Backwater Effect over Tailrace Water Level in Cascade Hydropower Plants

Brazil currently has more than one hundred medium and large hydropower plants that, together with small hydropower plants, produce on average 91% of the total electricity in the country. To use most of the hydropower potential of a basin, hydropower plants are built normally in cascade. In some cases, the reservoir operation can cause elevation of tailrace water level from upstream plants, the backwater effect. Is this case, the original water level-discharge relationship isn't valid, but depends also on the downstream reservoir level. In the future, this effect tends to be even more intense and frequent as new hydropower plants and reservoirs are built. This paper presents a study of the backwater effect on the productivity of upstream plants and evaluates where and how much it represents in the Brazilian hydropower system. We analyzed the system with 143 reservoirs and three different hydrological scenarios with planning horizons of five years, corresponding to periods of dry, wet, and medium inflows in historical series. The backwater effect was significant, resulting in reduced generation by approximately 400 MW, or 0.6% to 0.8% of the total hydropower production. This results in up to 8% more thermal dispatch in the country. Furthermore, the effect is concentrated in a small number of large hydropower plants. Some suggestions are made to incorporate this effect in reservoir operation models for hydropower generation.