Stream reaeration in nonuniform flow: macroroughness enhancement

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.