EFFECTIVENESS OF ROAD RIPPING IN RESTORING INFILTRATION CAPACITY OF FOREST ROADS

Many forest roads are being closed as a step in watershed restoration. Ripping roads with subsoilers or rock rippers is a common practice to increase the infiltration capacity of roads before closure. When cons idering the effectiveness of ripping for reducing runoff and erosion a nd the potential reduction in slope stability by saturating road fills , it is important to know how ripping changes the infiltration capacit y of forest roads. Hydrographs from simulated rainfall on 1 x Im plots were analyzed to find the saturated hydraulic conductivity, an indica tor of infiltration capacity. I examined saturated hydraulic conductiv ity for three treatments on two different soils. One road was built in a soil derived from the metamorphic belt series geology of northern I daho, a soil noted for its high rock fragment content. The second road was built in a sandy soil derived from decomposed granitics of the Id aho batholith. On each soil, five plots were installed on a road befor e ripping, and nine plots were installed on the same road segment foll owing ripping, four covered with a heavy straw mulch and five without. Three half-hour rainfall events with intensities near 90 mm/hr were s imulated on each plot. Results show that ripping increases hydraulic c onductivities enough to reduce risk of runoff but does not restore the natural hydraulic conductivity of a forested slope. The unripped road surfaces had hydraulic conductivities in the range of 0-4 mm/hr, wher eas ripped roads were in the range of 20-40 mm/hr after the second eve nt. Surface sealing and tilled soil subsidence processes are important in reducing the hydraulic conductivity of the soils with repeated wet ting. Subsidence appears to be important on the granitic soil, whereas surface sealing was more important an the belt series soil.