Dams are constructed to store excess river water during high flow periods and release it according to a regulated schedule. Spillways safely pass excess water during floods and include various types like free overfall, overflow, chute, and side channel spillways. Energy dissipators are structures like stilling basins that reduce the energy of outfalling water from spillways to prevent downstream river scouring. Selection of an energy dissipator depends on jump height from the spillway and tailwater depth based on discharge observations.

1. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
1
DAM (GENERAL),
SPILLWAYS & ENERGY
DISSIPATORS
BY SAGAR DODEJA, IES
• In order to harness the water potential of a river
optimally, it is necessary to construct two types
of hydraulic structures:
1. STORAGE STRUCTURE: usually a DAM, which
stores excess water of a river during periods of
high flows (as during the monsoons) and
releases it according to a regulated schedule.
2. DIVERSION STRUCTURE : may be a WEIR or a
BARRAGE that raises the water level of the river
slightly, not for creating storage, but for
allowing the water to get diverted through a
canal situated at one or either of its banks.
STRUCTURES FOR HARNESSING WATER RESOURCES OF A RIVER

2. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
2

3. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
3
• These types of dams have considerable
upstream curvature in plan and rely on an
arching action on the abutments through
which it passes most of the water load on to
the walls of the river valley.
• This type of dam is structurally more efficient
than gravity dams and greatly reduces the
volume of concrete required.
ARCH AND DOUBLE ARCH(CUPOLA) DAMS

4. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
4
• Earth-fill or earthen embankments –These
may be classified as dams in which compacted
soil is used for constructing the bulk of the
dam volume. An earth fill dam is constructed
primarily of selected engineering soils
compacted uniformly and intensively in the
relatively thin layers and at a controlled
moisture content.
• Rock-fill embankments – In these types of
dams, there is an impervious core of
compacted earth fill or a slender concrete or
bituminous membrane but the bulk of the
dam volume is made of coarse grained
gravels, crushed rocks or boulders.
BUTTRESS DAM

5. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
5
• This type of dams consist of a continuous
upstream face supported at regular intervals
by buttress walls on the down stream side.
• These dams are generally lighter than the
gravity dam but likely to induce slightly higher
stresses at the foundation since most of the
load now passes through the buttress walls
and not spread uniformly over the foundation
as in a solid gravity dam.
SPILLWAYS
• Passages constructed either within a dam or in
the periphery of the reservoir to safely pass
this excess of the river during flood flows are
called Spillways.
• The surface of the spillway should also be
such that it is able to withstand erosion or
scouring due to the very high velocities
generated during the passage of a flood
through the spillway.
• At the bottom of the channel, where the
water rushes out to meet the natural river, is
usually provided with an energy dissipation
device that kills most of the energy of the
flowing water.
• These devices, commonly called as Energy
Dissipators, are required to prevent the river
surface from getting dangerously scoured by
the impact of the outfalling water
• Usually, spillways are provided with gates,
which provides a better control on the
discharges passing through. However, in
remote areas, where access to the gates by
personnel may not be possible during all times
as during the rainy season or in the night
ungated spillways may have to be provided.

6. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
6
TYPES OF SPILLWAYS
1. Free Overfall Spillway
2. Overflow Spillway
• It has a crest shaped in the form of an ogee or
S-shape

7. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
7
3. Chute Spillway
• A chute spillway, variously called as open channel
or trough spillway, is one whose discharge is
conveyed from the reservoir to the downstream
river level through an open channel
• A side channel spillway is one in which the
control weir is placed approximately parallel to
the upper portion of the discharge channel
4. Side Channel Spillway
5. Shaft Spillway
• A Shaft Spillway is one where water enters
over a horizontally positioned lip, drops
through a vertical or sloping shaft, and then
flows to the downstream river channel
through a horizontal or nearly horizontal
conduit or tunnel

8. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
8
Energy Dissipators
• Different types of energy dissipators may be
used along with a spillway, alone or in
combination of more than one, depending
upon the energy to be dissipated and erosion
control required downstream of a dam.
• Broadly, the energy dissipators are classified
under two categories – Stilling basins or
Bucket Type Energy Dissipators
Stilling basin type energy dissipators
• HORIZONTAL APRON TYPE
Sloping Apron Type
Bucket type energy dissipators
SKI JUMP BUCKET TYPE

9. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
9
SELECTION OF AN ENERGY DISSIPATOR
An energy dissipator is selected based on the
following 2 parameters
1. JUMP HEIGHT – fixed for a particular
discharge intensity and height of the spillway
2. TAIL WATER DEPTH – calculated by actual
discharge observations

10. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
10
Now a graph is plotted between
1. Jump Height vs Discharge
2. Tail Water Depth vs Discharge
The above graphs are superimposed and the
following cases arise -

11. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
11

12. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
12

13. DAMS. SPILLWAYS AND ENERGY DISSIPATORS 12/08/18
13