Sediment resuspension of bottom boundary layer under waves and currents
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摘要: 海底沉积物再悬浮及其分布取决于海洋水动力、沉积物类型与床面形态之间复杂的相互作用，准确地理解和确定沉积物再悬浮过程对于沉积物输运的研究具有重要的意义。本文在祥云湾海洋牧场典型海域开展现场原位观测，获取研究区波浪、海流及悬浮沉积物浓度数据；分析了波、流作用下海底边界层悬浮沉积物垂向分布特征，并探究了海洋水动力和床面形态对悬浮沉积物垂向分布的影响。结果表明，研究区波流之间的相互作用不显著，沉积物再悬浮受控于风暴浪作用，风暴浪作用下底床切应力可以达到沉积物临界切应力的10～15倍，沉积物的再悬浮滞后于风暴浪作用2～3 h。在波浪荷载微小的情况下，悬浮沉积物垂向分布呈现“I”型，波浪荷载下，悬浮沉积物垂向分布呈现幂指函数分布，表现为“L”型；床面形态随波、流作用而演化，影响沉积物的再悬浮过程，
${u_{*w}}{\rm{/}}{u_{*c}} = 1.00$ 可作为波浪和海流起主导控制作用的床面形态的判别依据，纯波浪荷载作用下的${u_{*w}}{\rm{/}}{u_{*c}}$ 显著高于波浪主控作用下，但二者之间的界线随着波浪荷载的增加而升高。Abstract: Resuspension and its distribution of sediment depend upon three interacting components namely the characteristics of the mobile sediment, the bed forms and the forcing hydrodynamics. A good understanding of the process of sediment resuspension is important in sediment transport. In this paper, in-situ measurements of wave, current, and suspended sediment concentration profiles in the marine ranching of Xiangyun Bay were carried out. The vertical distribution characteristics of suspended sediment in the bottom boundary layer under the wave-current action were analyzed. The results show that the resuspension of seabed sediments in the study area is controlled by storm-waves. The bottom shear stress under storm wave is 10−15 times of the critical shear stress of sediment, resuspension of sediment lags behind storm-wave for 2−3 hours. The type of vertical distribution of suspended sediment is "I" under small wave load, the vertical distribution of suspended sediment in the bottom boundary layer presents a power exponential function, which is "L" type under storm-wave. Bedforms evolved with wave and current action, and which affected the resuspension process of sediments.${u_{*w}}{\rm{/}}{u_{*c}} = 1.00$ can be used as a criterion to distinguish the bedfroms under the dominant control of wave or current. The value of${u_{*w}}{\rm{/}}{u_{*c}}$ under wave load is higher than that under the dominant control of wave, and the cutoff value between them increased with the increase of wave load.-
Key words:
- bottom boundary layer /
- suspended sediment /
- waves /
- bedform /
- Xiangyun Bay
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图 1 原位观测位置及观测设备示意图
Fig. 1 Schematic diagram of field observation area and observation equipment
图 3 2017年9−10月现场长期原位观测结果
a图中阴影为风暴浪事件
Fig. 3 Results of long term in-situ observation during September to October, 2017
The shaded in a represent strom wave event
图 4 2017年9−10月海流单独作用下的底床切应力（τc）、波浪荷载导致的底床切应力（τw）及波流共同作用下最大底床切应力（τmax）
横虚线代表沉积物临界切应力τcr=0.148 N/m2
Fig. 4 Bottom skin friction shear-stresses of current alone (τc), waves alone(τw), and maximum wave-current (τmax) during September to October, 2017
The horizontal line is the threshold shear-stress for the initiation of movement for D50 of the bed
图 5 2017年9−10月海底边界层不同深度处悬浮沉积物变化
Fig. 5 Variation of suspended sediment of bottom boundary layer at different depth during September to October, 2017
图 6 风暴荷载下悬浮沉积物垂向分布与波高的关系
Fig. 6 Vertical distribution of suspended sediments and wave height during storm
图 8 2017年9−10月波、流摩阻流速比值
${u_{*w}}/{u_{*c}}$ 时间序列变化Fig. 8 The ratio of wave and current friction velocity
${u_{*w}}/{u_{*c}}$ plotted as a function of time during September to October, 2017表 1 海底边界层综合观测平台搭载仪器简介
Tab. 1 Introduction of instruments on integrated observing platform of bottom boundary layer
序号 仪器 厂家/型号 基本参数 参数设置 1 波潮仪 加拿大RBR公司/RBR virtuoso D|wave & Tidal 测量精度不小于0.05%，潮位、波浪采样频率1～6 Hz可调 采样频率6 Hz，采样周期15 min，距底高度165 cm 2 多普勒流速仪 美国Norterk公司/ADV 采样频率最大为250 Hz，最大输出频率为64 Hz，测量精度为0.1%±0.5 cm/s 采集频率8 Hz；采样周期10 min，距底高度113 cm 3 多参数浊度仪 加拿大RBR公司/XR-620 CTDTu 测量温度、电导、深度、浊度、溶解氧；浊度：0～125 FTU、0～250 FTU、0～500 FTU、0～4 000 FTU 1 min采集一次，连续采集，距底高度82 cm 4 高密度悬浮泥沙浊度剖面测量仪 德国Argus公司/ASM-4-N 悬浮沉积物浓度量程：沙0～50 000 mg/L；泥0～5 000 mg/L；剖面测量范围海底面以上1～2 m；沉积物浓度测量精度±10% 采样频率1 Hz，采样周期15 min，底端第一个探头距底高度17 cm 
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