A Computer Model to Differentiate Skidder and Cable-Yarder Based Road Network Concepts on Steep Slopes

Abstract

Road spacing on slopes depends on the underlying off-road transportation technology. One major decision in road network plan-ning is to determine under what terrain conditions ground-or cable based extraction systems should be applied. The present inves-tigation aims to develop a road spacing model for steep slope conditions and to implement a total cost model for skidder and cable-yarder based road network concepts. The study analyzes transportation and road geometry to specify the relationship between road density, slope gradient, and road spacing. Production functions for skidder and yarder-systems make it possible to derive trans-portation cost as a function of road density and slope gradient. A total cost function integrates road building cost, harvesting strat-egy, and production economics to derive optimal road density for the two network concepts. The difference between the cost lev-els at optimum road density is an indicator for differentiating cable and skidder-based extraction systems. The model was imple-mented as a Visual Basic add-in for Microsoft Excel spreadsheet software. This flexible approach makes future adaptations and changes very easy due to the modular concept. The validity of the model is limited to the production functions of the underlying off-road transportation technologies. Future work needs to develop production functions for the state-of-the-art technologies and to improve the road building cost model. The layout of a forest transportation network is a deci-sion that commits technical feasibility of off-road transport and harvesting cost for a long time. In flat terrain ground-based technology is the predominant approach to design forest har-vesting systems. On steep slopes cable-based and aircraft-based technologies are often advantageous alternatives. One of the most important problems of transportation planning in mountainous terrain is where to use ground-based, cable-based or aircraft-based extraction concepts. Optimizing road spacing is a problem in forest transporta-tion planning which dates back to the classical work of Matthews (1942). Almost all the scientific work treating road spacing was carried out for flat terrain conditions con-sidering specific off-road transportation technologies. In steep terrain Abegg's (1988) investigation seems to be the only attempt to differentiate ground-and cable based transportation concepts. An analytical approach formulating road spacing models for steep slope conditions is still missing. Such an approach would be very important to evaluate different har-vesting situations. Deriving the optimal concept may restrict the search for a spatial feasible solution to the most sensible idea. The present investigation aims (1) to develop an analytical road spacing model for steep slope conditions, and (2) to evaluate the model for specific harvesting situations. Landings are omitted because they are not relevant for European con-ditions. The model evaluation focuses on silvicultural and technological circumstances in Central Europe, but could easily consider conditions in other regions. The paper gives first a survey on previous work about road spacing models, then analyzes the new steep slope model and finally evaluates