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# How Does Decommissioning ForestRoads Effect Hydrologic and Geomorphic Risk?

Black, T.Luce, C.Cissel, R. M.Nelson, N.Staab, B.
> The US Forest Service is investigating road decommissioning projects tounderstand how treatments change hydrologic and geomorphic risks. Roadtreatment effect was measured using a before after control impact design(BACI), using the Geomorphic Road Analysis and Inventory Package(http://www.fs.fed.us/GRAIP). This suite of inventory and analysis toolsevaluates: road-stream hydrologic connectivity, fine sediment productionand delivery, shallow landslide risk, gully initiation risk, and risksassociated with stream crossing failures. The Skokomish River study siteis steep and wet and received a high intensity treatment including theremoval of stream crossing pipes and fills, all ditch relief pipes and afull hillslope recontouring. Road to stream hydrologic connectivity wasreduced by 70%. The treatments reduced fine sediment delivery by 21.8tons or 81%. The removal of the stream crossing culverts and largeassociated road fills eliminated the risk of pipe plugging relatedfailures and the eventual erosion of over 4,000 m3 of fill. The slopestability risk was assessed using a modified version of SINMAP (Pack etal, 2005). Risk below drain point locations on the original road wasreduced as water was redistributed across the hillslope to waterbars anddiffuse drainage. It is unclear; however, if landslide risk was reducedacross the entire treated road length because treatments slightlyincreased risk in some areas where new concentrated drainage featureswere added above steep slopes. Similarly, values of a gully index ESI(Istanbulluoglu et al, 2003), were reduced at many of the originaldrainage points, however some new drainage was added. ESI values stillexceed a predicted conservative initiation thresholds at some sites,therefore it is uncertain if gully risk will be changed. Mann Creekoccupies a moderately steep mid-elevation site in Southern Idaho. Thehigh intensity treatments removed all constructed road drainage featuresincluding stream crossing pipes and fills, and recontoured thehillslope. The length of road that was hydrologically connected tostreams was reduced by 2,923 m, or 97%. The model predicts that finesediment delivery was reduced by 98%, to 1.0 ton annually. The riskpresented by stream crossings becoming plugged was eliminated. Thepotential for streamflow diversion onto roads and hillslopes wasprecluded. The slope stability risk below drain point locations on theoriginal road was reduced as water was no longer concentrated anddischarged through a single drainage feature. Treatments are predictedto return slope stability to near undisturbed levels. Gully initiationrisks, already low prior to treatment, may be reduced to negligiblevalues. Results from these two case studies suggest that high intensityroad decommissioning can be effective at reducing the risk of roadsediment delivery, hydrologic connectivity and failures associated withstream crossings. Post storm monitoring will help validate thesepredictions and reduce uncertainty around the hydrology ofdecommissioned roads. If decommissioned roads continue to concentratewater and discharge it onto steep slopes, landslides and gully risk mayremain elevated.

Smithsonian Astrophysical ObservatoryNASA