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Turbid Bay: Sediment in Motion

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Jessie Lacy is a research oceanographer with the U.S. Geological Survey....Lissa Mac Vean is a USGS Mendenhall Fellow, she's recently completed her Ph.D in civil and environmental engineering with a focus on environmental fluid dynamics. Mendenhall studies address pressing scientific questions.


.....and the way it works is that scientists like myself write up what we call research opportunities and those are advertised and they outline a broad area of study and then the applicants write specific proposals in collaboration with the research advisors to say what they would like to do. And this project is actually what Lissa proposed for her Mendenhall Post-Doctoral Fellowship.


One topic, the one we're interested in is exactly how the sediment can get from the Bay, the shallow environments of the Bay out into the marsh ultimately and that's what it feeds into........ so we're looking at transport of sediment from areas that stay completely wet or are subtidal into intertidal areas that wet and dry... it's a major control on what sediment is able to access marsh, ultimately.


San Francisco Bay is a dynamic and complicated estuary. Millions of gallons of water are pumped in and out through the Golden Gate with each tidal cycle.

The Bays turbid murky waters are the product of the work of winds, waves and tides. Turbidity is the mixing of sediment and water. In some areas of the Bay the turbid waters resemble chocolate milk. Sediment on the bottom and sediment entering in streams is moved around and kept afloat with the constantly shifting waters.

Fringing the Bay are wetlands and tidal marshes that play an important role in the local ecosystem. Past human activity has wiped out over 90% of the Bays wetland habitat that existed before the 1850's and the gold rush.

Recent efforts to convert tens of thousands of acres of once commercial salt ponds in the Bay, back to natural and wetland habitat, mark a positive new trend for human impacts on the region. One of the important questions managers and scientists have is whether the Bay fringing marshes will survive in the face of accelerating sea level rise.

The theory on marsh preservation and restoration in the Bay is that tidal forces will carry sediment into the marshes at high tide and that sediment will fall out and add to the foundation of the marsh where plants can continue to thrive.

The problem is that if sea level rises faster than sediment can be deposited, the marshes will literally be drowned.

Narrator: Understanding how and when and how much sediment gets from the Bay into marshes is a complicated problem. An important part of scientific inquiry is breaking complicated problems into smaller questions, and designing and implementing studies to come up with answers.

Lissa MacVean's Mendenhall study addresses directly questions on how Bay sediment moves across intertidal's an important piece of a scientific puzzle, that will be integrated with the work of many other scientists, and will contribute answers on the fate of marshes fringing San Francisco Bay.


We're making field measurements of water velocities, and salinities and suspended sediment in order to determine exactly what's controlling how sediment moves in really shallow environments in an estuary. So, it's really sticky muddy sediment that's moving around and it's really important for how it accesses restoring marsh for instance on the fringe and how it gets there is controlled by how it goes between these kind of intertidal and subtidal shallow environments.


So, all of these platforms are going to have a lot of instruments to measure currents, waves and suspended sediment. And the reason we have more than one on each platform is because we're looking at different elevations above the sea floor and that's because the profile of currents and the profile of suspended sediment change a lot with height above the bed. And because of the physics that's understood about those processes if you can resolve those changes with depth you can actually learn something about the mechanisms that are bringing sediment up into the water column and then moving it around.


the bottom is really muddy so you've got a lot of sticky mud. Not too much sand or shell it's as uniform as we could find in the Bay. Water is really turbid and murcky so you can't see to the bottom even in a very shallow depth. It's sort of inhospitable for science which is really why it's interesting . There's still a lot that isn't known about how these systems work.


At four different locations we're going to have some of these tripods or platforms that are covered with instruments.

....... they're going to be positioned along a gradient from the channel all the way to the intertidal part of the mudflats so that's the part of the mudflats that actually dries out on every single tidal cycle.


A lot of the instruments in this experiment will actually be exposed at low tide. That's something that usually as oceanographers we kind of avoid doing because it's not that good for the pressure cases to heat up but this is key to the question that we're studying so we're just going ahead and doing it.


Over the six weeks the instruments will gather a massive amount of data. They'll measure quickly, as fast as ten times per second when they're operating. The scientists will learn how sediment concentrations and velocities are changing on a range of time scales....seasonally, when storms come and go, when it's windy or not windy and through each tidal cycle or even just from a passing wave.


and then even a shorter timescale is the turbulence timescale. And it turns out that while waves pick the sediment up off the bed it's the turbulence that actually mixes it up through the water column ---- in previous experiments we've been able to resolve that combination of processes. Which is hard to do and it allows you to do a lot of really interesting analysis. And in this experiment we'll be resolving that kind of thing right at this intertidal zone where there really are very few measurements like that.


Some wetlands restoration projects actually deposit sediment to bring the marsh plain elevation up to the appropriate level for plants but it's considered a much better option to rely on natural processes because that then is a sustainable restoration. And what that means is that they're assuming that the turbid waters of the bay will deposit enough sediment in the marsh to restore it.


But will that happen? If climate change and any other factors continue to cause sea level to rise these marshes will be an important buffer zone for communities next to the Bay. They also play a vital role as habitat for juvenile fishes and for the endangered California Clapper Rail. And as a feeding, resting point for birds travelling the Pacific Flyway.......Answers from Lissa MacVean's Mendenhall study will fill in pieces of the scientific puzzle on how to forecast and plan for changes in these Bay fringing marshes. Data collection in the Bay is really only the beginning of the much lengthier analytical process.


It will be collecting data autonomously for about six weeks so then we'll go and kind of do this process in reverse collect it all take it apart and clean everything . Data processing can take a variable length of time but I would say within about six months I should have some formulated concepts and hypotheses at least if not answers.


The Mendenhall Fellowship Program at the USGS has benefits on many levels...society benefits with useful new data.....the body of scientific knowledge grows.....the USGS accomplishes new important work.....and most importantly a young scientist is provided a starting point for their future in science.



Title: Turbid Bay: Sediment in Motion


USGS scientists from the Pacific Coastal and Marine Science Center explore how sediment moves across San Francisco Bay tidal flats.  The research team deploys a suite of large instrumented tripods to record sediment movements over a six-week period in early 2011.  Answers from this work will help determine whether deposition of sediment at high tide is occurring quickly enough to preserve marshes in the face of sea-level rise.  

The program also highlights the value and function of the USGS Mendenhall Fellowship Program.   The Mendenhall Program at the USGS provides recent PhD graduates an opportunity to conduct cutting-edge research on pressing scientific questions with the guidance and mentoring of established scientists.

Location: San Francisco Bay, CA, USA

Date Taken: 2/1/2011

Length: 9:59

Video Producer: Stephen M. Wessells , U.S. Geological Survey

Note: This video has been released into the public domain by the U.S. Geological Survey for use in its entirety. Some videos may contain pieces of copyrighted material. If you wish to use a portion of the video for any purpose, other than for resharing/reposting the video in its entirety, please contact the Video Producer/Videographer listed with this video. Please refer to the USGS Copyright section for how to credit this video.

Additional Video Credits:

Produced by:  USGS Office of Communications and Publishing & USGS Pacific Coastal & Marine Science Center

Directed, written, filmed and edited by: Stephen M. Wessells

Content Experts:  Jessie Lacy, Lissa MacVean, Rama Kotra, Helen Gibbons

Narrator:  Curtis Howard

Clapper Rail Photo: Peter LaTourrette


Access USGS - San Francisco Bay and Delta

South San Francisco Bay Salt Pond Restoration Project


Research Information:


USGS Mendenhall Research Fellowship Program


IPCC Fourth Assessment Report: Climate Change 2007


Napa Sonoma Marsh Restoration Project

File Details:

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Tags: MarineGeology Marsh MendenhallFellowship Restoration SanFranciscoBay SeaLevelRise Sediment SedimentMovement TidalFlat Turbidity WomenScientists


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