Roxburgh Dam Combined Discharge of Spillway and Sluice into the same Chute Peter Silvester – Civil Engineer Contact Energy Grant Webby – Principal Hydraulic Engineer Opus International Consultants

Roxburgh Dam - Location

Roxburgh Dam Commissioned 1956 50m high concrete gravity 320 MW power station Mean flow 507 cumecs PMF 7,000 cumecs

Roxburgh Dam Spillway Three Spillway Gates 15.2m wide x 13.4m high. Discharge capacity 4,750 cumecs Two diversion sluices 7.1m wide x 8.7m high. Discharge capacity 2,900 cumecs. Combined total discharge capacity 7,650 cumecs.

Roxburgh Dam Spillway Spillway chute 3 dewatered

Diversion Sluices Sluices 7.1m wide 8.7m high

Spillway – Sluice Long Section

Physical Hydraulic Model University of Auckland Tests covered: Sluice only operation - 25 Spillway only operation - 25 Combined operation - 125 Total tests 175

Physical Hydraulic Model Observations

Physical Hydraulic Model Results Maximum pressures along sluice

Physical Hydraulic Model Rooster Tails Sluice only operation

Gate Tests Jan 2013 Sluice 2 opened 6.77m (78%). Discharge 862 cumecs.

Sluice Gate Test

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).