Assessing Water Exchange Mechanisms in Complex Lake and Coastal Flows by Modelling the Spatial Distribution of Mean Residence Time

Water exchange mechanisms are assessed via modelling the spatial distribution of the mean residence time in lakes and coastal areas. A grid-based numerical solution of the deterministic advection-diffusion-reaction kinetics equation with the ageing of water as transport variable is applied. The composition of residence time from times spent over separate sub-domains is handled as multi-species transport. Numerical errors arising in the advection approximation at sharp gradients are reduced by robust upwind schemes. The spatial residence time distribution due to through-flow and wind-induced water exchange in a shallow sample lake with littoral reed is analysed by 2-D depth-integrated approach. Apart from the spatial distribution, the normalised volumetric distribution of the residence time is also considered. The impact of planned, large-scale landfill on the residence time in semi-enclosed bays at Helsinki coasts is evaluated by 3-D modelling.