River inflow mixing in a stratified reservoir

摘要

Limnologists have traditionally accepted that a negatively buoyant inflow entering a larger water body will plunge beneath the surface and, once submerged, it will flow downward along the bottom as a gravity-driven density current until it reaches the Level of Neutral Buoyancy (LNB) where it forms intrusions. Recent laboratory experiments, though, suggest that a significant fraction of inflow water entering linearly stratified bodies might be distributed throughout the water column above the theoretical LNB. These findings agree with previous field observations in Lake Mead, where it was estimated that 10-20% of river nutrients could be immediately available for primary production in the surface layers of the reservoir. We present and analyze the results of tracer release experiments conducted in a continuously stratified reservoir to understand the mechanisms causing the river inflow water to mix vertically reaching layers near the surface. Strong shear-driven mixing and dilution ratios of up to 10:1 at a distance of 200 m downstream from the inflow were measured. Traced river water was observed near the surface in the inflow basin, and appeared forming intrusions of nearly 10 m thick downstream. We explore the mixing processes affecting river inflows in the reservoir.