Prediction of local scour around circular piles under waves using a novel artificial intelligence approach

Abstract

The scour phenomena around vertical piles in oceans and under waves may influence the structure stability. Therefore, accurately predicting the scour depth is an important task in the design of piles. Empirical approaches often do not provide the required accuracy compared with data mining methods for modeling such complex processes. The main objective of this study is to develop three data-driven methods, locally weighted linear regression (LWLR), support vector machine (SVR), and multivariate linear regression (MLR) to predict the scour depth around vertical piles due to waves in a sand bed. It is the first effort to develop the LWLR to predict scour depth around vertical piles. The models simulate the scour depth mainly based on Shields parameter, pile Reynolds number, grain Reynolds number, Keulegan–Carpenter number, and sediment number. 111 laboratory datasets, derived from several experimental studies, were used for the modeling. The results indicated that the LWLR provided highly accurate predictions of the scour depths around piles (R?=?0.939 and RMSE = 0.075). Overall, this study demonstrated that the LWLR can be used as a valuable tool to predict the wave-induced scour around piles.     

Characteristics of Scour and Deposition in Front of Breakwaters Under Irregular Broken Clapotis

First the scour and deposition patterns of the sandy seabed in front of a vertical breakwater under the action of irregular broken clapotis are investigated experimentally and classified into five types: scour type Ⅰ , scour type Ⅱ, scour type Ⅲ, deposition type Ⅰ, and deposition type Ⅱ . Secondly, the processes of formation of scour and deposition patterns are probed in comparison with those induced by regular broken clapotis and standing waves. Thirdly, the criteria for distinguishing scour and deposition patterns under irregular broken clapotis are presented.     

Seasonal and Spatial Variations of Iceberg Drift off Dronning Maud Land, Antarctica, Detected by Satellite Scatterometers

Seasonal and spatial variations of iceberg drift were studied using continuous satellite scatterometer images off Dronning Maud Land, East Antarctica. Generally, iceberg drift speed showed a westward increase to the Greenwich Meridian. Seasonal variations of the drift speed were high in autumn—early winter and low in spring, and their magnitudes also increased westward. Seasonal variations of the drift speed were significantly correlated with variations of sea levels at Syowa and Mawson Stations, and hence qualitatively consistent with geostrophic current variations. Thus, the scatterometer data are demonstrated to be useful in monitoring iceberg trajectory and oceanic current variations. This revised version was published online in July 2006 with corrections to the Cover Date.     

新建水坝对下游已建水工建筑物的影响分析

分析了新建水坝及社会经济发展因素对坝下游水流及泥沙的影响，用一个简易河床模型来说明新建大坝后下游河床的演变规律，通过坝下游河床一般演变分析来阐述局部冲刷对已建水工建筑物基础的影响及对已建水工建筑基础的防护措施。     

A simulation model of morphological,vegetation and sediment changes in ephemeral streams

A model to simulate channel changes in ephemeral river channels and to test the effects of hydrological changes due to climate change and[sol ]or land use change was developed under the auspices of the EU funded MEDALUS programme (Mediterranean Desertification and Land Use). The model, CHANGISM (Channel Change GIS Simulation Model), is designed to simulate the effect of channel flow events and of climate conditions on morphology, sediment and vegetation, through sequences of events and conditions, over periods of up to several decades. The modelling is based on cellular automata but with calculations for water and sediment continuity. Process rules have both deterministic and stochastic elements. An important feature of the model is that it incorporates feedback elements between each event. The main aim of the model is to indicate the likely outcomes of events and combinations of conditions. It is linked to GIS for both input and output. The modelling is based on a channel reach and state is input as GIS layers of morphology (DEM), sediment and vegetation cover and state. Other initial conditions of soil moisture, groundwater level, and overall gradient are input. Parameters for processes are read from tables and can be easily changed for successive runs of the model. The bases for decisions on process specifications are discussed in this paper. Initial tests of the operation and sensitivity of the model were made on idealized reaches. The model was then tested using data from monitored sites in SE Spain. Simulations using clearwater flow worked well but initial simulations using events with sediment loads showed some tendency for excess deposition. Further tests and modifications are taking place. Overall, the model is one of the most sophisticated that simulates the interaction of flows with sediment and vegetation and the outcomes in terms of erosion, deposition, morphology, sediment cover, vegetation cover and plant survival over periods of up to 30 years for the scale of a channel reach. Copyright © 2005 John Wiley & Sons, Ltd.     

南海北部陆架早—中中新世古珠江河道发育特征及汇流冲刷模式

河道体系是侵蚀和沉积物搬运的重要通道,控制了大陆边缘源汇体系中砂体总体的输送和分布格局,并可有效指示古代构造活动、物源供给、气候变迁和海平面变化等丰富地质信息。研究基于高分辨率的三维地震、钻测井资料,结合层序划分方法和地震沉积学综合分析手段,揭示了南海北部珠一坳陷早—中中新世古珠江分流河道体系的类型和展布特征,并且以典型层序为例精细揭示了海平面变化控制下河道类型由辫状向曲流的转换。同时,在研究区地震剖面上识别到多处异常下切现象,经研究可解释为辫状河道内的汇流冲刷,其典型特征为：① 下切深度局部增大;② 平面近似圆形或椭圆形;③ 内部为砂质充填。虽然汇流冲刷结构在地震剖面特征上与“传统”意义的下切谷极易混淆,但在沉积特征和成因上存在较大差别,因此,对它的识别有助于避免层序界面的错误判别,并且其后期充填的河道砂体具有局限分布的特点,易于在后期海侵泥岩的覆盖下形成潜在的岩性圈闭新类型。     

Protection of bridge piers against scouring with tetrahedral frames

Armoring countermeasures, such as riprap stones, are the primary method used to protect bridge piers against scouring; however, these methods have not had definitive success. Recently, flow-altering countermeasures, such as sacrificial piles, have been tested as an alternative to armoring countermeasures. This study investigated the mechanics of an innovative flow-altering countermeasure device, frames in the shape of tetrahedrons that act as a pier-scour countermeasure. Results of measured characteristics for turbulence flow showed that the flow around the tetrahedral frames can be divided into three regions： （1） a deceleration region near the sediment bed; （2） an acceleration region in the middle of water depth region; and （3） a restoration region near the water surface. The velocity magnitudes, turbulent intensities and vorticities decreased in the deceleration region, increased in the acceleration region and reverted to that of the unprotected condition in the restoration region. This pier-scour countermeasure is innovative because of its ability to dissipate energy associated with the downflow and the horseshoes vortex generated around the bridge pier. The scour tests revealed that the frames protected the foundation of bridge piers against scour. The experimental results showed that the percentage reduction of scour depth decreased as the velocity ratio, U/U c , decreased, reaching a value of 50% for the range of parameters tested in this study. Moreover, its efficiency was dependent on the placement density of the frames. The data showed that the frames were more effective when η was larger. However, their influence becomes less significant when η≥ 0.16.     

清水冲刷条件下长江中游沙卵石河段局部卵石淤积成因

为探究三峡水库持续下泄清水条件下长江中游沙卵过渡段内卵石局部冲刷、搬运和淤积现象的成因,采用近期水文、泥沙和地形观测资料,结合河道平面二维水动力模型,计算了研究河段内各级流量下的泥沙起动粒径分布,分析了上游来流、下游水位变化的影响,辨析了河床冲淤的成因。结果表明:①45 000 m³/s以上流量时,粒径大于30 mm的卵石可沿洪水主流带连续搬运;流量低于15 000 m³/s时,大粒径卵石只能沿枯期主流在浅滩河段局部搬运;流量介于两者之间时,水流对大粒径卵石的输移动力相对较弱。②三峡建库后,洪水量级削减而枯水天数增多,不利于卵石长距离下移,而水位下降不断溯源传递,促使枯水流路上原本稳定的区域开始冲刷。③局部淤积现象由枯期水动力增强所导致,与长江中游沙卵过渡段特殊的地貌和沉积环境有关。卵石局部冲淤调整可能在河段内多个位置长期存在,需引起关注。     

Morphodynamics and depositional signature of low‐aggradation cyclic steps: New insights from a depth‐resolved numerical model

Bedforms related to Froude‐supercritical flow, such as cyclic steps, are increasingly frequently observed in contemporary fluvial and marine sedimentary systems. However, the number of observations of sedimentary structures formed by supercritical‐flow bedforms remains limited. The low number of observations might be caused by poor constraints on criteria to recognize these associated deposits. This study provides a detailed quantification on the mechanics of a fluvial cyclic step system, and their depositional signature. A computational fluid‐dynamics model is employed to acquire a depth‐resolved image of a cyclic step system. New insights into the mechanics of cyclic steps shows that: (i) the hydraulic jump is, in itself, erosional; (ii) there are periods over which the flow is supercritical throughout and there is no hydraulic jump, which plays a significant role in the morphodynamic behaviour of cyclic steps; and (iii) that the depositional signature of cyclic steps varies with rate of aggradation. Previous work has shown that strongly aggradational cyclic steps, where most of the deposited sediment is not reworked, create packages of backsets, bound upstream and downstream by erosive surfaces. Here, the modelling work is focussed on less aggradational conditions and more transportational systems. The depositional signature in such systems is dominated by an amalgamation of concave‐up erosional surfaces and low‐angle foresets and backsets creating lenticular bodies. The difference between highly aggradational cyclic steps and low‐aggradation steps can be visible in outcrop both by the amount of erosional surfaces, as well as the ratio of foreset to backset, with backsets being indicative of more aggradation.     

往复流作用下海底管道局部冲刷机制数值模拟

基于开源的计算流体力学模式REEF3D,建立了海底管道局部冲刷水槽数值模型,在验证单向流实验结果的基础上,进一步对往复流作用下的海底管道局部冲刷机制进行了研究,并作对比分析。研究表明,总体上往复流对管道所在海床的局部冲刷规模比单向流弱。当流向改变后,原先下游的堆积区转变成上游,优先受到冲刷,并填充到管道下方的冲刷坑,同时使水流在管道下方的作用减弱。这种回填过程使短周期下的往复流作用需要更长的冲淤平衡时间。在回填与冲刷的共同作用下,上下游的冲刷坑坡度会因流向变化而变化。