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<title>NASA ADS: How Does Decommissioning Forest Roads Effect Hydrologic and Geomorphic Risk?</title>
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<h1 class="title">
How Does Decommissioning <span class="a">Forest</span> <span class="a">Roads</span> Effect Hydrologic and Geomorphic Risk?</h1>
<div class="authors">
<a href="http://adsabs.harvard.edu/cgi-bin/author_form?author=Black,+T&fullauthor=Black,%20T.&charset=UTF-8&db_key=PHY">Black, T.</a>; <a href="http://adsabs.harvard.edu/cgi-bin/author_form?author=Luce,+C&fullauthor=Luce,%20C.&charset=UTF-8&db_key=PHY">Luce, C.</a>; <a href="http://adsabs.harvard.edu/cgi-bin/author_form?author=Cissel,+R&fullauthor=Cissel,%20R.%20M.&charset=UTF-8&db_key=PHY">Cissel, R. M.</a>; <a href="http://adsabs.harvard.edu/cgi-bin/author_form?author=Nelson,+N&fullauthor=Nelson,%20N.&charset=UTF-8&db_key=PHY">Nelson, N.</a>; <a href="http://adsabs.harvard.edu/cgi-bin/author_form?author=Staab,+B&fullauthor=Staab,%20B.&charset=UTF-8&db_key=PHY">Staab, B.</a></div>
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American Geophysical Union, Fall Meeting 2010, abstract #EP41B-0693</div>
<blockquote class="abstract">
The US <span class="a">Forest</span> Service is investigating <span class="a">road</span> decommissioning projects to
understand how treatments change hydrologic and geomorphic risks. <span class="a">Road</span>
treatment effect was measured using a before after control impact design
(BACI), using the Geomorphic <span class="a">Road</span> Analysis and Inventory Package
(http://www.fs.fed.us/GRAIP). This suite of inventory and analysis tools
evaluates: <span class="a">road</span>-<span class="b">stream</span> hydrologic connectivity, fine sediment production
and delivery, shallow landslide risk, gully initiation risk, and risks
associated with <span class="b">stream</span> crossing failures. The Skokomish River study site
is steep and wet and received a high intensity treatment including the
removal of <span class="b">stream</span> crossing pipes and fills, all ditch relief pipes and a
full hillslope recontouring. <span class="a">Road</span> to <span class="b">stream</span> hydrologic connectivity was
reduced by 70%. The treatments reduced fine sediment delivery by 21.8
tons or 81%. The removal of the <span class="b">stream</span> crossing culverts and large
associated <span class="a">road</span> 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 <span class="a">road</span> 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 <span class="a">road</span> 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 <span class="a">road</span> drainage features
including <span class="b">stream</span> crossing pipes and fills, and recontoured the
hillslope. The length of <span class="a">road</span> that was hydrologically connected to
<span class="b">streams</span> 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 <span class="b">stream</span> crossings becoming plugged was eliminated. The
potential for <span class="b">streamflow</span> diversion onto <span class="a">roads</span> and hillslopes was
precluded. The slope stability risk below drain point locations on the
original <span class="a">road</span> 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
<span class="a">road</span> decommissioning can be effective at reducing the risk of <span class="a">road</span>
sediment delivery, hydrologic connectivity and failures associated with
<span class="b">stream</span> crossings. Post storm monitoring will help validate these
predictions and reduce uncertainty around the hydrology of
decommissioned <span class="a">roads</span>. If decommissioned <span class="a">roads</span> continue to concentrate
water and discharge it onto steep slopes, landslides and gully risk may
remain elevated.
</blockquote>
<div class="keywords">
Keywords: [1815] HYDROLOGY / Erosion, [1834] HYDROLOGY / Human impacts, [1879] HYDROLOGY / Watershed
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