dfiles/OWS_summaries/evaluating_hydropower.md

In HARPgroup/hydro-tools: Data processing and accessors for VAHydro

Evaluating hydropower: In Virginia, beneficial use of water means both in-stream and off-stream uses. In-stream beneficial uses include the protection of fish and wildlife and habitat, and off-stream beneficial use includes electric power generation, which is a low emissions form of power. Hydropower dams are therefore a beneficial use of water, and are largely non-consumptive, that is, no net loss of water occurs with the exception of evaporation off the surface of impounded waters. The U.S. Department of Energy classifies 3 different types of hydropower: impoundment, diversion, and pumped-storage (see https://www.energy.gov/eere/water/types-hydropower-plants). Impoundment and diversion hydropower plants are typically operated in “run of river” fashion, with daily flow out of the project equal to daily inflows. For these “run of river” type projects, a typical analysis that considers flow-alteration or consumptive use in the river segment is not applicable. Instead, “run of river” hydro analysis uses the access to fish passage and management of power cycling rates as the primary quantitative measures of potential impacts.

• Diversion Hydropower: In these projects, water is diverted through a side channel for power generation, then returned further downstream, leaving a section of the stream that has reduced flows called the “bypass reach”. The bypass reach can provide habitat for resident species, which is evaluated with a weighted usable area or wetted perimeter study to show changes in total stream area for fish and non-mobile species under varying flows.
• Pump Store Hydropower: These projects may result in substantial changes to the net daily flow downstream of the project, and therefore, are evaluated with a standard flow-alteration type of approach which generally looks at limiting alterations of greater than 10-20% or utilizes a detailed flow:habitat model to determine acceptable levels of alteration.

HARPgroup/hydro-tools documentation built on July 4, 2025, 11:05 a.m.