Notes

Outline

Slide 1

Wood Production Costs 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 Planning Methods 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

Decision Support System 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 Work 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

Landslides and Roads 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

Project Goals 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

The Elliott State Forest 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

Elliott Location

The Elliott Relief Image 54º (138 %) Average slope 18º (32 %) Standard deviation Elevations from near 0 to 640 m (2100 ft)

Elliott Data Layers Roads Streams Culverts Digital Terrain Model Photogrammetrically derived

Topographic Index 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

Topographic Index 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

Applying the Index… 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

Factor of Safety 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" 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

Factor of Safety Assumptions 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

Factor of Safety Results

Factor of Safety Results 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

Project Results 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