Viii lid technology: case studies and watershed restoration
Figure 1. Typical profile along a stepped infiltration swale
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- Figure 2. Typical longitudinal profile illustrating
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Figure 1. Typical profile along a stepped infiltration swaleTo further promote long-term stability of the trail fill, buried woody debris dams, lodged in place within the trail cross section, are installed at irregular intervals along the filled trail profile (see Figure 2). These are intended to help buttress the fill; as they rot away, the ground surface will settle, creating microtopographic depressions mimicking natural root pits. Larger logs are also installed (preferentially along contours), which immediately create depression storage (and moist planting microsites) to prevent continuous surface runoff in the unlikely event of saturation-excess overland flow (Figure 2). Taken together, all of these measures re-create a hydraulically rough and water-receptive surface which is intended to mimic the deep forest floor and “pit-and-mound” microtopography characteristic of pristine temperate forest landscapes (Hack and Goodlett, 1960; Hewlett, 2003). By restoring hillslope depression storage and enhanced infiltration opportunity, rapid and erosive surface runoff is replaced by infiltration and slow subsurface flow within the former trail footprint. Contour Soakage Trench. One of the key methods of reducing runoff and erosion potential on active trails and unpaved roadways is to undulate the path or road surface with “rolling dips,” which are minor inflections in the travelway. These divert surface runoff from the compacted traveled surface into “turnouts” which then typically discharge to the naturally vegetated hillside. Successive rolling dips reduce erosion potential by reducing slope length and removing surface flow from the travelway before it can gather speed. 
Figure 2. Typical longitudinal profile illustratingtrail closure and re-naturalization techniquesWhen hillslopes are in a healthy condition hydrologically, the runoff diverted from the trail tread is quickly infiltrated and routed to shallow subsurface flow. Unfortunately, water-receptive hillside surfaces with the characteristics described above (deep forest floor, microtopographic complexity) are uncommon in WVP. Because of this, diverting surface runoff from trails or roadways onto such degraded slopes can simply move concentrated runoff and surface erosion from one location to another. The installation of what we call “contour soakage trenches” is intended to very locally restore some measure of the highly water receptive conditions associated with a pristine forest floor. This allows water to be diverted from the traveled path without destabilizing the adjacent hillside. In order to illustrate the utility of this technique, these treatments were employed along both filled trail segments (as a “fail safe” measure) and along unpaved travelways which must remain in active use. A contour soakage trench consists of a shallow hillside “trench” (linear depression) laid out along slope contours on one side of the trail. A dip in the trail tread is created so that trail runoff is quickly routed to the trench, whether directly or through a short turnout, which may or may not need to be armored. A constructed berm on the downslope side of the trench increases capacity and corresponds with the hump part of the rolling dip in the trail or roadway. A longitudinal profile through a typical contour soakage trench, taken a short distance away from and more or less parallel to the trail, is shown in Figure 3. To construct a contour soakage trench, the shallow depression is excavated first, with this material then used to construct the low downslope berm (Figure 3). The bottom of the trench is ripped to promote infiltration and the entire disturbed area is then covered with a deep layer of woody mulch. Ideally, downed logs, limbs, and plantings are added to this treatment to further naturalize it. All kinds of variations in this theme are possible. For example, large logs, shallowly embedded along the contour, can be used to create upslope depressions with little or no actual excavation. Existing natural trailside depressional areas, such as large old root pits, can also be exploited (and enlarged if necessary) for this purpose. ConclusionsForested hillsides in essentially undisturbed terrain have long been known to act as a veritable sponge, making them virtually immune to surface runoff and erosion (Hack and Goodlett, 1960; Hewlett, 2003). This is far less an effect of the trees themselves as it is related to conditions at ground level. Uncompacted forest soils with deep and extensive root systems, the high microtopographic roughness provided by fallen logs and the pits left by upturned trees, and a deep forest floor (duff and litter layer) are mainly responsible for runoff retardation, not the forest canopy. Unlike natural forests, forest-covered urban parks have usually been subject to extensive surface alterations by people. Microtopography has often been largely erased and soils substantially altered by past logging or grazing. Changes to the sub-canopy ecosystem may have also left the forest floor and ground layer vegetation significantly depleted. To this can be added the effects of historically altered hillslope profiles (cuts and fills), the presence of roads and parking areas, and often excessively extensive (and usually poorly planned) trail systems. As a result of all of these effects, semi-natural urban parks can be surprisingly significant sources of both rapid surface runoff and the fine sediment generated by surface water erosion. As demonstrated within WVP, a number of relatively simple techniques can be employed to minimize these impacts, thereby restoring the quality of such areas for all users and protecting downstream water quality. Stormwater-induced channels can be converted into rock-filled infiltration trenches to both prevent further gully enlargement and detain runoff. Stepped infiltration swales can be naturalized and disguised by adding large wood and plantings to their margins. Outside of channels, over-steep trails to be retired can be converted back to water receptive hillsides. Where trails and roadways must be retained, runoff can be diverted from these surfaces by installing rolling dips. 
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