Sediment trapping mechanism by salinity stratification in a river-dominted estuary: A case study of the Modaomen Estuary in flood season
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摘要: 河控型河口盐度混合和层化是控制悬沙输移扩散的重要动力机制。以珠江磨刀门河口为研究对象,基于2017年洪季三船同步大、小潮水文泥沙观测数据,分析河控型河口水体盐度层化结构的时空变化对悬沙分布的影响机制。结果表明:受径潮动力耦合时空变化影响,河口盐度垂向分布表现出时空差异,即受径流主导的M1站(挂锭角),河口盐度在涨落潮周期内垂向混合均匀,受径潮控制的M2站(口门)在整个潮周期内盐度层化结构明显,口门外侧的M3站,潮动力作用较强,盐度垂向分布随涨落潮变化而变化;悬沙空间分布与盐度分布关系密切,盐度混合均匀利于悬沙垂向均匀分布,而盐度层化则使悬沙倾向于滞留在底层水体中,且在盐度层结界面之下出现高悬沙浓度,悬沙浓度垂向分布曲线呈L字型或抛线型,纵向上表现为高浓度悬沙团抑制在盐水楔前端,盐度层化对悬沙的捕集效应明显。通过对比水体标准化分层系数与水流垂向扩散强度系数发现,两者呈现负相关关系,即标准化分层系数愈大,垂向扩散强度愈小,表明水体层化抑制悬沙垂向扩散强度,而且水体层化程度越高,悬沙垂向扩散抑制程度越大,进而促进了河口水体盐度层化对悬沙捕集作用。本研究有助于揭示河口细颗粒泥沙运动机制及河口拦门沙演变机制,并为磨刀门河口拦门沙治理提供科学依据。Abstract: Salinity mixing and stratification in the river-dominated estuaries are important dynamic mechanisms for controlling transport and diffusion of suspended sediment. Based on the synchronous field investigation with three surveying vessels in the flood season in 2017, covering the spring and neap tidal cycles, the influence mechanism of salinity stratification on suspended sediment distribution in the Modaomen Estuary was analyzed in this study. Vertical distribution of salinity in the estuary also displayed spatial differences under the influence of interaction between riverine and tidal dynamics. The salinity at M1 Station (Guading Jiao), dominated by runoff, was mixed well vertically over the tidal cycles; salinity stratification occurred at the M2 Station (outlet location) and at M3 Station (outside the mouth), which were influenced by interaction between runoff and tide over the tidal cycles. Spatial distribution of suspended sediment was closely related to spatial distribution of salinity. In general, salinity mixing promoted the vertical mixing of suspended sediment, while salinity stratification constrained the suspended sediment to be concentrated in the bottom water layer, and high suspended sediment concentration (SSC) tended to appear at the layer where salinity stratified occur. The vertical distribution curve of SSC was L-shape or paracurve shape, while high SSC always concentrated in the front of the salinity wedge in the longitudinal direction, indicating a significant sediment trapping effect caused by salinity stratification. Comparing the stratification ratio and vertical diffusion coefficient at three gauging stations, there was a negative relationship between them, the larger stratification ratio is, the smaller vertical diffusion coefficient is, indicating the suppression effect of stratification on vertical diffusion. Furthermore, the higher stratification is, the larger suppression effect is. Such mechanism contributes to the sediment trapping caused by stratification. This study is helpful to reveal the mechanism of fine sediment movement and the mechanism of evolution of mouth bar in a complicated estuary, and provide scientific basis for regulation of mouth bar in the Modaomen Estuary.
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1. 引言
全球河流每年向海洋输送泥沙100×108~200×108 t,其中大部分入海泥沙在近岸沉积发育而成河口三角洲[1]。作为陆海物质交换重要的环节,河口细颗粒泥沙运动受控于径流、潮流及波浪等动力耦合作用[2-4],泥沙在河口复杂动力环境下,经历着悬浮、输移、絮凝沉降、再悬浮等过程,不仅塑造了河口多样的地貌形态,而且对河口污染物、营养盐等物质的扩散具有重要影响[5-6]。因此,研究河口复杂动力耦合环境下细颗粒泥沙分布规律和扩散机制对于河口治理、岸线防护、污染防控等具有重要的指导价值。
河口悬沙垂向扩散作用与水体紊动密切相关,而盐度层化可以影响河口水体紊动特性[7-9]。不同类型河口由于受不同径潮耦合作用的影响,河口盐淡水混合呈现出不同的状态,如高度层化、部分混合及充分混合[10],其盐度层化对河口悬沙扩散输移过程影响作用也存在差异。目前,一些学者开展了河口盐度层化对悬沙垂向分布的影响作用研究,如长江河口[11-13]、伶仃洋河口[5]、甬江河口[14]等,以上研究对象具潮汐作用较强的特征,且盐淡水混合程度表现出较均匀的现象。然而,河流控制型河口(即河控型河口)受径流作用控制,潮汐作用弱,河口盐淡水混合以水体高度层化为特征,纵向上盐水楔发育,对河口悬沙输移扩散影响作用及其机制存在差异,该方面研究涉及较少;而且目前多数河口盐度层化对悬沙扩散影响的研究也多局限在单点垂向潮周期变化[7, 10-12],缺乏纵向空间上的研究。
作为粤港澳大湾区主要的泄洪通道,磨刀门河口是珠江水沙重要的入海通道,经20世纪90年代口门大规模围垦工程后,河口位置向海推进约16 km[15],并由河口湾型河口转变为渠道化河口,其细颗粒泥沙在“径−潮−波”动力耦合作用下,塑造了复杂的拦门沙地貌形态[16-17]。洪季磨刀门河口属于河控型河口,受径流影响和拦门沙障碍作用,径潮动力耦合表现出典型的时空变化特征,河口盐度混合和层化时空亦复杂多变[18],对河口悬沙分布会产生重要影响。以往研究对磨刀门河口盐度层化机制[19-20]、河口泥沙运动[21]及地貌演变[16, 22]等进行了分析,但缺乏河口盐度层化时空变化对细颗粒泥沙扩散影响机制的研究。本文基于磨刀门河口洪季水文泥沙观测数据,开展河控型河口水体盐度层化对悬沙扩散影响机制研究,有助于揭示河口细颗粒泥沙运动机制及河口拦门沙发育演变机制,并为磨刀门河口拦门沙治理提供科学依据。
2. 研究区域
珠江由西江、北江和东江等支流组成,其中西江是珠江最大支流,其多年(1954–2015年)平均年径流量、年输沙量分别为2 173×108 m3、59.60×106 t,分别占珠江总量的77%、76%[23],且其季节变化明显,洪季(4−9月份)径流量和输沙量分别占全年的76.9%、94%[15]。珠江流经三角洲后经八大口门入海(图1),其中磨刀门河口则是西江水沙最主要的出海通道,分别占珠江八大口门径流量和输沙量的25%、29%[24];磨刀门河口潮汐属于不正规半日潮,日不等现象显著,其口门处潮差小,潮流弱,年均潮差为0.86 m,居八大口门之末,山潮比为5.78;磨刀门河口常浪向为ESE和SE向,年平均波高为1.01~1.32 m,平均周期为5.15~5.70 s,洪季波高和波周期明显小于枯季。总体来说,磨刀门河口属于径流−波浪型河口,但洪季径流量大,波浪相对较弱,受径流主导[25]。
3. 数据与方法
3.1 数据来源
基于洪季磨刀门河口径潮动力相互作用特征,于2017年7月31日至8月8日在磨刀门河口布设了3个站点,位置分布在挂锭角M1、口门M2和口外M3(图1b),开展小潮(Neap Tide,NT)、大潮(Spring Tide, ST)26 h连续水文泥沙同步观测。其中M1站采用ADP(Acoustic Doppler Profile)、OBS-3A(Optical BackScatter Sensor)进行锚式观测,M2、M3采用ADCP(Acoustic Doppler Current Profilers)、OBS-3A进行锚式观测,并放置三脚架观测波浪、近底水沙。由于ADCP在水体表层存在盲区,M2、M3站表层流速流向分别采用其三脚架上ADCP、AWAC(Acoustic Wave And Current)流速补充。整点时刻采用OBS垂向观测,并采集水样,在实验室采用过滤法测定水样的悬沙浓度。观测期间,上游西江马口站平均流量约为9 400 m3/s,接近西江2009–2017年洪季多年平均流量(约9 900 m3/s)。
3.2 研究方法
国内外学者提出了多种表征河口盐度层化程度的参数[10, 26-33]以判定各种类型河口水体的层化状态。为更好地探讨磨刀门河口层化状态对悬沙浓度空间分布的影响,本文对比选取总体Richardson数、分层系数、势能差异和标准化分层系数等参数表征河口盐度层化状态。
(1)Richardson数
当水平方向流速的变化梯度远远小于垂向变化梯度时,梯度(局部)Richardson数(Ri)计算公式[26]为
$$ {R_i} = - \frac{{{g}}}{\rho }\frac{{\dfrac{{\partial \rho }}{{\partial z}}}}{{{{\left( {\dfrac{{\partial u}}{{\partial z}}} \right)}^2} + {{\left( {\dfrac{{\partial v}}{{\partial z}}} \right)}^2}}}, $$ (1) 式中,u、v分别为水流流速的东、北分量;z为各层水体距底床高度;重力加速度取g=9.81 m/s2;ρ为海水密度,有如下计算公式[27]:
$${\rho _0} = {\rho _{\rm{w}}}(1 + \beta S),$$ (2) $${\rho _{{\rm{sw}}}} = {\rho _0} + \left( {1{\rm{ - }}\frac{{{\rho _{\rm{0}}}}}{{{\rho _{\rm{s}}}}}} \right)C, $$ (3) 式中,ρ0为仅考虑海水盐度时海水的密度;ρsw为同时考虑海水盐度和悬沙浓度时海水的密度,在计算标准化分层系数Sr和总体Richardson数时采用式(2)计算海水密度。ρw=1 000 kg/m3为纯水密度;ρw =2 650 kg/m3为悬沙密度;盐水收缩率取β=0.000 78;S为海水盐度。
梯度Richardson数考虑了浮力频率(倾向于抑制水体湍流并将湍流能力转化为势能)和平均流速剪切力(促进湍流发展)的影响,较全面地反映混合层化水体的局部稳定性。在此基础上,有学者[28-29]引入了总体Richardson数(Ri0)的概念用以判断整个水柱的层结稳定性,计算公式如下:
$${R_{i0}} = - \frac{{\Delta \rho gh}}{{\rho {{(\Delta U{\rm{)}}}^{\rm{2}}}}},$$ (4) 式中,
$\Delta \rho $ 和$\Delta U$ 分别为表、底层水体的密度和水平流速的差值;h为水深。根据线性稳定性理论,流体主动混合的临界值为Ri0 = 0.25,当Ri0 ≤ 0.25时发生混合[30]。在本研究中,将Ri0 > 0.25作为河口发生层化的条件。
(2)分层系数
分层系数(N)是简便表征盐度层化程度的参数之一,其计算公式[31]如下:
$$N = \frac{{S_b - S_s}}{{\overline S }},$$ (5) 式中,Sb、Ss分别为底层和表层水体的盐度;
$\overline S $ 为垂向平均盐度。N数值越大,表明层化程度越高。(3)势能差异和标准化分层系数
势能差异(φ)是量化水体层化的发生和消散的参数,定量指示将一单位体积的层化水柱完全混合所需做的功(单位:J/m3),其数值越大,表征水体层化程度越明显,其稳定度越强。根据文献[32-33],φ可以用以下公式计算:
$$\varphi = \frac{1}{h}\int_{ - h}^0 {\left( {\overline \rho - {\rho _i}} \right)} gz{\rm{d}}z,$$ (6) 式中,−h < z < 0;
$\overline \rho $ 为水体垂向平均密度;${\rho _i}$ 为各层水体密度。考虑到要消除水深对势能差异的影响,本文引入了无量纲化的势能差异,即标准化分层系数(Sr),计算公式为[10]
$$\begin{split} Sr =& \frac{\varphi }{{\overline \varphi }} \times 100{\text \%} = \frac{{\dfrac{1}{h}\displaystyle\int_{ - h}^0 {\left( {\overline \rho - {\rho _i}} \right)gz{\rm{d}}z} }}{{\dfrac{1}{2}\overline \rho gh}} \times 100{\text \%}\\ & = \frac{{\displaystyle\int_{ - h}^0 {\left( {\overline \rho - {\rho _i}} \right)gz{\rm{d}}z} }}{{\displaystyle\int_{ - h}^0 {{\rho _i}gz{\rm{d}}z} }} \times 100{\text \%}. \end{split}$$ (7) Sr反映出了混合水体所需的能量占总势能的比值,比值越大,表示层化程度越高,层化状态越不容易打破。
4. 结果
4.1 流速潮周期垂向变化规律
观测期间磨刀门河口3站流速如图2所示,可见表层流速基本大于底层流速,大部分时刻表底层流速差异大于25 cm/s;由于受强径流的抑制作用,涨潮时期流速相对较小,部分时刻小于10 cm/s,而落潮时期流速较大,其垂线平均流速均比涨潮时期大15~35 cm/s。M1、M2、M3站在小潮落潮时期的表层流速变化范围分别为42~70 cm/s、85~145 cm/s和61~128 cm/s,流速峰值均出现在小潮的第二个落急时刻(第21 h);大潮落潮时期的表层流速变化范围分别为47~100 cm/s、75~160 cm/s和75~151 cm/s,流速峰值均出现在大潮的第一个落急时刻(第5 h)。
4.2 盐度潮周期垂向变化规律
观测期间,磨刀门河口3个站盐度时空变化规律明显(图3)。M1站受径流控制明显,盐度最小,多数时刻表底层盐度均小于1,且垂向盐度分布较均匀,仅在大潮第3~5 h底层水体出现约11的盐度(图3b)。M2站位于口门位置,受径潮耦合作用影响,盐度变化幅度较大,小潮期间变化范围在0~28之间(图3c),大潮变化区间为0~30(图3d),且受羽状流扩散影响,多数时刻水体表层盐度较小,底层盐度较大,垂向盐度差较大,仅在大潮的落憩时刻前后(第7 h)表底层盐度垂向差异较小。M3站位于拦门沙外侧,受潮汐影响作用增强,水体盐度在3个中最高,小、大潮平均值分别为15、17,且盐度变化幅度大;受洪季径流作用影响,表层盐度小于底层盐度,且表底层盐度差也变化较大,垂向平均盐度差变化范围为0.1~29,其中小潮的第一次落潮(第5~8 h)期间和第二次涨憩(第17 h)前后(图3e)以及大潮的第一次落潮(第0~3 h)阶段(图3f)表底层盐度均在22~29之间,盐度垂向分布相对较均匀,其他时间段表底层盐度差异均较大。洪季磨刀门河口盐度时空变化反映了河口径潮相互作用,从口外愈向河口上游,径流优势愈明显,而且从河口3个盐度纵向变化来看,河口盐水楔发育显著(图3),势必会对水体悬沙垂向分布产生影响。
4.3 悬沙浓度潮周期垂向变化规律
磨刀门河口观测期间悬沙浓度分布如图4所示,M1站位于河口上游,悬沙浓度峰值最小,小、大潮垂向悬沙浓度变化范围分别为0.01~0.05 kg/m3、0.01~0.06 kg/m3,总体来说其悬沙浓度较大值出现在落潮时期,且垂向分布差异不明显,表底层悬沙浓度差值变化为0~0.04 kg/m3。M2站小、大潮垂向平均悬沙浓度为0.03 kg/m3,表层悬沙浓度较低,基本保持在0.02 kg/m3以下,底层悬沙浓度较高,部分时刻大于0.10 kg/m3,表底层悬沙浓度差显著,且底层高悬沙浓度多出现在涨急时刻附近,表明该处底沙悬浮现象显著。M3站悬沙浓度峰值最大,其中小潮期间的峰值约为0.45 kg/m3,大潮期间的峰值约为0.40 kg/m3,出现在小、大潮第一次落急时刻(图4e第7 h、图4f第5 h),表、中层悬沙浓度同时也达到较大值,垂向分布相对均匀,此外涨急时刻前后M3测点底层水体也出现了0.18~0.39 kg/m3的较大值,底沙再悬浮现象明显,但此刻表中层悬沙浓度小于0.05 kg/m3,垂向分布不均匀,其他时间段表层和中层水体悬沙浓度值基本在0.10 kg/m3以下。从磨刀门河口3个站位悬沙浓度变化来看,磨刀门水道悬沙源于径流输运,底床泥沙再悬浮作用相对较弱;而口门处和拦门沙外侧床沙再悬浮显著,致水体底层出现悬沙浓度峰值。
图 4 磨刀门河口M1、M2和M3站小潮(NT)和大潮(ST)期间悬沙浓度时间过程图(横轴0点为观测周期起点时刻,白色线为盐度等值线)Figure 4. Time series of suspended sediment concentration profiles at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during neap tide (NT) and spring tide (ST) (point 0 on the horizontal axis is the starting time of the observation period, the white lines indicate the salinity contour lines)5. 讨论
5.1 盐度层化潮周期变化
河口水体盐度层化状态一般用分层系数、Richardson数、势能差异等参数表征。依据磨刀门河口观测水文泥沙数据,分别计算3个站位小、大潮分层系数(N)、标准化分层系数(Sr)及总体Richardson数(Ri0),结果如图5所示。从各站层化参数大、小潮时间变化过程来看,3个参数变化规律大体一致,均能在一定程度上表征河口盐度层化状态,但3个参数时间变化过程也存在一些差异。其中总体Richardson数Ri0波动变化明显,但在某些时刻与水体盐度垂向分布不对应,如从M2站大潮期间盐度等值线分布判断,水体盐度混合现象明显,而从总体Richardson数Ri0的变化来看,其值在大潮整个观测周期内均达到层化临界值;同样,分层系数N也与盐度垂向分布存在一些不对应,如M1站在小潮的第19~22 h其表底层水体盐度数值均小于5,而在大潮的第0~5 h其表底层水体盐度数值均小于11,水体层化程度较小,但其分层系数N均大于4,远大于M2站和M3站的分层系数;标准化分层系数Sr的时间变化过程不仅与盐度垂向分布一致,而且可以反映和对比盐度层化的空间变化,如3个站水体盐度层化状态。因此,本文选取标准化分层系数Sr来表征磨刀门河口盐度分层状态,并取Sr > 0.1%为判断发生盐度层化的充分条件。
图 5 磨刀门河口M1、M2和M3站小潮(NT)和大潮(ST)期间分层系数(N,蓝色线)、标准化分层系数(Sr,黑色线)和总体Richardson数(Ri0,红色线)时间过程图横轴0点为观测周期起点时刻,虚线为3个参数的层化临界值:N=1、Sr=0.1%以及lg(Ri0 /0.25) =0,即Ri0 =0.25Figure 5. Time series of the stratification ratio (N, blue lines), the normalized stratification ratio (Sr, black lines) and the overall Richardson number (Ri0, red lines) profiles at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during neap tide (NT) and spring tide (ST)Point 0 on the horizontal axis is the starting time of the observation period. The dotted lines indicate the stratification critical values of three parameters: N=1, Sr=0.1%, and lg (Ri0/0.25) =0, i.e., Ri0 =0.25按照上述分析,本研究选取标准化分层系数Sr作为量化磨刀门河口盐度层化的参数。从3个站标准化分层系数Sr时空变化来看,M1站Sr值最小,小、大潮均值分别为–0.002%、0.008%,且变化幅度小,水体表现出混合均匀,仅在大潮的第一次落急(第3~4 h)出现较弱的盐度层化;M2站Sr值最大,小、大潮均值分别为0.34%、0.256%,且在小潮时全程大于0.23%,表明在小潮时期均出现显著层化现象,大潮时变化幅度为–0.002%~0.419%,大潮时除第7~11 h外,水体层化现象显著;M3站Sr值小、大潮均值分别为0.191%、0.271%,且大、小潮周期内变化幅度较大,分别为–0.015%~0.419%和–0.05%~0.349%,从时间变化来看,小潮落潮阶段(第3~8 h、17~20 h)、大潮落潮阶段(第0~7 h)盐度层化不明显,其他时刻盐度层化现象显著。从磨刀门河口盐度层化空间变化来看,受径潮相互作用空间差异影响,口门处层化现象最为显著,口门外侧次之,水道内受径流控制,水体混合均匀。
5.2 盐度层化对悬沙浓度分布影响
5.2.1 垂向变化
从图4可以看出,悬沙浓度分布与盐度等值线分布具有明显的相关性,即盐度层结抑制了悬沙垂向扩散作用,而盐度均匀混合利于悬沙垂向分布均匀。为了更好地分析盐度层化与垂向悬沙分布的关系,选取3个测站特征时刻盐度层化与垂向悬沙浓度关系分析,如图6所示。同时,为量化各站点在不同特征时刻悬沙浓度垂向分布状态,采用统计学中的离散系数(CV)来表征每个特征时刻各层水体悬沙浓度的离散程度及平均数的代表性,CV数值越大,则表示垂向悬沙浓度分布越不均匀,其计算公式如下:
$$CV = \frac{{\sqrt {\dfrac{1}{{n{\rm{ - }}1}}\displaystyle\sum\limits_{i = {\rm{1}}}^n {{{\left( {{C_i} - \overline C } \right)}^2}} } }}{{\overline C }},$$ (8) 式中,n为水体层数(本文中n=6);
$\overline C $ 为垂向平均悬沙浓度;Ci为第i层水体的悬沙浓度,其中1 ≤ i ≤ 6,结果如图7所示。图 7 磨刀门河口M1,M2和M3站小潮(NT)和大潮(ST)特征时刻各水层悬沙浓度离散系数(CV,蓝色线)和标准化分层系数(Sr,黑色线)Figure 7. Time series of the coefficient of variation of suspended sediment concentration (CV, blue lines) and the normalized stratification ratio (Sr, black lines) profiles at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during the characteristic moments of neap tide (NT) and spring tide (ST)对比图6和图7,可发现M1站标准化分层系数Sr在大、小潮期间始终保持较小的数值,同时其悬沙浓度离散系数CV始终保持在0.3以下,变化幅度较小,表明悬沙垂向分布较均匀,大部分时刻悬沙浓度垂向分布曲线表现为近似垂线型;当M2和M3站标准化分层系数Sr较大时,如小潮的0 h、15 h和大潮的11 h、22 h,悬沙浓度离散系数CV的数值也较大,表明悬沙垂向分布不均匀,悬沙浓度垂向分布曲线表现为L字型;当M2和M3站标准化分层系数Sr较小时,如小潮的7 h和大潮的7 h,其悬沙浓度离散系数CV的数值也较小,表明悬沙垂向分布较均匀,悬沙浓度垂向分布曲线表现为近似垂线型。总体上,各站点悬沙浓度离散系数CV的变化趋势与标准化分层系数Sr的变化趋势基本一致,充分映证了盐度层结抑制了悬沙垂向扩散作用,而盐度均匀混合利于悬沙垂向分布均匀这一规律。
5.2.2 纵向变化
河口盐度层化对悬沙分布的影响不仅表现在垂向扩散作用,还对其纵向输移扩散产生重要影响。基于3个测站特征时刻纵向盐度、悬沙浓度,采用三次样条插值方法生成磨刀门河口纵向盐度、悬沙浓度分布,如图8所示。从图中可以看出,磨刀门河口由于洪季受径流作用主导,盐水楔发育显著,且纵向上盐度层结抑制了悬沙纵向上的扩散作用,如M2和M3站在小潮和大潮观测期间的2次涨急时刻(图8a、8e和图8c'、8g')都处于盐度层化状态,床沙再悬浮后无法穿过中底层水体的盐淡水交界面,高浓度悬沙团局限在M2和M3站之间的底层水体中;小潮期间第一次落急(图8c)和大潮期间第一次落急(图8a')时M2站处于层化状态而M3站处于混合状态,悬沙在垂向和纵向上都难以穿越M2站附近的盐淡水交界面,因此在M2站处存在极大的悬沙浓度梯度,而悬沙在M3站能向上扩散,在垂向上只存在较小的悬沙浓度梯度,且当M3站的标准化分层系数越小,即混合程度越高时,其底层水体悬沙浓度数值越高,悬沙垂向扩散也越强烈,在纵向上M3站中底层水体悬沙浓度与M2站差值越大。而大潮期间的第二次涨憩(图8h'第24 h)为两次观测周期内涨潮作用最强时刻,整个M1−M3纵剖面上的悬沙分布较均匀,剖面表底层水体悬沙浓度差异小于0.02 kg/m3。
图 8 小潮(NT)和大潮(ST)特征时刻纵向盐度、悬沙浓度分布黄色线为盐度等值线,黑色线为标准化分层系数Sr,黑色箭头为流速,箭头方向向右为径流方向Figure 8. Longitudinal distribution of salinity and suspended sediment concentration during the characteristic moments of neap tide (NT) and spring tide (ST)The yellow lines are the contour lines of salinity. The black lines are the standardized stratification ratio Sr. The black arrows are the velocity, and the arrow to the right indicates the direction of runoff5.2.3 泥沙捕集机制
从磨刀门河口盐度与悬沙浓度空间分布规律来看,垂向上表现为水体出现盐度层化时悬沙浓度在盐淡水交界面附近出现拐点,垂向悬沙浓度分布曲线呈L字型或抛线型,在纵向上表现为出现水体层化时高浓度悬沙团总出现在盐水楔前端,河口盐度层化对悬沙的捕集效应明显。
河口水体层化对动力结构和悬沙垂向分布产生明显影响作用[7-8]。为了分析河口盐度层化与悬沙垂向扩散的关系,对比河口水体层化参数与水体垂向扩散强度的关系,其中水体垂向扩散强度可以由涡动扩散系数Kz来表示[34-35]
$$K{\textit{z}} = \kappa {\left( {1 + \alpha \cdot {R_i}} \right)^{{\rm{ - }}\beta }},$$ (9) 式中,卡门常数
$\kappa $ =0.4,经验系数α和β参考文献[34]分别取α=3.5、β=1。此处梯度Richardson数Ri基于垂向剖面上盐度梯度最大的两层水体数据并采用式(1)和式(3)计算。式(9)表明水体的垂向扩散强度与Ri数成负相关,而Ri数表征了局部水体的层结稳定性,当水体层化加强时,垂向扩散受到抑制,当水体层化较弱时,垂向扩散强度较大。此外,对比磨刀门河口3个站标准化分层系数Sr和扩散系数Kz的关系来看(图9),两者呈现明显的负相关关系,其中M1站盐度混合均匀,Sr值较小,而Kz数值较大,多数时刻达到0.4;M2、M3站Kz则随标准化分层系数Sr变化,且在Sr>0.1%时,Kz基本小于0.2,在Sr<0.1%>时,Kz有半数大于0.2,表明水体盐度层化抑制了悬沙的垂向扩散。而且,从以上分析结果来看,水体层化程度越高,水流垂向扩散抑制程度越大。由此可见,水体层化通过影响水流垂向扩散强度而使泥沙在层结之下水体聚集形成高悬沙浓度,这种机制对于河口最大浑浊带和拦门沙的发育演变等过程产生深刻影响。图 9 磨刀门河口M1,M2和M3站小潮(NT)、大潮(ST)标准化分层系数Sr和垂向扩散系数 Kz 的散点分布(蓝色圈)和线性拟合曲线(红色线)Figure 9. Scatter distribution (blue circle) and linear fitting curve (red line) of standardized stratification coefficient (Sr) and vertical diffusion coefficient (Kz) at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during the neap tide (NT) and spring tide (ST)6. 结论
基于2017年洪季磨刀门河口水文泥沙观测数据,分析河控型河口盐度层化时空变化对悬沙分布的影响机制,得到以下主要结论:
(1)磨刀门河口洪季受径流主导,径流以羽状流形式向海输移;受径潮动力耦合时空变化影响,受径流主导的M1站(挂锭角),盐度潮周期均垂向混合均匀;受径潮控制的M2站(口门)盐度层化潮周期均现象明显;潮动力作用增强的M3站(口外)盐度垂向分布随涨落潮变化,河口盐度垂向分布表现出空间差异;
(2)磨刀门河口盐度混合均匀利于悬沙垂向分布均匀,盐度层化对悬沙的捕集效应明显,垂向上表现为悬沙滞留在底层水体,在盐度层结处之下水体出现高悬沙浓度,悬沙浓度分布曲线呈L字型或抛线性,纵向上表现为高浓度悬沙团总出现在盐水楔前端;
(3)磨刀门河口3站标准化分层系数和水流垂向扩散系数呈负相关,表明水体盐度层化抑制了悬沙的垂向扩散;而且水体层化程度越高,水流垂向扩散抑制程度越大。
以上结果表明,河控型河口水体层化可以抑制水流垂向扩散强度而使泥沙在层结之下水体聚集形成高悬沙浓度,形成明显的悬沙捕集效应,这种机制对于河口最大浑浊带和拦门沙的发育演变等过程产生深刻影响。
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图 4 磨刀门河口M1、M2和M3站小潮(NT)和大潮(ST)期间悬沙浓度时间过程图(横轴0点为观测周期起点时刻,白色线为盐度等值线)
Fig. 4 Time series of suspended sediment concentration profiles at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during neap tide (NT) and spring tide (ST) (point 0 on the horizontal axis is the starting time of the observation period, the white lines indicate the salinity contour lines)
图 5 磨刀门河口M1、M2和M3站小潮(NT)和大潮(ST)期间分层系数(N,蓝色线)、标准化分层系数(Sr,黑色线)和总体Richardson数(Ri0,红色线)时间过程图
横轴0点为观测周期起点时刻,虚线为3个参数的层化临界值:N=1、Sr=0.1%以及lg(Ri0 /0.25) =0,即Ri0 =0.25
Fig. 5 Time series of the stratification ratio (N, blue lines), the normalized stratification ratio (Sr, black lines) and the overall Richardson number (Ri0, red lines) profiles at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during neap tide (NT) and spring tide (ST)
Point 0 on the horizontal axis is the starting time of the observation period. The dotted lines indicate the stratification critical values of three parameters: N=1, Sr=0.1%, and lg (Ri0/0.25) =0, i.e., Ri0 =0.25
图 7 磨刀门河口M1,M2和M3站小潮(NT)和大潮(ST)特征时刻各水层悬沙浓度离散系数(CV,蓝色线)和标准化分层系数(Sr,黑色线)
Fig. 7 Time series of the coefficient of variation of suspended sediment concentration (CV, blue lines) and the normalized stratification ratio (Sr, black lines) profiles at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during the characteristic moments of neap tide (NT) and spring tide (ST)
图 8 小潮(NT)和大潮(ST)特征时刻纵向盐度、悬沙浓度分布
黄色线为盐度等值线,黑色线为标准化分层系数Sr,黑色箭头为流速,箭头方向向右为径流方向
Fig. 8 Longitudinal distribution of salinity and suspended sediment concentration during the characteristic moments of neap tide (NT) and spring tide (ST)
The yellow lines are the contour lines of salinity. The black lines are the standardized stratification ratio Sr. The black arrows are the velocity, and the arrow to the right indicates the direction of runoff
图 9 磨刀门河口M1,M2和M3站小潮(NT)、大潮(ST)标准化分层系数Sr和垂向扩散系数 Kz 的散点分布(蓝色圈)和线性拟合曲线(红色线)
Fig. 9 Scatter distribution (blue circle) and linear fitting curve (red line) of standardized stratification coefficient (Sr) and vertical diffusion coefficient (Kz) at the M1 Station, M2 Station and M3 Station in the Modaomen Estuary during the neap tide (NT) and spring tide (ST)
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