A reassessment of the role of tidal dispersion in estuaries and bays

The role of tidal dispersion is reassessed, based on a consideration of the relevant physical mechanisms, particularly those elucidated by numerical simulations of tide-induced dispersion. It appears that the principal influence of tidal currents on dispersion occurs at length scales of the tidal excursion and smaller; thus the effectiveness of tidal dispersion depends on the relative scale of the tidal excursion to the spacing between major bathymetric and shoreline features. In estuaries where the typical spacing of topographic features is less than the tidal excursion, tidal dispersion may contribute significantly to the overall flushing. In estuaries and embayments in which the typical spacing between major features is larger than the tidal excursion, the influence of tidal dispersion will be localized, and it will not markedly contribute to overall flushing. Tidal dispersion is most pronounced in regions of abrupt topographic changes such as headlands and inlets, where flow separation occurs. The strong strain rate in the region of flow separation tends to stretch patches of fluid into long filaments, which are subsequently rolled up and distorted by the transient eddy field. The dispersion process accomplished by the tides varies strongly as a function of position and tidal phase and thus does not lend itself to parameterization by an eddy diffusion coefficient.     

Analytical solution for salt intrusion in multiple-freshwater-source estuaries: application to Humen Estuary

Salt intrusion has some negative impact on the estuarine eco-environment as well as the water resource potential. The paper proposes an analytical model to describe salt intrusion in the estuaries with multiple freshwater sources. The impact of river discharge on the salinity distribution changes along the multiple-fresh-source estuaries, which is different from estuaries with single source of freshwater. Our analytical model is derived from the advection–dispersion equation for salinity while taking into account the hydrodynamic variation along the estuary. In this paper, we take the Humen Estuary, a strongly tide-dominated estuary with two major source of freshwater, as an example to illustrate the model. By testing against eight surveys over a complete spring-neap tidal cycle, the analytical model’s capacity to describe salt intrusion in the Humen Estuary is calibrated and validated. The results show that the analytical method can be used to compute the salinity distribution in the multiple-freshwater-source estuaries. In comparison with the field data in the Humen Estuary, the calculated results indicate that the salt intrusion process exhibits remarkable segmentation in the multiple-freshwater-source estuary, although the estuary’s inherent characteristic remains the same throughout the estuary. Moreover, by analyzing the multi-segmental features of the Humen Estuary, an efficient and effective model to predict the salt intrusion length of the Humen Estuary is presented and satisfactory results are obtained to illustrate its practical application.     

The calculation of estuarine turnover times using freshwater fraction and tidal prism models: A critical evaluation

Freshwater fraction and tidal prism models are simple methods for estimating the turnover time of estuarine water. The freshwater fraction method prominently features flushing by freshwater inflow and has sometimes been criticized because it appears not to include flushing by seawater, but this is accounted for implicitly because the average estuary salinity used in the calculation reflects all the processes that bring seawater into the estuary, including gravitational circulation and tidal processes. The model relies on measurable salinity differences among water masses and so must be used for estuaries with substantial freshwater inflow. Tidal prism models are based on flushing by flood tide inflow and ignore seawater inflow due to gravitational circulation. These models should only be applied to estuaries with weak or nonexistent gravitational circulation, which are generally those with little freshwater inflow. Using a framework that is less ambioguous and more directly applicable to the estimation of turnover times than those used previously, this paper critically examines the application of tidal prism models in well-mixed estuaries with complete tidal exchange, partial ebb return, or incomplete flood mixing and in partially mixed estuaries. Problems with self-consistency in earlier versions of these models also apply to the budgeting procedure used by the LOICZ (Land-Ocean Interactions in the Coastal Zone) program. Although freshwater fraction and tidal prism models are different approaches to estimating turnover times in systems with very different characteristics, consistent derivation shows that these models have much in common with each other and that they yield equivalent values that can be used to make comparisons across systems.     

Wave-forced resuspension of upper Chesapeake Bay muds

Moored instruments were used to make observations of near bottom currents, waves, temperature, salinity, and turbidity at shallow (3.5 m and 5.5 m depth) dredged sediment disposal sites in upper Chesapeake Bay during the winters of 1990 and 1991 to investigate time-varying characteristics of resuspension processes over extended periods. Resulting time series data show the variability of two components of the suspended sediment concentration field. Background suspended sediment concentrations varied inversely with salinity and in direct relation to Susquehanna River flow. Muddy bottom sediments were also resuspended locally by both tidal currents and wind-wave forcing, resulting in short-term increases and decreases in suspended concentration, with higher peak concentrations near the bottom. In both years, episodes of wave-forced resuspension dominated tidal resuspension on an individual event basis, exceeding most tidal resuspension peaks by a factor of 3 to 5. The winds that generated the waves responsible for the observed resuspension events were not optimal for wave generation, however. Application of a simple wind-wave model showed that much greater wave-forced resuspension than that observed might be generated under the proper conditions. The consolidated sediments investigated in 1990 were less susceptible to both tidal and wave-forced resuspension than the recently deposited sediments investigated in 1991. There was also some indication that wave-forced resuspension increased erodibility of the bottom sediments on a short-term basis. Wave-forced resuspension is implicated as an important part of sediment transport processes in much of Chesapeake Bay. Its role in deeper, narrower, and more tidally energetic estuaries is not as clear, and should be investigated on a case-by-case basis.     

Toward a unified theory of tidally-averaged estuarine salinity structure

Equations are developed for the tidally-averaged, width-averaged estuarine salinity and circulation in a rectangular estuary. Width and depth may vary along the length of the channel, as may coefficients of vertical turbulent mixing and along channel diffusion. The system is reduced to a single first-order, nonlinear, ordinary differential equation governing the section-averaged salinity. A technique for specifying the ocean boundary condition is given, and solutions are found by numerical integration. Under different assumptions for the diffusion it is possible to reproduce the few existing analytical solutions, in particular the Hansen and Rattray (1965) Central Regime solution, and Chatwin's (1976) solution. The mathematical framework allows easy comparison of the results of different channel geometries and mixing coefficients. Of particular interest is the along-channel distribution of the diffusive fraction of up-estuary salt flux. It is shown that the Hansen and Rattray solution is always diffusion-dominated near the mouth. A theory is presented for estimating the diffusion coefficient within a tidal excursion of the mouth. It is shown that the resulting rapid along-channel increase of diffusion may explain some observed patterns of salinity structure: a decrease in both stratification and along-channel salinity gradient near the mouth. The theory is applied to the Delaware Estuary and Northern San Francisco Bay, and shows reasonable agreement with observed sensitivities of salt intrusion distance to river flow.     

基于同伦分析法的非饱和土层固结问题的级数解

由于非饱和土的渗透系数是基质吸力的函数,使得控制方程带有强非线性的特征,进而使得控制方程的解析求解变得十分困难。同伦分析法对级数基函数和辅助线性算子的选择具有更大的自由性、灵活性,且收敛性的控制和调节更加容易实现,求解强非线性微分方程时在选择线性算子以及辅助参数上具有明显的优势。因此,针对非饱和土固结方程的非线性特征,对于处于地表浅层的非饱和土层,假设孔隙气压力为大气压力,在Richard经验公式与非饱和土一维固结理论的基础上,推导了非饱和一维固结无量纲控制方程;应用同伦分析法,通过选取适当的初始猜测解与辅助参数,将该非线性方程转换为线性的微分方程组并求解得到固结问题的级数解。此外,以压实高岭土为研究对象,在收集相关试验参数基础之上,将由同伦分析法求得的固结问题的近似解析解与有限差分法数值结果相对比,分析结果验证了解析解的正确性。     

Multispecies Mortality Patterns of Commercial Bivalves in Relation to Estuarine Salinity Fluctuation

Fluctuations in salinity may cause huge economic losses in estuaries with exploited commercial bivalves owing to their effect on mortality of these species. However, the same decrease in salinity does not affect all species in the same way, so it is interesting to study the effect of salinity from a multispecies standpoint. In the management of exploited bivalve beds, it is important to know the tolerance thresholds of the species, not only in cases of extremely low salinities but also over prolonged periods when salinities are low but not extreme. An analysis of mortality episodes of commercial bivalves in the Ulla River estuary (Galicia, NW Spain) from 1977 to 2009 revealed two mortality patterns related to how greatly the different species were affected. A mathematical model was designed to estimate salinity in the estuary based on weather conditions and tidal amplitude. By applying this model, it was possible to deduce the intensity and duration of the salinity decrease in the days prior to each mortality episode with the goal of relating these factors to mortality patterns. The two parameters found to be sufficient to explain the mortality observed were the minimum salinity at high tide and the number of consecutive days below a specific salinity threshold.     

The Effects of Wind,Runoff and Tides on Salinity in a Strongly Tidal Sub-estuary

Measurements over an annual cycle of longitudinal and vertical salinity distributions in a small sub-estuary, the Tavy Estuary, UK, are used to illustrate the dependence of salt intrusion and stratification on environmental variables. The interpretations are aided by vertical profiling and near-bed data recorded in the main channel and on the mudflats. Generally, high water (HW) salt intrusion at the bed is close to the tidal limit and is dominated by runoff and winds, with decreasing salt intrusion associated with increasing runoff and increasing up-estuary winds (or vice versa). Tidal effects are not statistically significant because of two compensating processes: the long tidal excursion, which is comparable with the sub-estuary length for all but the smallest neap tides, and the enhanced, near-bed, buoyancy-driven salt transport that occurs at small neap tides close to the limit of saline intrusion. The effect of wind on HW surface salt intrusion in the main channel is not statistically significant, partly because it is obscured by the opposing local and estuary-wide effects of an up-estuary or down-estuary wind stress. These processes are investigated using a simple tidal model that incorporates lateral, channel–mudflat bathymetry and reproduces, approximately, observed channel and mudflat velocities. Surface salinity at HW increases with tidal range because of enhanced spring-tide vertical mixing—a process that also reduces salinity stratification. Stratification increases with runoff because of increased buoyancy inputs and decreases with up-estuary winds because of reduced near-bed salt intrusion. Stratification and plume formation are interpreted in terms of the bulk and estuarine Richardson Numbers, and processes at the confluence of the sub-estuary and main estuary are described.     

Short-term water quality variability in two tropical estuaries,central Sumatra

We examined high frequency fluctuations in water quality parameters in two tropical coastal plain estuaries in response to changing tidal flow conditions. The variability in total suspended sediments (TSS), volatile suspended solids (VSS), total organic carbon (TOC) concentrations, and indicators of water quality, including pH, temperature, salinity, and dissolved oxygen, over one spring tidal cycle during the early wet monsoon season was measured in two estuaries in eastern Sumatra. The relatively high rainfall experienced throughout the year, in combination with the recent extensive vegetation clearing and modification of the landscape, resulted in significant concentrations of TSS, VSS, and TOC being discharged to coastal waters. Maximum values are reached on the ebb tide (TSS > 1,013 mg l⁻¹; VSS > 800 mg l⁻¹; TOC >60 mg l⁻¹). The influence of freshwater discharge and tidal flow on water properties of the lower estuaries is also marked by the variability in salinity, dissolved oxygen, and pH over the tidal cycle, with minimum values for each of these parameters following maximum current velocities and after the completion of the strong ebb tide. Estimation of seaward sediment fluxes, which are of significant interest in a region where rapid environmental change is occurring, would require further examination of sedimentary processes, such as resuspension and advection of sediment, as well as a consideration of neap-spring tidal variations and the effect of seasonality on estuarine circulation.     

Evidence of dynamics reversal in tropical estuaries, geomorphological and sedimentological consequences (Salum and Casamance Rivers, Senegal)

South Dakar Senegambian estuaries are subject to an unusual hydrodynamical regime caused by weak or absent run-off. In the Salum delta, each distributary lacks fresh water during most of the year. Only the tidal flows are responsible for geomorphological and sedimentological effects. The current distribution shows a net discharge upstream due to the extensive evaporation and evapotranspiration in mangrove swamps and tidal flats. Consequently the salinity is always higher towards the river than near the sea. A high salinity bottom layer suggests the occurrence of a supersaline wedge of reverse sense to the salt wedge of a normal estuary. Such an inverse pattern is similarly displayed by sedimentological features (double upstream turned spits) and by the external location of the turbidity maximum. A coherent reverse estuary model is suggested from our field observations.     

Transient hydrodynamic and salinity simulations in the Chesapeake Bay network

A transient network model is applied to the Chesapeake Bay and its tributary estuaries. Calibration of the model is based on only three external parameters: a friction factor that is spatially described, and two global constants required to calibrate a dynamic dispersion relationship that depends on both the local salinity gradient and hydraulic conditions. The transient hydrodynamics and the transient salinity distribution of the Bay and its tributary estuaries are simulated for the period of one month and comparisons made between calculated and observed salinities.     

Analysis of tidal marsh vegetation patterns in two Georgia estuaries using aerial photography and GIS

Aerial photographs and GIS analysis were used to map the distribution of tidal marsh vegetation along the salinity gradients of the estuaries of the Altamaha and Satilla Rivers in coastal Georgia. Vegetation maps were constructed from 1993 U.S. Geological Survey Digital Orthophoto Quarter Quads, 1∶77,000-scale color infrared photographs taken in 1974 and 1∶24,000-scale black and white photographs taken in 1953, Changes between years were identified using a GIS overlay analysis. Four vegetation classifications were identified and groundtruthed with field surveys: salt marsh (areas containing primarilySpartina alterniflora), brackish marsh (Spartina cynosuroides andS. alterniflora), Juncus (Juncus roemerianus), and fresh marsh (Zizania aquatica, Zizaniopsis miliacae, and others). There was no evidence for an upstream shift in marsh vegetation along the longitudinal axis of either estuary over the time frame of this analysis, which implies there has not been a long-term increase in salinity. Although the inland extent of each marsh zone was further upstream in the Satilla than the Altamaha, they corresponded to similar average high tide salinities in each estuary: areas classified as salt marsh occurred from the mouth up to where average high tide salinity in the water was approximately 15 psu;Juncus ranged from 21 to 1 psu; brackish marsh ranged from 15 to 1 psu; and fresh marsh was upstream of 1 psu. Approximately 63% of the 6,786 ha of tidal marsh vegetation mapped in the Altamaha and 75% of the 10,220 ha mapped in the Satilla remained the same in all 3 yr.Juncus was the dominant classification in the intermediate regions of both estuaries, and shifts between areas classified asJuncus and either brackish or salt marsh constituted the primary vegetation change between 1953 and 1993 (87% of the changes observed in the Altamaha and 95% of those in the Satilla). This analysis suggests that the broad distribution of tidal marsh vegetation along these two estuaries is driven by salinity, but that at the local scale these are dynamic systems with a larger number of factors affecting the frequently changing borders of vegetation patches.     

Expression of the Estuarine species minimum in littoral fish assemblages of the Lower Chesapeake Bay Tributaries

Species richness declines to a minimum (artenminimum) in the oligohaline reach of estuaries and other large bodies of brackish water. To date, observations of this feature in temperate estuaries have been largely restricted to benthic macroinvertebrates. Five years of seine data collected during the summers of 1990–1995 in the major tidal tributaries to the lower Chesapeake Bay were examined to see if this feature arose in estuarine fish assemblages. Estimates of numerical species richness (alpha diversity) and rates of species turnover between sites (beta diversity) were generated via rarefaction and detrended correspondence analysis. Two spatial attributes of the distribution of littoral fish species along salinity gradients in the tributaries of the lower Chesapeake Bay were revealed: (1) a species richness depression in salinities of 8–10% and (2) a peak in the rate of species turnover associated with the tidal freshwater interface (salinities of 0–2%). Expression of the minimum is influenced by the physical length of the salinity gradient and the interaction between a species’ salinity preferences and tendency to make long excursions from favorable habitats.     

Suspended-Sediment Flux and Retention in a Backwater Tidal Slough Complex near the Landward Boundary of an Estuary

Backwater tidal sloughs are commonly found at the landward boundary of estuaries. The Cache Slough complex is a backwater tidal region within the Upper Sacramento–San Joaquin Delta that includes two features that are relevant for resource managers: (1) relatively high abundance of the endangered fish, delta smelt (Hypomesus transpacificus), which prefers turbid water and (2) a recently flooded shallow island, Liberty Island, that is a prototype for habitat restoration. We characterized the turbidity around Liberty Island by measuring suspended-sediment flux at four locations from July 2008 through December 2010. An estuarine turbidity maximum in the backwater Cache Slough complex is created by tidal asymmetry, a limited tidal excursion, and wind-wave resuspension. During the study, there was a net export of sediment, though sediment accumulates within the region from landward tidal transport during the dry season. Sediment is continually resuspended by both wind waves and flood tide currents. The suspended-sediment mass oscillates within the region until winter freshwater flow pulses flush it seaward. The hydrodynamic characteristics within the backwater region such as low freshwater flow during the dry season, flood tide dominance, and a limited tidal excursion favor sediment retention.     

Numerical modeling of Persian Gulf salinity variations due to tidal effects

Numerical modeling of salinity changes in marine environment of Persian Gulf is investigated in this paper. Computer simulation of the problem is performed by the solution of a convection-diffusion equation for salinity concentration coupled with the hydrodynamics equations. The hydrodynamic equations consist of shallow water equations of continuity and motion in horizontal plane. The effects of rain and evaporations are considered in the continuity equation and the effects of bed slope and friction, as well as the Coriolis effects are considered in two equations of motion. The cell vertex finite volume method is applied for solving the governing equations on triangular unstructured meshes. Using unstructured meshes provides great flexibility for modeling the flow problems in arbitrary and complex geometries, such as Persian Gulf domain. The results of evaporation and Coriolis effects, as well as imposing river and tidal boundary conditions to the hydrodynamic model of Persian Gulf (considering variable topology rough bed) are compared with predictions of Admiralty Tide Table, which are obtained from the harmonic analysis. The performance of the developed computer model is demonstrated by simulation of salinity changes due to inflow effects and diffusion effects as well as computed currents.     

On the evaluation of sediment transport models in tidal environments

The application of numerical models for the simulation of coastal hydro-and sediment dynamics requires model verification, calibration and validation with field data. Yet, no commonly accepted rules for the evaluation of sediment transport models exist. This paper discusses the significance of statistical parameters and their limitations considering common time lags in tidal environments. It is shown that the occasionally used discrepancy ratio lacks quantitative and qualitative information on model performance, as the time context information on time series characteristics is lost. As an initial measure of association, the simple linear correlation coefficient r² is proposed. To account for time lag errors in suspended transport models, a separate cross-correlation analysis for the flood and ebb tidal phase is proposed. For a comparison with other model applications, a concluding rating of model performance can be expressed by a dimensionless error definition which takes into account the quality of field data.     

Transverse seismic kinematics of single piles by a modified Vlasov model

Within the framework of soil–pile interaction, a novel displacement scheme for the transverse kinematic response of single piles to vertically propagating S waves is proposed on the basis of the modified Vlasov foundation model. The displacement model contains a displacement function along the pile axis and an attenuation function along the radial direction. The governing equations and boundary conditions of the two undetermined functions are obtained in a coupled form by using Hamilton's principle. An iterative algorithm is adopted to decouple and solve the two unknown functions. In light of the governing equation of the pile kinematics, a mechanical model is proposed to evaluate the present method on a physical basis considering material damping. The coefficient of the equivalent Winkler spring is derived explicitly as function of the displacement decay parameter γ and soil Poisson's ratio. A parametric study is performed to investigate the effects of the soil–pile system properties on the kinematic response of single piles. The results show that the dimensionless pile length controls the transverse kinematics of piles. In terms of the theory of beams on elastic foundation, the classification limits of the dimensionless pile length may be π ∕ 4 and π, respectively. Copyright © 2014 John Wiley & Sons, Ltd.     

滨海多层含水层系统中海潮引起的地下水位波动

通过对一个滨海多层含水层系统的考察,该含水层系统由上、下弱透水层和介于中间的承压含水层组成,海底露头处被淤泥层覆盖.建立了描述该系统中地下水水头随潮汐波动的数学模型,并得到其解析解.该解析解与六个基本参数有关,分别是承压含水层的海潮传播参数,淤泥层的无量纲透水系数,上下弱透层与承压含水层贮水率的比率 (无量纲)和上下弱透层的无量纲越流.当这些参数取某些特殊值时,我们的解便化简为前人考虑的几种简单情形对应的解.分析表明,承压含水层中地下水水头波幅是上、下弱透水层贮水率和越流系数的减函数,是淤泥层相对透水系数的增函数;波动相位(时滞)是上、下弱透水层贮水率和越流系数的增函数,是淤泥层相对透水系数的减函数.     

外壁恒温条件下冻结管壁热流密度变化规律数值计算研究

应用相似理论将冻结温度场方程和参数无量纲化,然后建立无量纲的数值计算模型,研究了在外表面恒温情况下冻结管壁的无量纲热流密度与各无量纲影响参数间的关系.结果表明:无量纲热流密度随未冻土与冻土的导热系数之比线性增加,与冻土与未冻土的容积比热之比成线性关系,随无量纲结冰潜热线性增长,与无量纲盐水温度成线性关系;可表示为无量纲时间的负三分之一次方的线性函数.最后对167706次数值计算结果进行了回归分析,得到具有较高精度的回归计算公式.     

利用岸边水头动态确定含水层在临海方向上的边界

本文建立了潮汐效率与越流共同作用条件下承压水头动态的数学模型.利用复变函数的基本原理,考虑到地下水流动的连续性,推导出了相应的承压水头动态公式.以此动态公式为基础,提出了直接利用承压水头的潮汐动态来确定延伸到海底的含水层长度(边界条件)的计算方法.最后结合广西北海市的地下水头长观资料,对上述方法进行了应用和分析研究.