## 1 Introduction  
Hazardous and noxious substances (HNSs) have harmful effects on human health, biological resources, and the marine environment when they are spilled into the marine environment [1]. The volume of maritime trade of HNSs is increasing every year, thus increasing the possibility of HNS spill at sea. HNSs are distinguished by one or more of the following five characteristics according to their inherent properties: Flammable, explosive, toxic, corrosive, and reactive. Owing to these hazardous characteristics, a response strategy to, and rapid removal of, a HNS spill must be determined. However, it is difficult to cope with a HNS spill quickly and accurately because of its complicated characteristics [2; 3; 4; 5; 6; 7]. Furthermore, there is a lack of information on HNS propagation near ships over time under various accident scenarios. Thus, a more elaborate prediction method to measure HNS propagation characteristics is required that accounts for various effects of the HNS properties and environmental conditions. An experiment may be the best way to obtain the detailed propagation characteristics when an HNS spills in the ocean. [PERSON] et al. [8] conducted experiments under the limited condition, and made floating cells to monitor the residual concentration over time. In their experiment, they released a small amount of styrene, then captured the HNS behavior for several hours and a few days after. This result was taken under very limited conditions but it can be used for creating an emergency response guide for spill accidents. Despite this, due to the hazardous chemical properties, the study did not consider realistic scenarios, as releasing a HNS into the ocean may cause danger to the aquatic ecosystem as well as human safety. A computational study could be a promising alternative to the experiment for providing useful information on the propagation behavior of HNSs in more detail. [PERSON] et al. [9] numerically studied the management of oil pollution accidents in the Tuscany Archipelago region using a set of nested models with downscaled conditions. As a result, the model showed good agreement with the trajectories, captured by satellite. [PERSON] et al. [10] established three-dimensional numerical models by combining the hydrodynamic and oil spill models. They evaluated the predictions by comparison with the observed results and used the model to build a risk assessment map for an oil spill accident. [PERSON] et al. [11] developed the semi-empirical model, called OILTRANS, with theoretical equations and an empirical database to predict the spatial propagation. For oil spills, this model has been known to be accurate when compared with experimental observation. To the best of our knowledge, there are few computational fluid dynamics (CFD) studies for HNS propagation, especially regarding near-field characteristics, from the earlier published literature. Thus, more detailed information on the spatial change of HNS concentration over time is required to build a response strategy to an HNS spill, particularly in the early stage of the accident. As emphasized above, the experimental approach found it difficult to obtain useful and sufficient data related to HNS propagation, whereas the CFD approach could be useful for predicting the transient evolution of HNS concentration around a ship. If an accident occurs, all possible responses, e.g., setting up the risk zone, separation, evacuation, and chemical treatment, would be decided as quickly as possible. However, considerable additional time is required after the accident to provide HNS distributions over time. To overcome this limitation, it is necessary to develop a metamodel that predicts all information on the HNS propagation more quickly without CFD simulation. Furthermore, this model should be based on the CFD results for all possible scenarios, taken from literature and accident reports. Meanwhile, most simulations have been conducted for a space of a few hundred kilometers.