Roxburgh DamCombined Discharge of Spillway and Sluice

into the same Chute

Peter Silvester – Civil Engineer Contact EnergyGrant Webby – Principal Hydraulic Engineer Opus International Consultants

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Roxburgh Dam - Location

Roxburgh Dam

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Roxburgh Dam

Commissioned 1956

50m high concrete gravity

320 MW power station

Mean flow 507 cumecs

PMF 7,000 cumecs

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Roxburgh Dam Spillway

Three Spillway Gates15.2m wide x 13.4m high.Discharge capacity 4,750 cumecs

Two diversion sluices7.1m wide x 8.7m high.Discharge capacity 2,900 cumecs.

Combined total discharge capacity 7,650 cumecs.

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Roxburgh Dam Spillway

Spillway chute 3 dewatered

Diversion Sluices

Sluices

7.1m wide 8.7m high

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Spillway – Sluice Long Section

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Physical Hydraulic ModelUniversity of Auckland

Tests covered:

• Sluice only operation - 25

• Spillway only operation - 25

• Combined operation -

125

• Total tests

175

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Physical Hydraulic Model Observations

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Physical Hydraulic Model Results

Maximum pressures along sluice

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Physical Hydraulic Model

Rooster TailsSluice only operation

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Gate Tests

Jan 2013

Sluice 2 opened 6.77m (78%).

Discharge 862 cumecs.

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Sluice Gate Test

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Conclusions

• Physical hydraulic model investigations supported by prototype testing have successfully proved for the Roxburgh Dam case that simultaneous discharge from a spillway gate and a low level sluice gate into the same chute is feasible and will produce smooth merged flow lines.

• For sluice only operation the discharge will not pressurise the sluice tunnel.

• Spillway only operation will pressurise the sluice tunnel only at high lake levels and gate openings greater than 40%.

• Combined spillway and sluice operation will pressurise the sluice tunnel only at high lake levels and spillway gate openings greater than 40%.

• The water levels induced in the sluice gate chamber by the back pressure from spillway only or combined spillway and sluice operation are unlikely to flood the middle gallery in the dam where the sluice gate controls are located. However further prototype tests are desirable to confirm the extreme water level values observed in the model tests.

• Although the sluice is back pressured by the spillway the discharge from the sluice is not reduced.

• ‘Rooster tail’ behaviour along each side wall at the sluice exit into the spillway chute could threaten the access bridge to the power station under high sluice only flow conditions at a high lake level.

• Roxburgh Dam can safely pass the PMF inflow using the combined discharge from both the spillway and sluice gates fully open at a lake level of RL 132.3m (I.e. with 1.2m freeboard).