Enhancing urban flood resilience: interdisciplinary integration of climate adaptation, flood control, and land-use planning from 3PA to 4PA

Climate change has emerged as one of the most pressing global challenges, with far-reaching consequences for various aspects of our lives (Hoegh-Guldberg et al. 2019). One of the most tangible and immediate effects of climate change is the impact on flood disasters, particularly river and urban flooding (Eccles et al. 2019). Even seemingly minor alterations in rainfall patterns, their frequency, and intensity can significantly elevate the risks associated with flooding events (Tabari 2020). This phenomenon has prompted a heightened focus on the importance of flood control measures and urban resilience strategies (Cea & Costabile 2022).

Technically, flood control plays a crucial role in managing flood risks (Xia & Chen 2021). These works are designed to mitigate the impact of flooding by reducing the frequency, extent, and depth of inundation in areas prone to flooding (Luo et al. 2022). However, as climate change continues to drive more extreme severe weather events, we face a sobering reality: the existing flood prevention engineering projects may prove inadequate in the face of escalating challenges due to uncertainty scenarios (Schipper 2020).

In urban flooding, the responsibility extends well beyond the water resources agency (Rainey et al. 2021). It encompasses a web of governing authorities, including local government, land management authority, and regulatory bodies responsible for crafting and enforcing relevant laws and regulations. For instance, developing a detention basin, a vital flood control strategy, often necessitates using government-owned land (Li et al. 2022). Furthermore, the key to achieving urban flood resilience lies in the harmonious integration of engineering solutions and understanding the urban capacity to absorb and manage floodwaters. It requires a strategic alignment between engineering projects and land-use planning at various administrative levels (Avashia & Garg 2020).

In recent years, physicists have increasingly applied their methods to study societal phenomena, driven by interdisciplinary collaboration and the success of physics-based approaches (Jusup et al. 2022). In addition, Wang et al. (2020) proposed that the critical factor to respond to collective risks is the role of communication and behavioral types. Fratini et al. (2012) proposed the Three Points Approach (3PA) to formulate a proper solution to respond to the future flooding scenarios. Sørup et al. (2016) extended the 3PA to rainwater resources, urban stormwater drainage, and pluvial flood mitigation aspects, which provided a quantitative measure to help water resources management develop a suitable strategy to face flood risks. Lerer et al. (2017) developed 3PA-based modeling to enhance the flood control ability of the stormwater system and further improve the urban resilience against the 100-year event. Chen et al. (2021) introduced the sponge city concept and emphasized that flood protection targets and interdisciplinary tools are critical factors. In the past, integrating water resource management with land-use planning has often been overlooked in formulating relevant regulations, leading to outdated regulatory frameworks. Based on the above references, this study discusses the current water resources regulations in Taiwan and proposes improvements regarding the scope of land restrictions. Furthermore, this study extends the previously proposed 3PA strategy to a 4PA version, considering climate change scenarios, and presents a corresponding adaptation toolkit for urban planning reference. Notably, the concept of design for failure needs to be genuinely embedded in the urban planning core. Then, we can create resilient cities capable of weathering the storm of climate-induced disasters while continuing to flourish and thrive, which is a sustainable development city (Pour et al. 2020). Taiwan, susceptible to flooding disasters, was selected as the research area for this study. Notably, the 3PA to 4PA strategy can be generalized and applied to different research areas.

In Taiwan, several legal regulations are directly related to flood control, including the Water Act, Sewerage Act, Land Planning Act, Urban Planning Act, Regional Planning Act, and Wetland Act. Among these, the most directly related to flood control is Chapter 7-1 of the Water Act, titled ‘Runoff Allocation and Outflow Control’ (https://law.moj.gov.tw/Eng/), which was added by the Water Resources Agency of the Ministry of Economic Affairs on June 20, 2018.

In accordance with Article 83-2 and 83-7 of Chapter 7-1 (Please refer to the Supplementary data for detailed content), when a certain level of development area is reached, it becomes mandatory to undergo a review process. For instance, any land development project exceeding an area of 2 hectares must submit a comprehensive plan for review. Similarly, within urban planning zones, any construction or redevelopment project with a base area exceeding 300 m² necessitates the installation of permeable, water-retention, or flood control facilities.

From a flood management perspective, these regulations are put in place to ensure responsible and sustainable land development practices. By subjecting larger development projects to scrutiny and requiring the implementation of water management and flood control measures, authorities aim to mitigate potential environmental impacts and reduce the risk of flooding in urban areas. It is a proactive approach that helps strike a balance between urban development and environmental conservation.

However, relevant regulations may inadvertently impose restrictions on the construction of flood control facilities, and it is worth proposing improvement suggestions to further enhance the comprehensiveness of the legal provisions. The relevant discussions are described in Section 3. Note that while the discussion is based on Taiwan law, the related suggestions are applicable to a different research field.

From the urban planning perspective, the land development area within the urban planning zone is rarely greater than 2 hectares. In other words, there is little room for applying runoff allocation and outflow control. In fact, the primary purpose of implementing runoff allocation and outflow control in densely populated urban planning areas is to prevent flooding. The absence of large-scale land development can raise the flooding risk within the urban planning zone. Note that the implementation of runoff allocation and outflow control is barely linked to the size of the development area. Therefore, the limitation of a 2-hectare area violates the principle of legal reservation and has the aforementioned shortcomings. It should be promptly revised to delete such wording to fulfill the legislative purpose of the ‘runoff allocation and outflow control’. Otherwise, it is common for urban land and building base areas to be less than 300 m². Surprisingly, current regulations exempt them from the obligation to install permeable, water-retention, or flood control facilities. This exemption may increase the likelihood of flooding within the urban planning zone. In other words, the focus of executing runoff allocation and outflow control should be on whether the area is prone to flooding rather than solely assessing it based on the size of the developed area. Therefore, it is essential to revise and remove this restrictive provision promptly.

In addition, sewerage systems are a crucial tool for outflow control, and they should be adapted to extreme weather conditions by incorporating concepts and technologies related to wastewater recycling, expanding the application of wastewater testing and monitoring, and addressing the practical needs of sewerage management. Therefore, a comprehensive review and revision of sewerage system regulations should be undertaken promptly.

In response to the challenges posed by extreme weather conditions, Taiwan has taken several measures for managing flood-prone areas. Apart from the development of flood or disaster potential maps for flood-prone lands, Taiwan has adopted a practice similar to Austria's Hazard Zone Plan. The approach categorizes land into different color-coded zones based on the disaster risk and potential scale. Administrative controls are then implemented accordingly, aligning with the growth management strategies outlined in national land planning.

Runoff allocation aims to mitigate risk and disasters by dispersing flooding amounts. However, the current urban flooding incidents often result from short-duration heavy rainfall exceeding the design standards of urban stormwater drainage systems. Adopting measures that coexist with water or complement disaster avoidance strategies is essential. In cases where there is insufficient available land or flood storage space within a watershed, an evaluation should be conducted to determine if other land areas from different locations can share the burden or if road transport or storage can be utilized to enhance urban resilience. For instance, low impact development (LID) should be considered in flood management in future (Yang et al. 2020). It can be applied in the current road layout, such as making roads permeable to enhance the short-term flood retention capacity.

Furthermore, adopting best management practices and sustainable urban drainage systems in stormwater management is valuable. These techniques involve decentralized stormwater treatment emphasizing maximum infiltration, minimal runoff, and rainwater retention. They categorize urban wastewater into black, gray, and rainwater, treating each type differently. Through urban water recycling, sustainable ecosystem development, and urban space planning, these methods address drinking water, wastewater treatment and reuse, water collection, and self-purification technologies. They replace traditional mixed approaches with unified treatment, promoting rainwater reuse and reducing urban flooding. The above strategies could be considered and implemented to better cope with the challenges posed by extreme weather events and water resource management.

Technically speaking, flood resilience is changeable in different periods. For example, the runoff in the developing urban area should increase simultaneously; the extreme rainfall due to climate change raises the flood risk. Therefore, the existing flood prevention system needs to bear the extra risk of exceeding the flood amount, which is higher than the designed protection standard. Part of the responsibility for such risks belongs to the government, and the community residents also need to share the flood risk. Above all, the future flood scenario and the 3PA principle must be considered simultaneously to develop an appropriate approach, 4PA, to specifically define the responsibility attribution for the flood risk.

Based on the aforementioned responsibility concepts, this research divides the locations of the issues identified in the previous paragraph into four points, (i)–(iv), as shown in Figure 1. Under point (i), the residents maintain daily action and rarely do preparation for flood disasters. It can be defined as a normal period. Point (ii) belongs to the designed responsibility zone, and the government must confirm whether the precipitation can be stored or released from the existing system. To solve the exceeding runoff due to urban development and heavy rainfall, several adaptation strategies must be formulated under point (iii) to enhance flood resilience. The mentioned strategies are discussed in Section 4.2. When the existing and updated flood prevention system is challenged to protect against unexpected extreme rainfall, under point (iv), the concept of design for failure needs to be embedded, and the responsibility belongs to both the government and the residents. The evacuation procedure should be started to decrease the damage loss. Specifically, the responsibility of the government is to provide accurate and real-time flood information and evacuation measures. Citizens should then autonomously proceed with evacuation. During the design for failure phase, individuals may file for indemnification if they incur losses. In the other phases, they can file for damage compensation.

The adaptation strategy must be integrated with hydraulic engineering and land management and then designed to comprehensively consider flood prevention measures in points (iii) and (iv).

The first step involves determining the design rainfall and the preservation area under this rainfall, which represents the region that hydraulic-related agencies need to safeguard through engineering measures.

Step (ii) is to identify the excess rainfall and affordable areas that can be addressed through spatial planning, specifically by implementing measures like runoff dispersion to handle the water volume effectively.

Step (iii) entails selecting an extreme rainfall event as a target scenario and exploring land-related strategies to enhance the area's resilience. This involves using non-engineering approaches such as land planning to reduce the risks in this region during extreme rainfall events.

Step (iv) was rarely discussed before the climate change topic was raised because most people wish to live in a city that is not prone to flooding. When urban flooding occurs, water resources management is used to design more flood control infrastructures. However, new thinking is needed for urban flooding in the current situation with increasingly severe rainfall scenarios.

There is a common saying in the United States: ‘When your only tool is a hammer, every problem looks like a nail.’ In other words, we should avoid viewing every flooding problem as an engineering issue. We should consider reducing the cover area, decreasing the water depth, and shortening the flooding duration, as we proposed in step (iii).

Unfortunately, we may face the comprehensive failure of flood control infrastructure in the future under the extreme rainfall scenario. Therefore, we must focus on three key aspects: pre-disaster preparedness (Di Ludovico et al. 2023), disaster mitigation during events (Rana et al. 2021), and post-disaster relief efforts (Wang et al. 2022). (a) Pre-disaster preparedness: before a disaster strikes, it is crucial to establish a robust early warning system. This system should provide timely alerts and guidance to the public based on predefined operational procedures. Preparedness measures should include identifying high-risk areas, developing evacuation plans, and ensuring the population is well-informed about safety procedures. (b) Disaster mitigation during events: when a disaster occurs, it is essential to follow established standard operating procedures for evacuation and shelter. These procedures should prioritize safety and minimize risks. Effective coordination among relevant authorities and emergency responders is vital during this phase to ensure the efficient execution of disaster mitigation efforts. (c) Post-disaster relief: immediately deploying resources and personnel for relief efforts is crucial after a disaster. This includes providing emergency shelter, medical assistance, and essential supplies to affected communities. In addition, assessing and repairing damaged infrastructure is essential for a speedy recovery. Note that the key is to abandon traditional flood prevention concepts. The focus for the future should be on efficiently recovering the damaged city, which is the major spirit of 4PA.

Above all, striving to create a completely flood-free city may be unrealistic. Instead, the primary focus should be building a city with flood resilience capabilities. A resilient city can withstand and recover from disasters, ensuring sustainable development despite extreme weather events.

From an urban planning perspective, urban development areas within planning zones are typically small, often less than 2 hectares. This limits the application of runoff allocation and outflow control. However, these measures are primarily aimed at flood prevention in densely populated areas. The 2-hectare limit violates legal principles and should be removed. Regulations exempting areas less than 300 m² from installing flood control facilities may increase flood risks and should also be revised. In addition, sewerage systems must adapt to extreme weather conditions, incorporating wastewater recycling and expanded testing and monitoring, requiring comprehensive review and revision.

The concept of flood resilience is dynamic and changes over time. To address these evolving flood scenarios, a 4PA approach is proposed, considering the future flood scenario and the 3PA principle to define responsibility for flood risk.

The proposed methods are based on the 4PA concept, focusing on effective countermeasures. The adaptation strategy integrates hydraulic engineering and land management. Strategies encompass flood control, runoff regulation, dispersion, storage, road detention, land elevation management, flood risk management, building water barriers, enhanced flood tolerance, and other measures. These strategies can be adapted in the urban area, with specific measures determined by relevant authorities.

This study aims to deal with the challenges in urban development under the threat of flooding. Not only providing the regulatory revised suggestion, it also proposes the 4PA (prevention, protection, preparedness, and adaptation) approach to address evolving flood scenarios. We focus on the concept of integrating the future flood scenario to define responsibility for flood risk, offering the I-D-D adaptation tool kit for flood resilience. In the future, the comprehensive discussion of urban resilience could be investigated, especially the technical challenges and economic constraints.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.