The influence of in-stream structures on temperature dynamics via induced hyporheic exchange

Abstract

In-stream restoration structures, such as woody weirs, are intended to enhance hyporheic exchange, thus altering temperature dynamics in restored reaches. However, the relative effects of various in-stream structure types and streambed settings (i.e. the size, spacing, number and sediment hydraulic conductivity) remain poorly understood. Here, we conducted a field experiment with a channel-spanning weir in a stream to study its impact on hyporheic exchange and streambed and surface water temperatures. We subsequently applied numerical models to explore the effects of height, spacing and number of weirs and sediment permeability on hyporheic exchange and thermal distribution characteristics. Based on the field experiments and numerical models, the results showed that the addition of a weir enhanced hyporheic exchange and produced a characteristic temperature heterogeneity pattern in sediment that varied with height, spacing, number of weirs and sediment permeability. The spatial extent and rate of hyporheic exchange and temperature heterogeneity near the weir primarily increased with weir height and sediment permeability. With increasing spacing, the exchange flux and thermally heterogenous area increased, but the exchange intensity decreased, and the sediment temperature readily stabilized. With increasing number of weirs, the exchange flux first decreased and then increased, and the region exhibiting temperature heterogeneity increased. The interaction between the introduction of in-stream structures introduction and streambed settings must be understood to improve vertical connectivity in rivers, increase the thermal heterogeneity in the hyporheic zone and create localized but potentially valuable thermal refuge areas.