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