A New Approach to Quantify Shallow Water Hydrologic Exchanges in a Large Regulated River Reach

Abstract

Hydrologic exchange is a crucial component of the water cycle. The strength of the exchange directly affects the biogeochemical and ecological processes that occur in the hyporheic zone and aquifer from micro to reach scales. Hydrologic exchange fluxes (HEFs) can be quantified using many field measurement approaches, however, in a relatively large river (scale > 10 3 m), these approaches are limited by site accessibility, the difficulty of performing representative sampling, and the complexity of geomorphologic features and subsurface properties. In rivers regulated by hydroelectric dams, quantifying HEF rates becomes more challenging because of frequent hydropeaking events, featuring hourly to daily variations in flow and river stages created by dam operations. In this study, we developed and validated a new approach based on field measurements to estimate shallow water HEF rates across the river bed along the shoreline of the Columbia River, USA. Vertical thermal profiles measured by self-recording thermistors were combined with time series of hydraulic gradients derived from river stages and inland water levels to estimate the HEF rates. The results suggest that the HEF rates had high spatial and temporal heterogeneities over the riverbed, with predicted flux rates varied from +1 × 10 −6 m s −1 to −1.5 × 10 −6 m s −1 under different flow conditions.

Modulating factors of hydrologic exchanges in a large‐scale river reach: Insights from three‐dimensional computational fluid dynamics simulations

“…The CFD simulation results match the ADCP measurements very well. More details about the comparison between the CFD simulation results and the ADCP survey measurements are discussed in the Supporting Information and in Zhou et al (). Note that the CFD simulation used in the validation is a quasi‐steady state one controlled by the averaged river flow boundary conditions on February 19, 2016, while the ADCP measurements are almost instantaneous values.…”

“…The solid lines represent the CFD‐simulated flux along the red line marked in Figure b, and the bars are the iButton measurements at the five locations marked as red stars in Figure b or as red triangles in Figure . More details about the HEFs measurements can be found in Zhou et al (). The CFD simulations can capture the HEFs at the five iButton locations in different flow boundary conditions very well.…”

“…The river bed is about 100 m above the sea level based on the North American Vertical Datum of 1988 (NAD88). The alluvium layer, starting from the top of the river bed, is assumed to be 2 m (Zhou et al, ; Zhou et al, ), and its horizontal permeability is estimated from underwater video‐camera footage and accompanying grain‐size analyses (Fritz & Arntzen, ). The distribution of is shown in Figure c.…”

“…These transects were selected to cover the entire channel width (Transect C), main channel with deep bathymetry and relatively high flow velocity (Transects A and D) and secondary channel with shallow bathymetry and lower flow velocity (Transect B). More details about the measuring procedures and CFD simualtion validation can be found in Zhou et al (). HEF were derived from a set of iButtons®‐based temperature profiles along the reach. The five red triangles in Figure a mark the locations where the temperature sensor iButtons® were installed.…”

“…The temperature time series were then converted to HEFs using a hybrid modelling framework that combines the dynamic harmonic regression (Young, Pedregal, & Tych, ), an analytical solution (Turcotte & Schubert, ), and the local polynomial method with a maximum likelihood estimator model (Vandersteen et al, ). Interested readers are referred to Zhou et al () for details. A dynamically simulated HEFs time series was retrieved from a multiyear (i.e., 2011–2014) transient‐state simulation of the reactive transport model PFLOTRAN (Lichtner et al, ). Details of the PFLOTRAN simulation can be found in Bisht et al () and briefly summarized here for completeness.…”