Saturday, January 3, 2015

Measuring in a Continuum

How do I begin to explain the field work that goes on in my lab?

In essence, our goals are to characterize the physics of surf zone and nearshore processes. We hypothesize that waves have a large impact on both. For that matter, so do currents. And the interactions between waves and currents. Which mean tides become important in many cases. And don't even get me started on what happens when you throw silt-laden water into the mix...

I guess I should begin as simply as possible. So this post is going to focus on how we go about measuring some basic flow conditions in a nearshore environment. We are going to ignore sediment, bathymetry (i.e. water depth, mapping the sea floor), silt-laden water, significant changes in salinity and temperature, and anything else that goes beyond good ol' waves and currents meeting in shallow water.

This still leaves us with a lot to cover. So let's get to it.

In the last post, I gave the short version of my graduate project. I mentioned that my lab conducts field experiments to gather (as) accurate (as instruments will allow) data to understand how storms, waves and currents cause an inlet to migrate. I also mentioned that I use this data to validate a numerical model which also tracks how the inlet migrates.

Why do I need both data and model?

In an ideal world, field data would give us all of the answers. Because it is, in fact, as close to real life as you can get, field work can best approximate what happens when every variable imaginable impacts our system and moves the inlet. A model, by necessity of computation and limitations in our physical understanding, has to simplify a lot of the dynamics to calculate what is happening (or has happened, or might happen...more on that later). Fieldwork, in essence, extracts measurements directly from what we want to study. It is the only experimental tool we have to capture real world phenomena which keep every good fluid mechanist and climate scientist up at night.

The problem is that fieldwork is hard. And really expensive. So you're not likely to get anywhere near as much data as you need to answer some of Life's big fluid physics questions.

Let's give an example to drive the point home.

We use a variety of instruments to measure currents. One typical instrument is an ADCP, or Acoustic Doppler Current Profiler. The name alone gives a sense of what this does: it uses sound waves to measure velocities in the water over a range of depths. It looks something like this:

Mmmm, standard Nortek brochure images of three ADCP configurations.

Now, out of the water these instruments are not going to do very much. If you put them somewhere in water and point the acoustic receivers (black circles in the above picture) the right way, you can get measurements of how fast the current is flowing.

If you simply toss one of these in the ocean, however, you probably will never see it again. The currents it is measuring will sweep it right along and make your PI very sad, having just lost a couple grand in instrument funds.

If, on the other hand, you put this thing on a large, weighted plate at the seafloor, or mount it on a long pole which you then jam into the seafloor (the pole, not the ADCP), you have a good chance at being able to come back and find the thing again when you want the data. Or when it runs out of batteries.

The fact that the instrument remains in one place at all times means that it is an Eulerian measurement device. It can only monitor the water right above it, so you get a single depth profile at a point in space over your deployment time.

If you want to know what the current fields look like over a larger area (say, the entire inlet system), you need to deploy multiple ADCPs at different locations and pray that nothing exciting is happening anywhere you have not placed an instrument. The issue is that if something exciting is happening somewhere you are not measuring, you will probably never know.

Another issue, often overlooked in typical physical oceanography classes, is the deployment of said instruments. How do these things get jammed into the bottom of the seafloor?

Divers. Yup.
In my lab, people take the instruments down to 7 meters (~21 feet) or so depth, and then wrestle them into whatever sediment has decided to act the part of the seafloor for that day (and, yes, this can change). Of course, when you are throwing people overboard with an expensive instrument, you want to make sure that both they (primarily) and the instrument (secondarily) come out unharmed. This limits your deployment sites. A really interesting place with high currents is hazardous.

Katama, thankfully, has a fairly predictable tidal schedule. In fact, last summer our divers had all of 15 minutes of slack tide to get things in the water before 2 m/s currents swept them out to sea.

And you thought your work had strict deadlines.

The field team members who hang out with the grad students in my lab are (thankfully) incredibly competent at their job. They are divers, technicians, tenacious, and very dependable to get things done in a safe manner. I am fairly certain I would be too frightened to attempt some of the deployment techniques they come up with to get my instruments in the water. The best I can do is make sure I do not put them in a particularly dangerous spot, and to make sure that they have all of the equipment they need to get the job done safely.

Speaking of equipment, there is more where the ADCPs came from.

Boat which can carry an assortment of instruments, divers, slackers, and occasionally one dog.

We have several different instruments which measure, in addition to currents

PAROS, or pressure gages which measure the depth of water above the sea floor

Another stock photo, alas -- I do not seem to have many of these in action. AWAC (Acoustic Wave and Current profiler)
The trifecta of waves, currents, and pressure provide a thorough picture of what is happening at any given location in the Katama system. Locations like these:

Yes, this is how we plan field deployments. With large maps and pumpkin stickers.
We choose different locations each year based on results from the previous year and what the model suggests might be interesting spots. I've had the luxury of returning to the same area again and again to reconfigure instrument locations for maximum spacial coverage. Not many scientists get this option, which is another hitch when it comes to getting useful field measurements.

In essence, fieldwork boils down to safety, funding, a great field crew, and competent decisions made by the scientists running the deployment. All of these need to come together to gather a useful data set, but even when they do there is no guarantee everything will go as planned.

Despite the difficulties, fieldwork can be incredibly rewarding. Three years of deploying instruments in and around Katama Bay have yielded a fascinatingly complex picture of how the system works. My model helps to fill in some of the spacial and temporal gaps in our data, but the measurements alone give a sense of how this system behaves.

More on that...later. I need to go an finish a paper first. And I have not even begun to explain what you can do with field techniques that move, instead of staying in one place like all of the instruments mentioned above.

Indeed, it gets more interesting. I see I have a lot to cover in the near future on this thing.

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