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1.
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. 相似文献
2.
Physical, biological, and management responses to variable freshwater flow into the San Francisco Estuary 总被引：1，自引：0，他引：1
W. J. Kimmerer 《Estuaries and Coasts》2002,25(6):1275-1290
Freshwater flow is the principal cause of physical variability in estuaries and a focus of conflict in estuaries where a substantial fraction of the freshwater is diverted. Variation in freshwater flow can have many effects: inundation of flood plains, increase loading and advective transport of materials and organisms, dilution or mobilization of contaminants, compression of the estuarine salinity field and density gradient, increase in stratification, and decrease in residence time for water while increasing it for some particles and biota. In the San Francisco Estuary, freshwater flow is highly variable, and has been altered by shifts in seasonal patterns of river flow and increases in diversions from tidal and nontidal regions, entraining fish of several species of concern. Abundance or survival of several estuarine-dependent species also increases with freshwater outflow. These relationships to flow may be due to several potential mechanisms, each with its own locus and period of effectiveness, but no mechanism has been conclusively shown to underlie the flow relationship of any species. Several flow-based management actions were established in the mid-1990s, including a salinity standard based on these flow effects, as well as reductions in diversion pumping during critical periods for listed species of fish. The effectiveness of these actions has not been established. To make the salinity standard more effective and more applicable to future estuarine conditions will require investigation to determine the underlying mechanisms. Effects of entrainment at diversion facilities are more straightforward conceptually but difficult to quatify, and resolving these may require experimental manipulations of diversion flow. 相似文献
3.
ABSTRACT Laboratory observations regarding the limit conditions for particle entrainment into suspension are presented. A high‐speed video system was used to investigate conditions for the entrainment of sediment particles and glass beads lying over a smooth boundary as well as over a rough bed. The results extend experimental conditions of previous studies towards finer particle sizes. A criterion for the limit of entrainment into suspension is proposed which is a function of the ratio between the flow shear velocity and particle settling velocity. Observations indicate that particles totally immersed within the viscous sublayer can be entrained into suspension by the flow, which contradicts the conclusions of previous researchers. A theoretical analysis of the entrainment process within the viscous sublayer, based on force–balance considerations, is used to show that this phenomenon is related to turbulent flow events of high instantaneous values of the Reynolds stress, in agreement with previous observations. In the case of experiments with a rough bed, a hiding effect was observed, which tends to preclude the entrainment of particles finer than the roughness elements. This implies that, as the ratio between particle and roughness element sizes becomes smaller, progressively higher bed shear stresses are required to entrain particles into suspension. On the other hand, an overexposure effect was also observed, which indicates that a particle moving on a smooth bed is more prone to be entrained than the same particle moving on a bed formed by identical particles. 相似文献
4.
为了通过实验测量泥沙起动概率,基于封闭槽道内开展的泥沙起动实验,系统论述了基于高速摄影的起动概率测量方法所涉及的起动概率定义、相机采样频率、图像处理方法、起动比例与运动比例的关系等关键问题.根据泥沙在水流紊动猝发作用下起动的物理机制,定义实测起动概率为单个紊动猝发周期内,从一定面积床面上冲刷外移的泥沙比例.根据推移质运动概念模型,提出高速图片拍摄间隔应小于推移质运动的中间时间尺度,以捕获泥沙的每一次起动.在高速图像处理时,通过连续两帧图片之间的灰度差实现运动比例的无偏差提取.进一步分析表明,运动泥沙可处于起动、止动或滑翔状态,运动比例等于起动比例、止动比例和滑翔比例之和,起动比例与运动比例之比等于两张图片之间的时间间隔除以推移质运动的中间时间尺度.基于上述关键技术,将高速摄影测量方法用于泥沙起动概率的测量,结果表明,临界起动水流条件时,床沙处于少量起动现象,对应的临界起动概率与窦国仁所推导的2.28%接近. 相似文献
5.
A three-dimensional, intratidal sediment transport model is developed for the estuarine turbidity maximum (ETM) in the upper
Chesapeake Bay. The model considers three particle size classes, including the fine class mostly in suspension in the water
column, the medium class alternately suspended and deposited by tidal currents, and the coarse size suspended only during
the times of relatively high energy events. Based on the results of a box model, depth-limited erosion with continuous deposition
is employed for the medium and coarse classes by varying the critical shear stress for erosion as a function of eroded mass.
For the fine class, mutually exclusive erosion and deposition is employed with a small constant value for the critical shear
stresses for erosion and deposition to assure quick erosion of recently deposited fine particles but without allowing further
erosion of consolidated bed sediments. The model is run to simulate the annual condition in 1996, and the model generally
gives a reasonable reproduction of the observed characteristics of the ETM relative to the salt limit and tidal phase. The
model results for 1996 are analyzed to study the characteristics of the ETM along the main channel of the upper bay in intertidal
and intratidal time scales. Under a low flow condition, local erosion/deposition and bottom horizontal flux convergence are
the main processes responsible for the formation of the ETM, with the settling flux confining the ETM to the bottom water.
Under a high flow condition, a distinctive ETM is formed by strong convergence of the downstream flux of sediments eroded
from the upstream of the null zone and the upstream flux of sediments settled at the downstream of the null zone. Intratidal
variation of the ETM is mainly controlled by erosion and the tidal transport of eroded sediments for a low flow condition.
Under the direct influence of a high flow event, the ETM is mainly formed by erosion during ebbing tidal current strengthened
by large freshwater discharge and by convergence of ebbing freshwater discharge and flooding tidal current. During the rebounding
stage of a high flow event, intratidal variations are mainly controlled by tidal asymmetry caused by the interaction between
tidal currents, gravitational circulation, and stratification. 相似文献
6.
Physical and biological properties of the Chesapeake Bay estuarine turbidity maximum (ETM) region may influence retention and survival of anadromous white perch (Morone americana) and striped bass larvae (Morone saxatilis). To evaluate this hypothesis we collected data in five cruises, three during May 1998 and two during May 1999, in upper Chesapeake Bay. Time series of freshwater discharge, water temperature, wind, and water level explain differences in ETM location and properties between cruises and years. During high flows in 1998, a two-layer response to wind forcing shifted the ETM up-estuary, while a high discharge event resulted in a down-estuary shift in the salt front and ETM location. In 1999, extremely low discharge rates shifted the salt front 15 km up-estuary of its position in 1998. During 1999, the ETM was less intense and apparently topographically fixed. Gradients in depth-specific abundance of ichthyoplankton were compared with salinity and TSS concentrations along the channel axis of the upper Bay. During 1998, the high flow year, most striped bass eggs (75%) and most early-stage white perch larvae (80%) were located up-estuary of the salt front. In addition, most striped bass (91%) and white perch (67%) post-yolk-sac larvae were located within 10 km of maximum turbidity readings. Total abundance of white perch larvae was lower in 1999, a low freshwater flow year, than in 1998, a high flow year. In 1999, striped bass larvae were virtually absent. White perch (1977–1999) and striped bass (1968–1999) juvenile abundances were positively correlated with spring Susquehanna River discharge. The ETM regions is an important nursery area for white perch and striped bass larvae and life-history strategies of these species appear to insure transport to and within the ETM. We hypothesize that episodic wind and discharge events may modulate larval survival within years. Between years, differences in freshwater flow may influence striped bass and white perch survival and recruitment by controlling retention of egg and early-stage in the ETM region and by affecting the overlap of temperature/salinity zones preferred by later-stage larvae with elevated productivity in the ETM. 相似文献
7.
A turbidity maximum has been observed in the Kennebec estuary during mode rate and low flow conditions near the upstream limit of salinity intrusion. Hydrographic, ADCP, and transmissometer data were collected at different river flow levels and seasons during 1995–1998. The location of the tip of the salt intrusion changes dramatically and during high runoff may be flushed from the channel of the estuary along with the accumulated particles in the turbidity maximum. It is hypothesized that the estuarine turbidity maximum (ETM) was absent 18% of the time with occurrences in all seasons during 1993–1999 based on river flow volumes from the Kennebec and Androscoggin Rivers throughout the study period. When the flow is moderate and low, which occurred 73% of the time on average, a region of high turbidity can be found as far as 40 km upstream of the mouth. Suspended particulate loads are low in the ETM, on the order of tens of mg l−1 and may vary with the length of time that the ETM has been present. 相似文献
8.
Estimation of net physical transport and hydraulic residence times for a coastal plain estuary using box models 总被引：1，自引：0，他引：1
A box model based on salinity distributions and freshwater inflow measurements was developed and used to estimate net non-tidal physical circulation and hydraulic residence times for Patuxent River estuary, Maryland, a tributary estuary of Chesapeake Bay. The box model relaxes the usual assumption that salinity is at steady-state, an important improvement over previous box model studies, yet it remains simple enough to have broad appeal. Average monthly 2-dimensional net non-tidal circulation and residence times for 1986–1995 are estimated and related to river flow and salt water inflow as estimated by the box model. An important result is that advective exchange at the estuary mouth was not correlated with Patuxent River flow, most likely due to effects of offshore salinity changes in Chesapeake Bay. The median residence time for freshwater entering at the head of the estuary was 68 d and decreased hyperbolically with increasing river flow to 30 d during high flow. Estimates of residence times for down-estuary points of origin showed that, from the head of the estuary to its mouth, control of flushing changed from primarily river flow to other factors regulating the intensity of gravitational circulation. 相似文献
9.
PAUL D. KOMAR 《Sedimentology》1987,34(6):1165-1176
The concept of flow competence is generally employed to evaluate velocities and bed stresses of river floods from the sizes of the largest sediment particles transported. For the most part, this evaluation has been empirical, combining data from a number of separate flood events in different river systems. Those data are re-examined and compared with empirical equations for the selective entrainment of gravel from deposits of mixed sizes. It is found that the competence relationships trend counter to those obtained for selective entrainment, indicating that the competence evaluations are affected by varying degrees of selective size entrainment. Individual data sets which have been employed to establish the flow-competence relationships either show no trend on their own or yield a trend which runs counter to the competence equation, instead being more compatible with the selective-entrainment relationships. In most instances, the empirical competence equations greatly overestimate the hydraulics of flood flows, and it is suggested that the better established selective entrainment equations be used for competence evaluations as well. Empirical equations are available for this purpose, relating the dimensionless Shields entrainment function or the bed shear stress to the diameter of the largest grain moved and to the median diameter of the deposit as a whole. 相似文献
10.
Probabilistic analysis of landslide potential of an inclined uniform soil layer of infinite length: application 总被引：1，自引：0，他引：1
An extended probabilistic model that is a modification of the Chen et al. (2007,) model for evaluating the failure probability
of an inclined soil layer with an infinite length was developed in the present paper, and then applied to evaluate the occurrence
probability of landslide-related debris flow in Tungmen gully located in the eastern Taiwan, which occurred a devastating
debris flow in 1990. The statistical properties of hydrogeological parameters were collected and summarized, and then used
to evaluate the landslide-related debris-flow probabilities at various relative water depths for Tungmen gully by using the
probabilistic model. Under the assumption that the soil is saturated, the soil’s cohesion is negligible and the specific gravity
of the solid particles of soils is a constant, a simplified probabilistic critical slope equation for the stability of an
infinite slope of soils was also developed, and used to estimate the occurrence probability of debris flow. The result shows
that probabilistic landslide analysis for an infinite slope could provide a suitable approximation for the risk analysis of
debris flow mobilization at a given gully. 相似文献
11.
在明渠中开展不同水流条件下低强度均匀沙平衡输沙试验,基于灰度相减方法分析整个床面推移质运动特征,以探寻紊流结构与推移质运动之间的作用机理。结果表明:①受水槽横向方向水流强度分布特征影响,推移质运动概率从水槽中线到两侧壁逐渐变小,且基本呈对称分布;②在紊流相干结构作用下,推移质床面在紊流低速条带区形成凸槽,高速条带区形成凹槽,推移质运动概率沿水槽横向方向存在高低相间的带状分区;③随摩阻雷诺数增大,相邻两推移质运动高概率区域的间距值变化范围为0.13~0.24倍水槽宽度,其值随摩阻雷诺数增大而增大;④不同水流条件下,推移质运动高概率区域间距值约为水深的2倍,这与流向涡模型吻合,表明流向涡是诱导床面出现凹凸相间形态的重要因素。 相似文献
12.
Long-term observations in the Weser estuary (Germany) between 1983 and 1997 provide insight into the response of the estuarine turbidity maximum (ETM) under a wide range of conditions. In this estuary the turbidity zone is closely tied to the mixing zone, and the positions of the ETM and the mixing zone vary with runoff. The intratidal suspended particulate matter (SPM) concentrations vary due to deposition during slack water periods, subsequent resubsequent and depletion of temporarily-formed and spatially-limited deposits during the following ebb or flood, and subsequent transport by tidal currents. The corresponding time history of SPM concentrations is remarkably constant over the years. Spring tide SPM concentrations can be twice the neap tide concentrations or even larger. A hysteresis in SPM levels between the falling and rising spring-neap cycle is attributed to enhanced resuspension by the stronger spring tidal currents. There is evidence that the ETM is pushed up-estuary during times of higher mean water levels due to storms. During river floods the ETM is flushed towards the outer estuary. If river floods and their decreasing parts occur during times of relatively high mean water levels, the ETM seems to be maintained in the outer estuary. If river floods and their decreasing parts occur during times of relatively low mean water levels, the ETM seems to loose inventory and may need up to half a year of non-event conditions to gain its former magnitude. During this time seasonal effects may be involved. Analyses of storm events and river floods have revealed that the conditions in the seaward boundary region play an equally important role for the SPM dynamics as those arising from the river. 相似文献
13.
The Southwest Florida Water Management District has implemented a management approach for unimpounded rivers that limits withdrawals to a percentage of streamflow at the time of withdrawal. The natural flow regime of the contributing river is considered to be the baseline for assessing the effects of withdrawals. Development of the percent-of-flow approach has emphasized the interaction of freshwater inflow with the overlap of stationary and dynamic habitat components in tidal river zones of larger estuarine systems. Since the responses of key estuarine characteristics (e.g., isohaline locations, residence times) to freshwater inflow are frequently nonlinear, the approach is designed to prevent impacts to estuarine resources during sensitive low-inflow periods and to allow water supplies to become gradually more uvailable as inflow increases. A high sensitivity to variation at low inflow extends to many invertebrates and fishes that move upstream and downstream in synchrony with inflow. Total numbers of estuarine-resident and estuarine-dependent organisms have been found to decrease during low-inflow periods, including mysids, grass shrimp, and juveniles of the bay anchovy and sand seatrout. The interaction of freshwater inflow with seasonal processes, such as phytoplankton production and the recruitment of fishes to the tidal-river nursery, indicates that withdrawal percentages during the springtime should be most restrictive. Ongoing efforts are oriented toward refining percentage withdrawal limits among seasons and flow ranges to account for shifts in the responsiveness of estuarine processes to reductions in freshwater inflow. 相似文献
14.
ABSTRACT Particles projecting from the bed of an alluvial channel distort the fluid stream to produce a distinctive pressure field. This has considerable significance for both the entrapment and entrainment of other particles and is a primary cause of the widespread occurrence of pebble clusters and boulder shadows. Lift and drag forces are determined on clustered hemispherical particles of varying size. In the wake of an obstructing particle both forces are shown to vary directly with particle separation in a linear fashion. On the stoss side of the cluster, drag is uniform regardless of the separation of the component particles, but lift is shown to increase when particle separation is small, so affecting stability. This mutual interference of neighbouring clustered bed particles is a vital consideration of incipient motion and is shown by field evidence to cause a wide range in transport stage for particles of similar size and shape. On average, 46% of clustered particles are entrained by flood flow compared to 87% of particles in open plane-beds. The influence of clusters is a major determinant of sedimentary sorting. 相似文献
15.
Robert E. Ulanowicz John M. Lindsay William C. Caplins Tibor T. Polgar 《Estuaries and Coasts》1982,5(1):57-67
Previous simulations of potential ichthyoplankton entrainment by power generating stations on the Potomac estuary have not included the influence of lateral transport in distributing eggs and larvae over the nursery area. Therefore, two-dimensional, vertically-averaged hydrodynamic and kinematic models of passive organism transport were developed to represent advective and dispersive processes near the proposed Douglas Point Nuclear Generating Station. Although the more refined model did not substantially alter the estimate of ichthyoplankton entrainment, it did reveal that lateral inhomogeneities in hydrodynamics could engender severalfold differences in entrainment probabilities on opposite sides of the estuary. Models of higher resolution and greater biological detail did not project greater total entrainment by the Douglas Point plant, because the volume of nontidal flow past the site was large in comparison to the proposed rate of cooling water withdrawal. 相似文献
16.
This paper presents results of a small scale study that utilized particle-tracking techniques to evaluate transport of river
water through an alluvial aquifer in a bank infiltration testing site in El Paso, Texas, USA. The particle-tracking survey
was used to better define filtration parameters. Several simulations were generated to allow visualization of the effects
of well placement and pumping rate on flow paths, travel time, the size of the pumping influence zone, and proportion of river-derived
water and groundwater mixing in the pumping well. Simulations indicate that migration of river water into the aquifer is generally
slow. Most water does not arrive at the well by the end of an 18-day pumping period at 0.54 m3/min pumping rate for a well located 18 m from the river. Forty-four percent of the water pumped from the well was river water.
The models provided important information needed to design appropriate sampling schedules for bank filtration practices and
ensured meeting adequate soil-retention times. The pumping rate has more effect on river water travel time than the location
of the pumping well from the river. The examples presented in this paper indicate that operating the pumping well at a doubled
distance from the river increased the time required for the water to travel to the well, but did not greatly change the capture
zone. 相似文献
17.
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. 相似文献
18.
19.
横流中多孔射流的流动特性是揭示其输移扩散机理的基础。利用粒子图像测速技术PIV测量了横向流动条件下单孔、两孔和四孔射流的速度场,对射流主体范围内、射流的迎流面和各个射流之间的流场进行了研究。实验研究表明,多个射流中的第1个射流迎流面卷吸强度明显大于单个射流情况;两个射流之间的区域可分为两个部分,上游部分水体被卷吸进入上一个射流,下游部分水体被卷吸进入下一个射流;后面射流迎流面的横向流速明显低于第1个射流迎流面的横向流速,揭示了导致后面射流弯曲的横向有效流速沿程发展变化过程;揭示了横向有效流速的沿程变化规律。 相似文献
20.
The influence of wind and river pulses on an estuarine turbidity maximum: Numerical studies and field observations in Chesapeake Bay 总被引：1，自引：0，他引：1
The effect of pulsed events on estuarine turbidity maxima (ETM) was investigated with the Princeton Ocean Model, a three-dimensional
hydrodynamic model. The theoretical model was adapted to a straight-channel estuary and enhanced with sediment transport,
erosion, deposition, and burial components. Wind and river pulse scenarios from the numerical model were compared to field
observations before and after river pulse and wind events in upper Chesapeake Bay. Numerical studies and field observations
demonstrated that the salt front and ETM had rapid and nonlinear responses to short-term pulses in river flow and wind. Although
increases and decreases in river flow caused down-estuary and up-estuary (respectively) movements of the salt front, the effect
of increased river flow was more pronounced than that of decreased river flow. Along-channel wind events also elicited non-linear
responses. The salt front moved in the opposite direction of wind stress, shifting up-estuary in response to down-estuary
winds and vice-versa.
Modeled pulsed events affected suspended sediment distributions by modifying the location of the salt front, near-bottom shear
stress, and the location of bottom sediment in relation to stratification within the salt front. Bottom sediment accumulated
near the convergent zone at the tip of the salt front, but lagged behind the rapid response of the salt front during wind
events. While increases in river flow and along-channel winds resulted in sediment transport down-estuary, only reductions
in river flow resulted in consistent up-estuary movement of bottom sediment. Model predictions suggest that wind and river
pulse events significantly influence salt front structure and circulation patterns, and have an important role in the transport
of sediment in upper estuaries. 相似文献