Hydraulic roughness of reed stems in shallow surface flow under low current velocity is discussed. The roughness coefficient, defined as a Manning's formula, depends upon depth and velocity in this flow. Depth and velocity, in turn, are altered by roughness through water surface slope. The interaction between these three parameters must be solved simultaneously for the prediction of the surface flow in a reed wetland. Stem roughness can be expressed as a function of Reynolds' number, similar to that for a single cylinder. Assigning this function, an iteration method easily gives us a suitable set of three parameters.

Settling experiments show that stems promote vertical mixing of fine particles in shallow flow. The model of vertically uniform SS concentration is more suitable. Though the observed data fluctuated widely, the apparent settling velocity of fine particles in the flow was very close to the settling velocity in quiescent water.

Using the above two models, one for flow and the other for settling in shallow wetland, we can estimate the clarification potential of a reed field under given hydraulic conditions and particle load. Numerical experiments show that fine particles tend to accumulate on the bottom of a reed field, as often observed in natural wetlands.