Analytical representations of the Residence Time Distribution (RTD)
associated with Hyporheic Exchange beneath Dune-like bedforms at
different sediment bed depths
Abstract
The hyporheic exchange below dune-shaped bedforms has a great impact on
the stream environment. One of the most important properties of the
hyporheic zone is the residence time distribution (RTD) of flow paths in
the sediment domain. Here we evaluate the influence of an impervious
layer, at a dimensionless sediment depth of d_b^*=(2πd_b)⁄λ where λ
is the dune wavelength, on the form of the hyporheic exchange RTD.
Empirical RTDs were generated, over a range of d_b^(* ) values, from
numerical particle tracking experiments in which 10000 particles
sinusoidally distributed over a flatbed domain were released. These
empirical RTDs are best represented by the Gamma, Log-Normal and Fréchet
distributions over normalized bed depth of 〖0 <=d〗_b^(*
)≤1.2, 〖1.23.1, respectively. The depth dependence of the analytical
distribution parameters is also presented, together with a set of
regression formulae to predict these parameters based on d_b^(*
)with a high degree of accuracy (R^2>99.8%). These
results contribute to our understanding of the physical and mixing
processes underpinning hyporheic exchange in streams and allow for a
quick evaluation of its likely impact on nutrient and contaminant
processing (e.g., based on the magnitude of the Damköhler number).