Seabed Stability Research Of Typical Sea Areas

With marine resources explored, large numbers of marine engineering establishments are built. The marine geologists and the engineers focus on the research of seabed stability, on which the constructions are based. However in various marine environments, the seabed stability has different characters, and then collecting corresponding analytic methods are required in various geologic regions. Sand-bar and trough system in Huiquan Bay Beach in Qingdao, and sand ridges and sand waves in southeast Beibu Gulf are systemically researched in the thesis. Based on the analysis of surficial sediments, grain size trend analysis model, topographic and tidal current velocity measures are combined in Huiquan Bay Beach to analyze surficial sediment distribution pattern, transportation trend, transportation direction, deposition and erosion of the beach. Based on the use of bathymetric instruments and DGPS system, the relief patterns and geology regions are identified clearly. Relief maps of three years are compared to get migration velocity, and direction of sand waves, and deposition and erosion in the seabed. In the end, the effect on the submarine pipelines of seabed stability is discussed.149 surficial sediment samples were collected and analyzed over Huiquan Beach of Qingdao. In combination with hydrodynamic measures, surficial sediment distribution pattern and transportation trend of the beach are analyzed. It shows that there is a close relationship between the distribution pattern of sediment grain size and hydrodynamic conditions. The fine sands are transported towards east along shoreline driven by the alongshore current, while the coarse sands remain at the original position. The sediment sortation results in an ordinal pattern characterized by fine gravels, coarse sands, medium sands and fine sands from northwest to southeast in the area. In addition, the surficial sediment types are closely consistent with the mean grain size distribution. In Huiquan Bay Beach, the sediments are transported towards southeast from the high tide line to the sandbar, while towards northwest from the crest of sand bar to its offshore flank. Topographic measures show that the erosion area is distributed mostly near the crest of sand bar in March with the erosion volume up to 656.17m3. But in the same period, area near the trough is the main depocenter with the deposition volume up to 12415.96m3. Tidal current velocity and topography measures validate the results of grain size trend analysis and make up the disadvantages of grain trend size analysis in predicting erosion and deposition.Sand wave migration has an important effect on the seabed stability. Using multi-beam system, repetitive seabed micro-geomorphologic measures from 2004 to 2006 are analyzed. The investigated results show that the wavelength of sand wave ranges from 5.8 to 91.8m, wave height from 0.1 to 4.3, steepness from 0.013 to 0.12, and symmetry from 0.32 to 6.52. The sand wave migration velocity is up to 48.8m/a, migration direction is oriented SE in the west of the researched area, NW in the east, and to and fro in the mid. The migration velocity varies in different sections, and commonly the end of the sand wave migrates more rapidly. Reverse migration exists in a set of sand waves, and the axis line locates near the crest of the sand ridge, upon which the sand waves are superimposed, indicating that the reverse migration is not only related to bottom current but also to submarine topography. From 2004 to 2006, the seabed is under erosion, in the west and the erosion is up to 1.3m, while the seabed is under deposition, in the east, and the deposition is up to 2.8m.In the supralittoral zone and intertidal zone, in which sand-bar and trough system is developed, grain size trend analysis model, topographic and tidal current velocity measures are combined in Huiquan Bay Beach to do seabed erosion and deposition analysis. The results show the method makes up the disadvantages of grain trend size analysis in predicting erosion and deposition. In the shelf, on which sand ridges and sand waves is widely developed, three years relief maps are compared to get migration velocity and direction of sand waves. This technique is feasible in researching seabed stability, such as movement of sand waves.