Heat and dissolved oxygen transport processes in ice-covered lakes and the development of ice-preserving lake aeration

Hydrodynamics of ice-covered lakes are characterized by two distinctly different transport processes: (1) vertical unsteady diffusion in the absence of inflows and (2) lateral advection due to density currents associated with snowmelt. The most important parameter to quantify vertical diffusion in lakes is the vertical eddy diffusivity, K{dollar}sb{lcub}rm z{rcub}{dollar}. Using temperature measurements and a vertical unsteady heat transport model, K{dollar}sb{lcub}rm z{rcub}{dollar} was estimated to be 1.5 times molecular diffusivity adjacent to the ice cover and sediment surface during periods lacking significant inflows. In the upper portion of the water column, K{dollar}sb{lcub}rm z{rcub}{dollar} increased with distance from the ice to 6.5 times the molecular level at a point 2.5 meters from the ice and was a function of the density gradient. Near the bottom, K{dollar}sb{lcub}rm z{rcub}{dollar} increased linearly with distance from the sediment surface reaching 8.5 times molecular diffusivity 3 meters above the sediment. It is these upper and lower boundary layers that dominate under-ice transport processes.; To achieve reliable under-ice aeration (a technique used for winterkill prevention) without causing ice cover degradation, it is critical that the increase in vertical transport (mixing) caused by the aeration process be minimized for two reasons. First, the transport of oxygen to the sediment surface (by far the most significant sink of under-ice dissolved oxygen) reduces that which would otherwise be available for utilization by fish. Secondly, the ice thickness is strongly dependent on the rate of vertical heat transport from the underlying water and sediment which in turn is mediated by the diffusive near-ice boundary layer. Should the vertical diffusivity in this layer increase, ice cover degradation will result.; The insertion of aerated water under ice without causing water column mixing depends on the discharge diffuser being able to exhaust the required flowrate at low velocity with a minimum of turbulence. Both a linear and an axisymmetric manifold/diffuser were designed at model scale, manufactured full scale, and installed in winterkill prone lakes for field evaluation. Field testing of the linear system showed it capable of maintaining dissolved oxygen levels more than adequate for fish survival with no detectable degradation of the ice cover. The system has been in operation for five winters.