**1. Introduction**

The EU has actively promoted the use of Nature-based Solutions (NbS) to respond to disasters with the hope that NbS can be as effective as traditional engineering methods in preventing disasters while providing benefits for the ecological environment. NbS is a concept proposed by the International Union for Conservation of Nature (IUCN) and the World Bank in 2008 [1], and aims to respond to social challenges such as climate change, food and water safety, and public health by adopting nature-based methods to achieve resource sustainability and conduct effective disaster risk management measures. It also aims to provide other benefits, such as promoting human welfare and sustaining ecological diversity [2]. The European Commission (EC) defines NbS as actions inspired by, supported by, or copied from nature that aim to improve current or provide better methods for dealing with environmental, social, and economic challenges [3]. The United

**Citation:** Lo, W.; Huang, C.-T.; Wu, M.-H.; Doong, D.-J.; Tseng, L.-H.; Chen, C.-H.; Chen, Y.-J. Evaluation of Flood Mitigation Effectiveness of Nature-Based Solutions Potential Cases with an Assessment Model for Flood Mitigation. *Water* **2021**, *13*, 3451. https://doi.org/10.3390/ w13233451

Academic Editors: Momcilo Markus and Alban Kuriqi

Received: 2 November 2021 Accepted: 3 December 2021 Published: 5 December 2021

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Nations "World Water Assessment Programme" (WWAP) defines NbS as nature-inspired or mimicking actions that improve and contribute to water resource management [4]. In short, NbS refers to natural solutions and methods developed in response to various social or disaster challenges to achieve goals such as resource sustainability, effective disaster risk management, or disaster prevention and mitigation while providing social and environmental benefits simultaneously.

An ecosystem that is 100% natural may still not qualify as a NbS. To determine whether it is an NbS, it is necessary to consider whether the system can use natural processes to achieve water-related purposes [3], such as flood mitigation and water reservation. According to the EC's definition, NbS must also be able to provide added values when addressing the social, environmental, or economic challenge that is the main objective [4]. The IUCN pointed out that any solution must be an integrated concept for addressing one or more social challenges to be called NbS, and the solution must be able to maintain or promote biodiversity and human welfare [2].

The NbS of flood mitigation can be divided into small and large-scale solutions, where the small-scale solution refers to solutions for urban or local areas, and large scale refers to those for suburbs, river basins, or regional areas. The concept and facilities of small-scale NbS are like those of low-impact development; in fact, most related research, including those on facilities such as green roofs, rainwater harvesting systems, permeable pavements, bio-retentions, and rain gardens [5–7], were called as such in the past. Past studies have confirmed that small-scale NbS have significant effects on urban disaster reduction during small rainfall (2–5-year return period rainfall), and permeable pavement is one of the most effective devices of small-scale NbS [8]. However, in extreme rainfall events (such as 50-year or 100-year return period rainfall), the effectiveness of the permeable pavements is very limited [5]. The commonly applied large-scale NbS include wetland restoration, river restoration, flood detention ponds, forest restoration, and "Room for the River" [9–11]. Though large-scale NbS is similar to traditional engineering methods, large-scale NbS not only achieves disaster reduction by restoring the original appearance of the landscape or using natural materials, but increases biodiversity, improves environmental resilience, or provides added value such as an amenity-oriented environment to the general public. As a result, this study believes that grey measures combined with both small and large-scale NBS should be the future trend [12,13].

The literature to date shows that various social challenges can be addressed through NbS. Reducing flood risk [14], reducing surface runoff [13], reducing exposure to soil erosion and landslides [15], and limiting coastal erosion [16] are a few examples of such challenges. Such benefits help in reaching sustainable water management.

The percentage of Taiwan's population that is exposed to more than three types of natural disasters is as high as 73%, ranking first in the world [17]. With most of the population under such risks, disaster prevention, mitigation, and disaster recovery are considered to be some of the most important issues in need of addressing. In the past, Taiwan responded to floods with traditional engineering methods such as drainage systems planning and levee building. With the rapid development of cities resulting in impervious area increases, the scale and characteristics of flood disasters have become difficult to predict when coupled with the impact of extreme rainfall. Therefore, the concept of flood mitigation in Taiwan has gradually shifted from traditional flood control methods to comprehensive river basin management and land planning that integrates various engineering and non-engineering measures. For example, the concept of "Local Detention" reduces the required regional discharge capacity by lowering the water levels of fish farms and farmland in batches before typhoons and floods. This further reduces the probability of flooding during said events.
