Comparing Reservoir Refill For Power Generation and Flood Risk Management

Reservoirs with seasonally varying inflow and water demand vary their stored water content during the year. During portions of a year they draft while during other times they refill. This paper focuses on refill. Seasonal inflow variation can arise from varying precipitation but also from varying temperature. Varying precipitation can take the form of wet seasons and dry seasons. Varying temperature can cause precipitation to fall as snow during the cold season and not runoff until the snow melts when temperature increases. Snow-covered basins usually have a large portion of annual runoff occurring during a freshet in spring and early summer. Considerable effort goes into refill planning. Planning depends on project objectives. Most projects have multiple authorized uses. This analysis will focus on FRM (flood risk management) and power. Ideal FRM and power reservoir operations can be considered opposite. The ideal FRM operation holds reservoirs empty and fills as needed to control flooding. The ideal power operation holds reservoirs full to maximize head and drafts for reliability or economic needs. In a similar vein FRM planning aims to avoid flooding from high flows while power operations planning aims to avoid the inability to meet power demand due to low flow. Refill for both FRM and power operations attempt to minimize high flows. FRM aims to avoid flooding and power operations aim to avoid spill. Both power and FRM refill planning has moved to more probabilistic approaches over the past few decades. Probabilistic and deterministic planning for power and FRM present different tradeoffs. In FRM, the tradeoff is between minimizing flood damage from expected flows and avoiding low probability, but much more damaging future higher flows. In power operations, the tradeoff arises between an economically optimum operation and operating to minimize probability of inability of the power system to meet power demand. This paper presents results from modeling of reservoir operations that uses realistic data to illustrate and quantify the concepts described above. Emphasis is on snowmelt dominated systems using data from the Pacific Northwest region of the United States.