Étude des propriétés physiques de différents sédiments très fins et de leur comportement sous des actions hydrodynamiques

A study of the physical propreties of various forms of very fine sediment and their behaviour under hydrodynamic action

Ingénieur, Chef du Service de Sédimentologie du Laboratoire Central d'Hydraulique de France.

Résumé

A comparative study of the physical properties of pelitic sediment of the mud, silt or sludge type and of its behaviour under hydrodynamic action shows that in spite of the complexity of the phenomena involved, there are a certain number of simple relationships betwen the various representative parameters for soil-water complexes. The mineralogical, chemical and grain size characteristics of element particles must be known in establishing the origin of a deposit and making assumptions about the conditions under which it settled out at a given point. Such data can be confirmed by the detection of natural tracers in certain oligo-elements found in sediment, and by modern radioactive tracer techniques. In order to be able to define the dynamics of this very fine sediment in its natural water medium, however, it is necessary to estabIish precisely a certain number of basic physical properties. Laboratory research for various French and foreign government departments and private firms tends to show that the following are the three basic properties involved: (i) Flocculation of suspensions, and hence also the floc sinking rate Wf for various solid parti cle concentrations and water media; (ii) The bedding-down of deposits in time, the Characteristic feature of which is the mean particle concentration variation T8 and the concentration gradient with depth; (iii) The rheological properties of the deposits at the various bedding-down stages, and more especially initial rigidity ('ty)) and viscosity (v) variations with soIid particle concentration. Where these three physical properties are similar in different soil-water complexes, suspensions and deposits of the latter also behave similarly under hydrodynamic action. In parti cular, the slope of a submerged or emergent mud bank, tan α, for an assumed 40 cm height of deposit will be proportion al to the initial deposit rigidity ty, as follows: tan α = Kty where for given measurement conditions K is around 25/100 for a submerged deposit, and 7/100 for one above water, ty being in dynes/cm2• Similarly, it seems that the critical bed friction velocity U. can as a first approximation be related to initial rigidity by the following expression : U. = 0,5 ty1/2 where ty is in dynes/cm2 for deposits with an initial rigidity greater than 10 dynes/cm2 • This effect of initial deposit rigidity and viscosity is also found in the behaviour of various forms of pelitic sediment scattered within various water media by wave and tide current action. Flocculation, bedding-down, rigidity and viscosity of deposits invariably turn out to be characteristic parameters for the behaviour of soil-water complexes under various forms of hydrodynamic action. It should be noted, however, that accurate determination of this type of parameter still remains a difficult matter and that its quantitative values depend on measurment conditions, so that any extrapolations must remain subject to considerable caution.