Three-dimensional modeling for estimation of hydraulic retention time in a reservoir

A three-dimensional computational fluid dynamics model is used to estimate the hydraulic residence time for a portion of the Wachusett Reservoir in central Massachusetts. The basin under consideration has several major inflows and exhibits complex flow patterns. The basin is modeled using the FLUENT software package with particles used to track travel time in a steady-state flow field. A tetrahedral mesh with over 1.6 million cells is used with accurate depiction of basin bathymetry and inlet and outlet geometries. Modeling is performed to simulate behavior during a period when conditions are isothermal. It is determined that mean hydraulic residence time is 3-4 days; approximately half of what would be expected assuming strictly plug flow. The presence of a primary flow path, large scale eddies and stagnation zones contribute to the faster travel times. Reductions in inflow rates produce increased residence times and significant changes in flow patterns.