Particle trapping in stratified estuaries: Application to observations

Estuarine turbidity maxima (ETM) retain suspended particulate matter (SPM) through advection, settling, aggregation, and nonlinearities in bed processes, but the relative importance of these processes varies strongly between systems. Observations from two strongly advective systems (the Columbia and Fraser Rivers) are used to investigate seasonal cycles of SPM retention and the effects of very high flows. Results for the Fraser and Columbia plus literature values for 13 other estuaries illustrate the applicability of scaling parameters and the response of ETM phenomena to a range of river flow (U _r ) levels and tidal forcing. The most efficient trapping (represented by Trapping EfficiencyE, the ratio of maximum ETM concentration to the source SPM concentration) occurs for low ratios of river flow to tidal current amplitude (U_T), represented by low values of the Supply number S_r.E in the Columbia is found to be maximal in a null zone where advection or tidal asymmetry (represented by Advection numberA) is weak(A ∼ 0). The ratio of aggregation to disaggregation (the Floc number Θ) is maximal on neap tides, while the ratio of erosion to deposition (the Erosion number P) is maximal on spring tides. The ratio of settling velocity to vertical mixing (Rouse numberP) is relatively constant in the Columbia ETM(P ∼ 0.7), because particle settling velocity and turbulence levels adjust together. Assuming that this result applies broadly, scaling variables and data are combined to express ETM properties in terms of the friction velocity (U_*),U _r , andU _T , allowing a considerable simplification of the parameters used to describe ETM.     

平行层状岩体的自重应力场

将平行层状复合岩体视为等效的模观各向同性体，推导出自重荷载下地应力的理论解，并分析了两个水平主应力受岩层倾角和岩体弹性参数的影响，结果表明，与各向同性岩体相比，水平主应力的变化范围增大了，并且两上水平主应力可能不再相等，而岩层倾角对水平主应力之比影响显著，当倾角从0－90度变化时，两个水平主应力能从相等到相差内倍，这也说明在地应力研究中，考察岩体的各向异性是十分必要的，所导出的自重应力场解答对地下工程问题具有一定的实用价值。     

Progressive failure in stratified and jointed rock mass

Summary This paper describes a mathematical model for analyzing progressive failure in a stratified and jointed rock mass dipping parallel to the slope face. The relationship between the necessary support forces and permissible displacement in the rock mass is discussed. An analytical expression is provided for the minimum required support force corresponding to a critical displacement. This expression is used to evaluate the potential of unstable failure propagation. The effect of time on progressive failure is considered by using a simple rheological model for joints. A case study, involving an excavation in a stratified medium, is used to compare model prediction with monitored performance.     

Violence and conservation: Beyond unintended consequences and unfortunate coincidences

While the relationship between violence and conservation has gained increasing attention in both academic and activist circles, official and public discourses often portray their entanglements as (unlucky) overlapping phenomena. In this article, we show how, under specific practices of state territorialization, conservation becomes both the means and reasons for violence. Based on ethnographic research in Colombia’s emblematic Tayrona National Natural Park, we detail how both the war on drugs and tourism promotion shape these state practices, and how they have translated into everyday, yet powerful, means of dispossession in the name of conservation. By analyzing the effects of the production of peasants as environmental predators, illegal occupants and collateral damage, we show how official conservation strategies have justified local communities’ political and material erasure, and how they have resulted in the destruction of their lived ecologies and the erosion of their livelihood strategies.     

Control of phytoplankton biomass in estuaries: A comparative analysis of microtidal and macrotidal estuaries

Macrotidal estuaries (mean tidal range >2m) generally exhibit a tolerance to pollution with nitrogen-containing nutrients despite high loadings originating from freshwater outflows. These systems, which are characterized by high tidal energy, generally exhibit lower levels of chlorophylla than systems with lower tidal energy. A comparative analysis of data from 40 microtidal and macrotidal estuaries shows that mean annual chlorophylla levels are significantly lower in systems with high tidal energy even when nitrogen concentrations are equal to nitrogen levels in the microtidal systems. Tidal range and associated processes (e.g., tidal mixing, current velocity, light penetration, and sediment resuspension) influence phytoplankton biomass in some estuaries.     

Surface fronts in estuaries: A review

Recent advances in the understanding of the physics of estuarine fronts (small-scale regions of significant changes in hydrographic variables) are reviewed. These fronts may be classified in three groups—tidal mixing fronts, plume fronts, and shear fronts—on the basis of the mechanism dominating the local rate of change in the horizontal density gradient. These classes are not independent, however, since it appears that both tidal mixing fronts and shear fronts can produce buoyancy-driven flow structures that propagate away from the generation region as plume fronts. Field observations of typical examples of each class are presented and laboratory experiments and theoretical models which isolate the important physical processes are summarized. Some important unresolved questions are identified and suggestions for further research are offered.     

Aquaculture in estuaries: Feast or famine?

Living resources in estuarine systems have been so intensively harvested that many fisheries are severely depleted at a time when demand for these resources is rising. In addition, multiple use demands on estuaries often conflict, leaving unresolved management questions. With the movement of the population to the coastal regions, conflicts among uses and users of estuarine resources are increasing. Aquaculture offers one means to reconcile the conflicting demands for use of the estuarine resource. Yet, the nation has few, if any new policy or management approaches to address these conflicts. Management on the level of the watershed is one effective tool that should be more effectively used. Involvement of the full range of stakeholders or user groups in policy setting and management options has brought remarkable success in a small number of cases where this approach has been used. The nation will need to develop other tools and approaches if estuaries are to be restored and protected as protected as planned in federal, regional, and state laws and initiatives.     

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.     

层状岩体三维可视化构模与数值模拟的集成研究

由于数据结构的差异,用于岩土工程三维可视化的网格与用于数值模拟的计算网格之间存在本质差异,如何通过两种网格的共享实现三维地质模型与数值模拟之间的集成是岩土工程建模的重要内容。在对层状岩体三维可视化网格与数值模拟网格的特点进行剖析的基础上,提出了基于松散模式的三维规则格网与FLAC3D基本元素之间的转化方法,从理论上实现了“可视”与“可算”的结合。将其应用于我国某海底隧道的选线研究中,证明了该方法的合理性和有效性。     

层状边坡各向异性岩体渗流-应力耦合模型及工程应用

岩体渗流-应力耦合作用的研究是国内岩体渗流领域研究的热点问题,该问题在层状边坡工程中尤为常见。考虑到层状边坡岩体普遍存在的各向异性特征及渗流-应力耦合问题,基于等效连续介质模型和Louis经验公式,建立了层状边坡各向异性岩体渗流-应力耦合模型,应用COMSOL多物理场耦合软件对模型进行了数值计算。结果表明：该模型能够反映层状边坡岩体的各向异性变形及地下水渗流的非均匀性和各向异性特征。依托抚顺西露天矿南帮工程实例,通过采用不同的模型进行对比分析,研究了该边坡E800剖面地下水渗流情况。结果表明：采用该模型计算的潜水面与实际情况吻合较好,显示了该模型在层状边坡工程中良好的应用前景。     

多尺度节理岩体力学特性的颗粒流分析

以白云鄂博露天铁矿东矿岩质高边坡为工程背景,结合现场地质调查、室内岩石和节理力学试验等数据,采用等效岩体技术,构建能充分反映节理分布特征的实验室、现场原位试验和工程尺度等多尺度等效岩体模型。通过对各类等效岩体模型进行单轴压缩试验,研究岩体单轴抗压强度、弹性模量等力学特性的尺寸效应和各向异性。研究表明：节理的存在使岩体表现出尺寸效应和各向异性,且随着尺寸的增大,这种特性基本呈逐渐减弱的趋势;研究区域岩体的表征单元体积、单轴抗压强度和弹性模量分别为20 m×10 m×10 m、1.46 MPa和3.91 GPa;岩体单轴抗压强度、弹性模量与轴向尺寸的关系,近似符合渐进式指数函数关系,且该函数能直观地给出工程尺度岩体的力学特性。     

Retention of riverine iron in estuaries

Retention of Fe flocs, resulting from the mixing of river water and seawater, was examined in three Maine estuaries. Riverine Fe was found to remain fairly conservative with salinity, implying that the process of floccufation does not necessarily remove Fe from water parcels. Laboratory experiments corroborated the field data by demonstrating that neither gravity nor suspended sediment were very effective in removing flocculated Fe from suspension. However, input of a tannery effluent did appear to result in scavenging of Fe from estuarine waters. Flocculated riverine Fe was found to increase considerably the Fe concentrations of estuarine bottom sediments, with the amount of iron per sediment specific surface area dependent on mean river flow entering an estuary. While no long term retention efficiencies could be calculated for these estuaries, it seems likely that a significant portion of flocculated riverine Fe escapes to shelf waters.     

Field sampling in estuaries: The relationship of scale to variability

The spatial/temporal scaling problem (i.e., fitting a given research question to the dimensions of variability of the study area) is particularly pronounced in highly variable systems such as estuaries. Long-term, multidisciplinary studies in the Apalachicola Bay system were used to evaluate variation of different physical, chemical, and biological factors. Specific limitations of weekly, monthly, and quarterly sampling intervals were directly related to the efficiency of the sampling gear, the range of variation in the study parameters, and specific biological features (motility, recruitment, natural history) of infaunal macroinvertebrates and epibenthic organisms. There are families of spatial and temporal scaling phenomena that should be considered when establishing a given field sampling program. The dimensions of variation change along spatial/temporal gradients of salinity, habitat complexity, and productivity and among different levels of biological organization. The limits of variation define the needed sampling effort for a given level of estimation. Without an adequate evaluation of such variation, representative samples cannot be taken; the resulting inadequate sampling effort often precludes reliable comparisons and robust generalization. There is a continuum of scaling dimensions (and sampling problems) that ranges from small-scale experimental approaches to system-wide analyses. Misapplication of such scaling estimates has led to overgeneralization of experimental results. Currently, there is widespread misapplication of combinations of unrelated, limited sampling efforts to broad-scale resource problems. The loss of valuable estuarine resources is favored by the lack of adequate scientific databases that are consistent with the dimensions of the individual study areas. Unless experimental studies and field sampling programs are scaled to the dimensions of the research problem and the study area in question there will be a continued proliferation of trivial studies at one end of the continuum and the progressive deterioration of estuarine resources at the other.     

The mechanism of iron removal in estuaries

A survey of U.S. east coast estuaries confirms that large-scale rapid removal of iron from river water is a general phenomenon during estuarine mixing. The river-borne ‘dissolved’ iron consists almost entirely of mixed iron oxide-organic matter colloids, of diameter less than 0.45 μm, stabilized by the dissolved organic matter. Precipitation occurs on mixing because the seawater cations neutralize the negatively charged iron-bearing colloids allowing flocculation. The process has been duplicated in laboratory experiments using both natural filtered and unfiltered river water and a synthetic colloidal goethite in 0.05 μm filtered water. The colloidal nature of the iron has been further confirmed by ultracentrifugation and ultrafiltration. A major consequence of the precipitation phenomena is to reduce the effective input of ‘dissolved’ iron to the ocean by about 90% of the primary river value, equivalent to a concentration of less than 1 μmol per liter of river water.     

Geographical signatures of middle Atlantic estuaries: Historical layers

Estuaries of the middle Atlantic region can be characterized and viewed broadly against the backdrop of their geomorphologic features. While geomorphology is literally at the base of every estuary, these features do not necessarily yield regional signatures. A conceptual model, with layering in time and space, is proposed as an alternative to simplistic geomorphologic characterization. Humans have altered virtually every physical, chemical, and biological feature of middle Atlantic estuaries. A basic model premise is that middle Atlantic estuaries have a base of fundamental geomorphology features. Layered, in GIS fashion, on this base are the estuaries' components: climate, nutrients, watershed soils and vegetation, producers, and consumers. These components have been so strongly influenced by humans in time and space that the signature is anthropogenic. As a consequence, best management practices, stock assessment, and restoration have replaced concepts such as ecosystem integrity and stability. The focus of the layered model is the Chesapeake Bay watershed, and although middle Atlantic estuaries differ along climatic and latitudinal gradients, all reflect the detrimental effects of a massive human presence. The ability or inability of middle Atlantic estuaries to absorb human perturbation over the last 10,000 years gives them their signatures. From the Hudson-Raritan to the Pamlico-Albemarle estuaries, we have made some progress in curbing our impacts. Nearly everything we do affects our estuaries, and our actions are proportional to the number of humans living in the watersheds. Continued population growth on our coasts and many years of abuse may be irreversible as our estuaries lose their ability to be self-regulating, biological systems.     

The dynamics of conservative mixing in estuaries

Mixing plots, in which a dissolved constituent is plotted against salinity or chlorinity, are commonly used to interpret conservative and non-conservative processes in estuarine systems. A bend in the resulting curve is generally interpreted as indicative of the reactive or non-conservative nature of the constituent or the presence of multiple sources or sinks within the estuary. This paper demonstrates analytically that bends in mixing curves may also result from temporal variations in end-member (river or ocean) constituent concentrations even for conservative constituents. A one-dimensional dispersion equation is used to calculate the distribution of salinity and a conservative constituent in a model estuary. Both straight and bent mixing curves are shown resulting simply from changing the variability of the river constituent concentration. For no variability the curve is straight. For variability with a period much less than the flushing time, the average curve for a general data set straight, whereas the curve for a synoptic data set is bent. For variability with a period greater than the flushing time a bent curve results. Since bent mixing curves can occur for conservative properties, the use of these curves for interpretation of estuarine processes must be undertaken with an understanding of the temporal variability of the river and ocean constituents and their relationship to the estuary mixing properties and flushing time.     

Geochemistry of tin in rivers and estuaries

On the basis of measurements from a large number of rivers from pristine and polluted regions, we estimate the riverine fluxes of tin to the oceans to be 0.76 × 10⁶molyr⁻¹ for the dissolved fraction and 300–600 × 10⁶ mol yr⁻¹ for the paniculate fraction. The paniculate flux agrees with the flux calculated from denudation rates. Estuaries were found not to have a large effect upon the transport of tin to the oceans. Evidence for the remobilization of tin was found in an estuary that is highly polluted with tin from mining and smelting activities. Monobutyltin was found to be present in polluted estuaries and is presumed to be a degradation product of tributyltin additives to antifouling paint.     

Electromagnetic measurements of tidal transports in estuaries

Measurement of the electric potential produced across a tidal stream is an inexpensive and relatively maintenance free means of monitoring volume flow. Comparison of the electric signal to transports determined from current meter records is used to calibrate the system. A volume flow of 100 m³ per sec will induce a potential near 1 mV. Salinity and temperature changes at the electrodes will induce potentials of 500 μV per ‰ and 350 μV per °C. Transport estimates may need to be corrected for such effects. Examples of measurements made at two locations, Great Bay, New Hampshire, and Lake Pontchartrain, Louisiana, illustrate that the method is capable of yielding volume flow measurements with an uncertainty of about 15%.     

基于质量守恒的示踪测速模型及实证

示踪测井技术能准确测定地下水渗透流速,传统方法适用于没有垂向流的测井。总结了前人的研究,结合工程实际引入函数vv(z)和vh(z),并根据示踪剂质量守恒的原理建立了基于质量守恒的示踪测速模型,考虑了测井中水流垂向运动的影响。最后利用模型计算了存在较强垂向流的小浪底反调节水库6#井的渗透流速,计算结果与水库渗漏勘探结果一致,说明模型的计算结果是正确的,能排除垂向流的干扰,模型有较强的实际应用价值。