Unifying Advective and Diffusive Descriptions of Bedform Pumping in the
Benthic Biolayer of Streams
Abstract
Many water quality and ecosystem functions performed by streams occur in
the benthic biolayer, the biologically active upper (~5
cm) layer of the streambed. Solute transport through the benthic
biolayer is facilitated by bedform pumping, a physical process in which
dynamic and static pressure variations over the surface of stationary
bedforms (e.g., ripples and dunes) drive flow across the sediment-water
interface. In this paper we derive two predictive modeling frameworks,
one advective and the other diffusive, for solute transport through the
benthic biolayer by bedform pumping. Both frameworks closely reproduce
patterns and rates of bedform pumping previously measured in the
laboratory, provided that the diffusion model’s dispersion coefficient
declines exponentially with depth. They are also functionally
equivalent, such that parameter sets inferred from the advective model
can be applied to the diffusive model, and vice versa. The functional
equivalence and complementary strengths of these two models expands the
range of questions that can be answered, for example by adopting the
advective model to study the effects of geomorphic processes (such as
bedform adjustments to land use change) on flow-dependent processes, and
the diffusive model to study problems where multiple transport
mechanisms combine (such as bedform pumping and turbulent diffusion). By
unifying advective and diffusive descriptions of bedform pumping, our
analytical results provide a straightforward and computationally
efficient approach for predicting, and better understanding, solute
transport in the benthic biolayer of streams and coastal sediments.