Regeneration of sand waves after dredging

Sand waves are large bed waves on the seabed, being a few metres high and lying hundreds of metres apart. In some cases, these sand waves occur in navigation channels. If these sand waves reduce the water depth to an unacceptable level and hinder navigation, they need to be dredged. It has been observed in the Bisanseto Channel in Japan that the sand waves tend to regain their shape after dredging. In this paper, we address modelling of this regeneration of sand waves, aiming to predict this process. For this purpose, we combine a very simple, yet effective, amplitude-evolution model based on the Landau equation, with measurements in the Bisanseto Channel. The model parameters are tuned to the measured data using a genetic algorithm, a stochastic optimization routine. The results are good. The tuned model accurately reproduces the measured growth of the sand waves. The differences between the measured weave heights and the model results are smaller than the measurement noise. Furthermore, the resulting parameters are surprisingly consistent, given the large variations in the sediment characteristics, the water depth and the flow field. This approach was tested on its predictive capacity using a synthetic test case. The model was tuned based on constructed predredging data and the amplitude evolution as measured for over 2 years. After tuning, the predictions were accurate for about 10 years. Thus, it is shown that the approach could be a useful tool in the optimization of dredging strategies in case of dredging of sand waves.     

Recolonization and recovery dynamics of the macrozoobenthos after sand extraction in relict sand bottoms of the Northern Adriatic Sea

The long-term effects of sand extraction on macrozoobenthic communities were investigated in an offshore area in the Northern Adriatic Sea characterised by relict sands formed during the last Adriatic post-glacial transgression. Surveys were carried out before, during and 1, 6, 12, 18, 24 and 30 months after extraction at three impacted and seven reference stations. The operations did not influence the physical characteristics of the sediment, but they caused almost complete defaunation at dredged sites. Univariate and multivariate analyses highlighted that the macrozoobenthic community responses to the dredging operations were (1) a rapid initial recolonisation phase by the dominant taxa present before dredging, which took place 6-12 months after sand extraction; (2) a slower recovery phase, that ended 30 months after the operations, when the composition and structure of the communities were similar in the dredged and reference areas. This pattern of recolonisation-recovery fits well with the commonly encountered scenario where the substratum merely remains unchanged after marine aggregate extraction.     

Fish utilization of a salt marsh intertidal creek in the Yangtze River estuary, China

The structure and temporal variations of the fish community in salt marshes of Chinese estuaries are poorly understood. Fish utilization of a salt marsh intertidal creek in the Yangtze River estuary was studied based on quarterly sampling surveys in July and November, 2004, and February and May, 2005. Fishes were collected by consecutive day and night samplings using fyke nets during the ebbing spring tides. A total of 25,010 individuals were caught during the study. 17 families and 33 species were documented, and the most species-rich family was Gobiidae. Three species, Synechogobius ommaturus, Chelon haematocheilus and Lateolabrax maculatus together comprised 95.65% of the total catch, which were also the most important commercial fishery species in the Yangtze River estuary. The fish community was dominated by juvenile individuals of estuarine resident species. Time of year significantly affected fish use of salt marshes, but no significant effects of diel periodicity on the fish community were found except for fish sampling in July. These findings indicate that salt marshes in the Yangtze River estuary may play important nursery roles for fish community.     

Sediment transport on an estuarine intertidal flat: Measurements and conceptual model of waves, rainfall and exchanges with a tidal creek

Sediment (silt) transport on a wave-dominated estuarine intertidal flat dissected by a tidal creek that connects to the watershed freshwater network is investigated by analysing field data from Waikopua, New Zealand, and by applying simple models. The intention is to expand understanding beyond the case of the idealised, two-dimensional wave-dominated flat. During fairweather (no waves), there is a continuous exchange of silt between the bed of the tidal creek and the upper flat, and that exchange is controlled by the elevation of the flat with respect to the creek bed. Rainfall in the watershed does not fundamentally alter the way the intertidal flat and the creek interact, but it does increase silt loads in the creek, which in turn increases the amount of silt exchanged with the upper flat. Waves on the flat are fetch-limited, and frictional dissipation causes waves to reduce in height at the edge of the water body. Under some circumstances, a frictional-dissipation zone may occupy the entire middle-plus-upper flat. There is a maximum in wave-orbital speed at the bed (U_sigb) in the middle reaches of the flat, which arises from the particular balance between down-fetch wave growth, wave dissipation by bottom friction, and attenuation through the water column of wave-orbital motions under the short-period waves. There is a progressive decoupling of suspended-silt concentration (SSC) from U_sigb moving from the bottom to the top of the flat, such that SSC is highest towards the top of the flat, where U_sigb is virtually zero. We suggest that this is due to wave activity retarding the settling of suspended silt, and explore that idea with a simple model that is capable of reproducing the essential features of the data set. The results are assimilated in a conceptual model of the system, which shows the balances that control net silt transport in the creek and on the different parts of the flat, three different silt sources, and the role of waves and rainfall. The conceptual model also points at the feedbacks between sediment-transport processes and morphology that are inherent in the system. Implications of those feedbacks to long-term morphodynamics are essentially unexplored.     

垂直多井动态开采水合物边坡稳定性分析

天然气水合物是一种赋存在海底沉积物孔隙中的清洁能源,含量巨大,具有很好的开发前景和研究价值。垂直井作为开采水合物的一种主要方式,在开采过程中,会改变天然气水合物沉积层的环境条件,随着开采井释放出大量的气体和水,在地层中形成超压,过高的孔隙压力会降低沉积物的胶结强度,破坏沉积层的稳定性,诱发海底滑坡。借助Flac3D软件建立高精度的海底斜坡模型,基于有限差分法对垂直多井动态开采水合物过程中的边坡稳定性进行了数值模拟,模拟了不同开采方案条件下采用多井开采,水合物分解量、开采井压变化等不同影响因素产生的地层力学响应和位移变化,基于安全系数法进对于水合物开采引起的边坡稳定性进行了初步分析。结果表明：多井开采条件下,随着水合物分解程度的不断增大,海底斜坡稳定性逐渐降低,当水合物分解程度达到80%时,安全系数会降低到1.0以下,边坡会失稳;随着井压的不断降低,海底斜坡稳定性同样逐渐降低,当开采井压降低到4 Mpa以下时,安全系数会降低到1.05以下,边坡变为欠稳定状态,存在发生海底滑坡的风险。     

Effects of crab–halophytic plant interactions on creek growth in a S.W. Atlantic salt marsh: A Cellular Automata model

The Bahía Blanca Estuary (38° 50′ S, and 62° 30′ W) presents salt marshes where interactions between the local flora (Sarcocornia perennis) and fauna (Chasmagnathus granulatus) generate some kind of salt pans that alter the normal water circulation and condition its flow and course towards tidal creeks. The crab–vegetation dynamics in the salt marsh presents variations that cannot be quantified in a reasonable period of time. The interaction between S. perennis plant and C. granulatus crab is based on simple laws, but its result is a complex biological mechanism that causes an erosive process on the salt marsh and favors the formation of tidal creeks. To study it, a Cellular Automata model is proposed, based on the laws deduced from the observation of these phenomena in the field, and then verified with measurable data within macroscale time units. Therefore, the objective of this article is to model how the interaction between C. granulatus and S. perennis modifies the landscape of the salt marsh and influences the path of tidal creeks. The model copies the basic laws that rule the problem based on purely biological factors.The Cellular Automata model proved capable of reproducing the effects of the interaction between plants and crabs in the salt marsh. A study of the water drainage of the basins showed that this interaction does indeed modify the development of tidal creeks. Model dynamics would likewise follow different laws, which would provide a different formula for the probability of patch dilation. The patch shape can be obtained changing the pattern that dilates.     

Cementation of sand due to salt precipitation in drying process

Salt in soil can cause cementation effect and increase the shear strength and stiffness of soil during the drying process. In this paper, an experimental study is presented to explore the strength, stiffness, and particle level structure of a salt-cemented sand at the dry state. Unconfined compression tests were carried out on sands with various amounts of precipitated dry salt. Scanning electron microscopic and elemental analysis were also conducted. The study shows that the strength and stiffness of sand can increase significantly with salt content. The strength versus salt content curve displays a convex shape, instead of a concave shape as found in cemented or biocemented sands. This implies that the effect of dry salt on soil strength is strong at low levels of salt content. The microscopic and elemental analysis evidences that salt tends to precipitate at particle contacts and form bridges between particles at relatively low salt content. The results presented in the paper may explain why even small amount of salt can lead to an overestimation of soil strength.     

Changes in Venice Lagoon dynamics due to construction of mobile barriers

The MoSE project (construction of mobile barrier to safeguard the Lagoon of Venice) entails changes to the structure of the lagoon's inlets. This could have consequences for the areas near the inlets and for the dynamics of the lagoon ecosystem as a whole. In order to predict the effects of the proposed alterations on the hydrodynamics of the lagoon, a well-tested hydrodynamic-dispersion model was applied. Simulations were carried out considering both idealised and realistic tide and wind scenarios.     

江苏岸外辐射沙脊群烂沙洋潮流通道稳定性研究

烂沙洋潮流通道是江苏岸外辐射沙脊群内一条涨落潮流速大的潮流通道。利用其天然深水航道,可建设深水大港。为论证烂沙洋潮流通道的稳定性,在实测资料的基础上结合多种研究分析方法主要包括遥感影像分析、地理信息系统、沉积物粒度、主元素分析及水动力条件分析等对该地区进行了综合调查研究。研究结果表明。烂沙洋潮流通道从整体上讲是一条稳定的潮流通道,沉积动力条件较稳定,适宜建港,但在建港时仍需慎重地采取必要的工程措施以减轻西太阳沙北侧周期性冲刷现象。     

Radiation and diffraction of linear water waves by an infinitely long submerged rectangular structure parallel to a vertical wall

The radiation and the diffraction of linear water waves by an infinitely long floating rectangular structure submerged in water of finite depth with leeward boundary being a vertical wall are analyzed in this paper by using the method of separation of variables. Analytical expressions for the radiated and diffracted potentials are derived as infinite series with unknown coefficients determined by the eigenfunction expansion matching method. The expressions for wave forces and hydrodynamic coefficients are given. A comparison is made between the results obtained by the present analytical solution and those obtained by the boundary element method. By using the present analytical solution, the hydrodynamic influences of the submergence, the width, the thickness of the structure, and the distance between the structure and the wall on the wave forces and hydrodynamic coefficients are discussed in detail.     

Modelling of a medium-term dynamics in a shallow tidal sea, based on combined physical and neural network methods

The paper presents an approach towards a medium-term (decades) modelling of water levels and currents in a shallow tidal sea by means of combined hydrodynamic and neural network models. The two-dimensional version of the hydrodynamic model Delft3D, forced with realistic water level and wind fields, is used to produce a two-year-database of water levels and currents in the study area. The linear principal component analysis (PCA) of the results is performed to reveal dominating spatial patterns in the analyzed dataset and to significantly reduce the dimensionality of the data. It is shown that only a few principal components (PCs) are necessary to reconstruct the data with high accuracy (over 95% of the original variance). Feed-forward neural networks are set up and trained to effectively simulate the leading PCs based on water level and wind speed and direction time series in a single, arbitrarily chosen point in the study area. Assuming that the spatial modes resulting from the PCA are ‘universally’ applicable to the data from time periods not modelled with Delft3D, the trained neural networks can be used to very effectively and reliably simulate temporal and spatial variability of water levels and currents in the study area. The approach is shown to be able to accurately reproduce statistical distribution of water levels and currents in various locations inside the study area and thus can be viewed as a reliable complementary tool e.g., for computationally expensive hydrodynamic modelling. Finally, a detailed analysis of the leading PCs is performed to estimate the role of tidal forcing and wind (including its seasonal and annual variability) in shaping the water level and current climate in the study area.     

Interaction of linear waves with an array of infinitely long horizontal circular cylinders in a two-layer fluid of infinite depth

The linear water wave scattering and radiation by an array of infinitely long horizontal circular cylinders in a two-layer fluid of infinite depth is investigated by use of the multipole expansion method. The diffracted and radiated potentials are expressed as a linear combination of infinite multipoles placed at the centre of each cylinder with unknown coefficients to be determined by the cylinder boundary conditions. Analytical expressions for wave forces, hydrodynamic coefficients, reflection and transmission coefficients and energies are derived. Comparisons are made between the present analytical results and those obtained by the boundary element method, and some examples are presented to illustrate the hydrodynamic behavior of multiple horizontal circular cylinders in a two-layer fluid. It is found that for two submerged circular cylinders the influence of the fluid density ratio on internal-mode wave forces is more appreciable than surface-mode wave forces, and the periodic oscillations of hydrodynamic results occur with the increase of the distance between two cylinders; for four submerged circular cylinders the influence of adding two cylinders on the wave forces of the former cylinders is small in low and high wave frequencies, but the influence is appreciable in intermediate wave frequencies.     

Impact of mega-scale sand extraction on tidal dynamics in semi-enclosed basins: An idealized model study with application to the Southern North Sea

Long-term considerations of repeated and increasing sand extraction on the Netherlands Continental Shelf (North Sea) may lead to the creation of a mega-scale extraction trench in front of the Dutch coast (length hundreds of km, width over 10 km, depth several m). We investigate the impact of such a huge topographic intervention on tidal dynamics, which is a key aspect in hydrodynamics, and indirectly also affecting morphodynamics and ecology.     

Simulation and Analysis of a Gravity Cyh''''nder''''s Stability in Short-Crested Sea

Generally, the reliability of ami -sliding and anti-overturning stability of an isolated gravity cylinder in a certain working period can be evaluated only when the statistical properties of short term stability are given first. The authors used numerical method to simulate the stability state function of a cylinder in short-crested sea, and further to get the probabilitical characteristics of the structure's stability by time domain analysis. The external loads appeared in the state functions include horizontal wave force, lift force and the respective moments, and the loads are correlated by co- spectrum. The numerical method presented in this paper can be used not only to solve short term reliability problem directly, but to calculate and analyse the long term reliability problem as well. For circular cylinders, an example of simulation and analysis is displayed in this paper.     

Using a GIS to Examine Changes in the Bathymetry of Borrow Pits and in Lower Bay, New York Harbor, USA

Standard analyses with geographic information systems (GIS) and the publicly available GEODAS database were used to highlight bathymetric changes in the Lower Bay complex of New York Harbor. Dredging operations have deepened much of the Lower Bay complex. Approximately 6,580 hectares, or 20% of the bay bottom surveyed in 1934, was deeper in 1979/1982 than during 1934. Half of this deepening, 3,219 hectares or 10% of the bay bottom surveyed during 1934, was deeper by at least 2 m. Surveys conducted by the U.S. Army Corps of Engineers of three borrow pits in the central part of the Lower Bay complex were used to examine sedimentation over a 16-year period from 1979 to 1995. Results were consistent with studies conducted during the 1970s and 1980s that show the pits function as sediment traps. Between 1979 and 1995, sediment accumulated at rates of 6 to 12 cm per year in many portions of the borrow pits.     

Using a GIS to Examine Changes in the Bathymetry of Borrow Pits and in Lower Bay,New York Harbor,USA

Standard analyses with geographic information systems (GIS) and the publicly available GEODAS database were used to highlight bathymetric changes in the Lower Bay complex of New York Harbor. Dredging operations have deepened much of the Lower Bay complex. Approximately 6,580 hectares, or 20% of the bay bottom surveyed in 1934, was deeper in 1979/1982 than during 1934. Half of this deepening, 3,219 hectares or 10% of the bay bottom surveyed during 1934, was deeper by at least 2 m. Surveys conducted by the U.S. Army Corps of Engineers of three borrow pits in the central part of the Lower Bay complex were used to examine sedimentation over a 16-year period from 1979 to 1995. Results were consistent with studies conducted during the 1970s and 1980s that show the pits function as sediment traps. Between 1979 and 1995, sediment accumulated at rates of 6 to 12 cm per year in many portions of the borrow pits.     

Experimental study of wave loads on a small vehicle in close proximity to a large vessel

Operations involving the launch or recovery of a smaller vessel from a larger one are extremely dangerous in high sea states and, therefore, they are normally carried out in low to moderate sea states. However, this can be severely restrictive and in some situations, carrying out such operations in high sea states is unavoidable. Here we report on a detailed investigation of the interaction between two vessels of different size in order to characterise their hydrodynamic interaction under different conditions and to provide insight for operational purposes. Model experiments were conducted to investigate the hydrodynamic interaction between two vessels in close proximity in waves. Previous studies into this interaction have focused on two vessels with comparable size/displacement. This study focused on the interaction between vessels of very different sizes, a platform supply vessel and a lifeboat, at various separation distances between the two models and wave headings. It is found that the effect of the hydrodynamic interaction on the wave loads on the lifeboat model is substantial. The load responses show a strong non-linearity (high order harmonic components). In head waves, the effect of the hydrodynamic interaction on the wave loads is greater in the transverse modes (sway, roll and yaw) than in the longitudinal modes (surge, heave and pitch). The sheltering effects of the larger model on the lifeboat model were also evident from the experiments. The results of this investigation may be used to inform the planning of marine operations, such as the launch and recovery of a lifeboat or an Autonomous Underwater Vehicle (AUV) from a mothership and the transfer of equipment or personnel between vessels. The data will also provide a useful resource for validation of Computational Fluid Dynamics (CFD) codes and other numerical simulations, and can be used to better understand the limitations and potential widening of the operational weather windows and to ensure that operations are carried out safely.     

Numerical simulation of a partially buried pipeline in a permeable seabed subject to combined oscillatory flow and steady current

Hydrodynamic forces exerting on a pipeline partially buried in a permeable seabed subjected to combined oscillatory flow and steady current are investigated numerically. Two-dimensional Reynolds-Averaged Navier-Stokes equations with a k−ω turbulent model closure are solved to simulate the flow around the pipeline. The Laplace equation is solved to calculate the pore pressure below the seabed with the simulated seabed hydrodynamic pressure as boundary conditions. The numerical model is validated against the experimental data of a fully exposed pipeline resting on a plane boundary under various flow conditions. Then the flow with different embedment depths, steady current ratios and KC numbers is simulated. The amplitude of seepage velocity is much smaller than the amplitude of free stream velocity as expected. The normalized Morison inertia, drag and lift coefficients based on the corresponding force coefficients of a fully exposed pipeline are investigated. The normalized Morison force coefficients reduce almost linearly with the increase of embedment depth and that the KC only has minor effect on the normalized Morison coefficients. It is also found that the permeable seabed condition causes a slight increase on the inline force and has a little effect on the lift force, compared with corresponding conditions in an impermeable bed.