Process-based 2DH morphodynamic modeling of tidal inlets: A comparison with empirical classifications and theories

The economical and ecological importance of tidal inlets has fostered the development of empirical tools for inlet management during the last century. This study aims at confronting these empirical theories with results obtained with a process-based numerical model, MORSYS2D. This 2DH morphodynamic modeling system is applied to an idealized tidal inlet/lagoon system with different combinations of significant wave height, tidal range and tidal prism. The numerical model predictions are compared to the empirical models of Hayes, Bruun, O'Brien and FitzGerald and to morphologies observed at natural tidal inlets. The results present good accordance with observations as well as with some key behaviors predicted with the empirical theories. The predicted morphologies satisfy the relation of O'Brien between the tidal prism and the cross-sectional area, the model reproduces the conceptual model of sand by-passing by ebb-tidal delta breaching of FitzGerald and the classifications of Hayes and Bruun are generally respected. However, some inconsistencies between model results and Hayes classification highlight the limitations of applying this classification, which only considers the yearly-averaged significant wave height and tidal range, to a single tidal inlet case.     

潮滩前缘盐沼植被簇团与潮沟系统演变相互作用研究

潮滩前缘盐沼植被簇团可以通过改变水动力及泥沙运动等过程影响潮沟系统的地形地貌,而潮沟系统的地形特征也会影响盐沼簇团的生长、扩张与侵蚀,但对盐沼簇团与潮沟系统地貌演变的相互作用机制尚缺乏认识。针对这一问题,本文构建了考虑盐沼植被动态演变的潮滩生物动力地貌耦合模型,模拟了盐沼植被簇团生长扩张与潮沟系统地貌演变过程,分析了不同初始数量的盐沼植被簇团与潮沟系统的空间格局及形态参数间的双向反馈。结果表明,潮沟先迅速向海陆两侧延伸,后发育出大量分汊;盐沼簇团向周边扩张后未被潮沟切割区域逐渐连成片。少量盐沼簇团能够增加潮沟密度,促进边缘冲刷式潮沟系统的发育。潮沟的走向受盐沼簇团分布位置及数量的影响,多个盐沼植被簇团间的水流集中比单个簇团的边缘水流冲刷更易形成潮沟。在盐沼植被簇团与潮沟系统共同发育初期,潮沟系统发育受盐沼植被簇团的促进作用较大,后期潮沟内比簇团边缘更易形成水流汇聚,盐沼簇团的影响逐渐由促进作用转为稳定作用。此外,潮沟的存在限制了盐沼植被的横向扩散,切割了盐沼植被簇团,影响盐沼植被的空间分布格局。本研究揭示了盐沼植被簇团与潮沟系统地貌耦合演化机制,可为盐沼潮滩生态系统保护修复提供科学依据。     

Improvement of morphodynamic modeling of tidal channel migration by nudging

State-of-the-art process-based models have shown to be applicable to the simulation and prediction of coastal morphodynamics. On annual to decadal temporal scales, these models may show limitations in reproducing complex natural morphological evolution patterns, such as the movement of bars and tidal channels, e.g. the observed decadal migration of the Medem Channel in the Elbe Estuary, German Bight. Here a morphodynamic model is shown to simulate the hydrodynamics and sediment budgets of the domain to some extent, but fails to adequately reproduce the pronounced channel migration, due to the insufficient implementation of bank erosion processes. In order to allow for long-term simulations of the domain, a nudging method has been introduced to update the model-predicted bathymetries with observations. The model-predicted bathymetry is nudged towards true states in annual time steps. Sensitivity analysis of a user-defined correlation length scale, for the definition of the background error covariance matrix during the nudging procedure, suggests that the optimal error correlation length is similar to the grid cell size, here 80–90 m. Additionally, spatially heterogeneous correlation lengths produce more realistic channel depths than do spatially homogeneous correlation lengths. Consecutive application of the nudging method compensates for the (stand-alone) model prediction errors and corrects the channel migration pattern, with a Brier skill score of 0.78. The proposed nudging method in this study serves as an analytical approach to update model predictions towards a predefined ‘true’ state for the spatiotemporal interpolation of incomplete morphological data in long-term simulations.     

Morphodynamic modelling of open-sea tidal channels eroded into a sandy seabed, with reference to the channel systems on the China coast

In this contribution, the morphodynamics of open-sea tidal channels eroded into sandy seabed in regions of flow constriction is simulated by a one-dimensional model using the Bagnold formula for bedload transport rate, and accounting for the effect of bed slope. The results show that equilibrium conditions for such channels can be reached over a period of 10²–10³ years. Sediment eroded from the channel floor is transported in the direction of the dominant current, and deposited beyond the regions of flow constriction where the current looses competence due to spreading. In this way, the material remobilized from older strata in the channels is deposited in younger sand banks near the channel heads. Where several successive channels are incised along the current axis, they interact in their morphological evolution. The morphodynamic equilibrium of a tidal channel is reached once the combined interacting sedimentological and hydrodynamic factors, such as sediment particle diameter, tidal current velocity and flood/ebb dominance, are balanced. The model output shows that the equilibrium shape of the tidal channels appears to be related mainly to flow field characteristics and, to a lesser extent, to particle size. A positive correlation exists between the depth of the channels and their response times. The equilibrium water depths of the channels are more sensitive to current speed than to either particle size or the time–velocity asymmetry of the flow field. The response times for overall morphological equilibrium are sensitive to all of the above-mentioned parameters. In particular, sediment characteristics associated with critical current velocities have far-reaching effects on the morphodynamic behaviour of tidal channels.     

长江口扁担沙动力地貌变化过程研究

河口浅滩不仅为人类提供宝贵湿地资源,而且是调控河势演变的重要因素。研究河口浅滩动力地貌演变规律对航道整治、湿地生态开发及岸堤防护等具有重要价值。本文利用最近150多年的长江口历史海图资料、实测水深与水文泥沙数据,分析长江口南支最大的浅滩—扁担沙动力地貌演变格局及其变化机制。结果表明：(1) 1860−2016年期间,扁担沙反复历经淤积−冲刷−淤积,浅滩由最初水下阴滩发育出露而形成纺锤状沙体,随后演变为细长扁担状,沙尾切滩成爪状沙体,下扁担沙则伴随爪状缝隙被不断填充而淤长;(2)自1954年洪水到目前,扁担沙−2 m、−5 m等深线包络的面积与体积整体上均呈现增长态势,其中面积年均增长率分别为0.88 km²/a和0.81 km²/a,体积年均增长率分别为1.3×10⁶ m³/a和5×10⁶ m³/a;扁担沙浅滩在不同时期冲淤变化不同,其中1998年出现大幅度冲刷,平均冲刷厚度达到1.4 m;(3)扁担沙体积变化和长江入海泥沙的增减无直接联系,但与入海径流量的变化密切相关;(4)白茆沙“南强北弱”的河势、南北港分流工程以及东风西沙水库的建立导致扁担沙向北推移。     

全新世以来珠江三角洲海鸥沙形成过程的地貌动力学分析

海鸥沙是珠江三角洲一个极富特色的沉积砂体。本文基于钻孔资料并结合长周期“动力-沉积-形态”模型, 从沉积学和地貌动力学角度对全新世以来海鸥沙的形成演变过程进行了探讨。海欧沙在全新世的沉积层序自下而上分别为河流相、河口湾浅海相和三角洲相。全新世海侵盛期以来, 虎门涨潮射流和东北—西南向涨落潮流是影响海鸥沙形成演变的主要动力。6000—2500a BP, 受东北—西南向涨落潮流的影响, 海鸥沙中北部地区一直处于冲刷无沉积状态, 由虎门涨潮射流带来的泥沙主要在海鸥沙南部沉积, 沉积速率约为0.67mm·a ^-1; 2500—1700a BP, 随着番禺平原的发育, 东北—西南向涨落潮流逐渐消弱, 海鸥沙进入一个快速沉积期, 平均沉积速率约为15mm·a ^-1, 沉积由两端向中间发展; 1700—600a BP, 随着沙湾水道的形成, 海鸥沙中部迅速发展, 至600a BP左右, 海鸥沙基本形成并出露水面。     

Modelled channel patterns in a schematized tidal inlet

Tidal inlets in the Dutch Wadden Sea show typical morphological features, i.e. westward oriented main inlet channel and ebb-tidal delta. The objective of this study is to find the governing physical processes of these morphological features. The study uses a 2DH process-based morphodynamic model (Delft3D) on a schematized model domain, with dimensions similar to the Ameland inlet in the Dutch Wadden Sea.Starting from a flat bed the models are forced by tides only. Short-term simulations are made to explore the hydrodynamic characteristics and initial sedimentation and erosion patterns. Long-term morphodynamic simulations are employed to investigate the governing parameters of the main inlet channel and the ebb-tidal delta evolution. Sensitivity of the evolution is described in terms of initial inlet width (1.0 km and 3.5 km), direction and asymmetry of tidal forcing (M₂, M₄), transport formulations (Van Rijn, 1993; Engelund and Hansen, 1967) and relative position of the tidal basin with respect to the inlet (East (existing), Middle, and West).The results tend to produce morphological features typical to the Ameland inlet. The direction of tidal forcing is the main governing parameter to the present orientation of the main inlet channel and the ebb-tidal delta. The model results generally prove the conceptual hypotheses that describe the orientation of the main inlet channel and the ebb-tidal delta.     

FVCOM 在龙口海域潮汐潮流模拟中的应用研究

利用不规则三角网格和有限体积方法的FVCOM(finite-volume coastal ocean model)模式,建立了高分辨率的龙口海域三维潮汐潮流数值模型,模式结果与实测结果吻合良好。根据计算水位和流速得到了精细的龙口海域M2,S2,K1,O1分潮的同潮图、潮汐潮流类型分布图和潮流椭圆分布图等。结果显示,龙口海域潮汐类型主要为不规则半日潮;屺姆岛以北海域多为往复流而龙口湾内同时存在旋转方向不同的旋转流;最大可能流速分布与岸线和等深线几乎平行,最大值出现在桑岛南侧的狭窄水道;该海域的潮汐余流极大值出现在屺姆岛的西侧,余流流速可达25 cm/s。该结论对于了解龙口海域的动力过程具有重要意义。     

老铁山水道潮流能初步估算

采用ECOMSED海洋数值模型较好地模拟了渤海的潮流运动状况,估算了老铁山水道的潮流能的理论蕴藏量,并采用FLUX方法对该水道的技术可开发量进行了评估.结果表明,老铁山水道北侧近岸海域最大可能流速约2.5 m/s,平均功率密度超过500W/m2,大潮平均最大功率密度为3 700w/m2,小潮平均最大功率密度为1400w...     

东大港与豆腐渣腰门交汇区潮流动力特征分析

对江苏省辐射沙洲海域东大港与豆腐渣水道交汇处5号站位连续两个潮次流速测量资料进行分析,对该交汇区域的潮流特征和潮流过程有了近一步认识,拟合给出了潮汐过程流速垂线分布表达式。     

西北太平洋的一种潮汐数值同化模型

利用FVCOM海洋数值模式,在球坐标系统下考虑非线性效应和天体引潮力的影响,基于非结构的三角形网格建立了包括中国近海、日本海、鄂霍次科海和部分西北太平洋海域的高分辨率海洋潮汐数值模型,并采用趋近法同化84个沿岸验潮站的观测资料。模拟结果与175个验潮站的实测结果拟合良好,M₂,S₂,K₁,O₁四个主要分潮振幅和迟角的绝对平均误差分别为4.0 cm和5.6°,2.4 cm和7.5°,2.6 cm和6.3°,1.5 cm和5.0°。依据调和分析结果给出了4个主要分潮的同潮图分布,得到8个半日分潮和5个全日分潮的无潮点,证实了宗谷海峡全日潮无潮点的存在,首次模拟得到津轻海峡的全日潮无潮点;还给出了整个计算海域内最大可能潮差和潮汐余水位的分布特征。     

柘林湾多口门潮汐汊道动力地貌的演变

潮汐汊道是连接内湾和外海、维持港口通航和水体交换的枢纽,其演变特征是整个澙湖、海湾地貌体系研究的基础。选取典型多汊型潮汐汊道系统——柘林湾为研究对象,以实测资料和历年海图、地形图为基础,结合SELFE(semi-implicit Eulerian-Lagrangian finite-element)模型,研究多汊型潮汐汊道水动力结构、泥沙运动及自然和人类活动影响下的演变特征。结果显示,20世纪70年代澄饶大联围之前,共享柘林湾125km2纳潮水域的5个汊道同时维持稳定;围垦工程后,西侧2个汇潮口门封闭,纳潮面积减少50%,导致湾内动力减弱,韩江来沙中断,湾外浅滩冲刷,泥沙向湾内输送,30多年来三百门水道显著淤积,小金门稳定,大金门冲淤调整,小门水道淤积不稳。围垦工程后小金门水道的纳潮面积变化甚少,水动力及输沙仍为落潮占优,汊道环流发育,汊道断面冲淤微调,维持稳定;大金门汊道所控制的纳潮面积减小1/3,口门涨、落潮流速不对称性减弱,汊道冲淤调整;小门水道纳潮面积剧减一半以上,汊道延长弯曲,大金门与小门水道的环流结构加剧了该汊道的淤积和不稳定。     

北海银滩响应台风作用的动力地貌过程

极端高能事件影响下的海滩动力地貌过程直接关乎岸滩稳定及海堤安全。本文基于2020年16号台风“浪卡”前后北海银滩顺直岸段的剖面高程及表层沉积物等资料, 探究了中等潮差的顺直响应台风作用的地貌变化机制。结果表明: 1) 台风后海滩地貌表现为冲流带的大型沙坝消失, 后滨沙丘-滩槽体系被削平, 滩面坡度趋于平缓。2) 银滩沉积物均以中砂、细砂和极细砂为主, 三者占比超过95%; 台风后, 高潮位带沉积物变细, 低潮位带沉积物变粗, 沙坝附近则出现粗细交错式变化; 银滩中高潮位带滩面响应台风过程的特征与强潮型海滩相似, 呈现消散型海滩的特征, 而低潮位带滩面则因大型沙坝的作用而呈现弱潮型海滩特征。3) 台风期间波浪是影响海滩的主要动力因素, 海滩地形影响了近岸水动力的表现形式, 从而改变了沉积物的粒径分布。     

Non-assimilated tidal modeling of the South China Sea

The tides in the South China Sea were simulated using an established tidal model, with the purpose to evaluate if non-assimilated modeling of the area is feasible. Simulations were done for the locally dominating diurnal (K₁) and semi-diurnal (M₂) tidal constituents, and the model was shown to provide reasonably accurate results in terms of both elevations and levels of dissipation. However, this was only the case when a realistic tidal conversion parameterization was included in the model, and it is suggested that tidal conversion is a missing process in other model efforts of the area. Compared to observations, the modeled dissipation levels were slightly overestimated when integrated over the entire domain, and far larger in the model at topography with a slope which is supercritical for the baroclinic tidal waves. A crude, empirical correction of the tidal conversion rates at supercritical topography is suggested and implemented in the model and shown to improve the model results in terms of both elevations and dissipation rates. It is concluded that the presented model set up is suitable for investigations of how perturbations, e.g., future sea-level rise, will affect the tidal dynamics in the South China Sea.     

Morphodynamic response of nearshore bars to a shoreface nourishment

The autonomous nearshore bar behaviour along the barrier island of Terschelling, The Netherlands, is characterised by the presence of net seaward cyclic migrating sand bars generated near the shoreline. In 1993, a perturbation of the cyclic bar system was introduced by the implementation of a 2 Mm³ shoreface nourishment supplied to the nearshore bar zone, filling up the trough between the middle and outer bar. The morphodynamic response of the nearshore bars to the nourishment perturbation is investigated using a bathymetric data set with an alongshore extent of 12 km and sampled for 10 years. Bar behaviour is quantified in terms of bar crest position in relation to morphometric parameters such as bar depth, height, width and volume. Along with a pronounced development of a three-dimensional bar system unseen in the autonomous behaviour, the nearshore bars exhibited a 6–7 year arrest in their migrational behaviour during which bar morphology remained stable at immediate pre-nourishment morphometric values. At the subsequent onset of bar movement, bars resumed their migration at a rate predicted by autonomous behaviour in parallel development with morphometric parameters along their predicted trends. It is shown that the observed onshore transport of nourished sediment in the 6–7 year arrest results from a gradual deepening of troughs. Cross-shore sediment transport modelling is used to assess the effect of the nourishment on yearly averaged onshore (short-wave nonlinearity) and offshore (undertow) sediment transport rates. The gradual reappearance of the pre-nourishment bar-trough morphology is shown to engender a normalisation in the cross-shore distribution of sediment transport rates to pre-nourishment rates.     

Process-based modeling of morphodynamics of a tidal inlet system

The morphodynamic evolution of an idealized inlet system is investigated using a 2-D depthaveraged process-based model,incorporating the hydrodynamic equations,Englund-Hansen’s sediment transport formula and the mass conservation equation.The model has a fixed geometry,impermeable boundaries and uniform sediment grain size,and driven by shore-parallel tidal elevations.The results show that the model reproduces major elements of the inlet system,i.e.,flood and ebb tidal deltas,inlet channel.Equilibrium is reached after several years when the residual transport gradually decreases and eventually diminishes.At equilibrium,the flow field characteristics and morphological patterns agree with the schematized models proposed by O’Brien (1969) and Hayes (1980).The modeled minimum cross-sectional entrance area of the tidal inlet system is comparable with that calculated with the statistical P-A relationship for tidal inlets along the East China Sea coast.The morphological evolution of the inlet system is controlled by a negative feedback between hydrodynamics,sediment transport and bathymetric changes.The evolution rates decrease exponentially with time,i.e.,the system develops rapidly at an early stage while it slows down at later stages.Temporal changes in hydrodynamics occur in the system;for example,the flood velocity decreases while its duration increases,which weakens the flood domination patterns.The formation of the multi-channel system in the tidal basin can be divided into two stages;at the first stage the flood delta is formed and the water depth is reduced,and at the second stage the flood is dissected by a number of tidal channels in which the water depth increases in response to tidal scour.     

Methods for accurate estimation of net discharge in a tidal channel

Accurate estimates of net residual discharge in tidally affected rivers and estuaries are possible because of recently developed ultrasonic discharge measurement techniques. Ultrasonic measurement methods consist of: 1) the use of ultrasonic instruments for the measurement of a representative “index” velocity used for in situ estimation of mean water velocity and 2) the use of the acoustic Doppler current discharge measurement system to calibrate the index velocity measurement data. Methods used to calibrate (rate) the index velocity to the channel velocity measured using the Acoustic Doppler Current Profiler are the most critical factors affecting the accuracy of net discharge estimation. The index velocity first must be related to mean channel velocity and then used to calculate instantaneous channel discharge. Finally, discharge is low-pass filtered to remove the effects of the tides. An ultrasonic velocity meter discharge-measurement site in a tidally affected region of the Sacramento-San Joaquin Rivers was used to study the accuracy of the index velocity calibration procedure. Calibration data consisting of ultrasonic velocity meter index velocity and concurrent acoustic Doppler discharge measurement data were collected during three time periods. Two sets of data were collected during a spring tide (monthly maximum tidal current) and one of data collected during a neap tide (monthly minimum tidal current). The relative magnitude of instrumental errors, acoustic Doppler discharge measurement errors, and calibration errors were evaluated