Pathways of river water to the surface layers of stratified reservoirs

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 Beznar, 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 river—reservior 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, Ri12 = Γ × (ρ2 − ρ1)/(ρ0 − ρ1), where ρ0, ρ1, and ρ2 are densities of the inflow, the SML, and the metalimnetic layer underneath, respectively; and Γ is the total dilution through the plunge zone and subsequent entrainment. Splitting occurs when Ri12 ≥ 1, consistent with previous laboratory experiments. We use this theory to predict the seasonal fate of river water during the stratification of Lake Beznar and identify the conditions that favor the formation of intrusions near the base of the SML.