Danshuei River Estuary Research Articles

Modelling the effects of reservoir construction on tidal hydrodynamics and suspended sediment distribution in Danshuei River estuary

A vertical (laterally integrated) two-dimensional numerical model was implemented to study the hydrodynamics, saltwater intrusion, and suspended sediment in the Danshuei River–Tahan Stream due to the Shihmen Reservoir construction in the upriver reaches. The construction of the reservoir and water diversion in the upper reaches of the river system significantly reduces the freshwater inflow and drastically changes the river bathymetry. The model was calibrated and verified with the available hydrographic data measured in 1977, 1978, and 1999 as well as measured salinity and suspended sediment concentration in 1999. The overall performance of the model is in reasonable agreement with the field data. The validated model was then used to investigate the change in hydrodynamics, saltwater intrusion, suspended sediment distribution, and flushing time as a result of reservoir construction in upper river of Tahan Stream. The model simulations indicate that more tidal energy propagates into the estuarine system after reservoir construction because of the substantial increase in river cross-sections. The limits of saltwater intrusion after reservoir construction extended farther inland 2–3 km than those after reservoir construction. The modelling results also reveal that the suspended sediment concentration before reservoir construction was higher than that after reservoir construction along the river mouth to Kuan-Du due to the significant bathymetric change after the reservoir construction. The calculated estuary flushing time was strongly dependent on river flow and reduced 2.3–25 h under different river discharges after reservoir construction due to the change in river bathymetry.

Modeling the influence of river discharge on salt intrusion and residual circulation in Danshuei River estuary, Taiwan

A 3-D, time-dependent, baroclinic, hydrodynamic and salinity model was implemented and applied to the Danshuei River estuarine system and the adjacent coastal sea in Taiwan. The model forcing functions consist of tidal elevations along the open boundaries and freshwater inflows from the main stream and major tributaries in the Danshuei River estuarine system. The bottom friction coefficient was adjusted to achieve model calibration and verification in model simulations of barotropic and baroclinic flows. The turbulent diffusivities were ascertained through comparison of simulated salinity time series with observations. The model simulation results are in qualitative agreement with the available field data. The validated model was then used to investigate the influence of freshwater discharge on residual current and salinity intrusion under different freshwater inflow condition in the Danshuei River estuarine system. The model results reveal that the characteristic two-layered estuarine circulation prevails most of the time at Kuan-Du station near the river mouth. Comparing the estuarine circulation under low- and mean flow conditions, the circulation strengthens during low-flow period and its strength decreases at moderate river discharge. The river discharge is a dominating factor affecting the salinity intrusion in the estuarine system. A correlation between the distance of salt intrusion and freshwater discharge has been established allowing prediction of salt intrusion for different inflow conditions.

INVESTIGATION OF TURBIDITY MAXIMUM IN A MESOTIDAL ESTUARY, TAIWAN

ABSTRACT: To comprehend the distributions of salinity, temperature, and suspended sediment in the Danshuei River estuary in Taiwan, monthly field surveys were conducted in 2003. These included several high and low slackwater surveys and intensive surveys. The results show that the Danshuei River estuary is predominately a partially mixed estuary. The highest concentration of suspended sediment is typically observed at the Chung-Hsin Bridge, the most upstream sampling station. The suspended sediment concentration exhibits a general decreasing trend in the downstream direction. It may be concluded that the sediments mostly come from the upstream reach. A locally high concentration of suspended sediment is found at the Kuan-Du station because of the local deep channel bathymetry and two-layered estuarine circulation. A vertical two-dimensional hydrodynamic and sediment transport model is applied to investigate the tidally averaged salinity distribution, residual circulation, and suspended sediment concentration. The modeling results reveal that, under the Q75 flow condition (i.e., low flow), a turbidity maximum occurs at the Kuan-Du station due to the strong estuarine circulation. The model simulation with a much higher river flow condition results in a weaker residual circulation and weaker turbidity maximum.

Effect of channel connection on flow and salinity distribution of Danshuei River estuary

Numerical models are often used to evaluate the potential impact of human alteration of natural water bodies and to help the design the alternation to mitigate its impacts. A vertical (laterally integrated) two-dimensional hydrodynamic model was expanded to include the capability of simulating river loops as well as tributaries. The model was performed and applied to the Danshuei River estuarine system in northern Taiwan which consists of three major tributaries: the Tahan Stream, Hsintien Stream, Keelung River, and one river loop under the Chung-Hsin Bridge. The expanded model was reverified with observational field data of 2000. The verified model was then used to hindcast the river hydrodynamic conditions with a loop connection between the Danshuei River and Keelung River, which existed prior to 1965. It was found that the configuration of river loop connection has significant impacts on the residual transport along the connecting channel and the salinities in the connected river branches. The results show that the model may provide an ideal tool for management decision.

Water column light attenuation estimation to simulate phytoplankton population in tidal estuary

The penetration of sunlight into the water column plays a critical role in the aquatic ecosystem. The irradiance available for primary production in a water body depends on the incident light at the water surface, light extinction in the water column, and depth. In this study, the light attenuation through the water column of the Danshuei River–Keelung River estuary was estimated. The measurement of photosynthetically active radiation (PAR) indicates that the conventional exponential attenuation of light with depth is a very good model. A light attenuation coefficient may be derived from the PAR measurements at each location. The regression with salinity yields a good correlation, indicating that the fraction of seawater should be a good parameter for estimating the water column light attenuation coefficient (Kd). A laterally averaged two-dimensional finite difference model for hydrodynamic and water quality model was performed and applied to simulate the phytoplankton population at the lower reach of the Danshuei River estuary. In the process of phytoplankton population simulation, the regression model of Kd and salinity was incorporated in the water quality model. The simulated results show that the modeled concentration of chlorophyll a matched the measured values at the lower reach of the Danshuei River estuary.

Trace metal (Cd, Cu, Ni and Pb) partitioning, affinities and removal in the Danshuei River estuary, a macro-tidal, temporally anoxic estuary in Taiwan

Using physical (0.1 and 0.45 μm cut-off filters) and ion exchange (Chelex-100 and AG MP1 resins) separation techniques, detailed distribution patterns and chemical affinities of selected trace metals (Cd, Cu, Ni, Pb) were investigated under different redox conditions in the Danshuei River estuary, a macro-tidal, temporally anoxic estuary, passing though the metropolitan area of Taipei in Northern Taiwan. A distinct partitioning and removal behavior of the different trace metals was revealed, in that under high river flow and oxic conditions, a large fraction of these trace metals that were passing through a < 0.1 μm pore size filter was found to be Chelex-labile, while during the dry season, low river flow and anoxic conditions, elevated fractions of trace metals, especially Cd and Cu, were found to be strongly complexed to inert ligands such as sulfides, with higher percentages present also in the colloidal (0.1–0.45 μm), and particulate (> 0.4 μm) size fractions. Large fractions of both particulate and dissolved metals were removed within the estuary, especially during anoxic conditions in the dry season. Anoxic conditions in the upper Danshuei estuary resulted in very high sulfide concentrations that likely were responsible for the high removal of trace metals as metal-sulfides in the water column during that time. The variations in distribution and fractionation patterns of the different metals were reflected by both their affinities to ion exchange resins and their size distributions.

WATER COLUMN LIGHT ATTENUATION IN DANSHUEI RIVER ESTUARY, TAIWAN

The penetration of sunlight into the water column plays a critical role in the aquatic ecosystem. This study investi- gates light attenuation through the water column of the Danshuei River estuary and explores the models for quantifying it. The mea- surement of photosynthetically active radiation (PAR) indicates that the conventional exponential attenuation of light with depth is a very good model. A light attenuation coefficient may be derived from the PAR measurements at each location. The simpler mea- surements of Secchi depth (SD) provide an approximate estimate of the attenuation coefficients through an inverse linear relationship. The results also reveal that the amount and kinds of materials, which are either dissolved or suspended in the water, control light attenuation through the water column. However, the linear regres- sion with such a model results in very poor correlation, and a rela- tively large constant term for the Danshuei River estuary. The regression with salinity yields a good correlation, indicating that the fraction of sea water might be a good parameter for estimating light attenuation coefficient for practical application in the Dan- shuei River estuary. (KEY TERMS: light attenuation; Danshuei River estuary; Secchi depth; salinity; suspended sediment.)

Residence time of the Danshuei River estuary, Taiwan

The residence time of an estuary is defined in this study as the average time the initially existing water parcels reside in the system before they are flushed out. The residence time of the Danshuei River is calculated through a series of numerical experiments using a laterally integrated two-dimensional hydrodynamic eutrophication model (HEM-2D). The results show that the residence time is on the order of 1–2 days under the mean river flow to zero river flow condition, which is very short compared with most time scales of biogeochemical processes. A procedure is developed to quantify individual contribution to flushing by each of the three major physical transport mechanisms: tide, river discharge, and the density induced circulation. The results indicated that, in general conditions, tidal flushing exerts the greatest influence to the flushing of the Danshuei River estuary, while the density induced circulation hardly contributes any. Tidal transport contributes more than 50% of the flushing when river discharge is below its long-term mean. The suitability of applying the tidal prism method, the modified tidal prism methods, and the fraction of freshwater method in this estuary is also investigated. The relatively short residence time is likely to be one of the limiting factors that result in low phytoplankton biomass in spite of extremely high nutrient concentrations, and causes a significant fraction of pollutants to exert their effects in the coastal waters outside the estuary.