Numerical Age and Residence-Time Mapping for a Small Tidal Creek: Case Study

Abstract

A numerical modeling study, using an Eulerian tracer and Lagrangian particle-tracking methods, was carried out to map the age of water and residence-time distribution in the tide-dominated East Scott Creek Estuary, South Carolina. A coupled hydrodynamic, solute-transport, and particle-transport model was developed. The flow and solute-transport models were based on depth-integrated conservation equations and the particle-transport model was quasi-three-dimensional. The equations were discretized using the total variation diminishing finite-volume method. The numerical model predictions were verified against a set of field-measured hydrodynamic data, with the model-predicted water elevations and velocities in good agreement with the field measurements. Different methods of computing the age of water and the residence time that are applicable to tide-dominated estuaries and suitable for high-resolution flow and transport numerical modeling setup were investigated. An Eulerian method, based on the tracer-age theory, and a Lagrangian particle-tracking method were each used to map the age and the residence-time distribution throughout the main channel of East Scott Creek. The performance of the two methods in the given tide-dominated environment was investigated through a series of numerical experiments. The effects of the particle and tracer release time and tidal amplitude in the computation of tidal exchange-time scales were also investigated. The residence time computed by the Eulerian method scaled well with that of the Lagrangian method and produces a relatively smooth monotonic profile of the residence time in space. The Lagrangian method, unlike the Eulerian method, consistently showed a more detailed distribution of significant and varying nonmonotonic profile of residence times; yet, the two methods produce, in the tidally averaged sense, reasonably comparable residence-time distributions. The residence time computed by the Lagrangian method is very sensitive to the phase of the tidal forcing at the beginning of the computation.