Advanced flow simulations of estuarine water resources responding to changes in societal demands

Water resources of rivers and estuaries have traditionally been managed for local societal demands, including municipal, agricultural, and industrial usages, as well as for transportation as navigational channels. However, as societal demands have changed including urban expansion, economic growth, cultural transition, etc., the land uses also should be changed, and the civil necessities of water have been diversified, and this has raised concerns about the ecology, environment, and water quality issues. This study attempted to address three water resource management issues in South Korean rivers and estuaries, which occurred recently due to the changes in societal demands for water and explained how the most advanced scientific research techniques had been applied. The main issues were 1) agricultural water security and ecological environment restoration in Nakdong River Estuary, 2) water and soil quality degradation due to industrial effluents in Hyeongsan River Estuary, and 3) utilization of rivers as waterways and water quality and ecology issues in the ARA waterway. To find solutions to conflicts of water resources, the series of numerical simulations validated by the field observations allowed 1) assessments of the impact and extent of salt intrusion upon the opening of estuary dam gates, 2) estuarine physical processes to investigate the influence of industrial effluent, and 3) analysis water quality variations resulting from changes in the function of artificial channels. These analyses are expected to provide appropriate and science-based strategies of regional water management.


Introduction
Rivers serve various functions.South Korea had traditionally managed them, primarily focusing on water supply and flood control.Some rivers play roles in mitigating damages to society due to floods and others provide water for municipal, industrial, and agricultural needs, ensuring the availability of essential water resources.Such a traditional single-purpose targeted management of water resources has led to significant changes in the appearance of the river and affected the habitat environment for the wildlife residing within the river.Furthermore, due to urbanization, the increase of pollutants from 1313 (2024) 012015 IOP Publishing doi:10.1088/1755-1315/1313/1/012015 2 both point and non-point sources is an important factor contaminating the rivers.Along with the degradation of environmental qualities, as the economy and society developed, the public alertness of citizens has increased.As a result, environmental issues such as river ecology and water quality have become critical in natural ecology and civil communities.
Estuaries are unique systems where freshwater rivers merge with salty ocean waters, making dynamic environments.Estuaries can foster rich biodiversity serving as a breeding ground for many species, accommodate and precipitate soil, sand, and other sediments, and are suitable for the port facilities.Crucially, despite their diverse roles in filtering pollutants, improving water quality, and protecting coastal habitats, they have been overly exposed to pollutants from industrial and agricultural activities.In particular, water pollution affects aquatic ecosystems within estuaries, which can negatively impact fisheries.
Consequently, many scientific approaches have been conducted to solve these rivers and estuaries' ecological and environmental problems.For example, Kim and Hwang suggested a water quality monitoring system in a science-based optimized way to manage estuaries [9].In addition, Kim and Hwang proposed a more accurate method to estimate the river bed shear stress by integrating field observations and numerical modeling to identify sediment movement and morphological changes [11].Such integrated methods of modeling and observations have addressed issues related to the water quality, environment, and ecology of rivers and estuaries.Therefore, we want to introduce the investigations of three cases of rivers and estuaries in South Korea where environmental and ecological problems have occurred: Nakdong river, Hyeongsan river, and ARA waterway (Figure 1).

Managing water resources to ensure agricultural water security and restore the ecological environment: Case study of the Nakdong river estuary
Nakdong River Estuary holds significant hydrological importance in South Korea.The estuary covers a region with rural agricultural land, large modern cities, and maritime industrial complexes.Since more than 34% of the agricultural and 80% of municipal and industrial water demands in the entire Nakdong River watershed or basin are concentrated in the Nakdong River estuary, it is necessary to provide a stable water supply to meet these demands.Nevertheless, the seasonality of rainfall in the basin leads to a situation where the river discharge is insufficient to meet water demand during the dry season.Therefore, the Nakdong Estuary Dam was constructed in 1987 to secure fresh water from the Nakdong River reach upstream.
In the past, the primary focus of water resource management in the estuary was to ensure a stable water supply for agriculture.However, in recent years, there has been a growing societal interest and demand for water quality improvement and ecological restoration in the brackish water area that has been lost by the construction of the estuarine dam.In response to these social demands, the government has considered opening the gates of the estuary dam to improve the water quality and restore the estuarine ecosystem by circulating the coastal and fresh waters.However, opening the dam gates may significantly impact the ecological environment and social infrastructure that have adapted to the environmental changes brought about by the construction of dams over the past 35 years, particularly through saltwater intrusion.Therefore, it is necessary to assess the impact of opening the gates beforehand and consider measures to minimize any potential damage.
The most significant change in the estuarine environment resulting from the gates' opening is the river's stratification by seawater intrusion.The development of strong stratification in the river's upper reaches can impact the existing water environment and ecosystem.Therefore, it is imperative to consider the stratification by salty water intrusion and exclusion.In this case, three-dimensional hydrodynamic models were utilized, SUNTANS and Delft3D-Flow [6] [13].SUNTANS can analyze estuarine physical processes in three dimensions and has the advantage of accurately reproducing the stratification, Delft3D-Flow can simulate flow changes resulting from the dam gates operation.This work simulated seawater intrusions for various scenarios, including partial gate-opening continuously and gate operation to control the extent of salt intrusion.In addition, numerical simulations were conducted to consider various physical factors such as river flow, tides, density currents, and wind forcing, which can impact salt intrusion.The numerical experiments for partial gate-opening were designed to explore how the magnitude of river discharge, 102, 202, 302, and 402 m 3 /s, impacts the development of salt intrusion downstream in the river.Figure 2 illustrates the salinity distribution along the river's thalweg and the salt intrusion developments over time.With 102 m 3 /s of the river discharge, salinity concentrations of 0.3 PSU were intruded to a distance of 27 km on the 30th day after the gates were opened (Figure 2a and b).Since the major intake facilities for municipal and industrial waters and the withdrawals of the West Nakdong River for agricultural water are located 26 and 15 km upstream from the dam, these major facilities are susceptible to salinity damage under this river discharge condition (Figure 1b).In addition, the facilities within 15 km could be damaged by salt intrusion if the discharge is less than 202 m 3 /s.The seasonality of rainfall in the basin results in substantial variations of the discharge during a period between flood and drought, making it difficult to sustain a continuous discharge of more than 202 m 3 /s.Therefore, the facilities would experience damage from seawater intrusion if the dam gates were opened.
The numerical experiments for gate operations were also performed to give operating solutions that minimize the salinity impacts on the facility within 15 km and ecological restoration in the brackish water.The first operational strategy for maintaining a brackish zone within 5 km from the dam involves allowing a single saltwater inflow and twice the freshwater discharge as underflow through 1.5-meter opened gates during every two tidal cycles.The second operational strategy for maintaining a brackish zone within 10 km involves allowing twice the saltwater inflow and freshwater discharge as underflow through 1.5-meter opened gates during every two tidal cycles.When discharging freshwater into the sea, the opening is adjusted to maintain a water level at least 20 cm higher than the tidal level to mitigate salinity intrusion into the river.Additionally, it mentions the range of river water level (0.4-0.8 m) maintained to ensure an adequate river water volume.Figure 3 illustrates developments of salt intrusion length, river surface, and sea surface under two different gate operations to control the extent of the brackish zone to within 5 and 10 km.In the first operation, the 0.3 PSU of salinity concentration reached to 3.7 km from the dam, while in the second operation, it intruded up to 6.8 km.
The result indicates that the salt intrusion can be controlled to a specific distance by adjusting the gates instead of the 27 km of salt intrusion that occurs when the partial gates are opened continuously.The simulations presented here propose operational strategies aimed at preventing salt intrusion while simultaneously maintaining the circulation of freshwater and seawater through the dam.

Water and soil quality issues from industrial effluents: Case study of the Hyeongsan river estuary
Large industrial complexes have developed near estuarine areas due to the convenient transportation of materials by sea and rivers, the presence of freshwater sources, and the utilization of cooling or heating water discharge.In the past, water resource management in industrialized regions primarily prioritized water allocation for industrial purposes, with a particular focus on securing a stable supply of adequate quantities.Now, the societal needs for water are diversified including recreational usage, so water quality management is now necessary along with conventional purposes.For any purpose of using water, managing water resources necessitates scientific investigations of physical features, particularly the dynamics of rivers and ocean flows, especially in estuarine environments.
The largest steel industrial complex in South Korea is situated near the Hyeongsan River estuary, near the city of Pohang (Figure 1d).In June 2016, it was reported that mercury levels in a benthic organism collected from the Hyeongsan River exceeded the national standard.Subsequent investigations revealed significant mercury contamination in sediment samples from Gumu Creek (Figure 1d, yellow box).Discharging effluents from industrial wastewater treatment facilities into the creek may have substantially increased mercury concentration levels [2].The effluents from the creek could be discharged into the river, transported downstream along its main channel, and potentially impact both the river and Yeongil Bay.Therefore, scientific investigations were required to ascertain the cause of heavy metal contaminations and assess the extent of the contaminated sediment in order to take remediating actions in the region.Generally, it is employed to comprehend the physical processes within rivers or coasts by analyzing physical variables such as three-dimensional velocities, temperature, and salinity distributions, obtained through field observations or numerical modeling.In this work, a hydrodynamic model, Delft-3D, was used to comprehend the physical processes in the river estuary due to its advantages in reproducing and predicting flows and phenomena.In the past, depth-averaged two-or one-dimensional hydrodynamic models had limitations in understanding the three-dimensional complex flow structure and stratification phenomena in estuaries influenced by various external forces from rivers and oceans.However, with advancements in computer performance, it has become possible to conduct highresolution numerical simulations covering large estuarine areas in three-dimensions.The physical variables highly resolved and three-dimensionally simulated by the models contributed to understanding the physical processes in the estuary.To accurately reproduce flows in the region where sediment with absorbed mercury was detected, it should be taken into account: the physical characteristics of the Yeongil Bay and the Hyeongsan River basin, and the stratification resulting from the interaction between the river and the sea.
The flows within the study area is directly influenced by external forces, including tides, waves, ocean currents, and density currents originating from the ocean.The domain was extended to encompass the East Sea in order to model ocean circulations within the bay.As rivers typically exhibit hydraulically subcritical flow conditions, the water levels in channels are influenced by the downstream flow conditions.Therefore, tidal constituents were carefully configured downstream, and the discharge of freshwater was set at the upstream open boundary, respectively.Regarding the open boundary conditions on the ocean side, the results obtained from the global circulation model were used due to spatiotemporal resolution limitations that hindered observations from covering these regions (Figure 4).In addition, the hydrodynamic model was coupled two-way with a wave model, the Simulating Waves Nearshore (SWAN) [15], to account for the effects of waves.Freshwater discharge and sediment yields were simulated with a watershed model, the Soil and Water Assessment Tool (SWAT) [1], and then used as input data for the upstream boundary condition of the river.The river discharge, water levels, and sediment yields can be obtained through gauging stations or field measurements.However, in the absence of data, especially for sediment yields, the well-validated watershed model can assist in estimating these variables at the upstream open boundary.
The validation procedure has to be carried out to improve the accuracy and reliability of the model by tuning model parameters.The validation mainly focuses on external forces, physical variables, and parameters input into the open boundary and requires appropriate observational data for those variables.Typically, the model validation relies on time series data obtained from a few fixed observation points.This approach assesses the model's abilities to reproduce temporal variations but has limitations when it comes to capturing spatial variations.For instance, time series data of water temperature, salinity, and density measured at a fixed point are valuable for assessing the model's performance in simulating heat and salt transport over tidal cycles.However, validating the model for spatial variability, such as stratification, is impossible with using the time series data.In such a case, the Lagrangian observational methods, such as moving vessel measurement [11] or the Yoing Ocean Data Acquisition (YODA) profile measurement system [8][14], can provide valuable contributions to the model validation spatially (Figure 5).Furthermore, a High Frequency radar, which provides time series of near-surface ocean velocity field, and GPS surface drifters can be employed for the validation [12].Therefore, acquiring field observations capable of assessing both spatial and temporal variability becomes necessary.The well-validated model results were used to analyze and predict the physical processes in the river estuary.To assess the impacts on water quality, it is necessary to integrate this hydrodynamic model with the water quality module and conduct further model validation for water quality.Such thoroughly validated high-resolution models can establish a quantitative scientific foundation for addressing engineering problems and can significantly contribute to decision-making and response planning.

Utilization of rivers as waterways and water quality ecological issues: Case study of the ARA waterway
The ARA waterway is an artificial brackish waterway that connects the Yellow Sea and the Han River in South Korea (Figure 7).This waterway was initially designed to connect only with the Yellow Sea to prevent flooding in the Gulpo basin [7].As a great flood that occurred in the Gulpo basin in 1987 caused enormous loss of life and property, a flood control plan for the Gulpo Stream had to be established.However, if it is connected only to the Yellow Sea, it can only serve as a temporary waterway.Accordingly, the waterway was also connected to the Han River, making it possible to use it as a canal in normal times and in preparation for floods in the Gulpo basin [16].
However, contrary to expectations, the demand for the canal was significantly low, which necessitated the reduction of the canal's function.In addition, since living water, agricultural water, or industrial water in the ARA waterway basin are supplied by waterworks, this artificial channel does not function as direct water use.On the other hand, as urbanization progresses along the ARA waterway, the demand for cultural tourism naturally increases, which means that the function of this artificial waterway changes over time.However, the recent water quality of this artificial channel is not up to the level where waterfront activities are possible among river water quality grades in South Korea, and the occurrence of algae blooms is a particular problem [3].Therefore, further efforts are needed to utilize this waterway as a waterfront space for water leisure, sports, and tourism.For this reason, it is necessary to improve the water quality of the waterway in order to consider converting major facilities into cultural, tourism, and water-friendly spaces.The water elevation of the ARA waterway is usually maintained between 2.3 m and 3.4 m for waterway operations and should be at least 2.0 m for vessels to pass.Consequently, to sustain the water level, this artificial channel is controlled by fresh water from the Han River and seawater from the Yellow Sea.The fresh water and seawater are introduced into the channel through the drainage and lock gates, respectively, and water is drained toward the Yellow Sea to conserve water mass.Under these intricate operations, the brackish channel has complex water quality reactions and hydrodynamic processes like stratification.Hence, we analyzed the aquatic ecological phenomena that occur in the artificial channel using numerical simulations.
We first utilize a three-dimensional hydrodynamic model, Delft3D-FLOW, to reproduce the channel flow.The Arakawa C-grid and sigma coordinates were adopted for horizontal and vertical grids, respectively [4].This model is suitable for stable stratified shallow water flow such as rivers, estuaries, and coasts [10], and the bottom slope of the study area not steep, so applying the sigma coordinate system is appropriate.Before the 3-D model was introduced to academic society, one or two-dimensional models were available, which fit well when the research area is vertically homogeneous.However, areas with vertical changes such as stratification and thermocline that can occur due to salt intrusion, freshwater river discharge, and cooling water dispersion cannot be simulated using 2-D models.Therefore, we used the 3-D model to reproduce the artificial brackish channel, as the 3-D model can simulate vertical changes caused by wind forces, density differences, bed stress, etc.When the 3-D model was applied to the brackish channel considering the operating system, the model results were found to be very similar to observations (Figure 8).Based on the flow fields produced by the hydrodynamic model, a 3-D water quality model, Delft3D-WAQ, was then applied to simulate the movement and reaction of the substances.This model solves the advection-diffusion-reaction equation by considering user-defined biological and biochemical processes.Additionally, it can simulate up to 30 species of algae and includes multi-and inter-species competition [5].Since seawater and freshwater are introduced separately, marine diatoms and fresh diatoms should be specified separately, which can be simulated with this 3-D model.
Various scenarios according to input control can be simulated using the integrated numerical models of hydrodynamics and water quality, and water quality improvement measures for water leisure sports tourism can be proposed (Figure 9).The function of the artificial channel has changed over time from their original purpose, so unexpected water quality and ecological problems can arise.Nonetheless, we can find solutions by using 3-D numerical models to these problems.

Conclusion
The ecological, environmental, and water quality issues in South Korean rivers and estuaries were caused by changes of societal water resource demands, and we studied with the 3-D hydrodynamic models and field observations.The outcomes of each work can be summarized as follows: 1. Full opening of the dam gates in the Nakdong River could lead to extensive damage to water intake facilities for agricultural, municipal, and industrial water due to salinity intrusion.The brackish water region is expected to extend over 27 km from the dam.For securing agricultural water and simultaneously restoring the brackish water region, the numerical simulation recommends the dam gate operations hold the salt intrusion regions within 5 km and 10 km from the dam.2. To investigate the causes and effects of industrial wastewater effluents in the Hyeongsan River estuary, we performed comprehensive field observations and a high-resolution threedimensional hydrodynamic model.The models were configured to encompass hydrodynamic characteristics from the basin to the ocean, with the validation including spatiotemporal variability.The high-resolution data generated by the well-validated model have advanced the understanding of the physical processes in the study area and helped to analyze the environmental impacts.3. To mitigate water quality degradation in the ARA waterway, it is necessary to regulate waterway flows by controlling lock and drainage gates and effectively manage pollution sources entering the channel.The results obtained from the integrated model of the hydrodynamic and the water quality indicate the potential of improving water quality through regulating flow in the waterway.These findings are significant for guiding regional water management strategies, offering valuable insights into mitigating the identified challenges and ensuring sustainable water resource management in the studied areas.

Figure 1 .
Figure 1.Study areas: (a) map of South Korea; (b) map of Nakdong river estuary.The red line represents the location of the estuary dam, the yellow box denotes the location of intake facilities for municipal and industrial water, and the white box marks the location of the sluice gate used to supply agricultural water.;(c) the hydrodynamic model domain and its bathymetry; (d) map of Hyeongsan river estuary.The yellow box denotes the confluence of Hyeongsan river and Gumu creek, and white box indicate heavy industrial complex zone; (e) the grids and bathymetry of hydrodynamic model domain; (f) map of ARA waterway.The yellow and white box denote the location of sluice gates, and blue box marks the location of weir to prevent salt intrusion.

Figure 2 .
Figure 2. Distribution of salinity concentration and changes in maximum salt intrusion length; (a, b, c, d) the salinity distribution along the river's thalweg under 102, 202, 302, and 402 m 3 /s upstream discharge conditions.The red line indicates 0.3 PSU, the salinity concentration criterion that does not affect water intake facilities.(e, f, g, h) The maximum salt intrusion length changes over time under 102, 202, 302, and 402 m 3 /s upstream discharge conditions.

Figure 3 .
Figure 3. Changes in maximum salt intrusion length by the gates operations (a) to maintain a brackish zone within 5 km, specifying one instance of saltwater inflow and two instances of freshwater and (b) to maintain a brackish zone within 10 km, specifying two instance of saltwater inflow and freshwater discharge as underflow.

Figure 4 .
Figure 4. Configurations of the hydrodynamic model, incorporating watershed, wave, and global models.

Figure 5 .
Figure 5. Moving vessel and YODA profiler measurements and its results; (a) schematic diagram for the MVP methods; (b) spatial distribution of temperature and salinity using YODA profiler; (c) trajectories of moving vessel; (d) bathymetry estimation and (e) bed shear stress estimation using the MVP methods.

Figure 6 .
Figure 6.Coupled hydrodynamic model results for river flow conditions: (a, b, c) 35.4 m 3 /s for the drought flow condition, (d, e, f) 240.8 m 3 /s for the annual mean flow condition, and (g, h, i) 1147.3 m 3 /s for the flood flow condition in Hyeongsan River during 2021; (a, d, g) bed shear stress distribution; (d, e, h) suspended sediment load; (c, f, i) morphological changes.
Figure 6 illustrates bottom shear stress, suspended sediment transport, and morphological changes under various flow conditions, including average annual, drought, and flood ICERM-2023 IOP Conf.Series: Earth and Environmental Science 1313 (2024) 012015 IOP Publishing doi:10.1088/1755-1315/1313/1/0120158 flows in the river during 2021.The high-resolution simulation results can estimate suspended sediment load, amount of eroded and deposited sediment, and spatial trend of erosion and deposition.

Figure 7 .
Figure 7. Domain of the artificial brackish channel, the ARA waterway located in South Korea.

Figure 8 .
Figure 8. 3D comparison of (a) temperature and (b) salinity between observation data and model results.

Figure 9 .
Figure 9. Simulation results of water quality improvement scenarios using the 3D water quality model.