Bottom sediment transport in the western Laizhou Bay during strong wind events based on a tripod measurement
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摘要: 基于2018年10月21日至11月6日莱州湾西岸连续站观测数据，本文利用集合经验模态分解、希尔伯特−黄变换和小波分析法对底层单宽输沙率的小尺度特征做分析，并针对观测期间出现的大风天气对泥沙输运的影响进行了探究。结果表明，单宽输沙率在观测时间段内具有高频、潮周期、低频以及长周期尺度变化特征，周期尺度从小到大。其中高频和潮周期分量方差贡献率及所含能量最高，对输沙率的影响最强。边际谱显示东西方向输沙率的显著周期为13.3 h，南北方向大于11 h的周期较为显著。观测期间底层净泥沙通量分别为东向305.77 kg/m、南向597.25 kg/m，余流分量贡献最大，低频和高频分量贡献最小。上强迫风场主要在风速衰减期通过湍流和波浪影响输沙速率的时频分布，使其低频变化显著增强的同时，产生1 h周期左右的高频波动。交叉小波分析显示，风速和单宽输沙率在低频波段上相干性较强，且单宽输沙率会滞后风速1/4至1/2个周期。另外，风浪会增强泥沙输运的涨落潮不对称性，进而增加潮周期分量上的泥沙净输运。Abstract: Based on a tripod measurement in the Laizhou Bay from October 21 to November 6, 2018, the unit-width bottom sediment transport rate and the effects of strong winds on bottom sediment transport are analyzed using the ensemble empirical mode decomposition, the Hilbert-Huang transform and the wavelet analysis method. The resuluts show thta the unit-width sediment transport rate can be divided into four intrinsic mode functions with increased periods including the high-frequency, tidal-period, low-frequency and long-period. The high-frequency and tidal-period components have the highest variance contribution rate and energy, indicating their greatest impact on sediment transport. The marginal spectrum shows a significant period of 13.3 h in the east-west direction, and a period larger than 11 h in the north-south direction. The net bottom sediment fluxes in the east-west and north-south directions are 305.77 kg/m and 597.25 kg/m, respectively; and the high-frequency and low-frequency component contribute least while the residual-current component contributes most. Winds work on the time-frequency distribution of the unit-width sediment transport rate mainly through the turbulence and waves during the wind decrease periods, which enhances the low-frequency domain significantly, and further induces the high-frequency fluctuations with periods about 1 h. The cross-wavelet analysis shows the winds and the unit-width sediment transport rate have a strong coherence on the low-frequency band; and the latter lags behind the former by 1/4 to 1/2 period. In addition, the wind-waves enhance the asymmetric sediment transport between flood and ebb tidal phase, which increases the net sediment flux of tidal period.
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图 1 研究区域水深、连续站和风场数据站位
Fig. 1 Bathymetry map of the study area and the location of the measurement site and wind data site
图 2 ADCP观测的流速剖面U分量（a）和V分量（b），ADV观测的近底层流速U分量（c）和V分量（d）及临近层位的ADCP观测数据对比
Fig. 2 The ADCP measured U-component (a) and V-component (b) of the velocity profile, and the ADV measured U-component (c) and V-component (d) of the near-bottom velocity and corresponding ADCP measurement
图 3 风矢量（a），亚潮频率的流速剖面U分量（b）、V分量（c）和悬沙浓度（d）随时间的变化
Fig. 3 Temporal variation of wind vector (a), U-component (b) and V-component (c) of the sub-tidal velocity profile, and suspended sediment concentration (d)
图 4 单宽输沙率（a）和累积输沙率（b）随时间的变化
Fig. 4 Temporal variation of the sediment transport rate (a) and cumulative sediment transport rate (b) within unit-width
图 5 单宽输沙率EEMD分解重构结果
Fig. 5 The reconstructed intrinsic mode functions of the EEMD on unit-width sediment transport rate
图 6 东西方向（a）、南北方向（b）各分量显著性检验
Fig. 6 Significance test of each component in east-west direction (a) and north-south direction (b)
图 7 风速（a），东西方向（b）和南北方向（c）单宽输沙率的Hilbert谱，有效波高（d）和湍动能（e）随时间的变化
红色方框为风速增长期，绿色方框为风速衰减期
Fig. 7 Temporal variation of the wind speed (a), Hilbert spectrum of east-west (b) and north-south (c) unit-width sediment transport rate, significant wave height (d), and turbulent kinetic energy (e)
Red boxes indicate wind increase periods and green for wind decrease periods
图 8 东西方向（a）和南北方向（b）单宽输沙率的边际谱
Fig. 8 The marginal spectrum of the east-west (a) and north-south (b) sediment transport within unit-width
图 9 东西方向（a，b）和南北方向（c，d）单宽输沙率与风速的交叉小波功率谱（a，c）和小波相干谱（b，d）
Fig. 9 The cross-wavelet power spectrum (a, c) and wavelet coherence spectrum (b, d) between east-west (a, b) and north-south (c, d) unit-width sediment transport rate and wind speed
图 11 潮流分量涨潮、落潮和全潮净输沙量
Fig. 11 The net sediment transport of tide current component during each flood tide, ebb tide and full tide
表 1 测量仪器设置
Tab. 1 Settings of observation instruments
测量仪器 距海底距离/
cm采样周期/
min采样频率/
Hz测量参数 TD-WAVE 102 5 16 波高、波周期、水深 ADCP 向上 180 20 8 流速、流向、温度、压强 ADCP 向下 92 20 8 流速、流向、温度、压强 ADV 34 30 16 流速、流向 OBS1 102 20 8 浊度 OBS2 28 20 8 浊度 
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表 2 单宽输沙率EEMD分解各IMF分量平均周期和方差贡献率
Tab. 2 The period and variance contribution rate of each IMF of the EEMD decomposition on unit-width sediment transport rate
IMF分量 东西方向
平均周期/h东西方向
方差贡献率/%南北方向
平均周期/h南北方向
方差贡献率/%IMF1 1.04 1.52 0.98 7.19 IMF2 2.70 3.54 2.51 4.80 IMF3 6.99 56.30 5.96 12.34 IMF4 12.40 26.27 12.60 22.69 IMF5 24.80 6.33 23.29 14.52 IMF6 45.22 0.67 38.43 3.99 IMF7 109.81 1.12 109.81 1.32 IMF8 192.17 0.46 334.73 7.36 IMF9 384.33 0.08 380.61 2.66 Res / 3.70 / 23.11
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