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Technical Workgroups :: Modeling and Processes

Quantifying Combined Suspended and Bedload Sediment Discharge of the Lower Mississippi River to the Gulf of Mexico
Dr. Mead Allison, University of Texas Institute for Geophysics and Tulane University Center for Bioenvironmental Research

This research seeks to synoptically measure suspended load, bedload, and water discharge of the Mississippi River at the point where it enters the tidally influenced zone in its final reach to the Gulf of Mexico. The resulting datasets show that suspended sediment concentrations are quite variable within the confines of a reach. This variability calls into question standard sampling methods of measuring sediment discharge from a single cross-section.

Surficial Investigation of the Baton Rouge Fault Scarp
Dr. Nancye Dawers, Tulane University Department of Earth and Environmental Sciences

This project characterizes the morphology of numerous fault scarps along the Baton Rouge fault system in southeastern Louisiana. This fault system provides a natural laboratory for studying fault-related subsidence, as it marks the northern margin of the Pontchartrain Basin and has significant Late Pleistocene-Holocene vertical displacement across it. Moreover, it was recognized as an active fault by geologists prior to significant hydrocarbon production in south Louisiana, and therefore cannot be attributed to fluid-withdrawal.

Hurricane Katrina Impacts on Gulf Coast Forested Wetlands
Dr. Jeffrey Chambers, Tulane University Department of Ecology and Evolutionary Biology

Wetlands of the Gulf Coast Region are experiencing rapid loss rates due to a combination of rising sea level, high rates of subsidence, erosion from wave action, and a lack of new sediment input from the river. The loss of forested wetlands is particularly severe, with only a small fraction of this once widespread ecosystem remaining. Detailed quantitative assessments of forested wetland loss rates can provide a sensitive indicator of the inland migration of estuarine ecosystems. Of particular concern is how hurricane impacts may accelerate the replacement of forested wetlands by salt marsh. This project is taking a synthetic approach linking remote sensing image analyses and field investigations to quantify loss rates for forested wetlands impacted by hurricane Katrina. The overall objective is to quantify sub-pixel disturbance signals and correlate these remote sensing metrics with associated species-specific tree mortality rates from forest inventory plots. Forest disturbance maps coupled with extrapolation models for the entire Katrina impact zone predicted mortality and severe structural damage to ~320 million large trees totaling ~210 Tg C (105 Tg C), an amount equivalent to 60-100% of the total U.S. carbon sink in forest trees. For more please see: Chambers et al. (2007) Hurricane Katrina's carbon footprint on Gulf Coast forests Science 318:1107.

River and Estuarine Contributions to Coastal Hypoxia in the Northern Gulf of Mexico
Dr. Michael Dagg, Louisiana Universities Marine Consortium
Dr. Rodney Powell, Louisiana Universities Marine Consortium

The focus of our current field work is to examine the contributions of wetlands and marshes to the dissolved organic carbon content of Louisiana’s estuaries and coastal ocean. The metabolic status of these estuaries and coastal waters is one of net heterotrophy (Caffrey 2004), indicating an external source of organic material fueling respiration that is greater than photosynthesis. Our hypothesis is that a significant portion of this organic carbon is derived from the extensive marshes and wetlands along Louisiana’s coast. This hypothesis was developed in part from a LEAG cruise on the continental shelf away from the Mississippi plume. Nutrients from the Mississippi/Atchafalaya Rivers greatly stimulate biological production in the ‘classical’ foodweb on the inner shelf of the northern Gulf of Mexico. Portions of this production, especially large diatoms and zooplankton fecal pellets, sink and decompose in the bottom water, consuming oxygen and contributing to the annual development of an extensive zone of bottom water hypoxia, typically > 15,000 km2 since 1993. The microbial foodweb is also active in the Mississippi River plume but consists of small organisms that sink slowly. This ‘recycling’ foodweb has not been considered as a significant contributor to vertical flux and hypoxia. However, gelatinous zooplankton, especially pelagic appendicularians such as Oikopleura dioica, mediate the conversion of microbial web organisms to organic particles with high sinking rates. When pelagic appendicularians are abundant in coastal regions of the northern Gulf of Mexico, they stimulate the rapid vertical transfer of microbial web productivity in the surface layer, which is only 5 to 15 m thick in the coastal hypoxic region, to the sub-pycnocline layer that becomes hypoxic each summer.

Holocene Sea Level / Climate Relationships in the Central US Gulf of Mexico Coast
Dr. Torbjörn Törnqvist, Tulane University Department of Earth and Environmental Sciences

The overarching objective of this investigation is to determine the relationship between natural climate variability in the past and temporal variations in the rate of sea-level rise. Climate change in the past ~8000 years is closely linked to the evolution of the Earth’s large ice sheets, and understanding the connection between the cryosphere and sea level is critical to enable better-constrained predictions of sea-level rise under future scenarios of global warming. This project uses the unique sedimentary record of the US Gulf Coast as an archive of past sea-level change. Most of the work is focused in coastal Louisiana, but additional field sites in Florida and Alabama are also used.

Characterizing Biogeochemical Material Fluxes from River to Gulf
Dr. Brent McKee, University of North Carolina – Chapel Hill Department of Marine Sciences (LEAG work done while in Tulane University Department of Earth and Environmental Studies)

The primary objective of this project was to establish a monthly sampling regime for the Mississippi River at New Orleans to quantify the riverine flux of biogeochemical materials to the Gulf of Mexico. An accurate estimate of riverine carbon, micronutrient, macronutrient and sediment fluxes is essential to understanding coastal phenomena such as high primary production rates and hypoxic bottom waters. The large input of terrestrial materials (dissolved, colloidal and particulate) into shallow deltaic environments, such as the Mississippi River Shelf, is the primary reason that these river-dominated environments are so important to global biogeochemical cycles. Samples were collected at the Audubon Dock and chemical analyses were performed on these monthly samples (particulate and dissolved fractions) to quantify natural radioisotopes (Be-7, Cs-137 and Pb-210), which are used as terrestrial tracers in the coastal zone. What our data reveals is that when discharge gets much higher (late spring) the amount of Be-7 goes down. This indicates that under these high shear conditions within the river and its drainage basin, older sediments (dead to Be-7; i.e having contact with the atmosphere longer than 1 year prior) are delivered to the lower river. These older sediments are apparently stored somewhere within the drainage basin on time scales of years to decades. Any particle-associated pollutant would follow the same behavior. This data set helps us understand the delivery of particle-related contaminant that is delivered to the Mississippi somewhere in the upper drainage basin.

Inputs, Processing, and Fates of Nitrate and Suspended Sediment in Lower River and Plume
Dr. Michael Dagg, Louisiana Universities Marine Consortium
Dr. Rodney Powell, Louisiana Universities Marine Consortium

The Mississippi River currently delivers approximately 1.82 Tg N per year (1.3 x 1011 mol N y-1) to the northern Gulf of Mexico. This large input dominates the biological processes of the northern Gulf of Mexico. The “new” nitrogen from the river stimulates high levels of phytoplankton production, which in turn support high rates of bacterial production, protozoan and metazoan grazing, and fisheries production. A portion of the particulate organic matter produced in the pelagic food web sinks out of the euphotic zone and contributes to high rates of oxygen consumption in the bottom waters of the inner shelf, resulting in the development of an extensive zone of hypoxia each summer. The goal of this component of LEAG was to better quantify and understand the inputs, processes and fates of dissolved nitrogen, carbon, and suspended sediment in the Mississippi River as it flows into the Gulf of Mexico. The second major goal of this project was to examine some of the processes and fates of dissolved nitrogen and suspended sediments in the lower river and river plume regions. In June/July 2003, LEAG conducted a two-ship cruise to look at the dynamics of nitrogen, suspended particulate matter and phytoplankton in the lower river and river plume. In this Lagrangian experiment, we followed a parcel of water in the lower Mississippi river during its last 4 days of transit before discharge. We concluded that the direct effects of dissolved and particulate bio-reactive materials discharged by the Mississippi River were spatially and temporally restricted, at least as surface phenomena, during this period. After being transported through the lower river essentially unaltered, these materials were biogeochemically processed within days. More generally, mixing rates of plume water with receiving oceanic water has profound effects on the food web structure and biogeochemical cycling in the plume.

Multi-Level Chemical Biomarker Sampling in the Lower Mississippi River
Dr. Thomas Bianchi, Texas A&M University Department of Oceanography

This project’s overall goal was to understand carbon cycling at the MR/GoM estuary through measurement of key geochemical factors. Specific goals included 1) making monthly collections for consecutive years, establishing long-term trends in particulate, dissolved organic carbon (POC and DOC), and total dissolved nitrogen (TDN) in the lower Mississippi River; 2) comparing carbon concentrations with continuous measurements of nitrate (the dominant form of dissolved inorganic nitrogen in the river) using an in situ sampler (in collaboration with Dagg); and 3) making monthly collections of POC and DOC, using a continuous flow centrifuge (in collaboration with McKee). We discovered that river phytoplankton may be more abundant that previously thought, potentially changing our thinking about the dynamics of nutrient delivery to the coast. In addition, it is important to know the seasonal changes in carbon concentrations, especially DOC and TDN, when examining the fate and transport of trace metals.

Lower Mississippi River Biogeochemical Processes
Dr. Franco Marcantonio, Texas A&M University Department of Geology and Geophysics

Strontium isotope ratios of seawater precipitates are used in global climate change studies. Researchers need to know how Sr makes it through the river’s mixing zone “gauntlet,” and into the ocean. Through joint study of Sr with other trace elements, we can appreciate on a more thorough scale the role that particles and sediment dynamics play in regulating fate and transport of metals. Dissolved (<0.2 µm) and particulate (0.2 µm) Mn, Fe, U, V, Mo and Ba concentrations in the surface water column were studied to assess their behaviors during mixing of fresh water and salt water. Water column and sediment-porewater profiles at two sites in the mixing zone were also studied in order to further evaluate the effect of estuarine processes on the fate of fluvial trace metals that enter the MR-GOM mixing zone. Findings indicate that benthic flux from sediments deposited previously in the deltaic mixing zone provide a “new” source of Sr. Also, Sr isotope seawater anomalies are small (<40×10-6), and may not affect the application of Sr techniques in paleoclimatic techniques. However it does suggest a resolution limit for Sr stratigraphic dating of about 500 kyr.

Modeling Sediment Transport in Riparian, Estuarine and Coastal Environments
Dr. Efstathios Michaelides

The objective of this project was to conduct fundamental studies on sedimentation and re-suspension processes of groups of particles with attached organic and inorganic materials. We have examined these processes for irregularly shaped particles. The studies have shown the importance of the interactions of groups of particles on the sedimentation velocities and re-suspension rates. We have characterized these interactions with respect to the concentration of the particles and determined three distinct regimes for the interactions and the flow:

  • The dense flow regime, where the matrix of particles moves almost in unison.
  • The cluster formation regime, where formation and break-up is the main determinant of particulate behavior. We have established that flow inertia and the Reynolds number play an important role in particulate collisions and cluster formation and break-up.
  • The dilute flow regime, where formed clusters do not break-up and are advected by flow as groups of particles.
  • We have developed a new numerical technique, the Immersed-Boundary Lattice-Boltzmann Method (IB-LBM) to specifically solve complex problems with a large number of particles, inter-particle interactions and particle-boundary interactions. This method and the code developed have been validated by comparison with experimental data and other computational techniques.
  • We established that spherical as well as irregular particles slightly heavier than water do not settle in the bottom. Because of the influence of the bottom plane they hover at an “equilibrium distance,” which is of the order of magnitude of the diameter of the particles.

Complexity and Nonlinear Dynamics to Model Mixing, Transport, and Sedimentation in the Gulf of Mexico
Dr. Elia Eschenazi, University of the Sciences in Philadelphia Department of Mathematics, Physics and Statistics

The objective of this project was to use the Topological Approximation Method (TAM) [Rom-Kedar, Nonlinearity, 7,441, (1994)].and other techniques to study lobe dynamics and to quantify transport and mixing in unsteady fluid flows. The Kelvin-Stuart cat-eye flow was chosen as a prototype of driven flows displaying chaotic advection [Tsega, Y., E. Michaelides and E.V. Eschenazi, Chaos, 11,2, 351, (2001)].In this system generic features can be analyzed in order to apply the technique to more general numerically generated fields. In particular, the objective was to study how transport and mixing depend on external perturbations and driving factors. Previously steps for the implementation were described and results for other well-known systems obtained (such as forced cubic potential system). The major result recently obtained is the derivation of an analytical expression of the Melnikov function for the driven Kelvin-Stuart cat-eye flow. This very challenging effort and remarkable result will facilitate substantially the implementation of the TAM method. Given the importance of Kelvin-Stuart cat-eye flow for its possible generic features, the results create premises for larger number of applications. The understanding of mixing and transport in the driven cat-eye flow and their generic features is determinant to guide the study of transport on large scale (river or plume).

Modeling Transport in Rivers, Estuaries and Coastal, Environments
Dr. Cheryl Ann Blain, Naval Research Laboratory

The primary objective of this project was the development of a coastal modeling capability that could predict the water levels, three-dimensional current structure, and the hydrodynamic transport of biogeochemical species for the Mississippi River and its outflow in the northeast Gulf of Mexico in response to tides, wind, upstream river conditions, and offshore mesoscale event forcing. The application of a numerical model to the prediction of coastal circulation in rivers and estuaries is advantageous in interpreting the transport dynamics of observed distributions of biogeochemical species. A model that incorporates the range of important dynamical forcing and represents complex shoreline and bathymetry at fine scales can be exercised as a virtual laboratory for understanding river flow and coastal circulation and its impact on the transport of biogeochemical constituents in the water column. The finite element coastal circulation model, ADCIRC [Luettich and Westerink, 2003], was the core model for the prototype river and estuary system in all phases of this work. ADCIRC is a 2-D and 3-D, dynamically advanced model that accommodates forcing from tides, point sources, surface pressure, wind, waves, and density variations. The constructed numerical model (using the finite element-based ADCIRC) for the Mississippi River and its outflow onto the continental shelf was been applied to the period of June 20-30, 2003 in order to coincide with the first LEAG cruise. The model is forced by tides, hourly winds from the NOAA BURL1 buoy, and daily discharge at the USACE Tarbert Landing (Mississippi River) and Simmesport (Atchafalaya River) stations. The 100-200 m resolution of the model reveals a strong variability in the depth-averaged localized current structure. Winds are the dominant influence on currents in the region (in the absence of buoyancy forcing). Circulation fields correlate well to MODIS 250 beam attenuation imagery indicating a close correspondence between circulation-induced mixing and sediment re-suspension.

Data Management and Linkages Among Models, Monitoring and Processes
Dr. Michael Dagg, Louisiana Universities Marine Consortium

The objective of this work is to link LEAG’s field-measured empirical components with its modeling components. The larger goal, which is of critical importance, is to foster a liaison and linkages between the survey cruises and the coastal-circulation modeling activity ongoing in the estuary region. To stimulate group discussion on these themes, workshop and sessions were held in November 2002 and January 2004 at Tulane University.

Acoustic Mapping of Mississippi and Atchafalaya River Floors
Dr. Mead Allison, University of Texas Institute for Geophysics and Tulane University Center for Bioenvironmental Research

The first year of this project involved two research cruises in the lower Mississippi and Atchafalaya rivers to collect multibeam, sidescan SONAR, and CHIRP subbottom profiler acoustic data, with the goal of examining the extent, timing, and character of seasonal deposition of fine-grained sediment on the river channel bed and downstream migration of sand bedforms. Acoustic data were groundtruthed with core collection to determine the character of the sediment involved in the seasonal deposits. Year Two focused on measuring sand bedform migration rates at four new sites and at varying discharges, with the goal of quantifying bedload sand transport rates. Multibeam bathymetric mapping was conducted on two successive days (April 22-23, 2003) along a 1.5 km section of the Audubon Park grid to test the methods of measuring downstream sand bedform migration. Overlaying the two-day bathymetry sequence allows measurement of migration rates, which can then be used to develop quantitative measurements of bedload transport rates. Additionally, four 10-km-long river stretches were mapped from bank-to-bank during low discharge (October-November) when sediment storage is at maximum extent, and during rising-high discharge (April) when the seasonal layer was eroding. Study areas were on the main Mississippi channel at English Turn immediately downriver of New Orleans (freshwater meander loop), at Audubon Park immediately upriver of downtown New Orleans (freshwater straight reach), at Gramercy (freshwater meander loop), and immediately upriver of Donaldsonville, Louisiana (freshwater straight reach). Gravity cores and grab samples were collected at sites selected from the acoustic imagery for detailed examination of the sedimentological and geochemical character of the seasonal deposits. In Year Two of the project, the successful test of the multibeam bathymetry to measure sand bedform migration rates—and thus quantify bedload sand transport rates—led to the design of four additional field studies at low, rising, high, and falling Mississippi River discharge. Two large-scale studies (low and high discharge) were set up to characterize the bedload transport rates at three sites: Audubon Park, English Turn, and Venice. Two smaller scale studies (rising and falling discharge) provide additional coverage at the Audubon Park site only. Each study involves daily multibeam bathymetric mapping on at least two consecutive days of a 2-3 km long section of the river from bank to bank. Daily results are then compared to determine bedload migration rates. Data examination has revealed that bedload migration rates can be quantified from the data and that bedform morphology has changed dramatically at the Audubon Park site since the initial April 2003 survey.

Plume-Tracking Research Cruises at the River/Gulf Interface
Dr. Michael Dagg, Louisiana Universities Marine Consortium

This project aimed at developing quantitative linkages between the processes and events of the lower river and those within the discharge plume on the continental shelf, and to track processes within the river plume. Toward this end, three research cruises were carried out:

  • Lower river cruise, June 20-24, 2003
  • Plume tracking cruise, June 24 – July 29, 2003
  • Fate of plume materials cruise, August 9-12, 2004

Findings: Lower River Cruise, June 20-24, 2003 (in collaboration with Dr. Bianchi): Between mile 225 and mile 0, large particles appeared to be settling out of the surface water. Concentrations of chlorophyll containing particles > 20 μm declined 37%, TSM declined 43%, POC declined 42% and PN declined 57%. In deeper waters, concentrations of TSM were higher than in the surface. Concentrations of the smaller chlorophyll containing particles at the surface did not change suggesting only large particulate materials were settling. There was no measurable drawdown of dissolved N, P or Si, consistent with the observation that chlorophyll did not increase during the 4 day transit. Dissolved organic carbon (DOC) declined slightly (3%). There was no measurable autotrophic activity and possibly a slight amount of heterotrophic activity in this stretch of the lower Mississippi River at this time. We concluded that under these mid-range discharge conditions, large particulate materials were settling while smaller particulate materials and dissolved constituents were essentially unchanged while being transported through the lower river and into the open Gulf of Mexico.

Findings: Plume Tracking Cruise, June 25-July 29, 2003: During this cruise, the discharge plume from Southwest Pass was mapped three times in four days to determine the change in properties tracked during June 20-24 after river water was discharged and formed a buoyant plume in the northern Gulf of Mexico, and the stability of the plume structure over several days. Changes observed between the lower river and the plume were dramatic. The lower river was essentially inactive biologically, with no phytoplankton production and only a small amount of heterotrophic microbial activity. As we know from previous work and from this cruise, biological processing of riverine materials begins very shortly after formation of the buoyant plume. The plume mapping done on this cruise allowed quantification of these processes because we repeated our survey 3 times over the 4 days after the lower river water was discharged. Plume structure changed after day 2 so it appears that, during this study, the plume was stable for about 2 days and comparison the maps from day 1 and 2, allowing for estimates of rate processes.

Findings: Fate of Plume Materials Cruise, August 9-12, 2004: A large fraction of the water and materials from the Mississippi River is typically transported along the Louisiana continental shelf in a westward direction. The purpose of this cruise was to determine the connectivity between processes occurring on the shelf and the Mississippi River discharge.