Sediment Resuspension and Currents in Lake Manitoba

The physical resuspension of bottom sediment during periods of high winds is thought to be important for nutrient recycling in large shallow lakes. In order to study resuspension, prototype instrumentation designed to collect gram quantities of suspended sediment at 38.1-cm intervals in the water column was deployed in 4.2 m of water in the shallow southern basin of Lake Manitoba for 1-month periods in 1978 and 1981. At about 2.7 m depth a sharp discontinuity was observed in the vertical distribution of total mass of suspended sediment collected. The mean current speed varied linearly with depth above the discontinuity but was more uniform near bottom. Although the water column was not thermally stratified, drogues frequently displayed a two-layer structure in the currents with a high skew in direction between layers. The particle size distributions were similarly discontinuous in the vertical, with large quantities of sand in the upper layer samples and a high silt load in the lower. The unusual particle size distribution, which was explained in terms of the origin of resuspended sediments and subsequent transport by lake currents, suggests a decoupling of lake turbulence between the two layers during high winds.     

Currents and Temperatures in Green Bay,Lake Michigan

Current velocities and water temperatures were measured in the four main passages between Green Bay and Lake Michigan and at several sites within the bay during summer and fall 1977. Monthly resultant currents indicate there is anticlockwise circulation in the bay during dominant southwesterly wind and a reversal of this pattern during episodes of northeasterly wind. It is common for two layers to flow through the mouth of the bay in opposite directions during the stratified season. Cold hypolimnetic lake water entering through the mouth and extending far into the bay maintains stratification and promotes flushing. The effects of resonance of forced and free long wave disturbances are prominent in current records; these oscillations are coherent and in phase across the mouth.     

Soluble Element Concentrations and Complexation in Southeastern Lake Michigan

Water samples were collected from the epilimnion and hypolimnion of southeastern Lake Michigan during 1974 and 1975. The mean elemental soluble (< 0.45 μm) concentrations in log₁₀ (moles per liter) of the 88 samples collected from the epilimnion are: Ba = ?6.5, Ca = ?3.1, Co = ?7. 7, Cr = ?7.5, Cu = ?7.4, Fe =?6.9, K =?4.5, Mg= ?3.3, Mn =?8.1, Mo =?6.9, Na =?3.6,Ni = ?6.9, Sr=?5.9, Zn =?7.1, Cl = ?3.5, soluble reactive PO₄^2?-P = ?7.5, SO₄^2? = ?3. 7, and soluble reactive SiO₂ = ?5.3. During the month of July in 1974 and 1975, the only variable having significant concentration differences between the epilimnion and the hypolimnion was soluble reactive SiO₂. It was more concentrated in the hypolimnion.Using thermodynamic data and solving simultaneous equations, the degree of complexation of each ion has been estimated. Ions which have 10% or more of their activity accounted for by complexes include Co²⁺, Cu²⁺, Fe³⁺, Mn²⁺,Ni²⁺, CO₃^2?, SO₄^2?,and HPO₄^2?. Using the resulting free ion activities and calculating solubility products, epilimnetic lake water during July 1974 from southeastern Lake Michigan is supersaturated with respect to dolomite, malachite, hydroxylapatite, fluorapatite, hydroxoapatite, X-ray amorphous Fe(OH)₃, and goethite. It is undersaturated with respect to calcite, aragonite, rhodochrosite, barite, and strengite.     

Sediment Transport Model Validation in Lake Michigan

A multiple sediment type, three-dimensional hydrodynamic and sediment transport model was applied to Lake Michigan to simulate conditions during the Spring 2000 resuspension event. Model predictions were compared to data gathered by the EEGLE project including turbidity and downward mass flux. The model predictions for turbidity compared well to observed data, especially in capturing the distinctive peaks in turbidity due to advection that occurred in the area of the resuspension feature. The advection peaks seemed tied to the presence of a highly-resuspendable pool of sediments that was transported by weaker winds during early Spring 2000. The model predictions at depths of 40 m in the area of the resuspension feature were more problematic, as the observed data in one location showed a significant turbidity peak at the time of maximum winds. The model underestimated turbidity at that particular location, yet model predictions of a very similar turbidity peak were seen at a similar depth. The different turbidity predictions at these locations were due to underestimation of offshore flow by the hydrodynamic model. The model generally underestimated downward mass flux, though the predictions for the time-intervals that included the time of peak winds and the following week were good to excellent. These intervals generally showed the highest downward mass flux. This work highlights the importance of multiple sediment types, their associated critical shear stresses for resuspension, and the presence of a very easily resuspendable sediment layer. The availability of comprehensive data set was also important.     

基于悬移质泥沙分布理论的泥沙扬动流速公式

扬动流速是泥沙由间歇性滚动、滑动、跳跃运动转化为悬浮运动的临界流速,确定扬动流速对于预判沙质河床冲淤状态具有重要意义。在回顾前人研究成果的基础上,认为扬动流速概念适用的对象主要是非黏性沙质颗粒,它们的扬动状态同含沙量垂线分布图形存在的关系较为清晰,从而依据张红武及张罗号先后建立的含沙量垂线分布公式分别计算了不同悬浮指标下相对含沙量为0.85时的相对水深,得到一定水流条件下的颗粒悬浮高度拟合关系式,再通过假定泥沙扬动的临界状态为颗粒实际离开床面的运动高度已达相对水深0.03,即可得出泥沙扬动流速公式。通过与沙玉清和张罗号扬动流速公式的比较以及沙质河床实测水沙资料的检验,结果表明本文建立的泥沙扬动流速公式能较好地描述天然河流的泥沙扬动规律。     

泥沙悬浮的特征长度和悬移质浓度垂线分布

通过对紊动水流中混合掺长概念的类比,本文引入了泥沙作悬浮运动的特征长度的概念并讨论了其表达式的确定方法,从而在未经引入任何流速分布公式的情况下得到了一个悬移质浓度分布的统一公式,几乎所有已有各家公式均为本公式的特例。本文所采用的方法具有许多优点,采用本文的有关结果也便于从统一的高度来分析以往类似公式之间的内在联系、各公式的合理性及局限性。     

Sediment Distribution and Bottom Topography of Southeastern Lake Superior

Sediment distribution in the complex region of troughs and shoals in southeastern Lake Superior was studied by means of echo sounding, coring and surface sediment sampling. The primary factor governing sediment type is water depth with sediment source, water movement, trough morphology and lake history superimposed upon it.Thick accumulations of fine-grained sediment are confined to the trough bottoms whereas coarser sediments occur on the sides and crests. In the southern part of the region subaqueous erosion of red tills leads to the deposition of brown sandy sediments whereas farther offshore dark gray fine-grained sediments show the influence of eroding varved clays. Current action along the south shore and along the trough bottoms produces sediment sorting even in deep water. On slopes greater than one op two degrees, fine-grained postglacial sediment is very thin or absent and lag deposits overlie glacial sediments. Evidence of ice scour indicates that modifications to sediment distribution may have occurred during the low level Houghton stage about 8000 B.P.Stratigraphic evidence suggests that the most probable origin of the complex bottom topography is erosion by moving ice.     

Climatology of Large Sediment Resuspension Events in Southern Lake Michigan

Lake Michigan, particularly the southern basin, is subject to recurrent episodes of massive sediment resuspension by storm-induced waves and currents. The purpose of this paper is to investigate the climatology of these events for Lake Michigan, including an analysis of associated meteorological conditions. This paper begins by examining turbidity records from two water treatment plants (Chicago, IL and St. Joseph, MI) for which long-term records are available. The turbidity records from the two plants show significant differences indicating that turbidity measurements from a single location would probably not be representative of a basin-wide climatology. A one-dimensional sediment resuspension and deposition model for fine-grained sediments is then developed and calibrated with data from the water treatment plants. The one-dimensional model is applied at 15 points around the southern basin for a 45-year period for which Lake Michigan wave climatology is available and the results are averaged to obtain a basin-wide turbidity index (Southern Lake Michigan Turbidity Index, SLMTI). A frequency distribution of the turbidity index is presented and meteorological conditions associated with the largest events are examined. Our analysis indicates that significant resuspension events in southern Lake Michigan are usually caused by a strong cyclone passing to the east of the lake. The most likely time of the year for this to occur is October to April. There is an average of 1 event per year with SLMTI above 25 mg/L and each event typically lasts about 3 days. Our analysis indicates that events have occurred more frequently since the late 1980s as the number of winter storms has increased and ice cover has decreased.     

Stochastic Simulation of Daily Suspended Sediment Concentration Using Multivariate Copulas

An estimation of daily suspended sediment concentration (SSC) is required for water resource and environmental management. The traditional methods for simulating daily SSC focus on modeling the SSCs themselves, whereas the cross-correlation structure between SSC and streamflow has received only minor attention. To address this issue, we propose a stochastic method to generate long-term daily SSC using multivariate copula functions that account for temporal and cross dependences in daily SSCs. We use the conditional copula method to construct daily multivariate distributions to alleviate the complications and workload of parameter estimations using high-dimensional copulas. The observed daily streamflow and SSC data are normalized using the normal quantile transform method to relax the computationally intensive model of building daily marginal distributions. Daily SSCs can thus be simulated through the multivariate conditional distribution using previous daily SSC and concurrent daily streamflow values. The proposed method is rigorously examined by application to a case study at the Pingshan station in the Jinsha River Basin, China, and compared with the bivariate copula method. The results show that the proposed method has a high degree of accuracy, in preserving the statistics and temporal correlation of daily SSC observations, and better preserves the lag-0 cross correlation compared with the bivariate copula method. The multivariate copula framework proposed here can accurately and efficiently generate long-term daily SSC data for water resource and environmental management, which play a critical role in accurately estimating the frequency and magnitude of extreme SSC events.

     

垂线平均含沙量两种表述方法之比较

表达沿垂线平均含沙量的方法通常有流量法与面积法两种，但在实际运用中常发生混淆。本文讨论了两种垂线平均含沙量之间的关系，指出面积平均含沙量需要大于流量平均含沙量。含沙量及流速分布越不均匀，两者差别就越大，当悬浮指标２≤０．３时两者相差不到１０％，最后还对两种垂线平均含沙量的应用作了初步搪塞。     

Seasonal Features of Zooplankton Assemblages in the Nearshore Area of Southeastern Lake Michigan

Zooplankton in a nearshore area in southeastern Lake Michigan displayed strong seasonal and depth-related trends in abundance (#/m³) during the period 1971 to 1977. Depth-related assemblages defined by principal component analysis varied with season. From mid-spring to mid-autumn, zooplankton assemblages were strongly related to water depth. In general, zooplankton were least abundant in the inner region (5–10 m) where small organisms such as nauplii, Asplanchna spp., and Bosmina longirostris and epibenthic organisms such as Eurytemora affinis dominated. Zooplankton were most abundant between 20 and 30 m, with a peak July 1972–1977 mean density of 87,000/m³. Larger zooplankton, including Cyclops spp., Diaptomus spp., Daphnia spp., and Eubosmina coregoni dominated the outer region (20–50 m). During autumn, the inner region was dominated by cladocerans while the outer region was dominated by copepods. After thermal stratification disappeared in late autumn and before the thermal bar moved beyond the survey area in early spring, zooplankton tended to occur in higher densities in the shallow depths of the survey area. Also, in contrast to the mid-spring to mid-autumn period, zooplankton occurred in similar compositions over the survey grid. Factors implicated in producing seasonal differences in abundance patterns include temperature regimes, phytoplankton standing stocks, and predation by fish and invertebrates.     

Late Holocene Lake-level Variation in Southeastern Lake Superior: Tahquamenon Bay,Michigan

Internal architecture and ages of 71 beach ridges in the Tahquamenon Bay embayment along the southeastern shore of Lake Superior on the Upper Peninsula of Michigan were studied to generate a late Holocene relative lake-level curve. Establishing a long-term framework is important to examine the context of historic events and help predict potential future changes critical for effective water resource management. Ridges in the embayment formed between about 4,200 and 2,100 calendar years before 1950 (cal. yrs. B.P.) and were created and preserved every 28 ± 4.8 years on average. Groups of three to six beach ridges coupled with inflections in the lake-level curve indicate a history of lake levels fluctuations and outlet changes. A rapid lake-level drop (approximately 4 m) from about 4,100 to 3,800 cal. yrs. B.P. was associated with a fall from the Nipissing II high-water-level phase. A change from a gradual fall to a slight rise was associated with an outlet change from Port Huron, Michigan/Sarnia, Ontario to Sault Ste. Marie, Michigan/Ontario. A complete outlet change occurred after the Algoma high-water-level phase (ca. 2,400 cal. yrs. B.P.). Preliminary rates of vertical ground movement calculated from the strandplain are much greater than rates calculated from historical and geologic data. High rates of vertical ground movement could have caused tectonism in the Whitefish Bay area, modifying the strandplain during the past 2,400 years. A tectonic event at or near the Sault outlet also may have been a factor in the outlet change from Port Huron/Sarnia to Sault Ste. Marie.     

Food of Deepwater Sculpin,Myoxocephalus Thompsoni,from Southeastern Lake Michigan

Pontoporeia hoyi was the major food consumed by most sizes of deepwater sculpins (Myoxocephalus thompsoni) during spring and summer, 1981–82, at a 100-m station in southeastern Lake Michigan. There was no apparent seasonality in choices of food or volumes consumed. Mysis relicta made up less than 9% of the total food volume and averaged second to Pontoporeia volumetrically—a result which differed from previous investigations for Lake Michigan. Reduced consumption of Mysis in our study compared to previous work may be a function of lower prey density or station depth; mysids may have been farther above the lake bottom than at previously studied, more shallow stations. Ostracods were important in 6- and 7-cm fish diets. Mysis, chironomids, and fish eggs were only found in sculpins larger than 8 cm. Chironomid larvae, which typically burrow into sediments, and copepod and cladoceran zooplankton constituted a small percentage of the diet of all sculpins. These results indicated that the deepwater sculpins examined were primarily epibenthic predators at the sediment-water interface and apparently did not burrow for food or feed higher up in the water column.     

水库下游悬移质泥沙沿程恢复研究进展

水库下游悬移质泥沙沿程恢复是河流泥沙动力学研究的难点问题之一。对该问题的研究有助于推动学科发展,解决工程实践中的有关问题。针对此现状,归纳总结了平衡及不平衡输沙条件下悬移质浓度垂线分布、悬移质沿程恢复的天然河道实测资料分析和恢复饱和系数等方面的研究成果,认为在不平衡输沙条件下悬移质泥沙垂线分布研究及实测数据的获取、分析等方面还存在一定的不足。在此基础上,提出有必要进一步开展水库下游悬移质泥沙沿程恢复研究,特别是利用精准的测量仪器,开展科学合理、系统全面的水槽试验是丰富该研究成果的必要手段。     

Stimulation of Lake Michigan Plankton Metabolism by Sediment Resuspension and River Runoff

Previous work during a major sediment resuspension event (March 1988) in southern Lake Michigan demonstrated that nutrients and carbon derived from resuspended sediment stimulated intense winter heterotrophic production while simultaneously decreasing light availability and autotrophic biomass. However, the role of riverine inputs on plankton metabolism remained unclear. Here we present results from a simulated enrichment experiment (March 2000) designed to examine the influence of resuspended sediments and riverine inputs on Lake Michigan plankton dynamics. Lake water amended with realistic levels of river water, coastal resuspended sediment and river water + sediment all showed enhanced heterotrophic bacterial production and plankton respiration rates, relative to the lake water control. Bacterial production increased by approximately 4× in river water treatments and by a factor of 2.5× for the sediment only treatment compared to lake water controls. Rates of net primary production were stimulated by river water (8.5×) and resuspended sediment (3×), but most by a combination of river water + sediments (11×). Community respiration showed a similar response with rates approximately 8x higher in river water amendment treatments and 3.5× higher in the sediment treatment. Extrapolating experimentally determined production rates to the southern Lake Michigan basin indicated that heterotrophic and autotrophic production in this nearshore region may be enhanced by as much as 3× and 5.2× due to these source inputs. Indeed, field measurements throughout southern Lake Michigan from 1998–2000 support these experimental results. Experimental and field observations suggest that both seasonal riverine inputs and episodic resuspended sediments influence the regional scale ecosystem metabolism and biogeochemistry in Lake Michigan.     

Characterization of Growth and Winter Survival of Age-0 Yellow Perch in Southeastern Lake Michigan

Reduced growth and increased winter mortality are possible mechanisms for poor recruitment of age-0 yellow perch (Perca flavescens) in southeastern Lake Michigan. To assess the role of growth variability on recruitment of age-0 yellow perch, we quantified length-by-date and abundance during 1996–2002. Analyses revealed significant differences in length among years and sites. To resolve spatial trends, we contrasted the 1998 cohort from southeastern Lake Michigan with conspecifics from populations located within and beyond the Lake Michigan watershed. These analyses identified significant differences in length; trends suggested that the capacity for growth in the cohort from southeastern Lake Michigan was similar to these other populations. To characterize winter survival for the 1998 age-0 cohort, otoliths from age-1 perch in 1999 were used to back-calculate length (BCL) from the previous autumn. The BCL distribution was significantly larger than the 1998 distribution, but analyses led to the inference that this age-0 cohort experienced little mortality during the winter of 1998–99. We used a regression model to quantify risk of winter starvation for the 1998 cohort, using 1981–2001 winter water temperatures from southeastern Lake Michigan. This analysis suggested that risk of starvation for the 1998 cohort across this period was low (1.0–4.4%). Back calculation and regression analyses allow us to reject the hypothesis that reduced growth led to a high rate of winter mortality for the 1998 cohort. If we extrapolate our findings to past cohorts, the results would be similar unless these fish were significantly smaller than the 1998 cohort. Variation in age-0 length, growth, or winter mortality failed to fully explain yellow perch recruitment patterns in southeastern Lake Michigan. We hypothesize that factors acting before the juvenile stage is sampled during summer are shaping recruitment; resolving this requires further, stage-specific studies.     

非粘性悬沙垂向浓度分布的数值模拟及水槽试验

本文应用Ｋ－ε紊流模型，通过对悬沙运动扩散方程的数值求解，建立了计算悬沙浓度水深分布的数学模型。该模型未对扩散方程本身作任何简化，并采用速度场和浓度场耦合求解，因此精度较高，该模型应用方便、其结果与实验值吻合较好，是一种研究河流非粘性泥沙垂向浓度发布的新的尝试。     

Trophic Status of Southeastern Lake Michigan Based on the Chironomidae (Diptera)

Chironomids and oligochaetes were identified from a total of 2,432 samples collected at depths <42 m over a 9-year period in southeastern Lake Michigan. Each of 13 chironomid taxa was assigned a trophic indicator status based on density trends among samples characterized by oligochaete trophic indices in the ranges < 0.50 (Type 0), 0.50–1.50 (Type 1), and >1.50 (Type 2). A trophic index based on the chironomid fauna was generated for year, depth intervals, and combinations of years and depths. When averaged over all years, the trophic index at the 8- to 24-m depth interval was 1.26, indicating mesotrophic conditions in this portion of the littoral and sublittoral of southeastern Lake Michigan. Similarly, at depths >28 m the trophic index value was 0.41, suggesting oligotrophic conditions in the profundal. No annual trophic status trends were evident.