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When a stone/armor layer on a sand bed is exposed to flow, the sand underneath will be agitated by the flow turbulence. When the flow velocity reaches a critical value, the sand will be sucked (winnowed out) from between the armor blocks. In a previous investigation, we studied suction removal of sediment in steady currents. The present study is an extension of our previous investigation to waves. The critical condition for the onset of suction is determined. It is found that the onset of suction is governed by three parameters: (1) the sediment mobility number (based on the sediment size); (2) the ratio of sediment size to stone size, d/D; and (3) the Keulegan-Carpenter (KC) number, based on the armor block/stone size. The variation of the critical mobility number for suction as a function of d/D and KC is determined for the ranges of the parameters 0.001 < d/D [less than or equal to] 1 and 1 < KC < 50. The case of steady current is included as a reference case. The effect of waves superimposed on current, the effect of a multilayer stone cover, and the effect of regularly placed armor blocks are investigated. Suction of the base-bottom sediment will cause sinking of the armor layer and, therefore, general lowering of the bed. The time scale of the latter process and the downward displacement of armor blocks/stones are also investigated. CE Database subject headings: Lakes; Water quality; Acidic water; Flow simulation; Turbulence; Surface waters; Field tests; Wind speed.

This paper gives a state-of-the-art review of seismic-induced liquefaction with special reference to marine structures. The paper is organized in seven sections: (1) introduction; (2) seismic-induced liquefaction in which a general account of soil liquefaction is presented; (3) existing codes/guidelines regarding seismic-induced liquefaction and its implications for marine structures; (4) review of the Japanese experience, giving a brief history of earthquakes and design codes, describing the current design code/standard for port and harbor facilities including counter measures and remediation; (5) review of the liquefaction damage inflicted on marine structures in the 1999 Turkey Kocaeli Earthquake, including recommendations which draw on the lessons learned; (6) assessment of postliquefaction ground deformation (more specifically of lateral ground spreading); and (7) tsunamis and their impact. The present paper and the existing guidelines (CEN, ASCE, and PIANC) form a complementary source of information on earthquake-induced liquefaction with special reference to its impact on marine structures. DOI: 10.1061/(ASCE)0733-950X(2007)133:1(55) CE Database subject headings: Coastal structures; Earthquakes; Guidelines; Liquefaction; Offshore structures; Seismic effects.

This paper summarizes the results of an experimental investigation on wave scour around a circular pile in three kinds of soil, namely in dense silt (with a relative density of [D.sub.r] = 0.74), in medium dense silt (with [D.sub.r] = 0.38), and in sand (with [D.sub.r] = 0.23). The dense silt was achieved by wave liquefaction. Upon the completion of the liquefaction/compaction process, the pile was subjected to wave induced scour. The range of the Keulegan-Carpenter number (KC), tested in the experiments is from practically 0 to approximately 20. The Shields parameter in the experiments was such that the bed was live. The scour depth was increased by a factor of 1.6-2 when the bed soil was changed from medium dense silt or sand to dense silt. This is partly due to an increase in the angle of friction. The time scale of scour also was influenced by the density of the soil. The time scale was largest for the dense silt case, and smallest for the sand case in the experiments. DOI: 10.1061/(ASCE)0733-950X(2007)133:1(14) CE Database subject headings: Scour; Piles; Silts.

This paper summarizes the results of an experimental and theoretical investigation of: (1) pipeline flotation in a soil (liquefied under waves); and (2) density of the liquefied soil. In the experiments, the soil was silt with [d.sub.50]=0.078 mm. Pipeline models of 2 cm diameter were used. They were buried in the soil at different depths in the range 3-15.5 cm. The total depth of the silt layer was 17.5 cm. Waves (with 17 cm wave height and 1.6 s wave period, the water depth being 42 cm) were used to liquefy the soil. The pipes with specific gravity smaller than 1.85-2.0 floated when the soil was liquefied, the critical specific gravity for pipe flotation. The lower bound of the above range corresponds to the initial pipe position near the surface of the bed, and the upper bound to that near the impermeable base. Furthermore, the pipe floated (or sank) to a depth where the pipe specific gravity was equal to the previously mentioned critical specific gravity for flotation. The density of liquefied soil was determined in two different ways in the experiments: (1) from the aforementioned flotation tests where the pipe acted as a 'hydrometer', the instrument to measure fluid density; and (2) from the force balance equation for the pipe, in the vertical direction, corresponding to the critical condition for the pipe flotation. The results from these two methods agree well. The density of liquefied soil varied with depth; it was 1.85 near the surface of the bed and 2.0 near the impermeable base. A hydrodynamic model has been developed to predict the density of liquefied soil. The model is based on the force balance (in the vertical direction) for a soil grain of the liquefied soil. The model indicates that the density of liquefied soil is influenced by the soil category; the soil 'class'; the initial soil specific gravity; the specific gravity of soil grains; and the coefficient of lateral earth pressure. CE Database subject headings: Gas pipelines; Liquefaction; Underwater pipelines; Waves.

This paper presents the results of an experimental study on liquefaction around a pipeline buried in a soil exposed to a progressive wave. The soil used in the experiments was silt with [d.sup.50]=45 mm. The pore-water pressure was measured in the far field and on the pipe simultaneously. The tests indicate that the buildup of pore pressure and the resulting liquefaction in the soil are influenced by the presence of the pipe. The pore pressure builds up much more rapidly at the bottom of the pipe than in the far field (at the same level as the pipe bottom). By contrast, the buildup of pore pressure at the top of the pipe is not influenced radically by the presence of the pipe. The tests further indicate that as the liquefaction initially occurs in the very top layer and develops downwards, this picture changes in the vicinity of the pipe; in the latter case, the liquefaction initially occurs at the bottom of the pipe, and develops along the perimeter of the pipe upwards. The influence of the 'no-slip' condition at the pipe surface on the end results has been investigated and found very significant. The influence of the wave height, the influence of the pipe diameter, the influence of the no-liquefaction-regime conditions, and the influence of a sinking pipe have also been investigated. CE Database subject headings: Gas pipelines; Liquefaction; Underwater pipelines; Waves.

An experimental study on the control of scour at vertical circular piles under monochromatic waves and a steady current is presented. The experiments on wave and steady currents were carried out under live-bed and clear-water regimes, respectively. In waves, splitter plate attached to the pile along the vertical plane of symmetry and threaded pile (helical wires or cables wrapped spirally on the pile to form threads) were found to be effective to reduce the scour depth. For the Keulegan-Carpenter numbers 6-100, the vortex shedding is the main mechanism of scour under waves. The splitter plate and threaded pile disrupt the vortex shedding. The average reduction of the scour depth by the splitter plate was 61.6%. For threaded piles, different combinations of cable and pile sizes were tested, and the best combination was found for a cable-pile diameter ratio equaling 0.75, in which average scour depth reduction was 51.1%. The average reductions of scour depths for other cable-pile diameter ratios of 0.33 and 0.5 were 43.2 and 48.1%, respectively. On the other hand, in a steady current, the threaded pile proved to be effective to control scour depth to a great extent. Cables wrapped spirally forming threads on the pile help to weaken the downflow and horseshoe vortex, which are the principal agents of scour under a steady current. The experimental results showed that the scour depth consistently decreases with an increase in cable diameter and the number of threads, and with a decrease in thread angle. The maximum reduction of scour depth observed was 46.3% by using a triple threaded pile having a thread angle of 15[degrees] and a cable-pile diameter ratio of 0.1. The proposed methods of controlling scour are easy to install and are economical. DOI: 10.1061/(ASCE)0733-9429(2006) 132:3(270) CE Database subject headings: Scour; Erosion; Water waves; Wave velocity; Currents; Sediment transport; Hydraulics; Piles.

This paper summarizes the results of an experimental study on scour around spherical bodies and self-burial in steady current and in waves. The equilibrium scour depth below a fixed sphere in steady current for live-bed conditions was found to be S/D = O(0.3), D being the sphere diameter. The effect of the Shields parameter, [theta], and that of the position of the sphere with respect to the bed, e, were investigated. It was found that, under live-bed conditions, the effect of [theta] is quite weak. It was also found that the effect of e is significant. No scour occurs when e/D > 0.8. It was demonstrated that, in the case of waves, the Keulegan-Carpenter number, KC, is the main parameter that governs the equilibrium scour depth. S/D is always smaller in waves than in steady currents. Regarding the self-burial, the equilibrium self-burial depth in steady current was found to be e/D = O(0.5). The self-burial depth in waves is governed by KC, similar to the scour depth for a fixed sphere, e/D is always smaller in waves than in steady currents. The experiments indicated that the time scale of both the scour process and the self-burial process is a function of [theta] in the case of steady current, and [theta] and KC in the case of waves. DOI: 10.1061/(ASCE)0733-950X(2005)131:1(1) CE Database subject headings: Armor units: Buried structures: Mines; Scour: Protective structures; Currents.

This paper summarizes the results of an experimental study on the influence of an external turbulence field on the bed load sediment transport in an open channel. The external turbulence was generated by (1) a horizontal pipe placed halfway through the depth h; (2) a series of grids with a clearance of about one-third of the depth from the bed, and extending over a finite length of the flume; and (3) a series of grids with a clearance in the range (0.1-1.0)h from the bed, but extending over the entire length of the flume. Two kinds of experiments were conducted: plane-bed experiments and ripple-covered-bed experiments. In the former case, the flow in the presence of the turbulence generator was adjusted so that the mean bed shear stress was the same as in the case without the turbulence generator in order to single out the effect of the external turbulence on the sediment transport. In the ripple-covered-bed case, the mean and turbulence quantities of the streamwise component of the velocity were measured, and the Shields parameter, due to skin friction, was determined. The Shields parameter, together with the RMS value of the streamwise velocity fluctuations, was correlated with the sediment transport rate. The sediment transport increases markedly with increasing turbulence level. DOI: 10.1061/(ASCE)0733-9429(2003)129:8(585) CE Database subject headings: Bed load; Bed load movement; Turbulence; Sediment transport; Currents.

This paper presents the results of an experimental investigation on scour around piles exposed to waves. In addition to the actual scour tests, bed shear-stress measurements and a flow visualization study are carried out. The effects of lee wake and horseshoe vortex are demonstrated to be the two key elements in the scour process. The development of these flow structures mainly depends on the Keulegan-Carpenter (KC) number that hereby becomes the main parameter that governs the equilibrium scour depth on a live bed. Based on the present data, a design equation is established, relating the scour depth to the Keulegan-Carpenter number. For the values of the Keulegan-Carpenter number below six, the scour around the pile practically ceases to exist. The scour depth normalized by the pile diameter is found to increase with increasing Keulegan-Carpenter number and approaches its steady-current value for Keulegan-Carpenter numbers above approximately 100.