The influence of episodic events on transport of striped bass eggs to the estuarine turbidity maximum nursery area

The estuarine turbidity maximum (ETM) is an important nursery area for anadromous fish where early-life stages can be retained in high prey concentrations and favorable salinities. Episodic freshwater flow and wind events could influence the transport of striped bass (Morone saxatilis) eggs to the ETM. This hypothesis was evaluated with regression analysis of observational data and with a coupled biological-physical model of a semi-idealized upper Chesapeake Bay driven by observed wind and freshwater flow. A particle-tracking model was constructed within a numerical circulation model (Princeton Ocean Model) to simulate the transport of fish eggs in a 3-dimensional flow field. Particles with the sinking speed of striped bass eggs were released up-estuary of the salt front in both 2-d event-scale and 60-d seasonal-scale scenarios. In event scenarios, egg-like particles with observed specific gravities (densities) of striped bass eggs were transported to the optimum ETM nursery area after 2 d, the striped bass egg-stage duration. Wind events and pulses in river discharge decreased the number of egg-like particles transported to the ETM area by 20.9% and 13.2%, respectively, compared to nonevent conditions. In seasonal scenarios, particle delivery to the ETM depended upon the timing of the release of egg-like particles. The number of particles transported to the ETM area decreased when particles were released before and during wind and river pulse events. Particle delivery to the ETM area was enhanced when the salt front was moving up-estuary after river pulse events and as base river flow receded over the spawning season. Model results suggest that the timing of striped bass spawning in relation to pulsed events may have a negative (before or during events) or positive (after river flow events) effect on egg transport. Spawning after river flow events may promote early-stage survival by taking advantage of improved transport, enhanced turbidity refuge, and elevated prey production that may occur after river pulse events. In multiple regression analysis of observed data, mean spring freshwater flow rates and the number of pulsed freshwater flow events during the striped bass spawning season explained 71% of the variability in striped bass juvenile abundance in upper Chesapeake Bay from 1986 to 2002. Positive parameter estimates for these effects support the hypothesis that pulsed freshwater flow events, coupled with spawning after the events, may enhance striped bass early-stage survival. Results suggest that episodic events may have an important role in controlling fish recruitment.     

A physical model for the transport and sorting of fine-grained sediment by turbidity currents

Turbidite muds in cores from the outer Scotian continental margin, off eastern Canada, contain abundant thin silt laminae. Graded laminated units are recognized in parts of this sequence. These represent single depositional events, and show a regular decrease in modal grain size and thickness of the silt laminae through the unit. A similar fining trend is shown by both silt and mud layers over hundreds of kilometres downslope. Textural analysis of individual laminae allows the construction of a dynamically consistent physical model for transport and sorting in muddy turbidity currents. Hydraulic sorting aggregates finer material to the top and tail regions of a large turbidity flow which then overspills its channel banks. Downslope lateral sorting occurs with preferential deposition of coarser silt grains and larger mud flocs. Depositional sorting by increased shear in the boundary layer separates clay flocs from silt grains and results in a regular mud/silt lamination. Estimates can be made of the physical parameters of the turbidity flows involved. They are a minimum of several hundreds of metres thick, have low concentrations (of the order of 10^?3 or 2500 mg 1^?1), and move downslope at velocities of 10-20 cm s^?1. A 5 mm thick, coarse silt lamina takes about 10 h to deposit, and the subsequent mud layer ‘blankets’ very rapidly over this. A complete unit is deposited in 2-6 days which is the time it takes for the turbidity flow to pass a particular point. These thick, dilute, low-velocity flows are significantly different from the ‘classical’ turbidity current. However, there is mounting evidence in support of the new concept from laboratory observations and direct field measurements.     

A model study of turbidity maxima in the York River estuary,Virginia

A three-dimensional numerical model is used to investigate the mechanisms that contribute to the formation of the turbidity maxima in the York River, Virginia (U.S.). The model reproduces the basic features in both salinity and total suspended sediments (TSS) fields for three different patterns. Both the prominent estuary turbidity maximum (ETM) and the newly discovered secondary turbidity maximum (STM) are simulated when river discharge is relatively low. At higher river inflow, the two turbidity maxima move closer to each other. During very high river discharge event, only the prominent turbidity maximum is simulated. Diagnostic model studies also suggest that bottom resuspension is an important source of TSS in both the ETM and the STM, and confirm the observed association between the turbidity maxima and the stratification patterns in the York River estuary. The ETM is usually located near the head of salt intrusion and the STM is often associated with a transition zone between upriver well mixed and downriver more stratified water columns. Analysis of the model results from the diagnostic studies indicates that the location of the ETM is well associated with the null point of bottom residual flow. Convergent bottom residual flow, as well as tidal asymmetry, is the most important mechanisms that contribute to the formation of the STM. the STM often exists in a region with landward decrease of bottom residual flow and net landward sediment flux due to tidal asymmetry. The channel depth of this region usually decreases sharply upriver. As channel depth decreases, vertical mixing increases and hence the water column is better mixed landward of the STM.     

浊流沉积研究综述和展望

浊流理论的建立具有划时代的意义。浊流沉积的研究已经进行了半个多世纪,从理论到实践都取得了巨大的进展。本文首先讨论了浊流及其相关的几个概念,同时概述了浊流沉积的国内外研究历史和进展,重点介绍了浊积岩的识别标志及其沉积序列,指出下一步研究的重点应放在陆相湖泊浊流沉积及其含矿性上。     

A numerical study of fronts in random media using a reactive solute transport model

We simulate the random front solutions of a nonlinear solute transport equation with spatial random coefficients modeling inhomogeneous sorption sites in porous media. The nonlinear sorption function is chosen to be Langmuir type, and the random coefficients are two independent stationary processes with fast decay of correlations. The model equation is in conservation form, and the random fronts are similar to random viscous shocks. We find that the average front speed is given by an ensemble averaged explicit Rankine–Hugoniot relation, and the front position fluctuates about its mean. Our numerical calculations show that the standard deviation is of the order O( $\sqrt t $ ) for large time, and the front fluctuation scaled by $\sqrt t $ converges to a Gaussian random variable wih mean zero. We come up with a formal theory of front fluctuation, yielding an explicit expression of the root t normalized front standard deviation in terms of the random media statistics. The theory agrees remarkably with the numerically discovered empirical formula.     

Simulation of Holocene cooling events in a coupled climate model

Three potential mechanisms behind centennial-scale Holocene cooling events are studied in simulations performed with the coupled climate model ECBilt–CLIO: (1) internal variability, (2) solar forcing, and (3) freshwater forcing. In experiments with constant preindustrial forcings, three centennial-scale cooling events occur spontaneously in 15,000 years. These rare events represent an unstable internal mode of variability that is characterised by a weaker thermohaline circulation, a more southward location of the main site of deep-water formation, expanded sea-ice cover and cooling of 10 °C over the Nordic Seas. This mode is visited more frequently when the climate is cooled by abruptly reducing the solar constant by 5 or 3 Wm⁻². Prescribing a solar forcing of the same magnitude, but following a sinusoidal function with a period of 100 or 1000 years, does not result in any centennial-scale cooling events. The latter forcing does however result in more frequent individual cold years in the North Atlantic region that are related to local weakening of the deep convection and sea-ice expansion. Adding realistic freshwater pulses to the Labrador Sea is also able to trigger centennial-scale cooling events with temperature anomalies resembling proxy evidence for the cooling event at 8.2 kyr BP, suggesting that freshwater forcing is a valid explanation for early Holocene cooling events.     

地下水溶质运移数值模拟模型参数的局部灵敏度研究

采用局部灵敏度分析方法计算天津市滨海新区南港工业区内某研究区含水层渗透系数、降水入渗补给系数、纵向弥散度、孔隙度对地下水溶质运移数值模拟模型模拟结果的影响。结果表明,含水层渗透系数、纵向弥散度及孔隙度灵敏度较高,对模拟结果影响较大,降水入渗补给系数对模拟结果影响很小。三个灵敏参数的灵敏度排序为孔隙度>纵向弥散度>渗透系数。     

Assessment of extreme weather events on transport networks: case study of the 2007 wildfires in Peloponnesus

This paper deals with the 2007 wildfires that hit Peloponnesus, the southern peninsula of Greece, presenting an overview of the impacts in terms of infrastructural damages and human injuries and losses. Network performance and components’ criticality analyses are used to assess the effects of the fires in vehicular traffic and the overall transport network. The crisis and emergency management of the event are discussed in depth, highlighting potential gaps and possibilities for future improvement. The paper concludes with a presentation of the adaptation measures that succeeded the event in terms of recovery plans, national efforts on fire prevention programs and wildfire management.     

Preparing pollen concentrates for AMS dating – a methodological study from a hard-water lake in southern Sweden

Dating pollen concentrated from sediment samples is a way to improve lake-sediment chronology. The predominantly terrestrial origin of pollen assemblages minimizes, for example, the reservoir effect inherent in bulk sediment samples, especially from hard-water lakes. Pollen can be concentrated for dating by a combination of sieving and chemical treatment (Brown et al . 1989). This study illustrates the difficulty in applying a single, standard procedure, and demonstrates the need for flexibility depending on lake sediment characteristics and the particular pollen flora. Samples taken at the Ulmus decline were prepared and AMS-dated following different steps of the pollen concentration procedure. The results showed that both sieving and chemical treatment were needed to obtain an age close to the expected age for the Ulmus decline. The pollen concentrate dated c . 1000 years younger than a bulk date from the same level, but is close to the expected age based on correlation with dates for the Ulmus decline from Sphagnum peat sequences in north-western Europe. A compilation of bulk dates at the Ulmus decline implies that pollen concentrates would be a better material for dating than bulk samples for many lake sediments, not only for those from hard-water lakes.     

The use of a paired comparison model in ordering stratigraphic events

Data from lowest and highest occurrence events in several stratigraphic sections are analyzed by means of a paired comparison model with ties. The model produces an estimated relative geochronological ordering of these events. This ordering must be compared with actual observations for revision and interpretation.     

Tidal sediment transport versus freshwater flood events in the Konkouré Estuary, Republic of Guinea

In comparison to their temperate counterparts, sediment processes in tropical estuaries are poorly known and especially in African ones. The hydrodynamics of such environments is controlled by a combination of multiple processes including morphology, salinity, mangrove vegetation, tidal processes, river discharge, settling and erosion of mud and by physico-chemical processes as well as sediment dynamics.The aim of this study is to understand the sediment processes in this transitional stage of the estuary when the balance between river discharges and marine processes is reversing. Studying the hydrodynamics and sediment dynamics of the Konkouré Estuary has recently been made possible thanks to new data on bathymetry, sedimentary cover, salinity, water elevations, and current velocities. The Lower Konkouré is a shallow, funnel shaped, mesotidal mangrove-fringed, tide-dominated estuary, well mixed during low river discharge and stratified during high river discharge. The Konkouré Estuary is turbid despite the small amount of terrestrial input and its residual velocity at the mouth during low river discharges, landwards for two of the three branches, suggests a landward migration by tidal pumping of the suspended particulate matter. A Turbidity Maximum Zone (TMZ) is identified for typical states of the estuary with regard to fluvial and tidal components. Suspended sediment transport during a transitional stage between the rainy and dry seasons is known thanks to current velocity and Suspended Sediment Concentration (SSC) measurements taken in November 2003. The Richardson layered number calculation assesses that turbulence is the major mixing process in the water column, at least during the flood and ebb stages, whereas stratification occurs during the slack water periods. Tidal currents generate bottom erosion, and turbulence mixes the suspended sediment throughout the water column. As a result, a net sediment input is calculated from the western Konkouré outlet for two consecutive tidal cycles. Despite the net water export, almost 300 tons per tide reach the estuary through this outlet, for a moderate river flow.     

基于CFP的岩溶管道流溶质运移数值模拟研究

多重岩溶含水介质的复杂性导致岩溶地下水流动及溶质运移的数学模拟成为地下水研究难点之一。为了探讨岩溶多重含水介质中地下水流溶质运移特征,文章构建了管道流CFP水流模型和MT3DMS溶质运移三维耦合数值模型。在阐述管道流CFP和MT3DMS基本原理的基础上,通过建立水文地质概念模型算例（1个落水洞、4个直管道）,探讨岩溶管道水流及溶质运移规律,分析讨论不同水文地质参数对浓度穿透曲线的影响。研究结果表明:管道流CFP模型能够刻画岩溶管道与基岩裂隙水流交换特征,MT3DMS模型能够模拟穿透曲线的拖尾现象,符合实际岩溶区特征。随着水力梯度、管道直径及管道渗透系数增大,孔隙度减小,浓度曲线峰值越大,峰值到达时间越快,浓度穿透曲线越对称。得出结论:耦合CFP水流模型和MT3DMS溶质运移模型能够刻画岩溶管道流溶质运移规律,为研究岩溶复杂介质污染物运移特征提供一种思路和途径。     

Response of unconfined turbidity current to normal‐fault topography

Turbidity currents descending the slopes of deep‐water extensional basins or passive continental margins commonly encounter normal‐fault escarpments, but such large‐magnitude phenomena are hydraulically difficult to replicate at small scale in the laboratory. This study uses advanced computational fluid dynamics numerical simulations to monitor the response of large, natural‐scale unconfined turbidity currents (100 m thick and 2000 m wide at the inlet gate) to normal‐fault topography with a maximum relief of nearly 300 m. For comparative purposes, the turbidity current is first released on a non‐faulted pristine slope of 1·5° (simulation model 1). The expanding and waxing flow bypasses the slope without recognizable deposition within the visibility limit of 8 vol.% sand grain packing. Similar flow is then released towards the tip (model 2) and towards the centre (model 3) of a normal‐fault escarpment. In both of these latter models, the sand carried by flow tends to be entrapped in four distinct depozones: an upslope near‐gate zone of flow abrupt expansion and self‐regulation; a flow‐transverse zone at the fault footwall edge; a flow‐transverse zone at the immediate hangingwall; and a similar transverse zone near the crest of the hangingwall counter‐slope, where some of the deposited sand also tends to be reshuffled to the previous zone by a secondary reverse underflow. The near‐bottom reverse flow appears to be generated on a counter‐slope of 1·1°, increased to 2·0° by deposition. The Kelvin–Helmholtz interface instability plays an important role by causing three‐dimensional fluctuations in the flow velocity magnitude and sediment concentration. The thick deposits of large single‐surge flows may thus show hydraulic fluctuations resembling those widely ascribed to hyperpycnal flows. The study indicates further that the turbiditic slope fans formed on such fault topographies are likely to be patchy and hence may differ considerably from the existing slope‐fan conceptual models when it comes to the spatial prediction of main sand depozones.     

Sub-Milankovitch and Milankovitch forcing on a model Mesozoic carbonate platform – the Latemar (Middle Triassic, Italy)

ABSTRACT Basic shallowing-upward cycles (shu-cycles) and five-bundled megacycles in the Latemar platform have been widely regarded as a model example for precessional and eccentricity forcing in the Mesozoic. Based on bio-/chronostratigraphic data, recent studies questioned this particular type of Milankovitch forcing on the Latemar cyclic series. We present an integrated model that incorporates (i) new cyclostratigraphic data, (ii) new and existing bio-/chronostratigraphic data and (iii) new spectral analyses. The basic shu-cycles in the Latemar reflect sub-Milankovitch control. Cycle bundles of 1 : 4–5 (megacycles : shu-cycles) indicate precessional forcing. They do not reflect eccentricity superimposed on precessional forcing. Spectral analyses reveal highly significant ratios in the large-scale cycle bundlings. Stacking patterns of 1 : 9.9 and 1 : 24.0 shu-cycles represent obliquity and short eccentricity forcing. Both sub-Milankovitch and Milankovitch forcing potentially controlled shallow subtidal carbonate deposition in Mesozoic greenhouse times. Cyclostratigraphic models require an integrated approach including bio- and chronostratigraphic data.     

Modelling flood events with a cumulant CO lattice Boltzmann shallow water model

Natural Hazards - In this work the development of a semiautomatic procedure based on the coupled use of a GIS subroutine and a two-dimensional hydraulic lattice Boltzmann model solving the shallow...     

侵蚀性降雨识别的模糊隶属度模型建立及应用

根据降雨及其引发土壤侵蚀是一个连续变化过程的特点,基于次降雨雨量(P)和最大30 min雨强(I30)建立了侵蚀性降雨识别的模糊隶属度模型.模型分为3个部分:①推求次降雨各物理参数的模糊隶属度;②将各物理参数的模糊隶属度有机结合起来,获得次降雨的模糊隶属度;③依据次降雨模糊隶属度对自然降雨事件进行判别,筛选出其中的侵蚀...