Abstract
Long-term, continuous in-situ observation of seabed deformation plays an important role in studying the mechanisms of sand wave migration and engineering early warning methods. Research on pressure sensing techniques has examined the possibility of using the temporal characteristics of the vertical deformation of the seafloor to identify important factors (e.g., wave height and migration rate) of submarine sand wave migration. Two pressure sensing tools were developed in this study to observe the seabed deformation caused by submarine sand wave migration (a fixed-depth total pressure recorder (TPRFD) and a surface synchronous bottom pressure recorder (BPRSS)), based on the principle that as a sand wave migrates under hydrodynamic forcing, the near-bottom water pressure, bottom pressure and total fixed pressure synchronously change with time. Laboratory flume experiments were performed, using natural sandy sediments taken from the beach of Qingdao, China, to better present and discuss the feasibility and limitations of using these two pressure sensing methods to acquire continuous observations of seabed deformation. The results illustrate that the proposed pressure sensor techniques can be effectively applied in reflecting elevation caused by submarine sand wave migration (the accuracy of the two methods in observing the experimental bed morphology was more than 90%). However, an unexpected step-like process of the change in sand wave height observed by BPRSS is presented to show that the sensor states can be easily disturbed by submarine environments, and thus throw the validity of BPRSS into question. Therefore, the TPRFD technique is more worthy of further study for observing submarine sand wave migration continuously and in real-time.
Highlights
Submarine sand waves are approximately regular undulating landforms [1] formed by the movement of sandy sediments under various marine hydrodynamic forces, such as ocean currents [2], tidal currents [3] and internal waves [4]
According to the pressure change law acting on the sand wave profile during this process, the bottom pressure (PB) at the observation point mainly includes the hydrostatic pressure determined by the sea floor surface height (H) and dynamic water pressure created by the waves and current (∆PN)
The two methods exhibit the following two characteristics in terms of accuracy levels: (a) observations of the trough are more accurate than those of the crest, and (b) the bottom pressure recorder (BPRSS) method is more accurate than the TPRFD method. These findings show that while seabed elevation can be measured from the weight of overlying sand, this approach is less accurate than using water pressure to reflect elevation
Summary
Submarine sand waves are approximately regular undulating landforms [1] formed by the movement of sandy sediments under various marine hydrodynamic forces, such as ocean currents [2], tidal currents [3] and internal waves [4]. Under the action of ocean dynamics, sand waves undergo periodic migration movements, and rates can reach nearly 70 meters per year [12,13]. From this process, vertical deformation may spur the suspension or burial of submarine cables [3,14] and submarine pipelines [8,15], which can seriously damage them. DTM data obtained from multi-beam water depth measurements can be used to calculate the rate of seabed sand wave migration by profile analysis [13,18]. Because these methods depend on ship operations, they cannot be used to observe continuous changes in height, due to weather, sea conditions and cost constraints
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