Estimates of threshold erosion velocities have been obtained in the field and in the laboratory for marine mud relatively rich in organic matter and having an active infauna. A sea-floor flume was used to erode undisturbed sediments on the sea floor, and a laboratory flume was used to study the effects of sampling, redeposition, bioturbation, organic-matter content, and compaction time on erosion resistance.
Critical shear velocity, u*c, for erosion of muds in situ ranged from 0.32 to 0.84 cm/s, and u*c was apparently independent of season or bulk physical properties. Erosion was often but not always initiated in biogenically disturbed parts of the bed. Lee drifts around small bed-roughness elements were the most common constructional bedforms observed during the sea-floor experiments, and narrow and shallow longitudinal furrows were the most commonly observed erosional features. Tidal u*c values measured at the field site were of the same magnitude as the u*c values measured with the sea-floor flume.
Mean u*c for muds redeposited and bioturbated for 1 to 60 days in the laboratory flume (0.92 cm/s) was greater by a factor of two than mean u*c for muds eroded in situ (0.45 cm/s). As bioturbation time in the laboratory increased, u*c decreased. Increases in sediment organic content were found to increase u*c systematically in flat, nonbioturbated laboratory beds.