Joseph N. Suhayda

Processes of marine dispersal and deposition of suspended silts off the modern mouth of the Huanghe (Yellow River)

The processes responsible for the transport and deposition of concentrated suspended silts over the delta front of the Huanghe were observed during three cruises and have been modeled numerically. Suspended sediment concentrations in the lower Huanghe average about 25 kg m−3 and exceed 200 kg m−3 during flood stage. Cruises were conducted during normal discharge conditions in spring 1985 and summer 1986, and during low-discharge storm-dominated conditions in autumn 1987. During the first two cruises, the shallow delta-front top (depth≤ 5m) was covered by a turbid water mass with suspended sediment concentrations of 1–10 kg m−3. Strong (∼1m s−1) parabathic tidal currents resuspended newly deposited muds and advected them alongshore. Near a break in slope, the turbid layers plunged beneath the ambient water and descended the delta-front slope as gravity-driven hyperpycnal underflows. In 1987 the hyperpycnal underflows occurred only during an intense strom that resuspended delta-front sediments to produce underflows with concentrations on the order of 100 kg m−3. We infer that gravity-driven underflows constitute the most important mode of suspended sediment transport across isobaths. Concentrated and channelized “point source” underflows, apparently associated with flood conditions, were not observed but were inferred from morphological evidence and were modeled numerically. Modeling results show that the Coriolis force and ambient momentum should cause appreciable curvature to the paths of underflows, while entrainment of ambient mass contributes to underflow decay. Early extinction of all underflow types is suggested by field and modeling results, and is considered to be responsible for extremely rapid delta-front deposition and for the fact that most of the sediments discharged by the Huanghe remain close to the mouth.

Wave-current interactions on a shallow reef (Nicaragua, Central America)

Measurements of wave height and currents associated with normal trade-wind conditions have been made on a linear reef that parallels the northern and northeastern coast of Great Corn Island, eastern shelf of Nicaragua, Central America. Analyses indicate that waves breaking over the reef crest generate lagoonward flow normal to the reef. Average reef-normal flow was in the range of 10 to 20 cm/s; however, individual wave surges reached values of up to 180 cm/s. The strength of the over-the-reef flow is modulated by the tide. Lagoon currents are weak (2–5 cm/s) and change direction with the tide as the lagoon fills and drains. Long-period oscillations in water level (30 s to 20 min) and in the current were observed, and may be important in transporting fine-grained sediments out of the reef-lagoon system. Strong, short-duration surge currents ( <5 s) transport coarse sediment from the breaker zone to the seaward margin of the backreef lagoon.

Application of infinite slope analysis to subaqueous sediment instability, Mississippi delta

Abstract Submarine landslides are described on very low angle slopes (0.5°–1.7°) in the Mississippi prodelta area and are evaluated using infinite slope analysis. For instability to occur, pore water pressure ratios in excess of hydrostatic (u/γ w z) and approaching geostatic (u/γ′z) are needed. These calculated values, based on three sets of effective strength parameters and for two sediment depths to failure, are in close agreement with measured pore water pressure data from in situ piezometers. Ratios of (u/γ′z) as large as 0.986 have been monitored. Mud vents are also indicative of large internal pressures within the sediments, generated by rapid sedimentation, wave perturbation, and biogenic methane. The analysis suggests that the reduction in effective stress may be sufficient to cause failure by gravitational stresses alone.

Acoustical penetration and shear strength in gas‐charged sediment

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.

Surface waves and bottom sediment response

Abstract Field measurements of bottom oscillations and wave characteristics have been made in a study of the interaction of fine‐grained sediments and surface waves. A wave staff, pressure sensor, and accelerometer were used in East Bay, Louisiana, an area that has a fine‐grained clay bottom. The accelerometer contained three solid‐state accelerometers mounted at right angles. The instrument was placed about 0.3 m below the mudline. The results of the study indicate that bottom motions under wave action show well‐defined periodic features. The bottom sediments appear to be undergoing an elastic response to bottom pressures, such that the bottom is depressed under a surface wave crest. Under the range of bottom pressures measured, bottom displacement varied linearly with bottom pressure. Measured bottom pressures were up to 35% larger than predicted by linear wave theory. The effect of a movable bottom on wave pressure is considered. The energy lost from the surface wave to the bottom in forcing the bottom...

Soil response to ocean waves

Abstract Wave‐induced bottom pressure anomalies can result in soil motion, especially in soft deltaic deposits, that affect the design and performance of off‐shore facilities. A layered, vis‐coelastic model that includes wave‐seabottom interaction was used for a parametric study of the effects of wave degeneration, wave height, wave period, water depth, and soil properties on soil motions and the phase difference between the wave and soil motions. The effects of degradation to soil resistance due to cyclic loading are also shown with an example. The analyses were for conditions typical of the Mississippi Delta. The results presented in the paper should help the oceanographer, geotechnical engineer, and structural designer establish reasonable bounds for a preliminary design criterion and to narrow the number of analyses required to select a final design criterion.

Interaction Between Surface Waves and Muddy Bottom Sediments

Surface wave-induced bottom pressure fluctuations produce shear stresses in soft muddy bottom sediments that cause the sediments to undergo oscillatory motion. This motion can be described as a “mud-wave” and causes surface wave properties to vary from those that occur over a rigid bottom. Theoretical studies have attempted to describe this interaction using a variety of soil models, i.e., viscous fluid, elastic solid, viscoelastic material and nonlinear viscoelastic. Although the experimental basis for evaluating the validity of these assumptions is incomplete, it appears that a nonlinear viscoelastic soil model is required to describe the observed behavior. An example of the interaction of hurricane waves and soils found offshore of the Mississippi Delta is considered in detail. The soil is described using a model which is nonlinear in relating shear strain to shear stress and damping ratio. The surface wave-mud wave interaction for hurricane waves is significant and causes wave heights of 70 ft (21.3 m) and 80 ft (24.4 m) in deep water to decrease to values of from 10 ft (3.0 m) to 25 ft (7.6 m) at a water depth of 50 ft (15 m). Soil response during this wave-mud interaction is greatest at water depths of between 150 ft (45.7 m) and 250 ft (76.2 m). Maximum soil movements of 1.5 ft (.46 m) are predicted to occur under hurricane waves. As a means for making rough calculations of the wave-mud interaction a simplified technique for making engineering predictions is presented. The technique is based upon a nonlinear stress-strain and damping-strain soil model and predicts surface wave attenuation, soil shear stress and shear strain profiles.

Mass physical properties of Huanghe delta and Southern Bohai Sea near‐surface deposits, China

Abstract The Huanghe or Yellow River is the second largest river in the world in respect to sediment load, debouching approximately 1.1 × 109 tons annually into the Bohai Sea. Owing to the predominance of silt‐size material, 80–90% of the sediment is deposited within 20 km of the estuary mouth with only about 36% getting out beyond the delta front. Gross annual lateral growth of the delta may be as much as 4.5 km. The delta presents a complex depositional setting with considerable disruption of the deposits taking place because of the dynamics of this environment, where wave action, rapid loading‐induced mass failure, and underflows predominate. The upper 2 m of the delta front deposits are classified as clayey silts whose median diameters commonly range from 5 to 8⊘ (0.031–0.004 mm), with wet bulk densities of 1.52–1.98 Mg/m3, water contents of 26–98% dry weight, shear strengths of 1.6–8 kPa, and sensitivities of 2 to 6. As expected, these properties vary considerably both laterally and with depth. Withi...

Ocean wave attenuation due to soft seafloor sediments

Abstract Ocean waves traveling over soft bottoms attenuate as a result of energy being absorbed by the soft bottom. This wave attenuation is important in the design of offshore facilities. The magnitude of wave attenuation can be predicted with an analytical model that couples wave and seabottom motions and realistically models the nonlinear stress‐strain behavior of soft sediments. An extensive parameter study was performed to develop wave attenuation behavior. Soil and wave characteristics typical of the Mississippi Delta were used for these analysis. Predicted results are compared with field observations, and good agreement is found.

Estimating break-down pressure of upper marine sediments using soil boring data

Abstract This paper illustrates how soil boring data is used to determine the formation break-down pressure for the Green Canyon area of the Gulf of Mexico. Example soil boring data is integrated with deeper well log data to accurately estimate overburden stress. Values for the horizontal to vertical effective stress ratios are confirmed by measuring the actual in situ formation break-down pressure while collecting the soil samples. This study shows that upper marine clays for the Green Canyon Area example have horizontal to vertical effective stress ratios of about 1.0 rather than the extrapolated value of 0.33 often used for sands. The current objective of soil-boring geotechnical studies is to aid in the design of platform foundations. This study recommends that oil-boring geotechnical studies be extended to include obtaining data for designing shallow-gas well control systems.