2.1.2. Distance from Fault

Being located at the junction of the Eurasian continental plate and the Philippine Sea plate, Taiwan experiences frequent seismic activity. According to data collected by its Central Meteorological Bureau from 1991 to 2006, there are about 18,500 earthquakes in Taiwan each year, of which around 1000 are felt earthquakes. Major earthquakes often result in surface ruptures, rock folds and new faults. At present, scientists are unable to determine whether such faults cause earthquakes or vice versa, but their locations are identified in areas where seismic energy is strong [19]. Geological conditions in such areas are very fragmented due not only to the presence of fault gouges but also to broken zones near them and changes in the Earth's crust. The physical properties of the filler between the discontinuous surfaces of fault gouge or broken zone are usually poor, as is the degree of cementation, and this often causes engineering problems [16]. Lin observed that the rock mass on both sides of the Chenyulan fault is relatively broken, and weathered slate and metamorphic sandstone there, respectively provide fine-grained and coarse-grained material for earth-rock flows. The effects of faulting and river erosion also contribute to such flows [20]. The slopes of the terrain along both sides of the river are relatively steep, so the original weathered-soil layer collapsed due to by heavy rain and formed debris flows. During these flows, the rock plate was broken, and the broken pieces in the rock mass were drawn into them.

### 2.1.3. Distance from River

Wang's study of the Shaolai River concluded that the collapse percentages of susceptibility increased with proximity to the river's course. Specifically, the percentage of collapses within 200 m from the river channel was 42.7%; between 200 m and 400 m, 26.6%; between 400 m and 600 m, accounted for 19.4% [21]. Beyond 1600 m from the river channel, there were no collapses at all. Therefore, it can be inferred that large increases in the collapsed areas of adjacent rivers may be related to heavy typhoon rain causing water levels to surge, which in turn eroded the slope foot of the Shaolai River and accelerated the collapse of the riverbank [22]. In short, a closer distance to the river entails a higher level of risk. It is also a principle of hydrology that when the slope is closer to the river, it is nearer to groundwater; thus, water seeping into the ground will cause seepage pressure in the slope. If the soil structure is highly permeable, this process will greatly increase the probability that the stability of the slope will be negatively affected [23]. As Chang et al. observed, distance from the river channel determines flood impact [24]. The smoothness of a river channel determines its flood-discharge capacity, and the reclamation-area ratio of a reclaimed lake determines the flood-regulation capacity of the lake in the protection zone.

### 2.1.4. Potential Stream-Impact Quantity

According to a comprehensive assessment by Taiwan's Bureau of Soil and Water Conservation [15], natural streams or pits are likely to cause debris-flow disasters, though this likelihood is affected by local conditions including the presence or absence of protected objects. The Bureau uses two main criteria for judging the probable impact of potential debris flows. The first is that the slope of the stream bed is greater than 10 degrees, and that the catchment area above this point is greater than three hectares. The second is that, at the downstream exit or overflow point of the stream, there are more than three households or important bridges or roads that need to be protected. Assessment should be divided into four levels—"high", "medium", "low", and "continuous observation"—based on the characteristics of the site.

### *2.2. The River Terrace Itself*

### 2.2.1. Minimum Ratio

Specific height is defined as the height of the riverbank relative to that of the riverbed surface. Yoshiro divided Taiwan's terrain into eight types; from high to low, these were Highest Peneplain (HP), Old Piedmont (OP), Elevated Highland (EH), Young Piedmont (YP), Lateritic Highland (LH), Lateritic Terrace (LT), Fluvial Terrace (FT), and Fluvial Plain (FP) [25]. Through this perspective, the topographic evolution of the Taiwan River Valley can be explored. Lin subsequently provided a general description of the topographical features of Taiwan's important river systems, along with more detailed descriptions of the topographical categories defined by Tomita [25,26].
