| The costs of transporting timber from the forest to the mill are among the largest costs associated with wood production | |||
| Fixed and variable costs associated with forest roads | |||
| Road design | |||
| Road construction | |||
| Maintenance | |||
| The efficiency of a road network in meeting operation goals | |||
| Traditional transportation planning methods | ||
| Engineers work with topographic maps | ||
| Identify route alternatives | ||
| Rank route alternatives | ||
| Not efficient and sometimes not possible with large road networks | ||
| Time constraints | ||
| Network too large | ||
| Difficult to develop and select from a full set of alternatives | ||
| An automated system to assist planners in identifying and selecting transportation routes | ||
| The development of a decision support system might involve a combination of several capabilities | ||
| GIS to accommodate spatial considerations | ||
| Statistics to aid in evaluating decision outcomes | ||
| Heuristic to assist in developing and sorting through multiple alternatives | ||
| Previous efforts at building decision support systems | ||
| Reutebuch (1988) ROUTES | ||
| Shenglin (1990) Cost-benefit ratio | ||
| Liu and Sessions (1993) Minimizing construction, transport, and maintenance costs | ||
| Epstein (1999) PLANEX | ||
| None of these considered landslide-prone terrain | ||
| Of all forest land uses, roads, on a per unit-area basis, are the largest contributor to landslides (Sidle et al. 1985) | |||
| Forest landslide impacts include | |||
| Safety | |||
| People | |||
| Structures | |||
| Increased erosional processes | |||
| Increased delivery of sediment into streams | |||
| Loss of aquatic habitat | |||
| A decision support system for transportation planning that can take landslide prone terrain into account could help forest managers | |||
| Create a decision support system for the optimization of route selection based on operational constraints | ||
| Entry and destination points | ||
| Road construction, transportation, and maintenance costs | ||
| Incorporate landscape slope stability ratings as a support system parameter | ||
| Minimize routing through high-risk terrain | ||
| Reestablish or create a road network system in relatively stable terrain | ||
| Located in Oregon Coastal Range | ||
| 376 km2 (93,000 acres, 145 square miles) | ||
| 885 km (550 miles) of roads | ||
| An actively managed forest: | ||
| 41 million board feet harvested in FY 2000 | ||
| A well-developed and available GIS database | ||
| 54º (138 %) Average slope | |
| 18º (32 %) Standard deviation | |
| Elevations from near 0 to 640 m (2100 ft) |
| Roads | ||
| Streams | ||
| Culverts | ||
| Digital Terrain Model | ||
| Photogrammetrically derived | ||
| a / sin b | ||
| (Beven and Kirkby 1979) | ||
| a represents a grid cell’s upslope contributing area per contour length | ||
| sin b is the local slope of the grid cell | ||
| The index calculates drainage area and the ability of the landscape to accommodate hydrologic flow | ||
| The index defines potential areas of high saturation and runoff | ||
| The topographic index serves as the basis for many hydrologic models | ||
| The index tends to increase as upslope contributing area increases | |
| The index decreases as local slope decreases | |
| Grid cells that have similar values for (a / sin b) are expected to be similar hydrologically |
| DTM is necessary | |||
| A continuous hydrologic surface is created from the DTM | |||
| “Sinks” are removed so that hydrologic flow simulations do not become trapped in a portion of the landscape | |||
| Modeling constraints | |||
| Hydrologic flow constrained by roads | |||
| Flow paths were not allowed to cross roads in our simulations unless… | |||
| Culverts allowed to redirect hydrologic flow through roads | |||
| A Factor of Safety (FS) is a ratio of stabilizing to destabilizing forces that provides a relative rating of stability across a landscape | ||
| We used a FS formula developed by Pack et al. (2000) to provide a deterministic FS for all roads in the Elliott | ||
| Based on the infinite slope stability model | ||
| Requires a topographic index to assess hydrology | ||
| C = soil cohesion | |
| q = local slope | |
| T/R = transmissivity ratio | |
| a/sin b = topographic index | |
| wsr = water/soil density ratio | |
| f = internal soil friction angle |
| Soil cohesion, T/R, wsr, and the internal friction angle were constants throughout the study area | |||
| C = 0.25 | |||
| T/R = 0.00033 or 1/3000 | |||
| Wsr = 0.5 | |||
| f = 38° | |||
| Slope and topographic index parameter values varied according to local and upslope topography | |||
| FS values less than 0.5 indicate a slope that is more likely to fail | |||
| FS values in excess of 0.5 indicate slopes that are less likely to fail | |||
| Range was predominantly between 0.26 and 3 | ||
| Some extreme values (>3) were derived along ridge tops and where slopes were nearly flat (about 3.5% of cells representing roads) | ||
| With extreme values removed: | ||
| 0.88 FS average | ||
| 0.44 FS standard deviation | ||
| Roads along ridge tops and in areas of mild slopes tended to have higher (more stable) FS values | ||
| We combined a topographic index with a FS equation to rate the relative stability of Elliott Forest Roads | ||
| Used culverts to redirect overland flow paths | ||
| The road system appears to be relatively susceptible to slope failure | ||
| These initial results provide the pathway for assisting transportation planners | ||
| Identification of road network segments that are most prone to failure | ||
| Planners can lessen or avoid road use in these areas | ||
| Provide support for developing additional networks | ||
Future Optimization Applications
| Transportation planners could examine existing road networks | |||
| Minimize costs: | |||
| Maintenance (regrading, resurfacing) | |||
| Impacts to other resources | |||
| Aquatic habitat | |||
Future Optimization Applications
| Planners could also consider the design of new road networks | |||
| Optimization could be directed toward: | |||
| Minimizing travel distances | |||
| Avoid failure prone terrain | |||
| May involve trade-offs with minimizing travel distances | |||
| Potential maintenance costs | |||