The Yamadazeki Barrage, located in Fukuoka Prefecture, Japan, is an oblique weir that was constructed at the middle reach of the Chikugo River in 1790 and is still operational. The weir was ...recognized as a World Heritage Irrigation Structures by the International Commission on Irrigation and Drainage. This study used hydraulic analyses of the Yamadazeki Barrage to understand its function. Two-dimensional shallow water equations were utilized for the hydraulic analyses, and the calculated velocity distribution under ordinary flow conditions (approximately 40 m
3
/s) showed good agreement with the measured data. In addition, the bedload transport rate was calculated from the water velocity and depth distributions using the Ashida-Michiue formula to simulate riverbed variations. The results suggested that there was slight erosion and deposition in the vicinity of the weir under ordinary flow conditions. In contrast, under flood flows of approximately 3500 m
3
/s, riverbed variations occurred in most of the calculation area, with extensive erosion and deposition of more than 2.0 m over 24 h. To reevaluate the structural functions of the Yamadazeki Barrage, elimination scenarios of both the Southern Ship Way and the Sand Sluiceway, which are considered important sand removers, were determined and analyzed. These scenarios revealed that the flow became unstable, and riverbed variations were significant. Therefore, these structural elements stabilized the water flow and maintained a safe and reliable water withdrawal. Additionally, the relevance of the weir structures, which was carefully calculated and designed to match the river flow rate and water level, was evaluated numerically.
To effectively manage a eutrophic reservoir, short-term predictions of algae class-differentiated chlorophyll a (Chl-a) were conducted. The study adopted a three-layered feedforward artificial neural ...network using discrete water environment datasets collected through weekly observations from May to November over seven years (2012–2018). This network was constructed using supervised learning, and the available datasets of a certain observation day were set as the input variables to determine the total Chl-a, Chlorophyceae Chl-a, and cyanobacteria Chl-a that would exist after one week. From the viewpoint of the simplification of the network’s complexity to suppress overfitting, input variables were carefully selected by identifying the important variables related to the seasonal changes in Chlorophyceae and cyanobacteria and eliminating the duplicated expressions of water-quality parameters. However, network downsizing was found insufficient to suppress overfitting. To improve prediction accuracy, dropout was introduced, which stochastically deactivated some nodes in the input and hidden layers in the learning process. The analysis results showed that sufficient short-term predictions of total Chl-a and Chlorophyceae Chl-a may be achieved. The insufficient prediction accuracy of cyanobacteria Chl-a may be overcome using meteorological data as close as possible to the desired prediction day. Consequently, this model may serve as a useful tool for the management of eutrophic reservoirs because short-term predictions of the dominant phytoplankton can be achieved, and the necessary mitigation measures may be accordingly planned.
Massive freshwater discharges due to heavy rainfall have recently caused negative changes to coastal waters, such as red tides and anoxic water masses. In this research, a salinity analysis was ...conducted using a three-dimensional σ-coordinate model to evaluate the impact of large freshwater discharges on the horizontal and vertical distribution of salinity. Specifically, Hakata Bay was examined after a heavy rainfall with a six-year return period, beginning 16 September 2002, a common recurrence interval. The data were calculated for 11-27 September 2002. Tank models were applied to calculate river inflow discharges, which were considered river inflow in the hydrodynamic and salinity diffusion model for Hakata Bay. Model validation results showed high reproducibility, and the calculated tidal current and salinity agreed well with observed data. Results also showed that: (1) salinity was less than 15.0 psu at the river mouths about one day after heavy rainfall; (2) low-salinity water (< 28.0 psu) spread across the surface of the inner part of the bay; and (3) salinity differences between the surface and bottom were large (approximately 4 psu) and lasted three days after the heavy rainfall. It was concluded that red tides and anoxic water masses could be induced in the inner part of the bay.
This study focuses on a method for improving water quality via anoxification recovery using underwater LED treatment in an organically polluted reservoir. The main aim was to evaluate the effects of ...the optical spectrum and light intensity of LED irradiation on the maintenance of healthy aerobic conditions, as well as anoxification recovery, by promoting oxygen production via phytoplankton photosynthesis. Water quality was monitored via beaker and water tank experiments while using LED irradiation for 24 h (12 h on/12 h off) for 2 months in anoxic water, where the anaerobic decomposition of organic matter progressed under strong reductive conditions. As a result, red–green–blue (RGB) light was advantageous for promoting rapid oxygen production by phytoplankton photosynthesis compared with the mixed red and blue light. In particular, LED irradiation, including the green color, preserved the healthy dissolved oxygen environment without lowering the oxygen level in the dark. In addition, RGB irradiation (R:B:G = 1:1:1) did not only assist in preserving a healthy aerobic state in spite of low light intensities but also evidently decreased concentrations of total nitrogen and total phosphorous. In conclusion, the spatial effect of water quality improvement via LED irradiation was not limited to the vicinity of the light source but was applicable to a wide range, in which the light intensity contributed to one-fifth of the optimum photon intensity for phytoplankton photosynthesis.
Abstract
To protect Ho Chi Minh City (HCMC) from submergence due to the ongoing rapid sea level rise (SLR), the Vietnamese government have proposed the construction of a sea dike in Can Gio Bay. Can ...Gio Bay will be closed to regulate the storage and to control water levels in the drainage and sewer systems of HCMC. This could significantly impact the salinity distribution in the Bay and affect its mangrove forest. In this study, a set of scenarios will be analyzed using two-dimensional hydrodynamic and convective-dispersive models to assess the effects of SLR and the construction of a sea dike on salinity distribution in the Bay. The results reveal that the salinity in most areas of the Bay tended to increase positively with the SLR. The sea dikes significantly reduced seawater intrusion into half of the upstream area of the Bay. Considering the influence of SLR and the construction of a sea dike, the sea dike could result in the reduction of salinity. Furthermore, if the sea dike was operated for a long time, half of the Bay area would become freshwater, which would lead to adverse effects on the mangrove forest.
Hanoi, the capital city of Vietnam, is highly vulnerable to flooding from the Red River. A flood disaster in 1971 devastated the river basin and claimed the lives of 100,000 people. The Red River ...dike system was built to protect Hanoi from flooding of the Red River; however, thousands of households are outside the protected area inside the dike system and are highly vulnerable to flooding. Hanoi’s flood protection measures are classified according to the water level of the Red River at Long Bien station: warning levels 1, 2, and 3 correspond to water levels of 9.5 m, 10.5 m, and 11.5 m, respectively. However, risk-reduction strategies for residential areas based on a flood hazard assessment are still inadequate. This study modeled floods and mapped them using a two-dimensional depth-averaged hydrodynamic model to show the inundation depth levels for the entire area in high resolution. Residential areas were found to be highly vulnerable to flooding at all warning levels. Outside the area protected by the dike system, 40.17% (15.94 km2), 54.08% (21.44 km2), and 79.75% (31.60 km2) of the study area were inundated at warning levels 1, 2, and 3, respectively, and these areas contain a small number of households. These results can be used to develop flood prevention and mitigation plans for Hanoi.
The Red River dike system was built, and an emergency solution was implemented to protect Hanoi Capital city from flood disasters. In the solution, floodwater from the Red River is discharged through ...the Van Coc Gate, with an overflow point into Van Coc Lake, and is drained downstream through the Day Weir. However, an optimal scenario for the operating procedure of the emergency solution, based on a comprehensive evaluation of the flood risk level from the Red River to the residential areas located outside the protected areas of the dike system, has not been adequately addressed to date. In this study, we employed the latest digital elevation map in a high spatial resolution for the first time, and a two‐dimensional depth‐integrated hydrodynamic model was utilised to obtain the optimal operation for the emergency solution. The optimal scenario was determined to minimise the flood risk level to the residential areas and optimise the inflow discharge at the Van Coc Gate and the overflow point, and the outflow discharge at the Day Weir for ensuring the safety of the operating system. This study lays a promising foundation for designing risk‐reduction strategies in Hanoi Capital.
In closed water bodies with significant organic pollution, anoxification due to thermal stratification leads to the elution of nitrogen and phosphorus from the bottom sediment and the generation of ...sulfide, resulting in further degradation of the water environment. This study focuses on the water quality dynamics in an organically polluted reservoir exhibiting long-term anoxification using two approaches: (1) field observations of seasonal changes in vertical profiles of dissolved oxygen, nitrogen, phosphorus, and sulfide and (2) construction of a water quality prediction model based on an ecosystem model incorporated with anaerobic biochemical processes. Iron and sulfate reduction occurred simultaneously because nitrate–nitrogen was reduced by denitrifying bacteria after the anoxification, and iron reduction became the main factor of the increase in ammonium–nitrogen and phosphate–phosphorus. The redox state of the bottom sediment surface, when anoxification began to occur, greatly affected the water quality dynamics caused by gradual reductive reactions under anaerobic conditions. Furthermore, the calculation accuracy of ammonium–nitrogen, phosphate–phosphorus, and sulfide was highly improved by modifying the conventional model based on the field observations. The characteristics of water quality under anaerobic conditions were sufficiently reflected in the upgraded ecosystem model. The proposed water quality prediction model could be used to quantitatively estimate the water environment dynamics in organically polluted water bodies. The model could be developed further in the future to solve the problems caused by long-term anoxification.
Water scarcity and flooding constitute major problems for developing countries located within the tropical climatic region of Southeast Asia. In addition, regional water consumption is increasing, ...and water usage patterns have been changing recently. Therefore, an advanced water resource management framework that considers both water supply and flood control is needed. Multipurpose reservoirs are widely used to manage water resources efficiently; however, water-related problems occur with reservoirs in Southeast Asian watersheds because of inadequate operation rule curves. We developed a method for constructing optimal operation rule curves for Dau Tieng Reservoir, which is one of the largest multipurpose reservoirs in Vietnam. The reservoir is used for flood control, domestic water supply, industrial uses, environmental flows, and agricultural uses, in the order of priority. The operation rule curves of the Dau Tieng Reservoir comprise five reference water levels: the retarding water level, upper water level, lower water level, critical water level, and dead water level. Water release from the reservoir is determined based on the relationship between the reservoir level and the rule curves. In this study, the rule curves were newly determined using the shuffled complex evolution method of the University of Arizona (SCE-UA method). The objective function for optimization was defined by focusing on the improvement in insufficient supply for agricultural uses and environmental flow downstream of the Dau Tieng Reservoir. Inadequate solutions were prevented by introducing penalty functions into the objective function. Experimental results indicate that the proposed optimization method efficiently searches for optimal rule curves.
Van Coc Lake located in the Dan Phuong and Phuc Tho, Hanoi, Vietnam, has an area of 30.83 km2. It is usually dry and includes arable land and residential areas. However, to protect Hanoi City from ...catastrophic floods, in emergency situations, it receives the floodwater from the Red River as a regulating reservoir and drains the floodwater to the downstream Day River through the Day Weir. We performed numerical simulations to assess the movement of floodwater in this lake and evaluated the impact of floodwater on its residential areas. A two-dimensional depth-integrated hydrodynamic model was constructed to simulate inundation in this lake. The model was validated and the effects of Manning’s coefficient of roughness were examined. In case of emergency, it has to receive an inflow of 2,500 m3/s into the area. Inflow rates of 1,200 and 600 m3/s were also examined to compare the results. The simulation results indicate that this lake was entirely inundated in 13, 28, and 56 h by inflow rates of 2,500, 1,200, and 600 m3/s, respectively. Between 66% and 99% of residential areas were inundated. The areas with highest velocity were primarily focused behind the gate with a velocity of 1.5-1.8 m/s.
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