The effects of the moss Fissidens rigidulus (Fissidentaceae: Musci) on near‐bed flowstructure in an experimental cobble bed flume

Abstract

The effects of the aquatic moss Fissidens rigidulus on near‐bed flow structure in a cobble‐lined flume were investigated. Moss‐covered cobbles were placed in the flume, and velocities were measured at six vertical profiles upstream, above, and downstream of the cobbles. All measurements were replicated at three different discharges. An acoustic doppler velocimeter (ADV) was used to measure velocity fluctuations in three 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 background hydraulic conditions. We investigated mean local velocities, Reynolds stresses, total turbulence energy, relative turbulence intensities, and velocity spectra with and without the moss. Two flow 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 elements. The presence of Fissidens caused a reduction in average longitudinal velocity, Reynolds stress, and total turbulence energy. Relative turbulence intensity was much higher in the presence of the moss. An internal boundary layer (IBL) around the moss was also observed; this IBL was destroyed behind the moss by turbulent mixing in the roughness sublayer. The IBL thickness increased with increasing flow.