Riparian release and design discharge

Why riparian release is considered

Riparian release (environmental flow, compensation flow) is the minimum flow that must be left in the river below the diversion weir. It is deducted from the river flow before calculating the plant's design discharge.

• Aquatic life: keeps fish, insects and plants alive in the dewatered stretch between weir and tailrace; allows fish migration.
• Downstream water rights: people use river water for drinking, irrigation, water mills, washing and cremation (religious use) at ghats.
• Water quality and landscape: prevents stagnation, pollution and loss of scenic value.
• Legal requirement: in Nepal, the Hydropower Development Policy 2001 and EIA guidelines require releasing at least 10% of the minimum monthly average flow, or more if the EIA finds it necessary.

So available flow for generation $= Q_{river} - Q_{riparian}$.

Criteria for selection of design discharge

1. Flow duration curve: design flow is chosen at a set exceedance, e.g. $Q_{40}$–$Q_{45}$ for ROR in Nepal (older practice $Q_{65}$).
2. Riparian release deducted first.
3. Type of project: ROR uses an exceedance flow; PROR and storage projects can use larger flows for peaking.
4. Economics: design flow is increased step by step until the incremental benefit equals incremental cost (maximum NPV or acceptable IRR, B/C).
5. PPA and grid demand: the buyer's policy (NEA) on exceedance, dry/wet energy and plant factor.
6. Minimum turbine flow and number of units: the low-flow season must still run at least one unit.
7. Sediment and site constraints: tunnel size, desander capacity and geology limit the flow that can be conveyed economically.