Wave-Induced Seafloor Instability in the Yellow River Delta

Journal of Marine Science and Engineering

Article Wave-Induced Seaﬂoor Instability in the Yellow River Delta: Flume Experiments

Xiuhai Wang 1,2,3, Chaoqi Zhu 1,2,3,4,* and Hongjun Liu 1,2,3,* 1 College of Environmental Science and Engineering, Ocean University of China, Qingdao 266000, China; [email protected] 2 Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering, Qingdao 266000, China 3 Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266000, China 4 Shandong Provincial Key Laboratory of Marine Ecology and Environment & Disaster Prevention and Mitigation, Qingdao 266000, China * Correspondence: [email protected] (C.Z.); [email protected] (H.L.)

Received: 11 August 2019; Accepted: 1 October 2019; Published: 6 October 2019

Abstract: Geological disasters of seabed instability are widely distributed in the Yellow River Delta, posing a serious threat to the safety of offshore oil platforms and submarine pipelines. Waves act as one of the main factors causing the frequent occurrence of instabilities in the region. In order to explore the soil failure mode and the law for pore pressure response of the subaqueous Yellow River Delta under wave actions, in-lab flume tank experiments were conducted in this paper. In the experiments, wave loads were applied with a duration of 1 hour each day for 7 consecutive days; pore water pressure data of the soil under wave action were acquired, and penetration strength data of the sediments were determined after wave action. The results showed that the fine-grained seabed presented an arc-shaped oscillation failure form under wave action. In addition, the sliding surface firstly became deeper and then shallower with the wave action. Interestingly, the distribution of pores substantially coincided with that of sliding surfaces. For the first time, gas holes were identified along with their positioning and angle with respect to the sediments. The presence of gas may serve as a primer for submarine slope failures. The wave process can lead to an increase in the excess pore pressure, while the anti-liquefaction capacity of the sediments was improved, causing a decrease in the excess pore pressure resulting from the next wave process. Without new depositional sediments, the existing surface sediments can form high-strength formation under wave actions. The test results may provide a reference for numerical simulations and engineering practice.

Keywords: wave; seaﬂoor instability; pore pressure; slide surface; gas distribution

1. Introduction The Yellow River Delta is one of largest-scale deltas with the fastest progradation rate in the world. The Yellow River brings abundant high-concentration sediments from the Loess Plateau, which are rapidly deposited in the subaqueous delta of the estuary, forming a high-moisture under-consolidated silty seabed. It is estimated that the average annual sediment transport volume exceeds 800 million tons from 1950 to 2005 [1], and over 70–90% of the sediments were deposited in the sea area within 30 km from the estuary [2], gradually forming the modern subaqueous Yellow River Delta. Such newly formed seabeds under the influence of waves and storm surges are prone to instability, thereby forming unstable seabed landforms of subaqueous delta in the estuary of the Yellow River. Previous studies reported that various ocean dynamic processes can affect the delta of the Yellow River [3–5]. The yearly winter storms coming from the northwest have the most powerful effect from

J. Mar. Sci. Eng. 2019, 7, 356; doi:10.3390/jmse7100356 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, 356 2 of 10

October to March. During the winter season, the prevailing northwest winter winds becomes stronger than grade 8 on average 6.4 times each year. The wave height generated by winter storms can reach 7 m [3]. In addition, the weak southeastern winds prevail from July to August. Occasionally, hurricanes or typhoons occur in a given year. For example, Typhoon Lekima meandered over the Yellow Sea and Bohai Sea in 2019. Available survey results showed that geological disasters such as liquefaction, landslide, subsidence and depression, gully, scarp, disturbed strata, and erosion are common across the subaqueous Yellow River Delta [6–9]. The Yellow River Delta is rich in oil and gas resources and is the location of Shengli Oilfield, currently the largest offshore oil field in China. Seabed instability disasters seriously threatens the safety and stability of the offshore oil platform, submarine pipeline, wharf, and embankment [10–12]. For example, on December 3, 1998, the CB6A-5 oil production platform of Chengdao Oilfield collapsed, and the casing pipes at the bottom of the oil well ruptured, causing a material oil spill lasting half a year [13]. In November 2003, a submarine landslide occurred under the action of ocean power near the oil production platform CB12B, resulting in the interruption of two submarine cables [14]. In 2010, Operation Platform No. 3 of Shengli Oilfield was overturned due to the instability of the seabed stratum during a storm surge, leading to the death of two people and a direct economic loss of RMB 5.92 million [15]. There are two main types of seabed instability under wave action. In one, the shear stress is greater than the shear strength of the soil, causing the seabed to be unstable; in the other, it is the variation in the pore water pressure in the seabed under wave action that causes the seabed to undergo liquefaction, resulting in the loss of effective stress in the soil, thereby leading to instability. With respect to the studies on the instability of the seabed under wave action, especially in the subaqueous Yellow River Delta, plenty of field surveys, numerical analyses, laboratory tests, and in situ observation studies have been carried out [16–21]. However, few wave flume experiments have comprehensively investigated landforms, failure mode, gas distribution, pore pressure response, and sediment strength. In addition, failure of the seabed has been understudied, even though some interesting phenomena have been identified. For example, previous studies have confirmed fine particle migration due to wave action. In order to deeply explore the soil failure mode, gas distribution, and pore pressure response law under wave action, in-lab wave flume experiments were designed and carried out in this study. For the first time, gas holes were identified along with their positioning and angle with respect to the sediments. The presence of gas may serve as a primer for submarine slope failure.

2. Materials and Methods

2.1. Experiment Process The flume used in the experiment was 120 cm 50 cm 120 cm. In order to observe the variation × × in soil mass, the outer wall of the flume was made of transparent glass. In the flume experiment, the sediment thickness was set to be 45 cm, and the depth of overlying water was set to be 40 cm. The sediments used in the experiment were taken from the Yellow River Delta, and its median particle size was 0.058 mm. The particle-size distribution curve of these sediment samples is shown in Figure1. Prior to each experiment, powdery sediments were taken, and their contents were controlled at around 40%, we then added water to have a water content of 40%, and the powdery sediment was mixed with the stirrer. The deposited sediments were then transferred to the flume until the thickness of the sediment hit 45 cm; and the pore pressure sensors were then buried with a burial depth of 0 cm, 25 cm, 35 cm, and 40 cm, respectively, as shown in Figure2. Afterward, water was filled into the flume, avoiding the formation of erosion pits on the surface of the soil mass during the water-filling process due to high water-filling speed, and we stop adding water when the height of the water filling reached 40 cm. We kept the model flume still for 24 hours after its preparation was finished in order to complete drainage consolidation. We then performed manual wave generation with the duration of 1 hour each day and proceeded for 7 consecutive days. Simultaneously, we measured the pore water pressure and observed the experimental phenomenon. We discharged the overlying water body J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 3 of 10

J. Mar. Sci. Eng. 2019, 7, 356 3 of 10 when the experiment ended, and determined the penetration strength of the soil mass with a micro penetrometerJ. Mar. Sci. Eng. 2019 at, di 7,ﬀ xerent FOR PEER depths. REVIEW 3 of 10

Figure 1. Grading curve of sediment size.

Prior to each experiment, powdery sediments were taken, and their contents were controlled at around 40%, we then added water to have a water content of 40%, and the powdery sediment was mixed with the stirrer. The deposited sediments were then transferred to the flume until the thickness of the sediment hit 45 cm; and the pore pressure sensors were then buried with a burial depth of 0 cm, 25 cm, 35 cm, and 40 cm, respectively, as shown in Figure 2. Afterward, water was filled into the flume, avoiding the formation of erosion pits on the surface of the soil mass during the water-filling process due to high water-filling speed, and we stop adding water when the height of the water filling reached 40 cm. We kept the model flume still for 24 hours after its preparation was finished in order to complete drainage consolidation. We then performed manual wave generation with the duration of 1 hour each day and proceeded for 7 consecutive days. Simultaneously, we measured the pore water pressure and observed the experimental phenomenon. We discharged the overlying water body when the experiment ended, and determined the penetration strength of the soil mass with a Figure 1. Grading curve of sediment size. micro penetrometer at differentFigure depths. 1. Grading curve of sediment size.

Figure 2. Schematic diagram of wave ﬂume. Figure 2. Schematic diagram of wave flume. 2.2. Wave Generation

A hollow cylinder with a diameter of 10 cm and height of 40 cm was used in the experiment to artificially create waves in the flume. The cylinder was placed horizontally on one end of the flume and then it was moved up and down the water surface to create the first wave as shown in Figure3a; when the first wave propagated to the other end of the flume, a reflected wave was formed and then propagated in the opposite direction. The cylinder was moved up and down the water surface to produce the second wave as shown in Figure3b; when two waves collided in the middle of the flume, a superposition occurred (Figure3c); then, the two waves continued to advance in their respective directions (Figure3d), and a reflected wave was formed simultaneously at each side wall of the flume.

Figure 2. Schematic diagram of wave flume.

J. Mar.J. Mar. Sci. Sci. Eng. Eng. 2019 2019, 7, ,x 7 FOR, x FOR PEER PEER REVIEW REVIEW 4 of 4 10 of 10

2.2.2.2. Wave Wave Generation Generation A hollowA hollow cylinder cylinder with with a diameter a diameter of of10 10cm cm and and he ightheight of of40 40cm cm was was used used in inthe the experiment experiment to to artificiallyartificially create create waves waves in thein the flume. flume. The The cylinder cylinder was was placed placed horizontally horizontally on onone one end end of theof the flume flume and and thenthen it was it was moved moved up up and and down down the the water water surface surface to createto create the the first first wave wave as asshown shown in Figurein Figure 3a; 3a; when when thethe first first wave wave propagated propagated to tothe the other other end end of ofth eth flume,e flume, a reflecteda reflected wave wave was was formed formed and and then then propagatedpropagated in inthe the opposite opposite direction. direction. The The cylin cylinderder was was moved moved up up and and down down the the water water surface surface to to J.produce Mar.produce Sci. the Eng. the second2019 second, 7, 356wave wave as asshown shown in Figurein Figure 3b; 3b; when when two two waves waves collided collided in thein the middle middle of theof the flume,4 offlume, 10 a superpositiona superposition occurred occurred (Figure (Figure 3c); 3c); then, then, the the two two waves waves continued continued to toadvance advance in intheir their respective respective directionsdirections (Figure (Figure 3d), 3d), and and a reflected a reflected wave wave was was form formed edsimultaneously simultaneously at eachat each side side wall wall of theof the flume. flume. WhenWhenWhen thethe the firstfirst first wavewave wave waswas was locatedlocated located atat thetheat the hollowhollow hollow cylinder,cylinder, cylinder, thethe the cylindercylinder cylinder was was moved moved up up and and down down at thetheat the frequencyfrequencyfrequency of theofthe the wavewave wave toto increaseincreaseto increase thethe the amplitudeamplitude amplitude ofof theofthe the wavewave wave oror toorto supplementsupplementto supplement thethe the energyenergy energy lostlost lost duringduring during thethethe propagationpropagation propagation (Figure (Figure3 3e).e). 3e). TheThe The secondsecond second wavewave wave collidedcollided collided withwith with thethe the firstfirst first wavewave wave inin theinthe themiddle middle middle ofofthe ofthe theflume flume flume afterafter being being reflectedreflected reflected by by the the side side wall wall of theof the flumeflume flume andan dan thend then they they separated separated (Figure (Figure3 3f);f); 3f); wewe we supplementedsupplemented supplemented thethethe energyenergy energy lostlost lost duringduring during thethe the propagation propagation in thein the same same way. way. Continuous Continuous waves waves with with didifferentff differenterent frequencies frequencies werewerewere formedformed formed in thein the flume flume flume in thisin this way. way. way.

Figure 3. Schematic diagram showing the wave generation. FigureFigure 3. Schematic 3. Schematic diagram diagram showing showing the the wave wave generation. generation. In the flume experiment, the length of the flume was short (120 cm), and the wave propagated fromIn one Inthe sidethe flume flume wall experiment, of experiment, the flume the to the length the length other of of withinthe the flume fl aume short was was time,short short so(120 that(120 cm), twocm), and waves and the the werewave wave superposedpropagated propagated togetherfromfrom one one in side the side wall flume wall of toofthe formthe flume flume the to standing tothe the other other wave. within within Only a short a the short waveformstime, time, so sothat that in two Figure two waves waves3e,f were could were superposed be superposed seen in thetogethertogether flume in during thein the flume theflume experiment,to formto form the the standing as standing shown wave. in wave. Figure Only Only4. the the waveforms waveforms in inFigure Figure 3e, 3e, f could f could be beseen seen in in thethe flume flume during during the the experiment, experiment, as asshown shown in Figurein Figure 4. 4.

(a) (wavea) wave crest crest (b) (waveb) wave trough trough

FigureFigure 4. Experimental 4. Experimental photographs photographs showing showing the the wave wave generation. generation.

3. Results and Discussion

3.1. Slide Surface and Ripples No signiﬁcant movement of the soil mass occurred within 5 min upon the wave application action, while the water body began to become turbid, and the 15-cm-thick sediment started to exhibit a weak waveform oscillation motion with a frequency and phase consistent with those of the water wave when J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 5 of 10

J.3. Mar. Results Sci. Eng. and 2019 Discussion, 7, x FOR PEER REVIEW 5 of 10

3.3.1. Results Slide Surface and Discussion and Ripples No significant movement of the soil mass occurred within 5 minutes upon the wave application 3.1. Slide Surface and Ripples action, while the water body began to become turbid, and the 15-cm-thick sediment started to exhibit J. Mar. Sci. Eng. 2019, 7, 356 5 of 10 a weakNo waveformsignificant oscillationmovement motionof the soil with mass a freque occurredncy andwithin phase 5 minutes consistent upon with the thosewave ofapplication the water action,wave when while thethe wavewater actedbody forbegan approximately to become turbid, 3 minu andtes. the The 15-cm-thick soil mass showed sediment distinct started arc-shaped to exhibit aslidingthe weak wave waveformsurface acted in for oscillation20 approximately minutes. motion The 3sliding min.with a Thesurfaces freque soilncy masslocated and showed phasein the distinctconsistentmiddle arc-shapedof withthe flume those sliding andof the near surfacewater the wavewallin 20 ofwhen min. the flume Thethe wave sliding were acted higher, surfaces for andapproximately located those inat the30 3cm middleminu andtes. 90 of Thecm the approximately soil flume mass and showed near from the distinct the wall left of arc-shaped wall the flumeof the slidingflumewere higher, weresurface lower. and in those20 The minutes. atsediment 30 cm The and on sliding the 90 cmsliding surfaces approximately surf locatedace reciprocated fromin the the middle left the wall wave, of the of theand flume flumethe andlower were near part lower. the of walltheThe slidingof sediment the flume surface on were the was sliding higher, almost surface and stat thoseionary. reciprocated at As30 cmthe theandduration wave,90 cm of andapproximately the the wave lower action part from increased, of the the left sliding wallthe surface slidingof the flumesurfacewas almost were constantly lower. stationary. The expanded sediment As the durationdeeper, on the sliding ofand the the wavesurf thicknessace action reciprocated increased, of the the moving the wave, sliding andsoil surface themass lower constantlyincreased part of thecontinuously,expanded sliding deeper,surface forming andwas the aalmost distinct thickness stat "W"ionary. of shaped the movingAs sliding the duration soil surface. mass of increasedWhen the wave the continuously, wave action application increased, forming was the a stopped,sliding distinct surfacethe"W" concentration shaped constantly sliding of surface. expandedsuspended When deeper,sediments the wave and in application thethe wate thicknessr gradually was stopped,of thedecreased. moving the concentration In addition,soil mass of fine-grained suspendedincreased continuously,sedimentarysediments in formation theforming water a gradually occurreddistinct "W" decreased.on shapedthe surface sliding In addition, layer surface. (Figure fine-grained When 5a). the sedimentary wave application formation was stopped, occurred theon concentration the surface layer of suspended (Figure5a). sediments in the water gradually decreased. In addition, fine-grained sedimentary formation occurred on the surface layer (Figure 5a).

Figure 5. Experimental photographs showing the slide surface. (a) First day; (b) Seventh day. The dottedFigure lines 5. Experimental are used to photographs show the slide showing surface. the slide surface. (a) First day; (b) Seventh day. The dotted Figurelines are 5. usedExperimental to show thephotographs slide surface. showing the slide surface. (a) First day; (b) Seventh day. The dottedSimilar lines phenomenon are used to show still theoccurred slide surface. in the course of the wave application in subsequent days, whileSimilar the maximum phenomenon depth still of occurredthe sliding in thesurface course gradually of the wave decreased, application and inthe subsequent sliding surface days, graduallywhileSimilar the maximummoved phenomenon upwards; depth still of thethe occurred sliding surface surfacein ofthe the course gradually soil ofmass the decreased, startedwave application andto undergo the sliding in localsubsequent surface erosion gradually days, and whilesedimentation,moved the upwards; maximum and the ripples depth surface appear of of the theed. sliding soil Similar mass surface experimental started gradually to undergo phenomena decreased, local were erosion and also the and observed sliding sedimentation, insurface other graduallyexperimentsand ripples moved appeared. [22,23], upwards; Similarbut “V”-shaped the experimental surface sliding of phenomena the surfacessoil mass were occurred, alsostarted observed torather undergo in otherthan experimentslocal“W”-shaped. erosion [22 andThe,23], sedimentation,maximumbut “V”-shaped expansion and sliding ripples depth surfaces appear of the occurred, slidinged. Similar surface rather experimental was than cl “W”-shaped.ose tophenomena the interface The were maximum of water also observed and expansion the soil in other depthwhen experimentstheof the last sliding wave [22,23], application surface but was was“V”-shaped close made, to the and interfacesliding the soil ofsurfaces mass water did andoccurred, not the further soil rather when oscillate. thethan lastFinally, “W”-shaped. wave a applicationcontinuous The maximumarraywas made, of ripples expansion and thewith soil depth the mass wavelength of didthe notsliding further of surface5–10 oscillate. cm was formed cl Finally,ose onto thethe a continuousinterface surface ofof arraythewater soil ofand mass ripples the (Figuresoil with when the6). theAdditionally,wavelength last wave of application sedimentation 5–10 cm formed was occurredmade, on the and surfacein the the soil middle of themass soil position did mass not (Figurefurtherof the flume6 oscillate.). Additionally, and Finally,the position sedimentation a continuous near the arraywalloccurred ofof theripples in flume, the with middle and the the positionwavelength erosion of occurred the of ﬂume5–10 at cm andsites formed theapproximately position on the nearsurface 30 the cm of walland the of90soil thecm mass ﬂume,away (Figure from and the 6). Additionally,lefterosion wall. occurred The sedimentationsurface at sites shape approximately ofoccurred the soil inmass 30 the cm tendedmiddle and 90 toposition cm be away consistent of from the theflumewith left the and wall. sliding the The position surface, surface near shapeand the W- of wallshapedthe soilof the forms mass flume, tended appeared. and to the be erosion consistent occurred with the at slidingsites approximately surface, and W-shaped 30 cm and forms 90 cm appeared. away from the left wall. The surface shape of the soil mass tended to be consistent with the sliding surface, and W- shaped forms appeared.

Figure 6. Experimental photographs showing the ripples. (a) Top view; (b) Side view. Figure 6. Experimental photographs showing the ripples. (a) Top view; (b) Side view. 3.2. Gas Distribution Figure 6. Experimental photographs showing the ripples. (a) Top view; (b) Side view. A sample portion of the soil approximately 10 cm in the upper layer was taken after the end of

the experiment. It was found that there was a large number of gas holes in the vertical section of the soil sample (Figure7). The gas holes in the soil were distributed in a horizontal or inclined and zonal manner rather than being uniform (Figure8), and even a gas channel existed. Within a depth J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 6 of 10 J.J. Mar.Mar. Sci.Sci. Eng.Eng. 20192019,, 77,, xx FORFOR PEERPEER REVIEWREVIEW 6 6 of of 10 10

3.2.3.2. GasGas DistributionDistribution AA samplesample portionportion ofof thethe soilsoil approximatelyapproximately 1010 cmcm inin thethe upperupper layerlayer waswas takentaken afterafter thethe endend ofof the experiment. It was found that there was a large number of gas holes in the vertical section of the J.the Mar. experiment. Sci. Eng. 2019 ,It7 ,was 356 found that there was a large number of gas holes in the vertical section of6 of the 10 soilsoil samplesample (Figure(Figure 7).7). TheThe gasgas holesholes inin thethe soilsoil werewere distributeddistributed inin aa horizontalhorizontal oror inclinedinclined andand zonalzonal mannermanner ratherrather thanthan beingbeing uniformuniform (Figure(Figure 8),8), andand evevenen aa gasgas channelchannel existed.existed. WithinWithin aa depthdepth rangingranging rangingfromfrom 33 cmcm from toto 44 3 cmcm cm andand to 4 fromfrom cm and 88 cmcm from toto 1010 8 cmcm,cm, to thethe 10 numbernumber cm, the ofof number porespores waswas of pores greatergreater was thanthan greater 100,100, thanwhilewhile 100, thethe while holesholes theinin otherother holes depthdepth in other rangesranges depth werewere ranges lessless were thanthan less 30.30. than TheThe 30. arareaea The wherewhere area wherethethe porespores the poreswerewere weredenselydensely densely distributeddistributed distributed waswas wasconsistentconsistent consistent withwithwith thethe slidingsliding the sliding surfacesurface surface ofof thethe of soilsoil the mass. soilmass. mass.

FigureFigure 7. 7. PhotographsPhotographs showing showing the the gas gas holes holes in in different didifferentﬀerent scales.scales.

Figure 8. Photographs showing the ribbon of gas holes. FigureFigure 8.8. PPhotographshotographs showingshowing thethe ribbonribbon ofof gasgas holes.holes. The gas holes in the soil sample were distributed in an oblique and zonal manner at a distance of The gas holes in the soil sample were distributed in an oblique and zonal manner at a distance 100–110The cm gas from holes the in left the wallsoil ofsample the flume, were atdistributed an angle ofin approximatelyan oblique and 30 zonal◦–40 ◦mannerwith the at horizontala distance of 100–110 cm from the left wall of the flume, at an angle of approximately 30°–40° with the horizontal plane.of 100–110 The closercm from they the were left wall to the of surface the flume, of the at soil,an angle the smaller of approximately the inclination. 30°–40° The with distribution the horizontal of gas plane. The closer they were to the surface of the soil, the smaller the inclination. The distribution of holesplane. (Figure The closer9) was they substantially were to th consistente surface of with the thatsoil, of the sliding smaller surfaces. the inclination. Previous The studies distribution confirmed of gas holes (Figure 9) was substantially consistent with that of sliding surfaces. Previous studies fine-particlegas holes (Figure migration 9) was due substantially to external actions, consistent such with as waves that of [22 sliding,24,25]. surfaces. However, Previous this experiment studies confirmed fine-particle migration due to external actions, such as waves [22,24,25]. However, this oconfirmedffered the firstfine-particle evidence migration of possible due gas to migration external due actions, to wave such action. as waves The presence[22,24,25]. of However, gas may serve this experiment offered the first evidence of possible gas migration due to wave action. The presence of asexperiment a primer foroffered submarine the first slope evidence failure of [26 possible]. gas migration due to wave action. The presence of gasgas maymay serveserve asas aa primerprimer forfor submarinesubmarine slopeslope failurefailure [26].[26].

Figure 9. Schematic diagram showing the direction of gas distribution.

FigureFigure 9.9. SchematicSchematic diagramdiagram showingshowing thethe directiondirection ofof gasgas distribution.distribution. 3.3. Pore Pressure The pore pressure is an important indicator of the properties of soil masses [27,28]. A total of seven wave application processes each with a duration of approximately 1 hour were performed. It was found from Figure 10 that the pore pressure in the seabed soil body signiﬁcantly accumulated J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 7 of 10

3.3. Pore Pressure

J. Mar. Sci.The Eng. pore2019 pressure, 7, 356 is an important indicator of the properties of soil masses [27,28]. A total7 of of 10 seven wave application processes each with a duration of approximately 1 hour were performed. It was found from Figure 10 that the pore pressure in the seabed soil body significantly accumulated andand could could reach reach the the maximum maximum cumulative cumulative pore pore pressure pressure within within a shorta short time time (approximately (approximately 5 min)5 andminutes) remained and stable.remained The stable. ﬁne-grained The fine-grained seabed hadseabed low had permeability, low permeability, and it wasand it di wasﬃcult difficult to quickly to dissipatequickly dissipate the excess the pore excess water pore pressure water pressure caused bycaused the wave. by the wave.

FigureFigure 10. 10.Excess Excess porepore pressurepressure during the wave wave action. action. The The curves curves thorough thorough from from (a) ( ato–g ()g) represent represent the excessthe excess pore pressurepore pressure curves curv fores the for first the to first seventh to seventh wave applicationwave application process; process; (h) excess (h) excess pore pressure pore ofpressure the seabed of the for seabed different for wave different application wave ap processesplication processes at a depth at of a 20depth cm; of (i) 20 excess cm; ( porei) excess pressure pore of thepressure seabed of at the a depth seabed of at 35 a cmdepth from of di35ff cmerent from wave different application wave application processes. processes.

FigureFigure 10 10 shows shows the the excess excess pore pore pressure pressure curve curve at aat depth a depth of 20of cm20 andcm and 35 cm 35 incm the in seabed the seabed during sevenduring wave seven application wave application processes. processes. The figures The fromfigures 10a from to 10g 10a show to 10g that show the excessthat the pore excess pressure pore at apressure depth of at 35 a cmdepth was of slightly 35 cm was greater slightly than greater that at than a depth that at of a 20 depth cm, indicatingof 20 cm, indicating that the greater that the the depthgreater was, the the depth harder was, it the was harder for the it excesswas for pore the pressureexcess pore to dissipate.pressure to The dissipate. excess poreThe excess pressure pore data frompressure the seven data wavefrom the applications seven wave at theapplications same depth at the (Figure same 10 depthh,i) indicated (Figure 10h,i) that theindicated maximum that excessthe poremaximum pressure excess formed pore from pressure the process formed offrom the the last proc waveess application of the last wave was application lower than was that lower formed than from previousthat formed wave from applications. previous wave For applications. example, the Fo maximumr example, porethe maximum pressure atpo aredepth pressure of 20at a cm depth of the seabedof 20 cm hit of approximately the seabed hit approximately 1.4 kPa in the 1.4 first kPa wave in the application, first wave application, approximately approximately 1.0 kPa in 1.0 the kPa third wavein the application, third wave and application, approximately and approximately 0.2 kPa in the seventh0.2 kPa wavein the application.seventh wave The application. maximum excessThe poremaximum pressure excess inside pore the pressure soil mass inside showed the soil a decrease mass showed trend a with decrease the increase trend with in thethe numberincrease ofin the wave actions, indicating that the whole soil mass tended to be stable. It was consistent with the experimental phenomenon that the sliding surface gradually moved up and became shallow with wave application process, and that the thickness of the oscillation layer reduced. Each wave application process might lead to an increase in the excess pore pressure of the seabed sediment. However, the liquefaction J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 8 of 10 number of wave actions, indicating that the whole soil mass tended to be stable. It was consistent with the experimental phenomenon that the sliding surface gradually moved up and became shallow with J.wave Mar. Sci. application Eng. 2019, 7 ,process, 356 and that the thickness of the oscillation layer reduced. Each wave8 of 10 application process might lead to an increase in the excess pore pressure of the seabed sediment. However, the liquefaction resistance of the sediment improved upon each wave action process, resultingresistance in a decrease of the sediment in the excess improved pore pressure upon each caused wave by action the next process, process resulting of wave in aaction. decrease This in the mightexcess be due pore to pressurethe fact that caused the bywave the action next process can promote of wave the action. rearrangement This might of be sediment due to the particles. fact that the The waveexperiment action canresults promote show thethat rearrangement the seabed is of the sediment most unstable particles. in The the experiment initial stage results of rapid show that depositionthe seabed of sediment, is the most and unstable it is most in likely the initial to be stagedestroyed of rapid under deposition wave action. of sediment, Due to andthe limitations it is most likely of laboratoryto be destroyed experiments under [29,30], wave action. more Duefield towork the limitationsis required of to laboratory confirm these experiments experimental [29,30 results.], more field work is required to confirm these experimental results. 3.4. Sediment Strength 3.4. Sediment Strength The overlying water mass was discharged after the seventh wave action, and the entire soil sample wasThe subject overlying to a water penetration mass was strength discharged experiment after the with seventh a micro wave penetration action, and instrument. the entire soil The sample diameterwas subjectof the mini to a penetrationpenetrometer strength was 3 experimentcm, the horizontal with a micro separation penetration distance instrument. between Thetest diameterpoints of was 15the cm, mini and penetrometer the vertical depth was 3 interval cm, the was horizontal 5 cm. As separation shown in distance Figure 11, between the penetration test points strength was 15 cm, of theand soil the mass vertical was substantially depth interval greater was 5 than cm. As50 kN shown in the in depth Figure range 11, the from penetration 0 cm to 20 strength cm, and of the the soil penetrationmass was strength substantially of the greatersoil mass than ranging 50 kN from in the 8 depth cm to range 13 cm from was 0 greater, cm to 20 with cm, anda mean the penetration value greaterstrength than 80 of kN, the indicating soil mass there ranging was from a high-stren 8 cm togth 13 cmformation was greater, at this with depth. a meanThe soil value mass greater had a than relatively80 kN, large indicating variation there in wasstrength a high-strength gradient, with formation the strength at this depth.of the Thesoil soilmass mass decreasing had a relatively from 60 large kN tovariation 30 kN in strengththis 7-cm-thick gradient, range. with theThe strength penetration of the strengths soil mass decreasingof the soil frommass 60with kN tothe 30 depth kN in this ranging7-cm-thick from 20 range. cm to The 40 cm penetration were less strengths than 30 ofkN, the and soil most mass withof the the values depth were ranging approximately from 20 cm to10 40 cm kN, werebeing less less than than 30 kN,the andpenetration most of thestrength values in were the approximately hard formation 10 kN,of the being surface less than strength. the penetration By comparingstrength the in strength the hard distribution formation of law the for surface sliding strength. surface Byand comparing soil penetration, the strength it was distributionfound that the law for depthsliding of the surfacehigh-strength and soil formation penetration, was it slightly was found less thatthan the the depth depth of (15 the cm) high-strength of maximum formation sliding was surface,slightly that is, less the than high-strength the depth (15 formation cm) of maximum was located sliding above surface, the sliding that surface. is, the high-strength This indicates formation that exitingwas surface located sediment above the undergoes sliding surface. multiple This indicateswave actions, that exiting and its surface strength sediment is improved undergoes in multiplethe absencewave of actions,new deposition and its strength conditions. is improved in the absence of new deposition conditions.

FigureFigure 11. Sediment 11. Sediment penetration penetration strength strength profile proﬁle after the after experiment. the experiment.

4. Conclusions4. Conclusions The instabilityThe instability behavior behavior of the of fine-grained the fine-grained seabed seabed under under wave wave action action was simulated was simulated in in-lab in in-lab flumeflume experiments experiments in this in work. this work. The main The main experimental experimental results results are summarized are summarized as follows: as follows: (1) The fine-grained seabed presents an arc-shaped oscillation failure form under wave action. The sliding(1) The surface fine-grained becomes seabeddeeper and presents then shallower an arc-shaped as the oscillationwave action failure continues. form The under distribution wave action. of the gas Thepores sliding is substantially surface the becomes same as deeper that of andthe sliding then shallowersurfaces. as the wave action continues. The distribution of the gas pores is substantially the same as that of the sliding surfaces. (2) We found the first evidence for possible gas migration due to wave action. The presence of gas may serve as a primer for submarine slope failures, which suggests a new mechanism for seafloor instability in the Yellow River Delta. J. Mar. Sci. Eng. 2019, 7, 356 9 of 10

(3) Each wave process may cause the perforation pressure of the sediment to increase, and the liquefaction resistance capacity of the sediment increases during each wave process, leading to a decrease in the excess pore pressure caused by the next wave process. The seabed is the most unstable and most susceptible to damage under wave action in the initial stage of rapid deposition of the sediment. (4) Without new depositional sediments, the existing surface sediments can form high-strength formation under wave actions.

Author Contributions: Conceptualization, H.L.; Data curation, X.W. and C.Z.; Formal analysis, X.W. and C.Z.; Funding acquisition, H.L.; Writing—original draft, X.W. and C.Z.; Writing—review & editing, X.W., C.Z., and H.L. The final manuscript has been approved by all the authors. Funding: This research was funded by the National Natural Science Foundation of China (grant number 41877223) and Shandong Provincial Key Laboratory of Marine Ecology and Environment & Disaster Prevention and Mitigation (grant number 201801). Acknowledgments: Authors acknowledge the experimental and technical support provided by Minsheng Zhang. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Wang, H.; Yang, Z.; Li, G.; Jiang, W. Wave climate modeling on the abandoned Huanghe (Yellow River) delta lobe and related deltaic erosion. J. Coast. Res. 2006, 22, 906–918. [CrossRef] 2. Saito, Y.; Yang, Z.S.; Hori, K. The Huanghe (Yellow River) and Changjiang (Yangtze River) deltas: A review on their characteristics, evolution and sediment discharge during the Holocene. Geomorphology 2001, 41, 219–231. [CrossRef] 3. Yang, Z.; Ji, Y.; Bi, N.; Lei, K.; Wang, H. Sediment transport off the Huanghe (Yellow River) delta and in the adjacent Bohai Sea in winter and seasonal comparison. Estuar. Coast. Shelf Sci. 2011, 93, 173–181. [CrossRef] 4. Bi, N.; Yang, Z.; Wang, H.; Fan, D.; Sun, X.; Lei, K. Seasonal variation of suspended-sediment transport through the southern Bohai Strait. Estuar. Coast. Shelf Sci. 2011, 93, 239–247. [CrossRef] 5. Johnson, M.E.; Gudveig Baarli, B. Geomorphology and coastal erosion of a quartzite island: Hongdo in the Yellow Sea off the SW Korean Peninsula. J. Geol. 2013, 121, 503–516. [CrossRef] 6. Prior, D.B.; Suhayda, J.N.; Lu, N.Z.; Bornhold, B.D. Storm wave reactivation of a submarine landslide. Nature 1989, 341, 47–50. [CrossRef] 7. Guo, Z.; Zhou, W.; Zhu, C.; Yuan, F.; Rui, S. Numerical Simulations of Wave-Induced Soil Erosion in Silty Sand Seabeds. J. Mar. Sci. Eng. 2019, 7, 52. [CrossRef] 8. Zhu, C.; Liu, X.; Shan, H.; Zhang, H.; Shen, Z.; Zhang, B.; Jia, Y. Properties of suspended sediment concentrations in the Yellow River delta based on observation. Mar. Georesour. Geotech. 2018, 36, 139–149. [CrossRef] 9. Xu, G.; Sun, Y.; Wang, X.; Hu, G.; Song, Y. Wave-induced shallow slides and their features on the subaqueous Yellow River delta. Can. Geotech. J. 2009, 46, 1406–1417. [CrossRef] 10. Jia, Y.; Zhu, C.; Liu, L.; Wang, D. Marine Geohazards: Review and Future Perspective. Acta Geol. Sin. Engl. 2016, 90, 1455–1470. [CrossRef] 11. Sun, Y.; Dong, L.; Song, Y. Analysis of characteristics and formation of disturbed soil on subaqueous delta of Yellow River. Rock Soil Mech. 2008, 29, 1494–1499. 12. Zhu, C.; Cheng, S.; Li, Q.; Shan, H.; Shen, Z.; Liu, X.; Jia, Y. Giant Submarine Landslide in the South China Sea: Evidence, Causes and Implications. J. Mar. Sci. Eng. 2019, 7, 152. [CrossRef] 13. Chang, F. Study on Mechanism of Wave-Induced Submarine Landslide at the Yellow River Estuary, China. Ph.D. Thesis, Ocean University of China, Qingdao, China, 30 June 2009. 14. Zheng, C. Shengli Chengdao Oilfield Submarine Power Cable Fault Type and Causes Analysis. Saf. Health Environ. 2016, 16, 6–10. 15. Du, F. Research on the Geological Causes of Sheng Li Well Workover Platform III Overturning Accident. Ph.D. Thesis, Ocean University of China, Qingdao, China, 30 June 2013. 16. Jia, Y.; Zhang, L.; Zheng, J.; Liu, X.; Jeng, D.; Shan, H. Effects of wave-induced seabed liquefaction on sediment re-suspension in the Yellow River Delta. Ocean Eng. 2014, 89, 146–156. [CrossRef] J. Mar. Sci. Eng. 2019, 7, 356 10 of 10

17. Keller, G.H.; Prior, D.B. Sediment dynamics of the Huanghe (Yellow River) delta and neighboring gulf of Bohai, People’s Republic of China: Project overview. Geo Mar. Lett. 1986, 6, 63–66. [CrossRef] 18. Sumer, B.M.; Hatipoglu, F.; Fredsoe, J.; Sumer, S.K. The sequence of sediment behaviour during wave-induced liquefaction. Sedimentology 2006, 53, 611–629. [CrossRef] 19. Tong, L.; Zhang, J.; Sun, K.; Guo, Y.; Zheng, J.; Jeng, D. Experimental study on soil response and wave attenuation in a silt bed. Ocean Eng. 2018, 160, 105–118. [CrossRef] 20. Wang, Z.; Jia, Y.; Liu, X.; Wang, D.; Shan, H.; Guo, L.; Wei, W. In situ observation of storm-wave-induced seabed deformation with a submarine landslide monitoring system. Bull. Eng. Geol. Environ. 2018, 77, 1091–1102. [CrossRef] 21. Wang, H.; Liu, H.; Zhang, M. Pore pressure response of seabed in standing waves and its mechanism. Coast. Eng. 2014, 91, 213–219. [CrossRef] 22. Liu, X.; Jia, Y.; Zheng, J.; Wen, M.; Shan, H. An experimental investigation of wave-induced sediment responses in a natural silty seabed: New insights into seabed stratification. Sedimentology 2017, 64, 508–529. [CrossRef] 23. Xu, G.; Liu, Z.; Sun, Y.; Wan, X.; Lin, L.; Ren, Y. Experimental characterization of storm liquefaction deposits sequences. Mar. Geol. 2016, 382, 191–199. [CrossRef] 24. Zhang, S.; Jia, Y.; Wen, M.; Wang, Z.; Zhang, Y.; Zhu, C.; Li, B.; Liu, X. Vertical migration of fine-grained sediments from interior to surface of seabed driven by seepage flows—‘sub-bottom sediment pump action’. J. Ocean Univ. China 2017, 16, 15–24. [CrossRef] 25. Yang, Q.; Cheng, K.; Wang, Y.; Ahmad, M. Improvement of semi-resolved CFD-DEM model for seepage-induced fine-particle migration: Eliminate limitation on mesh refinement. Comput. Geotech. 2019, 110, 1–18. [CrossRef] 26. Li, A.; Davies, R.J.; Yang, J. Gas trapped below hydrate as a primer for submarine slope failures. Mar. Geol. 2016, 380, 264–271. [CrossRef] 27. Yang, Q.; Ren, Y.; Niu, J.; Cheng, K.; Hu, Y.; Wang, Y. Characteristics of soft marine clay under cyclic loading: A review. Bull. Eng. Geol. Environ. 2018, 77, 1027–1046. [CrossRef] 28. Zhang, Y.; Jeng, D.S.; Zhao, H.Y.; Zhang, J.S. An integrated model for pore pressure accumulations in marine sediment under combined wave and current loading. Geomech. Eng. 2016, 10, 387–403. [CrossRef] 29. Barry, M.A.; Boudreau, B.P.; Johnson, B.D. Gas domes in soft cohesive sediments. Geology 2012, 40, 379–382. [CrossRef] 30. Williams, S.C.P. Skimming the surface of underwater landslides. Proc. Natl. Acad. Sci. USA 2016, 113, 1675–1678. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).