AN INDICATOR FOR ESTIMATING THE ARRANGEMENT OF RURAL ROADS IN A TERRAIN RELIEF AND ITS DIGITAL IMPLEMENTATION TO GIS ON THE EXAMPLE OF GRODARZ STREAM WATERSHED

Rafał Wawer

ABSTRACT

The work presents theoretical bases of an indicator for estimating the arrangement of rural roads in a terrain relief. The indicator bases on the comparison between azimuths of agricultural roads and terrain aspect. The physical interpretation of the indicator is meant as planar angle between the road and the theoretical line ideally perpendicular to slope. Thanks to the physical, non-expert basis of the indicator it is possible to choose arbitrary ranges and thresholds by classifying roads to perpendicular-, diagonal- and along- slope arrangement classes.

Key words: rural road, arrangement in relief, GIS, spatial planning, IARRR, terrain relief, loess upland, Grodarz stream..

INTRODUCTION

In strongly eroded terrains, a considerable part of agricultural roads are transformed into ravines, as a result of their inappropriate location in the terrain relief [10, 16]. In some regions, road ravines have typical and general forms testifying large intensification of water erosion. This concerns especially terrains of Polish south-eastern foothills and uplands, with very diverse terrain profiles of ten crumbled by the chessboard of plots as well as majority of loess soils- the most susceptible to superficial rinse-off and wash away.

The arrangement and hardening of rural roads is an essential component of formation of arable areas on eroded terrains [6, 8, 9]. Correctly traced and hardened roads facilitate not only husbandry, but also fulfil the role of anti-erosion meliorations, distracting superficial flows or discharging the excess of waters. The literature recommends locating the rural roads transversely to the slope of a terrain [11, 12, 13]. This is correct from the anti-erosion point of view, although such roads are less functional than those led obliquely to the slope. In practices, however, the diagonal roads, slope-along roads and roads on bottoms of valleculae are the most strongly eroded ones since the most of superficial waters reach them [6, 11].

Recent investigations regarding erosion have been conducted with help of arduous techniques, applied both to test-stand and field investigations, defined conventionally as “analog” ones. The weak side of these techniques is large time consumption as well as limited number of variables, which narrow range of analyses. Together with progressive computerization of scientific investigations as well as the appearance of technology of Geographical Information Systems, new technical and analytic possibilities opened for explorers with practically unrestricted range of theoretical and practical applications. The integrity as well as interactiveness of the GIS environment makes it possible to realize automatic analysis of complex variable combinations [1, 15]. The use of GIS systems in spatial investigations enables also realization of compiled analyses of large areas between drainage basins.

The present article presents results of spatial analyses utilizing an indicator of the arrangement of rural roads in a terrain relief on the example of Grodarz stream watershed on Nałęczów Plateau.

METHODS OF INVESTIGATIONS AND MATERIALS

Area of investigations

The investigations were conducted in 1999-2002 on an area of the upland drainage basin area of Grodarz stream. According to Jahn, the area of Grodarz watershed lies between 2 geomorphologic sub regions of Kazimierz Plateau [7]. The first sub-region presents Kazimierz's loess hills, while the one- loess-less lowering between Męćmierz and Niezabitów villages. According to Maruszczak, the upper part of Grodarz watershed is situated on Bełżycka Plain, while middle and lower parts lie on Nałęczów Plateau [7, 15]. The larger part of the drainage area lies within the borders of Kazimierski Landscape Park and remains under control of Provincial Conservator of Nature. The places with relics of culture and art are additionally under the supervision of Provincial Conservator of Monuments.

The relief of the terrain is diverse, mainly due to distinction of geological surface cover formations which conditioned the development of different geomorphologic forms. The loess-less area of Bełżycka Plain, lying within the watershed limits, is characterized by weakly varied relief, small inclinations and presence of complex of dune forms. The loess areas of Grodarz watershed within Nałęczów Plateau instead, are characterized with strongly varied relief and micro-relief as well as huge inclinations of the slopes.

In majority, the area of Grodarz stream watershed is used as an agricultural land. A large part is also covered by afforestations, particulaly occuring in direct nearness of gullies and on ornate slopes, while the share of pastures and orchards is relatively small. The arrangement of the land demonstrates characteristic chessboard features (see phot. 1).

There are 181.8 km of agricultural roads with diverse location in the terrain relief appearing on the watershed area. The roads with a perpendicular-to-slope arrangement are most often the cause of formation of road gullies (phot. 2) and sometimes of also slope- and valley- gullies. The dynamics of sinking of roads into land surface on loess terrains amounts to about 5 cm/year [10]. The road gullies act as deep draining ditches that make tillage difficult, while the earth material washed from the ravines silts meliorative devices, ditches as well as transportation system of the Town of Kazimierz Dolny located in the valley of Grodarz stream [7, 15].

Source of data

The basis of spatial analyses of Grodarz stream watershed was its digital model consisting of different groups of spatial layers created through digitalisation and digital processing of analog data [2, 14], such as:

1. topographical map of 1:10 000 scale;

2. aerial ortho photo map from a target run in May 1996, with scale 1:26 000 and digital resolution of 1m;

The analog data were transformed into a digital form through:

1. Digitalisation of "the road net on the screen” from a topographical map in scale 1:10 000 by the ArcView Š 3.1 programme with division into individual utility classes ofroads [2, 15];

2. "On- screen” digitalisation [2, 14] of contour lines- to create a base for DEM with spatial resolution of 10m, by malining use of the TOPOGRID method [4].

Method of investigations

The proposed method of tracing agricultural roads in a terrain relief bases on comparison between azimuths of internodal road sections and terrain aspect. The aspect is understood as the steepest downward direction from each cell to its neighbours [2, 5]. The values of the aspect grid represent the compass direction of the aspect; 0 is true north, 90 degree aspect is east, and so forth. For purposes of the analysis of arrangement of agricultural roads in the relief, a semi-automatic digital implementation was worked out and adopted to the ArcView GIS 3.2 system.

Vectorized agricultural roads with the ground surface were divided initially into sections through breaking in points of intersection of contour lines, between which the terrain slope is assumed to be constant. The accuracy of the method depends tightly on the assumed contour-cut, which should be matched suitably to the variability of the terrain relief.

For Grodarz stream watershed, the cut was assumed at a level of 5 meters, which is a compromise between good representation of longitudinal slopes of the roads in the flat and wavy terrain characteristic for the plateu of Bełżycka Plain as well as the bottom of Grodarz valley and the character of strongly wrinkled and cut loess upland of Nałęczów Plateau. One should emphasise that all analyses were conducted with the same cut of agricultural roads by contour lines maintained.

The information about azimuths of internodal sections of agricultural roads was obtained automatically through utilization of the script Line.FromandToAz [3].

Next, every internodal section of the agricultural road was covered with an aspect grid layer, assigning values of the terrain aspect to the road database. Values of azimuths of agricultural roads and terrain aspect were transformed to range from- 90 to 90 degrees, with 0 for the N direction. In the next step, the difference between the road azimuths and terrain aspect in the central point of individual sections of roads was calculated. The resulting difference represents a flat horizontal angle between a segment of the road and terrain aspect, i.e. the line with the highest slope, which can be considered as the line lying ideally with the slope. Through transposition it is possible to import this value to an angle between the road section and line ideally perpendicular to slope, representing simultaneously the zero slope.

The angle assumed arbitrary values in the range from 0 to 90 degrees. The roads with the angle of 0 degrees should be accepted as slope-perpendicular, whereas roads with the angle of 90 degrees as ideally slope-along.

The resulting angle represents the Indicator of Arrangement of Rural Roads in Relief (IARRR).

Results of investigations

Basing on the Digital Terrain Model as well as on the vectorial layer representing a net of agricultural roads, with the methodology worked out by author, one has received a map of the arrangement of non-hardened agricultural roads in the terrain relief.

Analyses (Figs. 1, 2), carried out with the semi-automatic method show large differentiations in internodal sections of agricultural roads. For the assumed range of IARRR on the level of 22.5 degrees, the total length of sections along- and perpendicular- to slope amounts to 38.8 km and 33.4 km, respectively (Tab. 1), whereas for the tolerance on the level of 10 degrees to 29.6 km and 12.3 km (Tab. 2). The length of sections ideally slope-along (the angle equals 90 degrees) and slope-perpendicular (the angle equals 0 degrees) amounts to 900 m and 1213 m, respectively.

DISCUSSION OF RESULTS

The analyses show a considerable scale of the problem of creating a road net on the area of Grodarz watershed. The road sections with the slope-along arrangement (with angle below 10 degrees) occur as much as 19% of all agricultural roads. The share of slope-perpendicular roads is found to be 7.5 %.

The digital implementation of the indicator of the arrangement of agricultural roads in the terrain relief, using the Surface Tools for Points, Lines and Polygons extension, assumes intermediation of road azimuth and terrain aspect values basing on the weight of the length. The results of investigations, however, do not show any occurrence of irregularities in the projection of roads arrangement in the relief (see Fig. 3).

The applied method and its digital implementation poses an improvement of the method of defining the arrangement of roads through a supervised classification (15). Both methods have- upon diametrically different solution of digital implementation- analogous theoretical basis, however the results differ considerably, both with respect to the values, and their statistical distribution. Those differences arise, first of all, from large errors in the estimation of the azimuth of roads in the considered method, amounting 22.5 degrees (15).

CONCLUSIONS

1. The used indicator allows for maling free assumptions on thresholds, and developing criterions for positioning of agricultural roads in a terrain relief.

2. Results of application of the indicator for Grodarz stream watershed do not show incorrectnesses in the assumed method and its digital implementation.

3. Results indicate a substantial scale of the problem of location of roads in the relief on the studied area, the more so as the loess part of Grodarz stream watershed is strongly susceptible to superficial water erosion.

4. The method can be used in spatial investigations connected with complex anti-erosion management of upland areas.

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