4.2 Energy dissipation devices

The water flowing over the spillway acquires a lot of kinetic energy by the time it reaches near toe of spillway. Hence arrangements have to be made to dissipate this huge kinetic energy or it will scour near toe of dam and away from it. These arrangements are known as energy dissipaters.

In general it is dissipated in to two ways:

· Converting supercritical flow in to subcritical flow by hydraulic jump.

· Directing flow of water in to air and then making it fall away from toe.

Hydraulic Jump Formation

When a steam of water moving with high velocity and low depth, strikes another stream of water low velocity and high depth. This phenomenon of hydraulic jump accompanies by large scale turbulence, dissipating most if the kinetic energy of supercritical flow.

Depending up on tail water depth there can be fine solutions depending up too hydraulic depth. If a graph is plotted between discharge intensity and tail water depth (Y1) the curve obtained is known as tail water curve.

Y1= pre jump depth

Y2= post jump depth

CASE 1: TWC coinciding with Y2 curve at all discharges:

This is most ideal condition for jump formation. Hydraulic jump will form at toe of spillway at all discharges. In such a case simple concrete apron of length s(Y2-Y1) is sufficient.

CASE 2: TWC lying above the Y2 curve at all discharges:

The jump will form on slopping apron where depth equal to Y2 is available slope of apron is made in such a way that jump will occur somewhere on apron at all discharges.

Apron is upturned sharply at ends two main rollers are formed which dissipate the energy due to internal turbulence.

CASE 3: TWC laying below the Y2 curve at all discharges:

If tail water is very low the water may shoot up out of above bucket and fall harmlessly into river at some distance d\s of the bucket. This bucket is then known as ski jump bucket.

Construction of subsidiary dam below main dam will increase tail water depth and cause a jump to form at toe of main as shown in fig.

CASE 4: TWC laying above Y2 curve at smaller discharges and laying below Y2 curve at larger discharges:

Auxiliary energy Dissipating Devices:

Chute Blocks

Chute Blocks are kind of serrated device and provided at the entrance of stilling basin. The incoming jet of water is furrowed and partly lifted from the floor producing a shorter length of jump then what would have been without them.

Sills and Dentate Sills

Sill or more preferably detented sill is generally provided at the end of stilling basin. The dentated sill diffuses the residual portion of high velocity jet reaching the end of basin.

Baffle Piers:

They are blocks placed the basin across the basin floor. They help in the breaking the flow and dissipate energy mostly by impact, sometimes called as friction blocks. They are however unsuitable for large works due to cavitations under high velocity jets.