THE EFFECTS OF THE MOSS FISSIDENS RIGIDULUS (FISSIDENTACEAE, MUSCI) ON NEAR-BED FLOW STRUCTURE IN AN EXPERIMENTAL COBBLE BED FLUME

The effects of the aquatic moss Fissidens rigidulus on near-bed flow s tructure in a cobble-lined flume were investigated. Moss-covered cobbl es were placed in the flume, and velocities were measured at six verti cal profiles upstream, above, and downstream of the cobbles. All measu rements were replicated at three different discharges. An acoustic dop pler velocimeter (ADV) was used to measure velocity fluctuations in th ree dimensions at each point, at a 0.04-s sampling interval for 2 min. Following this, the moss was removed without disturbing the cobbles' spatial arrangement, and measurements were repeated to characterize ba ckground hydraulic conditions. We investigated mean local velocities, Reynolds stresses, total turbulence energy, relative turbulence intens ities, and velocity spectra with and without the moss. Two how layers were observed in all our experiments: an upper logarithmic layer and a lower roughness sublayer. Velocities in the upper layer followed the standard logarithmic law, whereas velocities in the roughness sublayer did not, because of interaction between flow and the roughness elemen ts. The presence of Fissidens caused a reduction in average longitudin al velocity, Reynolds stress, and total turbulence energy. Relative tu rbulence intensity was much higher in the presence of the moss. An int ernal boundary layer (IBL) around the moss was also observed; this IBL was destroyed behind the moss by turbulent mixing in the roughness su blayer. The IBL thickness increased with increasing flow.