FOREST ROADS SYSTEM IN MOUNTAIN RELIEF: INDICATOR EVALUATION OF WATER-EROSION RISK
DOI:10.30825/5.EJPAU.21.2017.20.1

Paweł B. Dąbek¹, Romuald Żmuda¹, Tomasz Kowalczyk¹, Jolanta Dąbrowska², Andrzej Moryl², Ewa Kucharczak-Moryl^3
1 Institute of Environmental Protection and Development, Wrocław University of Environmental and Life Sciences, Poland
² Institute of Environmental Engineering, Wrocław University of Environmental and Life Sciences, Poland
³ Department of Biochemistry, Pharmacology and Toxicology, Wrocław University of Environmental and Life Sciences, Poland

ABSTRACT

According to existing scientific assessments, water erosion is one of the main processes responsible for the degradation of the natural environment in the Polish Sudety Mountains. The anthropogenic activity in forested areas, as well as forestry management, requires a dense network of internal roads, which limits the protective functions of the forest in terms of preventing soil erosion. An important factor contributing to the intensification of water erosion in forested areas is the improper location of roads in relation to land relief. While designing the routing of forest roads, their influence on the intensity and spatial extent of water-erosion is often omitted.
    On the basis of the Digital Terrain Model (DTM) and spatial data for the Polish part of the Western Sudety, an effort was made to evaluate the existing forest road networks with respect to their placement in land relief. The location of roads was evaluated using an analysis that measures road orientation with regard to the direction of slope gradient. This was expressed using the indicator IARRR (Indicator of Arrangement of Rural Road in Relief). This method unequivocally characterises the location of roads with regard to land relief. It simultaneously serves as the basis for erosion risk evaluation alongside roads, as well as one of the risk evaluation factors for linear water erosion in a catchment. The results indicate a large share of along-slope roads (ca 10.0%) which tend to get easily deepened by the traffic, and a vast majority of road sections running diagonal to the slope (ca 76.0%) tend to collect and intensify superficial runoff, thereby increasing linear erosion.

Key words: water erosion, forest roads, arrangement in relief, IARRR, GIS, Sudety Mountains.

INTRODUCTION

Linear erosion is one of the most dangerous forms of water erosion of soils. Concentrated runoff carries an increased risk due to the energy of flowing water. The prevention of the occurrence and intensification of erosion depends, among other things, on treatments that limit the speed and volume of runoff, as well as on securing potential lines of drainage (e.g. trails and forest roads) [11, 16, 27]. Water erosion processes basically depend on the type of rainfall as well as on the amount and intensity of heavy and torrential rain, which are measured in Poland between 10–45 mm·h^-1 and 45–120 mm·h^-1, respectively [6]. The Chomicz classification does not include regional disparities, dependent on geographic location and climatic conditions. However, the intensity of rainfall can vary throughout its occurrence; therefore, a temporary high intensity of rainfall and the occurrence of long lasting low intensity rain greatly influence erosion processes [8, 18]. These processes result in the saturation of soil, reducing its stability and increasing its susceptibility to erosion. This, in turn, results in land degradation, e.g. changes of the thickness of soil, formation of rills, accumulation in depressed terrain, as well as delivery of debris to rivers [17, 24].

In mountains and terrains with rich relief, water erosion could cause the transition from generally uniformly distributed superficial to concentrated linear forms. Rural roads and other linear structures, when inappropriately designed, can lead to large concentration of runoff and hence to intense, in some cases even catastrophic, erosion events [24].

Studies on hydrology and erosion in Poland and other countries indicate that forests have best soil-protective functions, efficiently retaining water and reducing the concentration of runoff [4, 14, 23, 24]. Under appropriate conditions and healthy forests, water erosion, even in mountainous terrain, is practically absent (or occurs locally). However, dense forest roads and trails network, which results from necessity to conduct the forest works and from the forest management, negatively affect the forest’s protective functions [7]. The location of roads in the land relief has a particular significance [9, 30]. However, road networks in mountainous terrain are generally poorly planned. Soil protection in forest regions can be said to be fundamentally dependent on proper planning and securing forest trails and roads [29].

Road construction and drainage systems are all very important in the context of preventing erosion. The density of the road network within forest area and their location with respect to topography are important issues as well. Forest road infrastructure is primarily designed with the economic maintenance of the logging industry and forest management in mind [2]. Among the many functions that the forest roads fulfill, the economic functions are of utmost importance. The location of the forest roads in the area is dictated primarily making available the forest stands [10]. This results in a dense network of roads which are mostly located perpendicularly or obliquely to the slope; in other words, in line with the largest gradient of the slope, where the strongest runoff and erosion occur, as in rural dirt roads [32]. The design decisions relating to the parameters of roads (eg. the longitudinal and transverse inclination of the road sections, drainage, road surface) result from obligatory technical normative (eg. specified maximum inclination for particular categories of roads, localisation of the roads in the area, density of the forest road network) [10]. While designing the forest road network, their influence on the hydrology of the area is not taken into account. Intensive runoff and its concentration increase the occurrence of water erosion, thus degrading the natural environment and increasing the cost of forestry management, as well as lowering the safety of tourism and increasing the risk of flooding [15, 25]. The prevention of these phenomena should be based on appropriate design of the road network. The evaluation of existing road network plays an important role, with the objective in identifying the potential risk and the analysis of prevention measures.

The aim of the study presented here was to characterise the existing forest roads infrastructure, to analyse their location with regard to the hydrological-erosion conditions and water erosion risk processes resulting from roads location with respect to land relief. The main task was the evaluation of the road network as characterised by the IARRR indicator [28, 29, 30]. Applying the IARRR method at the stage of planning the forest roads network could affect on the final location of roads in the area among others due to the potential the threat of erosion phenomenon on roads with high values of IARRR. It is an effort to apply to mountain forest regions a method which has until now been used for rural roads [28]. For the subject of the study, a mountain area with a known occurrence and intensity of water erosion processes was selected, it being also a region of significant forest cover, where regular tree harvesting takes place, as well as the transport of timber using forest roads. The importance of remote sensing as a data acquisition source should be mentioned, and simultaneously the possibility of using it in later stages of processing and analysis, which greatly increases the scope of erosion studies, especially in protected and difficult to reach regions [5, 12, 21].

CHARACTERISTICS OF THE STUDY REGION

The administrative region of the Szklarska Poręba Forest District, located in the Karkonosze Mountains, was chosen as the study area. Within Polish borders the Karkonosze Mts. cover areas of about 185 km² which, while the Pogórze (the Czech part) encompasses an area of about 80 km². The most important climatic features that characterise the Karkonosze Mts. are high rainfall, rapid winds, and a highly dynamic weather regime. In the upper parts of the Karkonosze Mts. (above 1400 mamsl) the average annual rainfall surpasses 1500 mm, whereas in the north at the foothills (350–400 mamsl) the average annual rainfall is about 700 mm. The Szklarska Poręba Forest District covers an area of about 14 200 ha, and forest cover amounts to about 58.0%. The District has a typical mountain character – only the north-eastern part is made up of highlands. In its entirety, the area has a richly varied topography, with numerous steep slopes and varying aspects of mountain slopes. The specific topography and climate make forest maintenance a difficult task in this area.

The terrain analysed in the study is located north of the town of Szklarska Poręba. It has an area of about 800 ha, situated at an elevation range from 480 to 1090 mamsl. The average terrain slope in this area is about 13° (ca 22.0%). The average terrain slop of the area was calculated as arithmetic mean from slope GRID which was created from DTM.  That conditions almost qualifies this area for cableway skidding (recommended for forest mountain area with average terrain slope more than 25.0%) [10]. The aggregate length of forest roads in this area is over 24 km. For the VII Sudetian natural-forest land, the density of roads should equal 25.8–28.3 m·ha^-1; in the case of the analysed region, this indicator amounts to about 30.0 m·ha^-1 [10].

METHOD

The following analysis is based on the Forest District's Digital Terrain Model (DTM). The DTM contains elevation data and an interpolation algorithm, thus allowing the recreation of land relief [19]. The basic material to conduct the analysis was obtained through remote sensing. Remote sensing methods are currently becoming more popular for projects concerning environmental assessment and management. In this case, for the remote sensing, an aerial laser scanning (ALS) was commissioned by the Forest District in 2007. Based on its results, an ArcGIS generated DTM was completed in 10 m resolution (10 m pixel size), which is an adequate resolution concerning the area characteristics and scope of research [11, 20, 31]. The DTM was completed with spatial data containing information including the location of watercourses, forest roads line, and areas of forest complexes.

The study is based on indicator analysis which characterises the location of forest road network in land relief. To create the road pattern characteristics the IARRR (Indicator of Arrangement of Rural Road in Relief) was used [28–30]. This method is unequivocal in describing the location of the roads in relief in terms of greatest slopes, which is the basis for further evaluation of the potential effects of erosion on the slopes. The study is the first serious analysis of forest roads location in the Polish Western Sudety regarding land relief, while simultaneously examining the adequacy of applying the indicator to non-agricultural roads.

The IARRR indicator is expressed as the angle between the contour line and the selected road section; likewise, it describes the inter-nodal position of forest road segments in relief. In this study, an indirect method of IARRR indicator determination was used which based on the juxtaposition of the road segment azimuth and aspect of the slope. The aspect of the slope (exposition) map was generated from the DTM (10 m pixel size).The road network was illustrated on a vector map and divided into sections resulting from intersections with contour lines of 25 m intervals (by the XTools Pro v.7.1 extension). Then the resulting line was divided into sections by existing vertices, this operation was necessary because of programme capabilities. The applied threshold of 25 m elevation difference results from the amount of the road network, as well as the elevation difference of the terrain. In rural areas and in less-variable terrain, it is possible to apply a division using smaller elevation intervals or a division using uniform length road sections [29].

The next stage was the designation of the average azimuth value of each road segment. To this the Graphics and Shapes Tools v.2.1.85 plug-in was utilised within ArcGIS. The designation of the slope surface aspect in the area where the road segment is located was also necessary. This operation was completed using the Hawth's Analysis Tools v.3.27 extension which, from the previously defined slope surface aspect raster layer, defines the average exposition of the given terrain on which there is a road segment. Using the aforementioned variables, i.e. road segment azimuth value and slope surface aspect under the road, arithmetic operations were conducted which identified the value of the IARRR coefficient. After the final transformation of the data, a numeric value of the coefficient was obtained, it being the absolute value of the angle between the road segment (direction in which the road is headed) and the contour line (the angle perpendicular to the line of largest slopes).

The IARRR coefficient assumes values from 0 to 90, where 0 is a situation in which the road runs in perfectly perpendicular to the slope gradient line, and the value of 90 identifies a road running perfectly along the slope [28, 32]. For the purposes of characterising the location of forest roads in mountainous area, the IARRR values were grouped into six classes (Tab. 1) allowing the road to be described as perpendicular, along or diagonal to the slope, with division of diagonal slopes into two subclasses reflecting their tendency to be of more perpendicular or parallel orientation to the slope. This will have a specific meaning in the case of surfaces with a large longitudinal slope, which in turn may directly affect the intensity of erosion processes. The increase of classes of the IARRR coefficient is also noted in other scientific studies [29, 30].

RESULTS AND DISCUSSION

The results of the analysis with the use of the IARRR indicator for road network on mountain terrain forest complexes of the Szklarska Poręba Forest District in the Western Sudety (Poland) indicate a relatively unfavourable localisation in relation to terrain (Tab. 1). A detailed information of the classification of roads with the IARRR coefficient is presented in Figures 3–4, and the location of given sections within the region of analysis in Figure 5.

Table. 1. The location of the forest roads in land relief according to IARRR in the Szronowiec Forestry (The Szklarska Poręba Forestry District)

IARRR	Road arrangement in relief	Road length
		[m]	[%]
0	Ideally perpendicular to slope (at an angle of 90° to slope)	0	0.0
1–10	Perpendicular to slope (at an angle of 80–89° to slope)	3173	13.5
11–45	Diagonal to slope (at an angle of 45–79° to slope)	11 736	49.9
46–80	Diagonal to slope (at an angle of 10–44° to slope)	6224	26.5
81–89	Along slope (at an angle of 1–9° to slope)	2390	10.2
90	Ideally along slope (at an angle of 0° to slope)	0	0.0
Total		23 523	100.0

Fig. 1. Slope map of the study area (The Szklarska Poręba Forestry District)

Fig. 2. Aspect map of the study area (The Szklarska Poręba Forestry District)

Fig. 3. Histogram of IARRR values in the Szronowiec Forestry (The Szklarska Poręba Forestry District)

Fig. 4. Diagram of IARRR values in the Szronowiec Forestry (The Szklarska Poręba Forestry District)

Fig. 5. The IARRR of the forest roads in the Szronowiec Forestry (The Szklarska Poręba Forestry District)

The combined length of segments with unfavourable parallel orientation to the slope is almost 2.4 km. From over 23.5 km of roads, only about 3.2 km have a favourable (according to intensification of erosion phenomena) perpendicular orientation to the greatest slope line; this amounts to 13.5% of the entire road network in this region. At the same time, no road segments were found to have an orientation perfectly perpendicular to the slope.

For the most part, road segments of diagonal orientation to the slope were found. The angle of these segments to the line of slopes is in the interval of 79 to 10° (IARRR = 11–80). Their share of the roads in the road network amounts to 76.4%, which taken together sums to a length equal to about 18 km. However, road segments diagonal to the slope, but with a tendency to be oriented more parallel to the slope, sum up to a length of over 6.2 km, which amounts to 26.5% of all the roads in the analysed region.

Forest road network is designed primarily for carrying out forestry work. Often, this results in a dense network of roads which are most often located perpendicularly or obliquely to the slope. When designing a forest road system, their impact on the hydrological-erosion conditions of forests mountain catchment is often ignored [10]. In the forest the most eroded are roads along and obliquely to slopes, where the intensity of runoff is the greatest [3]. This specificity can be compared to the intensity of the processes taking place on dirt roads of agricultural land [26]. Also research carried out in forest catchments in the Polish Sudety Mts. [22, 23] showed that linear erosion on the slopes have mostly been initiated by anthropogenic activities. The main factor contributing to the intensification of soil erosion in these areas is the incorrect location of roads and trails in relation to the terrain (local relief).

The IARRR indicator describes the location of road section with regard to land relief. The assessment of risk of intensification of erosion processes on a dirt road surface can be more precise after simultaneous analysis of the relationship of the roads with regard to land relief and the slope map (Fig. 1). For example, an overwhelming part of road sections with an unfavourable orientation (IARRR > 45) is found in the central part of the analysed region, where slopes usually have a northerly exposure (Fig. 2) and their slope gradient is more than 6°, and in large parts of the region it is more than 10°. A simultaneous overlap of these three conditions may create a risk of runoff concentration which, in its primary phase, will intensify erosion processes on road surfaces, but can also create a flood risk for the town lying downhill from this region [1]. Also Chang [4] draws attention to the fact that as much as 90.0% of the debris deposited in streams originating from forest areas may have come from forest roads. In areas of heavy relief, erosion occurring on scarps of roads located on the slopes can generate 70.0–90.0% of soil loss from the entire network of roads in the area [11, 13].

CONCLUSION

The analysis allowed an evaluation of the road network with regard to land relief, which remains one of the primary steps in the risk assessment of erosion process on road surfaces. The indicator assessment applied showed that the road network is not adequately prepared with regard to erosion process countermeasures. It revealed tendencies of road segments to be oriented diagonally to the slope, or even parallel. In regions of high slopes and high degrees of potential water erosion risk, the prevention of erosion processes should be one of the main considerations – in forestry policy decision-making as well.

The collected results of spatial analysis using the IARRR indicator, as compared to visual examinations, indicate the correct application of the method for displaying and assessing forest road network in mountainous terrain. Terrain reconnaissance confirms the accurate depiction of road orientation with regard to the slope, as well as the presence of water erosion processes on roads with unfavourable orientation, primarily roads running parallel and diagonally to the slopes.

The IARRR method gives positive results when implemented in mountain conditions for the assessment of forest roads. It allows a relatively fast and automatic acquisition of spatial information necessary for effective decision-making. The application of this method in the assessment of forest road network with regard to land relief easily identifies road segments most at risk of degradation and places of potential runoff concentration, which may create risk not limited in scope to forestry operations. The method has a great potential as a tool at the planning stage of forest road network, in the verification of planning solutions, as well as in the reconstruction and expansion of the network and also in crisis situations.

Another important aspect is the fact that a full picture of potential erosion process occurrence on forest roads may be achieved through a complex analysis of topographical-hydrological conditions or test by modelling. In the case of terrain conditions, a risk assessment of erosion occurrence should take into account the location of roads in land relief, as well as longitudinal slope and soil conditions. Among the hydrological-meteorological factors, of most importance are the amount and intensity of rainfall, and the concentration and volume of runoff.

Research of the intensity of hydrological and erosion phenomena in forest areas, with special focus on the surface of roads and skid trails, forms a part of the current research trends and Forestry issues. The issue of forest roads, exploitation safety and dynamics of changes is intensely discussed in the scientific community. The research relates to the erosion phenomenon on road surface, the dynamics of hydrological processes, stability of the structure, both in terms of field studies and modelling.

Since 2015 The State Forests National Forest Holding starts a strategic program aimed at analysis and improvement of the conditions of forest road engineering. In particular, the project aims are focused on unpaved roads, density of the road network, their location related to land relief and their construction. The project results define new guidelines for designing and constructing forest roads. This should contribute to an increase of transport safety and a reduction of road maintenance costs, including reducing erosion processes.

The main conclusions of this study are as follows:

1. The IARRR method gives positive results in mountain areas for the erosion risk assessment on forest roads. The method permits relatively fast and automatic acquisition of spatial information necessary for effective decision-making and easily identifies road segments most at potential risk of degradation.
2. The results of the analysis with the IARRR indicator of the road network on forest mountain terrain of a part of the Szklarska Poręba Forest District indicate a relatively unfavourable location of the forest roads in relation to terrain (local relief). The analysis has shown tendencies of road segments to be oriented diagonally or even parallel to the line of maximum slope.
3. Location of the forest roads, especially in the mountain areas and the ones with a high risk of erosion phenomena, should closely follow the technical guidelines for location and construction of forest roads in force for the State Forests. In order to maintain the good condition of the forest roads, their proper maintenance and prevention of degradation should be adequate, in this it can also help the analysis by the IARRR method.
4. Geomorphological risk conditions should be taken into account during the planning of the location of forest roads networks. Identification of places potentially threatened with the risk of intense erosion processes may contribute to the protection against the degradation of these areas.

ACKNOWLEDGEMENT

The research is supported by the cooperation agreement signed by the Szklarska Poręba Forest District and the Institute of Environmental Protection and Development at Wrocław University of Environmental and Life Sciences.

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Accepted for print: 23.01.2017

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Paweł B. Dąbek
Institute of Environmental Protection and Development, Wrocław University of Environmental and Life Sciences, Poland
pl. Grunwaldzki 24
50-363 Wrocław
Poland
email: pawel.dabek@up.wroc.pl

Romuald Żmuda
Institute of Environmental Protection and Development, Wrocław University of Environmental and Life Sciences, Poland
pl. Grunwaldzki 24
50-363 Wrocław
Poland

Tomasz Kowalczyk
Institute of Environmental Protection and Development, Wrocław University of Environmental and Life Sciences, Poland
pl. Grunwaldzki 24
50-363 Wrocław
Poland
email: kowal@miks.ar.wroc.pl

Jolanta Dąbrowska
Institute of Environmental Engineering, Wrocław University of Environmental and Life Sciences, Poland
pl. Grunwaldzki 24
50-363 Wrocław
Poland

Andrzej Moryl
Institute of Environmental Engineering, Wrocław University of Environmental and Life Sciences, Poland
pl. Grunwaldzki 24
50-363 Wrocław
Poland

Ewa Kucharczak-Moryl
Department of Biochemistry, Pharmacology and Toxicology, Wrocław University of Environmental and Life Sciences, Poland
ul. C.K. Norwida 31
50-375 Wrocław
Poland

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