渤海湾海面变化

【研究目的】海面上升,是渤海湾泥质海岸带地区全新世海陆变化最主要的影响因素。通过重建海面变化历史,有助于预测今后的变化趋势。【研究方法】全球基于实际调查的建模研究,恢复了过去数万年来较高分辨率的海面变化历史、分析了导致这种变化的各类分量的贡献。我们通过地区性相对海面变化研究,着重对地区性和区域性分量或做了定量评估、或进行了半定量推理分析,并做了具体的案例研究。【研究结果】这些研究表明,距今10~7 ka时全球海面平均上升速率是~9 mm/a,同期渤海湾西岸相对海面平均上升速率是~5.6 mm/a。这一较快的上升造成了包括渤海湾在内全球沿海平原海岸线后退(以及渤海海盆-渤海湾可能的数道沿岸堤被淹没)。但是,7 ka至今,全球与冰融水等效海面(ice-equivalent sea level,ESL)的平均上升速率降至~0.64 mm/a;5 ka以来,甚至不再上升。7 ka以来,渤海湾西岸相对上升速率是~0.46 mm/a;5.5 ka之后,进一步减弱到~0.18 mm/a。上述全球和渤海湾西岸海面变化,显示出明显的“二段式”上升特点。以~7 ka为转折点:之前与之后的上升速率相差一个数量级。对于渤海湾西岸而言,叠加在这个长达~7000年的明显减速但仍缓慢上升背景上的,是均衡掀升分量与下沉(新构造下沉+沉积自压实下沉)分量的相互抵消。于是,全球性海面的缓慢上升、区域性均衡调整造成的掀升与局地下沉,这三者的复杂博弈,形成了长达~7000年的、经常受高潮水影响的渤海湾西岸障壁岛型贝壳堤-潟湖盐沼洼地周期性交替的沿海地形地貌格局。1870 CE至今的150余年间,全球海面平均上升速率~1.7 mm/a,近10余年来更加速至3.7 mm/a,渤海湾同时期I堤海岸线向海侧的前凸岸段平均蚀退1~3 km。【结论】我们推断1870CE以来的全球气温上升-海面上升与渤海湾同时期海岸线蚀退可能具因果关系。21世纪全球变暖及因此引起海面~6~8 mm/a上升的预测,是1870 CE至今全球加速升温-海面加速上升的必然延续。从地质学角度,这将导致渤海湾今后的“可容空间”逐步增加,为海岸线-潮间带蚀退、盐沼湿地退化提供条件,从而将可能导致持续了~7000年的贝壳堤障壁岛-潟湖盐沼地貌过程的终止,转入与7 ka之前相似的海面上升加速时的沿岸堤-沿海低地模式。以上基于全球研究及渤海湾实际调查结果做出的推断,将对渤海湾泥质海岸带的中长期发展产生影响。

波流作用下海底边界层沉积物再悬浮与影响因素研究

海底沉积物再悬浮及其分布取决于海洋水动力、沉积物类型与床面形态之间复杂的相互作用,准确地理解和确定沉积物再悬浮过程对于沉积物输运的研究具有重要的意义。本文在祥云湾海洋牧场典型海域开展现场原位观测,获取研究区波浪、海流及悬浮沉积物浓度数据;分析了波、流作用下海底边界层悬浮沉积物垂向分布特征,并探究了海洋水动力和床面形态对悬浮沉积物垂向分布的影响。结果表明,研究区波流之间的相互作用不显著,沉积物再悬浮受控于风暴浪作用,风暴浪作用下底床切应力可以达到沉积物临界切应力的10~15倍,沉积物的再悬浮滞后于风暴浪作用2~3 h。在波浪荷载微小的情况下,悬浮沉积物垂向分布呈现"I"型,波浪荷载下,悬浮沉积物垂向分布呈现幂指函数分布,表现为"L"型;床面形态随波、流作用而演化,影响沉积物的再悬浮过程,u*w/u*c=1.00可作为波浪和海流起主导控制作用的床面形态的判别依据,纯波浪荷载作用下的u*w/u*c显著高于波浪主控作用下,但二者之间的界线随着波浪荷载的增加而升高。

Vertical Migration of Fine-Grained Sediments from Interior to Surface of Seabed Driven by Seepage Flows–‘Sub-Bottom Sediment Pump Action'

A scientific hypothesis is proposed and preliminarily verified in this paper: under the driving of seepage flows, there might be a vertical migration of fine-grained soil particles from interior to surface of seabed, which is defined as ‘sub-bottom sediment pump action' in this paper. Field experiments were performed twice on the intertidal flat of the Yellow River delta to study this process via both trapping the pumped materials and recording the pore pressures in the substrate. Experimental results are quite interesting as we did observe yellow slurry which is mainly composed of fine-grained soil particles appearing on the seabed surface; seepage gradients were also detected in the intertidal flat, under the action of tides and small wind waves. Preliminary conclusions are that ‘sediment pump' occurs when seepage force exceeds a certain threshold: firstly, it is big enough to disconnect the soil particles from the soil skeleton; secondly, the degree of seabed fluidization or bioturbation is big enough to provide preferred paths for the detached materials to migrate upwards. Then they would be firstly pumped from interior to the surface of seabed and then easily re-suspended into overlying water column. Influential factors of ‘sediment pump' are determined as hydrodynamics(wave energy), degree of consolidation, index of bioturbation(permeability) and content of fine-grained materials(sedimentary age). This new perspective of ‘sediment pump' may provide some implications for the mechanism interpretation of several unclear geological phenomena in the Yellow River delta area.

Wave flume experiments on the contribution of seabed fluidization to sediment resuspension

Sediment resuspension is commonly assumed to be eroded from the seabed surface by an excess bottom shear stress and evolves in layers from the top down. Although considerable investigations have argued the importance of wave-induced seabed fluidization in affecting the sediment resuspension, few studies have been able to reliably evaluate its quantitative contribution till now. Attempt is made to preliminarily quantify the contribution of fluidization to resuspension using a series of large-scale wave flume experiments. The experimental results indicated that fluidization of the sandy silts of the Huanghe Delta account for 52.5% and 66.8% of the total resuspension under model scales of 4/20 and 6/20（i.e., relative water depth： the ratio of wave height to water depth）, respectively. Some previously reported results obtained using the same flume and sediments are also summarized for a contrastive analysis, through which not only the positive correlation is confirmed, but also a parametric equation for depicting the relationship between the contribution of fluidization and the model scale is established. Finally, the contribution of fluidization is attributed to two physical mechanisms：（1） an attenuation of the erosion resistance of fluidized sediments in surface layers due to the disappearing of original cohesion and the uplifting effect resulting from upward seepage flows, and（2） seepage pumping of fines from the interior to the surface of fluidized seabed.

Development and Application of an in Situ Penetrator for Rapid Strength Testing of Submarine Sediment

In marine engineering, the strength of a submarine sediment is an indispensable parameter for assessment of construction. In this study, a free-fall cone penetrator named IPen was developed to realize a rapid and efficient measurement of sediment strength. The equipment is characterized by modular design and self-contained data acquisition. It is equipped with an acceleration sensor, a water pressure sensor, and a piezocone penetration test(CPTu) probe. It is designed to be released from near seabed surface with a releaser and then fall freely to provide a higher penetration velocity. Its maximum working depth is approximately 2500 m and maximum penetration depth is approximately 3 m. To derive the correlation between penetration resistance and sediment strength, a calibrator was devised to determine the penetration-rate factor. In addition, the factor applicable to in situ test points was determined in laboratory experiments. In June 2016, the IPen was tested in situ in the South Yellow Sea, China, during a shared voyage funded by the National Science Foundation. Meanwhile, undisturbed column samples were collected for laboratory tests. Based on the in situ test results, it was demonstrated that the IPen could accurately record the working states of various sensors during the freely falling course. IPen test results reliably reflected the sediment strength at all the testing points when compared with laboratory calibration tests, in situ vane tests and penetration tests, laboratory penetration tests, and unconsolidated and undrained triaxial compression tests.

Experimental Researches on the Accuracy of Micro- Electromechanical Accelerometer-Array in Seafloor Failure Deformation Measurement

At present,there are few technologies applied to in situ observation of seabed deformation,among which the micro-electromechanical accelerometer-array(hereinafter referred to as accelerometers array)is a very advantageous measurement method,with both commercial products and successful application cases.However,the coupling effect between accelerometer-array and sur-rounding soil and the linkage effect of accelerometer-array itself during the deformation may influence the accuracy and reliability of the measurement data.A simulation test chamber was designed and processed,and four groups of simulation tests were carried out to explore the coupling effect and linkage effect of accelerometer-array in the soil with different degree of consolidation.The results show that the accelerometer-array and the soil coupled well,and the coupling effect is positively correlated with the degree of soil consoli-dation.The ratio of accumulative deviation to soil lateral deformation is high at the initial stage of deformation(0-50 mm)and reduced with the continuous increase of deformation(>100 mm).In the process of liquefied soil deformation,the linkage effect of accelerometer array can be ignored,and is negatively correlated with the degree of soil consolidation.A concept to improve the measurement accu-racy of accelerometer-array in different seafloor failure deformation modes is proposed.The research results provide references for the modification of accelerometer-array and the improvement for other flexible rod-shaped deformation sensors.