Laboratory measurements of uniform longshore currents

Experiments on uniform longshore currents in a wave basin are described. The measurements were performed in a basin with a pumped recirculation through openings in the wave guides. Minimal return flows in the offshore region of the basin are found to be accompanied with longshore currents which are virtually uniform alongshore. Three-dimensional longshore current velocity distributions were measured with much attention to quality control. Detailed experimental results are presented for different wave fields, two beach slopes and two beach roughnesses.     

海岸沿岸流垂向分布实验研究

进行海岸沿岸流模型实验,利用ADV(acoustic doppler velocimeter)测量了沿岸流流速沿垂直岸线方向多个断面的垂向分布。采用Faria等的方法对实验结果进行了分析,将沿岸流沿水深分为上层和下层两部分:下层采用对数分布来表达沿岸流的垂向分布;上层考虑了波浪引起的自由表面的波动,流速分布为修正的对数分布。最后对沿岸流垂向分布特征做了分析,并与Visser和Hamilton的实验结果做了对比。     

沿岸流不稳定运动研究进展

沿岸流不稳定运动目前已成为一个研究热点,对于沿岸流不稳定的研究对于近岸泥沙、污染物输移以及海岸养护等都有很重要的意义。本文详细介绍了沿岸流不稳定运动的研究进展,具体包括沿岸流不稳定运动现场以及实验室实验研究进展,沿岸流线性不稳定理论,非线性不稳定理论以及沿岸流不稳定运动产生的其它理论分析。结果表明国外对于该问题的研究(主要是有关不稳定理论方面)已经取得很多成果,但国内目前很少,并且现在对沿岸流不稳定研究比较成熟的理论基础是不稳定理论,该理论都假定增长率最大的不稳定模式决定着沿岸流的波动特性。但对于多模式不稳定运动的研究还基本没有,对于沿岸流不稳定运动三维特性的研究(包括数值研究及实验研究)也基本没有报道。所以这两个方面可能是以后对该问题研究的重要方向。     

One-dimensional modelling of individual waves and wave-induced longshore currents in the surf zone

A probabilistic model ( -model) was developed to describe the propagation and transformation of individual waves (wave by wave approach). The individual waves shoal until an empirical criterion for breaking is satisfied. Wave height decay after breaking is modelled by using an energy dissipation method. Wave-induced set-up and set-down and breaking-associated longshore currents are also modelled. Laboratory and field data were used to calibrate and verify the model. The model was calibrated by adjusting the wave breaking coefficient (as a function of local wave steepness and bottom slope) to obtain optimum agreement between measured and computed wave height. Four tests carried out in the large Delta flume of Delft Hydraulics were considered. Generally, the measured H_1/3-wave heights are reasonably well represented by the model in all zones from deep water to the shallow surf zone. The fraction of breaking waves was reasonably well represented by the model in the upsloping zones of the bottom profile. Verification of the model results with respect to wave-induced longshore current velocities was not extensive, because of a lack of data. In case of a barred profile the measured longshore velocities showed a relatively uniform distribution in the (trough) zone between the bar crest and the shoreline, which could to some extent be modelled by including space-averaging of the radiation force gradient, horizontal mixing and longshore water surface gradients related to variations in set-up. In case of a monotonically upsloping profile the cross-shore distribution of the longshore current velocities is reasonably well represented.     

沿岸流不稳定运动传播特性的小波相干谱分析

沿岸流不稳定运动属于超低频运动,研究它的传播特性,有助于深入理解其对岸滩演变、污染物、鱼卵等输移、迁移的影响。本文基于小波相干谱对所选实验波况进行了研究,分析了规则波、随机波入射情况下沿岸流不稳定运动传播特性,并讨论了入射波高、周期、坡度等对其的影响。结果表明,不规则波更易诱导出沿岸流不稳定运动,且在不规则波情况下,不稳定运动在沿岸方向相距4 m的两个断面上产生的相位差都约为±30°,与波浪入射角相近;随着入射波高的增加,非线性随之增强,更易诱导出不稳定运动,生成的沿岸流不稳定运动周期范围将增大;入射波周期对沿岸流不稳定运动的传播特性影响较小;坡度越陡越易诱导出超低频的不稳定运动。     

Cross-shore distribution of longshore sediment transport: comparison between predictive formulas and field measurements

The skill of six well-known formulas developed for calculating the longshore sediment transport rate was evaluated in the present study. Formulas proposed by Bijker [Bijker, E.W., 1967. Some considerations about scales for coastal models with movable bed. Delft Hydraulics Laboratory, Publication 50, Delft, The Netherlands; Journal of the Waterways, Harbors and Coastal Engineering Division, 97 (4) (1971) 687.], Engelund–Hansen [Engelund, F., Hansen, E., 1967. A Monograph On Sediment Transport in Alluvial Streams. Teknisk Forlag, Copenhagen, Denmark], Ackers–White [Journal of Hydraulics Division, 99 (1) (1973) 2041], Bailard–Inman [Journal of Geophysical Research, 86 (C3) (1981) 2035], Van Rijn [Journal of Hydraulic Division, 110 (10) (1984) 1431; 110(11) (1984) 1613; 110(12) (1984) 1733], and Watanabe [Watanabe, A., 1992. Total rate and distribution of longshore sand transport. Proceedings of the 23rd Coastal Engineering Conference, ASCE, 2528–2541] were investigated because they are commonly employed in engineering studies to calculate the time-averaged net sediment transport rate in the surf zone. The predictive capability of these six formulas was examined by comparison to detailed, high-quality data on hydrodynamics and sediment transport from Duck, NC, collected during the DUCK85, SUPERDUCK, and SANDYDUCK field data collection projects. Measured hydrodynamics were employed as much as possible to reduce uncertainties in the calculations, and all formulas were applied with standard coefficient values without calibration to the data sets. Overall, the Van Rijn formula was found to yield the most reliable predictions over the range of swell and storm conditions covered by the field data set. The Engelund–Hansen formula worked reasonably well, although with large scatter for the storm cases, whereas the Bailard–Inman formula systematically overestimated the swell cases and underestimated the storm cases. The formulas by Watanabe and Ackers–White produced satisfactory results for most cases, although the former overestimated the transport rates for swell cases and the latter yielded considerable scatter for storm cases. Finally, the Bijker formula systematically overestimated the transport rates for all cases. It should be pointed out that the coefficient values in most of the employed formulas were based primarily on data from the laboratory or from the river environment. Thus, re-calibration of the coefficient values by reference to field data from the surf zone is expected to improve their predictive capability, although the limited amount of high-quality field data available at present makes it difficult to obtain values that would be applicable to a wide range of wave and beach conditions.     

An experimental study of the spatial structures of the instabilities of longshore currents on plane beaches

A laboratory experiment on alongshore currents is conducted for two plane beaches with slopes 1:40 and 1:100 to investigate the instability of alongshore currents.The dye release experiment is also performed synchronously in surf zone.Complicated and strongly unstable motions of alongshore currents are observed in the experiment.To examine the spatial and temporal variations of the shear instabilities of longshore clearly,dye batches are released in surf zone.The deformation of the dye patch is observed efficiently and effectively with charge coupled device(CCD) system.Some essential characteristics of the shear instability are validated from the results of image analyses of the temporal variation of the dye patch.The influences of alongshore currents,Stokes drift,large-scale vorticity and the shear instabilities on the transport of dye are analyzed using the collected images.The spatial structure of the instabilities of longshore currents is studied by analyzing collected images of the dye patch.And the phase velocity of the meandering movements is obtained through measuring the movement distances of the oscillations of dye patch in alongshore direction with time.The results suggest that the propagation speed of the shear instability is approximately 50%-75% of maximum of mean alongshore currents for irregular and regular waves.The calculated propagation speed using a linear instability analysis theory is compared with the experimental results.The comparison shows agreements between them.     

不规则波情况下沿岸流不稳定运动传播方向的研究

本文以沿岸流不稳定运动实验中流速仪采集到的不规则波的流速时间历程为基础,利用相关性分析,得到两个不同位置处对应流速时间历程的最大相关系数和延迟时间,进而分析沿岸流不稳定运动在1∶40和1∶100平直斜坡上的传播方向,并且分析了其随入射波高、周期的变化。结果表明沿岸流不稳定运动在海岸垂向环流系统的作用下,其传播方向会向海方向偏移,与岸线角度大约在0°到45°之间,且在1∶40和1∶100坡情况下,沿岸流不稳定运动的偏移角分别随入射周期和波高的增加而增加,但1∶100坡的增加幅值较小,大约为1∶40坡情况下的1/2。     

背景流场对台风风暴潮数值预报的影响研究

选取了3个穿过台湾海峡的台风引起的台风风暴潮为算例,分别用SODA月平均流速资料做垂向平均后构建了台湾周边海域的大尺度背景环流场,利用国家海洋环境预报中心业务化的台风风暴潮模式对经过该海域的台风个例进行风暴潮数值模拟。将结果与验潮站潮位实测资料进行对比发现,在加入了背景流场后,能更好地模拟整个风暴潮过程,模拟结果更接近真实值,特别是对于原模式风暴增水峰值容易偏大的问题得到了改善,而且整体相对误差更小,提高了风暴潮模式的业务稳定性。     

Laboratory study and analysis of the instability of alongshore currents

A laboratory experiment on the instability of alongshore currents was conducted on a plane beach with slope 1:40. Low-frequency fluctuations of alongshore currents with the period of approximately 100 s were observed. The dominant frequency and amplitudes of the oscillations of alongshore currents were determined using the maximum entropy method and the regression method of trigonometric function. The variations across the beach cross-section of the oscillation amplitudes of the alongshore current were given. The linear shear instability theory was used to analyze the mechanism of the oscillation, and the calculated results agreed with measurements. This confirms that the observed fluctuation of alongshore currents is due to the shear instability of alongshore currents.