The transport of granular matter by a fluid flow is frequently found in nature and in industry. When the shear stresses exerted by the fluid flow on a granular bed are bounded to some limits, a mobile granular layer known as bed-load takes place, in which the grains stay in contact with the fixed part of the granular bed. Under these conditions, an initially flat granular bed may be unstable, generating ripples and dunes, such as those observed in deserts, but also in pipelines conveying sand. There are evidences that these forms have a typical length correlated to their initial wavelength. So, the length-scale of the initial linear instabilities is a key point to understand the typical structures observed. This paper presents a theoretical study of the initial instabilities on a granular bed sheared by a turbulent liquid flow without free-surface effects, when bed-load is present. This study consists of a linear stability analysis, taking into consideration fluid flow, relaxation and gravity effects, and it is compared to published experimental data. It is proposed here, differently from many previous studies, that the initial wavelength of bed-forms varies with flow conditions when the fluid is a liquid.324460467Petrobras [0050.0045763.08.4]Petrobras [0050.0045763.08.4]