Measuring And Modeling Mean Flow, Turbulence, And Hydraulic Residence Time In Shallow Surface Water Systems Occupied By Submerged And Emergent Aquatic Vegetation

Abstract

Predicting mixing and transport of pollutants and nutrients in natural surface waters requires consideration of many different scales, from the smallest scales of turbulence dissipated in the wakes of aquatic vegetation to the scales of transit through an entire aquatic system such as a lake, river, or wetland. Here we address two topics pertaining to the general problem of mixing and transport: the first is modeling of vertical mixing by turbulence in systems that may be occupied by aquatic plants, and the second is tracerbased measurement of hydraulic residence time (HRT), a bulk quantity defined as the time water remains in an aquatic system. We develop a numerical model that predicts vertical turbulent eddy viscosity in flow through aquatic vegetation, employing a k -[epsilon] approach. The model is unique in its treatment of turbulent dissipation in plant wakes, outperforming existing models in predicting experimental results from emergent and submerged rigid cylinders (model vegetation) in two laboratory studies. The model is applicable to real vegetation, but validation in real vegetation is pending as ongoing experiments are completed. The model can be readily incorporated into larger two- or three-dimensional hydrodynamic solvers that predict momentum and scalar transport in natural systems. Focusing on the system scale, we develop new methods for measuring mean HRT. A standard technique is the passive tracer pulse release, in which a known mass of neu- trally buoyant tracer is released all at once into a system, and its flux out of the system is monitored. The first temporal moment of tracer flux equals the mean HRT. We propose new methods for extrapolating flux in a way that is consistent with conservation of mass, correcting for photolytic decay of fluorescent water tracing dyes (commonly used tracers), and estimating uncertainty in mean HRT measurements. We review the literature on sorption of Rhodamine WT (a popular tracer), exposing knowledge gaps that must be filled before sorption can be predicted in field studies. We evaluate the advantage of carefully measuring velocity profiles across an outlet (vs. point measurement) and suggest techniques for measuring concentration over long times in particle-laden systems.