Effect of self-stratification on sediment diffusivity in channel flows and boundary layers: a study using direct numerical simulations

DUTTA, SOM; CANTERO, MARIANO I.; GARCÍA, MARCELO

Earth Surface Dynamics

Copernicus Gesellschaft mbH

Año: 2014 vol. 2 p. 419 - 431

Sediment transport in nature comprises of bedload and suspended load, and precise modelling ofthese processes is essential for accurate sediment flux estimation. Traditionally, non-cohesive suspended sedi-ment has been modelled using the advection?diffusion equation (Garcia, 2008), where the success of the modelis largely dependent on accurate approximation of the sediment diffusion coefficients. The current study exploresthe effect of self-stratification on sediment diffusivity using suspended sediment concentration data from directnumerical simulations (DNS) of flows subjected to different levels of stratification, where the level of strati-fication is dependent on the particle size (parameterized using particle fall velocity V  ̃ ) and volume-averagedsediment concentration (parameterized using shear Richardson number Ri τ ). Two distinct configurations wereexplored, first the channel flow configuration (similar to flow in a pipe or a duct) and second, a boundary-layerconfiguration (similar to open-channel flow). Self-stratification was found to modulate the turbulence intensity(Cantero et al., 2009b), which in turn was found to reduce vertical sediment diffusivity in portions of the domainexposed to turbulence damping. The effect of particle size on vertical sediment diffusivity has been studied inthe past by several authors (Rouse, 1937; Coleman, 1970; Nielsen and Teakle, 2004); so in addition to the effectof particle size, the current study also explores the effect of sediment concentration on vertical sediment diffusiv-ity. The results from the DNS simulations were compared with experiments (Ismail, 1952; Coleman, 1986) andfield measurements (Coleman, 1970), and were found to agree qualitatively, especially for the case of channelflows. The aim of the study is to understand the effect of stratification due to suspended sediment on verticalsediment diffusivity for different flow configurations, in order to gain insight of the underlying physics, whichwill eventually help us to improve the existing models for sediment diffusivity.