A multiscale model for integrating hyporheic exchange from ripples to meanders

Abstract

[1] It is necessary to improve our understanding of the exchange of dissolved constituents between surface and subsurface waters in river systems in order to better evaluate the fate of water-borne contaminants and nutrients and their effects on water quality and aquatic ecosystems. Here we present a model that can predict hyporheic exchange at the bed-form-to-reach scale using readily measurable system characteristics. The objective of this effort was to compare subsurface flow induced at scales ranging from very small scale bed forms up to much larger planform geomorphic features such as meanders. In order to compare exchange consistently over this range of scales, we employed a spectral scaling approach as the basis for a generalized analysis of topography-induced stream-subsurface exchange. The spectral model involves a first-order approximation for local flow-boundary interactions but is fully three-dimensional and includes the lateral hyporheic zone in addition to the flow directly beneath the streambed. The primary model input parameters are stream velocity and slope, sediment permeability and porosity, and detailed measurements of the stream channel topography. The primary outputs are the distribution of water flux across the stream channel boundary, the resulting pore water flow paths, and the subsurface residence time distribution. We tested the bed-formexchange component of the model using a highly detailed two-dimensional data set for exchange with ripples and dunes and then applied the model to a three-dimensional meandering stream in a laboratory flume. Having spatially explicit information allowed us to evaluate the contributions of both gravitational and current-driven hyporheic flow through various classes of stream channel features including ripples, dunes, bars, and meanders. The model simulations indicate that all scales of topography between ripples and meanders have a significant effect on pore water flow fields and residence time distributions. Furthermore, complex interactions across the spectrum of topographic features play an important role in controlling the net interfacial flux and spatial distribution of hyporheic exchange. For example, shallow exchange induced by current-driven interactions with small bed forms dominates the interfacial flux, but local pore water flows are modified significantly by larger-scale surface-groundwater interactions. As a result, simplified representations of the stream topography do not adequately characterize patterns and rates of hyporheic exchange.

Crossing Turbulent Boundaries: Interfacial Flux in Environmental Flows

“…Thus, the effects of the outer region likely manifest as predictive uncertainty or, more concernedly, as uncharacterized bias when results from the laboratory are applied to the field. Such historical constraints are being overcome by dramatic improvements in the spatial resolution and frequency with which fluid velocity and mass concentration fields can be measured near the sedimentÀwater interface, 29,44 and computing advances that permit computational fluid dynamic simulations of flow paths within stream sediments 30,35,42,45 and direct numerical solutions of the nonlinear NavierÀStokes equations that govern multiphase flow in the turbulent boundary layer. 46 These experimental and theoretical advances, together with careful reanalysis of existing data sets, should pave the way for better characterization, and ultimately prediction, of interfacial mass transport in turbulent environmental flows from both inner and outer region variables.…”

Fractal patterns in riverbed morphology produce fractal scaling of water storage times

“…Since rivers have fractal topography and interactions between river flow and boundary topography drive hyporheic exchange [Harvey and Bencala, 1993;Stonedahl et al, 2010;Tonina and Buffington, 2011;Kiel and Cardenas, 2014;Gomez-Velez and Harvey, 2014], the ubiquitous fractal properties of rivers should produce fractal patterns in hyporheic flow paths and fractal scaling in the associated residence time distributions [Worman et al, 2007;Stonedahl et al, 2012]. Other mechanisms that can produce broad travel time distributions include subsurface heterogeneity and nested flow paths in homogeneous systems [Kirchner et al, 2001;Scher et al, 2002;Cardenas, 2007].…”

Sinuosity-Driven Water Pressure Distribution on Slope of Slightly-Curved Riparian Zone: Analytical Solution Based on Small-disturbance Theory and Comparison to Experiments

“…For instance, aided by an exaggerated superelevation near the bend apex of a curved riparian zone, a turbidity current may split [2][3][4][5][6], and hyporheic exchange occurs over a wide range of topography scales of meandering riparian zones [7][8][9][10]. It is evident that these flow patterns are much more complicated than those in straight rivers [11][12][13][14].…”

“…We insert Equation (7) into Equations (4)-(6) and the second-order terms are ignored because they are too small. Then, Equations (4)-(6) can be reformulated as Equations (8)- (10), which are referred to as small-disturbance equations.…”