Search

Transfer of low-solubility gas between air and flowing water is studied experimentally and through analysis of previous studies. The smallest turbulent motions are seen to have a theoretical advantage in overcoming interfacial resistance, and measurements of oxygen absorption rates are found to agree with small-eddy scaling. Though recent studies have emphasized large motions, this conflict is resolved by a conceptual framework incorporating both scales in terms of the distribution of near-surface turbulent energy dissipation rates. At higher Reynolds numbers, such as in natural streams, transfer rates are expected to be controlled by mean dissipative scales.

Models of dissolved oxygen in streams require accurate estimation of absorption rates of atmospheric oxygen, or 'reaeration rates.' These depend upon gas transfer coefficients, which engineers predict as functions of mean flow depth, velocity, and slope. Though many such equations have been developed, errors remain large, pointing to a need for an additional degree of description. Noting that streams vary widely in hydraulic geometry, a set of laboratory studies finds that reaeration rates are enhanced by 'macroroughness,' large bed variations that produce depth-scale form drag. This enhancement is accurately described by a theory that was successful in smooth-channel flows, upon accounting for increases in near-surface turbulent energy dissipation rates caused by macroroughness. In addition, a set of runs featuring bubble entrainment exhibits very high absorption rates, which are characterized by a Froude number based on macroroughness size.

Numerous equations employing depth, velocity, and slope have been developed to estimate the stream reaeration coefficient. These have been evaluated previously using statistics based on differential errors, which are shown to be biased toward underprediction. A new metric, the mean multiplicative error (MME), overcomes this defect and offers other advantages, including identical results for both reaeration and gas transfer coefficients and less sensitivity to extreme errors. It is equal to the geometric mean of the factors, greater than unity, by which the estimates would have to be multiplied or divided to equal the corresponding measurements. With the use of the MME to test 10 selected equations, against a compilation of field measurements based on gas tracers, current equations are shown to be of little value at low slopes, whereas some frequently used equations are shown to have little general value. Slope is found to be an essential component of reaeration equations. Recommendations are made for estimating the reaeration coefficient.

The authors discuss the problem of water quality and stream reaeration. The effect of air bubble entrainment on the stream reaeration is evaluated.

The equilibrium alluvial stream channel has a geometry that allows it to pass the water and sediment supplied from the watershed. At the same time, the equilibrium alluvial channel is built and maintained by the flows and sediment delivered to it. A prerequisite for understanding the creation of the equilibrium channel is an understanding of the sediment conveyance and competence of the flows the channel receives. This study describes the bed-load transport regime as it is linked to hydrology and geomorphology in 23 headwater gravel-bed streams in snowmelt-dominated parts of central and northern Idaho. At sites, drainage areas range from 1.29 to 381 [km.sup.2], stream gradients range from 0.0042 to 0.0747, and median bed surface particle sizes range from 4 to 207 mm. Stream architecture includes riffle-pool, planar, and step-pool beds. The bed load is much finer than the surface and subsurface material, suggesting selective transport of the finer sizes. Nonetheless, the majority of the load is sand at all flow discharges. Progressively coarser sediment was collected as flow discharge increased, and painted rock experiments documented the transport of coarser particles at higher discharges. The supply of sediment to the streams appears limited, as indicated by observed clockwise hysteresis in bed-load transport rates during each spring snowmelt and by the coarse surface armor observed at sites. Flows above bankfull discharge move 37% of the bed load, whereas flows between mean annual discharge and bankfull move 57% of the bed load. The bed-load effective discharge has a recurrence interval that averages 1.4 yr and the magnitude of effective discharge averages 80% of bankfull discharge. The recurrence interval of bankfull discharge averages 2.0 yr. The ratio of effective discharge to bankfull discharge is independent of basin size, grain size, and gradient, although the ratio increases with the relative magnitude of large infrequent events.