Surface storage dynamics in large rivers: Comparing three-dimensional particle transport, one-dimensional fractional derivative, and multirate transient storage models

Large rivers are major conduits for sediment and nutrient transport and play an important role in global biogeochemical cycles. While smaller rivers received attention in recent decades for hyporheic exchange and nutrient uptake, fewer studies have focused on the dynamics of surface storage zones in large rivers. We investigate transport dynamics in the St. Clair River, an international river straddling the U.S.-Canadian border, using a combination of modeling and dye tracer studies. We describe a calibrated three-dimensional hydrodynamic model to generate (synthetic) breakthrough data to evaluate several classes of 1-D solute transport models for their ability to describe surface storage dynamics. Breakthrough data from the 3-D particle transport model exhibited multimodal behavior and complex dynamics that could not be described using a single first-order exchange coefficient-an approach often used to describe surface storage in transient storage models for small rivers. The 1-D models examined include multirate transient storage (MRTS) models in which storage zones were arranged either in series or parallel as well as 1-D models based on fractional derivatives. Results indicate that for 1-D models to describe data adequately, the timing of solute pulses that correspond to various in-channel features such as sandbars, islands or meander bends should be taken into account. As a result, the MRTS model with storage zones arranged in series (i.e., exchange rates triggered sequentially) provided the best description of the data. In contrast, fractional derivative models that assume storage zones were arranged in parallel failed to capture the multimodal nature of the breakthrough curves.