**1. Introduction**

A properly maintained network of forest roads provides the forest accessibility necessary for the sustainable use of forest resources, including forest maintenance, timber harvesting, hunting management, and recreational activities [1]. On the other hand, forest roads gradually lose their functionality due to erosion caused by use and exposure to rainfall after establishment, and if the degree of erosion becomes severe enough, they are damaged and become dysfunctional. The forms and factors of erosion and damage vary. Accumulation of sediment and organic matter in culverts crossing forest roads reduces the drainage capacity of culverts [2], causing gully of the road surface due to overflow of stream water [3], culvert failure, and road body failure at stream crossings. Overflow of road surface runoff water causes road body failure by significantly changing the shape of the rutted area itself. Sediment accumulation on the road surface due to soil avalanches from the cut slope or its upper slope makes the road impassable for vehicular traffic.

Soil erosion, in which forest roads are directly or indirectly involved, is an important source of sediment to streams in forested watersheds, affecting the hydrologic system of the forested watershed [4]. Sediment inputs to forested stream systems can have adverse effects on water quality, such as turbidity, increased nutrient concentrations, and reduced water clarity. Therefore, studies have examined the effects of rainfall, vegetation, and season on sediment runoff generated by the existing road network [5,6], as well as studies on the interaction between the road network and the river network [7].

Facilities for proper drainage are of paramount importance in road design [8]. Therefore, studies have been conducted on the layout and dimensioning of appropriate drainage

**Citation:** Watanabe, M.; Saito, M.; Toda, K.; Shirasawa, H. Rain-Driven Failure Risk on Forest Roads around Catchment Landforms in Mountainous Areas of Japan. *Forests* **2023**, *14*, 537. https://doi.org/ 10.3390/f14030537

Academic Editor: Chong Xu

Received: 15 February 2023 Revised: 7 March 2023 Accepted: 8 March 2023 Published: 9 March 2023

**Copyright:** © 2023 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/).

<sup>1</sup> United Graduate School of Agricultural Sciences, Iwate University, Morioka 020-8550, Japan

facilities, taking into account the topography and hydrological factors of the hillsides and the road structure [2,9]. From the viewpoint of minimizing the amount of road surface erosion, guidelines have been given for appropriate cross-sectional trench spacing according to the longitudinal gradient [10]. In recent years, forest road design and maintenance methods have been proposed that take into account the impact on the forest environment as well as economic efficiency; Aruga et al. [11] attempted to reduce sediment deposition to rivers and the total cost of forest road alignment planning; Dodson et al. [12] studied the environmental impact on rivers, the cost both environmental and economic to the river, the occurrence of road failures that may lead to both environmental and economic costs, and the total cost of owning a forest road, they examined a method for scheduling maintenance and improvement activities on forest road segments to minimize these three costs. Saito et al. [13] studied an automatic forest road design model that considers shallow landslides using LiDAR data. Pellegrini and Grigolato [14] examined how the integrated use of GIS and Analytical Hierarchy Process (AHP) analysis can be used to determine priorities in road network maintenance work to minimize sediment generation from road surfaces and maximize the social value of roads.

In Japan, studies have analyzed the risk of forest road failure from the perspective of topography and forest road structure. Located on the eastern edge of monsoon Asia, one of the world's rainiest regions, Japan has an average annual precipitation of 1718 mm, which is about twice the world average (880 mm) [15]. The rainy season in June and typhoons in autumn bring torrential rains. Geologically, the terrain is complex and steep due to its location in an orogenic belt where the Pacific Plate is subducting toward the continental plate, and there are many mountainous areas with active erosion. Therefore, the placement of forest roads on unstable slopes may cause slope failure. Kondo and Kamiya [16] surveyed 100 forest road failures that occurred on three forest road routes over a 13-year period from 1977 to 1989 and analyzed the characteristics of damage-prone stream crossings using Quantification II. Surface failures on hillsides occur most frequently in concave landforms (zero-order basin) where seepage water and weathered sediments are concentrated and have not yet developed in the primary valley [17]. Yoshimura and Kanzaki [18] analyzed slope failure factors by analysis of variance using only topographic factors legible from topographic maps. In the analysis, inclination, cross-section slope, turning point inclination, and catchment area were used as the factors estimating the risk of slope failure. They reported that the risk of failure is particularly high on concave slopes. Yoshimura et al. [19] also examined forest road alignment geometry in slope failure areas and reported that the risk of failure was significantly higher on inner curves where water had been concentrated, followed by a higher risk of failure at outer curve inflection points (the starting and ending points of a curve on a curve located on a ridge). Kondo and Kamiya [16] and Yoshimura et al. [19] suggest that the topography and structural configuration of forest road sections have some influence on the risk of forest road failure in Japan, although they do not consider the layout of drainage facilities. In Japanese forest road engineering textbooks, the rising water level of the stream parallel to the forest road has also been cited as a cause of damage on the valley side of forest roads [20].

Forest road failures in Japan are more likely to occur in forest road segments with topography, such as stream crossings, stream sides, and zero-order basins [16–20]. However, all of the existing studies in Japan are only the results of a few lines of investigation in some areas. Thus, for example, Kondo and Kamiya [16] surveyed 100 forest road failures that occurred on three forest road routes over a 13-year period from 1977 to 1989. They reported that failures at stream crossings accounted for 72% of the total in the research site and reported a characteristic form of damage, but it is not known to what extent this trend applies in Japan. Wemple et al. [21] reported that of 103 sediment transport events that occurred near forest roads in the western Cascade Range region of Oregon due to heavy rainfall in February 1996, three-quarters were mass movements, debris flows, cut slope slides, fill slope slides, and Phillips [22] investigated 116 landslides that occurred during two heavy rainfall events on the east coast of the North Island and found 17 landslides

associated with forest roads. Sidle et al. [23] studied mass wasting along a road in Yunnan, China, to determine the extent to which landslide erosion was caused by cut-and-fill slope failure or dry ravel. The results show that the landslide erosion was caused by a cut slope, embankment slope failure, and dry ravel. However, only one or two heavy rainfall events were reported in each case.

Factors and countermeasures for forest road failure differ depending on the type of failure that occurs, but the statistical understanding of the type of damage events is not sufficient because of the limited study area in the existing studies. Since we do not know the macroscopic occurrence trends of failures, it is also impossible to determine whether there are sufficient guidelines and research on the important countermeasures for failures that occur in the target area. In addition to stream crossings, other failure hazards have been identified in concave terrain, but it is not known to what extent the susceptibility to failure differs. If there is a difference in the likelihood of failure even in the same category of damage-prone areas, the weight should be changed when selecting routes and scheduling maintenance and management. If there is a pattern of failure that tends to require high repair costs, then the most important issue from the viewpoint of reducing repair costs is to prevent the occurrence of such a pattern of failure. Since the sensitivity to rainfall intensity may differ depending on the damage pattern, it is important to understand the relationship between the failure pattern and rainfall intensity in order to consider the maintenance of forest roads under climate change. By understanding the overall trend of forest road failure, more strategic maintenance may be possible. In Japan, forest road failures occur frequently due to heavy rains caused by typhoons and rainy season fronts, which predominantly occur from July to October. The total number of damaged segments of forest roads and the total amount spent on repair in Japan in the last 3 years has been 8181 (19.3 billion yen) in the 2017 fiscal year, 13,241 (39.8 billion yen) in the 2018 fiscal year, and 12,448 (34.1 billion yen) in the 2019 fiscal year [24]. However, the latest Basic Plan for Forestry and Forest Products states that the desired length for forest roads is 250,000 km, compared to the current 190,000 km [25]. If the frequency of heavy rainfall and the length of forest roads increase in the future, the number of failures caused by heavy rainfall and the cost of repair is very likely to increase [26]. In recent years, there has been a shortage of forest engineers who can formulate repair plans [24]. If the number of failures is not reduced, it may not be possible to provide the labor required for failure repair in the future. If the number of failures increases, the time required for failure recovery may become longer, with forest roads remaining impassable for a longer period and forestry activities potentially being severely hampered. As the intensity and frequency of rainfall events increase, it is important to avoid planning routes in unstable terrain and to take appropriate precautionary measures in hazardous areas on existing routes. This is particularly important to promote appropriate forest road maintenance in the context of limited human and economic resources. To achieve this, it is necessary to identify what types of forest road failure are important to address.

Therefore, in order to provide statistical information on forest road failures, this study collected data on a large number of forest road failures that occurred in mountainous areas in Japan and statistically analyzed the characteristics of each topographic type at the points where failures occurred. Specifically, forest road segments were classified into four categories: stream crossings, streamside, zero-order basin, and others, and comparisons were made regarding the length of forest road failure, the relative probability of occurrence, repair costs, and induced rainfall intensity in each category.

The terms "erosion" and "failure" to forest roads are not strictly defined. In this study, the difference between the two terms is the degree of erosion, and the term "failure" is used when a forest road is so eroded that it cannot fulfill its design purpose and becomes dysfunctional (Figure 1). In this study, "failure" includes shallow landslides, slope failures, road surface erosion, and road body failure.

**Figure 1.** Image of forest road failure. A culvert at a stream crossing was destroyed by heavy rain, and the road body was eroded by stream water, making it inaccessible to vehicles.

#### **2. Materials and Methods**
