The behaviour of cohesive sediment beds by currents and waves has great importance from the engineering, biological and chemical standpoints. The erodibility and the deposition of cohesive sediments are related, for instance, with the release of nutrients from beds, the maintenance of minimum depth (in tidal rivers, estuaries, and water ways), and the degrading water quality and the resulting harm to aquatic organisms.

In nature, the physical characteristics of a cohesive bed is affected by many factors (of biological, chemical, and physical origins) which change the behaviour of the sediment. To be able to measure those effects, it is necessary to have in the first place a good understanding of the sediment behaviour under abiotic and inorganic conditions. Once the response of certain cohesive sediment under known conditions is known, it is possible to study the changes caused by natural variables on the sediment.

For this reason, the original propose of this investigation was to carry out experiments under controlled conditions on a standard cohesive bed. Then to introduce external factors and to measure and describe any possible changes in the stability of the bed. At the same time, the purpose of this work was to evaluate the accuracy of the methodology usually used to measure the stability of cohesive sediment beds under both laboratory and field conditions.

According with Parchure (1984), and Hunt and Mehta (1985), a cohesive sediment deposited from a suspension will have a decrease in erodibility downwards (Type I profile), whereas a placed sediment bed (homogeneous properties with depth) will show no variation in the erodibility with depth (Type II profile). In estuaries, Type I profiles typify the superficial layers of sediments, which are frequently resuspended by the action of waves and currents. The present study is focused on the behaviour of the top sediment layers (profile Type I) under the action of unidirectional currents.