Drainage System, Rainwater Flooding and Underground Inundation in Urban Area

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1 Drainage System, Rainwater Flooding and Underground Inundation in Urban Area T. Ishigaki 1 *, T. Ozaki 1, T. Inoue 1, H. Shimada 1 and K. Toda 2 1 Kansai University, , Yamate-cho, Suita, Osaka, , Japan 2 Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, , Japan * ishigaki@kansai-u.ac.jp ABSTRACT Torrential rains have been observed frequently in recent years in Japan and these rainfalls caused inundations in urban and rural areas. Intensity of such rainfalls was sometimes excess over the design rainfall of drainage systems and rainwater was running over the road. In these floods, some portion of rainwater intruded into underground spaces and the users faced the danger of underground flooding. In this paper, rainwater flooding in densely urbanized area was investigated by 1D-2D urban drainage model (InfoWorks CS), and underground flooding in a mega-underground mall was investigated by using 2D shallow flow model with structured mesh. By using the calculated results, evacuation from underground space was also studied with the criteria of safe evacuation obtained by the previous evacuation tests. It is found that the results are useful to make an evacuation plan from inundated underground space. KEYWORDS Evacuation; InfoWorks; rainwater flooding; underground inundation; urban drainage INTRODUCTION Most of urbanized areas in Japan are susceptible to floods because they locate in lowlands which are below flood water level of rivers. Fifty percent of Japanese population live in ten percent of the land where are on alluvial plains, and seventy-five percent of the national property are in these areas. These areas are prone to floods caused by rainwater, river-water and sea-water. River-water flooding commences with over topping and bank breach of rivers, and sea-water flooding comes from storm surge or tsunami. On the other hand, rainwater flooding is caused by heavy rainfall which exceeds over the design rainfall. Rainwater flooding is discussed in this paper. Frequency of rainfalls over the capacity of drainage system is increasing and underground inundations occurred in some Japanese cities of Tokyo, Fukuoka, Nagoya etc., in recent years. It is an urgent matter that managements of underground spaces plan for safe evacuation of users. In order to make the plan, they should know when and from which entrances the rainwater will flow into there. In this paper, rainwater flooding in densely urbanized area with a mega-underground mall is simulated to investigate the evacuation plan from underground space considering the criteria of safe evacuation obtained in our previous works. The evacuation from underground space has been investigated by using a real size model of staircase (Ishigaki et al., 2005, 2006). On the route of evacuation, there are two points those should be considered. The first point is whether people can walk through flooded corridors. Ishigaki et al. 1

2 And the second point is whether they can go up stairs. These points are concerned with water depth, h, and flow velocity, u. From the results of our previous works, it is found that the specific force per unit width, M 0 =u 2 h/g+h 2 /2=0.125 m 3 /m, is the male s criterion of safe evacuation through corridors and stairs (Ishigaki et al., 2008a). Where g is the gravity acceleration, g=9.8m/s 2. Acting force on each person is different under the same condition of flow, however, a valid criterion in deciding the safety of evacuation from the flow condition is necessary. This is the reason why M 0 is used in this paper. The relation between M 0 values and evacuation speed was discussed on the basis of experimental data obtained by evacuation tests and the assessment of safety on evacuating routes during underground flooding has been carried (Ishigaki et al., 2008b, Onishi et al., 2009). The evacuation of aged persons have been discussed by using experimental data and numerical simulations (Ishigaki et al., 2010, Asai et al., 2010), and the criteria of safe evacuation for elderly female, M 0 =0.08 m 3 /m, is used in this paper. Table 1 shows the criteria of safe evacuation for male and female by using the specific force per unit width, M 0. And the criteria are shown by water depth and velocity in Figure 1. Table 1. Criteria of safe evacuation presented by the specific force per unit width, M 0 (m 3 /m). Limit of safe evacuation Difficult without any help Male Elderly male Female Elderly female Figure 1. Criteria of safe evacuation described by water depth and velocity. DRAINAGE SYSTEM AND RAINWATER FLOODING Figure 2 is the study area of 12.15km 2. This also shows the ground level of the area and the location of a mega-underground mall. As the area is surrounded by two rivers on the north, east and south sides, this area is vulnerable to river-water floods. However, rainwater is drained only by the drainage system as shown in Figure 3. Drainage water is pumped up to the surrounding rivers by four pumps. Therefore, this area is also vulnerable to rainwater floods. In this paper, rainwater flooding and inflow discharge into the underground mall were calculated by using 1D-2D urban drainage model (InfoWorks CS). Flow in drainage pipes was calculated by 1D model of Preissman Slot model considering pipes of larger than 0.2 m diameter, and flow on roads was treated by using 2D shallow flow model with unstructured 2 Drainage system, rainwater flooding and underground inundation in urban area

3 mesh. Inundation into houses was not considered. The entrances of underground mall were treated as rectangular weirs and inflow discharge was calculated with discharge formula. Figure 2. Study area in Osaka, Japan and the elevation map. (Total area: km 2 ) Figure 3. Drainage system (D>200mm) and roads network (right figure) in the study area. Figure 4. Heavy rainfall observed on 29, August, 2008 in Okazaki of middle Japan. Figure 4 shows the heavy rainfall observed on 29th of August, 2008 in Okazaki of middle Japan. Total rainfall was mm in only 3 hours and the maximum intensity was mm/hr. This rain is the seventh record in Japan. The hyetograph shows the intensity of precipitation for each 10 minutes to use in calculation. As the design rainfall for the study area is 60 mm/hr, severe rainwater flood happens in this case. Figure 5 is the distributions of Ishigaki et al. 3

4 manhole from which the drained water overflows over road. It is found that there is a time-lag between the rainfall and flood peaks and the flooding near the mega-underground mall continues for many hours. The peak of flood occurs after rain and the maximum inundation depth goes up to over 1 m as shown in Figure 6. This shows the distribution of calculated water depth at T=3.00 hr. It indicates that evacuation on the road becomes difficult and dangerous. Figure 5. Distribution of overflowed manholes. Figure 6. Distribution of inundation depth on roads at T=3.0 hours from the beginning of rainfall. 4 Drainage system, rainwater flooding and underground inundation in urban area

5 UNDERGROUND INUNDATION The mega-underground mall is the largest underground space in Japan including four railway stations and three subway stations. Over 0.6 million people use the space per day for commuting and shopping. This space was opened in 1963, and railway stations and shopping malls has been expanded continuously. Consequently, the floor of each areas has different elevation as shown in Figure 7. There are so many entrances directly or indirectly connected to the space, however, only the former entrances are considered here because the latter entrances connect to the space through the basement of buildings. 129 entrances in Figure 8 were considered in the calculation of underground inundation. Figure 7. Surveyed floor level of passages in the mega-underground mall. Figure considered entrances of underground mall for calculation of inundation. In the case of Okazaki rainfall mentioned before, some portion of rainwater flood intrudes into the underground through 45 entrances and the total amount of inflow discharge is 275,727 m 3. Figure 9 is the hydrographs for the subtotal inflow discharge of 6 blocks and Ishigaki et al. 5

6 underground parking lots. The maximum peak of discharge occurs in the area A and the magnitude goes up to 20 m 3 /s. The important consideration is that the time-lag between discharge and rainfall peaks is about one hour. This means that people have a chance of staying in the underground after rain and coming up against the danger of underground inundation. And it indicates that managements of underground space have to consider this thing to make an evacuation plan. Figure 9. Hydrographs of inflow discharge and the location of entrance blocks. Figure 10. Distribution of inundation depth when the total volume is 50,000 m 3 (left) and when it is 150,000 m 3 (right). Underground inundation was simulated by using 2D shallow flow model as shown in Figure 8. Square mesh size was 2 m and the number of them was 18,299. This model included only passages, and did not include shops, subway stations and rails. The time subtraction was 0.01 second and the total calculation time was 27,000 seconds. Manning s n was set at Figure 10 shows the two distributions of water depth in the mega-underground mall when the amount of inflow volume is 50,000 m 3 and 150,000m 3. The inundation commences around 4,000 seconds from the beginning of rainfall. Over 60 percent of the whole area is inundated when the inflow volume is 50,000 m 3 and the percent goes up to over 95 percent when the volume is 150,000 m 3. Water depth rises over 0.9 m in many areas and evacuation becomes difficult and unusually dangerous as shown in Figure 11. Figure 11 shows when elderly 6 Drainage system, rainwater flooding and underground inundation in urban area

7 females in the each area could not evacuate safely. This assessment is based on the criteria of safe evacuation for elderly female, M 0 =0.08 m 3 /m, mentioned in the introduction. The time of legend means a lapse time in seconds from the beginning of inundation. The result is very important for drawing up an evacuation plan from underground spaces. Figure 11. Difficulty of evacuation for elderly female obtained on the basis of criteria of safe evacuation by using the specific force per unit width, M 0 =0.08 m 3 /m. CONCLUSIONS Rainwater flooding and the safety of evacuation in fully urbanized area with a megaunderground mall were discussed in this paper. Main conclusive remarks are as follows; 1) inundation depth on ground level relates not to ground elevation but mainly to drainage system, 2) the maximum inundation depth on roads appears after the rain, 3) massive rainwater intrudes into the mega-underground mall and spreads over the mall, 4) the criteria of safe evacuation is useful to make an evacuation plan from underground spaces, 5) drainage system should be considered to simulate rainwater flooding on roads and underground spaces. Results of the calculation mentioned here is not enough to prevent the flood disaster in urban area. It is necessary to investigate the rainwater flooding in this area for another case of rainfalls on the next step of this study. In the calculation of underground flooding, only passages were considered in this paper. Therefore, shops and subways should be considered in future investigation to simulate more details of underground inundation. It goes out saying that effects of counter measures such a stop board setting at entrances will be investigated in our future study by using the method discussed here. ACKNOWLEDGEMENT The authors really appreciate Ujigawa Open Laboratory, Disaster Prevention Research Institute, Kyoto University to provide the experimental facilities. Ishigaki et al. 7

8 REFERENCES Asai Y., Ishigaki T., Baba Y. and Toda K. (2010). Safety analysis of evacuation routes considering elderly persons during underground flooding, Journal of Hydroscience and Hydraulic Engineering, JSCE, Vol.28, No.2, pp Ishigaki T., Baba Y., Toda K. and Inoue K. (2005). Experimental study on evacuation from underground space in urban flood, Proc. of 31st IAHR Congress, CD-ROM, Seoul. Ishigaki T., Toda K., Baba Y., Nakagawa H. and Shimada H. (2006). Difficulty of evacuation from underground space in urban flood, Proc. of 7th International Conference on Hydroinformatics, HIC 2006, France, Vol. I, pp Ishigaki T., Onishi Y., Asai Y., Toda K. and Shimada H. (2008a). Evacuation criteria during urban flooding in underground space, Proc. of 11th ICUD, CD-ROM, Scotland, UK. Ishigaki T., Kawanaka R., Onishi Y., Shimada H., Toda K. and Baba Y. (2008b). Assessment of safety on evacuation route during underground flooding, Proc. of 16th APD-IAHR, Nanjing, China, pp Ishigaki, T., Asai, Y., Nakahata, Y., Shimada, H., Baba, Y. and Toda, K. (2010). Evacuation of aged persons from inundated underground space, Water Science & Technology, IWA Publishing, 62.8, pp Onishi Y., Ishigaki, T., Baba, Y. and Toda, K. (2009). Criterion and its application for safety evacuation during underground flooding, Journal of Hydraulics Engineering, Vol.27, No.1, pp Drainage system, rainwater flooding and underground inundation in urban area