Investigation of the fecal coliform plumes induced by river discharge and wind stress using a three-dimensional model

A three-dimensional hydrodynamic, suspended sediment, and fecal coliform transport model was developed and applied to characterize the fecal coliform plume in the Danshuei River estuarine system and its adjacent coastal sea. The numerical model was calibrated and validated with time-series water levels and spatial distributions of suspended sediment and fecal coliform concentrations in 2012 and 2013, respectively. Three indices (i.e., mean absolute error, root-mean-square error, and skill score) were used to compare the predicted results and measured data for assessing the performance of the model. Quantitatively, there was good agreement between the simulation and the measurement results. The validated model was later applied to explore the influence of river discharge and wind stress on the fecal coliform plume in the Danshuei River coastal area. In the absence of wind stress, no cyclonic circulation occurs as a result of river discharge under flood conditions. However, anticyclone circulation occurs on the east coast of the Danshuei River mouth at low river discharge. The plume distances of fecal coliform from the Danshuei River mouth reach 17 km and 5 km under flood flow and Q50 flow conditions, respectively. In the absence of river discharge, the plume distance of fecal coliform is confined to the Danshuei River mouth. Consequently, the wind directions control the upwelling and downwelling plumes. If the river discharge and wind stress are included in the model simulations, the surface fecal coliform concentrations are flushed out of the Danshuei River mouth and move to the southwestern coast as a result of the wind’s favoring downwelling. Conversely, the fecal coliform concentrations are flushed out of the Danshuei River mouth and flow to the northeastern coast, with the wind favoring upwelling. Comparing simulated results with and without wind stresses, the plume distance/area of fecal coliform decreases with the inclusion of wind stress.