*Article* **Risk Assessment of Riverine Terraces: The Case of the Chenyulan River Watershed in Nantou County, Taiwan**

**Ji-Yuan Lin <sup>1</sup> , Jen-Chih Chao 1,\* and Yung-Ming Hsu <sup>2</sup>**


**Abstract:** The purpose of this study is to establish a method of hazard assessment for the river terraces along the Chenyulan River and use 40 of them as protected objects. Using a geographic information system, the researchers extracted nine parameters for such terraces. These are length to attack shore, distance away from fault, distance from river channel, number of creeks and streams with possibility of debris flows, height above stream level, average slope degree, geology, number of erosion ditches, and distance from landslide area behind. Next, the weightings identified by analytic hierarchy process analysis were used as the basis for grading the various factors affecting river terraces. Hazard assessment for the river terraces then proceeded via totaling of the potential trends of the various factors and the protected objects, as well as comparison of historical disaster conditions and satellite images. The results showed that there were 8 high-risk river terraces, 14 medium–high-risk river terraces, 14 medium–low-risk river terraces and 4 low-risk river terraces. The evaluation of the current conditions of the settlement environment through parameter weighting has a certain accuracy and reference value in reducing the disaster impact of the riverine terrace settlement.

**Keywords:** geographic information system; hazard assessment; river terraces; risk assessment

### **1. Introduction**

Taiwan is located at the junction of the Eurasian continental plate and the Philippine Sea plate. Formed through the Penglai orogeny, the Nanao orogeny and crustal changes, it is a mountainous terrain with flat land accounting for only 25% of its total area. Due to population growth and rapid industrial and commercial development in Taiwan in recent years, the use of flat land has become saturated, and development of hillside areas, especially river terraces, has become common. The existence of river terraces indicates frequent geological changes, high erosion rates, abundant sources of silt, and strong river scour [1], but their formation is also affected to some extent by climate change and human activities [2]. As Taiwan is surrounded by the sea on all sides, it receives abundant rainfall throughout the year, about 2500 mm, more than two and a half times the world annual average of 970 mm. During the rainy season (from 1 May to 30 November each year), the region is prone to typhoons, each of which tends to increase the intensity of rainfall within a short period of time, and this can result in landslides and mudslides in mountainous areas, as well as rapid rises in the water levels in rivers, which often results in flooding and the erosion and collapse of riverbanks. To prevent loss of life, residents of river terraces have to evacuate when typhoons occur. On 7 August 2009, when Taiwan was struck by a moderate-strength typhoon, Morakot, heavy rainfall led to a series of disasters in southern Taiwan. Due to the collapse of Xiandu (Xianto) Mountain, Xiaolin Village was destroyed, and a short-term barrier lake that was formed endangered the lives and property of residents downstream.

**Citation:** Lin, J.-Y.; Chao, J.-C.; Hsu, Y.-M. Risk Assessment of Riverine Terraces: The Case of the Chenyulan River Watershed in Nantou County, Taiwan. *Appl. Sci.* **2022**, *12*, 1375. https://doi.org/10.3390/ app12031375

Academic Editors: Andrea Chiozzi, Elena Benvenuti and Željana Nikoli´c

Received: 16 November 2021 Accepted: 19 January 2022 Published: 27 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Due to undercutting and erosion by rivers, river terraces remain above the water surface during normal floods and are distributed in steps on the slopes of the river valleys [3]. Chang and Shi defined river terraces as land along rivers, consisting of terraces and cliffs [4]. The terraces' surfaces were the riverbed or floodplain surfaces in a former period, while the cliffs below them, facing the valley axis, were formed by both down erosion and lateral erosion [2]. The terraces of Xiaolin Village were formed by ancient and recent landslides. Their geology is extremely unstable, and the village was destroyed largely due to its location on dangerous low-lying ones. According to our survey, among the 144 mountain settlements and aboriginal tribal villages in southern Taiwan, 92 are located on river terraces, similar to the situation of Xiaolin. Therefore, the hazard and risk assessment of river terraces demand special attention to avoid similar disasters.

Various explorations of the topographic evolution of river terraces and the reasons they are formed in different regions have been conducted [5–8]. Scholars have also investigated the risks to river-terrace settlements posed by rainfall-induced landslides, based on historical data regarding the potential risk range of debris flows and the areas where landslides occur [9–11]. The Chenyulan River, in particular, has been the subject of multiple studies focused on collapses' potential indicators and locations [12–14], due to the multiple disasters that have struck the terraces of its lower reaches. However, due in part to strong variation in the reasons for the formation of river terraces and the hazards they face, residents of river-terrace settlements tend to have low awareness of disaster risks and are thus unable to effectively mitigate them. The people who live in settlements on the river terraces in Chenyulan today could face disaster at any time. Therefore, levels of danger to such terraces are estimated by risk assessment, so that when a typhoon is about to strike Taiwan, local residents can be quickly moved to safe places and disaster-relief facilities.

This study focuses on factors that may harm river terraces, derived from special questionnaires to establish index weights, and uses a geographic information system (GIS) overlap to allocate these factors to particular river terraces. Then, the scores of these potential factors and preservation factors are summed to estimate the terraces' risk, and establish a risk map of the area, with the wider aim of disaster prevention and reduction in disaster losses. This study uses the analytical features of AHP multilevel evaluation to decompose the elements of the river terraces' environment and construct a model of potential factors of the river terraces. Using the GIS data and AHP model, a matrix of judgement is established based on the corresponding criteria to derive the corresponding element weights, and a spatial analysis of the river terraces' hazard trend map is used to provide a solution to reduce the impact caused by the disaster.

### *Study Area*

Li's survey of the Chenyulan River noted that its inland river terraces, alluvial fans and landslides were highly developed, and that there were 46 fan-shaped terraces and alluvial fans. The large number of these features implies rapid geological change [1].

According to the Bureau of Soil and Water Conservation, part of Taiwan's Council of Agriculture, there are more than 1700 potential soil and rock flows [15]. Within our study area, as shown in Figure 1, there are 49 such potential flows.

### **2. Literature Review**

The potential hazard factors affecting river terraces can be summarized into three latent-sensing categories. The first, in front of the terrace, comprises four factors: attack shore, distance from fault, distance from river, and potential stream-impact quantity. The second, of the river terrace itself, consists of three factors: minimum ratio, average slope and geology. Additionally, the third, behind the river terrace, includes two factors: number of erosion ditches and number of collapses from the rear. Each category is analyzed in the following sections.

**Figure 1.** Study area. **Figure 1.** Study area.

**2. Literature Review** 

the following sections.

The potential hazard factors affecting river terraces can be summarized into three

shore, distance from fault, distance from river, and potential stream-impact quantity. The second, of the river terrace itself, consists of three factors: minimum ratio, average slope and geology. Additionally, the third, behind the river terrace, includes two factors: number of erosion ditches and number of collapses from the rear. Each category is analyzed in
