Bottom Outlets

CE 458 Design of Hydraulic Structures

3.1

3. BOTTOM OUTLETS


3.2

Purpose of bottom outlet Bottom outlet is the outlet structure with the lowest elevation which can serve for

various purposes:

Filling of the reservoir

Drawdown of the reservoir

Flushing of sediments

Discharging surplus water

A bottom outlet must be provided in every dam as a safety structure to control reservoir

level and for emergency operations.


3.3

The load test of a dam is performed during the first filling of the reservoir.

The filling must be made progressively by accounting for the stability and the

watertightness of both the dam and its surroundings at certain levels of the

reservoir.

The load test procedure during the first filling is only amenable if a bottom outlet is

available.

The bottom outlet must thus be designed that the reservoir level can be kept

constantly under arbitrary levels


3.4

Drawdown of reservoir

Qo is the reservoir approach flow corresponding to an average value over a certain

time period such as several weeks or even months.

The choice of Qo depends on inflow hydrograph and reservoir storage

characteristics.

Qo

Qb

Fp h

zo


3.5

Qb > Qo

Qb is the design discharge of the bottom outlet.

The hydrostatic pressure force on the dam face is

2/BhF 2p

A drawdown of the reservoir level is followed by a stress relief. Accordingly,

drawing down an endangered dam is highly efficient.

Drawdown by the bottom outlet has to be fast


3.6

Design guidelines for emergency drawdown:

Rapid drawdown of a reservoir within the shortest time lapse, the maximum Qb.

The tailwater valley should not be damaged by Qb.

Qb < QL

where QL is the limit tailwater discharge.

The drawdown should not cause shore slides.

Qb < (Qo + Qs)

where Qs is the allowable discharge for shore protection.


3.7

The allowable discharge Qs can be estimated from the limit drawdown velocity us

t

hus

and

Qs = us A where A is the reservoir surface area. The limit drawdown velocity is related to allowable pore water velocity of the

groundwater flow along the shores of the reservoir.


3.8

Optimization of head-discharge relation for design discharge of bottom outlet.

h

Qb

Qo QL

Qo+Qs

h=h(Qb))

Solution domain

Acceptable bottom outlet dimensions


3.9

For flood conditions, the approach discharge Qo is large and the limit discharge QL

is small. Then, domain of solution shrinks, and can even vanish.

During flood periods, a drawdown is thus impossible.

Usually, one would choose average discharge conditions for the design.

The drawdown time is an important parameter. Under emergency, one would like

to draw down the top layers of the reservoir only, in a few days or weeks.

Bottom outlet is not a structure for permanent use due to limitations regarding

cavitation, hydrodynamic forces, abrasion and vibrations. It should, however, allow

a complete emptying of the reservoir when necessary.


3.10

Flushing of sediments

In certain cases, bottom outlets are used for sediment flushing.

Continuity of the sediment transport is important. If it deposits in the tailwater the

bottom outlet may be submerged and thus endangered.

The ratio between water and sediment has to be such that it corresponds to the

transport capacity of the tailwater.


3.11

Flood and residual discharge

Combined use of overflow spillway and the bottom outlet may be allowed for flood

conditions.

For this purpose multiple bottom outlets may be constructed.

A bottom outlet designed according to previous criteria is normally too large to fit

for the control of residual discharge.

It had to be operated permanently with an extremely small opening not suited

hydraulically. Therefore, a small outlet is often added to satisfy the needs for

residual discharge.


3.12

Design Principles

A useful design is the combination of diversion tunnel and bottom outlet.

Karakaya dam: 1 Intake structure, 2 Diversion tunnels, 3 Bottom outlet chambers, 4 Upstream cofferdam, 5 Service building, 6 Outlet works, 7 Powerhouse.


3.13

For smaller dams, a culvert type bottom outlet may also be considered because of the

simple design.

Possible arrangements for the bottom outlet

a) Diversion tunnel used as bottom outlet, with access through a shaft.


3.14

b) Bottom outlet culvert, not accessible except for minimum reservoir level, with two

gates close to the inlet.


3.15

c) Diversion tunnel used as combined spillway and bottom outlet for morning glory spillways.


3.16

d) Gravity dam with bottom outlet that is much shorter than for an earth dam.


3.17

Technical requirements for a bottom outlet

Smooth flow for completely opened structure.

Excellent performance for all flows under partial openings.

Effective energy dissipation at terminal outlets.

Structure without leakage.

Simple and immediate application.

Easy access for maintenance and service.

Economic and useful design.

Long life.


3.18

Hydraulic requirements for a bottom outlet

Pressurized flow upstream of the gate.

Free surface flow downstream of the gate.

A trash rack is provided at the inlet.


3.19

The tunnel often has a horseshoe profile.

The tunnel section contracts to a rectangular cross-section shortly before the gate

chamber.

Downstream from the gate chamber, the tunnel is expanded both laterally and at

the tunnel ceiling.

For long tunnels an aeration conduit behind the gate chamber may be required.

The air supply conduit has to be designed so that the gate chamber is safe against

submergence from the tunnel.

It is imperative that submergence of the bottom outlet is inhibited. The discharge is

then fully controlled with the gates.

Submergence will cause the mechanical equipment be wetted in the gate section.


3.20

Hydraulic phenomena that must be considered in the design of bottom outlet

Average velocity at the bottom outlet is large.

Cavitation

Abrasion

Aerated flow

Sediment flow due to reservoir sedimentation

Floating debris or sediment deposition

Gate vibration


3.21

Air entrainment

Free gate outflow reduces the potential of gate vibration and cavitation.

A bottom outlet should always be designed for free surface flow.

Aeration of flow just after the gate is required for rapid energy dissipation and to

reduce the risk of cavitation.

The aeration of flow may originate from three different sources


3.22

1) Tunnel outlet in a counter-current air flow along the outlet roof.

2) Air supply conduit by which the underpressure of the surface air is reduced.

3) Bottom aerator that counter problems with cavitation damage.


3.23

Flow types (without bottom aerator)

a) Spray flow for relative gate

opening below 10%, with an

extremely high air entrainment.

b) Free flow as typical for bottom

outlets, and accompanied by features

of supercritical flow, such as shock

waves and two-phase flow.

.


3.24

c) Foamy flow for a tunnel almost

full with air-water flow.

d) Hydraulic jump with a free

surface tailwater flow due to

tailwater submergence.


3.25

In applications, cases e and f should be avoided because of possibility of

dangerous surging in the channel.

e) Hydraulic jump with transition to

pressurized tailwater flow.

f) Fully pressurized flow caused by a

deep tailwater submergence.


3.26

Gate Controlled Discharge

Gate flow may be either free or submerged. For free gate flow, the space behind the

gate is filled with air of pressure head ha. If the efflux is into the atmosphere, ha=0.

a0

a Cca

Qa

Q


3.27

Based on energy considerations, the underflow discharge of a gate is given by

2/1acec )haChH(g2abCQ

where Cc is the contraction coefficient,

a the gate opeing,

b the gate width, and

H-Δhe the head on the gate with Δhe the head loss from the entrance to the

gate section.

The contraction coefficient is dependent on the gate geometry and opening. For quick calculations of flow rate as function of gate opening one can assume:

40c )a/a(2.08.0C


3.28

Cavitation Cavitation is defined as formation of bubble or void in a liquid.

Cavitation occurs by decreasing the local pressure under constant temperature.

The local pressure reduction in a fluid flow can be caused by:

a decrease of total energy head because of increase in elevation,

a local increase of velocity and

turbulence, vortices or large scale separation.


3.29

The water flowing in hydraulic structures contains air bubbles of various sizes and with

numerous impurities. These conditions are necessary to initiate cavitation.

The hydrodynamic parameter describing the cavitation process is cavitation index

where

p is local pressure,

pv is vapor pressure and

V0 is the reference velocity typically of the upstream flow.

g2/V

hh

2/V

pp20

v20

v


3.30

Temperature (oC)

p v(k

Pa)

0 10 20 30 40 50 600

5

10

15

20

Vapor pressure for water


3.31

Example: a sudden into-the-flow offset

> 1.8 : no cavitation

0.3 < < 1.8 : Developed cavitation < 0.3 : Supercavitation

= 1.8 : Incipient cavitation where cavitation bubbles occasionally can be observed.


3.32

If cavitation occurs close to flow boundaries, then cavitation damage may happen, and

the structure can seriously be damaged.

Isolated roughness elements causing cavitation

Offset into-the-flow Offset away from-the-flow

Groves Protruding joints


3.33

Incipient cavitation for into-the-flow offset.


3.34

Incipient cavitation index for chamfered offset.


3.35

Control of cavitation Cavitation damage can be controlled by two methods:

Control of cavitation index by geometry

Control of cavitation damage by aeration


3.36

Steps in cavitation control

Prediction of water surface profiles.

Determination of pressure head curves.

Computation of cavitation index curves.

If there are locations where cavitation is predicted, then the geometry of the

structure or smoothness of the boundary has to be improved.

If both approaches fail, the flow has to be aerated.

Capacity of a structure with an aerated flow has to be larger because of the

increase of air-water discharge.

Bottom Outlets

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