my study

At times the day moves by way too slowly. I look at the clock constantly to see that only 1 hour has passed, see how many dozens of measurements still left before I can complete my workday, and I wail. Who doesn't have those kinds of days? But in the big picture, time is really moving by quite quickly. It seems like I have barely completed one day of work, go home to cook/eat, and relax for a bit, then it is to bed and back up at it again before I know it. Even though this pattern has been true all my life (though sometimes substitute play for work), I am still amazed at how time passes.

Work is going well these days. The sampling can be a bit monotonous, but I believe I am seeing the kind of results that I was expecting. This is good! That means that I am on schedule. I still have some pretty large decisions to make before I know whether it all will work out, but I have confidence that everything will be okay. Some of you have wondered about what it is I am doing, so here it goes:

In stream restoration work, one of the most common practices is to plant vegetation. These plants are important for many ecological reasons such as cover for animals or nutrients, but also for many physical reasons. Plants act as natural barriers to water flowing past them, slowing down currents and reducing bank erosion. Of course there are many other functions that plants provide, but my focus is on the last thing I mentioned - how plants act as natural resistance to flow.

So what does that mean exactly? I study how different patterns of plants growing on the toe of a bank influence how fast water is flowing over that bank. This is difficult to study in the field because you have to wait for different water levels and velocities, which are impossible to replicate, making comparisons difficult. That is why I set up an experimental study in a flume. In the flume I can control factors such as discharge, flexibility of plants and depth of plant submergence, which makes it easier to isolate the influence of plant patterns.

My flume is approximately 25 feet long (I'll update with a picture soon, for now you can look back through other blog entries). The first couple of feet are for the flow straighteners. The next 16 feet include an insert where about 1/3 of the bed is a flat "main channel" and the other 2/3 represents the sloped toe of a bank. I use two different bank toe inserts - a 15 degree slope and a 30 degree slope. About 9 feet of the bank toe has holes drilled in it for vegetation. The plants are spaced so that when scaled to a real bank toe, there would be about 24 per square meter. At the end of the flume is a weir. The flume empties into a stilling basin with a v-notch weir where I take my discharge measurements.

I am collecting measurements on 10 experimental set-ups. Two are a "no vegetation" condition. The remaining eight are split evenly between the two different bank toe slopes. I have four different combinations of high/low density of plants and high/low projected area. Projected area is the "density" of vegetation that the water sees. In other words, when we normally think of plant density, it is as if we are above the plot of land looking down on it. You could measure it by counting the number of plants per area (just one of many ways). For projected area you would be standing on the ground looking into that plot of land. This time you are measuring by counting the number of leaves over the area you are looking at (also one of many ways).

With this study, I am hoping to quantify the difference in resistance due to the plants and the difference in shear stress applied on the bank toe. To gather this information, I collect 3-D flow velocity data along the bed and at 0.6 depth. With the velocity data I look more closely at turbulence along the bed, forces on the bed, and average flow velocity within the vegetation versus the main channel. I also look at how surface water elevation changes depending on the vegetation.

Back to the big picture - what will this help us understand? I am hoping that a project like this will help inform restoration activities in the future. There are two ways - one, by telling us what will give us the biggest bang for our buck. For instance, let's say you want to plant a bunch of saplings along a bank. It would be useful to know if a few sparsely planted stems with a lot of branches would give you a similar amount of protection as a densely planted bank of willow whips. Or say the bank is already sparsely planted, when it drops it's leaves, how much more vulnerable will it be to erosion than when the leaves were on? If they are different, that flood that happens in late spring will be different than the mid-winter flood.

So that's my project, boiled down to the basics. Throw in a bunch of fluid mechanics and a dash of geomorphology and plant biomechanics, and that's about it. I've seen a fun term describing the field of study for projects kind of like this (though generally a bit more field based) - hydrobiogeomorphology. Maybe I should start calling myself a hydrobiogeomorphologist just to see strange looks on people's faces!