Run-of-River Hydropower

A run-of-river hydroelectric project generates electricity mainly from the natural flow and fall of a river, with little capacity to store water for later use. Water is diverted or passed through turbines under a hydraulic head, and the turbines drive generators. Such a project may have a small pond or limited pondage, so “run-of-river” does not necessarily mean complete absence of storage.

How electricity is generated

The layout varies with terrain: a diversion project may carry water away from the river channel, while a low-head project may place the powerhouse near a weir or barrage.

• A weir, barrage or small pond raises the water level and directs part of the river flow into an intake.
• A headrace channel or tunnel conveys the diverted water while preserving the elevation difference needed for generation.
• A penstock carries water under pressure to the turbine at a lower elevation.
• The moving water rotates the turbine, which drives an electrical generator.
• The tailrace returns the water to the river downstream after generation.
• Power output depends principally on the discharge passing through the turbines, the available hydraulic head and the efficiency of the equipment.

Role of limited live storage

Live storage is the usable volume of water that can be released by lowering the reservoir within its normal operating range. In a run-of-river project, this volume is absent or small compared with a storage-reservoir project.

• Small pondage can regulate short-term variations in river inflow and electricity demand.
• Pondage also helps maintain the water level required for reliable intake and turbine operation.
• When inflow exceeds turbine capacity and available pondage, the excess water must pass through spillways or other river outlets.
• When river discharge falls, generation must generally fall because the project cannot draw upon substantial seasonal storage.
• Limited pondage may permit short-duration peaking, but it cannot ordinarily provide the seasonal regulation associated with a large storage dam.

Relevance to the Indus Waters Treaty

The treaty uses “run-of-river” as a specific legal and engineering category, making project design, not merely its descriptive label, important.

• The Indus Waters Treaty defines a Run-of-River Plant as a hydroelectric plant that develops power without live storage as an integral part, except for pondage and surcharge storage.
• The treaty permits India to construct run-of-river plants on the Western Rivers subject to the conditions and design criteria in Annexure D.
• Treaty-compliant pondage is intended for limited operational regulation rather than large-scale seasonal storage.

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