Nathan Hawley

A model of sediment resuspension and transport dynamics in southern Lake Michigan

A quasi-three-dimensional suspended sediment transport model was developed and generalized to include combined wave-current effects to study bottom sediment resuspension and transport in southern Lake Michigan. The results from a three-dimensional circulation model and a wind wave model were used as input to the sediment transport model. Two effects of nonlinear wave-current interactions were considered in the sediment transport model: the changes in turbulence intensity due to waves and the enhancement of induced bottom shear stresses. Empirical formulations of sediment entrainment and resuspension processes were established and parameterized by laboratory data and field studies in the lake. In this preliminary application of the model to Lake Michigan, only a single grain size is used to characterize the sedimentary material, and the bottom of the lake is treated as an unlimited sediment source. The model results were compared with measured suspended sediment concentrations at two stations and several municipal water intake turbidity measurements in southern Lake Michigan during November–December 1994. The model was able to reproduce the general patterns of high-turbidity events in the lake. A model simulation for the entire 1994–1995 two-year period gave a reasonable description of sediment erosion/deposition in the lake, and the modeled settling mass fluxes were consistent with sediment trap data. The mechanisms of sediment resuspension and transport in southern Lake Michigan are discussed. To improve the model, sediment classifications, spatial bottom sediment distribution, sediment source function, and tributary sediment discharge should be considered.

The partitioning of 7beryllium in fresh water

Abstract Field observations and experimental measurements of the partitioning coefficient ( K d ) of 7 Be in fresh water show that it varies inversely with the solids concentration at typical environmental values (up to 30 mg/1). This behavior is similar to that of many other metals and organic pollutants, which means that 7 Be may be useful as a tracer of the movement of these substances in the water column. However, the wide range in the percentage of 7 Be adsorbed by solids over this range of concentrations (over 50%) means that in order to use 7 Be either to measure total sedimentation rates or to trace lateral sediment movement it will be necessary to monitor changes in sediment concentration over the area and time period of interest. The wide scatter in our data at both high and low solids concentrations suggests that other factors also affect K d . Until these factors are identified, application of our results to other systems will be risky. At high (greater than 100 mg/1) solids concentrations over 90% of 7 Be is associated with the solid phase, so it may be a useful tracer of reworking rates in bottom sediments. 7 Be has a slightly greater affinity for the solid phase in fresh water than in seawater.

Development of recurrent coastal plume in Lake Michigan observed for first time

NOAA CoastWatch satellite imagery from early 1996 captured the initiation, development, and decay of a recurrent coastal plume in southern Lake Michigan (Figure 1). For the past 4 years intermittent satellite coverage has revealed a late winter-early spring plume in the lake, a feature also observed by Mortimer [1988]. In 1996, clear weather conditions allowed researchers to observe the plumes development for the first time and they also collected water samples from helicopter and a small boat.

Lake Erie hypoxia prompts Canada‐U.S. study

Because of its size and geometry, the central basin of Lake Erie, one of North Americas Great Lakes, is subject to periods in the late summer when dissolved oxygen concentrations are low (hypoxia). An apparent increase in the occurrence of these eutrophic conditions and ‘dead zones’ in recent years has led to increased public concern. The International Field Years for Lake Erie (IFYLE) project of the Great Lakes Environmental Research Laboratory (GLERL, a U.S. National Oceanic and Atmospheric Administration (NOAA) laboratory), was established in 2005 in response to this increase. This project is investigating the causes and consequences of hypoxia in the lake. As part of the effort, scientists from the United States and Canada conducted an extensive field study in 2005 to gather more information on the duration and extent of the hypoxic zone and its effects on the biota in the lake. This article gives a brief history and description of the problem and presents initial results from the field study.

Particle Transport, Nutrient Cycling, and Algal Community Structure Associated with a Major Winter-Spring Sediment Resuspension Event in Southern Lake Michigan

Abstract Over the past decade, intermittent satellite imagery revealed the presence of an extensive plume of resuspended sediments in late winter-early spring with a clear offshore projection coinciding with the region of maximum sediment accumulation in the lake. The large scale of the plume implied that this process was important in sediment, and associated constituent, cycling and transport, but it had never been sampled due to severe conditions. The onset of the 1996 event coincided with a major March storm. Within a few days the plume was approximately 10 km wide and over 300 km in length, implying that the source of the reflective materials was widely distributed. An estimate of the total mass of resuspended sediment, 12 days after the storm, was similar to the annual external load of (sand-free) particulate material to the southern basin. The high turbidity plume persisted for over a month, progressing northward along the eastern shore with a major offshore transport feature. Sediment traps within this feature recorded a major mass flux event. The plume was sampled on two occasions and was found to contain 5 to 10 times as much suspended matter as open-lake locations outside the visible plume. Total particulate phosphorus was high within the plume making this episodic process important in sedimentwater exchange. The diatom community structure within the plume was significantly different from outside the plume and was characteristic of more eutrophic waters. Abundance of non-diatom phytoplankton and microbial food web organisms were highest at the plume edge. The episodic nature of this process makes it difficult to sample, but the scale makes it important in designing monitoring programs and massbalance modeling efforts.

Modeling summer circulation and thermal structure of Lake Erie

] Athree-dimensionalprimitiveequationnumericalmodelwasappliedtoLakeErieona2kmgridtostudyitssummercirculation andthermalstructure.Modelresultswerecomparedtolong-termobservationsofcurrentsandtemperaturemadein2005at severallocations,mostlyinitscentralbasin.IntheshallowandmostlyunstratifiedwesternbasincirculationisdrivenbyDetroitRiverinflow(modifiedtosomeextentbywind)andisfromwesttoeast. Inthecentralbasin(which isofintermediate depthandhasarelativelyflatbottom),themodeledcirculationisanticyclonic (clockwise),drivenbyanticyclonicvorticityinthesurfacewind,andthethermocline isbowl-shaped,inlinewithobservations.Inthedeeppartoftheeastern basin,the thermoclineisdome-shapedandcirculationiscyclonic (counter-clockwise),duetodensitygradients(aconfigurationtypicalforotherlargedeeplakes),whileshallower areasareoccupiedbyanticyclonic circulationdrivenbyanticyclonic windvorticity.Inthe centralbasin,modeledtemperatureandcirculationpatternsare quitesensitivetothespecificationofthe windfield.Anticyclonicwindvorticityleadstothinningofthehypolimnion inthecentralbasinandearlierdestratificationinthefall.

Development of the Great Lakes Ice-Circulation Model (GLIM): Application to Lake Erie in 2003–2004

ABSTRACT To simulate ice and water circulation in Lake Erie over a yearly cycle, a Great Lakes Ice-circulation Model (GLIM) was developed by applying a Coupled Ice-Ocean Model (CIOM) with a 2-km resolution grid. The hourly surface wind stress and thermodynamic forcings for input into the GLIM are derived from meteorological measurements interpolated onto the 2-km model grids. The seasonal cycles for ice concentration, thickness, velocity, and other variables are well reproduced in the 2003/04 ice season. Satellite measurements of ice cover were used to validate GLIM with a mean bias deviation (MBD) of 7.4%. The seasonal cycle for lake surface temperature is well reproduced in comparison to the satellite measurements with a MBD of 1.5%. Additional sensitivity experiments further confirm the important impacts of ice cover on lake water temperature and water level variations. Furthermore, a period including an extreme cooling (due to a cold air outbreak) and an extreme warming event in February 2004 was examined to test GLIMs response to rapidly-changing synoptic forcing.

Flow in Cylindrical Sediment Traps

Abstract Flow behavior in cylindrical settling traps was observed in steady flow for trap Reynolds numbers (Re) between 1,600–30,500. The effects due to aspect ratio, wall thickness, a funnel at the bottom of the trap, and the presence of a mooring line were investigated. Although the data are semiquantitative, they show that upwelling frequency and intensity increased as Re increased and as the aspect ratio decreased. The relationships between the thickness of the bottom layer, the frequency of upwelling, and the settling velocity of a particle determines whether deposition occurs through the bottom tranquil layer or the viscous sublayer. Since the thickness of the bottom tranquil layer can be scaled by the trap diameter, the settling behavior of particles cannot be modeled simply by Re and the aspect ratio, as can flow behavior, but also depends on the ratio of flow velocity to settling speed. Flow behavior induced by wave action is similar in many ways to that generated by steady flow at the same Re. Depending upon flow conditions in the trap, either the measured flux or the size distribution of the material collected may differ substantially from that outside the trap. Traps that are designed so that upwelling does not occur during their deployment will give the best estimates of flux and particle distribution. This means that in most cases several small traps are preferable to a single large trap.

Sediment Resuspension Near the Keweenaw Peninsula, Lake Superior During the Fall and Winter 1990–1991

Abstract During the winter of 1990–1991 time series measurements of current velocity, temperature, and attenuation (a measure of water transparency) were made at a site in 91 m of water near Copper Harbor, MI. The observations show that bottom resuspension occurred several times during the unstratified period. The resuspension is the result of the interaction between high bottom current velocities and surface waves generated by strong winds. Transport during the storms was almost entirely alongshore, although some offshore transport of material occurred. Calculations show that suspended material could have been transported eastward several hundred km during the unstratified period.

Wave grouping characteristics in nearshore Great Lakes II

The recently advanced approach of wavelet transform is applied to the analysis of wave data measured in the nearshore areas of the Great Lakes. The conventional spectrum analysis of wave time series in the frequency domain can be readily generalized to the frequency and time domain using the wavelet transform. The traditional Fourier transform approach has not been able to directly assess the time localized nature of wave groups. With the application of wavelet transformation, the relatively unexplored wave grouping characteristics come to light as the predominant feature of wave processes.