# Model help:FallVelocity

## FallVelocity

This model is used to calculate Particle fall velocity.

## Model introduction

This model calculate fall velocity for spheres. Uses formulation of Dietrich (1982).

## Model parameters

Parameter Description Unit
Input directory path to input files
Site prefix Site prefix for Input/Output files
Case prefix Case prefix for Input/Output files
Parameter Description Unit
kinematic viscosity of water (ν) m2 / s
Submerged specific gravity of sediment (R) -
Grain size (D) mm
Acceleration due to gravity (g) -
Parameter Description Unit
Model name name of the model -
Author name name of the model author -

## Uses ports

This will be something that the CSDMS facility will add

## Provides ports

This will be something that the CSDMS facility will add

## Main equations

• Explicit particle Reynolds number
 ${\displaystyle Re_{p}={\frac {{\sqrt {RgD}}D}{\nu }}}$ (1)
• Dimensionless fall velocity
 ${\displaystyle R_{f}=exp\left(-b_{1}+b_{2}ln\left(Re_{p}\right)-b_{3}\left(ln\left(Re_{p}\right)\right)^{2}-b_{4}\left(ln\left(Re_{p}\right)\right)^{3}+b_{5}\left(ln\left(Re_{p}\right)\right)^{4}\right)}$ (2)
• fall velocity
 ${\displaystyle R_{f}={\frac {v_{s}}{\sqrt {RgD}}}}$ (3)

## Notes

b1, b2, b3, b4, b5 are all parameters from Dietrich(1982).

This formulation is only valid for Reynold’s numbers less than or equal to 2.5·106. If Rep is greater than this upper limit, the function will alert the user, and exit the program.

## Examples

An example run with input parameters, BLD files, as well as a figure / movie of the output

Follow the next steps to include images / movies of simulations: