Coastal Benthic Boundary Layer Research Articles

Turbulence and fine sediment dynamics in a benthic boundary layer in Fujian Coastal Sea, SE China

A short-term (∼ 25 h) intensive field observation focused on flow, turbulence, suspended sediment concentration, and floc/particle size distributions was conducted in a coastal benthic boundary layer to understand flocculation and erosion/deposition processes of fine sediment. Observations showed suspended particles were mainly flocculated into the size range of microflocs (50-200 μm). Macroflocs (> 230 μm) occurred at slack tides and SPM (suspended particulate matter) concentrations decreased as they settled faster. Twofold intra-tidal variations in floc size and settling velocity were observed with a larger medium floc size of 140 μm and a settling velocity of 1.3 mm/s occurring at slack tides than peak flood/ebb tides with their corresponding values of 70 μm and 0.7 mm/s. This variability can be ascribed to the turbulence modulation in that low turbulence enhances aggregation to form large flocs but they turn to break down by high turbulence. The inverse relationship was found between floc size and turbulence shear rate, SPM concentration, and their product. SPM concentration covaried with bed stress and the low critical erosion stress (∼ 0.02 Pa) indicates that the cohesive bed was primarily eroded as aggregates which formed a fluff layer above the consolidated bed. The fine sediment dynamics behave as a loop from resuspension, through to breakup and re-aggregation, and to settling in response to intro-tidal turbulence variability. This looped behavior is vital for understanding the sequestration of organic carbon and pollutants in fine sediments, awaiting more detailed and longer-term field observations and the assemblage into numerical models.

Data-model comparisons for scattering by a bubbly sediment

As part of the Coastal Benthic Boundary Layer Program, seafloor backscattering measurements were made at 40 kHz in Eckernfoerde Bay, Germany, in areas of high concentration of gaseous methane. Although it had long been known that methane bubbles have a profound influence on acoustic scattering, this was the first time that contemporaneous measurements were made of acoustic scattering and bubble size distributions. The latter measurements were part of a collaboration between Aubrey Anderson’s group and NRL-SSC and made detailed model-data comparisons possible. Acoustic interferometry was used to conclude that the dominant scattering layer was about 1 m below the sediment–water interface. A single scattering model accounted for the angular dependence and level of scattering, however, the model gave unphysical results at very small grazing angles. A multiple scattering model has been developed to remedy this defect. [Work sponsored by ONR.]

Geotechnical characterization of calcareous sediments from the Dry Tortugas Keys and Marquesas Keys CBBL SRP study sites, Lower Florida Keys

Geotechnical characteristics of carbonate sediments from two test sites (Dry Tortugas Keys and Marquesas Keys) in the Lower Florida Keys were investigated as part of the Coastal Benthic Boundary Layer Special Research Program, through an extensive field coring and laboratory testing program conducted by the Marine Geomechanics Laboratory of the University of Rhode Island. Based on results from physical measurements, water content and wet bulk density values for both sites generally showed large variations in the upper 25 cm and little variation below this depth. Sediment samples exhibited low plasticity or nonplastic characteristics. Constant-rate-of-deformation consolidation test results showed strong apparent overconsolidation (stress state ratio >7.5) in the surface sediments (upper 50 cm) at the Dry Tortugas Keys, and light overconsolidation (stress state ratio <1.5) below 50-cm depth at the Marquesas Keys site. In-situ permeability values were between 10–4 and 10–7 cm/s at both sites and showed no strong depth dependence in the upper 2 m. Undrained shear strength profiles for Dry Tortugas Keys sediments indicated a marked stiffening with depth, whereas the Marquesas Keys sediments showed a gradual increase with depth. Consolidated isotropically undrained triaxial shear strength test results indicate that the undisturbed sediments had an average effective angle of internal friction of 38°, which is not fully realized until large axial strains on the order of 11% have accumulated. Evidence of cementation was not found in triaxial compression or consolidation test results. The general behavior and characteristics of these sediments are similar to those of granular materials, which is primarily due to their high calcium carbonate contents and lack of cementation.

The influence of gas bubbles on sediment acoustic properties: in situ, laboratory, and theoretical results from Eckernförde Bay, Baltic sea

Acoustic turbidity caused by the presence of gas bubbles in seafloor sediments is a common occurrence worldwide,but is as yet poorly understood. The Coastal Benthic Boundary Layer experiment in the Baltic off northern Germany was planned to better characterize the acoustic response of a bubbly sediment horizon. In this context, in situ measurements of compressional wave speed and attenuation were made over the frequency range of 5–400 kHz in gassy sediments of Eckernförde Bay. Dispersion of compressional speed data was used to determine the upper limit of the frequency of methane bubble resonance at between 20 and 25 kHz. These data, combined with bubble size distributions determined from CT scans of sediments in cores retained at ambient pressure, yield estimates of effective bubble sizes of 0.3–5.0 mm equivalent radius. The highly variable spatial distribution of bubble volume and bubble size distribution is used to reconcile the otherwise contradictory frequency-dependent speed and attenuation data with theory. At acoustic frequencies above resonance (>25 kHz) compressional speed is unaffected by bubbles and scattering from bubbles dominates attenuation. At frequencies below resonance (<1 kHz) ‘compressibility effects’ dominate, speed is much lower (250 m s -1) than bubble-free sediments, and attenuation is dominated by scattering from impedance contrasts. Between 1.5 and 25 kHz bubble resonance greatly affects speed and attenuation. Compressional speed in gassy sediments (1100–1200 m s -1) determined at 5–15 kHz is variable and higher than predicted by theory (<250 m s -1). These higher measured speeds result from two factors: speeds are an average of lower speeds in gassy sediments and higher speeds in bubble-free sediments; and the volume of smaller-sized bubbles which contribute to the lower observed speeds is much lower than total gas volume. The frequency-dependent acoustic propagation is further complicated as the mixture of bubble sizes selectively strips energy near bubble resonance frequencies (very high attenuation) allowing lower and higher frequency energy to propagate. It was also demonstrated that acoustic characterization of gassy sediments can be used to define bubble size distribution and fractional volume.

Using the Biot theory to establish a baseline geoacoustic model for seafloor sediments

By modeling the soft, fine-grained sediments of Eckernförde Bay, Germany, as porous viscoelastic materials filled with a viscous fluid, the Biot theory is used to establish a baseline geoacoustic model that is useful as a starting point for studies of scattering and other phenomena caused by the introduction of free gas into the sediment. The basic parameters of the model are derived from a wide variety of field measurements made during the Coastal Benthic Boundary Layer research program. The low permeability revealed by these measurements indicated that the overall fluid mobility would be small and therefore it would be necessary to include the effects of local fluid motion in “hidden” coordinate systems in order to model the dispersion and attenuation that is observed over a wide fre-quency range. This was accomplished by using a very general viscoelastic model (Cole–Cole model) to describe the response of the skeletal frame in a water environment. By adjusting the three free parameters of this model it was possible to match both overall attenuation and velocity in both high- and low-frequency ranges, however, because of the fact that compressional attenuation and velocity were only measured at two relatively high frequencies and shear wave attenuation determined only at low frequency, it is not clear whether the best rheological model was chosen to model the skeletal frame. This suggests that more specially designed experimental studies are desirable if the objective is to define the most general baseline model, applicable over a wide frequency range.

High-frequency bistatic scattering from sediments—experiments and modeling

Typically, for frequencies in the 10–100 kHz range, the interaction of sound with the ocean bottom is a scattering process. Use of bistatic geometries can give insight into the scattering process not possible via traditional monostatic experiments. The trade-off is that such experiments are more difficult and more data are needed to get statistically meaningful results to compare with models. During the last 5 years, the Coastal Benthic Boundary Layer (CBBL) program allowed the opportunity to develop and fine-tune one method of obtaining bistatic data. That evolution will be briefly described and the data from three sites (one mud, one sand, one sand-silt-clay) shown. The sand and sand-silt-clay site data are compared with a model based on surface roughness and volume perturbation scattering. The mud site had methane bubbles embedded within it and the data are compared with a model based on bubble scattering. The physical insight derivable from the model/data comparison in the mud case will be discussed. [Work supported by ONR.]

Bistatic bottom scattering: Model, experiments, and model/ data comparison

A model is presented for bistatic scattering from ocean sediments. It treats scattering due to both roughness of the seabed and volume inhomogeneities within the sediment. Accordingly, the scattered intensity is assumed to be a sum of two terms, one proportional to the roughness-scattering cross section and the other proportional to the volume-scattering cross section. The model is tested against data acquired as part of the Coastal Benthic Boundary Layer (CBBL) research program. As part of that program, an autonomous, circularly scanning sonar system was deployed in well-characterized regions. This sonar operated at 40 kHz, had a 5° horizontal beam, and acquired backscattering data over a 50-m radius. During part of the deployment, it operated in conjunction with a mobile receiving array so as to acquire bistatic data. The experimental apparatus and procedures are presented, and results are compared with model predictions.

Geophysical sea-floor sensing in a carbonate sediment regime

Within the Coastal Benthic Boundary Layer (CBBL) Program’s Key West Campaign, a series of complementary geophysical experiments were undertaken in the Lower Florida Keys. The primary objective was the collection of data to assist in the physical characterization of the sea-floor carbonate sediments. Digital subbottom profiler data collected along closely spaced lines were processed to produce maps of sea-floor reflectivity strength and variance. These were subsequently used to help map the spatial distribution of the Holocene sediment and to identify and delineate the outcropping Pleistocene limestones and Holocene patch reefs. Analysis of a complementary data set, comprised of shear wave velocities collected with a bottom-towed sled, provided additional valuable information on the physical characteristics of the soft silty sediments making up the surficial cover at the CBBL Program’s experimental tower site.

Stress‐strain behavior of surficial carbonate sediments from key west, Florida, USA

A triaxial strength testing program was conducted to determine the stress strain and strength behavior of a surficial calcareous sediment from the Dry Tortugas study site west of Key West, Florida, which is part of the Coastal Benthic Boundary Layer Special Research Program. The sediment at the study site is comprised largely of Halimeda algal grains resulting in carbonate contents near 93% by mass. Sediment grains range from sand‐sized shells and shell fragments to carbonate muds. Isotropically consolidated undrained triaxial tests were conducted at consolidation stresses no greater than 34 kPa on undisturbed and remolded sediment samples from box cores. It was found that the sediments behave like a loose sand during shear and are characterized by failure envelopes with friction angles between 37° and 39° with no cohesive intercepts. Initial elastic tangent Young's moduli, determined by the hyperbolic method, range from 130 kPa to 18,873 kPa, and exponential relations between the elastic moduli and confining stress are found to exist for the undisturbed and remolded sediments. Particle size distributions were found for the sediments at various stages of testing and results show that no particle crushing occurred due either to the stresses or the remolding procedures employed in this investigation. Although apparent overconsolidation was present, there was no evidence of cementation during shear, as a brittle failure mechanism was not present. Comparisons of the strength behavior of the undisturbed and remolded sediments suggest that particle mobility plays an important role as a mechanism of strength development.

Environmental overview of Eckernf�rde Bay, northern Germany

This environmental overview of Eckenforde Bay (northern Germany) summarizes the results of previous studies relevant to the Office of Naval Research's Coastal Benthic Boundary Layer (CBBL) Baltic field exercise conducted during 1993-1994. Significant environmental characteristics include the following: (1) surface sediment distribution is related to water depth, dictated primarily by hydrodynamic reworking of older glacial deposits: (2) the origin and characteristics of small-scale sedimentary structures depend on storm-generated waves and currents; (3) the proximity of the sea surface and sediment -water interface results in a pelagic-benthic coupling that drives biogeochemical processes and produces organic-rich, acoustically turbid sediments and (4) the bay floor is complicated topographically by pockmarks and manmade sedimentary structures.

Benthic boundary layer processes in coastal environments: An introduction

This special issue ofGeo-Marine Letters “Benthic Boundary Layer Processes in Coastal Environments” includes 20 papers devoted to results of recent near-shore experiments supported by the Coastal Benthic Boundary Layer (CBBL) program. Experiments were conducted in gas-rich muddy sediments of Eckernforde Bay of the Baltic Sea and on relict sandy sediments of the West Florida Sand Sheet. In this introductory paper we present scientific justification for experiments and summarize preliminary results.

Geotechnical characterization of surficial high-porosity sediments in Eckernf�rde Bay

Geotechnical studies of high-porosity, finegrained sediments from Eckernforde Bay, part of the Coastal Benthic Boundary Layer program, included coring, subsampling, and on-board testing during three field experiments, and a laboratory testing program to determine the mechanical properties of the seabed. The sediments consist of high-porosity (86–94%), organic-rich (10–20%) silty clays with varying amounts of methane gas. The surface 5–10 cm are characterized by unusually high water contents (400–600%), very low shear strengths ( 1 kPa).

High-frequency acoustic observations of benthic spatial and temporal variability

A bottom-mounted sonar operating at 40 kHz has been used to measure the variation of bottom acoustic scattering over extended time intervals at two shallow sites as part of the Coastal Benthic Boundary Layer Special Research Project. The acoustic data were analyzed using a correlation method that measures the spatial and temporal dependence of benthic change. The rate of decorrelation was two orders of magnitude more rapid at a sandy site near Panama City, Florida, USA, than at a silty site in Eckernforde Bay, Germany, and both sites were characterized by hot spots or localized regions of activity.

Geophysical ground-truthing experiments in Eckernf�rde Bay

During the 1994 Coastal Benthic Boundary Layer Project research cruise in Eckernforde Bay, multichannel digital seismic and electrical resistivity data were collected using surface- and bottom-towed arrays. Profiling with a bottom-towed sled yielded shear wave velocity and electrical resistivity data indicative of the structural strength of the sediment and of the properties of the pore space. Shear wave velocities for the gassy mud were, as expected, extremely low, ranging from < 10 m s−1 at the surface to around 16 m s−1 at 2 m. Variations in electrical properties were correlatable with lithological change. It is anticipated that analysis of reflection responses will provide significant additional geotechnical ground-truthing.

Measurements of the acoustic (200 kHz) backscatter from a carbonate sediment at low grazing angles

A multibeam sonar, mounted on a remotely operated vehicle, and operated at 200 kHz was used to acquire bottom backscatter data at nine different sites in the Dry Tartugas near Key West, Florida. The bottom in this area is carbonate mud and biogenic carbonate detritus (hermitific corals, shells, etc.). Bottom backscattering strength as a function of grazing angle for each site, supported by bottom core sample analyses, is presented. A wide range of bottom backscattering strength values were measured and some correlation between bottom type and backscattering strength was found. [Work sponsored by ONR under management by NRL/SSC under the Coastal Benthic Boundary Layer Special Research Program.]

High-frequency bistatic scattering by sub-bottom gas bubbles

The acoustic scattering by sub-bottom inhomogeneities has received increased attentions in recent years. However, most of the previous studies were restricted to monostatic or backscattering. To better understand the complicated scattering process, a bistatic scattering involving both grazing and azimuth angles dependence is desirable. As part of the Coastal Benthic Boundary Layer Special Research Program (CBBLSRP), along with a previously reported backscattering experiment, a bistatic scattering field experiment using Mobile Benthic Receiver Arrays (MBRA) operated at 40 kHz was conducted at Eckernfoerde Bay, Germany in April 1993. It has been found in previous studies that high-frequency backscattering measured is due to methane gas bubbles buried about a meter beneath the seafloor. In this paper, a bistatic scattering model is proposed as an extension of the previously developed backscattering model to account for the three-dimensional (out-of-plane) scattering effect. In the model, as in the backscattering model, gas bubbles were assumed to be oblate spheroids with varying aspect ratios. While the parameters used in the model to fit backscattering data are kept unchanged, the model prediction for the bistatic scattering is satisfactory. It is found that the scattering strength exhibits a mild azimuthal dependence and is sensitive to the scattering geometry. [Work sponsored by ONR.]

High-frequency acoustic penetration of ocean sediments

Two experiments were conducted to investigate acoustic penetration of ocean sediments: one in a muddy site in the Baltic Sea and the other in a sandy site in the Gulf of Mexico. In both experiments, acoustic sensors were placed in the sediment to measure the acoustic signals. The signals are processed coherently to give direction and speed of the acoustic waves. The data from the two sites give very different results. In the muddy site, the sediment sound speed is very close to that of the water, the situation in the sandy site is more complicated. The medium is treated as a poro-elastic solid governed by Biot’s theory of acoustic propagation. For the sandy site, the theory predicts two acoustic waves, one faster than the sound speed in water, and the other slower. Comparisons are made between theory and experiment. [Work supported by Office of Naval Research, under the Coastal Benthic Boundary Layer (CBBL) Special Research Program.]

The investigation of millimeter scale heterogeneity in Coastal Benthic Boundary Layer sediments using microresistivity and x-ray imaging of ‘‘diver’’ cores

A micro-resistivity imaging technique, developed for use on 70-mm ‘‘half-round’’ slabbed core has been adapted for use with larger ‘‘diver’’ cores, 33 mm thick, 350 mm wide, and 440 mm long. This resistivity technique is shown to have a resolution of 5 mm and to operate in a manner which is complimentary to x-ray photography. Theoretical and practical examples are presented showing responses to individual heterogeneity such as shells. The benefits of 3-D investigations are explored via numerical models are shown to be beneficial even in cores such as those above having a ‘‘flat’’ aspect ratio. The microresistivity method is shown to be particularly sensitive to layered structures and the presence of shells within the very high porosity surface layers in Eckernforde Bay (89% porosity 0–10 cm below sea floor). [Work supported by U.S. Navy.]

Geophysical ground-truthing experiments in Eckernfoerde Bay

During the 1994 Coastal Benthic Boundary Layer Special Research Program’s (CBBLSRP) experiment in Eckernfoerde Bay, multichannel digital seismic and electrical resistivity data were collected using surface and bottom towed arrays. Profiling with a bottom towed sledge yielded shear wave velocity and electrical resistivity data indicative of the structural strength of the sediment and of the properties of the sediment’s pore space. Shear wave velocities for the gassy mud were, as expected, extremely low, ranging from &amp;lt;10 at the surface to around 16 m/s at 2 m. Variations in electrical properties were correlatable with lithological change. It is anticipated that analysis of reflection responses will provide significant additional geotechnical ground-truthing.

Microstructural modeling of clay sediments in Eckernfoerde Bay

A finite-element microstructure analysis code was used to model the geoacoustic properties of shallow marine clay sediments in Eckernfoerde Bay which has been the subject of an intensive geoacoustic and geotechnical study (Coastal Benthic Boundary Layer—Special Research Program). Uniaxial strain and pure shear were applied to two-phase microstructure models with generalized, periodic plane-strain boundary conditions to generate anisotropic bulk and shear moduli. Analyses were carried out on two length scales: clay microfabric (μm) and gas-filled inclusions (mm). The clay microstructure was modeled as a random lattice of Illite platelets with a density and porosity equivalent to average values observed in shallow Eckernfoerde Bay sediments. Saturated and dry framework moduli were calculated and the effective medium velocities were compared with observed velocities. The analysis suggests that shear wave propagation in these sediments is not supported by the framework structure but by differential compression of fluid-filled pores. Radiographic images of pressurized cores containing gas-filled inclusions with dimensions of a few millimeters were also analyzed. The analysis indicates a shear wave velocity anomaly related to the inclusion structure. The calculated effective medium velocities are compared with analytical models of spheroidal inclusions. [Work supported by ONR.]