Jumps

Need for Energy Dissipaters

When water is released over the spillway, the potential energy is converted into kinetic energy at the base of spillway. This energy must be dissipated in order to prevent the possibility of severe scouring of downstream . For this purpose energy dissipators must be used which perform the energy reduction by converting the kinetic energy into turbulence and finally into heat.

Hydraulic Energy Dissipation DevicesWater CushionBaffle WallsBiff WallsDeflectorsStaggered blocksRibbed PitchingHydraulic Jumps

Water Cushion

Baffle Walls

It is an obstruction constructed across the canal downstream of the fall. It is in the form of a wall of low height.It heads up the water just upstream of it. Thus it tries to create a water cushion on the upstream.

Biff WallsIt is an end wall of the cistern. It is a vertical wall with a horizontal projection extending in the cistern.Due to the projection, the flow of water returns back in the cistern. It creates an obstruction to the fast moving water down the fall. As a result the energy of flow is dissipated.

DeflectorsIt is a short wall constructed at the end of a downstream side.This end wall deflects the high velocity flow of water. Due to deflection the velocity of flow in the direction of motion is reduced.

Staggered BlocksThey are nothing but rectangular blocks or cubes generally made of concrete. They are arranged in a staggered manner on the downstream horizontal apron. They deflect the high velocity flow in a lateral direction. It provides an obstruction to high velocity downstream flow and the energy of flow is dissipated effectively.

Jumps

For a given value of specific energy, the critical depth gives the greatest discharge, or conversely, for a given discharge, the specific energy is a minimum for the critical depth. For rectangular channels, the critical depth, dc ft (m), is given by dc=[Q2/b2g]1/3Where dc =critical depth, (m)Q= quantity of flow or discharge, (m3/s)B= width of channel, (m)

Theoretical depth after jump alternate depth

Actual depth after jump sequent depth

Length of jump about 5-7 times the subcritical depth

Uses of Hydraulic Jumps Hydraulic jump is used to dissipate or destroy the energy of water where it is not needed otherwise it may cause damage to hydraulic structures.

It may be used for mixing of certain chemicals like in case of water treatment plants.

Hydraulic jump usually maintains the high water level on the down stream side. This high water level can be used for irrigation purposes.

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Case 1

This is the ideal case in which the horizontal apron provided on the riverbed downstream from the toe of the spillway would suffice. The length of the apron should be equal to the length of the jump corresponding to the maximum discharge over the spillway.

Class 1Jump height rating curve is always above tailwater rating curve. In this class, the depth of in the river is insufficient for all discharges for the formation of a jump at the toe of the structure. The jump will form at certain place for downstream (Case 2). The energy dissipation can be achieved in any of the following ways: 1. Lowering the floor level downstream of the dam in order to make the tailwater depth in the stilling basin equal to the jump height for all discharges. 2. Stilling basin with baffles or sills at bed level. 3. Stilling basin with a low secondary dam downstream. 4. Bucket type energy dissipators (ski-jump).

Ski Jump

Sloping Apron below the Bed

Dam Construction

Note Length is same for all. You cannot break any of matchsticks.

Class 2The jump height curve is always below tailwater rating curve. This means that Case 3 occurs at all times and the jump will move upstream consequently, little energy will be dissipated. A method of energy dissipation can be achieved by: 1. Sloping apron. 2. Roller bucket type energy dissipator.

Sloping apron above river bed

Roller Bucket Energy DissipationA roller bucket energy dissipator consists of a circular arc bucket tangent to the spillway face terminating with an upward slope. This geometry when located at an appropriate depth below tailwater will produce hydraulic conditions consisting of a back roller having a horizontal axis above the bucket and a surge immediately downstream from the bucket. Solid and slotted buckets have been used successfully.

Class 3Jump height curve is above tailwater rating curve at low discharges and below at higher discharges. An effective method of dissipating energy is by: 1. Stilling basin for forming a jump at low discharges and to combine with the basin a sloping apron for developing a satisfactory jump at high discharge. 2. Stilling basin with baffle piers or dentated sill.

Apron partly above & below G.L.

Class 4Jump height curve is below tailwater curve at low discharges and above at high discharges. An effective method to insure a jump is to increase the tailwater depth sufficiently high by providing stilling pool (basin), this forming a jump at high discharges.

Siphon SpillwayContains siphon pipeOn end at upstream end and other end at downstream end

Cistern or Water Cushion

Baffled WallsThe baffled chute spillway relies upon multiple rows of baffles to aid in dissemination of energy flowing down a spillway chute. The USBR has developed a set of design guidance which can be used in preliminary design of such a structure. Model studies are recommended for design verification when the design discharge exceeds 50 ft3/sec and/or the slope is steeper than 1V on 2H.

Presented By RAHUL GUPTA (D4 CE A 120091)