Ch-3. There are several equations useful in understanding hydraulics and sediments deposits.

Reynolds NumberFroude NumberThese numbers helps us to understand the relationship between fluid flow, the type of bedforms produced along the surface, and the mechanism by which entrained particles move.

Reynolds Number

addressed the problem of how laminar flow changes to turbulent flow. the transition from laminar to turbulent flow occurs as velocity increases, viscosity decrease, the roughness of the flow boundary increases, and/or the flow becomes less narrowly confined.

Froud Number

The Froud Number is the ratio between fluid inertial forces and fluid gravitational forces.

Froud Number

It compares the tendency of a moving fluid (and a particle borne by that fluid) to continue moving with the gravitational forces that act to stop that motion.The force of inertia express the distance traveled by a discrete portion of the fluid before it comes to rest.Like reynolds Numbers, Froud numbers are dimensionless.

The equation for the Froud number is

Fr = fluid inertial forces . gravitational forces in flowFr= flow velocity . (acceleration of gravity)x(force of inertia)Fr= V , _/(gD)

The equation for the Froud number is

Fr= V , _/(gD)

V= velocity, D= depth, g= gravitational constant

The relationship among the bedforms or surface waves (ripples and dunes) produced beneath moving currents of wind or water, the flow streamlines within the current itself, and surface waves developed on the upper surface of the fluid changes with the Froud numbers; so too does the type of the flow.

Froud number

When the Froud number is less than 1, the velocity at which wave moves is greater than the flow velocity, and waves can travel upstream - (tranquil, streaming, or subcritical).When the Froud number exceeds 1, waves do not flow upstream, (rapid, shooting, or supercritical.So, Froud number of 1 represent the threshold between tranquil and rapid flow.

When tranquil flow give way to rapid it occurs in an smooth transition.When rapid flow suddenly decreases to a tranquil flow, there is an abrupt change known as Hydraulic jump (a sudden increase in depth accompanied by much turbulence, smoke).

Entrainment, transport, and deposition of clasts.

Entrainment

Bernoullis principle- the sum of velocity and pressure on an object in a flow must be constant if the velocity increase then the pressure must decrease, and vice versa.

Entrainment

Two main forces are involve in how particles get picked up:Fluid drag forceFluid lift force

How do sediments move, once they have been lifte?

Traction- they are rolled and dragged along the base of a moving fluid. Saltation- they abruptly leave the bottom and are temporarily suspended.Traction and saltation load together constitute bedload.

How do sediments move, once they have been lifte?

Supension constitues a third mode of transportat.It concist of those grains that more or less continually float within the moving fluid.

Clast size has an important effect on sediment entrainment, transport, and settling velocity, the factors that control deposition.This figure shows the critical velocity for movement of quartz grains on a plane bed.