A multiscale model for integrating hyporheic exchange from ripples to meanders

Abstract

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‐form‐exchange 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.