress is more typical of the critical stress for bed erosion than for mass failure, but the low-density, high pore-water and high (>90%) silt-clay content of the sediment may have contributed to the apparent failure of the bed. Is it also possible that the pressure fluctuations due to the oscillating grid are sufficient to fluidize or otherwise mobilize particles to greater depths in this sediment core. For example, Vollmer and Kleinhans (2007) show how pressure gradients within the bed can contribute significantly to particle mobilization. Therefore, these results suggest that there may be some potential temporal and small scale spatial pressure gradients within the BEAST. Once the second phase of erosion is reached, turbidity levels off as the bed yields less sediment. Erosion rate also shows a distinct change at this time, peaking at the boundary between these phases of erosion and declining thereafter. This decline likely represents depth-limited erosion to harder underlying sediments, typically associated with the limit of the first phase of erosion. Nonetheless, we find it more practical to demarcate this distinction on the basis of surficial layer events, first phase of erosion (flocs, biofilms, surface mm of sediment) compared to the second phase of erosion (failure of the sediment surface to cm-scale depths), indicated by a steepening of the transmissivity curve, i.e. sharp increase in turbidity (Fig. 7a). During turbidity calibration the SPM concentration vs % transmission had a tight linear relationship (r 2 = 0.91, n = 6) (Fig. 7b). For each incremental increase in oscillation speed (or time interval change) there is an approximate

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a

b

Fig. 7. (a) Time series of an erosion sequence in a core from the Beaufort Sea (September 30 2003, Site Number, CA06; 255 m depth; Location, 70° 37.808′ N, 127° 15.168′ W, near Cape Bathurst; (Walker et al., 2008)). Distinction between the different phases of erosion (⃝) is defined in the text. The lowest % transmission (•) corresponds to a suspended sediment concentration of 2467 mgL−1 . (b) Relationship of % transmission measurement with SPM concentration in the Beaufort Sea core (r 2 = 0.91).

step wise increase in SPM concentration. The SPM concentration at each step are additive and as a result the lowest % transmission (highest turbidity) corresponded to a cumulative SPM concentration of 2467 mg L−1 at the end of this erosion sequence. These observations reiterate previous studies which demonstrate that there are multiple erosion thresholds, dependent on sediment texture and the vertical distribution of shear strength (Amos et al., 1992; Tolhurst et al., 2000). Our results also demonstrate the multiple sediment transport variables derived by the BEAST compared to its previous iterations, with erosion rate and particle size distributions (latter not shown) serving as additional signatures for two phases of erosion. The clear PlexiglasTM core allowed for detailed observations of sediment erosion and when combined with

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the simultaneous measurement of turbidity and particle size analysis, provides a comprehensive set of erosion measurements. 4. Discussion Previous studies (Tolhurst et al., 1999, 2000; Widdows et al., 2007; Bohling, 2009) examined different erosion devices, but found that comparisons proved difficult due to different operating parameters including the size and duration of the shear velocity increments, sediment area covered by the device, and the type of flow exerted. There are a number of difficulties associated with quantitative prediction of erosion of sediments. These include the multitude of erosion devices available and the different protocols used to calculate the derived erosion parameters (Sanford and Maa, 2001; Sanford, 2006). Sanford (2006) emphasized the time dependence of erosion as an additional variable; not always logged since some devices are not able to collect time series data. The choice of erosion devices is largely dependent on the research questions. Our results demonstrate that the BEAST serves as a useful proxy for sediment erosion by quantifying multiple variables such as erosion rate, erosion threshold, and particle size information (Walker et al., 2008; Walker and Grant, 2009). The BEAST however, also has certain limitations. As noted earlier, the device has variations in stress temporally for a given oscillation rate, with radial distance, and with distance between plunger and sediment, although these have been noted by others [e.g., Gust and Müller, 1997; Vollmer and Kleinhans, 2007], and are not unique to this specific device. Another potential limitation may be the potential of the high frequency pressure fluctuations to influence deep sediment mobilization, because at these oscillation rates, the pressure and stress fluctuations may not be that different from wave forcing. However, the BEAST remains a useful field-device for quantification of sediments providing researchers take the necessary care to minimize these potential sources of error. The clear PlexiglasTM cores also allowed detailed observations of sediment erosion, along with simultaneous measurements of % transmission (to derive turbidity) and particle videography. For example, in our work with microbial mats (Walker and Grant, 2009), visual observation of the sediment surface during erosion is essential, a feature absent in some in situ devices. Calibration to absolute flow measurements (pressure) using hot film measurements for calibration of the BEAST was an obvious advantage in this study, but in order to compare the BEAST to other disparate methods, it is useful to emphasize the advantages of the BEAST which include: correspondence between expected and observed flow characteristics, ample sediment surface for observation of erosion, thorough mixing of turbidity in the water column for erosion rate determination, portability for field applications, and potential for particle imagery. In addition, we found correspondence between predicted and measured erosion of sand-sized sediments, an important standard for calibration of erosion devices (Widdows et al., 2007). On this basis, our study demonstrates that the erosion device first proposed by Tsai and Lick (1986), now re-configured and calibrated, can be used to quantify sediment erosion thresholds, erosion rate, and particle size response under a range of conditions. Given that the BEAST is robust, inexpensive and portable, sediments from different sites may be accurately compared. Whilst the BEAST poses variations in stress for a given oscillation rate, these variations are predictable, and given the strong relationship between u∗ and RPM will provide users of the device with confidence to measure and compare sediment erosion from different sampling sites. We have conducted erosion trials with this device from East, West, and Arctic coasts of Canada in both intertidal and subtidal sediments, and are able to confirm its reliability and portability (Walker et al., 2008; Walker and Grant, 2009; Walker et al., 2005; Lintern et al., 2005). In addition, with storage of cores at appropriate light and temperature conditions we are able to collect multiple samples in spare core liners allowing high throughput in erosion experiments. This feature is particularly important in enabling replication of cores from single sites. Recent studies using the BEAST have confirmed that cores collected by hand using divers had similar erosion characteristics to those subsampled from within Ekman grabs (Walker and Grant, 2009). Although circulation in the chamber has not been fully elaborated, the magnitude of observed shear velocity is consistent with predictions of the force on the water by the oscillating plunger, and erosion thresholds measured for sand of known sizes. Secondly, we have quantified radial distribution of stress as well as the effects of piston height on bed stress. While further studies of

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fluid behavior in the chamber could be undertaken, the present level of calibration provides a fully functional erosion device. In fact, if the chamber were reproduced with identical specifications, it would be ‘pre-calibrated’, a significant benefit to other users. The low cost of the BEAST materials (< $1000 USD) make reproduction easily attainable. Visual observation of erosion stages (prior to excess turbidity in the chamber) has proved valuable in understanding resuspension from biologically active sediments. This is particularly true of erosion studies with microbial mats where armoring is so important to the erosion process (Walker and Grant, 2009). We were also able to photograph the core surface prior to erosion as well as sample the sediment following the experiment for grain size and other textural measurements. Although the relatively small size of the core liner is advantageous for these reasons, it limits the incorporation of larger features such as sand ripples or larger biogenic features. 5. Conclusions Our work in modeling the role of resuspension in benthic–pelagic coupling is limited by accurate estimates of erosion characteristics, whose prediction is often mired in complex biophysical interactions. Ready access to direct measurements of erosion via the BEAST provides the empirical data to improve mechanistic understanding of these processes and further strengthen predictive capability in modeling efforts. Our studies indicate that the oscillation rate of the piston in the BEAST is approximately linearly related to shear velocity on the bottom of the device, as expected from theoretical considerations. In addition, the predicted erosion threshold of sand-sized particles corresponds to measurements made within the device. Coupled with verified applicability of stress sensor calibration, these sources of BEAST validation demonstrate its capability to provide quantitative measurements of transport parameters from intact natural marine sediments. Acknowledgments The re-design and building of the BEAST formed part of the equipment necessary for studies in the Canadian Arctic Shelves Exchange Study (CASES), a Research Network funded by the Natural Sciences and Engineering Research Council of Canada (NSERC). We thank Bryan Schofield and Mark Merrimen for fabrication of the BEAST and making subsequent modifications and improvements. We are thankful to Marie-Claude Archambault for assistance with the laboratory foundry sand analysis. J.G., T.R.W., P.S.H. and D.G.L. contributed equally to every aspect of this study. References Aberle, J., Nikora, V.I., Walters, R., 2004. Effects of bed material properties on cohesive sediment erosion. Mar. Geol. 207, 83–93. Amos, C.L., Feeney, T., Sutherland, T.F., Luternauer, J.L., 1997. The stability of fine-grained sediments from the Fraser river delta. Estuar. Coast. Shelf Sci. 45, 507–524. Amos, C.L., Grant, J., Daborn, G.R., Black, K., 1992. Sea carousel-A benthic, annular flume. Estuar. Coast. Shelf Sci. 34, 557–577. Black, K.S., Cramp, A., 1995. A device to examine the in situ response of intertidal cohesive sediment deposits to fluid shear. Cont. Shelf Res. 15, 945–954. Bohling, B., 2009. Measurements of threshold values for incipient motion of sediment particles with two different erosion devices. J. Mar. Syst. 75, 330–335. Brumley, B.H., Jirka, G.H., 1987. Near-surface turbulence in a grid-stirred tank. J. Fluid Mech. 183, 235–263. Dade, B.W., Davis, J.D., Nichols, P.D., Nowell, A.R.M., Thistle, D., Trexler, M.B., White, D.C., 1990. Effects of bacterial exoploymer adhesion on the entrainment of sand. Geomicrobiol. J. 8, 1–16. Davis, W.R., 1993. The role of bioturbation in sediment resuspension and its interaction with physical shearing. J. Exp. Mar. Biol. Ecol. 171, 187–200. De Deckere, E.M.G.T., Tolhurst, T.J., de Brouwer, J.F.C., 2001. Destabilization of muddy intertidal sediments by benthos. Estuar. Coast. Shelf Sci. 53, 665–669. Denny, M.W., 1988. Biology and the Mechanics of the Wave-swept Environment. Princeton Univ. Press. Dyer, K.R., 1989. Sediment processes in estuaries: future research requirements. J. Geophys. Res. 94, 14327–14339. Eisma, D., Bernard, P., Cadee, G.C., Ittekkot, V., Kalf, J., Laane, R., Martin, J.M., Mook, W.G., Put, A., van Schuhmacher, T., 1991. Suspended-matter particle size in some west-European estuaries; Part 2: a review on floc formation and break-up. Netherl. J. Sea Res. 28, 215–220. Fujisawa, N., Takizawa, Y., 2003. Study of feedback control of edge tone by simultaneous flow visualization, control and PIV measurement. Meas. Sci. Technol. 14, 1412–1419.

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Size sorting of fine-grained sediments during erosion: Results from the western Gulf of Lions. Cont. Shelf Res. 28, 1935–1946. Lintern, D.G., Hill, P.R., Solomon, S., Walker, T.R., Grant, J., 2005. Erodibility, sediment strength and storm resuspension in Kugmallit Bay, Beaufort Sea, in: Canadian Coastal Conference 2005, Conférence canadienne sur le littoral, Dartmouth, NS, pp. 13. Lintern, D.G., Macdonald, R.W., Solomon, S.M., Jakes, H., 2011. Beaufort Sea storm and resuspension modeling. J. Mar. Syst. http://dx.doi.org/10.1016/j.jmarsys.2011.11.015. Lintern, D.G., Sills, G.C., Feates, N., Roberts, W., 2002. Erosion properties of mud beds deposited in laboratory settling columns. In: Winterwerp, J.C., Kranenburg, C. (Eds.), Fine Sediment Dynamics In the Marine Environment. Elsevier, pp. 343–357. Maa, J.P.Y., Kwon, J.I., Hwang, K.N., Ha, H.K., 2007. Critical bed shear stress for cohesive sediment deposition under steady flows. J. Hydraul. Eng. 134, 1767–1771. 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Tolhurst, T.J., Black, K.S., Paterson, D.M., Mitchener, H.J., Termaat, G.R., Shayler, S.A., 2000. A comparison and measurement standardization of four in situ devices for determining the erosion shear stress of intertidal sediments. Cont. Shelf Res. 20, 1397–1418. Tolhurst, T.J., Black, K.S., Shayler, S.A., Mather, S., Black, I., Baker, K., Paterson, D.M., 1999. Measuring the in situ erosion shear stress of intertidal sediments with the Cohesive Strength Meter (CSM). Estuar. Coast. Shelf Sci. 49, 281–294. Tolhurst, T.J., Gust, G., Paterson, D.M., 2002. The influence of an extracellular polymeric substance (EPS) on cohesive sediment stability. In: Winterwerp, J.C., Kranenburg, C. (Eds.), Fine Sediment Dynamics In the Marine Environment. Elsevier, pp. 409–425. Tsai, C.-H., Lick, W., 1986. A portable device for measuring sediment resuspension. J. Gt Lakes Res. 12, 314–321. Vignaga, E., 2012. The effect of biofilm colonization on the stability of non-cohesive sediments, PhD thesis, University of Glasgow, p. 263. Vollmer, S., Kleinhans, M.G., 2007. Predicting incipient motion, including the effect of turbulent pressure fluctuations in the bed. Water Resour. Res. 43, W05410. Walker, T.R., Grant, J., 2009. Quantifying erosion rates and stability of bottom sediments at mussel aquaculture sites in Prince Edward Island, Canada. J. Mar. Syst. 75, 46–55. Walker, T.R., Grant, J., Cranford, P., Lintern, D.G., Jarvis, P., Barrell, J., Nozais, C., 2008. Suspended sediment and erosion dynamics in Kugmallit Bay and Beaufort Sea during ice-free conditions. J. Mar. Syst. 74, 794–809.

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Walker, T.R., Grant, J., Hill, P.S., Cranford, P., Lintern, D.G., Scofield, B., 2005. Measuring particle dynamics in Arctic and mussel aquaculture environments, in: Canadian Coastal Conference 2005, Conférence canadienne sur le littoral, Dartmouth, NS, pp. 11. Watts, C.W., Tolhurst, T.J., Black, K.S., Whitmore, A.P., 2003. In situ measurements of erosion shear stress and geotechnical shear strength of the intertidal sediments of the experimental managed realignment scheme at Tollesbury, Essex, UK. Estuar. Coast. Shelf Sci. 58, 611–620. Wiberg, P.L., Smith, J.D., 1987. Calculations of the critical shear stress for motion of uniform and heterogeneous sediments. Water Resour. Res. 23, 1471–1480. Widdows, J., Brinsley, M.D., Bowley, N., Barrett, C., 1998. A benthic annular flume for in situ measurement of suspension feeding/biodeposition rates and erosion potential of intertidal cohesive sediments. Estuar. Coast. Shelf Sci. 46, 27–38. Widdows, J., Friend, P.L., Bale, A.J., Brinsley, M.D., Pope, N.D., Thompson, C.E.L., 2007. Inter-comparison between five devices for determining erodability of intertidal sediments. Cont. Shelf Res. 27, 1174–1189. Williamson, H.J., Ockenden, M.C., 1996. ISIS: An instrument for measuring erosion shear stress in situ. Estuar. Coast. Shelf Sci. 42, 1–18. Wolter, J.D., 2005. Drag Measurements of Porous Plate Acoustic Liners, 43rd Aerospace Sciences Meeting, American Institute of Aeronautics and Astronautics, Reno, Nevada, National Aeronautics and Space Administration Report Number NASA TM2005-213570, AIAA-2005-0803. p. 15.