Visualizing the Shields Parameter

Thomas Hickson
University of St. Thomas
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The Shields parameter is a fundamental dimensionless variable that embodies the complex factors that interact to initiate motion on a sediment bed. Many students have trouble grasping this (and most other) dimensionless numbers, yet they are fundamental to understanding the controls on sediment transport and deposition. Here I provide a dynamic, interactive equation that plots points on the Shields diagram, thus allowing students to explore the many variables that impact initiation of motion.

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I use this in a junior level, required sedimentology and stratigraphy course.

Skills and concepts that students must have mastered

They have been introduced to grain size and some basic ideas of turbulent versus laminar flow.

How the activity is situated in the course

I would use this spreadsheet as part of a short in-class or take-home assignment on the initiation of motion.


Content/concepts goals for this activity

I want students to be able to explain the range of factors that govern the movement of a particle on the bed and I want to dispel the notion that velocity is the most important variable. I also want to give them a bit of introduction to dimensionless numbers.

Higher order thinking skills goals for this activity

Qualitative analysis of what appears to be a complex 'equation' (in their words).

Other skills goals for this activity

Description of the activity/assignment

This is not so much an activity as it is a useful tool to build an activity around. I created an Excel spreadsheet that calculates the Shields parameter and the grain Reynolds number for a given set of conditions, then plots the resultant values on the Shields diagram. Thus, students can tweak the boundary shear stress value to calculate the stress required to move a given grain size in any fluid, under any gravitational conditions. It is a great way for them to understand how changes in fluid density and viscosity (water vs. air) affect initiation of motion, as well as the effects of grain size. I might use this in a lecture on initiation of motion, asking them to answer some basic questions (see attached example).

Determining whether students have met the goals

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