�ke Sundborg later modified the Hjulstr�m curve to show separate curves for the movement threshold corresponding to several water depths, as is necessary if the flow velocity rather than the boundary shear stress ( as in the Shields diagram ) is used for the flow strength.
2.
Assuming a single, well-mixed, homogeneous fluid and a single acceleration due to gravity ( both are good assumptions in natural rivers, and the second is a good assumption for processes on Earth, or any planetary body with a dominant influence on the local gravitational field ), the only two variables that determine the boundary shear stress are the depth and the slope.