Predicting Total Dissolved Gas Travel Time in Hydropower Reservoirs

Large spills at hydropower facilities can produce environmentally unfavorable supersaturated total dissolved gas (TDG) conditions. Enhanced coordination between adjacent hydropower facilities is required when a large, brief spill creates a pulse of TDG that is transported into a downstream reservoir. A premium is placed on quick and simple methods and relationships that can support short-term decision making under dynamic conditions. This paper presents an empirically based methodology to quantify and assess the relationship between mean hourly river flow and TDG travel time between an upstream hydropower spillway and a downstream dam. The analysis is conducted for reservoirs in the mid-Columbia River system and is limited to summer months when spill events that create high TDG are most common. A two-step filter and crosscorrelation procedure is used to isolate high-strength TDG events and characterize the time delay between when the TDG event is initiated upstream and when it is detected downstream. Spill flow, powerhouse flow, and TDG measurements are used to develop a bulk parameter estimation of mean TDG time delay as a function of mean river flow. The approach reveals an inverse power law relationship between mean river flow and mean TDG travel time. The relationship provides hydropower operators with a new, simplified decision support tool. The generalized methodology could be applied to other reservoir systems for which TDG minimization is a critical management objective.