The influence of river discharge on salinity intrusion in the Tanshui estuary, Taiwan

The Tanshui River system is the largest estuarine system in Taiwan, with dr ainage basin including the capital city of Taipei. It consists of three maj or tributaries: the Tahan Stream, Hsintien Stream and Keelung River. A vert ical two-dimensional numerical model is refined and expanded to handle trib utaries 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 distribut ions have been compared with model results to calibrate the turbulent diffu sion coefficients. The overall model verification is suggested to be achiev ed with comparisons of residual currents and salinity distribution. The agr eement between observed data and computed results put the stamp of approval on the model. The model is shown capable of reproducing the prototype wate r surface elevation, currents and salinity distributions. This paper emphas izes model applications. As an example of model utilities, the calibrated a nd verified model is used to calculate the salinity distributions under var ious conditions. The salinity distributions are simulated and compared unde r 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 parti ally mixed estuary in most instances and its salinity is very sensitive to river discharge. The model is also used to investigate the salinity respons e 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 calcul ated estuary flushing time was strongly dependent on river flow and varied between hours to a month.