Particle dispersal due to interplay of motions in the surface layer of a small reservoir

Horizontal dispersion mechanisms must be well defined in order to determine the origin and fate of heterogeneous distributions of biogeochemical material. Here data from an intensive field campaign in Valle de Bravo Reservoir, Mexico, was used to run a three-dimensional hydrodynamic model, and the simulated flow field was used to characterize the horizontal motions and investigate the dispersal of waterborne particles driven by the dominant circulation patterns. The surface layer horizontal motions included an oscillatory component associated with two basin-scale internal wave modes and a spatially and temporally complex flow field associated mostly with topographic circulation. Modeling of particle advection by the horizontal motions led to areal dispersal rates of O(1) to O(10) m(2) s(-1), comparable with bulk dispersion rates observed in other surface layer studies. The aperiodic topographic component of the horizontal flow field, not the oscillatory motion or a combination of both, sustained most of the particle dispersal. In particular, local flow features, such as stagnation points, were shown to be a significant driver for horizontal dispersion and should be taken into consideration when analyzing horizontal distributions of waterborne material.