# Visualizing the Shields Parameter

Thomas Hickson
,
University of St. Thomas
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#### Summary

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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## Context

#### Audience

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.

## Goals

#### 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).

## 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 