The influences of weir construction on salt water intrusion and water quality in a tidal estuary--assessment with modeling study.

A three-dimensional hydrodynamic and water quality model was developed and applied to predict the changes in physical and biochemical processes that would result from the proposed Dadu Weir construction in the Wu River estuary, located in central Taiwan. A high-resolution unstructured grid was constructed to represent the narrow channels in the tidal estuary. The model was calibrated and verified with available hydrographic and water quality data measured in 2011. The overall performance of the model is in reasonable agreement with the measured water level, salinity, and water quality state variables. The model was then used to investigate the changes in salt water intrusion and water quality as a result of weir construction under low-flow conditions. The model simulations indicate that more tidal energy will propagate into the estuary after weir construction because of decreased freshwater discharges. The limits of salt water intrusion before and after weir construction coincide at a distance of 11.6 km from the mouth of the Wu River. This salt water intrusion limit is the reason that the Dadu Weir will be constructed at a distance of 12 km from the mouth of the Wu River. The weir will become a barrier to salt water intrusion. The simulation results indicate that the concentrations of dissolved oxygen and nutrients will decrease slightly after weir construction, while the chlorophyll a concentration in the middle reach will increase after weir construction. However, construction of the weir is predicted to have little influence on water quality conditions downstream of the Wu River estuary. The results of this case study provide quantitative estimates of the physical and biochemical changes expected to occur in this nature system due to human action.

Modelling diagnosis of heavy metal (copper) transport in an estuary.

A vertical (laterally-averaged) two-dimensional heavy metal transport model was developed to simulate the fluxes of copper in the Danshuei River estuarine system. The toxic model was incorporated into the hydrodynamic, the salt transport and the sediment transport modules. The model has been validated with observed time series data of water surface elevation, current, salinity, and suspended sediment concentrations in 2001 and 2002. An exponential relationship was established to relate variation in partition coefficient of copper to salinity and suspended sediment concentrations in the Danshuei River estuary. Comparisons of the total, dissolved, and particulate copper concentrations calculated by the numerical model and field data along the Danshuei River-Tahan Stream show good agreement. It is noteworthy that the model requires a point source of total copper with the strength of 250 kg/day to account for the observed persistently high total copper concentration near Hsin-Hai Bridge in the Tahan Stream, about 24 km upriver from the river mouth. The huge garbage dump site near the Hsin-Hai Bridge is likely the source of heavy metal pollution. The validated model was then applied to investigate the tidally averaged salinity distributions, residual circulation, suspended sediment, and total copper concentrations under low flow condition. The residual circulation is characterized by the upriver movement of denser saline water in the lower layer and the downriver movement of fresher water in the upper layer. The circulation pattern is driven by the longitudinal gradient of salinity distribution. The residual circulation occurs in the deep channel of Kuan-Du Bridge and Taipei Bridge. Under low flow condition, the limits of salt intrusion are located at Hsin-Hai Bridge. The tidally-averaged sediment concentration distribution exhibits a local maximum concentration around the null point in the Danshuei River-Tahan Stream. A local maximum of total copper concentration, which is nearly as high as that near the point source, occurs around the null point, which is potentially a hot spot of heavy metal accumulation.

Modelling of hydrodynamics and cohesive sediment transport in Tanshui River estuarine system, Taiwan.

A laterally averaged two-dimensional numerical model is used to simulate hydrodynamics and cohesive sediment transport in the Tanshui River estuarine system. The model handles tributaries as well as the main stem of the estuarine system. Observed time series of salinity data and tidally averaged salinity distributions have been compared with model results to calibrate the turbulent diffusion coefficients. The overall model verification is achieved with comparisons of residual currents and salinity distribution. The model reproduces the prototype water surface elevation, currents and salinity distributions. Comparisons of the suspended cohesive sediment concentrations calculated by the numerical model and the field data at various stations show good agreement. The validated model is applied to investigate the tidally averaged salinity distributions, residual circulation and suspended sediment concentration under low flow conditions in the Tanshui River estuarine system. The model results show that the limit of salt intrusion in the mainstem estuary is located at Hsin-Hai bridge in Tahan Stream, 26 km from the River mouth under Q75 flow. The null point is located at the head of salt intrusion, using 1 ppt isohaline as an indicator. The tidally averaged sediment concentration distribution exhibits a local maximum around the null point.