Pathways of river water to the surface layers of stratified reservoirs

Abstract

This study uses tracer experiments to characterize the fate of a cold plunging river into a Mediterranean reservoir under different stratification conditions. Three tracer experiments in Lake Béznar, Spain, demonstrate that a significant fraction of the inflow may be quickly entrained into the surface mixed layer (SML), rather than flowing to lower depths. We observe that a fraction (and possibly all) of river inflow entrains into the SML when the density current forms intrusions at the top of the metalimnetic layer. Intrusions near the base of the SML form due to lighter interfacial fluid at the top of the density current, with intermediate properties between riverreservior water when there is sufficient dilution of the cold inflow, and abrupt density changes at the base of the SML. Consequently, a layer of fluid splits from the denser part of the current and intrudes just below the SML, while any remaining fluid flows to lower depths. We parameterize this splitting behavior in terms of the transition Richardson number, Ri 12 5 C 3 (r 2 2 r 1 )/(r 0 2 r 1 ), where r 0 , r 1 , and r 2 are densities of the inflow, the SML, and the metalimnetic layer underneath, respectively; and C is the total dilution through the plunge zone and subsequent entrainment. Splitting occurs when Ri 12 $ 1, consistent with previous laboratory experiments. We use this theory to predict the seasonal fate of river water during the stratification of Lake Béznar and identify the conditions that favor the formation of intrusions near the base of the SML.

Effect of thermal stratification on interflow travel time in stratified reservoir

“…The existing thermal stratification influences the pathway or fate of a contaminant in the reservoir, including the patterns of the formed density current, intrusion occurrence and its depth, the region or scope of contaminant in ambient water if intrusion occurs, and the travel time of contaminant flow or residence time of contaminant in reservoirs (Ahlfeld et al, 2003;Rueda et al, 2006;Rueda and MacIntyre, 2010;Cortés et al, 2014b). In this study, we mainly consider the influence of thermal stratification on the travel time of contaminant in a reservoir when interflow occurs.…”

“…If the ambient water is thermally stratified, the inflow separates from the bottom slope and penetrates horizontally into the ambient water as an interflow where it becomes neutrally buoyant (Alavian et al, 1992;Fernandez and Imberger, 2008;Cortés et al, 2014a). Thus, the contaminant can be transported to different depths of a reservoir by different flow patterns (Wells and Nadarajah, 2009;Cortés et al, 2014b).…”

Tributaries affect the thermal response of lakes to climate change

“…The intrusion model used here was compared (Figs. A2c-A5c) to the intrusion scheme proposed by Cortés et al (2014), which produced an inferior result for LB. Figure A6.…”

Environmental water body characteristics in a major tributary backwater of the unique and strongly seasonal Three Gorges Reservoir, China

“…As tracer experiments in a Spanish stratied reservoir have shown, this can happen very rapidly by partial mixing at the interface between the density current and the overlying water. 37 Particularly, when a density current intrusion enters right below the surface mixed layer, corresponding tracers could quickly reach the water surface as well. 37 This was the case here in both September and November scenarios.…”

“…37 Particularly, when a density current intrusion enters right below the surface mixed layer, corresponding tracers could quickly reach the water surface as well. 37 This was the case here in both September and November scenarios. As such, dissolved nutrients originating from either the Xiangxi River headwater or the Yangtze River main stream are very likely to reach the surface water layer and aggravate eutrophication problems, there.…”