AErosion of Cohesive Sediments: A conceptual error in its formulation.

GOMEZ, E. A.; AMOS, C. L.

San Carlos de Bariloche

Congreso; IV Congreso Latinoamericano de Sedimentolog¨ªa, XI Reuni¨®n Argentina de Sedimentolog¨ªa; 2006

AAS

Cohesive sediment prediction in the nearshore is important for the evaluation of nutrient, chemical and pollutant mobilization from the bed, for the morphodynamics of navigational channels and harbours (Nichols, 1986, 1993; West, 1994) and for the evaluation of water quality and the effects on aquatic organisms (Wiltshire et al., 1998). However, the most frequently used models and particularly those involving bed erosion are usually of low reliability. Recently-deposited cohesive beds generally demonstrate an increase in shear strength with sediment depth. In order to predict erosion, an equation such as that of Parchure and Mehta (1985) is usually used: ln (e/ef) = a(to-tb(z))n  , where e is the erosion rate, to is bed shear stress, tb(z)  is the critical value of sediment shear strength which normally increases with depth (z), ef is the erosion rate when to = tb(z); n is an empirical value equal to 0.5 and a is a constant for a given sediment. The underlying principle of this equation is that, under a given steady current, the bed will undergo erosion until to equals tb at which point erosion ceases. Experiments carried out on kaolinite in an annular flume under controlled conditions, showed that sediment erodibility was controlled by the magnitude of pre-threshold currents and also on the bed thickness (G¨®mez and Amos, 2005). This latter effect was minimized by using just a 2 mm bed thickness instead of the 2 cm originally used by Parchure and Mehta (1985). It was also observed that the equilibrium condition e = ef ¡Ö 0 did not occur, and that erosion appeared to continue regardless of eroded depth.  This paper attempts to explain why this occurs and how results from such experiments may be used gainfully in applications to nature. The relationship to - e for different bed depths was obtained in the form ln (e) = a to + b, where a decreases with depth following an inverse relationship with b. These relationships lead to a general equation which is almost identical in its form to the one given by Parchure and Mehta. However, such formulation conceptually differs in that a is variable with depth (a(z))  and would depend mainly on the sediment density, whilst the critical shear strength (tb) is constant with depth for a sediment vertically homogeneous in grain size and mineralogy.  Standard methodologies employed to define cohesive sediments erosion may in such cases be wrong. .