C103

Modeling Flow and Pollutant Transport of Urban Areas under Flood Disaster〇Congji HAN, Kenji KAWAIKE, Kazuki YAMANOI, Takahiro KOSHIBA

Although research on the transport of pollutants in natural rivers has been extensively conducted, there are still
few studies on the transport of pollutants in urban inundation areas. In this research, the development andimplementation of a two-dimensional, numerical pollutant transport model is delineated. Unstructured grids areemployed to facilitate grid generation and localized refinement, providing clear boundaries for different land uses
such as buildings, roads, and others. This allows for a more accurate assignment of the Manning coefficient,exchange coefficient, diffusion coefficient, and other attribute information to grids. It is then applied to model thedistribution and transport of pollutants in Uji Campus, Kyoto University when urban inundation happened. The
results from the numerical simulations illustrate that the pollutants will have an impact on residential andagricultural areas in the south and west.
1. Introduction
Urban inundation disasters caused by heavy rain,
tsunamis, and floods have become increasingly
frequent in recent years. The Global Assessment
Report 2019 released by the United Nations Office forDisaster Risk Reduction pointed out that secondary
disasters caused by the impact of flood hazards occur
frequently but are often overlooked, such as thepollutants leakage incidents that occurred during the2002 European floods, and 2005 Atlantic hurricane
season[1]. Therefore, it is necessary to study thetransport of pollutants in urban areas under flood
disasters.
This research employs unstructured grids tofacilitate grid generation in the research area, anddiscretizes the transport equation on the unstructured
grid, proposing a two-dimensional numerical model
for pollutant transport. Using the proposed model tosimulate pollutant transport on Uji Campus, Kyoto
University and its surrounding areas under flood
disaster.
2. Research Contents
This research is divided into two parts: urban
inundation simulation and pollutant transport
simulation. The urban inundation simulation utilizes
the unstructured grid model proposed by Kawaike[2],with the transport equation being discretized on thesame unstructured grid. An example of unstructured

grids is illustrated in Figure 1. Delineating boundaries
between different land uses through unstructured girds
allows for accurate assignment of attributes andinformation.

Figure 1 Example of unstructured grids(1) Urban Inundation Simulation
This research simulates the urban inundation causedby the breach of the Uji River. The study area is thehazardous material storage room at the Uji Campus,
Kyoto University, enclosed by two waterways leading
to the Uji River and the Keihan Uji Line. There are nowater flux exchanges along the boundaries, except at
the breach point. The length of the breach point is20m, and a total of 𝑄 = 4.5*106 m3 of water flows into
the study area within 5 hours. The study area andbreach point are illustrated in Figure 2.
(2) Pollutant Transport Simulation
The two-dimensional pollutant transport equation
used in this research is given by:
𝜕(𝐶ℎ) 𝜕(𝐶𝑀) 𝜕(𝐶𝑁)𝜕𝜕𝐶𝜕
𝜕𝐶++=(𝐷ℎ ) +(𝐷ℎ )
𝜕𝑡
𝜕𝑥
𝜕𝑦
𝜕𝑥
𝜕𝑥
𝜕𝑦
𝜕𝑦

research utilizes rivers’
coefficient 𝐷 = 6.43ℎ𝑢∗ .

Figure 2 Location of the study area, storage room, andbreach point

longitudinal

diffusion

3. Results and Discussions
The water depth in the storage room reaches 0.3m
approximately 23050 seconds after the breach, leading
to the release of hazardous materials. Figure 4
illustrates the distribution of hazardous materials
30000 seconds after the breach. These materials
generally move southeast with the flow, spreading
simultaneously. Residential areas to the south andwest of Uji Campus, Kyoto University, as well as
agriculture lands, are affected. The Uji Municipal
Nanbu Elementary School, designated as anemergency shelter, is also within the impact zone.
Therefore, it is essential to recognize the secondary
disaster of hazardous materials leakage during flood
disasters.

Figure 3 Discretization on unstructured grids
Where 𝐶 is the mass concentration; ℎ is the waterdepth; 𝑀 , 𝑁 are the flux of 𝑥 ,y direction; 𝐷 is thediffusion coefficient. The transport equation is
discretized on grids shown in Figure 3 and can be
expressed as:
𝐶𝑖𝑛+3 ℎ𝑖𝑛+3 − 𝐶𝑖𝑛+1 ℎ𝑖𝑛+12∆𝑡𝑚𝑖
1+ ∑{𝐶𝑙̅ 𝑛+1 𝑀𝑙𝑛+2 (∆𝑦)𝑙 − 𝐶𝑙̅ 𝑛+1 𝑁𝑙𝑛+2 (∆𝑥)𝑙 }
𝐴𝑖𝑙=1
𝑚𝑖

1𝐶𝑁𝑛+1 − 𝐶𝑃𝑛+1𝑑𝑥(∆𝑦)𝑙= ∑ ℎ𝑙𝑙𝑛+1 𝐷 {222𝐴𝑖√𝑑𝑥 + 𝑑𝑦 √𝑑𝑥 + 𝑑𝑦 2
𝑙=1−𝐶𝑁𝑛+1 − 𝐶𝑃𝑛+1𝑑𝑦√𝑑𝑥 2 + 𝑑𝑦 2 √𝑑𝑥 2 + 𝑑𝑦 2(∆𝑥)𝑙 }Based on the analysis of stored hazardous materials,
this research assumes that a total of 220 kg ofconservative substance, which can dissolve in water
immediately, will be released into the water when thewater depth in the storage room reaches 0.3m. This

Figure 4 Pollutant distribution 30000s after breach
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