| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | August 4, 2016 |
| Latest Amendment Date: | August 4, 2016 |
| Award Number: | 1634480 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Baris Uz
bmuz@nsf.gov (703)292-4557 OCE Division Of Ocean Sciences GEO Directorate for Geosciences |
| Start Date: | September 1, 2016 |
| End Date: | August 31, 2020 (Estimated) |
| Total Intended Award Amount: | $669,987.00 |
| Total Awarded Amount to Date: | $669,987.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
266 WOODS HOLE RD WOODS HOLE MA US 02543-1535 (508)289-3542 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
266 Woods Hole Road Woods Hole MA US 02543-1053 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | PHYSICAL OCEANOGRAPHY |
| Primary Program Source: |
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| Program Reference Code(s): | |
| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Mixing between fresh and salt water is the fundamental process that is common to all estuaries. However well-mixed estuaries present an intriguing paradox: how can mixing occur if the estuary is already mixed? This project addresses that paradox and how it relates to the broader problem of estuarine dispersion in the examination of two well-mixed estuaries with similar tidal and freshwater forcing but highly contrasting geometries, one a narrow estuary with a single, uniform channel and the other a broad lagoon-type estuary with a complex, dendritic channel network. The study will address the coupled momentum and salt balances of these two estuaries with a combination of intensive field measurements and high-resolution numerical modeling. The mixing and exchange processes affecting well-mixed estuaries are fundamentally important to the ecology and life histories of nearshore organisms, to the biogeochemistry (including the carbon budget) of coastal environments, and to the morphodynamic stability of coastal embayments. The results of this study will thus inform research and management related to marine resource management, carbon sequestration, and coastal sustainability. The project will focus a large fraction of its resources on the training of a graduate student. It also includes support for a field methods short-course to provide training for students from different graduate programs in advanced estuarine field methods.
Whereas the dynamics of partially mixed and highly stratified estuaries have a firm theoretical framework, well-mixed estuaries are subject to a wide range of hypothesized regimes, such as lateral shear dispersion, chaotic advection and tidal-asymmetry-induced circulation, but with no consistent synthesis or scaling of these different processes. In the case of the narrow estuary, mixing is hypothesized to result from brief, episodic restratification events localized in space and time. For the broad estuary mixing may exhibit a much more complex spatial structure, dominated by lateral fronts at the edges of the channels and intensified at channel junctions. This study will examine the roles of these different mechanisms in well mixed estuaries with strongly contrasting geometries, using observations and realistic simulations to identify the processes, and using model sensitivity studies with varying geometries to parameterize the scaling of the different mechanisms and to generalize the results.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Estuaries are water bodies where fresh water from rivers mixes with salt water from the ocean. The mixing between the fresh and salt water result in a complex suite of dynamics that vary depending on the geometry of the estuary and the relative strength of the tides and river flow. Salt marsh estuaries typically have a small amount of river flow and strong tidal currents, leading to weak vertical stratification. This study investigated the transport processes in the North River in Marshfield, MA, a salt marsh estuary, in order to determine the mechanisms exchange between salt and fresh water, and to examine the influence of the geometry of the estuary on those processes.
One of the most notable results of the research is related to the flow around bends. In rivers, the pattern of flow around bends is well known, and a certain pattern of lateral circulation occurs consistently in river bends, with surface flow toward the outside of the bend and near-bottom flow toward the inside. Our study revealed the same pattern of flow for the ebb tide, but a much more complicated regime occurred during the incoming tide (Fig. 1). This difference in lateral circulation is caused by the along-channel salinity gradient in the estuary, which affects the flow in opposite ways during the different phases of the tide. This difference in flow structure affects the transport of salt and sediment as well as altering the dynamics of the estuary.
Motivated by these distinctive processes related to bends in the estuary, the research team investigated the influence of bends on the dynamics, specifically their contribution to the drag force that opposes the dynamical forcing that drives the tidal flow. Drag can be one of two types: skin friction and form drag. Skin friction is the slowing down of the fluid by direct interaction with the bottom and sides of the estuary, whereas form drag is caused by the separation of the flow by obstacles, including the sharp meanders of the estuary (Fig. 2), which produce a retarding force on the tidal flow. We performed both a numerical model study and analysis of our field data to determine the contribution of bends in the estuary to form drag. We found that the drag may be enhanced by as much as a factor of 5 by the form drag induced by flow separation around the bends. Whereas this process was already identified in rivers, the more complex patterns of circulation in estuaries lead to asymmetry in the drag between flood and ebb, and also changes related to the changes in the strength of the tides and river flow.
A third element of the study was an investigation of the influence of side channels on the exchange of salt and fresh water within the estuary. We looked at a process called ?tidal trapping?, in which water enters side embayments at one phase of the tide and comes out at another phase of the tide, leading to enhanced exchange between the salt and fresh water. This study provided one of the most detailed studies to date of this trapping process, using a variety of techniques including drone imagery (Fig. 3) in combination with conventional oceanographic methods to quantify the dispersion processes.
A fourth aspect of this project was a field methods class that provided undergraduate and graduate students the opportunity for hands-on oceanographic research experience (Fig. 4).
Last Modified: 12/28/2020
Modified by: Wayne R Geyer
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