STREAM REAERATION IN NONUNIFORM FLOW: MACROROUGHNESS ENHANCEMENTA

The authors presented two papers which are a welcome addition to the topic of water quality and stream re-aeration. The renewal theory and small eddy model provides interesting results for smooth and small-roughness channels. The discussers feel however that the second paper highlights some limits of the method, particularly when free-surface aeration takes place. The present discussion provides additional material on the problem of stream re-aeration in presence of 'white waters ' (i.e. air bubble entrainment). It complement the original paper and some references are added including large-scale data. MASS TRANSFER EQUATION AND AIR-WATER INTERFACE AREA The mass transfer rate of a chemical across an interface varies directly as the coefficient of molecular diffusion and the negative gradient of gas concentration. If the chemical of interest is volatile (e.g. oxygen), the transfer is controlled by the liquid phase and the gas transfer of the dissolved chemical across an air-water interface is rewritten usually as