High Energy Benthic Boundary Layer Experiment ...

EOS, vol. 63, no. 31, August 3, 1982

The Oceanography Report temporal and spatial variability, and to predict the response of the natural biologically altered surficial, fine sediment to imposed stresses. The transport of sediment as bedload, which results in dramatic bedforms, and the vertical flux and advection of suspended sediment, which gives rise to high concentrations of material in the mixed layer and high transport rates, are the consequences of these imposed forces. The entrainment and transport of sediment will be examined, and dynamic influences of this moving sediment will be incorporated into a general, verified preH.M.S. CHA LLEXGE R l'R E PA RIX G TO SO'G XD 1872. dictive model of sediment transport in a turbulent, stratified Ekman layer. The Oceanography Report The focal point for physical, chemical, geological, and bioA site has been chosen for the experiment logical oceanographers. which has small slope, a lack of periodic bedforms, and which shows evidence of very reAssociate Editor: Arnold L. Gordon, Lamont-Do- cent erosion or deposition. It is at 4820 m on herty Geological Observatory, Palisades , New York, the Scotian Rise (48°27'N,62°20'W). Deep 10964 (telephone 914/359-2900, ext. 325) Tow was used for a I month survey of the site so that the regional topography can be described accurately. Extensive stereo-photogrammetry is being done on each cruise to permit detailed measurement of local roughness and topography at the site. The lack of knowledge about the temporal variability of shear stresses sufficient to entrain material from the bed in the deep ocean means that HEBBLE has need for a structured program that will evaluate both the stress required to move such marine muds and the frequency of currents of the necessary magnitude. Hence, HEBBLE will produce an expository model, using developed Ekman layer models and incorporating new PAGES 594, 595 theoretical formulations for fine material transport. Arthur R. M. Nowell, Charles D. Manipulative laboratory studies designed to evaluate the influence of organism activity on Hollister, and Peter A. jumars sediment properties, and the resultant effects on entrainment rates, must be carried out in What is HEBBLE, and Why is it order to produce a realistic sediment transport model. Long-term studies will characterUnique? ize the Ekman layer and will relate its strucHEBBLE's precise aim is to develop and to ture to the mesoscale physical oceanographic test explicit predictions about the response of forcing. Short-term measurements will paadhesive/cohesive marine sediments to imrameterize the spatial variability in the logaposed and controlled stresses [Hollister et al., rithmic region. The scales of variability of the 1980; Kerr, 1980). Pursuit of this goal has ne- stress, velocity, and sediment concentration cessitated a co-ordinated, interdisciplinary effields will be measured and modeled so that fort, to date including physical oceanograsubsequently the time-dependent structure of phers, sedimentologists, radiochemists and the boundary layer may be predicted. biochemists, and biological oceanographers. Two major studies will be undertaken folCurrent produced bed features reflect siglowing a detailed survey of the sedimentologinificant momentum exchange between the cal and biological properties of the site. In fluid boundary layer and the sediment sur1983, a series of short-term (-3 days) deployface. From photographs, and the few current ments will be made of rapidly sampled velocimeter records available, it appears that vast ty, Reynolds stress, and suspended sediment areas of the deep sea are presently being concentration sensors in order to describe the modified by energetic flows. The bed forms spatial variability of the high frequency proprange in scale from kilometers to millimeters erties of the inertial sublayer. In 1984, a and are found where near bottom currents long-term (-months) deployment of 5 moorhave been delineated by maxima in near bot- ings around a central mooring will be carried tom potential temperature. Moreover, on the out to measure the magnitude, frequency, Scotian Rise for example, many of these bed- and forcing of the sediment transporting forms are being produced by present day events. The central mooring will measure currents because rapid destruction of the fea- momentum and sediment fluxes throughout tures by benthic organisms is evidenced in the Ekman layer, and the sensors will be stereo-photographs. switched to a high sampling rate during maThe objectives of HEBBLE are, at a carejor sediment transporting periods. fully chosen site, to quantify the magnitude The true experiment within HEBBLE will of deep ocean currents, their forcing, their occur after the major Ekman layer observa1

High Energy Benthic Boundary Layer Experiment: HEBBLE

0096-3941182/0031-0025$1.00 Copyright 1982 by the American Geophysical Union.

tions and laboratory studies have been carried out. Specifically, in 1985 and 1986 we will test, using SEADUCT, explicit predictions of sediment entrainment and flux rates as functions of imposed fluid shear stress. SEADUCT is a recirculating, inverted flume that can impose known and controlled shear stresses on the bottom boundary of the deep sea. The studies in 1983 and 1984 may be defined as 'representative studies,' for they do not qualify as scientific experiments since there is no effective control over the subject of interest. However, they are crucial in that they yield knowledge upon which the true experiment in SEADUCT is based. The SEADUCT deployments represent narrowly focused, exactly prescribed experiments from which strong inferences may be made. Thus, the unique contributions of HEBBLE are threefold : (I) It represents one of the few true experiments, as defined above, ever carried out in marine geology; (2) it provides the first test of a sediment transport model with typical deep-sea sediments, i.e. , adhesive silty clays; and (3) it yields an unparalleled data set with which to examine, at a well described site, the utility and accuracy of presently available Ekman layer models.

Objectives of HEBBLE The specific near term objectives are (1) to measure the Ekman layer thickness and structure (three components of velocity, stress, and density including suspended sediment concentration) to test alternative formulations for turbulent Ekman layers; (2) to measure the geostrophic flow and its variability over scales large enough to study the coupling of the Ekman layer to the interior at eddy resolving scales; (3) to monitor changes of the bed, its roughness, and its properties in response to imposed stress, and to measure the dynamical properties of the transported materials for input to sediment transport models; (4) to analyze sedimentological, faunal, and geochemical data in part to parameterize spatial variability of these properties and in part to aid in the selection of suitable analogs for laboratory simulation and measurement of sediment entrainment; ·(5) to carry out laboratory experiments with shallow-water analogs to develop suitable parameterization of macrofaunal, meiofaunal, and microbial effects on sediment transport; (6) to build a recirculating, ocean-bottom flume (SEADUCT) to test, in situ, a sediment transport model based on a priori local information. Along the way, a number of zero-order problems will be addressed in 1983 and 1984, and these include the following:

Physical Oceanography I. Internal wave interaction with the Ekman layer, resolved to above Brunt-Vaisala frequency. Armi and D'Asaro [ 1980) could only look at inertial waves and were not able to resolve to the Brunt-Vaisala frequency. Furthermore, their measurements were generally outside the Ekman layer. 2. Escape of well-mixed bottom water ('blobs' into the interior), which leads to

EOS, vol. 63, no. 31, August 3, 1982 thicker mixed layers than are predicted by simple steady Ekman layer scaling. 3. Coupling of Ekman layer response to variations in the geostrophic field up to the base of the thermocline in a region of strong surface currents. 4. Tests of state-of-the-art Ekman layer models for neutral and stably stratified Ekman layers including the influence of time dependence over known and simple topography.

This leads to specific-process-oriented hypotheses that will be supported or falsified by an experiment. To test the hypotheses from a well-developed theory requires (2) definition of the properties of interest and operational statements about the measurements being made, (3) a formalized schedule of measurements, and (4) a specified scheme for analysis to discriminate between or among the alternate hypotheses.

Benthic Biology

HEBBLE qua Experiment

I. Covariance of local community composition in relation to differing roughness properties, and stress history, of the bed. 2. Long-term evolutionary effects of sediment transport on community structure. 3. Test of species diversity models in a hydrodynamically active region.

Few oceanographic studies (e.g., MODE, POLYMODE, GATE, FGGE) qualify in Church's terms as experiments, despite the ubiquitous 'E' in their acronyms. At best they are purposive measurements of selected features, or they may be usefully called, if they were truly successful, representative studies. HEBBLE, on the other hand, is designed to proceed from a type III to a type I experiment. The sequence for the program is to carry out a representative study; that is, to obtain a test case of the extant Ekman layer models and then in conjunction with the spatial sampling scheme (type Ill) to carry out the type I experiment by using SEADUCT. Let us examine the steps from proposition I and the attendant requirements of points 2-4 mentioned above. The conceptual model to be tested is a coupled Ekman layer-sediment transport model: it can be broken up into three parts; namely, an Ekman layer model, a suspended sediment model, and a bedload model. These parts are discussed below. The specific hypotheses to be tested in SEADUCT can only be formulated precisely after we have obtained point 2 above, namely, accurate definition of the properties of interest. At worst, the hypotheses will be ordinal in scale, but the intent is to work at the ratio level. Conceptual model of the processes. For a horizontally uniform flow, the Reynolds equations are reduced to

Sediment Dynamics I. The response of natural, biologically modified marine sediment to fluid stresses. 2. The change in settling velocity and floc characteristics between erosion and later stages of deposition. 3. Spatial variability of roughness on small scales as reflected in changes in faunal assemblage. 4. The magnitude of bedload transport in regions of natural (biologically modified) clays and the flux of fine material into suspension. 5. The effects of mucous and bacterial adhesion on entrainment properties of sediment.

Methodology The Meaning of an Experiment We have held as our central methodological tenet that progress in any science is most rapidly and predictably made when competing theories are proposed and are then mortally endangered [Platt, I964; Laudan, I977]. The key element is to develop an experiment that will allow us to test the predictions of the theory. Church [I98I] observed that there are three classes of 'experiments.' In the traditional experiment (type I), one or two parameters are allowed to vary under control, all other factors held constant. This type is the classical experiment most often performed in the laboratory. Type II experiments are classified as purposive measurements of selected dynamical factors, and these may be classified as 'representative studies.' However, they do not qualify as true scientific experiments since there is no control over the subject of interest. Often these are correlational studies; a series of parameters is selected, and 'goodness of fit' tests to postulated models are carried out. Type III experiments utilize spatially stratified sampling schemes whereby a number of samples are examined in a manner that permits examination or removal of variability not of central concern to the experiment. Sound statistical design thus represents the basis for the type III experiment. An experiment consists of (I) a conceptual model of the processes that is to be supported or refuted by an experiment (i.e., a conceptual model from which predictions can be deduced in the form of testable hypotheses).

-au - fv

ap ax I ap - p ay I p

at av - + fu at

-u'w' = K

a (u ,w ') az a , ') - (v w az

= - - -

- -

(I)

= -

-

(2)

and the temperature transport equation

ae at

-

a

=-

az

(-e'w')

(3)

The equations are written for an f plane where f is the Coriolis paramet&, u and v are the x andy components of velocity, p is the mean density, p the mean pressure, and e the temperature relative to an initial temperature. We assume that the bottom roughness is described by a length parameter z0 • Laboratory measurements on bed replicas from the site will be used to proscribe the range of values of a topographic Zo· Various means are used to close these equations: The turbulent kinetic energy equation may be written for our case as

-,-, au

-,-, ov

~u w oz - v w az - g[~ew

- (p, - p) Cn'w'] p

The first two terms are the production of turbulent kinetic energy. The third term represents the buoyancy flux expressed as a combination of contributions from temperature and suspended sediment. The righthand side of the equation is the rate of energy dissipation taken as equal to the kinetic energy to the 3/2 power divided by turbulence length scale l. A transport equation is developed for this length scale. An alternate model (8) uses an eddy coefficient closure

= vV 2 u· = .i._ '

151

(4)

m

ou OZ

(5)

m

av az

(6)

-v'w' = K

where Km = KU• ze-