Design of Dam Bottom Outlets - CH-3
Design of Dam Bottom Outlets - CH-3
Design of Dam Bottom Outlets - CH-3
Bottom outlet is the outlet structure with the lowest elevation which can
serve for various purposes:
Drawdown of reservoir
The load test of a dam is performed during the first filling of the reservoir.
Qo
Fp
h
The filling must be made progressively by accounting for the stability and Qb
the water tightness of both the dam and its surroundings at certain levels of
zo
the reservoir.
The load test procedure during the first filling is only amenable if a bottom
outlet is available. Qo is the reservoir approach flow corresponding to an average value
over a certain time period such as several weeks or even months.
The bottom outlet must thus be designed that the reservoir level can be The choice of Qo depends on inflow hydrograph and reservoir storage
kept constantly under arbitrary levels
characteristics.
Design guidelines for emergency drawdown:
Drawdown by the bottom outlet has to be fast The drawdown should not cause shore slides.
Acceptable bottom
outlet dimensions
The allowable discharge Qs can be estimated from the limit drawdown
h=h(Qb)
velocity us h
Solution
Qo+Qs
h domain
us
t
and
Qs = us A
During flood periods, a drawdown is thus impossible. In certain cases, bottom outlets are used for sediment flushing.
Usually, one would choose average discharge conditions for the design. Continuity of the sediment transport is important. If it deposits in the
tailwater the bottom outlet may be submerged and thus endangered.
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 The ratio between water and sediment has to be such that it
weeks. corresponds to the transport capacity of the tailwater.
Design Principles
Flood and residual discharge
A useful design is the combination of diversion tunnel and bottom outlet.
Combined use of overflow spillway and the bottom outlet may be
allowed for flood conditions.
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 1) Tunnel outlet in a counter-current air flow along the outlet roof.
Qa
a0
a Q Cca
where
Cc is the contraction coefficient,
a the gate opeing, Cavitation occurs by decreasing the local pressure under constant
b the gate width, and temperature.
H-∆he the head on the gate with ∆he, the head loss from the
entrance to the gate section.
The contraction coefficient is dependent on the gate geometry and The local pressure reduction in a fluid flow can be caused by:
opening.
•a decrease of total energy head because of increase in elevation,
For quick calculations of flow rate as function of gate opening one can
assume: •a local increase of velocity and
p pv hh
2 v
pv (kPa)
2
V0 / 2 V0 / 2g 10
where 5
p is local pressure,
pv is vapor pressure and
V0 is the reference velocity typically of the upstream flow.
0
0 10 20 30 40 50 60
o
Temperature ( C)