Sediment transport in the coastal ocean : a retrospective evaluation of the benthic tripod and its impact , past , present and future

In-situ observations of near-bed flow and sediment transport have been carried out for approximately 50 years. The observational technique employs a benthic tripod which can remain submerged on the seafloor for an extended period of time upon which are mounted various oceanographic instruments. The instruments transmit their signals to shipboard via connecting cable or more frequently record their output in accompanying data loggers. During the early years of benthic tripod work, measurements emphasized nearbed current velocity and associated suspended sediment profiles over periods of tidal cycles. The objectives of these studies were to evaluate existing boundary-layer flow and sediment transport theories under natural marine conditions. More recently, instrument capabilities and tripod deployment times have increased dramatically. Research objectives have expanded to include documentation of the physical processes active in the coastal ocean and their contribution to along-and across-margin sediment transport. Tripod-based studies continue to be a significant component of many comprehensive field programs. These long-term studies provide a way to document seabed physical processes and sediment transport over the full range of environmental conditions and continue to shape our concepts of sediment transport on continental shelves. Additionally, these boundary-layer studies have provided critical input for boundary-layernumerical model development. Benthic tripods and self-contained instrumentation also represent a base of expertise that will help with design and implementation of coastal observatories in the future.


INTRODUCTION
Marine sedimentologists studying modern physical processes always have had difficulty in interpreting or observing sedimentary conditions in the marine environment.The reasons for this are severalfold.Early geological studies lacked appropriate instrumentation and have historically relied on grain-size distributions and geochemical signatures to interpret marine sedimentary conditions both past and present.Theoretical studies from the 1930s along with observations from laboratory flumes and rivers clearly showed that sediment grains moved primarily near the bed.This further complicated observational techniques and rendered instruments available in those early years ineffective for marinenearbed measurements.
As instrumentation designed for marine use became available through the 1960s, it became possible to make spot, or short-term observations of some important physical parameters (e.g.water level, currents).It was not possible, however, to continue observations over the times when energetic conditions occurred in the sea (e.g.major storms, winter periods) due to ship operational difficulties and instrument limitations.
The advent of micro-electronics with concomitant low power requirements and solid-state characteristics signaled a major revolution in marine instrumentation.Through the 1970s marine instruments and associated control and data logging capabilities were developed which allowed marine scientists to deploy instruments at sea that could (1) remain submerged and operational for extended periods of time; (2) record data internally; and (3) be recalled to the surface by acoustic signals.Among the early users of such technology were physical oceanographers intent on monitoring tides and ocean currents over extended time periods (0-1 yr) using submerged instruments attached to taut-wire moorings with subsurface floats (e.g.Collins et al., 1966;Hopkins, 1971).Moorings could be deployed and recalled when sea conditions allowed but measurements continued during the extended deployment intervals.
Marine sedimentologists also capitalized on this new instrumentation by building stable platforms to be deployed on the seabed over extended periods.These stable platforms, or "benthic tripods", were self-contained with battery power and recovery floats/ropes connected to acoustic-release mechanisms.A variety of instruments and sensors could be mounted on benthic tripods depending on the study and although the benthic tripod concept has remained relatively unchanged, sensor technology has changed greatly over recent years.Benthic tripods have been used throughout the coastal ocean including inland waters, estuaries, lagoons, and continental shelf and slope environments.
It seems very appropriate to include a retrospective evaluation of the benthic tripod instrumentation system in this special issue commemorating the 50th anniversary of this Journal.The reason for this is because the development and use of benthic tripods began approximately 50 years ago and continues today.The information gained from near-bed observations has greatly improved our concepts of physical processes and associated sediment transport in the sea and results have application that extend into all facets of oceanography, not just marine sedimentology.

METHODS
Early investigations of marine boundary layers were restricted to the measurement of velocity profiles in the vicinity of the sea floor that extended over minutes to hours (Grace, 1937;Mosby, 1947Mosby, , 1949;;Lesser, 1951;Bowden and Fairbairn, 1952;Duxbury, 1956;Bowden et al., 1959;Charnock, 1959).These studies focused on the nature of velocity profiles and the average values of bed roughness length, bottom shear stress, and the drag coefficient that characterize various shallow marine environments.Accompanying observations of seabed sediment or sediment transport conditions were not carried out.
In the mid-1960s, Sternberg and Creager (1965) described an instrumentation system lowered to the seabed from an anchored ship.The tripod was powered from shipboard and transmitted data signals back to the surface (Fig. 1).This instrumented tripod contained five miniature current rotors within 1.7 meters above the seabed, pressure transducer, suspended sediment samplers, downward-looking video camera and stereo cameras.Sensor signals were previewed and recorded on deck in real time and the tripod could remain submerged at the discretion of the operators.
Although the tripod described by Sternberg and Creager (1965) was an important step in increasing our knowledge of marine boundary layers and associated sediment transport, it was limited in a practi-cal sense to several tidal cycles (several days) of operation because it required an anchored ship in a three-point mooring.A subsequent benthic tripod was constructed for remote use over more extended periods (~30 days) to depths up to 300 m (Sternberg et al., 1973).It contained instruments to measure near-bed currents, pressure fluctuations, bed configuration, the onset of sediment movement (time lapse camera), a beam transmissometer, and water/suspended sediment samplers (Fig. 2).These early tripods were made possible by advances in marine instrumentation and techniques for deploying and recovering instrument systems from the seabed.These included acoustic releases, internally recording data loggers, and smaller, low powered instrumentation.As instrument capabilities continued to improve, new benthic systems were developed by numerous scientific groups around the world.The major increase of benthic measurement systems began in 1979 by scientists at the U.S. Geological Survey (Butman and Folger, 1979;Cacchione and Drake, 1979) followed by other scientific groups through the 1980s and continuing today.A brief summary of a few benthic tripods in use today is given in Table 1.These benthic systems provide a range of capabilities and are operated by research groups from institutions in a number of countries.For example, some systems tend to monitor currents and sediment concentration at a single level over extended periods (e.g.USGS) or at two levels above the seabed (STRATAFORM tripod) while others have multiple sensors mounted at different elevations to evaluate velocity and suspended sediment concentration profiles.Although benthic tripods generally are constructed to operate at continental shelf depths (200-300 m), the BIOPROBE is designed for continental slope studies (2500 m) and the BASS tripod operates from shelf to deep-sea depths (Table 1).
The benthic tripod platform is basically a stable platform that can remain submerged for extended periods and contains release mechanisms, (e.g.recovery floats with connecting ropes), and battery power.Sensor capabilities change continually.For example, the descriptions of some of the tripod systems in Table 1 are from the original publications cited (e.g.Sternberg et al., 1973;Butman and Folger, 1979); however, the present versions of these tripods and others such as those listed in  available for use on tripods is very broad as summarized in Table 2. Examples of the changes occurring in benthic tripods due to sensor capabilities are seen by comparison of the GEOPROBE system (Fig. 3; Table 1), the TRIPOD-New Zealand system (Fig. 4, Table 1), and the TRIPOD-EUROSTRATAFORM, or ICM tripod, which is the most recent tripod on the list (Fig. 5; Table 1).GEOPROBE contains individual current meter and suspended sediment sensors mounted at five distinct elevations on masts positioned within the tripod frame.This causes noticeable clutter and the legs present some flow disturbance to the sensors (while protecting the sensors) and makes observations of gradients from the five elevations.The VIMS-New Zealand tripod has the sensors swung outboard from the platform legs to minimize flow disturbance from the legs but exposes the sensors to possible damage.The ICM tripod uses acoustic sensors mounted within the upper frame that acoustically profile the water column below.As a result, there is no physical presence of current meters or suspended sediment concentration sensors within the measurement volume (the legs have a minimal profile to decrease flow disturbance).Velocity and suspended sediment concentration profiles are reconstructed from time-sequence "bins" virtually stacked from the seabed to the upper area of the tripod frame.The array of tripods used around the world is extensive and is always undergoing modification as sensors and scientific objectives evolve.

TRIPOD APPLICATIONS
The mix and individuality of tripod systems used within the marine sediment transport community (e.g.Table 1) provides a significant mechanism for interactive studies and comprehensive results.Indeed, the high cost of construction, deployment, and data analysis has promoted or strongly supported multi-investigator shelf studies.Benthic-tripod groups commonly deploy their instruments in coordinated arrays, share data, and/or interact in published results.Examples of several modes of tripod deployments are summarized in Table 3.This summary is not meant to be all-inclusive but to serve as an illustration of the breadth of benthic tripod work that is used to support comprehensive continental margin studies.
Inspection of    pathways and are contributing to a third.These include (1) validation and application of boundarylayer and sediment transport theory in the sea; (2) documentation and identification of the range of physical processes and associated sediment transport active on continental shelves; and (3) data input for development and evaluation of numerical models of shelf circulation and sedimentology.

Boundary layer processes
When benthic tripods were initially developed and deployed, the objectives were to measure velocity profiles within 1-2 m of the seabed and to determine if they followed the classic "law of the wall" form.It was important, as an initial step, to explore whether marine boundary layer flows followed theoretical forms developed in the 1920s and 1930s.Prior documentation of boundary-layer flows and sediment transport only had been carried out in flumes and rivers (e.g.Gilbert, 1914;Nikuradze, 1933;Hjulstrom, 1939;White, 1940;Guy et al., 1966).The verification of the log-profile in the sea provided a way to evaluate constants and parameters used to understand such physical concepts of boundary shear stress, drag coefficients, threshold of grain motion, and turbulent exchange of particulates and chemical species across the sediment-water interface.
The results of early field investigations in the sea were very promising.Log profiles were observed in marine boundary layer tidal flows and parameters such as the boundary roughness length (Sverdrup et al., 1942;Lesser, 1951;Dyer, 1970), and frictional drag coefficients over various boundary types (e.g.Sternberg, 1968Sternberg, , 1976) ) were determined.Additionally, sediment transport characteristics were evaluated in terms of the threshold of grain motion (Sternberg, 1971;Sternberg and Larsen, 1975) and bedload transport (Kachel and Sternberg, 1971;Sternberg, 1972).
Recent benthic tripod observations using the latest generation of instruments has continued to advance our knowledge of boundary-layer flows under a wide range of marine conditions.This includes, for example, evaluation of suspended sediment concentration profiles (e.g.Kineke and Sternberg, 1989;Guillén et al., 2002); influence of high concentrations of suspended sediment on boundary layer flows (Friederichs and Wright, 1997;Friederichs et al., 2000;Wright et al., 2001), fluid mud formation on the continental shelf (Cacchione et al., 1995;Kineke and Sternberg, 1995;Kineke et al., 1996;Sternberg et al., 1996;Ogston et al., 2000;Traykovski et al., 2000), biological mediation of threshold of grain motion (Nowell et al., 1981;Wright et al., 1997) and boundary layer processes (Drake et al., 1992).Studies of boundary layer flows and associated sediment transport dynamics represent the means by which marine scientists have improved our ability to link physical process to sediment response, predict the impact of various physical factors on coastal seas (e.g.crossshelf transport, patterns of erosion and deposition, development of event stratigraphy) and provide a conceptual and theoretical base for numerical models.

Physical process identification
The first tripods deployed for extended periods were designed to remain submerged for periods of approximately one month and to be redeployed as possible over winter months to document nearbed conditions under storm conditions when ship operations were not possible.The objectives of these early studies were expressed by a series of questions: -When does sediment move?-What are the directions of transport?-How much is transported?-What physical processes cause sediment movement?
-What conditions control the distribution of modern shelf sedimentary deposits?
Beginning in the 1970s, the above objectives were addressed with benthic tripod deployments on various continental shelves of the United States (e.g.Smith and Hopkins, 1972;Sternberg and McManus, 1972;McClennen, 1973;Gordon, 1975;Sternberg and Larsen, 1976;Sternberg et al., 1977;Butman et al., 1979;Vincent et al., 1981) and continuing into the present (e.g. in European waters, Green et al., 1995;Palanques et al., 2002).Results of these studies highlighted the importance of waves, tidal flows, currents, and seasonal storm events in forcing shelf sediment transport and addressed a number of these original questions on numerous continental shelves (e.g.summarized in Sternberg, 1986;Sherwood et al., 1994;Sternberg and Nowell, 1999).
In more recent decades the availability of instruments (Table 2) and capability for extended deployments has led to the documentation and identification of numerous other processes that contribute and at times dominate shelf sediment transport.A summary of these processes is given in Table 4 along with some example references.A noteworthy aspect of the list in Table 4 is its potential impact on all facets of the continental shelf marine environment, not just sedimentology.As other oceanographic disciplines participating in large interdisciplinary shelf studies have gained the ability to visualize, document, and measure the range of processes at work, new levels of understanding of physical, chemical, and biological processes have emerged.One example of the interdisciplinary potential of these studies is illustrated in the documentation of fluid mud formed on the Amazon shelf (nearbed sediment suspensions >10 g/l).The observed fluid mud, once identified, mapped, and sampled, was found to strongly influence shelf sediment transport and depositional patterns (Faas, 1986;Kineke and Sternberg, 1989;Cacchione et al., 1995;Kineke et al., 1995;Kuehl et al., 1996).Fluid mud also was found to significantly influence tidal sediment dynamics in the river mouth (Jaeger and Nittrouer, 1995), mud flat sediment storage (Allison et al., 1995), tide propagation on the open shelf (Beardsley et al., 1995), tide-induced mixing (Geyer, 1995), chemical signatures and budgets (Moore et al., 1996), sediment remineralization, recycling, and storage (Aller et al., 1996), and the benthic community structure (Aller and Stupakoff, 1996).So these broadly based and extensive shelf studies have significant interdisciplinary implications.

Numerical modeling
Because benthic studies document such a wide range of oceanic conditions, they have been influ-ential in both development and validation of time dependent, numerical models of shelf circulation and sediment transport.Examples of models include 1D resuspension models of Wiberg and Smith (1983), Wiberg et al. (1994), andCacchione et al. (1999), 2D, time-dependent circulation models incorporating wind stress, river discharge, and bathymetry reported by Pullen andAllen (2000, 2001) and 2D, time-dependent models incorporating the advection-diffusion equation to calculate suspended sediment concentration, net erosion, deposition and seabed modification during resuspension events (Harris andWiberg, 2001, 2002).Considering the elapsed time from field identification and documentation of various processes (e.g.Table 4) to the incorporation of new concepts with appropriate physics into numerical models, it is not surprising that major model development lags field observation.Model development, therefore, tends to proceed in steps.The extensive comprehensive models of Harris andWiberg (2001, 2002) do not yet include, for example, the physics of fluid mud formation and its potential role in cross-margin transport.The physics of fluid mud, however, has been and is being modeled from observations made off the Amazon shelf (Trowbridge and Kineke, 1994), and the northern California shelf (Scully et al., 2002).Presumably these process-specific models will eventually be incorporated into shelf-wide models as well as other processes such as those summarized in Table 4 et al., 1994;Ogston andSternberg, 1999 Tides Gordon, 1975;Jaeger and Nittrouer, 1995Currents Ogston et al., 2000Wind band (dy-wks) Jiménez et al., 1999;Guerra, 2004;Ogston et al., et al., 1995;Kineke and Sternberg, 1995;Mulder and Syvitski, 1995;Kineke et al., 1996;Traykovski et al., 2000;Wright et al., 2002 inner shelf fluidization Fredericks and Wright, in press Source/sink connections Baker and Hickey, 1986;Milliman and Syvitski, 1992;Wheatcroft et al., 1997; (e.g.river discharge; submarine canyons) Wheatcroft and Borgeld, 2000;Mullenbach and Nittrouer, 2000;Puig et al., 2003 FUTURE IMPACTS Benthic tripod studies have contributed significantly to our knowledge of shelf sediment transport, and the data base from these studies has been important as shelf scientists expand their efforts from field observation to numerical modeling and associated predictive capabilities.To the extent that the range and mix of physical forcing mechanisms that drive sediment transport is still being discovered, benthic tripods should continue to play a role in shelf studies into the foreseeable future.
An additional aspect of future marine science is the consideration of ocean observatories for carrying out studies over decades rather than years.Examples include the Longterm Ecosystem Observatory located on the New Jersey inner shelf (LEO-15) operated by Rutgers University for over a decade.LEO-15 is a cabled network on the seabed that transmits to shore, in real time, visual images and measurements of temperature, currents, and pressure.Another cabled network in the planning stage is NEP-TUNE which will be a fiber-optic/power network of distributed sensors on the scale of an oceanic plate off the west coast of the United States and Canada (Delaney et al., 2001).Other considerations of long-range observatories have been made by the National Oceanic and Atmospheric Administration of the U.S. (Jahnke et al., 2003), the Ocean Observatories Initiative (OOI) of NSF, and the Ocean Research Interactive Observatory Networks (ORION) program under the auspices of the U.S. National Science Foundation (NSF), Joint Oceanographic Institutions (JOI), and the Consortium for Oceanographic Research and Education (CORE).ORION is an overarching program that encompasses OOI and NEPTUNE (e.g.see http://orionprogram.org) and also includes a wide range of ocean observatory concepts.As future initiatives and plans for seabed observatories are being considered, the instrumental capabilities developed and used in benthic tripod investigations are providing a base of knowledge and experience for these evolving initiatives.An example of this instrument crossover is seen in the Science White Paper #1, "Cross-margin particulate flux studies associated with NEPTUNE" (C.A. Nittrouer, 2004) which incorporates many of the sensors used on present-day benthic tripods.

CONCLUSIONS
The development of the benthic tripod as a stable platform on the seabed has evolved over 50 years in response to advances in deployment/retrieval capabilities and individual sensor technology.Although the basic tripod concept has not changed substantially from the early years, the sensors mounted on these platforms are continually changing, reflecting individualized study goals and major advances in instrument development.
The scientific results from benthic tripod deployments have been far-reaching.These include the understanding of marine boundary-layer flows and resulting sediment transport; identification of a wide range of physical processes which contribute to shelf and slope sedimentology and often play a dominant role; and the collection of a necessary data base for predictive model development and verification.The breadth of new knowledge and concepts resulting from benthic tripod studies has had a major influence on our understanding of the coastal ocean.These findings also have application to all of the oceanographic disciplines, not just the physical and sedimentological aspects.It is expected that use of benthic tripods has relevance into the future and will continue.Additionally, the technical aspects of these instrumentation systems will provide a base of experience as the scientific community considers new initiatives for the future, such as expansion into long-term submarine observatories.
FIG. 5. -Photograph of the tripod designed by Puig, Palanques, and Guillén at the Institute de Ciencias del Mar, Barcelona, Spain, presently deployed off the east coast of Spain in cooperation with the EUROSTRATAFORM program (SeeTable 1 for details).
. 1. -Photograph of the early tripod of Sternberg and Creager (1965) deployed from an anchored vessel.(SeeTable 1 for details.) FIG. 2. -Photograph of the early tripod of Sternberg et al. (1973) designed for remote operation on the continental shelf.(See Table 1 for details.) A RETROSPECTIVE EVALUATION OF THE BENTHIC TRIPOD 45 FIG

TABLE 1 .
-Examples of tripod characteristics in terms of tripod, organization, reference, and elevations above the bed of current meters and suspended sediment concentration sensors or samplers.All tripods include pressure sensors and combinations of instruments as summarized in Table2.The terms used include Savonius Rotor (rotor), electromagnetic current meter (EMCM), acoustic current meter (ACOUSTIC), transmissometer (XMISS), optical backscatterance sensor (OBS), acoustic backscatter sensor (ABS), acoustic doppler velocimeter (ADV), pulse coherent acoustic doppler profiler (PCADP).
Table 1 include numerous sensors.The range of sensors 46 R. STERNBERG *example of tripod deployed at 60 m depth.

TABLE 2
. -Examples of sensors used on benthic tripods.
Table 3 shows individual long-term deployments either with single tripods operating for (Trembanis et al., 2004)9)EOPROBE tripod ofCacchione and Drake (1979)deployed on many continental shelves over the past 25 yr (See Table1for details).FIG.4.-Photograph of the tripod designed at the Virginia Institute of Marine Sciences(Trembanis et al., 2004)and used on the New Zealand shelf (photograph courtesy of Bob Gammisch).(SeeTable1fordetails).

TABLE 3
.-Limited examples of a range of tripod studies and cooperative studies to monitor and study shelf sedimentary processes.Under each study is listed the geographic region and funding agency.

TABLE 4 .
. -Shelf sediment transport: processes of importance based on concepts of the 1970s and the present.A few example references are also given for each process.