冲流带海滩高频振动探讨

在粤东汕尾寮嘴口岬间海滩冲流带设置两条剖面,利用2003年10月13日一个潮周期实测高频剖面数据(采样间隔为1min和6min),分析了海滩的高频振动特征。并结合同期实测碎波带水位波动,探讨了海滩高频振动的动力原因。分析表明,此海滩过程主要以堆积为主,并表现有显著周期的振动：其日内变化受潮汐控制,表现为涨潮堆积、落潮侵蚀;波能对海滩高频振动过程有着重要的影响。     

Tracer studies on the updrift margin of a complex inlet system

Three sediment transport studies using tracers were performed at Ancão Inlet (southern Portugal). The objectives of the experiments were to understand the sediment transport pathways and to determine their magnitudes on the updrift margin of an inlet. In order to apply the traditionally used Space Integration Methodology to the tracer experiments, adaptations were required. The study area was divided into four morphologically defined sectors and this was found to be a key factor for the applicability of tracers in a complex area. The four sectors are as follows: sector A is the straight part of the updrift beach; sector B is the upper area of the swash platform; sector C is the lower area of the swash platform; and sector D represents the inner parts of the inlet margin. The integrated analysis of all collected data (forcing mechanisms, tracer distribution and topographic evolution) led to the determination of the sediment pathways. A semi-quantitative conceptual model was developed in order to explain the sediment transport pathways and magnitudes that a known mass of sediment would follow after arriving at the swash platform. It was found that the areas with the largest sediment accumulation were sectors B and C, while almost no sediment was retained in sector D, which experienced significant erosion. According to the model, 53% of the initial mass of sediments remain in the system after two tidal cycles. It is hypothesised that sediment losses are caused by sediment transport towards the ebb delta and by sediment bypassing occurring from the ebb delta to the downdrift beach through swash bar processes. The herein defined conceptual model represents a useful tool that could be applied to other tidal inlets under similar conditions, facilitating sediment budget studies around tidal inlets.     

粤东后江湾冲流带滩面高频振动及动力作用分析

利用涌浪影响下短时段内的冲流带滩面高频高程数据和碎波带波流资料,在奇异谱分析(SSA)的基础上,对比研究了不同形态滩面的冲淤变化趋势、趋势分布形状、冲淤变化周期和冲淤变化强度,以及同一条剖面不同桩点间各因素间的变化关系;用交叉谱方法探索了每分钟滩面高频冲淤变化与碎波带长重力波间的作用关系。分析结果表明,滩角韵律地形引起的冲流分流作用促进了滩脊向滩谷的泥沙转运,冲流带滩面存在明显的长重力波频段的周期性冲淤振动,滩面冲淤振动强度由滩面下部向上部递减,碎波带长重力波对滩面高频冲淤变化起重要作用。     

Infra-gravity wave generation by the shoaling wave groups over beaches

A physical parameter, μb, which was used to meet the forcing of primary short waves to be off-resonant before wave breaking, has been considered as an applicable parameter in the infra-gravity wave generation. Since a series of modulating wave groups for different wave conditions are performed to proceed with the resonant mechanism of infra-gravity waves prior to wave breaking, the amplitude growth of incident bound long wave is assumed to be simply controlled by the normalized bed slope, βb. The results appear a large dependence of the growth rate, α, of incident bound long wave, separated by the three-array method, on the normalized bed slope, βb. High spatial resolution of wave records enables identification of the cross-correlation between squared short-wave envelopes and infra-gravity waves. The cross-shore structure of infra-gravity waves over beaches presents the mechanics of incident bound- and outgoing free long waves with the formation of free standing long waves in the nearshore region. The wave run-up and amplification of infra-gravity waves in the swash zone appear that the additional long waves generated by the breaking process would modify the cross-shore structure of free standing long waves. Finally, this paper would further discuss the contribution of long wave breaking and bottom friction to the energy dissipation of infra-gravity waves based on different slope conditions.     

Hindered settling of sand grains

The sedimentation rate of sand grains in the hindered settling regime has been considered to assess particle shape effects. The behaviour of various particulate systems involving sand has been compared with the widely used Richardson–Zaki expression. The general form of the expression is found to hold, in as much as remaining as a suitable means to describe the hindered settling of irregular particles. The sedimentation exponent n in the Richardson–Zaki expression is found to be significantly larger for natural sand grains than for regular particles. The hindered settling effect is therefore greater, leading to lower concentration gradients than expected. The effect becomes more pronounced with increasing particle irregularity. At concentrations around 0·4, the hindered settling velocity of fine and medium natural sands reduces to about 70% of the value predicted using existing empirical expressions for n. Using appropriate expressions for the fluidization velocity and the clear water settling velocity, a simple method is discussed to evaluate the sedimentation exponent and to determine the hindered settling effect for sands of various shapes.