May the Shields theory be extended to cohesive and adhesive benthic sediments?

A threshold criterion for incipient motion of cohesive-adhesive sediments, based on moment balance and dimensional considerations, is here developed. The criterion discriminates between single particles and flocs critical conditions and induces a modification to the traditional Shields curve, commonly adopted for noncohesive particles. This modification is particularly effective for the smaller size particles but tending to vanish for the larger ones. The proposed approach is validated on the basis of experiments performed on sediment cores, sampled at different depths, from seven lakes with different trophic conditions and organic matter content. The incipient motion conditions are evaluated by means of an original procedure based on image analysis techniques, which enables the entrainment between single particles and flocs to be distinguished. The adhesion force, which is mainly due to the biological activity at the sediment water interface, is estimated following the proposed criterion and it is found to depend on the organic matter content. In particular the cores sampled in the littoral photic zone are characterized by a bell shaped dependence between adhesion coefficient and organic matter content, which is typical for indicators of biological activity of the littoral ecosystems.