Rebuild, retreat or resilience: urban flood vulnerability analysis and simulation in Taipei
Authors: Su Yu-Shou 
Published: March 2017

Purpose
This paper aims to propose the practice of urban resilience to flooding in a vulnerable Asian city, Taipei. It conducts Geographic Information System (GIS)-based simulations to assess Taipei’s vulnerability under a set of varying flooding scenarios and likelihoods. This research concludes by proposing remedies to fill the gaps these flood simulations reveal and, in doing so, promotes urban resilience in Taipei. This paper provides an example of urban resilience to flooding for other cities in Asia.

Design/methodology/approach
The paper undertakes a case study of Taipei to survey current practices and historic analyses as a tool to evaluate a chronology of policies implemented to make Taipei resilient to flooding. It also conducts flood simulation and scenario analysis through the technology of GIS by using ArcMap 10.2.2 software to gauge the vulnerabilities in Taipei. These GIS-based data sets are collected from Taiwan’s central and local governments. This paper analyzes the vulnerability of population, land value, residential properties, GDP and critical facilities, such as major subway stations, medical centers, public schools, major public buildings, electric power substations and gas/oil stations. Additionally, it analyzes the likelihood and cost and benefit of different flooding scenarios based on typhoon and rainfall data sets in 1975-2014 period. After a thorough analysis of vulnerability, likelihood of flooding and cost–benefit analyses, this research develops Taipei resilience policies to address the vulnerabilities.

Findings
The findings indicate that Taipei case study, a chronology of policies implemented to prevent flooding, explains that costly engineering structures, rebuilding and fortification against floods eventually created a false sense of security, which has encouraged more intensive residential and commercial developments in flood-prone areas, and led to a higher level of vulnerability. Additionally, flooding simulations indicate that 40 per cent of Taipei City is located in flood risk areas in an extreme weather scenario. This percentage is higher than other global cities such as London’s 15 per cent, Tokyo’s 10 per cent and New York City’s 25 per cent. Based on the 10 per cent of total flooding areas above 0.5 m, the vulnerable population is estimated at 200,000 people, or 7 per cent of the total population. The GDP impact will be more than $28bn. More than $67bn of land value is vulnerable. A least one million subway passengers will be affected each day. Further, there is little evidence that the urban poor are particularly vulnerable to floods. On the contrary, some neighborhoods with high-income households face a higher risk of floods. Very few medical centers, oil and gas stations and electrical power substations are located in flood-prone areas, but a large number of public schools, administrative buildings and major subway stations are susceptible. Additionally, the likelihood analysis of flooding in an extreme rainfall scenario concludes that the possibility will be five times that of the existing assumption with a flood in every 200 years. Thus, Taipei City’s infrequent once-in-two-century floods are likely to occur more frequently.

Originality/value
This paper provides a thorough analysis of vulnerability, likelihood of flooding and cost–benefit analyses in Taipei. It also develops Taipei resilience policies to address the vulnerabilities. In the future, rather than strengthening and rebuilding costly structures, Taipei should focus on land-use and environmental planning for resilience. Urban policies should include environmentally responsible development in the face of continued population and economic growth, and being resilient regarding natural disasters. Most important is the need for a strong political commitment and leadership to initiate and implement urban policies toward resilience.