Wave-current bottom shear stresses and sediment re-suspension in the mouth bar of the Modaomen Estuary during the dry season

On the basis of the measurement data pertaining to waves, current, and sediment in February 2012 in the mouth bar of the Modaomen Estuary, the Soulsby formulae with an iterative method are applied to calculating bottom shear stresses （BSS） and their effect on a sediment resuspension. Swell induced BSS have been found to be the most important part of the BSS. In this study, the correlation coefficient between a wavecurrent shear stress and SSC is 0.86, and that between current shear stresses and SSC is only 0.40. The peaks of the SSC are consistent with the height and the BSS of the swell. The swell is the main mechanism for the sediment re-suspension, and the tidal current effect on sediment re-suspension is small. The peaks of the SSC are centered on the high tidal level, and the flood tide enhances the wave shear stresses and the SSC near the bottom. The critical shear stress for sediment re-suspension at the observation station is between 0.20 and 0.30 N/m2. Tidal currents are too weak to stir up the bottom sediment into the flow, but a WCI （wave-current interaction） is strong enough to re-suspend the coarse sediment.

Effects of the Anthropogenic Activities on the Morphological Evolution of the Modaomen Estuary, Pearl River Delta, China

Owing to the intensive human activities, the Modaomen Estuary has been significantly modified since 1950s, which has resulted in considerable changes of hydrodynamics and morphodynamics in the area. In this paper, the effects of the anthropogenic activities on the hydrodynamics and morphological evolution in the estuary at different stages are systematically assessed based on the detailed bathymetric data and field survey. The results show that the human activities have caused the channelization of the enclosed sea area in the Modamen Estuary;fast seaward movement of the mouth bar with high siltation;expansion of the channel volume due to channel deepening. The paper also highlights the main hydrodynamic changes in the estuary, including the rise of the water level;the distinguishing changes of tidal range before and after the 1990s （decrease and increase respectively）; as well as the increase of the divided flow ratio. It is found that reclamation is the main factor promoting the transition of nature of the estuary from runoff dominant to runoff and wave dominant, and sand mining activities are mainly to strengthen the tidal dynamic and to low the water level. The results provide useful guidance for better planning of the future developments in the estuary and further research in the area.

A Long-Term Numerical Model of Morphodynamic Evolution and Its Application to the Modaomen Estuary

Because of the influence of human activities, the evolution of the Modaomen Estuary is no longer a purely natural process. We used a long-term morphodynamic model （PRD-LTMM-10） to study the evolution of the estuary from 1977 to 1988. The model incorporated modules for riprap-siltation promotion and waterway dredging. The model can simulate the morphodynamic evolutionary processes occurring in the Modaomen Estuary during the period of interest. We were able to isolate the long-term influences of various human engineering activities and the roles of natural factors in estuarine evolution. The governance projects had the largest effect on the natural development of the estuary, resulting in larger siltation on the west side. Installation of riprap and reclamation of submerged land resulted in scouring of the main Hengzhou Channel causing deep trough out-shift. Severe siltation narrowed the upper end of the Longshiku Deep Trough.

Impacts of human activities on morphological evolution in the Modaomen Estuary, China

The morphology of the Modaomen Estuary(ME)has undergone drastic changes in recent decades,and quantifying the contribution of human activities and natural processes is crucial for estuary management.Using Landsat images,chart data,and hydrological and meteorological data,this study analyzed the evolution of the shoreline and subaqueous topography of the ME and attempted to quantify the extent of the contributions of human activities.The results show that local human activities dominated morphological evolution in some periods.From 1973 to 2003,the shoreline advanced rapidly seaward,resulting in approximately half of the water area being converted into land.Human activity is critical to this process,with the direct contribution of local land reclamation projects reaching more than 85%.After 2003,the shoreline remained relatively stable,probably due to a decrease in land reclamation projects.Regarding the evolution of subaqueous topography,the shoals in the estuary were heavily silted and gradually disappeared during 1983–2003,and the waterways narrowed and deepened.The average siltation rate decreased from 15.43 mm/a to-1.02 mm/a,indicating that the ME changed from sedimentation to slight erosion.By detecting variations of sediment load,we found that upstream human activities reduced river sediment,while downstream human activities significantly increased sediment input to the ME,leaving little change in the actual sediment input to the ME for a relatively long period.In addition,based on the empirical relationship between the sediment input and siltation rate,local human activities influenced the shift in the siltation state more than upstream and downstream human activities did.These findings suggest that more attention should be paid to local human activities to improve the estuarine management in the ME.

Dynamics of saltwater intrusion in the Modaomen Waterway of the Pearl River Estuary

Observations indicate an abnormal characteristic of saltwater intrusion in the upper Modaomen Waterway of the Pearl River Estuary,i.e.,the maximum salinity occurs during the neap tide or the coming moderate tide instead of during the spring tide.To explore the associated dynamic mechanisms,a high resolution three-dimensional numerical model was set up based on the Finite Volume Coastal Ocean Model(FVCOM),which covered the entire river network,the Pearl River Estuary,and the adjacent sea.Numerical experiments illustrated that the upper Modaomen Waterway is significantly influenced by the saltwater intrusion from the Hongwan Waterway,a narrow and shallow channel connecting the Modaomen Waterway to the sea.Specific topography,spring-neap tidal variation,local wind stress,and their interaction drive an up-estuary residual current in the Hongwan Waterway,which is much stronger during the neap tide than during the spring tide.As a result,more saltwater in the Hongwan Waterway is spilled over into the Modaomen Waterway during the neap tide or the coming moderate tide.This is the inherent dynamic mechanism why the saltwater intrusion in the upper Modaomen Waterway reaches its maximum during the neap tide or the coming moderate tide.Besides,we also found that the winter prevailing wind can pronouncedly enhance the saltwater intrusion in the Modaomen Waterway.

典型大洪水下珠江磨刀门河口拦门沙演变模式研究

磨刀门是珠江八大口门中输水、输沙量最大的口门,也是珠江最重要的泄洪通道,在珠江三角洲水系中占有极其重要的地位,研究其拦门沙冲淤演变模式对磨刀门河口地貌演变趋势分析及未来治理开发工程规划具有指导意义。以2005年6月典型大洪水过程为例,建立二维波流耦合的泥沙数学模型,分析磨刀门河口拦门沙冲淤演变特征。结果显示,典型大洪水条件下,上游来水来沙丰富,波浪对泥沙的转运作用相对被弱化,能够在较短时间内促进浅滩等沙体发育,形成新的地貌单元,并促进两侧沙体以沙嘴的形态快速向前淤进,伴随发育河口次级分汊地貌单元。研究表明,影响河口冲淤演变特征的因素包括河道地形、流场、余流余沙场及水流挟沙力等,上游来水来沙量、潮汐、波浪等因素则主导河口拦门沙演变模式。

西北江径流量变化对感潮河口咸潮上溯的影响

近年来,珠江三角洲地区咸潮灾害频发,区域供水安全受到威胁,快速、准确地预测咸潮上溯距离对潮汐影响河段取水口优化布置具有重要意义。基于盐度对流扩散方程,推求河口咸潮上溯距离与径流的理论关系并通过构建珠江口水动力-盐度模型,分析咸潮上溯对径流变化的响应特征,提出磨刀门咸潮上溯距离与径流响应模型,继而比较不同咸潮上溯半经验模型在相同水文条件下的应用差异性,评估不同预测模型在磨刀门的适用性。结果表明:在不同的流量动力组合下,珠江口咸界均呈现S形分布特征。咸潮上溯距离L与上游径流量Q在理论上呈现幂指函数关系,利用半经验模型预测磨刀门的咸潮上溯距离是可行的。Savenije模型和L-Q响应模型较好地考虑了实际地形和动力特征的影响,并且模型的计算精度和效率高,可实现磨刀门咸潮上溯距离的快速预测。

盐水楔河口絮凝体分布规律及控制因素——以珠江磨刀门为例

细颗粒泥沙絮凝是影响河口泥沙输移扩散的关键过程,受控于河口复杂动力结构,其中河口水体层化环境下絮凝体空间分布规律亟待探究。针对该问题,本文基于2020年珠江磨刀门河口枯季大面积走航原型观测数据,分析磨刀门河口絮凝体时空分布特征,探究不同动力对其影响作用,揭示水体层化下絮凝体分布规律。结果表明,观测期间磨刀门河口絮凝体中值粒径介于1.87~395.53μm,体积浓度介于20.29~1 495.67μL/L;垂向上,中表层水体中值粒径多大于底层,平面上最大值多位于中心拦门沙和西侧。采用多峰分解方法可将磨刀门河口絮凝体分解为基本颗粒和絮凝核(统称微絮团)、小絮团、大絮团等组分,大絮团占比最大;总体来讲,中表层水体微絮团和小絮团体积浓度小于底层,底层大絮团体积浓度小于中表层。这与盐水楔河口动力结构密切相关,强盐度层化抑制絮凝体在各水层间的交换,导致表层水体大絮团占比远高于中底层,而底层絮凝体受湍流剪切强度影响,以解絮为主,小絮团和微絮团占比高。本研究不仅有助阐明复杂动力结构下细颗粒泥沙絮凝机制,也能为磨刀门河口拦门沙治理、水沙调控及航道疏浚等提供技术支撑。

基于地貌信息熵的磨刀门河口拦门沙系统演变规律分析

河口拦门沙演变受高强度人类活动的影响呈现新局面。珠江磨刀门河口经历综合整治后发生重大变化,其拦门沙系统演变过程存在不确定性。基于磨刀门河口不同年代地形数据,采用数字高程模型和地貌信息熵等方法,量化分析拦门沙不同地貌单元演变特征,分析拦门沙系统水深-面积关系的变化过程,揭示其系统状态转变规律。结果表明:磨刀门河口拦门沙系统呈现典型滩槽结构,不同地貌单元表现出阶段性变化特征;拦门沙东汊系统地貌演变呈有序发展,近年来沉积动力结构不断变化,有序性继续增强,拦门沙西汊系统地貌结构演变复杂,一直处于自适应的无序发展状态,不确定性较大。

磨刀门咸潮上溯对河口拦门沙地形变化的响应研究

采用磨刀门河口拦门沙地形变化前后的2005年和2020年实测地形资料,基于FVCOM三维数值模型,对比分析了咸潮上溯对河口拦门沙地形变化的响应,并探讨了拦门沙萎缩对盐分输运的影响机制。结果表明:拦门沙萎缩导致磨刀门水道各取水口表、底层盐度差增大,水体层化程度增强,下游至上游层化增强程度逐渐减小,广昌泵站势能异常最大值增大25.2 J/m^(3),马角水闸、平岗泵站、竹洲头泵站分别增大8.3、2.9、0.2 J/m^(3);拦门沙萎缩后未改变磨刀门水道咸潮上溯规律,半月潮周期盐度峰值依然出现在小潮后的中潮期;拦门沙对咸潮上溯有较大阻碍作用,底层15 psu等盐度线最大上溯距离向陆推进5.1 km,对于取水更为重要的表层0.5 psu等盐度线最大上溯距离向陆推进2.9 km;稳定剪切盐通量显著增大是拦门沙萎缩后咸潮上溯加剧的主要原因,并叠加潮汐震荡盐通量增大的影响。

波流共同作用下磨刀门河口余流特征分析

磨刀门河口水动力条件复杂,波浪作用明显,波浪影响下磨刀门河口余流特征的研究较少。本文基于SCHISM模型,建立二维波流耦合数值模型,通过比较有波浪和无波浪影响的河口余流特征,分析波浪对余流的影响,并从余流角度定性分析交杯四沙演变特征。结果表明:磨刀门河口西汊余流动力较强,西侧龙屎窟水道出口处余流较小,四沙水域受波浪作用较明显,余流动力介于两者之间。四沙水域余流流向在波浪作用下发生较大改变;交杯四沙在西汊强劲余流以及波浪掀沙作用下,四沙脊线有向西顺时针偏转趋势。

基于浮标站观测的磨刀门洪枯季径潮动力作用规律及相关性分析

为揭示磨刀门主要动力因子对潮流运动的作用规律,采用主潮通量算法计算洪枯季半月不同时间尺度下的潮流动力特征值,采用多元线性回归法对特征值的相关性进行拟合分析。结果表明,磨刀门水道内平均落潮流速洪季比枯季总体大约14%,但涨潮平均流速仅为枯季一半;水道内洪季余流流速很大,枯季可忽略不计;口门外东汊和西汊涨潮流态洪季存在相互顶托,枯季均为西南向,洪枯季余流流速西汊均大于东汊,近底层水深范围存在逆时针半环流结构,且枯季尤为明显。潮汐动力和海面风对水道内余流影响较小,径流动力洪季影响显著、枯季影响很小。由于洪季径流动力强劲,东汊和西汊净潮通量变化规律一致;枯季磨刀门水道净通量主要输运至东汊水域,随沿岸流朝西南方向输运进入西汊,然后随离岸流朝南侧外海方向输出,符合口门外逆时针半环流结构的输运特征。

珠江口咸潮入侵分析与经验模型——以磨刀门水道为例

收集了2004—2006年珠江口磨刀门水道咸潮发生时测站（1-7）逐日定时观测的的含氯度、水位与流量数据，分析了各监测站含氯度与水位的日变化与年变化，导出了咸潮演变各过程中，含氯度与径流、潮流、河口地形等的关系式，建立了珠江口地区磨刀门水道咸潮入侵的经验模型。据此，模拟了2006年1月12日的磨刀门地区的咸潮入侵态势，经过和沿途各观测点验证发现与实测数据非常吻合。以含氯度等于250mg／L（饮用水的含氯度最大值）的点作为咸潮入侵的最远点，用简化修改后的盐度模拟模型计算了磨刀门咸潮入侵最大距离，并根据2006年1月12～20日的河口含氯度与最近的上游天河站的径流量实测数据计算出相应的咸潮入侵最大距离。研究表明，在河流枯水期（珠江河口通常是12月至翌年3月），只要获得当天河口的含氯度和上游测站的径流量数据，就能利用此经验模型估算出河流各点的含氯度，作出盐度模拟图，并估算出相应的咸潮入侵最大距离。

磨刀门水道咸潮入侵及其变异分析

随着区域经济及人口的进一步增长,珠江三角洲磨刀门水道已成为江门、中山、珠海、澳门等城市重要的水源地,相应咸潮入侵对供水安全造成的危害也越来越严重。本文依据实测资料和现场调研,对磨刀门水道咸潮入侵的潮相变化、径流—咸潮的响应模式、风对咸潮入侵的影响及咸潮入侵的历史演变趋势及其变异、最小压咸流量及最佳压咸时机等进行了研究。研究结果表明,影响咸潮入侵的主要因素是径流,有利于咸潮入侵的风向为北风～东北风。以思贤滘流量(即马口水文站与三水水文站流量之和)为参照,为保障珠海、澳门供水安全,磨刀门水道最小压咸流量范围为2200-2700m3/s,平均流量为2450m3/s,最佳压咸补淡的时机为大潮转小潮期,也就是朔望(半月)周期的落潮期。

磨刀门水道盐度混合层化机制

基于Simpson方法和磨刀门水道2009年枯季水文实测资料,选取上、下游两个站位的径流层化、潮汐混合、风致扰动3个影响河口水体分层的主要因素进行盐度混合的层化机制分析。研究表明:由于M1站位处上游,径流作用相对占优,分层不明显,只在涨潮急流时出现微弱的盐度分层;M2站则水体分层明显,小潮期间径流作用占主导,水体呈持续性分层,当由小潮转为中潮后,潮流作用增强,出现周期性分层现象,大潮以后,由于上游径流增加,潮流与径流作用相当,仍为周期性分层,但分层有所加强。层化的发育程度依赖径流致层化作用与潮汐、风致混合作用的博弈。

珠江口磨刀门盐水入侵的现状与成因分析

通过对珠江口磨刀门水域包括潮位、径流量、河道深泓、海表水温、盐度、气温、风速、风向、降雨等参数的长期基础历史资料的整理、转换与统计分析,探讨其盐水入侵加强的主要原因。结果表明:珠江口磨刀门盐水入侵受到潮位、径流、降雨、气温、风速的等诸多因素的综合影响,但最直接、最明显的影响因素是径潮相互作用。多因素的综合作用,使得珠江口磨刀门的盐水入侵活动规律复杂多变,给预报和治理工作带来一定的困难。

磨刀门拦门沙区域近期地貌演变和航道整治研究

应用近几年的水文和地形实测资料,研究了磨刀门拦门沙区域近期的动力和地貌演变特点。近期地貌演变的主要特点为:深槽外移扩展,一主一支分汊明显;拦门沙内坡冲刷外移,外坡变化不大,纵向宽度变窄;拦门沙西区淤积,拦门沙中心区和拦门沙东区冲刷;地貌轴线总体向南和西南方向偏转。从水动力和地貌演变的角度来看,磨刀门拦门沙区域航道整治应关注下列问题:1)注意河口性质的变化,着重解决径流输沙,波流共同输沙这两个主要问题;2)在工程布置中,应与口门外动力场的特点和地貌演变的总体趋势相适应,以建设双导堤为宜;3)西汊、东汊均是可选择的航线,从综合因素考虑宜选择西汊;4)注重河口分汊的自然规律,注意保护东支汊;5)注意稳定拦门沙中心区。

珠江口磨刀门枯季表层沉积物特征

根据2003年12月在珠江口磨刀门附近水域采集的河口表层沉积物粒度分析资料,研究了磨刀门河口表层沉积物的组成、分布、运动等特征及其动力环境指示作用。研究表明,磨刀门口枯季表层沉积物组成以粉砂为主,其次为粘土和砂。表层沉积物平均中值粒径为6.356(0.0122mm),表层沉积物的级配与悬沙的级配较接近。口门附近表层沉积物的中值粒径、分选性沿程没有明显的变化。枯季表层沉积物的形成以悬移质沉积为主,跃移和推移运动形式很少。磨刀门由于受人类活动的影响枯季表层沉积物与历史相比出现明显的细化,磨刀门口附近枯季表层沉积物的特征在一定程度上反映了枯季以潮流作用为主的相对较弱的动力环境。

枯季磨刀门水道咸潮活动与压咸控制分析

为进一步认识珠江河口咸潮活动规律和提供调水压咸有关控制参数,本文分析枯季磨刀门水道的盐度监测资料,发现咸潮半月周期内存在入侵期和退落期,在咸潮入侵强烈时上游河段一般的来流量(<4000m^3/s)难以保证取水要求,控制上游增加一定的来流量只有在咸潮退落期才具有压咸实际意义;用日最小盐度反映咸潮发展的不同阶段较有代表性,而日最低潮位是决定日最小盐度的主要因素。利用三灶站加权日最低潮位过程可初步判断咸潮入侵期与退落期,同时可利用该潮位过程曲线谷值所对应的时间预报控制上游加大流量的最佳时机起点。资料表明,这一方法比现有方法更符合实际。

近四十年西江磨刀门水道河床演变分析

将西江磨刀门水道1962、1977、1999年的河道地形图数字化,建立磨刀门水道三个年份的水下数字高程模型(DEM),在此基础上将河道分为上、中、下三段进行了河床演变及其成因分析。结果表明:磨刀门水道1977年前后河道发生过明显的河道冲淤转化;1962年至1977年磨刀门水道处于淤积状态,淤积量达9.92×106 m3,河道上游向窄深发展,下游则向宽浅演变;而从1977至1999年磨刀门水道则处于冲刷状态,冲刷量为25.53×106 m3,河道平均水深增加,逐渐向窄深演变。磨刀门水道河床演变是自然环境变化和人类活动共同作用的结果,其中人类活动的影响最为强烈。