Reaeration in a turbulent stream system Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/zg64tp14q

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  • The oxygen concentration in a stream is an important parameter of water quality. Changes in oxygen concentrations can affect various stream organisms including fish. Foresters have become concerned with predicting the impacts of forest activities on oxygen levels in streams. Slash, which accumulates in streams as a result of harvesting activities, is a food source for stream organisms. During aerobic respiration, oxygen is utilized. Under some conditions the oxygen concentration can be depleted below acceptable levels. Large, fish bearing streams are generally well protected by forest practice regulations. For smaller streams without fish populations, the issue is one of downstream impairment of water quality as deoxygenated water enters fish-bearing reaches. A natural process counteracting oxygen depletion is reaeration. Reaeration is the exchange of gases between the atmosphere and water. This process operates to maintain oxygen near the saturation concentration. The change in the oxygen deficit in a stream is a function of the existing deficit and the reaeration rate coefficient. The objective of this study was to develop a predictive equation for the reaeration rate coefficient based on the hydraulic characteristics of stream channels. This is a a first step in developing guidelines to regulate harvesting residues in streams. Seven natural stream sites were selected in Oregon. These sites represented a wide range of hydraulic conditions. The stream reaches were segregated into segments of uniform hydraulic characteristics. Sodium sulfite was injected into the stream to artificially deplete the oxygen concentration. The recovery of the oxygen concentration was used to determine the reaeration rate coefficient. Several models for the reaeration process were tested using regression techniques. Some were models proposed by other investigators and some were developed independently. The predictive equation which fit the data best is a function of the maximum unit energy dissipation rate (ED) and a depth parameter (HD): [equation-see PDF] This equation is consistent with theoretical descriptions of gas exchange phenomena. As the rate of energy dissipation increases in a segment, the turbulence in the segment also increases. Turbulence promotes an increase in the liquid-atmosphere interface area and in the exchange rate of volume elements in the interface. Reaeration is stimulated when deaerated water from the bulk flow of the stream replaces the oxygen saturated water in the surface film. As the area of liquid-atmosphere contact increases, the total flux of oxygen molecules into the depleted fluid volume increases. As the fluid volume increases, the change in concentration for a specific flux of molecules decreases. The depth term (HD) can be used to describe the ratio of the surface area to the volume of fluid in the segment. In this study, the depth term used was the discharge divided by the mean width and maximum velocity. This approach adjusts for dead zones that do not actively mix with the bulk flow. For field applications, predicting the reaeration coefficient for any temperature (T) requires that the slope (s), active width (WD), maximum velocity (UD), and discharge (Q), be measured for uniform stream segments. These variables are combined in the following equation: [equation- see PDF] Using the predicted reaeration rates, estimates of mean segment velocities, biochemical oxygen demand loading, and rates of oxygen demand decay, it is possible to predict the oxygen concentration of a stream moving through and downstream from a harvesting site. The reaeration rate influences the maximum deficit and time required for recovery and can be used to evaluate the risks that debris accumulations pose to water quality.
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