Dynamics of flow exchange and heat transport in hyporheic zone due to the dam-induced river temperature fluctuations

Hyporheic zone (HZ) plays a dominant role in maintaining the ecosystem equilibrium on the river catchment scale. Assessing the temperature changes occurring in the HZ is critical although it is likely to be more important than other factors. As metabolic activity within HZ may be strongly affected by the thermal regime of the stream water, and also plenty of literatures have already showed that the surface water temperature may be changed greatly by the low temperature water (LTW) discharged from the lower part of a deep reservoir; especially the elevation of water-intakes of electronic station is within the mid-lower layer of deep reservoirs. However, how and to what extent the LTW in stream will impact on the HZ temperature regime and ulteriorly, on the full range of the river ecosystem are still not understood widely. This work focuses on the temperature changes and flow rates of infiltration and seepages in the HZ as the stream water influenced by LTW. Water discharged from up reservoir usually contains less sediment and the sediments within the water need to get saturated when it is running along the downstream. Therefore, the hyporheic zone porous structure will be much different at large scale due to the scour activities. Three kinds of HZ structure conception models were built for analysis due to the sediments transport and settlement characteristics along the downstream. VS2DH simulations shows water infiltrates into HZ then laterally mixes with the groundwater, gradually causing a low temperature environment in the HZ. The extent of temperature changes varies with different particle size distribution within the riverbed but it is generally according to the temperature period of stream water. We also examine the grade of infiltration surface impacts on the heat transfer and water velocity distribution in the three different models. The heat and water transport rate increases with the infiltration surface slope angle increasing, it indicates that the local riverbed topography has an assignable on the hydrodynamics in the HZ as well as the thermal phenomenon. The sensitivity of the sediment distribution regime to the hydrodynamic conditions increases with increasing water pressure head and with the sediment permeability.