Fitting Emptying Rates of Conservative Contaminants in a Two-Storage Model of a Lateral Cavity in a Stream

Abstract:

Recirculating in-stream regions in river flows play a significant role in many physical and biogeochemical processes that control the water quality and fate of contaminants, producing surface storage zones with lower velocities and stresses in the stream. Understanding the behavior of contaminants in these regions of recirculation can provide insight on the impact of lateral cavities on transport in the entire stream. In this investigation, we develop an upscaled, effective model to describe the emptying rate of a contaminant inside a lateral cavity in a channel flow and apply it to data from 3D numerical simulations, based on a DES approach, coupled with an advection-diffusion model to simulate the transport of a passive scalar. Current models of contaminants exiting recirculation zones usually consist of an equation with a first-order mass exchange process between the open channel and the lateral cavity, where the contaminant transfer is modeled as proportional to the mean concentration difference between the cavity and the main channel. Our three-dimensional simulations show that such a model does not work, as the cavity does not appear to be well mixed at all times, but rather is made up of two distinct regions – a central recirculating core and an outer flow. We improve upon the first-order model, by including both storage zones and accounting for exchanges between them. By including these additional exchange processes we are able to successfully reproduce numerical observations over the full range of observed timescales.

This research has been supported by Fondecyt grant 1130940 and the Notre Dame Department of Engineering.