The Influence of River Discharge on Salinity Intrusion in the Tanshui Estuary, Taiwan

Abstract

The Tanshui River system is the largest estuarine system in Taiwan, with drainage basin including the capital city of Taipei. It consists of three major tributaries: the Tahan Stream, Hsintien Stream and Keelung River. A vertical two-dimensional numerical model is refined and expanded to handle tributaries as well as the main stem of an estuarine system, and applied to the Tanshui River estuarine system. Observed time series of salinity data and tidal-averaged salinity distributions have been compared with model results to calibrate the turbulent diffusion coefficients. The overall model verification is suggested to be achieved with comparisons of residual currents and salinity distribution. The agreement between observed data and computed results put the stamp of approval on the model. The model is shown capable of reproducing the prototype water surface elevation, currents and salinity distributions. This paper emphasizes model applications. As an example of model utilities, the calibrated and verified model is used to calculate the salinity distributions under various conditions. The salinity distributions are simulated and compared under several scenarios of wastewater diversion and under various hydrological conditions to examine their response to the amount of freshwater inflows in the Tanshui River system.

We find that the Tanshui River system is a partially mixed estuary in most instances and its salinity is very sensitive to river discharge. The model is also used to investigate the salinity response to the pulse of high freshwater discharge. The salinity has a very quick response and recovers to its original condition in about 7 days. The calculated estuary flushing time was strongly dependent on river flow and varied between hours to a month.