Microtopographical roughness of shallow-water continental shelves

High-resolution (<1 cm) roughness height measurements were made of the seafloor at seven locations on continental-shelf sediments on water depths ranging from 18 to 50 m. Roughness profiles of the sediment-water interface were digitized primarily from stereo photogrammetric measurements of varying pathlengths and increments. The data show that the root-mean-square roughness height varies from 0.3 cm for flat, featureless bottoms to 2.3 cm for rippled bottoms. Slopes of the roughness power spectra were calculated to be -1.5 to near -3.0 and depended to a large extent on contributions in higher spatial frequencies due to coarse sediments. Correlation lengths of different bottom types were estimated by using the Weiner-Khintchine theorem and examining the low-frequency behavior of the roughness spectra derived from the longest roughness profiles     

Evaluation of Laser Scanning and Stereo Photography Roughness Measurement Systems Using a Realistic Model Seabed Surface

The topography of the seabed is influenced by sediment transport due to wave motion, current disturbance, and biological activities. The bottom roughness generated by these processes can substantially alter acoustic wave penetration into and scattering from the bottom, and therefore, it is essential to make accurate measurements of the bottom roughness for such acoustic applications. Methods to make direct measurements of bottom roughness include stereo photography, laser line scanning, and sediment conductivity. Roughness can also be measured indirectly using high-frequency sound backscatter. For optically-based methods, the accuracy of these measurements is typically evaluated using the elevations, lengths, or diameters of simple surface features of known dimensions. However, for acoustic applications, the statistical characteristics of the surface, e.g., the roughness spectrum, are more meaningful. In this paper, we present a fabricated rough surface milled into a 40 $,times ,$60 cm $^{2}$ plastic block for use as a benchmark in the assessment of two in situ roughness measurement systems: a laser scanning system and a digital stereo photography system. The surface has a realistic roughness power spectrum that is derived from the bottom roughness measured during the 1999 Sediment Acoustics Experiment (SAX99) and was fabricated by a computer numerical controlled milling machine. By comparing the fabricated surface spectrum to the measured spectrum, a determination of the accuracy of the roughness measurement is evaluated, which is of direct relevance to acoustic applications.      

In situ permeabilities of selected coastal marine sediments

An in situ permeameter probe was deployed off Fort Walton Beach, Florida in shallow-water coastal sandy sediment. Stations were occupied in a 600 m /spl times/ 600 m area. Intrinsic permeability in sand at 17 stations varied from 0.3 to 6.1 /spl times/ 10/sup -11/ m/sup 2/ to subbottom depths of 50 cm. Permeability decreased with increasing subbottom depth and minimal compaction. The sediment is a well-sorted, medium quartz sand with a mean grain size of approximately 0.34-0.52 mm and with 5%-8% carbonate shells and shell fragments. The probe was tested in a slightly finer-grained sediment near the main study area and revealed permeabilities of 0.1-3.2 /spl times/ 10/sup -11/ m/sup 2/. The permeabilities measured are reasonable for the observed sandy and slightly finer-grained sediments.     

Seafloor Roughness Measured by a Laser Line Scanner and a Conductivity Probe

To support modeling acoustic backscatter from the seafloor, a conductivity probe and a laser line scanner were deployed jointly to measure bottom roughness during an experiment off the New Jersey coast in summer 2006. The conductivity probe in situ measurement of porosity (IMP2) is impervious to water turbidity and yields a 1-D profile with 10-mm horizontal spacing and 1-mm resolution in the vertical direction. The laser line scanner is limited by water visibility but it provides 2-D grid points with resolutions 0.3 mm across track, 0.5 mm along track, and 0.3 mm in the vertical direction. Two sets of data, suitable to model mid- to high-frequency acoustic backscatter, were collected from two sites 900 m apart on August 14 and 17, 2006. The roughness spectra obtained from the laser scanning were compared to those measured by the IMP2. The spectra from the two methods are consistent over wave number range 0.0188–3 rad/cm, which are the wave number range common to both methods. The efficacy of the laser scanner is also confirmed by showing the spectral line created by the IMP2's periodic probing marks. The 2-D spectra generated from the laser scan data show that the bottom roughness at these sites is azimuthally isotropic, but significant spatial heterogeneity is observed.      

The morphodynamic modelling of tidal sand waves on the shoreface

An artificial sand wave on the Dutch shoreface of the North Sea has been studied in conditions with relatively strong tidal currents in the range of 0.5 to 1 m/s and sediments in the medium sand size range of 0.2 to 0.5 mm. The sand wave is perpendicular to the tidal current and has a maximum height and length of the order of 5 m and 1 km, respectively. The sand wave is dynamically active and shows migration rates of the order of a few metres per year. A numerical morphodynamic model (DELFT3D model) has been used to simulate the morphological behaviour of the sand wave in the North Sea. This model approach is based on the numerical solution of the three-dimensional shallow water equations in combination with a surface wave propagation model (wind waves) and the advection–diffusion equation for the sediment particles with online bed updating after each time step. The model results show that the sand wave grows in the case of dominant bed-load transport (weak tidal currents; relatively coarse sediment; small roughness height; low waves) and that the sand wave decays in the case of dominant suspended transport (strong currents, relatively fine sediment, large roughness height; storm waves).     

Mobile sand deposits and shoreface sediment dynamics in the inner German Bight (North Sea)

This paper presents a conceptual model for the net bedload transport regime on the shoreface of the German Bight. The model is based on the spatial distribution of the surficial sediment cover (North Sea sands) which is identical to the uppermost layer in the seismic recordings. Sediment thickness was measured using very high resolution seismic profiling (chirp sonar) and vibrocoring. The three-dimensional sediment distribution was estimated using geostatistical methods (cokriging). The results demonstrate a longshore sand distribution with three distinct zones. In Zone 1 (0–10 m water depth) the sediments attain their maximum thickness of 10±2.5 m. Between 10 and 15 m water depth a relatively thin sand layer of 0.4–1.5 m is observed within Zone 2. The seaward adjacent Zone 3 (15–20 m water depth) is characterized by an averaged sand thickness of 2–3 m with local maxima of 5–6 m. Further offshore, the sand layer decreases to about 1–2 m thickness. The net bedload transport directions inferred from this sediment zonation comprise a longshore sediment bypassing in Zone 1 which results in a substantial sediment supply to the innermost part of the German Bight due to bedload convergence. Shore-normal bedload transport shifts sand to and fro across the coastal profile although the net directional transport is seawards. This results in sediment depletion between the 10 and 15 m-isobaths (Zone 2) and an adjacent sediment accumulation in deeper waters (Zone 3).     

Fine-scale volume heterogeneity measurements in sand

As part of the effort to characterize the acoustic environment during the high frequency sediment acoustics experiment (SAX99), fine-scale variability of sediment density was measured by an in situ technique and by core analysis. The in situ measurement was accomplished by a newly developed instrument that measures sediment conductivity. The conductivity measurements were conducted on a three-dimensional (3-D) grid, hence providing a set of data suited for assessing sediment spatial variability. A 3-D sediment porosity matrix is obtained from the conductivity data through an empirical relationship (Archie's Law). From the porosity matrix, sediment bulk density is estimated from known average grain density. A number of cores were taken at the SAX99 site, and density variations were measured using laboratory techniques. The power spectra were estimated from both techniques and were found to be appropriately fit by a power-law. The exponents of the horizontal one-dimensional (1-D) power-law spectra have a depth-dependence and range from 1.72 to 2.41. The vertical 1-D spectra have the same form, but with an exponent of 2.2. It was found that most of the density variability is within the top 5 mm of the sediment, which suggests that sediment volume variability will not have major impact on acoustic scattering when the sound frequency is below 100 kHz. At higher frequencies, however, sediment volume variability is likely to play an important role in sound scattering.     

长江河口区边界层参数的观测与分析

2003年11月在长江口南槽用ADCP进行定点水文观测,结果表明研究区为不规则半日潮,在水流转向期流速较低时常出现悬沙浓度峰值。根据流速对数剖面分布模型与悬沙分布模型,分别计算了海底边界层参数,其中潮周期内摩阻流速可达0.15 m/s,粗糙长度为0.01~1.2 m,拖曳系数为10-3~10-4,边界层厚度为2~4 m,悬沙的沉降速率为0.2~6 mm/s。     

Porometric properties of siliciclastic marine sand: a comparison of traditional laboratory measurements with image analysis and effective medium modeling

During the 1999 sediment acoustics experiment (SAX99), porometric properties were measured and predicted for a well sorted, medium sand using standard laboratory geotechnical methods and image analysis of resin-impregnated sediments. Sediment porosity measured by laboratory water-weight-loss methods (0.372 /spl plusmn/ 0.0073 for mean /spl plusmn/1 standard deviation) is 0.026 lower than determined by microscopic image analysis of resin-impregnated sediments (0.398 /spl plusmn/ 0.029). Values of intrinsic permeability (m/sup 2/) determined from constant-head permeameter measurements (3.29 /spl times/ 10/sup -11/ /spl plusmn/ 0.60 /spl times/ 10/sup -11/) and by microscopic image analysis coupled with effective medium theory modeling (2.78 /spl times/ 10/sup -11/ /spl plusmn/ 1.01 /spl times/ 10/sup -11/) are nearly identical within measurement error. The mean value of tortuosity factor measured from images is 1.49 /spl plusmn/ 0.09, which is in agreement with tortuosity factor determined from electrical resistivity measurements. Slight heterogeneity and anisotropy are apparent in the top three centimeters of sediment as determined by image-based porometric property measurements. However, the overall similarity for both measured and predicted values of porosity and permeability among and within SAX99 sites indicates sediments are primarily homogeneous and isotropic and pore size distributions are fairly uniform. The results indicate that an effective medium theory technique and two-dimensional image analysis accurately predicts bulk permeability in resin-impregnated sands.     

Time-evolution of high-resolution topographic measurements of the sea floor using a 3-D laser line scan mapping system

A laser line scan system was used to characterize the high-resolution spatial variability and temporal evolution of the sea floor as a function of environmental conditions for a small section of the sea floor in the Gulf of Mexico during a period of nine days. High-resolution bathymetric profiles (<1 mm) covering about two sand ripple periods on the sea floor were acquired over a one-dimensional transect 1.35 m in length. The system was also used to measure reflectance and to produce three-dimensional bottom maps of the test area. Over the nine-day period, the sand ripple peak-to-trough height was observed to decrease slowly from about 2.5 cm to about 2.0 cm. Similar gradual changes we also observed in the time-evolution of bottom profile correlations and of the bottom roughness spectra. However, when smaller sections were examined individually, the time-evolution of the profile correlation was observed to vary acutely and in a transitory manner in some regions but not in others, and without preference for crests or troughs. In general, bottom roughness for spatial frequencies greater than 0.044 cycles/cm completely decorrelated within 20-30 h. However, this gradual trend was also marked by acute and transitory changes in bottom topography believed to be primarily from fish feeding on epibenthic prey.     

Joint perturbation scattering characterization of a littoral oceanbottom reverberation: theory, scattering strength predictions, and datacomparisons

A joint surface roughness/volumetric perturbation scattering theory is utilized to characterize the reverberation from a littoral ocean bottom. The result is a reflected field spectrum that consists of specular and off-specular components. The predicted scattering strength from the off-specular component is shown to be comprised of interface roughness scattering, sediment inhomogeneity volumetric scattering, and interface roughness/sediment inhomogeneity correlation scattering. The sediment inhomogeneity volumetric scattering is shown to contain two contributions that are due to fractional variations in sediment densities and sound velocities. Both contributions are shown to be affected by the interface effect by a round-trip transmission coefficient factor. These two fractional variations are shown to contribute differently to scattering strength but similarly to backscattering strength. Inversely predicted roughness spectra from various sets of backscattering strength data are shown to be consistent with a generally known roughness spectrum. Both inversely predicted roughness and volumetric scattering physical property spectra are found to be self-consistent. However, the use of only ocean bottom backscattering strength data is found to be insufficient to judge whether the roughness or the volumetric scattering dominates. Reverberation characterizations using bistatic scattering strength data and signal spread data are planned for future studies     

Sediments in the East China Sea

This paper describes measurements of sediments during the 2000-2001 Asian Seas International Acoustic Experiment in the East China Sea. A number of techniques were used to infer properties of these sediments, including gravity and piston cores, subbottom profiling using a water gun, long-range sediment tomography, and in situ measurement of conductivity. Historical data from echosounder records and cores showed two regions of surficial sediments in the experimental area: a silty area to the west and a sandy area to the east. The tomography, cores, and water-gun measurements confirm the two surficial sediment regions seen in the historical data and also indicate that the subbottom structure at the experimental site consists of a thin (0-3 m thick) layer of sandy sediment directly beneath the sea floor. Below this layer, there is an extensive package of sediment with relatively uniform acoustic attributes. Core analysis shows that the surface sediment layer varies in compressional wave speed from a low near 1600 m/s in the west side of the experiment area to 1660 m/s in the east side of the experiment area. Long-range sediment tomography inversions show a similar spatial variation in the surface layer properties. In addition, the layer thickness as determined from tomography is consistent with the estimates from subbottom profiling.     

Field Observation and Analysis of Wave-Current-Sediment Movement in Caofeidian Sea Area in the Bohai Bay, China

In order to study the mechanism of flow-sediment movement, it is essential to obtain measured data of water hydrodynamic and sediment concentration process with high spatial and temporal resolution in the bottom boundary layer (BBL). Field observations were carried out in the northwest Caofeidian sea area in the Bohai Bay. Near 2 m isobath (under the lowest tidal level), a tripod system was installed with AWAC (Acoustic Wave And Current), ADCP (Acoustic Doppler Current Profilers), OBS-3A (Optical Backscatter Point Sensor), ADV (Acoustic Doppler Velocimeters), etc. The accurate measurement of the bottom boundary layer during a single tidal period was carried out, together with a long-term sediment concentration measurement under different hydrological conditions. All the measured data were used to analyze the characteristics of wave-current-sediment movement and the BBL. Analysis was performed on flow structure, shear stress, roughness, eddy viscosity and other parameters of the BBL. Two major findings were made. Firstly, from the measured data, the three-layer distribution model of the velocity profiles and eddy viscosities in the wave-current BBL are proposed in the observed sea area; secondly, the sediment movement is related closely to wind-waves in the muddy coast area where sediment is clayey silt: 1) The observed suspended sediment concentration under light wind conditions is very low, with the peak value generally smaller than 0.1 kg/m3 and the average value being 0.03 kg/m3; 2) The sediment concentration increases continuously under the gales over 6-7 in Beaufort scale, under a sustained wind action. The measured peak sediment concentration at 0.4 m above the seabed is 0.15-0.32 kg/m3, and the average sediment concentration during wind-wave action is 0.08-0.18 kg/m3, which is about 3-6 times the value under light wind conditions. The critical wave height signaling remarkable changes of sediment concentration is 0.5 m. The results show that the suspended load sediment concentration is mainly influenced by wave-induced sediment suspension.     

Atmospheric forcing of fine-sand transport on a low-energy inner shelf: south-central Louisiana,USA

Hydrodynamic and sediment transport measurements from instrumentation deployed during a 54-day winter period at two sites on the Louisiana inner shelf are presented. Strong extratropical storms, with wind speeds of 7.8 to 15.1 m s^-1, were the dominant forcing mechanism during the study. These typically caused mean oscillatory flows and shear velocities about 33% higher than fair weather (averaging 12.3 and 3.2 cm s^-1 at the landward site, and 11.4 and 2.7 cm s^-1 at the seaward site, respectively). These responses were coupled with mean near-bottom currents more than twice as strong as during fair weather (10.3 and 7.5 cm s^-1 at the landward and seaward sites, respectively). These flowed in approximately the same direction as the veering wind, causing a net offshore transport of fine sand. Weak storms were responsible for little sediment transport whereas during fair weather, onshore sand transport of approximately 25-75% of the storm values appears to have occurred. This contradicts previous predictions of negligible fair-weather sediment movement on this inner shelf.     

海南岛西部岸外沙波的高分辨率形态特征

利用SIMRAD-EM3000多波束探测系统和DGPS定位系统,对海南岛东方岸外的沙波沙脊区进行了高精度探测,分析结果表明:从海岸到陆架底形具有明显的分带性,依次出现弱侵蚀底形段、沙波沙脊底形段和平坦底形段。沙波仅发育于沙波沙脊段,介于水深20~50 m之间,沙波形态有二维与三维两种,沙波波高多为0.7~2.5 m,波长20~70 m,沙波指数(L/H)为20~60,对称指数为1~3;沙波沙脊区沉积物的搬运方向有明显的规律性,在沙脊的西侧,沉积物主要向北搬运;在沙脊的东侧,沉积物主要向南搬运;沙波的形成和发育主要受潮流场控制,热带风暴对其有改造作用。     

Puzzling mass movement features in the Navarinsky Canyon head,Bering Sea

Two types of morphologic features in the head of Navarinsky Canyon are attributed to mass movement of near-surface sediment. A series of pull-aparts is located downslope of large sand waves. These pull-aparts, possibly induced by liquefaction, affect the upper 5 to 10 m of sandy sediment (water depths 350 to 600 m) on a 1^o slope. A hummocky elongate mound of muddy sand (water depths 550 to 800 m) contains chaotic internal reflectors to a subbottom depth of 30 to 40 m and possibly is the product of a shallow slide. We speculate that Holocene seismicity is the likely triggering mechanism.     

黄河口现行流路入海水沙特征对流域的响应

基于1976-2012年利津站实测水沙资料,选取1976-1986年、1987-1999年及2000-2012年作为特征时段,探讨了黄河口现行入海水沙条件变化的阶段性特征,重点研究了入海泥沙粒度特征对入海水沙变化的响应情况。结果表明:(1)与1976-1986年年均入海流量1 020 m~3/s相比,1987-1999年与2000-2012年入海流量大幅度减小,分别为473.7 m~3/s和504.7 m~3/s;(2)与1976-1986年年均入海含沙量24.4 kg/m~3相比,1987-1999年大幅度减小至15.9 kg/m~3,2000-2012年仅为4.1 kg/m~3;(3) 1976-1986年入海悬沙中值粒径为0.022 mm,1987-1999年减小至0.016 mm,2000-2012年又变粗至0.021 mm,反映水库正常运行一段时间后,下游河床冲刷造成入海泥沙粗化;(4) 2000年后利津站与花园口站悬沙粒度特征变化趋势趋于一致。研究还揭示了利津站床沙特征与流域前4年来沙系数存在较好相关性,利津床沙粒径呈现随来沙系数减少而增大的趋势,表明流域来沙量减少会造成利津站床沙粗化。     

The reverse sediment transport trend between abandoned Huanghe River (Yellow River) Delta and radial sand ridges along Jiangsu coastline of China-an evidence from grain size analysis

To reveal the sediment transporting mechanism between the abandoned Huanghe River(Yellow River) Delta and radial sand ridges,"End Member" Model and grain size trend analysis have been employed to separate the "dynamic populations" in the surficial sediment particle spectra and to determine the possible sediment transporting pathway.The results reveal four "dynamic subpopulations"(EM1 to EM4) and two reverse sediment transporting directions:a northward transport tend from the radial sand ridges to mud patch,and a southward transport trend in deep water area outside the mud patch.Combined with the published hydrodynamic information,the transporting mechanism of dynamic populations has been discussed,and the main conclusion is that the transporting of finer subpopulations EM1 and EM2 is controlled by the "anticlockwise residual current circulation" forming during tidal cycle,which favor a northward transporting trend and the forming of mud patch on the north of radial sand ridges,while the transporting of coarser EM3 is mainly controlled by wind driven drift in winter,which favors a southward transporting direction.     

An overview of SAX99: acoustic measurements

A high-frequency acoustic experiment was performed at a site 2 km from shore on the Florida Panhandle near Fort Walton Beach in water of 18-19 m depth. The goal of the experiment was, for high-frequency acoustic fields (mostly In the 10-300-kHz range), to quantify backscattering from the seafloor sediment, penetration into the sediment, and propagation within the sediment. In addition, spheres and other objects were used to gather data on acoustic detection of buried objects. The high-frequency acoustic interaction with the medium sand sediment was investigated at grazing angles both above and below the critical angle of about 30°. Detailed characterizations of the upper seafloor physical properties were made to aid in quantifying the acoustic interaction with the seafloor. Biological processes within the seabed and the water column were also investigated with the goal of understanding their impact on acoustic properties. This paper summarizes the topics that motivated the experiment, outlines the scope of the measurements done, and presents preliminary acoustics results