Wetland Dynamics and Design
Coastal Wetland Sediment Transport
​
Background
During my freshman year at MIT, I had the opportunity to conduct research in Professor Heidi Nepf's Environmental Fluid Mechanics Lab in the Parsons Environmental Engineering and Science Laboratory. Now-PhD Judy Yang supervised and guided my research. My project focused on modeling coastal wetland systems to examine how emergent vegetation affects sediment flow. By doing so, I hoped to determine how vegetation in coastal wetlands influences erosion rates in order to find a way to minimize the adverse effects of erosion.
​
​
Running a sediment transport experiment in the Nepf Lab, with wooden rods to mimic submerged aquatic vegetation (2016).
Part I: Measuring Bed Topography
In order to examine how erosion occurs spatially, it is necessary to be able to accurately represent channel bed topography. As a precursor to sediment transport experiments, my task was to find a method to efficiently and precisely measure bed topography. To do so, we conducted a series of experiments using a laser that was designed to measure distance of solid surfaces. The challenges we faced were two-fold:
(1) being able to measure bed height with the presence of modeled vegetation (wooden rods) distributed across the bed surface, and
(2) adjusting the laser measurements based on refraction from the water surface.
​
​
A MATLAB model of the sand bed surface in the flume tank. The model was used to verify that the laser measurements were consistent with the physical surface appearance.
Urban Wetland Design
​
Background
One of my favorite times of the year is MIT Independent Activities Period, known as IAP. During the month of January, students have the option to do whatever they want -- take a class for a few weeks, spend more time at home, travel, you name it. During IAP 2017, I took a class called 11.S952 Infrastructure for Green Cities: Designing Urban Constructed Wetlands. The class was co-taught between a professor in the Department of Civil and Environmental Engineering, Heidi Nepf, and a then-graduate student in the Department of Urban Studies and Planning, Celina Balderas-Guzman. I had already worked in Professor Nepf's lab for a year (see above), so I knew about sediment transport in coastal wetland ecosystems. The class gave me the chance to move from the experimental side of wetlands into planning and design, in collaboration with urban design students.
Four measured volumes were tested to see if the derived equation for the height of the bed surface produced an accurate calculated volume. The black line shows a 1:1 ratio between measured and calculated volumes.
Part 2: Measuring Flow Velocity
This is where I will put info and a video.
Overall site design
After learning about the multifaceted functions of a wetland -- to slow water flow, encourage recreation and promote biodiversity -- we prepared a high level design for the site. The site was an area adjacent to a bayou in Houston, so we considered the local context. I researched what vegetation was native to the area and chose plantings that would attract migratory birds. As a group, we decided to create platforms above the wetland to allow for recreational activities, especially since the area around the wetland was economically depressed and lacked resources to develop other green spaces.
Wetland design testing
I helped design the wetland islands to try to achieve plug flow through the system. In plug flow conditions, each parcel of water spends the same amount of time in the system. This creates the longest cumulative travel time, which allows sediment and pollutants to settle before stormwater enters a larger body of water. To test out two alternatives, we sketched designs that would disperse water entering the system in different ways. The first design incorporated teardrop-shaped islands that would force flow to travel back and forth slowly. The second design had increasingly small islands from the upstream to downstream facing ends in order to break up flow into smaller and smaller parcels as it traveled.
Our goal was to be able to settle pollutants out of the first flush of rainfall, which contains the highest concentration of pollutants.
The architect in our group drafted up the two designs using CAD software so we could test flow through the structures. After printing them with a mill and lathe, we tested flow through the islands in a laboratory flume using fluorescent dye. The trials looked like this:
Results
Using fluorescent dye allowed us to model how pollutants would flow through the wetlands. We used computer software to turn the images into graphs of concentration over time. Our goal was to create wetlands with long retention times, which would be the case for plug flow. Under plug flow conditions, the concentration over time graph has one peak at the time when all flow is traveling through the system at the same rate. Our designs ended up being fairly comparable to each other, with similar retention times. Based on the retention times calculated and the our goal to capture the first flush of rainfall, we would need two sets of either wetland design to allow pollutants to settle. Since our draft design already incorporated two sets of wetland islands, we considered the project a success!
