INTRODUCTION

Hydraulic structural construction such as a water retention behind a dam may result in extreme high pressure on the foundation of the construction.

To reduce the hydrostatic pressure water is channeled out through a spillway or other structure

Rapid flow in the upstream may cause erosion dan destruction of the down stream channel. Therefore the downstream channel will require flow control to slow down the flow and not to cause any erosion.

Energy dissipation structure will be capable to reduce the extremely high energy of the flow to a lower energy.

This energy reduction within a short distance and fast is known as Rapid Varid Flow RVF

HYDRAULIC JUMP

A water flow that experience a change from a high energy fast flow velocity supercritical flow to a low energy slow velocity subcritical flow immediately within a short distance by dissipating or releasing its energy is known as Hydraulic Jump

The function of hydraulic jump in engineering include;

Reducing destruction of channel by scouring or erosion of the channel bed.

To increase the upstream water depth for a critical flume or canal to canal for irrigation.

To reduce drowned hydraulic structure uplift by uplift pressure due to supercritical flow water over the concrete structure.

Before hydraulic jump;

Supercritical flow with flow depth y1 < yc and Fr1> 1

After hydraulic jump;

Subcritical flow with flow depth y2> yc and Fr2> 1

Diagram shows the forces in a hydraulic jump

Theory and Assumption taken for hydraulic jump;

Channel is prismatic & no change in shape and cross section size and bedslope.

Friction on channel wall and bed is ignored because the length is short.

The bedslope is assume horizontal

Velocity in the sections of the channel is assume uniform

Shear stress on the wall and bed channel is ignored.

Force of flow

F= ρQv

Static force

ρghA

Equillibrium of forces acting on the hydraulic jump is;

FOR RECTANGULAR CHANNEL ONLY

Therefore;

FOR TRAPEZOIDAL CHANNEL

For triangle and rectangle part distance from the centeroid to water surface is hj

FOR TRIANGLE CHANNEL

LOCATION OF HYDRAULIC JUMP

The location of of hydraulic jump depends on the tail water depth i.e. the depth of flow at the downstream side of the channel. Hydraulic occurs when yos>yc>yom

Yos is normal depth on the steep (s) slope, yom is normal depth on the mild slope (m) & yc is critical depth

Case 1 y2 > yom

Hydraulic jump occurs on the downstream mild slope

Therefore for any channel section;

or;

Case 2 y2 = yom

Hydraulic jump occur at the meeting point of the two slopes

Case 3 y2 < yom

Hydraulic jump occur on upstream steep slope

THE DEPTH IMMEDIATELY BEFORE & AFTER THE HYDRAULIC JUMP

where;

The estimated distance from the meeting point of the slopes to the hydraulic jump L is

where;

and;

where average velocity vm is

and Rm is the average hydraulic radius upstream and downstream of channel length L

HEAD LOSSESS IN THE HYDRAULLIC JUMP

Substituting v2/2g with

therefore;

h is the distance from water surface to the center of gravity of the water in the channel. For rectangular channel h=y/2 and area A= By, so;

and finally energy loss is;

THE POWER LOSS IN THE HYDRAULIC JUMP

and the unit is Nm/s or Watts