Considerations for managing reservoir outflows from siphon drawdown schemes

Permanent siphons are increasingly being employed to increase the discharge capacity at existing reservoirs to ensure the available drawdown provision is in line with current guidance. Routine ‘wet’ testing of reservoir drawdown systems, during which flows are discharged from the system, is considered fundamental to reservoir safety to ensure that there is confidence that such systems can be relied upon to operate in emergency situations. This is particularly true for siphon systems which can be more complex to operate than other gravity systems. However, reservoir discharges to the downstream watercourse may be constrained by requirements for pollution and flood risk mitigation to downstream receptors. The paper outlines a range of options considered and employed across multiple sites to manage the discharge of outflows from permanent siphon systems. It discusses constraints encountered – actual and theoretical – and outlines potential lessons learned for future reservoir siphon schemes. The discussion considers methods of siphon priming; the philosophy of ‘wet’ test operations; how outflows can be terminated; and if and by what means outflows are conveyed to the downstream watercourse or are otherwise managed.