I find that a lot of my conversations revolve around, in some ways, "extracurricular" stuff. Hobbies, relationships, travel. I want to get better at asking people about their work—what they spend doing for hours on weekdays—through questions that probe deeper than making small-talk.
There seems to be some friction in doing so because talking about work requires sharing the nitty-gritty of what you're working on, and there is the feeling that this doesn't matter to someone who isn't in the same field. I'm trying to remind myself that every conversation is an opportunity to learn about some unknown realm of ideas I wasn't previously aware of. There are juicy nuggets tucked into just about anything.
So, this week, I want to tell you a little about why I find sediment transport so interesting. This is my deep-dive!
Firstly, when I say "sediment transport," what I mean is the movement of granular material (e.g. pebbles, sand, dust particles) via a fluid (water or air, usually). Specifically, what I am studying is how sediment is moved by rivers, through estuaries, and along coastlines. Sediment ranges in size from boulders to mud particles, and these particles get pushed or carried by water moving due to wind, waves, tides, upwelling, and river flows. These flows apply a stress to the bed, at the interface between the water and the sediment, and this stress moves the sediment around. Exactly how to characterize where this interface exists is a fairly open question, especially for coarse sediment.
Imran 2007
This is the start of the messiness of sediment transport. Some particles are light enough to be essentially buoyant in a turbulent water flow—so once they're picked up ("entrained"), they will flow until the water goes still—then they will settle out to the bottom. Some particles are too heavy for this, and roll or "saltate" along the bottom bed. Particles bump into each other, change the properties of the water that carries them, affect how the bed is textured (i.e. making ripples), shield each other from the water that tries to pick them up, and occasionally clog their conduits. The particles are rarely homogeneous, meaning they will all move at different stresses and in different ways, and some particles
flocculate together only to later get pulled apart again.
Kondolf 1997
This is to say, even under laboratory conditions, predicting exactly how sediment is going to be moved is pretty tough. We make some huge assumptions when trying to think analytically. ("Assume all particles are the same size and shape... assume the flow is unchanging and smooth... assume the particles have a constant value of friction along the bottom...")
The above is a micro-scale understanding of sediment movement. However, a more zoomed-out approach is often more useful, and is often referred to as "sediment budgeting."
It is what it sounds like: if we examine a certain geographic area or feature, how much sediment is entering, how much is leaving, and, thus, is sediment accumulating or eroding? We don't need to have a flawless understanding of how each individual particle moves around, we just need to know that sediment gets picked up and moved around my flows, waves, and tides. A shoal in a riverbed may accumulate for some time, and then erode when the flow regime (and suspended sediment in the flow) changes—if we perform a sediment budget on the same timescale as the regime change, we'll capture these dynamics. Sediment budgeting is useful in dredge management, habitat maintenance, beach replenishment, and more. For instance, budgeting gives evidence that many
bays around the world are slowly filling in with sediment. The same is
definitely true of the world's dams.
The
project I'm working on this summer is using field data to calculate two specific properties of sediment-water dynamics in North San Francisco Bay: 1. its erodibility (given a stress applied by waves, currents, and tides, how much does the bed erode?) and 2. its roughness (how much friction does the water "feel" as it flows over the bed?). By answering these questions, we'll have ballpark values to inform understandings of sediment movement around the Bay (to help keep Delta smelt alive) and to use in more formal modeling work.
Day-to-day, for me, this mostly just looks like a lot of data management, looking at sediment samples, and conversations/readings on the topic to dig deeper into understanding and relevance.
Ask me about sediment transport and budgeting any time!!!
Muddy,
Lukas