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Planning the Most Appropriate Forest Road Network Considering Soil Drainage and Stability Using GIS in Shafaroud Watershed- Guilan
Ramin Naghdi1, Iraj Bagheri2 , Esmael GhaJar3, Kambiz Taheri1,Iraj Hasanzad1
1Assistant Professor, Department of Forestry, Faculty of Natural Resources, University of Guilan, Sowmehsara, PO Box
1144.
Iran.naghdir@yahoo.com.
2Senior Lecturer, Faculty of Agricultural Sciences, Dept. of Agronomy and Plant Breeding, University of Guilan, Rasht, PO Box
41335-3179, Iran
4Msc student, Dep. of Forestry, College of Natural Resources, University of Guilan, Sowmeasara, P.O.Box 1144,Iran.
Abstract
In this study, the capability of utilizing GIS in improving the accuracy in quality of forest road planning based on soil stability was investigated. Therefore, Janbehsara- a forest district under the administration of Shafaroud watershed authority of Guilan province, northern Iran- was selected as the study area, covering approximately 1849 ha. Extracting and overlaying the slope, aspect, soil texture, and drainage maps was carried out using Arc-GIS ver. 8.3. In order to prepare stability map for five classes, soil texture and drainage maps were merged based on FAO guideline. A questionnaire was prepared in order to get the experts view on effective factors of road planning which include, slope, aspect, altitude, stock per hectare and soil stability. The collected factors were analyzed. These factors were given a value (an impact factor) by comparison method. Based on these given values the road crossing capability map were prepared for three levels of crossing, high, low and medium. Then PEEGER software was used to plan roads on land crossing capability map. The GIS planned road network was compared with current road network. The results show that the GIS planned road network crosses 75.5% of stable areas, while the current road network covers 60% of stable areas. With regards to drainage, the GIS planned road network covers 70% of good drainage areas while the current road network only covers 55.8%.
Introduction
Planning appropriate roads in order to connect the forestry units located in mountainous areas through conventional route projection with dividers methods based mainly on the contour lines would be a very time consuming job. Today, integrative management of available information in the maps and considering these factors in road planning has been possible through GIS. Passing the roads through low to moderate slopes and areas with high level of stability, reduces excavation, earth filling, reduces land stabilization cost, excavation gable roofs and earth filling, and thus reduces construction costs (Kunwoo, 1990).
Using available tools, forest road planners can analyze a variety of road variants rapidly and evaluate environmental and economical conditions using GIS (Rogers, 2005). Many attempts have been made to use GIS in road planning process and it perform different function in each of these projects. In many studies, GIS has been used to include different factors in environmental impact assessment resulted from construction of any potential variant (Ghodsipour, 2002). In other studies conducted recently for forest and mountainous road planning, technical issues and observing road geometric principles are considered (Akay and Karas, 2004).
Reutebuch (1988) presented a software entitled “ROUTES” to estimate the longitudinal slope length, forest length and routing possible variants using DEM.
Akay and Session (2005) have presented the 3D forest path finding model named “TRACER” for rapid evaluation of different paths, aiming to help the planners to plan the primary roads. TRACER is a decision making tool which prepares the planner to rapidly evaluate different paths. Rogers and Schiess (2001) developed the PEGGER software to automatically trace the forest roads using GIS. PEGGER is a strong tool for rapid analyzing of various road variants; and acts based on the percentage of road’s longitudinal slope which is determined by the planner. Ahmadi et al. (2005) in a study named “Path-finding the road based on environmental principles using GIS to construct the eastern Tehran’s ring road” tried to evaluate some features including geology, erosion, soil, slope, land use, water streams, faults and altitude based on questionnaires and in a relative manner.
Musa and Mohamed (2002) implemented the forest road planning using GIS. They compared the GIS based road planning to other methods including field based, office based planning, and the best road’s analysis for a forest road network. Hosseini (2003) in a study which was carried out in kairoodkenar Noshaer (Iran) showed that the road network designed using GIS was more suitable and better than the existing road network in terms of accessibility, skidding distances, environmental damages, construction costs and excavation. Jha (2000) and Ichihara et al (1996) with the use of GIS and the use of generic algorithm were able to design the most suitable road route which had the least construction cost.
With regards to the provided research facilities, and necessity of forest road planning using GIS capabilities, the present study aimed to take into account various factors effective in forest road planning including soil type and texture, drainage and soil stability condition and slope; and utilize them in GIS to present a method for forest road network planning.
Materials and Methods
The study was carried out in Janbehsara, a forest district covering approximately 1849 ha of Shafaroud watershed in Guilan province- northern Iran. The area is located between and of Longitude, and and of latitude. The altitude ranges between 100 to 850 m above sea level.
The methodology selected for the study was the Systemic Analysis (Makhdoum, 1999). Regarding that, base maps and information of the watershed were prepared after a complete orientation of the entire area. The research was done around two main issues: essential and effective parameters on road construction, and stability/ non- stability of the ground surface.
The important factors in road construction which should be considered in the planning phase include slope and aspect. In addition, the factors concerning the ground stability/non-stability mainly include factors such as slope, altitude, aspect, and soil texture. Land classification based on the soil stability and considering it for road planning in order to maximize road coverage (appropriate road density) on the most stable soils requires mapping the associated attributes. Thus, the mentioned factors were mapped using Arc GIS ver 8.3 to be used in road network planning of the district.
Because of relatively large area of the district, it covers almost two adjacent map sheets which were used for contour line extraction as the first step. The extracted layer was capable of being transferred directly to GIS environment for producing slope, aspect, and altitude maps which were produced and classified according to the network planning requirements.
In order to produce soil texture map, information related to land form was needed which was provided by merging slope, aspect, and altitude of the district (Map 2). A number of 18 land form units with distinctive characteristics were formed. In each unit, soil texture test was done based on field survey method. Besides, some samples were transferred to the lab for hydrometric soil texture test.
Therefore, the soil texture map of the study area was yielded after transferring the soil texture results to the land form unit map. In addition, the drainage and soil stability maps were prepared according to FAO guideline and soil texture and aspect of the area. In path finding phase, it was tried to pass the potential paths through positive points (low slope, high soil drainage, high soil stability, high forest stand density and etc.) taking into account the available road density. Pegger extension in Arc view ver 3.1 was used to plan forest road network, this was carried out in confined allowable slopes (3-8%).
Finally, to compare the two road variants (current and GIS based networks), the percentage of their crossing on stable areas based on the drainage and soil stability maps was used.
Results
After collecting required information and processing them using GIS, a series of maps including topographic, slope, altitude, aspect, and land form were extracted. The 5-class slope map to determine the site characteristics for path finding, together with a table showing the distribution of different classes on the slopes were yielded. The altitude map of the area was also prepared in accordance to the low / moderate land border (approximately 600 m above sea level). The aspect map and its associate variance distribution table were yielded.
Land form unit map
In order to produce the soil texture, drainage, and stability maps, the land form unit map was composed of merging slope, aspect, and altitude maps. An initial number of 24 land form units were composed of which 6 units had the area of less than 1 cm2 which equals to 6.25 ha and hence were not considered in planning. So they were merged with their nearest adjacent units. A number of 200 soil samples for soil texture determination based on field survey, and 18 soil samples for soil texture based on hydrometric lab test were selected in the entire study area. The information examined in the remaining 18 units is shown in (Fig 1).
Soil texture map
Transferring the soil texture information to the land form map, the area’s soil texture map was produced. Results show that four soil types exist in the area. The variance distribution of soil texture is shown in table 1.
Drainage map
The 3- class drainage map of the study area ( high – medium and low ) was yielded applying FAO (1984) taking into account slope, aspect, and soil texture data from borehole tests carried out in land form units.
Soil stability map
The soil stability map was prepared merging the drainage and soil texture maps of the studied district (Fig 2). According to the results yielded from merging soil texture and drainage maps and with regards to table 2, classes 2, 3, and 4 are currently available in the area.
Results of investigating different road network variants
Comparison of two road variants (the existing road network and the proposed GIS based network) using drainage and soil stability (FAO, 1984) criterion are shown in fig 3, table 3 and fig 4, table 4. With reference to the mentioned figures and tables the results of this research showed that the road network designed by using GIS compared to the existing road network passes through more areas that have medium and high drainage (34.3% and 33.5% in GIS designed road network and 24.9% and 28.7% in existing road network).Also in GIS designed road network more of the roads pass through high stable soil area (75.5% in GIS designed road network and 60.4% in existing road network). This shows clearly that the new method of designing road network with GIS is preferable to the traditional method. In order to properly compare the two roads networks the optimum road density and other attributes of road quality are important. Therefore effort was made to get the considered density close to available density. In other words the total length of existing roads was 40.49 kilometers and their density was 23 meter per hectare and the total length of suggested roads was 40.86 km and its density was 23.21 m ha-1.
The percentage of network (E %) for existing roads and suggested roads was 61.52 % and 78.13% respectively. The average longitudinal slope for existing roads and suggested roads was 6.5% and 7.2% respectively.
Discussion
Forest road planning is associated with technical and principal points which entail a logical and comprehensive decision making that enables one to achieve several goals in forest. To this end, concentrating on the application of the best methods is essential, particularly in middle management levels of which the technical road construction is a part.
In Iran, in most forestry plans, the traditional method is still being used for path finding and establishing road network. In traditional method the topographic and maybe soil maps are used individually to recognize the area’s features and positive/ negative points. In this research the study and identification of the site for path finding was carried out considering all effective factors which reduces time and costs of road planning.
To find the efficient paths in order to plan the forest road network, geographic information system and the maps derived from that were utilized. Merging the initial maps, the drainage and soil stability maps were yielded. The initial maps were produced aiming to find paths in the areas featuring suitable growing and ground condition to reach the forest road network in the district. It should be noted that it was attempted to pass as much paths as possible on stable areas. On account of the fact that calculating the optimal road density was not of the research objectives, the current existing road density was implemented in the study.
The purpose of the study was to investigate the Janbehsara district in terms of soil drainage and its stability and robustness for forest road construction. The study carried out by Chunkunwoo (1990) in mountainous forest roads showed that passing the road from low sloped areas reduces the excavation and fewer trenches are made in the forest and if more trenches are made then more excavation are needed. Therefore, determining the stability or non stability of different points of an area can considerably help to predict and reduce the construction costs which were achieved in the present study using slope, drainage, and soil stability maps of the study area. In this research the above factors were taken into consideration when carrying out the study and the necessary maps were presented.
In order to avoid road passage from areas featuring steep slopes, low drainage, and low stability the planed paths were possibly adjusted to pass the areas featuring low slope, high drainage, and higher stability. Nevertheless, the permissible slopes were taken into account with regards to forest road type. Finally, a network was attained based on both drainage and soil stability maps. This network has got the appropriate distribution in terms of crossing relatively more stable areas. Considering the areas associated with lower slopes, more soil drainage, stable area and passing the road through high biologic potential areas can improve the prediction of low maintenance and constructing cost of roads, and thus higher efficiency of road construction. The results of this study show that road network designed using GIS can be more accessible than the existing road network. In other words skidding to the side of the roads is carried out in shorter distances. With regard to this Hosseini (2003) in his research in Khiroodkenar Noshar (Iran) also showed that the road network designed using GIS are more accessible than the existing road network and are therefore superior.
Therefore due to more accessibility and also shorter skidding distances in road network designed using GIS, the skidding cost can be expected to be less than the existing road network. On account of the results achieved, the appropriate condition of the planned road network based on the drainage and soil stability maps was approved as long as the rate of road passage through high stable areas has shown to be 60 percent in the current road network, and 75.5 percent for the GIS based network. Moreover, the percentage of the new planned road passage from less stable areas is less than the current road. Owing to this fact, suitability of the planned road in terms of passing more stable points was approved. Due to Experiences and studies show that the paths passing through more stable areas will have less maintenance costs after construction (Sarikhani, 1999). In terms of drainage, it has been concluded that the rate of passage from the areas with suitable drainage is 34 percent for the planed road, comparing to 25 percent for the current existing road. In addition, it was attempted to pass less roads through low drainage classes in planned road network than that of the current existing road. With respect to this Musa and Mohamed (2002) and Hosseini (2003) also in their research showed that road network designed using GIS passes through more area with high drainage and high soil stability and therefore the road network are improved and superior which confirms the efficiency and competence of GIS for quick and more desirable method of designing road network.
References
Ahmadi, H., A.Darvishsafat, M.Makhdom and S.Abolghasemi. (2005). Designing of road based on environmental issues using GIS, In: proceedings of the Geomathic congress, Tehran, Iran, p8.
Akay, A.E., I.R. Karas, J.Sessions, A.Yuksel, N.Bozali, R.Gundogan. (2004). Using high- resolution digital elevation model for computer-aided forest road design, In: proceedings of Geo-Imagery Briding Continents XXth ISPRS congress. Istanbul, Turkey, pp.1090-1095.
Akay, A.E. and J. Sessions. (2005). Apply the decision support system, TRACER, to forest road design, Western Journal of Applied Forestry, 20(3): 184-191.
Kunwoo, C. (1990). Studies on forest road construction in mountain forest, kangwoon national university, Research bulletin of the experimental forest, 16:1996.109-131.
Ghodsipour, S.H. (2002). Analytical Hierarchy Process (AHP), Amirkabir University press, Tehran, Iran.
FAO. (1984). The terrain classification for forestry.Tim/EFC/Wp.1/R.51-34.
Hosseini, S.A. (2003). Planning of forest roads network using GIS, Khiroudkenar forest. Ph.D. thesis, Tarbiat Modares University, Tehran, Iran, P145.
Ichihara, K., T. Tanaka, I. Sawaguchi, S. Umeda, and K.Toyokawa. (1996). The method for designing the profile of forest roads supported by genetic algorithm, The Japanese Forestry Society, Journal of Forestry Research, 1:45-49.
Jha, M.K. (2000). A geographic information system based model for highway design optimization, Ph.D. thesis, University of Maryland, College Park.
Makhdom, M. (1999). Fundamental of land use, Tehran University press, Tehran, Iran. Musa, M.K. and A.N. Mohamed, (2002). Alignment locating forest road network by best-path modeling method, In: proceedings of 23rd Asian conference on remote sensing, Nepal, p 6.
Reutebuch, S. (1988). ROUTES: A computer program for preliminary route location. United States Department of Agriculture, Forest Service, Pacific Northwest Research Station, General Technical Report, PNW-GTR-216, p18.
Rogers, L.R. and P. Schiess. (2001). PEEGER and ROADVIEW: A new GIS tool to assist engineers in operations planning, In: proceedings of the international mountain logging and 11th Pacific Northwest skyline symposium, University of Washington, Seattle, pp.177-182.
Rogers, L.W. (2005). Automating contour based road projection for preliminary forestry road designs using GIS, MSc. thesis, Washington State University, College of Forest Resources, p 59.
Sarikhani, N. (1999).Planning and implementing forest road network guideline, Plan and Budget Organization of Iran, technical report, Tehran, Iran, p110.
Fig1. Land form unites map of Janbesara
Fig 2. Field stability map of Janbesara
Figure 3. Comparison of length of road in percentage on different drainage classes map for two road networks.
Figure 4. Comparison of length of road in percentage on different soil stability classes map for two road networks
Table 1. Frequency distribution of soil texture factor.
Table 2. Classification of soil stability
Table 3.Comparison of two road networks (existing and GIS planned roads) based on the path of different drainage classes.
Table 4. Comparison of two road networks (existing and GIS planned roads) based on the path of different soil stability classes.
Ramin Naghdi1, Iraj Bagheri2 , Esmael GhaJar3, Kambiz Taheri1,Iraj Hasanzad1
1Assistant Professor, Department of Forestry, Faculty of Natural Resources, University of Guilan, Sowmehsara, PO Box
1144.
Iran.naghdir@yahoo.com.
2Senior Lecturer, Faculty of Agricultural Sciences, Dept. of Agronomy and Plant Breeding, University of Guilan, Rasht, PO Box
41335-3179, Iran
4Msc student, Dep. of Forestry, College of Natural Resources, University of Guilan, Sowmeasara, P.O.Box 1144,Iran.
Abstract
In this study, the capability of utilizing GIS in improving the accuracy in quality of forest road planning based on soil stability was investigated. Therefore, Janbehsara- a forest district under the administration of Shafaroud watershed authority of Guilan province, northern Iran- was selected as the study area, covering approximately 1849 ha. Extracting and overlaying the slope, aspect, soil texture, and drainage maps was carried out using Arc-GIS ver. 8.3. In order to prepare stability map for five classes, soil texture and drainage maps were merged based on FAO guideline. A questionnaire was prepared in order to get the experts view on effective factors of road planning which include, slope, aspect, altitude, stock per hectare and soil stability. The collected factors were analyzed. These factors were given a value (an impact factor) by comparison method. Based on these given values the road crossing capability map were prepared for three levels of crossing, high, low and medium. Then PEEGER software was used to plan roads on land crossing capability map. The GIS planned road network was compared with current road network. The results show that the GIS planned road network crosses 75.5% of stable areas, while the current road network covers 60% of stable areas. With regards to drainage, the GIS planned road network covers 70% of good drainage areas while the current road network only covers 55.8%.
Introduction
Planning appropriate roads in order to connect the forestry units located in mountainous areas through conventional route projection with dividers methods based mainly on the contour lines would be a very time consuming job. Today, integrative management of available information in the maps and considering these factors in road planning has been possible through GIS. Passing the roads through low to moderate slopes and areas with high level of stability, reduces excavation, earth filling, reduces land stabilization cost, excavation gable roofs and earth filling, and thus reduces construction costs (Kunwoo, 1990).
Using available tools, forest road planners can analyze a variety of road variants rapidly and evaluate environmental and economical conditions using GIS (Rogers, 2005). Many attempts have been made to use GIS in road planning process and it perform different function in each of these projects. In many studies, GIS has been used to include different factors in environmental impact assessment resulted from construction of any potential variant (Ghodsipour, 2002). In other studies conducted recently for forest and mountainous road planning, technical issues and observing road geometric principles are considered (Akay and Karas, 2004).
Reutebuch (1988) presented a software entitled “ROUTES” to estimate the longitudinal slope length, forest length and routing possible variants using DEM.
Akay and Session (2005) have presented the 3D forest path finding model named “TRACER” for rapid evaluation of different paths, aiming to help the planners to plan the primary roads. TRACER is a decision making tool which prepares the planner to rapidly evaluate different paths. Rogers and Schiess (2001) developed the PEGGER software to automatically trace the forest roads using GIS. PEGGER is a strong tool for rapid analyzing of various road variants; and acts based on the percentage of road’s longitudinal slope which is determined by the planner. Ahmadi et al. (2005) in a study named “Path-finding the road based on environmental principles using GIS to construct the eastern Tehran’s ring road” tried to evaluate some features including geology, erosion, soil, slope, land use, water streams, faults and altitude based on questionnaires and in a relative manner.
Musa and Mohamed (2002) implemented the forest road planning using GIS. They compared the GIS based road planning to other methods including field based, office based planning, and the best road’s analysis for a forest road network. Hosseini (2003) in a study which was carried out in kairoodkenar Noshaer (Iran) showed that the road network designed using GIS was more suitable and better than the existing road network in terms of accessibility, skidding distances, environmental damages, construction costs and excavation. Jha (2000) and Ichihara et al (1996) with the use of GIS and the use of generic algorithm were able to design the most suitable road route which had the least construction cost.
With regards to the provided research facilities, and necessity of forest road planning using GIS capabilities, the present study aimed to take into account various factors effective in forest road planning including soil type and texture, drainage and soil stability condition and slope; and utilize them in GIS to present a method for forest road network planning.
Materials and Methods
The study was carried out in Janbehsara, a forest district covering approximately 1849 ha of Shafaroud watershed in Guilan province- northern Iran. The area is located between and of Longitude, and and of latitude. The altitude ranges between 100 to 850 m above sea level.
The methodology selected for the study was the Systemic Analysis (Makhdoum, 1999). Regarding that, base maps and information of the watershed were prepared after a complete orientation of the entire area. The research was done around two main issues: essential and effective parameters on road construction, and stability/ non- stability of the ground surface.
The important factors in road construction which should be considered in the planning phase include slope and aspect. In addition, the factors concerning the ground stability/non-stability mainly include factors such as slope, altitude, aspect, and soil texture. Land classification based on the soil stability and considering it for road planning in order to maximize road coverage (appropriate road density) on the most stable soils requires mapping the associated attributes. Thus, the mentioned factors were mapped using Arc GIS ver 8.3 to be used in road network planning of the district.
Because of relatively large area of the district, it covers almost two adjacent map sheets which were used for contour line extraction as the first step. The extracted layer was capable of being transferred directly to GIS environment for producing slope, aspect, and altitude maps which were produced and classified according to the network planning requirements.
In order to produce soil texture map, information related to land form was needed which was provided by merging slope, aspect, and altitude of the district (Map 2). A number of 18 land form units with distinctive characteristics were formed. In each unit, soil texture test was done based on field survey method. Besides, some samples were transferred to the lab for hydrometric soil texture test.
Therefore, the soil texture map of the study area was yielded after transferring the soil texture results to the land form unit map. In addition, the drainage and soil stability maps were prepared according to FAO guideline and soil texture and aspect of the area. In path finding phase, it was tried to pass the potential paths through positive points (low slope, high soil drainage, high soil stability, high forest stand density and etc.) taking into account the available road density. Pegger extension in Arc view ver 3.1 was used to plan forest road network, this was carried out in confined allowable slopes (3-8%).
Finally, to compare the two road variants (current and GIS based networks), the percentage of their crossing on stable areas based on the drainage and soil stability maps was used.
Results
After collecting required information and processing them using GIS, a series of maps including topographic, slope, altitude, aspect, and land form were extracted. The 5-class slope map to determine the site characteristics for path finding, together with a table showing the distribution of different classes on the slopes were yielded. The altitude map of the area was also prepared in accordance to the low / moderate land border (approximately 600 m above sea level). The aspect map and its associate variance distribution table were yielded.
Land form unit map
In order to produce the soil texture, drainage, and stability maps, the land form unit map was composed of merging slope, aspect, and altitude maps. An initial number of 24 land form units were composed of which 6 units had the area of less than 1 cm2 which equals to 6.25 ha and hence were not considered in planning. So they were merged with their nearest adjacent units. A number of 200 soil samples for soil texture determination based on field survey, and 18 soil samples for soil texture based on hydrometric lab test were selected in the entire study area. The information examined in the remaining 18 units is shown in (Fig 1).
Soil texture map
Transferring the soil texture information to the land form map, the area’s soil texture map was produced. Results show that four soil types exist in the area. The variance distribution of soil texture is shown in table 1.
Drainage map
The 3- class drainage map of the study area ( high – medium and low ) was yielded applying FAO (1984) taking into account slope, aspect, and soil texture data from borehole tests carried out in land form units.
Soil stability map
The soil stability map was prepared merging the drainage and soil texture maps of the studied district (Fig 2). According to the results yielded from merging soil texture and drainage maps and with regards to table 2, classes 2, 3, and 4 are currently available in the area.
Results of investigating different road network variants
Comparison of two road variants (the existing road network and the proposed GIS based network) using drainage and soil stability (FAO, 1984) criterion are shown in fig 3, table 3 and fig 4, table 4. With reference to the mentioned figures and tables the results of this research showed that the road network designed by using GIS compared to the existing road network passes through more areas that have medium and high drainage (34.3% and 33.5% in GIS designed road network and 24.9% and 28.7% in existing road network).Also in GIS designed road network more of the roads pass through high stable soil area (75.5% in GIS designed road network and 60.4% in existing road network). This shows clearly that the new method of designing road network with GIS is preferable to the traditional method. In order to properly compare the two roads networks the optimum road density and other attributes of road quality are important. Therefore effort was made to get the considered density close to available density. In other words the total length of existing roads was 40.49 kilometers and their density was 23 meter per hectare and the total length of suggested roads was 40.86 km and its density was 23.21 m ha-1.
The percentage of network (E %) for existing roads and suggested roads was 61.52 % and 78.13% respectively. The average longitudinal slope for existing roads and suggested roads was 6.5% and 7.2% respectively.
Discussion
Forest road planning is associated with technical and principal points which entail a logical and comprehensive decision making that enables one to achieve several goals in forest. To this end, concentrating on the application of the best methods is essential, particularly in middle management levels of which the technical road construction is a part.
In Iran, in most forestry plans, the traditional method is still being used for path finding and establishing road network. In traditional method the topographic and maybe soil maps are used individually to recognize the area’s features and positive/ negative points. In this research the study and identification of the site for path finding was carried out considering all effective factors which reduces time and costs of road planning.
To find the efficient paths in order to plan the forest road network, geographic information system and the maps derived from that were utilized. Merging the initial maps, the drainage and soil stability maps were yielded. The initial maps were produced aiming to find paths in the areas featuring suitable growing and ground condition to reach the forest road network in the district. It should be noted that it was attempted to pass as much paths as possible on stable areas. On account of the fact that calculating the optimal road density was not of the research objectives, the current existing road density was implemented in the study.
The purpose of the study was to investigate the Janbehsara district in terms of soil drainage and its stability and robustness for forest road construction. The study carried out by Chunkunwoo (1990) in mountainous forest roads showed that passing the road from low sloped areas reduces the excavation and fewer trenches are made in the forest and if more trenches are made then more excavation are needed. Therefore, determining the stability or non stability of different points of an area can considerably help to predict and reduce the construction costs which were achieved in the present study using slope, drainage, and soil stability maps of the study area. In this research the above factors were taken into consideration when carrying out the study and the necessary maps were presented.
In order to avoid road passage from areas featuring steep slopes, low drainage, and low stability the planed paths were possibly adjusted to pass the areas featuring low slope, high drainage, and higher stability. Nevertheless, the permissible slopes were taken into account with regards to forest road type. Finally, a network was attained based on both drainage and soil stability maps. This network has got the appropriate distribution in terms of crossing relatively more stable areas. Considering the areas associated with lower slopes, more soil drainage, stable area and passing the road through high biologic potential areas can improve the prediction of low maintenance and constructing cost of roads, and thus higher efficiency of road construction. The results of this study show that road network designed using GIS can be more accessible than the existing road network. In other words skidding to the side of the roads is carried out in shorter distances. With regard to this Hosseini (2003) in his research in Khiroodkenar Noshar (Iran) also showed that the road network designed using GIS are more accessible than the existing road network and are therefore superior.
Therefore due to more accessibility and also shorter skidding distances in road network designed using GIS, the skidding cost can be expected to be less than the existing road network. On account of the results achieved, the appropriate condition of the planned road network based on the drainage and soil stability maps was approved as long as the rate of road passage through high stable areas has shown to be 60 percent in the current road network, and 75.5 percent for the GIS based network. Moreover, the percentage of the new planned road passage from less stable areas is less than the current road. Owing to this fact, suitability of the planned road in terms of passing more stable points was approved. Due to Experiences and studies show that the paths passing through more stable areas will have less maintenance costs after construction (Sarikhani, 1999). In terms of drainage, it has been concluded that the rate of passage from the areas with suitable drainage is 34 percent for the planed road, comparing to 25 percent for the current existing road. In addition, it was attempted to pass less roads through low drainage classes in planned road network than that of the current existing road. With respect to this Musa and Mohamed (2002) and Hosseini (2003) also in their research showed that road network designed using GIS passes through more area with high drainage and high soil stability and therefore the road network are improved and superior which confirms the efficiency and competence of GIS for quick and more desirable method of designing road network.
References
Ahmadi, H., A.Darvishsafat, M.Makhdom and S.Abolghasemi. (2005). Designing of road based on environmental issues using GIS, In: proceedings of the Geomathic congress, Tehran, Iran, p8.
Akay, A.E., I.R. Karas, J.Sessions, A.Yuksel, N.Bozali, R.Gundogan. (2004). Using high- resolution digital elevation model for computer-aided forest road design, In: proceedings of Geo-Imagery Briding Continents XXth ISPRS congress. Istanbul, Turkey, pp.1090-1095.
Akay, A.E. and J. Sessions. (2005). Apply the decision support system, TRACER, to forest road design, Western Journal of Applied Forestry, 20(3): 184-191.
Kunwoo, C. (1990). Studies on forest road construction in mountain forest, kangwoon national university, Research bulletin of the experimental forest, 16:1996.109-131.
Ghodsipour, S.H. (2002). Analytical Hierarchy Process (AHP), Amirkabir University press, Tehran, Iran.
FAO. (1984). The terrain classification for forestry.Tim/EFC/Wp.1/R.51-34.
Hosseini, S.A. (2003). Planning of forest roads network using GIS, Khiroudkenar forest. Ph.D. thesis, Tarbiat Modares University, Tehran, Iran, P145.
Ichihara, K., T. Tanaka, I. Sawaguchi, S. Umeda, and K.Toyokawa. (1996). The method for designing the profile of forest roads supported by genetic algorithm, The Japanese Forestry Society, Journal of Forestry Research, 1:45-49.
Jha, M.K. (2000). A geographic information system based model for highway design optimization, Ph.D. thesis, University of Maryland, College Park.
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Fig1. Land form unites map of Janbesara
Fig 2. Field stability map of Janbesara
Figure 3. Comparison of length of road in percentage on different drainage classes map for two road networks.
Figure 4. Comparison of length of road in percentage on different soil stability classes map for two road networks
Table 1. Frequency distribution of soil texture factor.
Table 2. Classification of soil stability
Table 3.Comparison of two road networks (existing and GIS planned roads) based on the path of different drainage classes.
Table 4. Comparison of two road networks (existing and GIS planned roads) based on the path of different soil stability classes.