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1.
Abstract

SEASWAB is one element of the Delta Project of the U.S. Geological Survey, a cooperative effort with several universities and other governmental agencies to investigate the processes that cause marine‐sediment instability. The basic purpose of the SEASWAB experiment was to obtain field measurements of sediment motion and pore‐pressure variations in soft sediment affected by wave‐pressure perturbations. This article serves as an introduction to the six papers that follow and that together make up a report on the results of SEASWAB.  相似文献   

2.
Abstract

Methane concentrations and sediment shear strengths were measured in three foundation borings taken from areas of variable acoustical penetration in the Mississippi river delta front. Acoustically impenetrable or “turbid”; zones were associated with sedimentary methane concentrations above about 30 ml/liter, measured at atmospheric pressure. Sediments in the high‐gas, acoustically turbid zones demonstrated a smaller percentage increase in shear strength with depth than in zones of low gas concentration. The results indicate that a 3.5‐kHz system used for sub‐bottom profiles is unable to determine the thickness of gas‐charged sediments.  相似文献   

3.
Abstract

Where undissolved gas occurs within fine‐grained marine sediments it usually takes the form of discrete bubbles that are much larger than the normal void spaces. The possibility of buoyancy‐induced movement of these relatively large bubbles must be included when considering the transport of gas through marine sediments. A theoretical analysis shows that, under static loading conditions, bubbles larger than a critical size should have sufficient buoyancy to move upward through a fine‐grained sediment stratum, whereas bubbles smaller than the critical size should remain fixed in position. The critical radius is directly proportional to sediment shear strength, and bubbles of a realistic size should move upward only in extremely weak sediments. Further theoretical analysis shows that the critical bubble size is reduced under cyclic loading conditions, but movement of typical‐sized bubbles should still be restricted to sediments of low shear strength. A simple laboratory experiment provides support for the conclusions of the theoretical analysis. The results indicate that buoyancy‐induced movement of relatively large gas bubbles in fine‐grained sediments is most likely to occur under storm loading conditions and is unlikely to occur at depths greater than a few meters below the seabed.  相似文献   

4.
Abstract

The superficial marine sediment is an important boundary of ocean acoustic propagation. So, the acoustic property of seafloor surface is always research hotspot. The acoustic property of sediment is affected by temperature and pressure which is not considered by conventional lab acoustic measurement. A new type of system, called “Small-scale Geo-acoustic Physical Model Pilot System” (SGPMP) has been developed. The system measures geo-acoustic property of sediment under specific temperature, pressure and frequency conditions which can be controlled conveniently and accurately. The components, structure, measurement principle, error analysis and application example of this system are introduced in this article. As a laboratory platform, the system makes it convenient for us to study the relationship between the temperature, pressure, frequency and acoustic properties of marine sediment.  相似文献   

5.
Abstract

Subbottom profiling (3.5 kHz) in the uppermost slope of the outer shelf of the northwestern Aegean Sea has shown downslope slumping and intense sediment deformation as well as a succession of microslumping within the Late Pleistocene delta sequences. Postdeposition bottom erosion by currents and the overconsolidated sediments (Cu/Po > 1) indicate the termination of deformations a long time ago. Deformation processes are estimated to have been active 18,000–10,000 B.P. Large‐scale active downslope deformations with clockwise sediment block rotation have affected the area as well. These longer‐period deformations are associated with regional neotectonics and older unconformities (i. e., Plio‐Pleistocene) as potential slide planes.

Geotechnical properties reflect mainly textural variations and locally (within sapropelic layers) high organic matter content. Relatively high values of shear strength were measured (5–29 kPa) with intermediate sensitivities (2–5), whereas relatively low values of water content (33–81%) were found along the five selective sediment cores.

Infinity slope stability analysis revealed that the slopes in the study area are most stable, although theoretical evaluations (Cu estimated from linear regression analysis) indicate relative instability for the potential glide plane at 20 m depth. The outer shelf is covered by compact relict sands, and their gentle and low‐angle (<0.2°) slopes are the most stable region of the investigated area.  相似文献   

6.
Abstract

Fine‐grained sedimentary deposits on the Kodiak continental shelf and upper slope comprise three distinct compositional types: terrigenous mud, diatomrich mud, and ash‐rich sandy mud. The sediment types can be distinguished on the basis of geotechnical properties as well as by composition. The terrigenous mud has properties largely within the normal range for fine‐grained marine sediment, except for the low compressibility of many samples. This sediment underlies the walls of canyons that incise the upper slope, and analyses of undrained static and cyclic loading indicate potential instability in the steepest areas. The diatom‐rich mud has high water content, plasticity index, and compression index but low grain specific gravity. The ash‐rich sandy mud is nonplastic and has low water content and compressibility. It has high drained and undrained static strength but is extremely weakened by cyclic loading. Extensive deposits of sedimentary bedrock and coarsegrained glacial sediment in the region apparently are relatively stable, but low sediment strength or high compressibility may be encountered at the local sites of soft sedimentary deposits.  相似文献   

7.
Sediment transport near the Tauranga entrance to Tauranga Harbour   总被引：2，自引：2，他引：0  
Abstract

Sediment transport at the Tauranga Entrance was studied in relation to tidal currents and waves. Bedforms resulting from tidal flow were investigated with scuba divers and echo‐soundings. The alignment and scale of bedforms indicated the direction and approximate rate of sediment transport. Sediment transport was measured directly using sediment traps, and results were compared with rates calculated by another method. Maximum sediment transport rates of 20 000–30 000 g.m?1 per half tidal cycle occur near the inlet gorge, but rates vary considerably in time and space, depending mainly upon power of tidal currents. A model of sediment transport for this inlet has been evolved based on tidal flow streamlines, bedform features, and the measured and calculated rates of sediment transport.  相似文献   

8.
Abstract

From September 1975 to April 1976 offshore production Platform V in South Pass, Block 28 (East Bay, Louisiana), was instrumented to measure the effect of storm waves on the soft sediments typical of the Mississippi delta (in a project given the acronym SEASWAB). A portion of this project consisted of four identifiable units of instrumentation (see note): (1) an accelerometer package buried 1 m in the sediment to measure three‐dimensional sediment accelerations and an associated pressure transducer, which measured wave‐induced pressures; (2) an array of instruments that included a wave staff, electromagnetic current meter, and a pressure transducer to examine various relationships between wave properties; (3) a wave‐, current‐, and wind‐measuring station 3.35 km inshore of Platform V to determine the transformation of the waves as they moved over the sediments; and (4) a transponder buried in the mud, the position monitored so that long‐term mudflow could be measured. The direct measurement of seafloor oscillations required the unique instrumentation of the accelerometer system. Three Bruel and Kjaer 8306 accelerometers mounted at right angles to each other made possible the measurement of small oscillations (～0.01 m) at low frequencies (0.1–0.3 Hz). The acoustic method of measuring long‐term mudflow was subject to problems associated with sound propagation in shallow water. The range of the system was found to be 2.74 km, apparently independent of depth. Multiple returns received after single interrogations of the transponder decreased the accuracy of the system.  相似文献   

9.
Abstract

Concerns about the potential adverse effects of Spartina invasion on New Zealand estuaries, such as habitat loss and degradation, have led to herbicide use to control its spread. An experiment was conducted in the Manukau Harbour (New Zealand) to determine sediment remobilisation rates (?E) from herbicide‐treated S. anglica patches. Paired treatment and control patches (≤25 m diam.) were monitored at two sites 500 m apart with different wave exposures. Above‐ (AGB) and below‐ground (BGB) biomass decomposition and AE were determined from repeat surveys over c. 2.5 years. Work done by waves (W), which integrates wave‐energy dissipation at the bed over time, was used as a surrogate measure of sediment transport potential. Complete AGB loss occurred in both treated patches within 3 months of initial herbicide application, whereas BGB (a matrix of roots and rhizomes) persisted much longer. An exponential decay model (r 2 = 0.6) fitted to the site‐two treatment data predicted an c. 85% reduction in BGB after 5 years, whereas the BGB changes for the larger and more heterogeneous site‐one treatment were more complex. BGB was a better predictor of average bed elevation than AGB in multiple regression models and, although bed elevation decreased through time and with waves, BGB was positively related to bed elevation at both sites. AE was similar for both treatment patches (c. 0.05 and c. 0.06 m yr‐1, P < 0.001, r2 ≥ 0.97) and complete sediment loss was predicted to occur within 6–10 years. The site‐one control patch accumulated sediment (0.01 m yr‐1 , P < 0.001, r 2 = 0.81) whereas the more wave‐exposed site‐two control eroded (0.01 m yr‐1, P = 0.0017, r 2 = 0.61). The potential for adverse sediment effects resulting from Spartina control at wave‐exposed sites will be minimal if the treated areas are small relative to the size of the estuary.  相似文献   

10.
Abstract

Potential sediment mass movement was analyzed at ten locations on the continental slope off Peru and northern Chile, using samples obtained from up to 3 m below the seafloor. Shear strength parameters were obtained from consolidated‐undrained triaxial compression tests. Sediment behavior in these tests reflects the influence of organic matter, which is concentrated in the slope deposits by coastal upwelling. High water content of the organic‐rich sediments and the high de‐formability of organic matter contribute to the prevalent ductile behavior. Aggregation of clays by organic matter is apparently responsible for the high friction angles, up to 44°, displayed by the slope deposits. Sediment stability was assessed using infinite slope analyses. These analyses indicate that gravitational forces alone are not sufficient to cause sediment failure at any of the slope locations. Sediment accumulation on the slope is not rapid enough to generate excess pore pressure and reduce the resistance to gravitational sliding. Effects of earthquakes on slope stability were evaluated by modeling earthquake‐induced inertia forces as static forces and estimating pore pressures developed during cyclic loading. This analysis shows that sediments of the lower slope off Peru possess the highest susceptibility to failure during earthquakes. Earthquake accelerations on the order of 0.2 gravity are sufficient to trigger slumping at all ten slope locations. Indirect evidence suggests that creep and mass flows initiated at shallower water depths are factors that might contribute to sediment failure on the slope.  相似文献   

11.
Abstract

The improvement of sensors such as various high‐resolution seismic and navigational systems and side‐scan sonar, of offshore shallow‐water drilling techniques, and of laboratory analyses has allowed the marine geologist to make more accurate identifications and maps of the distribution of numerous types of marine sediment instabilities, as well as to determine the mechanisms responsible for their occurrence. A large number of data on the continental shelf and upper continental slope off the modern delta of the Mississippi river have been compiled; these data will be used to document the major types of slope instabilities. The continental shelf and slope off the modern Mississippi river delta display various types of sediment instability. High rates of sedimentation (up to 80 m per century), weak, high‐water‐content clays, and differential weighting of clay sediments characterize this region. The major types of sediment instabilities that have been documented include (a) Peripheral slumping, with dimensions of slumps ranging from 200 to 1000 m; slumping often occurring in multiple stairstep arrangement; and downslope movement as high as 700 m per year. (b) Shallow diapiric intrusions, ranging in size from a few hundred meters to 2 km in diameter; vertical displacement ranging from 200 to 500 m; rate of sediment movement several meters per year; and intrusions caused by differential sediment loading, (c) Radial graben (tensional faulting), with widths from 50 to 500 m and lengths of several kilometers; both vertical and downslope lateral movements occurring; and downslope movements of surface material as much as 5 m per year common. (d) Circular collapse depressions, with diameters of depressions ranging from 50 to 500 m; topography of depression interiors, hummocky; and depressions possibly caused by dewatering or degassing of sediments under the influence of cyclic wave loading. (e) Surface mudflows, thick (often more than 35 m) masses of surface sediment flowage; often bounded by abrupt seaward slope; mudflows often extending laterally for distances in excess of 100 km; movement sporadic and lobate and rates of movement as much as several hundred meters per year; often being associated with extremely hummocky topography and mud volcanoes; and with extrusion of sediments the possible mechanism. (f) Shelf‐edge arcuate slumps, with large arcuate slumps displacing several hundred meters of sediment; slippage planes are commonly concave. Finally, (g) Various deep‐seated faults, with faults extending from deep horizons up to modern sediment surface; commonly being associated with abrupt scarps on the seafloor; numerous contemporaneous faults; and local slumping associated with fault scarps.  相似文献   

12.
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13.
Abstract

An analysis of the relationship between organic matter and liquid and plastic limits, and grain‐specific gravity of a marine sediment was accomplished by making a stepwise adjustment in the organic content of that sediment. The sample used was from Santa Barbara Basin (off southern California) and is typical of fine‐grained marine sediments: it is a clayey silt with a common suite of minerals and other constituents. During the experiment, texture and composition (except organic content) were constant; only the quantity of natural, indigenous organic matter was changed.

A strong linear relationship exists between the independent variable, the amount of organic carbon present in the sample, and the dependent variables. Liquid limit, plastic limit, and plasticity index all increased with increasing organic content over the range studied (0.57–3.20% organic carbon). Grain‐specific gravity decreased. All had linear correlation coefficients (r) greater than |0.90| and r 2 values greater than 90%, except the plasticity index (83%).

Comparing the results of regression analyses from this and several similar studies shows that although there is good qualitative agreement, there are quantitative inconsistencies. In particular there is considerable overall variability in the regression coefficients. Among studies on marine sediments the inconsistencies are less pronounced, yet still evident. The increase in liquid limit as organic carbon increased by 1 % sediment dry weight ranged from 9 to 28% water content; in the plastic limit the range was from 4 to 18%. However, in these marine studies regression coefficients are relatively close in value in some cases, levels of significance of the regressions are high in most cases, and in all cases the relationships appear to be linear over the range of organic carbon percentage studied. Finally, we believe that a relatively clear relationship between plasticity and organic carbon begins to emerge when the latter exceeds a value of 2%.  相似文献   

14.
Abstract

This report describes the instrumentation, initial results, and progress of an experiment designed to measure and monitor submarine sediment pore water and hydrostatic pressures in a selected area of the Mississippi Delta. The experiment also is intended to monitor significant pressure perturbations during active storm periods. Initial analysis of the data revealed excess pore water pressures in the silty clay sediment at selected depths below the mudline. Continuous monitoring of the pore water and hydrostatic pressures was expected to reveal important information regarding sediment pore water pressure variations as a function of the geological processes active in the Mississippi Delta.  相似文献   

15.
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16.
Abstract

On shingle beaches, changes in foreshore elevation and sediment distribution landward of the break point are produced largely by variations in the uprush and backwash of waves. However, very little is known about the forces active in this zone.

A field instrument system which senses and records some of the parameters thought to influence beach erosion and deposition in this zone has been constructed. The equipment is also suitable for the investigation of a number of other shore and nearshore processes including erosion on sandy and rocky shores, and flow processes affecting littoral biological communities.

In the swash zone two sensing heads, a dynamometer and a depth recorder, sense variations in uprush and backwash velocities, energies, discharges, and depths of flow. Both devices are electromechanical and are coupled to a recording unit on land by PVC‐insulated cable. The dynamometer (two force plates mounted back‐to‐back on a compression spring and coupled to variable resistances) has been calibrated, statically and in a flume, to obtain velocity determinations accurate to within 10 cm . sec?1 of true flow speed. Average swash zone velocities lie between 100 and 300 cm . sec?1.

A parallel‐wire resistance gauge mounted an a stilling tube records flow depths. As water level rises and falls in the tube it alters resistance in a control circuit. The land unit, amplifiers and a strip‐chart recorder, receives the output from the dynamometer and flow depth gauge. The recorder is equipped with a trip‐pen so that analysis of wave periods or other variables is possible in the field. With poles at known spacings across the shore and the trip‐pen records, velocity distributions across the swash zone can be obtained. Measurements of velocity made near the bed with the dynamometer can then be related to the local surface velocity profile.

Problems with the instrument system include inability to record velocities at several points simultaneously, and unreliable records of backwash parameters with low breakers on shingle beaches because of the small volume of flow and rapid percolation of water into the beach face.  相似文献   

17.
Abstract

Foundations for offshore structures present a design problem resulting from two primary sources. First, the normally consolidated sea‐floor sediments are often weak and compressible compared to relatively competent terrestrial soils. Second, the technical difficulties of either obtaining relatively undisturbed sea‐floor samples or of performing in situ mechanical tests can be more problematic than with terrestrial soils. Any technique that might improve the assessment of sediment behavior is worthy of investigation.

This paper describes some preliminary experiments which measure P and S velocities during conventional soil testing procedures. Computed static and dynamic moduli, measured from tests with different strain levels, have been compared, revealing that although two orders of magnitude separate the moduli, there are similarities in the way certain moduli change with increasing stress levels. It is suggested that the use of a “frame‐wave”; velocity in the calculation of dynamic moduli may reduce drastically the differences that exist between the static and dynamic moduli for saturated sediments. In this way it is thought that results from tests, such as those described in this paper, not only provide empirical correlative data for a given sediment, but may also contribute to a clearer understanding of its behavior under loading conditions.  相似文献   

18.
Abstract

The high-frequency acoustic properties of seafloor sediments are very significant in seafloor study and underwater acoustic study field. In order to measure the sound speed and the attenuation for the small-scale sediment cores more accurately, this study developed a water coupled acoustic laboratory measurement system based on Richardson-Briggs technique. This method used the correlation comparison of waveforms received in sediment core and in identical reference tubes filled with water to measure sound speed and attenuation. The sound speed and attenuation of a clayey silt sediment sample were measured using the water coupled acoustic laboratory measurement system. This frequency dependence of the sound speed and attenuation showed that the clayey silt sediment has a weak positive sound speed dispersion, while the attenuation increases with a strong positive gradient within the measurement frequency range. This study also noted that the measured sound speed ratio match well with the empirical equations from literature. The measured attenuation factor data can fall in the Hamilton’s empirical prediction range.  相似文献   

19.
Abstract

Surface sediment samples (60 μm‐2 mm, and < 60 μm fractions) from the Waiwhetu Stream, Lower Hutt City, New Zealand, were leached with dilute HCl to remove the mobile heavy‐metal fraction. Cd, Cr, Cu, Pb, Zn, Al, Fe, and Mn analyses of the leachates show that sediments of the upper reaches area of the stream are generally uncontaminated. In contrast, sediments in the lower reaches area are highly contaminated, despite clean‐up measures in the late 1970s. Lead and Zn are the most significantly enriched metals with concentrations over 1000 mg kg?1 adjacent to some drains. In the polluted zone, metals originate from either upward remobilisation from underlying sediment (contaminated in the early 1960s and 70s) or more recent industrial spillages. Similar trends exhibited by Cd, Cu, Cr, Zn, Pb, and Al indicate that a proportion of the metal enrichment downstream probably results from a naturally higher clay content of sediments near the mouth of the stream. Fe and Mn show reverse trends to the other metals, indicating that a high proportion of these two metals occurs in substrate different to that of the other metals. The orange brick‐coloured sediments of the upper reaches of the stream indicate oxides upstream; the black sediments in the lower reaches indicate sulphides downstream.  相似文献   

20.
Abstract

A nonlinear pile‐soil interface model incorporated in a boundary element analysis is presented to simulate both the static and cyclic behavior of piles embedded in cemented and noncemented calcareous sediments. Based on the soil parameters derived from model test data, theoretical predictions are made for a few field tests. Finally, theoretical solutions are obtained for a full‐scale hypothetical pile embedded in homogeneous and layered calcareous sediments.  相似文献   

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