Hydraulic and thermal effects of in‐stream structure‐induced hyporheic exchange across a range of hydraulic conductivities

Abstract

AbstractIn‐stream structure‐induced hyporheic exchange and associated thermal dynamics affect stream ecosystems. Their importance is controlled by spatial variability of sediment hydraulic conductivity (K). We calibrated a computational fluid dynamics (CFD) model of surface and groundwater hydraulics near a channel‐spanning weir (represents log dams, boulder weirs) to field data and varied K from 10−7 to 10−2 m/s (silt to gravel). Surface water stopped cresting the weir for K > 10−3 m/s. Non‐Darcy hyporheic flow was also prevalent for K > 10−3 m/s, and velocity errors using non‐CFD models ranged up to 32.2%. We also modeled weir‐induced heat transport during summer. As K increased from 10−7 to 10−3 m/s, weir‐induced hyporheic heat advection steadily increased. Cooling and buffering along hyporheic flow paths decreased with increasing K, particularly above K = 10−5 and 10−4 m/s, respectively. Vertical heat conduction between surface water and groundwater near the weir decreased with increasing K, particularly for K > 10−5 m/s. Conduction between hyporheic flow paths and adjacent groundwater helped cool hyporheic flow. Downstream surface water cooling by hyporheic advection increased steadily with K as increases in hyporheic flow overwhelmed decreases in cooling along hyporheic flow paths. Yet such effects were small (0.016°C) even at K = 10−3 m/s. The largest thermal effect of weir‐induced exchange was therefore spatial expansion of subsurface diel variability (particularly for K > 10−5 m/s) which affects benthic habitat and chemical reactions. The specific values of K where such trend shifts occur is likely variable among streams based on flow conditions, but we expect the presence of such trend shifts to be widespread.