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How Does Decommissioning Forest Roads Effect 

Hydrologic and Geomorphic Risk?

Black, T.; Luce, C.; Cissel, R. M.; Nelson, N.; Staab, B.

American Geophysical Union, Fall Meeting 2010, abstract #EP41B-0693

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 midelevation 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. 

NASA ADS: How Does Decommissioning Forest Roads Effect Hydrologic and Geomorphic Risk? Page 1 of 2

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Keywords: [1815] HYDROLOGY / Erosion, [1834] HYDROLOGY / Human impacts, [1879] HYDROLOGY / Watershed 

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 The ADS is Operated by the Smithsonian Astrophysical Observatory under NASA Grant NNX09AB39G 

NASA ADS: How Does Decommissioning Forest Roads Effect Hydrologic and Geomorphic Risk? Page 2 of 2

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