Abstract
Osaka Bay is a semi-enclosed water body suffering from water quality deterioration caused by numerous borrow pits being excavated during the era of the industrial revolution. To mitigate such conditions, backfilling is under consideration, and performance assessment of backfilling requires accurate numerical simulations of hydrodynamics. However, it is challenging to reproduce the hydrodynamics in a borrow pit due to its smaller size, and a coarse-resolution simulation is not enough to accurately capture bathymetry as well as the flow characteristics on a spatial scale. In this study, a non-hydrostatic 3-D hydrodynamic model is successfully used to accurately reproduce the hydrodynamics on both coarse and high-resolution mesh configurations. The advantage of this modeling framework is the inclusion of the building cube method that can locally modify the uniform structural grid to a part of the full simulation domain. This capability made this model quite handy and less time-consuming when it comes to high-resolution simulations focusing on small-scale barrow pits.
You have full access to this open access chapter, Download conference paper PDF
Keywords
1 Introduction
In recent years, the urbanization and development of new infrastructure increased the demand for sand by threefold, and as per United Nations, the global annual demand for sand is 50 billion tons [1]. Sand is needed for land reclamation and beach nourishment in coastal regions for the expansion of land and replenishment of eroding coastlines and often acquired from borrow pits dredged in the shelf and offshore regions [2]. Dredging in the offshore region often leaves borrow pits that are substantially different from the pre-dredging and neighboring environment [3]. Dredging of borrow pit can contribute to physical changes by altering the density stratification of water column within the borrow pits and restricting the vertical mixing. In Japan, extensive dredging projects were executed particularly for reclaimed lands as Japan has one of the longest coastline, ranked 6th in the world and major economic centers are located along the coastal areas. There was a paramount development contribution to the economy by Japanese ports and harbors, leading to sand requirements for construction and landfilling projects [4]. There are many dredged depressions in Osaka Bay, which is a subject study area in this research. From the 1960s to 1970s, Osaka Bay also had several reclamation projects for industrial purposes and sand was mined from coastal regions which left several dredged depressions with stagnant water leading to a higher occurrence of blue tides [5]. An experiment was conducted in Osaka Bay to assess the sediment quality in borrow pits and it was found that the sediments in borrow pits are highly contaminated and the oxygen consumption in borrow pits was 2–10 times that of outside neighboring sediments [6]. It is somewhat challenging to reproduce the hydrodynamic conditions in borrow pits due to the limitation of borrow pit size. Hence, a state-of-the-art modeling framework is required that has the capability of conducting high-resolution simulations. In this study, a non-hydrostatic 3-D hydrodynamic model is used to accurately reproduce the hydrodynamics on coarse and high-resolution mesh configurations. This modeling framework can locally modify the uniform Cartesian mesh system to a certain area of interest.
2 Methodology
2.1 Study Area
Osaka Bay as shown in Fig. 1 is located in the western part of Japan almost in the middle of the main island. It is the eastern part of the mighty Seto Inland Sea. The Seto Inland Sea is the biggest semi-enclosed coastal sea with over 700 small islands. It is stretched over a length of 500 km with an average water depth of 30 m [7]. Osaka Bay has an oval shape and it runs over 60 km with a 30 km width and an average water depth of 28 m. A strait called Akashi Strait which is situated between the Japanese islands of Honshu and Awaji has a width of 4 km connects Osaka Bay at the western end with the neighboring areas of Seto Inland Sea, while on the southern side, the other Kitan Strait with a width of 10 km connects it to the Kii Channel which is further connected to the Pacific Ocean.
2.2 Study Area
A 3-D non-hydrostatic model called “EcoPARI” [8] was employed to simulate the coarse and high-resolution hydrodynamic conditions of Osaka Bay. In the past, this model was utilized to successfully simulate the hydrodynamics and ecosystem variables for different study sites in Japan [9,10,11]. The hydrodynamic model is primarily comprised of a basic continuity equation, momentum equations, sea state equation, and scalar transport equations. Turbulent kinematic viscosity and eddy diffusivity in the horizontal direction (XY Plain) employed a large eddy simulation (LES) model, while in the vertical direction (Z, Plain) an upgraded turbulent diffusion approach model was used [12].
EcoPARI meshes the geometry through a uniform Cartesian grid system. This mesh system has a limitation in the case of high-resolution simulations as it will become computationally too expensive. To counter this issue, two-dimensional building cube method (hereafter BCM) was introduced in the EcoPARI, which can increase grid resolution level by level by subdividing into child cubes via insertion of root cubes using a quadtree structure. This BCM was originally proposed by Nakahashi, and since then, it has been successfully used in several computational fluid dynamics applications [13]. A typical example is shown in Fig. 2; an adaptive mesh refinement can be seen around a borrow pit through BCM. In a quadtree structure, the parent or root cube contains the average data of four child cubes, and deeper traversing in the tree will yield more detailed and precise results. Level 1 has further refined mesh size as compared to level 0, and similarly, all the nodes in level 1 have a further 4 nodes as shown by level 2. Hence, by increasing the layers of the quadtree structure, the levels can be increased so does the mesh size. BCM applies the governing equations that describe the physical phenomena being simulated to each cell within the mesh. These equations pertain to fluid dynamics, heat transfer, or other physical processes. In case of EcoPARI, it employs finite difference method to discretize the governing equations within each cell. These methods approximate derivatives and integrals in the equations. The numerical solutions are computed iteratively over the entire mesh. The solution process is iterative, where the values are repeatedly updated until the solution converges to a stable state or meets a specific convergence criterion.
2.3 Numerical Experiments
The first numerical experiment comprised of low-resolution hydrodynamic simulation covering the entire model domain while in the second numerical experiment, BCM was employed in the borrow pit region to reproduce the detailed high-resolution hydrodynamics. The objectives are devised by keeping the size of the borrow pit under consideration as 100 m mesh was able to trace the shape and bathymetry while a low-resolution (800 m) bathymetric map was unable to replicate the actual geometric and bathymetric conditions. As shown in Fig. 3, the shape and bathymetry of the borrow pit are also very different in both coarse and high-resolution maps. In the case of 800 m grid configuration, the borrow pit is only represented by a few grids and it looks rectangular in shape while in the case of 100 m mesh configuration, it is represented by several grids and it signifies the actual field conditions.
3 Results and Discussions
The coarse-resolution simulation model was evaluated by comparing its results with observed data from monitoring stations, and regression results are shown in the following table. The "B-3" station near a major river mouth showed anticipation of surface salinity but had higher fluctuations and the highest RMSE. Surface temperature was well captured with high R2 and low RMSE. Bottom salinity was overestimated at “B-3” during summer months. The “A-2” station showed similar performance, but with improved surface salinity. The "A-6" station had the smallest RMSE for bottom salinity and temperature. "A-10" had less surface salinity fluctuation. The “A-11” station near the bay mouth showed good reproducibility of water temperature and salinity. Overall, the model's performance varied among stations, with varying levels of agreement with observed data (Table 1).
Figure 4 represents the time series comparison of both simulations with observation and subplot “a” is showing the comparison of the “B-5” station while subplot “b” is showing the comparison of the “B-P” station. The major discrepancy was found in high-resolution simulation in the surface layer, and sometimes model was unable to reproduce the surface salinity and temperature, especially in August. During this period, the surface salinity was overestimated while the surface temperature was underestimated. As reflected by the observed data, the warm fresh water on the surface might be coming from the major rivers located in the proximity of the borrow pit, and in high-resolution simulation, this warm freshwater was pushed away from the borrow pit. Contrary to high-resolution simulation, the coarse-resolution simulation was able to capture this trend. However, the high-resolution simulation results were promising in the lower layer; especially in the summer season, the bottom water temperature and salinity were well reproduced as compared to coarse-resolution simulation.
To see the spatial comparison, a cross-sectional distribution of hydrodynamic parameters was also plotted for a particular date as shown in Fig. 5; it was the time when bottom water temperature and salinity within the borrow pit showed larger discrepancy between coarse-resolution and BCM simulation. This discrepant period is shown in Fig. 4(b). It was evident from the results that in case of coarse-resolution simulation a thick warm freshwater layer existed in the surface while in case of BCM it was rather thin. Furthermore, for the bottom layer, coarse resolution showed existence of denser water as compared to BCM simulation. During this particular time step, the BCM results were in good agreement with observation as compared to coarse-resolution simulation. Apart from that, the flow magnitude was also different between both simulations and a detailed flow field was obtained within the borrow pit.
Instantaneous cross-sectional comparison of borrow pit for 1st of August. The upper 1st and 2nd subplots are showing the salinity and temperature for 800 m simulation while lower two subplots are showing the same parameters for BCM simulation. Furthermore, quivers are depicting the flow magnitude and direction
Despite facing some discrepancies in high-resolution simulation, it is still indispensable to use the 100 m bathymetric map to trace the actual borrow pit shape and bathymetry. BCM resolved this issue by multilevel simulation with overall coarse-resolution simulation over a major part of the model domain while the high-resolution simulation only in the borrow pit region. To compare the calculation cost, both coarse and high-resolution (BCM) simulations were conducted on the same supercomputer (Oakbridge-cx) having a CPU clock frequency of 2.70 GHz. In case of low resolution simulation, the model has the freedom to choose the computational resources (number of cores) for parallel computations depending upon the distribution of grids in both “X” and “Y” directions, while in the case of BCM simulation the grid distribution system is autonomous and the model automatically selects the number of cores from given computational resources. Hence, a fixed 130 cores were utilized for coarse-resolution simulation to simulate one full year, i.e., 2015, while in the case of BCM 112 cores were automatically allocated to execute the same simulation period. In the case of coarse resolution, the simulation was completed in 4.35 h while the BCM simulation was completed in 11.00 h. The BCM simulation execution time was 2.50 times as compared to coarse-resolution simulation, but still, it was quite fast. Furthermore, practically, it will be computationally too expensive to simulate the entire model domain with just a 100 m mesh configuration, which has a 64 times greater number of grids as compared to an 800 m bathymetric map. The BCM simulation reasonably worked twofold by reducing the computational cost and increasing the model resolution.
4 Conclusion
In this study, a 3-D non-hydrostatic hydrodynamic model with the inclusion of BCM was successfully employed to accurately reproduce the spatial hydrodynamics on a high-resolution mesh configuration. The purpose of BCM was to locally modify the Cartesian mesh configuration from coarse resolution to high resolution to get the detailed hydrodynamic conditions, especially in the bottom layer of a borrow pit. The high-resolution simulation results were promising in the bottom layer; especially in the summer season, the bottom water temperature and salinity were well reproduced as compared to coarse-resolution simulation.
References
UNEP (2019) Sand and sustainability: finding new solutions for environmental governance of global sand resources
Crowe SE, Bergquist DC, Sanger DM, Van Dolah RF (2016) Physical and biological alterations following dredging in two beach nourishment borrow areas in South Carolina’s coastal zone. J Coast Res 32(4):875–889
Palmer TA, Montagna PA, Nairn RB (2008) The effects of a dredge excavation pit on benthic macrofauna in offshore Louisiana. Environ Manage 41(4):573–583
Horii R (1996) Historical background and recent trends in dredging and reclamation technologies in Japan. Mar Georesources Geotechnol 14(1):19–36
Nakatsuji K, Irie M, Shibata T (2007) Field surveys of currents and water quality around dredged hollow places in port of Han-nan, Osaka Bay. Proc Coast Eng 54:1096–1100
Irie M, Nakatsuji K, Teranaka K (2007) Study of sediment quality at dredged hollow places in port of Han-nan, Osaka Bay. Proc Coast Eng 54:1091–1095
Nagai S, Yoshida G, Tarutani K (2011) Change in species composition and distribution of algae in the coastal waters of Western Japan. In: Global warming impacts-case studies on the economy, human health, and on urban and natural environments, no October
Tanaka Y, Suzuki K (2010) Development of non-hydrostatic numerical model for stratified flow and upwelling in estuary and coastal areas. Report Port Airport Res Inst 49(1):3–25 (In Japanese)
Matsuzaki Y, Inoue T (2022) Perturbation of boundary conditions to create appropriate ensembles for regional data assimilation in coastal estuary modeling. J Geophys Res Ocean 127(4)
Hafeez MA, Inoue T (2021) Determination of flow characteristics of Ohashi River through 3-D hydrodynamic model under simplified and detailed bathymetric conditions. Water 13(3076):1–21
Hafeez MA et al (2021) Integration of Weather Research and Forecasting (WRF) model with regional coastal ecosystem model to simulate the hypoxic conditions. Sci Total Environ 771:1–40
Nakamura Y, Hayakawa N (1991) Modelling of thermal stratification in lakes and coastal seas. In: Hydrol national manmade lakes. Proceedings of symposium Vienna, no 206, pp 227–236
Nakahashi K, Kim L (2012) Building-cube method for large-scale, high resolution flow computations. In: 42nd AIAA aerospace sciences meeting and exhibit
Author information
Authors and Affiliations
Corresponding author
Editor information
Editors and Affiliations
Rights and permissions
Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.
The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
Copyright information
© 2024 The Author(s)
About this paper
Cite this paper
Hafeez, M.A., Inoue, T., Matsumoto, H., Sato, T., Matsuzaki, Y. (2024). Application of Building Cube Method to Reproduce High-Resolution Hydrodynamics of a Dredged Borrow Pit in Osaka Bay, Japan. In: Tajima, Y., Aoki, Si., Sato, S. (eds) Proceedings of the 11th International Conference on Asian and Pacific Coasts. APAC 2023. Lecture Notes in Civil Engineering, vol 394. Springer, Singapore. https://doi.org/10.1007/978-981-99-7409-2_26
Download citation
DOI: https://doi.org/10.1007/978-981-99-7409-2_26
Published:
Publisher Name: Springer, Singapore
Print ISBN: 978-981-99-7408-5
Online ISBN: 978-981-99-7409-2
eBook Packages: Earth and Environmental ScienceEarth and Environmental Science (R0)