An Evaluation of Flood Risk Reduction Utilizing Green Infrastructures in the Greater Tokyo Area

The Sustainable City IX, Vol. 2 1267

An evaluation of flood risk reduction utilizing green infrastructures in the Greater Tokyo Area

K. Tanji Faculty of Environment and Information Studies, Keio University, Japan

Abstract

The purpose of this study is to evaluate the effects of runoff control by green infrastructures in densely urbanized residential areas in the Greater Tokyo Area (GTA) in order to reduce the risk of urban flooding. In this research, we make comparisons of the effects of runoff control between green infrastructure and the measures projected by the Tokyo metropolitan government, which are composed of a mixture of porous pavement, seepage pit, and trenches for infiltration. This paper reveals that a green infrastructure plan gives feasible measures considering land use conditions, but is inferior to that of a seepage pit and soakage trench plan in terms of the total essential site area and the required expense. However, theoretically, the expense of the green infrastructure is improved by creating incentives for residents. Keywords: flood in urban area, green infrastructure, densely built-up area.

1 Introduction

The Greater Tokyo Area (GTA) is one of the most urbanized areas in the world. 32 million people live in this area and the population density is 14,669 per square kilometer. In terms of geographic conditions, the GTA is situated in a river delta area, downstream of the River Tone and the River Ara. 1.76 million people live in areas below sea level. Therefore there are growing concerns about increasing risks of urban flooding and storm surge caused by extreme weather. The main reason why so many people are exposed to the risk is rapid urbanization in the past half century. Because of population growth, which has increased 3.2 times compared to 1945, the urban area has expanded 4 times as large as it was in 1945. As a result,

WIT Transactions on Ecology and The Environment, Vol 191, © 2014 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) doi:10.2495/SC141062 1268 The Sustainable City IX, Vol. 2 green space per capita in the GTA became narrow (5.0 square meters) compared to other major cities (New York: 23.2 square meters, Berlin: 27.0 square meters, Paris: 11.7 square meters). In other words the GTA has lost its infiltration and storage function against heavy rains in the water circulation. The trend of precipitation has not changed for the past 100 years, which is 1610 mm per year in average, however precipitation patterns have gradually changed according to observed heavy rains defined as more than 50 mm per hour. 20 times as heavy rains were observed in 1991, on the other hand 66 times as heavy rains were observed in 2005 based on observation station installed in 117 sites around the GTA. Therefore it is essential measures are taken for the GTA to reduce the risk of urban flooding. The Tokyo Metropolitan Government (TMG) has developed action plans to mitigate impacts concentrated heavy rains for each major watershed. Objects of them are 1st to realize prevention flooding in whole GTA against 60 mm per hour of precipitation by 2037, 2nd to mitigate damages caused by inundation above and below a floor level as much as possible against 75 mm per hour of precipitation, and 3rd to implement concrete action plan to prevent any damage against 55 mm per hour of precipitation by 2017. Dominant responses to the action plan relate to the allocations made between the sewerage system and rain water storage-infiltration. Sewerage systems have to be enhanced, so that their draining capacity is 50 mm per hour of precipitation. Rain water storage-infiltration capacity is supposed to be charge of rest 5 mm per hour precipitation by 2017. Therefore the purpose of this study is to make a comparison of performance between current measures which is composed of rain water storage-infiltration and green infrastructure in terms of capacity of drainage, cost, and local adaptability.

2 Materials and method

2.1 Study area

Kanda river runs through the central part of GTA from west to east, its stream length is 24 km, the size of watershed is 105 km2 (Fig. 1, Fig. 2). It flows down urban area especially the housing district from the source of the river where Inogashira Park is in Mitaka city and finally meets the Sumida River which discharges itself into Tokyo bay [1]. Table 1 shows detailed land use information around the Kanda River. In past observations, precipitations exceeding 50 mm per hour have concentrated west of watershed such as the Shakujii River, the Shibuya River, the Nokawa River, and the Kanda River, because heavy rain tends to be concentrated to the west of Tokyo. In particular, the Kanda River’s 800 housings have suffered damage of flooding, and flood damage has reached 30 million US$ annually for 15 years from 1993 to 2008 [2]. Actually when heavy rain had fallen exceeding 100 mm/h in 2005, 1,500 housings were damaged by inundation above a floor level and 1,000 housings were damaged by inundation below a floor level in Kanda sub-watershed. Thereby we set kanga watershed as a study site to discuss enhancing the capacity to storage/infiltration and green infrastructure.

Figure 1: Tokyo Metropolitan area and the Kanda River.

Figure 2: Land use of the Kanda River.

Table 1: Detailed land use information around the Kanda River.

classification (ha) classification (ha) Forest area 114 Office and commercial 2,001 Crop field 169 Road 2,177 Bare land 40,781 Park 716 Industrial site. 98 Other public facilities 1,274 Ordinary low level house 3,924 River and lake 120 Densely low level house 802 Office and commercial 2,001 Mid to high rise housing 496 Road 2,177 Total 52,672

2.2 Model and date

Many methods have been suggested to make an analysis of the risk of flood around the Kanda River [1, 3]. In this study, hydrological analysis is carried out following procedure based on the analysis method conducted by Inoue [4]: 1) Date generation of elevation including height of developed land as well as elevation;

2) the analysis of flow direction and stream line of drainage; and 3) the analysis of flow accumulation. We utilized hydrological analysis tools added in ArcGIS and basic data for this analysis are made from investigation of current land use in GTA. In this analysis, flow paths of rain water and the volume of total out flow are estimated. To make elevation for hydrological analysis including height of developed land, we made overlay the some maps, such as digital map 5 m grid elevation, digital districts and buildings maps published as fundamental geospatial data on a scale of 1:2500. We added 0.5 m to consider height of developed land, and all height of housings are set 10 m for making analysis simple. Equation to estimate water drainage of each sell against precipitation is shown (1) which is considered water discharge coefficient by land cover.

n kk Friii R S * k1 where i is Number of grid, k is types of land cover, Fri is water discharge in grid, R is Precipitation, Ski is area of land cover grid i, type k, δi is Water discharge coefficient (WDC) grid I type k. Types of land covers are classified by utilization land use map recoding present situation published by local government in 2009, and scanned it to get figure of each land use and estimate area. To verify the accuracy of them, we made comparison between the product and aerial photographs taken within three years. Table 2 shows Land cover classification of the watershed and WDC. With this procedure, we can set detailed figure of each land cover data and WDCs for whole of the watershed. WDCs are set based on guideline of drainage road construction.

2.3 Scenario setting of installing measures

Three types of scenario are set to evaluate capacity of drainage, cost, and local adaptability. Scenario A is “permeable pavement of minor streets” which is adopted by local government, Scenario B is “conventional water storage/infiltration technology”, and Scenario C is “Green infrastructure”.

Scenario A “Permeable pavement of minor streets” Permeable pavement is easy to install because roads are widespread in study area and it can catch precipitation and infiltrate them to underground. In order to achieve a goal, we set diffusion rate of permeable pavement is 46% of total area of all road except for main street.

Scenario B: “Conventional rainwater infiltration trench/ rain water seepage pits” This scenario is water storage and infiltration measures adopted by local government. Concretely rain water seepage pits (lengthwise 3.0 m × breadth wide 3.0 m) are supposed to be installed 2,231 buildings and 6% of parking place in the catchment area. The difference of scenario A is to carrying out the measure in private land in contrast with implementing Permeable pavement in public land.

Scenario C: “Green infrastructure”

Table 2: Water discharge coefficient by land cover.

Land cover classification WDC governmental facility 0.2 Land for public use Educational and cultural facilities 0.2 Public welfare medical facilities 0.8 Office buildings 0.8 Commercial facilities 0.8 Land for private use Mixed use of commercial and housing 0.7 Hotel and amusement 0.8 Sports facility 0.8 Detached housing 0.7 Housing Housing complex 0.7 Factory 0.85 Industry Mixed use of factory and housing 0.7 Store house 0.8 Outdoor parking space 0.85 Park, athletic ground 0.2 Unused ground 0.2 Railway 0.2 Graveyard 0.2 Forest or grove of trees surrounding a residence 0.4 Permeable pavement 0.7 Road 0.85 Shopping arcade 0.9

This scenario is to utilize green infrastructure in order to control drainage. Additional green infrastructures shown in Table 2 are needed to install around the watershed to accomplish the goal. Establishing green infrastructures are required but there are some places where they are not supposed to be installed, in minor streets whose width is less than 4 m. There are some facilities that have capacity to install green infrastructures such as main streets, square parks, primary schools in the watershed, these facilities are chosen for appropriate sites of green infrastructures. Table 3 shows detailed information about each performance of infiltration, Table 4 shows a composition of scenarios and infiltration measures.

Table 3: Performance of each infiltration measure.

Performance Measures Unit of infiltration ① Greening roof, thickness of soil 12 cm 0.020 ② Greening roof with perlite 30 cm 0.060 ③ Vegetated street swale 0.532 ④ Infiltration planter 0.232 ⑤ Turf 0.050 ⑥ Planting 0.050 m3/m2 ⑦ Bare land 0.002 ⑧ Developed land 0.002 ⑨ Permeable pavement (pathway) 0.020 ⑩ Permeable pavement (road) 0.050 ⑪ Permeable interlocking concrete block 0.020 ⑫ Infiltrate side ditch 0.100 small：0.3 m × 0.3 m 0.250 ⑬ Rain water seepage pit medium：0.6 m × 0.6 m 0.618 m3/unit large：1.0 m × 1.0 m 1.710 Small：0.25 m 0.247 ⑭ Rain water seepage Medium：0.40 m 0.365 m3/m hr trench Large：0.75 m 0.658

Table 4: Composition of scenarios.

Performance Measures Site and volume of installation (m3) A ⑩ 46% of narrow street 1,268.5 ⑬ 2,231 housing 557.7 B ⑩ 6% of parking space 710.6 ④ 1230 m of pathway of main street 285.0 ⑨ Rest of pathway of main street 83.0 ③ 100 m of total circuit in public park 53.0 ③ 84.5 m2 of station square 8.0 ④ 2800 m of boundary between housings 650.0 C ③ Elementary school 64.0 ③ 1,177 m2 of public park 1.0 ①, ⑤ Elementary school 8.0 ③ More than 0.85 m in public park 0.0 ① 6,208 m2 of total roof of building 124.0

3 Results

As a result of hydrologic analysis, there are three paths of drainage in this watershed. The first path is directly flowing into the main river, which is one of major branch of Kanda river Second path is running into north-east direction from south-west. Third path is running into south direction from north direction. It is found that these three paths join together at some points. According to flood information generated in the past, many flooding are reported in the joining point of water paths in this area. Additionally the rainwater flowed into the area with narrow minor streets whose width is less than 4 m in a house crowd place through rain water paths. These narrow and minor streets are not able to drain when heavy rain occurs because they are covered by impermeability materials. It is also clarified that all scenario is supposed to control additional 5 mm rain fall by accumulation and infiltration. A green infrastructure plan is feasible for measures are considered from land use conditions, but they are inferior to a seepage pit and soakage trench plan in terms of total essential area for installing them and the expense. However, the expense of the green infrastructure is more likely to be improved. Also the required wider area for green infrastructure doesn’t mean demerit because green infrastructure reproduce a natural water function by linking each unit. A green infrastructure plan has the complex effect that a private dry well and soakage trench plan do not have, and, in long-term city planning, it is desirable for the wide area compactly in close cooperation with the civil society to realize a rainwater outflow suppressant effect by the green infrastructure. In terms of required installation area, owing to performance of permeable pavement of minor streets, scenario A is needed for the widest area compared to other scenarios. However a policy target is supposed to be accomplished with only installing them on public road (except for private road) in Scenario A. Required installation area is smallest in scenario B, because main measures are rainwater infiltration trenches and rain water seepage pits. Scenario C is needed wider area than Scenario B. because feature of this watershed land use is densely built housing area, therefore infiltration planter should not be used, we used green roof planting. And both of private and public facilities are needed (Figure 3).

Million $ 8.00 7.00 Private facilities Public facilities 6.00 5.00 4.00 3.00 2.00 1.00 0.00 Scenario A Scenario B Scenario C Figure 3: Comparison of required cost in each scenario.

In terms of cost for installation, Scenario A is on the same level with Scenario B. Green infrastructure is the highest cost of the three scenarios (Figure 4).

1,000ha 30 Private facilities 25 Public facilities

0 Scenario A Scenario B Scenario C

Figure 4: Comparison of required area in each scenario.

4 Conclusion

The present result suggests three points of conclusion. Green Infrastructure can be installed on roofs, garden, parking places, however it is inferior to conventional rainwater infiltration trench/rain water seepage pits and, permeable pavement of minor streets in terms of required installation area and total cost. Except for Scenario A, both Scenario A and B are required efforts of private sectors. It is impossible to realize target of flood risk reduction and making district full of green without private sector. Scenario C (Green infrastructure) is supposed to create attractive district based on its character compared to other scenarios. It can make incentives for private sector to pay additional cost by attractive design, or to make them by themselves. In this way additional cost can be reduced compared to this research. Only conventional rainwater infiltration trench/ rain water seepage pits are approved as flood control measures, because Tokyo metropolitan government has thought that flooding measures is separate from tree planting policies. Therefore subsidies for tree planting are utilized for enhancing green space ratio per person or making townscape. Further studies are needed in order to establish green infrastructure of mega cities of Japan and East Asia where have features such as growth of population, mosaic of narrow partitioned area, densely built housing. To accomplish them, we should many experiments of green infrastructure, and collect their performance, choose and design original Green Infrastructure.

References

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