Sea Carousel—A benthic, annular flume

Estuarine, Coastal and Shelf Science (1992) 34, 557-577

Sea C a r o u s e l - - A Benthic, A n n u l a r F l u m e

C a r l L. A m o s a, J. G r a n t b, G. R. D a b o r n c a n d K. B l a c k d aGeological Survey of Canada, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia, Canada, bDepartment of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada, CAcadia Centre for Estuarine Research, Acadia University, Wolfville, Nova Scotia, Canada and aDepartment of Earth Sciences, University of Wales, Swansea, Wales, U.K. Received 25 March 1991 and in revised form 27 December 1991

A benthic annular flume (Sea Carousel) has been developed and tested to measure in situ the erodibility of cohesive sediments. The flume is equipped with three optical backscatter sensors, a lid rotation switch, and an electromagnetic (EM) flow meter capable of detecting azimuthal and vertical components of flow. Data are logged at rates up to 10-66 Hz. Erodibility is inferred from the rate of change in suspended sediment concentration detected in the annulus. T h e energy-density/wave number spectrum of azimuthal flow showed peaks in the energy.spectrum at paddle rotation wave numbers (k) of 14 and 7 m-~ (macroturbulent time scales) but were not significant, Friction velocity (U,), measured (1) at 1 Hz using a flush-mounted hot-film sensor, and (2) derived from measured velocity profiles in the inner part of the logarithmic layer gave comparable results for U , < 0-064 m s- i. At higher values of U , , method (2) underpredicted by up to 20%. Method (1) showed radial increases in ~r, in the annulus for ~/y> 0-32 m s- 1. Radial velocity gradients were proportional to (Uy-- 0"32 m s- ~). Maximum radial differences in U , were 10% for Uy = 0-5 m s - i. Suspended sediment mass concentration (S) in the annulus resulted in a significant decrease (10"5 %) in 0 , derived by method (1) over the range 0 < S < 208 m g l -l. These decreases were not evident in method (2). Method (1) may, therefore, be subject to changes in stress sensor calibration with changes in S. Subaerial deployments of Sea Carousel caused severe substrate disturbance, water losses, and aeration of the annulus. Submarine deployments produced stable results, though dispersion of turbid flume water took place. Results clearly demonstrated the existence of ' T y p e I ' and ' T y p e I I ' erosion documented from laboratory studies.

Introduction T h e m a j o r i t y o f studies o n the e r o d i b i l i t y o f n a t u r a l cohesive s e d i m e n t s have b e e n c a r r i e d o u t in l a b o r a t o r y flumes w h e r e i n s u s p e n d e d s e d i m e n t m a s s is u s e d as an i n d e x o f b e d erosion. T h e a p p l i c a t i o n o f such l a b o r a t o r y - f l u m e results to n a t u r a l settings m a y n o t be valid d u e to t h e c o m p l e x i t y o f n a t u r a l b e d s e d i m e n t s a n d the d e p e n d e n c e o f s e d i m e n t p r o p e r t i e s on in situ w a t e r p r o p e r t i e s ( N o w e l l & J u m a r s , 1987). T h e e r o s i o n a n d e n t r a i n m e n t o f a cohesive s e d i m e n t d e p o s i t , a c c o r d i n g to L i c k (1982), are c o m p l e x a n d i n f l u e n c e d by: ( ! ) t h e t u r b u l e n t n a t u r e o f b e d shear stresses ( S h e n g & V i l l a r e t , 1989); (2) t h e h i s t o r y o f 0272-7714/92/060557 + 21 $03.00/0

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bed deposition and subsequent consolidation, densification and dewatering (Postma, 1967; Partheniades et al., 1968; Southard et al., 1971; Ariathurai & Arulanandan, 1978; Thorn & Parsons, 1980; Hawley, 1981; Parker, 1986); (3) the mineralogy, particle size distribution and organic content of bed sediment (Yingst & Rhoads, 1978); (4) the distribution of bed properties with depth (Partheniades et al., 1968; Dyer, 1989); and (5) activity of benthic organisms and bacteria (Rhoads et al., 1978; Self et al., 1989). Also important are: (6) the effects of the chemistry and valency of cations in the sediment pore water on physico-chemical bonding of particles or aggregates (Lambermont & Lebon, 1978; Hydraulics Research Station, 1980; Gularte et al., 1980; McCave, 1984; Otsubo & Muraoka, 1988); (7) the mass and grain size spectrum of the particles suspended in the eroding fluid (W. Kamphuis, pets. comm., 1989); and (8) water temperature and salinity of the eroding fluid (Kelly & Gularte, 1981). The complexity of the natural erosion process leads us and others (Lavelle & Mofjeld, 1987; Wilkinson & Jones, 1988, Maa, 1990) to believe that realistic measures of this erosion process can only be undertaken within the natural environment. One method of measuring natural erodibility is to use field flumes. Several have been developed and tested. Black (1989) recently described eight flumes developed for field use, including Scoffm's (1968) flow-through flume, Peirce's et al. (1970) recirculating annular flume, SEAFLUME (Young, 1977), SEADUCT (Nowell et al., 1985), and Black's own mobile recirculating flume (MORF). Only the SEAF L U M E and SEADUCT were capable of submerged, remote deployment, and only SEAFLUME logged data in a digital form. The shape of the above flumes varied between linear, oval, or annular patterns. We chose to build an annular flume as this shape has some clear advantages (Partheniades et al., 1968; Mehta & Partheniades, 1979; Creutzberg & Postma, 1979; Fukuda & Lick, 1980; Lee et al., 1981; Lick, 1982; Kusuda & Umita, 1982; Burt, 1984; Wilkinson & Jones, 1988; Sheng & Villaret, 1989; Kuijper et al., 1989; Maa, 1990). It is argued that a constant channel geometry and infinite flow length results in a fully developed benthic boundary layerman essential prerequisite to the application of flume-derived bed erosion studies to the natural environment. Furthermore, analyses of the total (radial and tangential) bed stress across an annulus made by Hydraulic Research Limited (1987), Maa (1990) and by T. E. R. Jones (pers. comm., 1989) show only minor increases radially. In fact Maa predicted that the total (radial and tangential) and average bed stresses were within 20% of each other for the central 80% of the annulus. Unfortunately, the extant annular flumes were not capable of continuous and remote monitoring of flow character and bed response. Nor were they equipped with sensors to detect (1) the nature of bed failure at the floe level, and (2) the flow kinematics and dynamics leading to failure. This paper describes the development of a benthic annular flume capable of logging and quantifying in situ flow character and bed response at high (up to 10 Hz) frequencies. The flume is evaluated through laboratory calibration trials and field deployments. Background on bed erosion

There are several controversies surrounding the nature of bed erosion. The first concerns the cause of the apparent exponential decrease to zero in sediment mass (/14) erosion rate (dM/dt), observed at constant flows during bed erosion: the ' Type I ' bed erosion of Mehta and Partheniades (1982). On the one hand, Parchure and Mehta (1985) and later Sheng and Villaret (1989) defined' Type I ' o r ' floe' erosion rate as: dM/dt = E = E o e x p [ a { z o -- zb(z)}½].

(1)

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Bed erosion rate (E) under an applied fluid bed stress (to) is eventually balanced by an increase in bed shear strength (Zb) with depth (z) as erosion takes place; E o and a are coefficients given values of 4-78x 1 0 - 5 k g m - I s - l and 2-78x 10 -3 (m 2 P a - l ) °s. T h e y equate bed shear strength with the critical bed stress for erosion (r~) as t approaches longtime scales. T h i s process is independent of particle settling rate or mass deposition and implies a time-variable erosion rate and an increase in bed strength with depth (Mehta, 1989). Lick (1982), on the other hand, explains' T y p e I ' erosion as a balance between the upward flux of suspended particles from erosion (E), which remains constant, and the downward flux through settling (D) which increases through time in proportion to suspended sediment mass concentration (S). F o r flocculated, well mixed, suspended particles he proposed: d M / d t = E -- D

(2)

D is deft_ned as WsS, where W s is the mass settling velocity; E, by contrast, is less easy to define. Most researchers postulate that a critical shear stress is needed for erosion of a sediment exhibiting a Bingham yield strength (see Zeman, 1983 for a review). Erosion rate is expressed by Owen (1975) and Sheng and Lick (1977) and Villaret and Paulic (1986) in the form of: d M / d t = E = B ( r o - zb)"

(3)

where r b is the critical stress for bed erosion, and B is the erosion rate constant. Note that E is a function of the excess bed stress and shows no time dependency. T h a t is, erosion is constant and continuous: ' T y p e I I ' erosion of Mehta and Partheniades (1982). Ariathurai and Arulanandan (1978) and Ariathurai and K r o n e (1976) also ascribe to this type of erosion, but of th..e form: d M / d t = E = B(zo/r b - 1).

(4)

Notice that equations 3 and 4 do not account for changes in bed strength with eroded depth nor for changes in S. H o w do we reconcile' T y p e I I "erosion with these differences of opinion? T h e uncertainty of the time-dependency of E is a major obstacle to the prediction of free-grained sediment stability. In-so-far as most of the data on which the above controversies are based are derived f r o m laboratory experimentation, we concluded that it was first necessary to show that the two T y p e s of erosion occur in nature. Instrumentation and calibration S y s t e m configuration

Sea Carousel, named after the carousels of Postma (1967) and Hydraulic Research Limited (Burt, 1984), is a benthic annular flume designed for field use in intertidal and subtidal settings. T h e carousel is 1-0 m in radius with an annulus 0.15 m wide and 0-30 m high (Figure 1). It weighs approximately 150kg in air and 4 0 k g in water and is made entirely of aluminium. Flow in the annulus is induced by rotating a movable lid that is driven by a 0-35 lap m o t o r powered f r o m the surface. Eight small paddles, spaced equidistantly beneath the lid, induce a flow of water in the annulus. T h e width of the annulus (D) was made 0.15 m to give a relative roughness (e/D) ~ 0.004 (where the wall roughness, e~,0.0006 m; after Shames, 1962). T h e water depth in the annulus was minimized to 0.25 m to ensure conditions for Nikuradse's ' rough-pipe zone of flow ' wherein changes in wall friction factor with changes in Reynolds n u m b e r are at a m a x i m u m (Shames, 1962).

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Figure 1. A schematic diagram in cross-section of Sea Carousel giving its general dimensions. A schematic diagram of the Sea Carousel configuration is shown in Figure 2. I t is equipped with three optical backscatter sensors (OBS's; Downing, 1983). T w o of these arc located non-intrusively on the inner wall of the annulus at heights of 0.03 and 0.18 m above the skirt (the skirt is a horizontal flange situated around the outer wall of the annulus 0.04 m above the base; it was designed to standardize penetration o f the flume into the seabed; see Figure 1). T h e third OBS detects ambient particle concentration outside the annulus, or it may bc used to detect internal sediment conccntration at a height between the other two. T h e OBS sensors give linear responses to particle concentration (of a constant size) for both m u d and sand over a concentration range o f 0.1 to 50 g 1(Downing & Beach, 1989). T h e y arc unaffected by flows below 1-5 m s -1 and arc stable through time. A sampling port is situated in the outer wall of the annulus at a height of 0-2 m above the skirt through which water samples can be drawn to calibrate the three sensors under well mixed conditions. A M a r s h / M c B i m e y current m e t e r (model 511) is located on the ccntrcline o f the annulus at a height of 0-16 m above the skirt. I t was used to detect the instantaneous azimuthal and vertical components of flow within the annulus (Uy and U~ respectively). Mean tangential lid rotational speed (Lrr) is detected through a magnetic reed switch triggered by the passage o f 12 magnets spaced equidistantly around the lid. Controllcr boards for each sensor and the necessary power (12 vdc) arc derived from an underwater pod located above the annulus. O u t p u t voltages from all sensors are digitized and transformed to scientific units on a Campbell Scientific CR10 data logger and stored on a Campbell Scientific SM192 storage module (storage capacity of 96 000 data values), also located in the underwater pod. T h e data logger is interrogated and p r o g r a m m e d f r o m the surface using a microcomputer linked to the data logger through an RS232 interface M a x i m u m sampling rate of all channels is approximately 2 Hz, whereas Uyand U,~may be logged at rates up to 10-66 Hz. All channels m a y be monitored and displayed on the surface computer allowing the operator to control the experiment interactively. Bed shear stress is varied in time by varying the power supplied to the underwater m o t o r up to 350 watts via a surface power supply. T h e data stored f r o m each deployment m a y be downloaded remotely through the RS232 cable at the end of each experiment and the storage module re-initialized. A window is located in the inner flume wall for purposes o f observing and recording the mechanics of bed failure. A perspex wedge at the base of the window sections the sediment

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upon deployment. T h u s the upper 20 m m of sediment and the lower 10 cm of the water column can be viewed in section. Visual observations are made using a Sony Handycam 8 m m video recorder model C C D - V 1 1 held in an Amphibico Amphibian V l l underwater housing. Light is provided by two 100 watt underwater lights powered from the surface. T h e housing has a lens that corrects for underwater geometric distortions and so is suitable for accurate image scaling. T h e camera lens is located approximately 20 m m from the

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window. Horizontal and vertical scale lines are present on the window and situated within the field of view. T h e camera images 100 frames/s. A co-axial cable connects the camera to a surface monitor for real-time detection. Video records are stored on a standard V H S video cassette recorder also at the surface. Sequential video images are digitized for particle trajectories at varying heights above the bed. F r o m these, velocity profiles are constructed. From such profiles, thicknesses of the logarithmic part of the benthic boundary layer are determined (for comparison with results o f equation 7) and friction velocities computed (using equation 8). These latter values were then compared with laboratory measures.

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Flume Reynolds numbers (Re) for 0.1 < ~/y < 0.5 m s- 1are 4,7 calculated by the expression Re = (pr]yRh)/~t

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where the fluid density p = 1026 kg m -3, the absolute viscosity p is normalized to 10 °C = 0-00013 kg m - t s -~, and the flume hydraulic radius R h = 6 0 ram. Eroding flows are thus fully turbulent. Variations in Reynolds number within this range result in only a 3% variation in the wall boundary layer velocity profile, and so time variations in wall effects on our flow measurements are ignored. Azimuthal velocity profiles carried out by Fukuda and Lick (1980) in an annular flume showed that the thickness of the viscous sublayer (~') is defined by the same relationship as in straight channels or ducts

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where ~/, is the time-averaged friction velocity. Similarly, the maximum thickness (fi) of the logarithmic part of the benthic boundary layer is defined by Gust (1976) as pU.,~lp. ~ 300.

(7)

T h e predicted thicknesses of the benthic viscous sublayer and the logarithmic layer in the flume for ~ r , = 16 m m s -I (a typical friction velocity for bed erosion; Nowell et al., 1981) are 0-9 m m and 30 m m respectively. I f we approximate a typical mudflat roughness as the floc diameter of material forming it (Yingst & Rhoads, 1978): 0-03 ram, then bed erosion would begin under dynamically smooth turbulent flow conditions. Although this has the advantage of allowing a simplification of bed roughness in the computation of bed stress, it is sensitive to changes in thickness of the viscous sublayer induced by changes in S. Measurements of the viscous sublayer made by Gust (1976) and Buchholtz-Ten Brink et al. (1989) show that turbidity expands this layer by a factor of 2 at 500 mg l -~ and 3 at 10 g 1-I. T h e impact of this on our study is addressed in the results. T h e majority of the flume is expected to be occupied by the turbulent outer layer where time-averaged velocity, ~ry, is a function of the velocity-defect law. T h e velocity profile for the centreline clear-water velocity (Uy) in the logarithmic part of the overlap layer may be approximated by ~_lflU. = 1/k ln(y/yo)

(8)

where k is yon Karman's constant (0-4) a n d y o is the roughness length (0.4 mm; see later section). T h e outer layer is independent of turbulent scales, absolute viscosity and bed roughness

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where y is the height of the measurement of ~ry and d is flow depth. F r o m the laboratory measures of U , , the constantfwas evaluated as 5-94 ( + 0.35). Uy is transformed to friction velocity ( 0 , ) through a laboratory-derived empirical relationship (see later section). Finally, the resulting time-average bed stress may be determined from the relationship ro = p 0 . 2

(10)

Results

Optical backscatter sensors T h e optical backscatter sensors on Sea Carousel were calibrated in situ in a saline waterfilled test tank using known suspended sediment concentrations (S) of Minas Basin mud. Sediment concentration was derived by vacuum filtration and determined gravimetrically. T h e sensor response to S was linear. Calibration fits are shown in Figure 3. All three sensors yielded high calibration variances (r 2 = 0-96). T h e response line for the ambient OBS (2) passes through the origin, whereas the other two sensors (1 and 3) record a zero offset. This indicates that they are affected by backscatter from the opposite flume wall as no clear water offset was evident when removed from the flume. These effects were accounted for in the derivation of S. Calibrations of the OBS sensors were also made during each deployment due to the sensitivity of the OBSs to changes in grain size. This was achieved by sampling through the sample port at intervals throughout an experiment. Lid rotation and azimuthal current T h e verification of lid rotational speed ( 0 r) was carried out by timing 10 revolutions o f the lid at a range of speeds. N o apparent long-term drift in lid rotation occurred nor is there evidence of excessive signal noise. Lid rotation speed was predicted within + 4%. T h e E M current meter was calibrated in a test tow tank at Bedford Institute of Oceanography. Results showed a linear stable calibration in both X and Y coordinates. It

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Figure 4. A scattergram of the time-averaged (20 s) azimuthal current speed (Uy)derived from the EM flow meter and from measures of current speed derived from suspended particle trajectories digitized from video imagery. A good correlation between the two data sets is seen. 0, Station 6 and O, Station 7. Scatter is presumed due to turbulent fluctuations in the video results which were digitized in 0-01 second increments.

was also tested in place against measures of short-term (0.05 s) particle trajectories scaled from video images. T h e correlation between azimuthal current speed measured between the E M current meter (time-averaged over 20 s) and video observations (for the same height above the bed) is shown in Figure 4. T h e data are based on results of two field deployments on natural estuarine mud. A good correlation exists over the range of velocities tested. T h e scatter is considered to be the result of turbulent fluctuations in velocity detected in the video records which includes both the mean (time-averaged) component of flow and the turbulent component. Time-averaged azimuthal current was generally stable and showed a clear relationship to lid rotation. This relationship is linear over the entire test range. Uy = 0-574[/, + 0.025 (m s - l ; r 2 = 0.92; n = 180)

(11)

T h e relationship is unaffected by changes in suspended sediment concentration (up to 208 mg 1-1) Figure 5(a) or temperature (between the 4-5 and 18 °C) Figure 5(b). We have, therefore, neglected, changes in fluid viscosity resulting from changes in S or water temperature. I t is however affected by salinity. Measures of Us are consistently 0-05 to 0-1 m s-~ greater in saline water (34 ppt) than in brackish water (0.16 ppt). This offset is presumably the result of changes in response to the E M flow meter over the range in salinities tested. Day to day variations in the zero offset of the E M meter were up to +__0-02 m s-~. Still-water calibrations are therefore necessary prior to each experiment. Turbulence High-frequency (10.66 I-Iz) sampling of Uy and U~ was undertaken to evaluate system noise and the turbulent structure in the annulus (salinity = 34 ppt; temperature = 4.5 ° and 18 °C). An example of the still-water time-series is shown in Figure 6. A zero-offset of approximately 0.01 m s-~ is apparent in the azimuthal velocity. Also, fluctuations in both components of velocity were detected with an amplitude SD of 4.7 x 10 -3 m s -1.

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Figure 5.(a). A scattergram of lid rotation vs. azimuthal current speed for a range of sediment concentrations (0 < S < 208 mg 1-'; salinity= 0.16 ppt). There is a strong relationship between the two variables irrespective of S. The scatter at low rotation and the zero-flow offset of the EM flow meter are evident. Figure 5(b). A scattergram of lid rotation vs. azimuthal current speed for two water temperatures (O, 4'5 and O, 18 °C; salinity = 34 ppt). Water temperature has little effect on the relationship of the two variables.

T h e t u r b u l e n t c h a r a c t e r o f t h e flow was d e t e r m i n e d o v e r a w i d e r a n g e o f lid r o t a t i o n s a n d lid accelerations. T h i s was e x p r e s s e d as a c u r r e n t s p e e d s t a n d a r d deviation, w h i c h for t h e h i g h e s t flows in Sea C a r o u s e l was 2-6 x 10 -2 m s - 1 for U~, a n d 0.16 m s - i for [/y. I t s h o w e d o n l y a w e a k r e l a t i o n s h i p to Uy a n d n o a p p a r e n t r e l a t i o n s h i p to c u r r e n t a c c e l e r a t i o n o r d e c e l e r a t i o n . T h i s i m p l i e s t h a t the flow t u r b u l e n c e g e n e r a t e d b y changes in p a d d l e r o t a t i o n is n o t e n h a n c e d d u r i n g n o r m a l use. S p e c t r a l analysis o f Uy reveals an o r d e r l y s t r u c t u r e to flow m a c r o t u r b u l e n c e . F i g u r e 7 s h o w s s p e c t r a l d e n s i t y p l o t t e d against w a v e n u m b e r (k = 2re/~_fyt)for a m e a n s p e e d o f 0-42 m s - 1. T h e p a d d l e w a v e n u m b e r (t = 1 / p a d d l e f r e q u e n c y ) for an a z i m u t h a l c u r r e n t s p e e d o f 0.42 m s -1 is 14 m -1. N o t i c e t h a t t h e r e is a n a r r o w p e a k in t h e e n e r g y d e n s i t y at this f r e q u e n c y . T h i s indicates t h a t t h e p a d d l e s g e n e r a t e m e a s u r a b l e t u r b u l e n c e . I t is h o w e v e r , small c o m p a r e d to t h e e n e r g y o f t h e s p e c t r u m as a whole. I t is n o t e d t h a t t h e influence o f t h e p a d d l e s will increase t h e likelih o o d o f ' b u r s t i n g ' a n d ' s w e e p i n g ' o f t u r b u l e n t e v e n t s in t h e viscous s u b - l a y e r at t i m e scales in excess o f 1 s. A s e c o n d p e a k in t h e s p e c t r u m (at k = 7 m - ] ) c o r r e s p o n d s to 1/2 t h e f r e q u e n c y o f p a d d l e passage a n d m a y b e r e l a t e d to e c c e n t r i c i t y o f lid rotation. T h e

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Figure 7. T h e log spectral energy density from a fast Fourier transform o f azimuthal flow ( U~ = 0-42 m s- i) plotted against log wave number. Spectral energy decreases systematically with increasing frequency. A peak in the spectrum at k = 14 m - i is correlated with paddle rotation. A peak at k = 7 m - ~is also apparent.

majority of the energy of the flow is found at wave numbers less than 10 m - 1 i.e. within macroturbulent events. These events are controlled by the geometry of the flume and by time variations in paddle rotation. T h e illustrated spectrum is consistent for all water salinities and temperatures tested. It is unclear if the falloffin spectral energy at high wave numbers is related to detection limits of the 38. I m m head of the current meter, or to a

Sea Carousel--a benthic, annular flume

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genuine reduction in energy found in natural benthic boundary layers (West et al., 1986; Veth, 1990).

The logarithmic layer, roughness length, and friction velocity T h e thickness of the logarithmic layer and the evaluation of roughness length and friction velocity was derived under 18 differing flow conditions during two field experiments. Examples of the velocity profiles are shown in Figure 8. Measures were taken between heights of 10 m m and 10 cm above the bed. Each value is the average of 10 separate measures of particle velocity. Note the existence of logarithmic profiles close to the bed, and the scatter in results above. T h e logarithmic layer varies in thickness from 3 to 10 cm. T h i s range encompasses the values reported b y Middleton and Southard (1984). Equation 8 is fitted to these data. T h e mean roughness length (Yo) derived from the velocity profiles is 4 m m . T h i s is larger than the observed bed roughness (0.1 m m ) and supports the presence of smooth turbulent flow conditions throughout the flume experiments. Total U , was measured at a 1 H z sampling rate for a series of differing flow velocities at five positions radially across the annulus. Measurements were made using an omnidirectional, flush-mounted, hot-film probe of G u s t (1988) embedded in the base of a smooth fibre glass test tank. Voltage output of the hot-film probe was calibrated directly to friction velocity which was determined from the pressure gradient in pipe flow (J. Grant, unpubl, data). Results from this sensor are compared with values o f azimuthal U , calculated from velocity profiles derived from the video observations. T h e unfiltered hot-film measurements of friction velocity reveal significant fluctuation with time even at constant flows. Figure 9 shows results recorded 0.05 m from the outer flume wall at four different values of ~Ty.T h e standard deviation in friction velocity (0.41 cm s -1) shows no relationship to its mean value. However, the time-averaged friction velocity yields stable results that illustrate a variation in bed stress radially across the annulus. Figure 10 shows measurements of time-averaged friction velocity across the annulus for four differing values of Uy. Notice that the cross-channel gradient in friction velocity increases with azimuthal current speed.

C.L. Amos et al.

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The root-mean-square value of the time- and width-averaged friction velocity (~7.rms) is correlated with ~Tyin Figure 11. The relationship of ~7.rms and ~Tyis good, and takes the form

Sea Carousel--a benthic, annular flume

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~/,rms = 0.0167 + 0-097[/y (m s-i; r 2 = 0.96)

(12)

T h u s [ / , r m s is used as the standard hydrodynamic measure in this study. By manipulation of the quadratic stress law (equation 14), a drag coefficient (C arelating Uy and [ / , is derived U.rms = and

thus

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(15)

This results in a value of Ca= 4.0 x 10 2 and a corresponding Calooof 1"3 x 10 -3. T h e latter coefficient falls within the range of values detected over flat bed in the field observations of Sternberg (1972). T h e friction velocity measured above is compared to values derived from the observed velocity profiles plotted in Figure 12. Notice that there is good agreement for ~7,y < 0"09 m s-1. Above this value field measures consistently underpredict. T h e field derived measures of [7,y are based upon the azimuthal component of flow, whereas the laboratory bed stress is the maximum (resultant) value. We propose therefore, that the under-prediction reflects the cross-channel component of flow. We have suggested that this flow becomes significant when Uy > 0.32 m s- 1 This corresponds to a value of ~7,rms (from equations 13 and 14) of 0 - 0 6 4 m s -l. This is close to the value where underprediction of the field method was seen to begin (see Figure 12). Assuming this to be the case, we may evaluate the radial component of friction velocity ([7, r) in terms of the azimuthal component of friction velocity ([/%): ~/., = 0"38~/.y;

U*y > 0"064m s -1

(16)

570

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Date: 19 July, 1990

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T h i s corresponds to a veering of the resultant friction velocity vector approximately 20 ° from azimuthal. T h i s conclusion fits with observations of Butt (1984) who measured radial flow in a laboratory annular flume. Ambiguities due to the presence of secondary flows are avoided in this study by use of the total stress ([J.rms).

The effect of S on friction velocity T h e effects of suspended sediment concentration on the time-averaged, hot-film friction velocity measured near the centre of the annulus are evaluated for a constant Oy (0-43 m s - 1). Sensor drift during the experiment was eliminated by a still-water test before and after the experiment. Results of tests at five differing concentrations (in m g 1- i) are illustrated in Figure 13. A clear decrease in friction velocity is detected over the concentration range and follows the exponential form

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(17)

Notice that the greatest reduction in U , occurs for 0 < S < 50 m g 1-1. I t is unclear from G u s t ' s papers whether the decrease in friction velocity is the result of a genuine stress reduction or due to changes in sensor response. W e m a y evaluate this by reference to the video-derived friction velocities which include the effect of S (0 < S < 300 m g 1-~). T h e friction velocity derived from equation 12 is for clear seawater. We transform this value using equation 17 and plot the result against the video-derived friction velocity. T h e S-induced changes in stress are indicated by arrows in Figure 12. C o m p a r e results for station 6 in this Figure to the uncorrected values. Notice that now the laboratory friction velocities significantly underpredict compared to the field measures. Such bed stress reductions should be associated with a 2-3 times thickening of the viscous sub-layer (to 2-3 cm) and due to buoyancy effects related to high turbidity in this layer (Sheng & Villaret, 1989). We saw no such effects in the video recordings of the field deployments.

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We conclude that the increases in bed stress indicated by the hot-film sensor is an artifact, induced by turbidity.

Dispersion of suspended sediment Dispersion of suspended sediment out of the rotating annulus was observed on the video to take place during submerged deployments of Sea Carousel. Dispersion results from exchanges of water mass between the annulus (at concentration S~) and the open marine environment (at concentration So) where S I > S o. T h e rate of diffusion of mass (M) may be defined per unit cross-section area as a M / a t = - 6 aS/dx

(18)

where 8 is the coefficient of diffusivity ( L 2 T - i) and x is a typical horizontal length scale, which in our case in unknown. Similarly, the change in mass in Sea Carousel m a y be defined as a M / a t = - 8 aS/ax Ae/V

(19)

where A is the area over which diffusion takes place (0.012 m2), V is the volume of Sea Carousel (0-218 m3), and e is an efficiency term dependent on the azimuthal velocity (ear Uy). Measurements of a M / a t at different constant azimuthal velocities yield a concentration of half-life (S½) of 2400 s, setting e = Uy and ~S/dx to (S~ - So), the quotient ( - dA/~x) is derived 0M = - 3.3 x 10-3(S~ - So)Uy at

(20)

T h e loss of mass through dispersion, calculated using equation 20, is added to measured annulus mass ( S I r) to derive a measure of the total mass [Figure 14(b)].

Field measures of erosion rate T h e method used to collect information on bed erosion and erosion rates is similar to that proposed by Mehta et al. (1982), T h o r n and Parsons (1980), Kuijper et al. (1989) and

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Sea Carousel--a benthic, annular flume

573

Sheng and Villaret (I 989). T h a t is, m increase near bed flow and hence bed stress in steps through time and to hold the bed stress constant between each step (At) of 10-20 min. Lid rotation, azimuthal velocity, vertical velocity, and the three measures of suspended sedim e n t concentration were sampled and logged at 1 Hz. T i m e series of each parameter were generated after time-averaging (20 s) and despiking the data between 2 SDs of the 20 s mean values. T r a n f o r m s of velocity to bed stress were carried out based on the calibration results above and assuming a smooth bed. T o t a l suspended mass (M) was computed each 20 s. T h e erosion rate was determined from the differential concentration (EooAS/At) of successive 20 s time-averaged measures of S. T h e depth of erosion (z) was estimated as z = M/Aps, where A is the annulus area (0-873 m 2) and Ps is the measured sediment bulk density (in the case presented ps = 1850 kg m -s, Christian et al., 1990). Surface critical shear stress is determined following Mehta and Partheniades (1982) and Mehta et al. (1982) i.e. by assuming that the measured bed shear stress is equivalent to the aggregate shear strength at the depth in the sediment at which erosion ceases. T h e surface critical shear stress (z¢) is derived by extrapolation of the least squares best-fit line to the surface. T h i s method can only be used under conditions o f ' T y p e I ' erosion. U n d e r other conditions, z¢ may be derived by examining erosion rate as a function of applied bed stress on a series of fresh surfaces, zc is found from the zero intercept of erosion of a least squares regression line of the two variables. T h e results of a 1.8 h submerged deployment o f Sea Carousel on an intertidal mudflat in Minas Basin is shown in Figures 14(a-d). L i d rotation speed was increased in nine steps of equal power output. Associated increases in azimuthal and vertical (downward-directed) velocity are apparent in Figure 14(a). T h e time-series of S (raw and corrected for diffusion as discussed above) is shown in Figure 14(b) (all three O B S ' s were used in the annulus). Note that erosion appears to take place at all velocities and that b o t h ' T y p e I ' a n d ' T y p e I I ' erosion are evident..There is no clear evidence of stratification in S. ' T y p e I ' erosion typifies flows where Uy<0.45 m s -~, ' T y p e I I ' erosion prevails at stronger flows. Bed erosion is more clearly demonstrated by reference to Figure 14c. Here, a rapid peaking in erosion rate within 40 s of flow acceleration is evident in the seven highest of the nine flow accelerations.' T y p e I ' peak erosion rate is approximately 2-0 x 10 -4 kg m -2 s -~ and does not appear correlated with azimuthal velocity. Also the rate of erosion drops quickly back towards zero in an exponentially decaying fashion. ' T y p e I I ' peak erosion rate shows an increase with ~Ty.T h i s peak is less clearly developed and drops back towards a constant erosion rate that also increases with Lry. N o t e that the scatter of the background signal of ' T y p e I I ' erosion is significantly greater than that f o r ' T y p e I ' erosion [Figure 14(c)]. T h e significance of this is evident in the video observations discussed in the next section.

Discussion T h e measured total (azimuthal and radial) friction velocity increases radially in a linear fashion over the central 70% of the annulus (where measurements were possible) for ~Ty> 0-32 m s - l ( • , = 0-048 m s - 1). In-so-far as m o s t tested sediments eroded below this value, the radial stress gradient appears u n i m p o r t a n t in defining the threshold for erosion. It is however, an important influence on bed erosion rate at high flows during the later parts of the experiments. T h e change in U , (at the highest measured velocity, Uy = 0.5 m s - l ) is approximately 10 % for an increase in radius of 0.13 m. T h i s is equivalent to an associated radial increase in bed stress of 21%.

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T h e wall boundary layer was not evident beyond 0.02 m of the annulus walls. This suggests that the pipe-flow model for boundary layer development (discussed earlier) may not be applicable to our case, the wall boundary layer being m u c h smaller than anticipated. Flow measurements made by Hydraulics Research Limited, H R L (1987) in their annular flume showed the same linear radial increases in azimuthal current velocity (10% over 0.15 m) and equally small wall boundary effects. T h e s e data agree despite a marked difference in aspect ratios of their and our annuli (height-to-width ratios of 0.25 and 1.7 respectively), and the absence of paddles in the H R L case. O u r results conflict with theoretical predictions by M a a (1990) of bed stress across an annulus with an aspect ratio of 0-20 in a system without paddles. His results, based on a laminar flow model, suggested that the total bed stress should peak near the centre of the annulus and should decrease hyperbolically to the walls. T h e 1 H z time-variability (SD = 0.41 cm s - l ) in bed friction velocity, measured during the calibration with hot-film probes, is diagnostic ofpertubations in the viscous sub-layer. T h i s fluctuation appears to be associated with flume macroturbulence (Figure 9) and is therefore not controllable. T h i s is important when flows are at or near threshold. I n such cases, increases in S may be apparent even when the mean friction velocity is less than the actual threshold velocity in the manner outlined by Lavelle and Mofjeld (1987). T h e observed trends in S for the presented field experiment follow closely those observed in the laboratory (Owen, 1977; Lee et al., 1981; Kusada & Umita, 1982; Mehta & Partheniades, 1982; T h o r n & Parsons, 1980). ' T y p e I ' and ' T y p e I I ' erosion are apparent in Figures 14(b, c). In-so-far as our sediments are naturally deposited, the observed trends cannot be artifacts of bed preparation in the laboratory. Peak erosion rates f o r ' T y p e I ' erosion vary between 1.5 and 2.5 x 10 -4 kg m -2 s - i. T h e s e values are similar to the erosion rate of 3.3 x 1 0 - 4 k g m - 2 s - l reported by Kusada and U m i t a (1982). R e m e m b e r that actual values of S in our test were decreasing through time due to dispersion [Figure 14(b)] yet this trend appears to have no effect on E. O u r initial observations, therefore, support the proposal of M e h t a and Partheniades (1982) that there is a physical difference in the erosion process b e t w e e n ' T y p e I ' a n d ' T y p e I I ' erosion that is independent of S. This conclusion is also evident in the difference between the scatter of t h e ' T y p e I ' a n d ' T y p e I I ' time-series of erosion rate shown in Figure 14(c). ' T y p e I ' erosion shows the characteristics of rapid initial erosion rate followed by an exponential decay to zero, as predicted by Sheng and Villaret (1989).

Conclusions This paper describes the benthic flume Sea Carousel, which was developed for the evaluation of erodibility of m u d s in intertidal or submerged settings. T h e system developed is an operational success although a n u m b e r of improvements and refinements appear necessary. T h e conclusions of the study are: (1) Sea Carousel is best deployed and operated in submerged settings (even when studying intertidal flats) to minimize mudflat disturbance, water losses and aeration of the annulus. (2) T h e r e is a consistent and linear relationship between lid rotation and azimuthal current speed for a salt water (34 ppt) temperature range of 4.5 to 18 °C and for a range in suspended sediment concentrations up to 208 m g 1-1. Changes in salinity (from 34 to 0-16 ppt) result in a decrease in measured azimuthal velocity, Uy, by a constant 0-05 m s -1. T h i s appears to be due to changes in sensor response. (3) Spectral analysis of burst-sampled currents in the annulus show that turbulence structure is similar to that measured in the field. T h e r e is, however, evidence of

Sea Carousel---a benthic, annular flume

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energy input at the frequency of paddle rotation. (4) Instantaneous bed friction velocity, measured with a hot-film probe, shows a high degree of time variability at all tested current speeds. Systematic results are obtained by time-averaging measures of friction velocity over 20 s. T h e results show that when ~ry> 0-32 m s-~ friction velocity increases radially across the annulus. T h i s increase is linear and in proportion to (~ry-0.32 m s-1). T h e rms value of the spatially varying friction velocity show a high correlation with measured azimuthal velocity. (5) Dispersion of turbid water via the annulus lid was detected, and resulted in a S half-life of 2400 s. Time-series of S m u s t be corrected by this dispersion rate in order to determine true values, and subsequently the erosion rates. (6) Consistent and systematic trends in S and erosion rate were produced. Results show that both ' T y p e I ' and ' T y p e I I ' erosion of Mehta and Partheniades (1982). In the first case, erosion rate peaked within 40 s of current acceleration and thereafter dropped in an exponentially-decaying fashion to zero. In the second case, a similar, but less well developed peak in erosion rate was measured but the decay was less clearly defined and the time-variability in erosion was large.

Notation A area of annulus footprint, m2; C a dimensionless drag coefficient; d depth of flow, m; D annulus width, m; e flume wall roughness scale, m; E bed mass erosion rate, kg m - 2 s - ~; M sediment mass, kg; R h flume hydraulic radius, m; S suspended sediment mass concentration, mg 1- l; Ur lid rotation speed, m s - 1; Uy azimuthal current speed, m s - 1; Uw vertical current speed, m s - 1; U , friction velocity, m m s - ~; V annulus volume, m3; y height above seabed, m; z depth within sediment, m; d dimensionless coefficient of diffusivity; d' thickness of viscous sub-layer, ram; e dimensionless efficiency factor; r o applied bed shear stress, N m-2; re critical.shear stress for bed erosion, N m-2; Ps sediment density, kg m-3; p fluid density, k g m - 3 ; a,t/ coefficients of proportionality; Ix absolute fluid viscosity, kgms -l

Acknowledgements T h i s p a p e r is the result of a strong team effort. O u r thanks go to: R. Vine who produced mechanical designs and machine drawings from rough notes; J. H o m e who built Sea Carousel and made n u m e r o u s suggestions to help improve operation; A. At.kinson who interfaced and installed the electronic and electrical system; B. F. L o n g ( I N R S Oceanologie) who supplied us with the underwater pod; A. Robertson who built the launch pontoon and helped with deployment; F. Jodrey who provided laboratory support during calibration; and H. Christian who provided geotechnical input into data interpretation. T h e paper was reviewed by Drs D. Gillespie, D. Willis, D. Cacchione and C. Paola. It was funded under G S C project 89053 and Unsolicited Proposal Contract No. 23420-8-M565/01 from Canada D e p a r u u e n t of Supply and Services to Acadia Centre for Estuarine Research.

References Abde1-Rahman, N. M. 1962 The effect of flowing water on cohesive beds. Ph.D. Thesis presented to Laboratory for Hydraulic Research and Soil Mechanics, Swiss Federal Institute of Technology, Zurich, Switzerland.

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