Viii lid technology: case studies and watershed restoration
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- Background
- Selected Rehabilitation Techniques
- Figure 1. Typical profile along a stepped infiltration swale
Coupling Stormflow Attenuation with Gully and Trail Stabilization, Wissahickon Valley Park, PhiladelphiaTodd Moses1, M.A., Michael Lower2, P.E., Gerald Longenecker3, P.E., Daniel Aungst4, P.E. 1 Geomorphologist and Restoration Specialist, Skelly and Loy, Inc., 449 Eisenhower Boulevard, Harrisburg, PA 17111; tmoses@skellyloy.com 2 Environmental Engineer, Skelly and Loy, Inc., 449 Eisenhower Boulevard, Harrisburg, PA 17111; mlower@skellyloy.com 3 Vice President of Environmental Engineering, Skelly and Loy, Inc., 449 Eisenhower Boulevard, Harrisburg, PA 17111; glongenecker@skellyloy.com 4 Environmental Engineer, Skelly and Loy, Inc., 449 Eisenhower Boulevard, Harrisburg, PA 17111; daungst@skellyloy.com Abstract A variety of trail and channel stabilization and runoff reduction techniques were implemented within Philadelphia’s Wissahickon Valley Park in Philadelphia. In addition to “conventional” bioinfiltration systems (not discussed here), the methods employed and described in some detail below include 1) the replacement of actively enlarging gullies below stormwater outfalls with stable “stepped infiltration swales”; 2) the obliteration and hydrologic restoration of steep, improperly routed trails; and 3) the installation of trailside microtopographic basins, “contour soakage trenches”, designed to accept and infiltrate runoff from active trails and unpaved roadways. These best practices have wide application within semi-natural urban nature parks generally, many of which are seriously impaired hydrologically because of land use history, intensive human use today, and ongoing urban development along their periphery. BackgroundLocated in north Philadelphia, lower Wissahickon Creek flows through a deep gorge which has long been preserved within an 1,800-acre semi-natural urban nature reserve known as Wissahickon Valley Park (WVP). Although thickly forested, with many very large and old trees, both rapid runoff and erosion are prominent in many areas of this park. Gully erosion, along with channel erosion in steep tributary streams, has occurred because of stormwater drainage into the park from intensively developed areas located along its perimeter. Surface water erosion is occurring on unpaved utility roads within the park (which also serve as multi-use trails) and gullies have formed where stormwater, gathered from paved roads and parking areas, has been allowed to discharge onto unprotected slopes, which in WVP are mostly very steep. WVP is also laced with 50 miles of dedicated trails, the majority of which are natural surface paths which are heavily utilized by pedestrians, bicyclists, and equestrians. Many paths are essentially unplanned social trails, and the combination of poor trail routing and heavy mountain bike and pedestrian traffic on the steepest trail segments has led to serious and widespread trail erosion within the park. Many fall-line trails (steep paths routed across slope contours), especially those which traverse natural hillside hollows, have developed into deep, water-concentrating gullies. All of these circumstances are speeding delivery of water and fine sediment to Wissahickon Creek, which has already been significantly impaired by excess sediment before it enters the park. In order to counter these problems, the Philadelphia Parks and Recreation Department (PPRD), the Friends of the Wissahickon (FOW), and the Philadelphia Water Department (PWD) have joined forces and are all presently engaged in stream channel, gully, and trail rehabilitation projects throughout this historic park. As funds become available, unstable trails are being rebuilt, re-routed, or eliminated by the PPRD and FOW. The PWD’s interest in the WVP stems from the fact that it manages and maintains many miles of water and sanitary sewer lines within park. Both gully erosion and stream erosion threaten this vital infrastructure, which for the most part is very old. The FOW, a nearly 90-year old conservation organization, has as its focus the preservation and enhancement of Wissahickon Valley Park in its entirety. The simple stormwater management (SWM) and gully and trail stabilization measures described here have been designed and installed by Skelly and Loy, Inc. as part of a project sponsored by the FOW. The methods developed by Skelly and Loy are robust and are intended to cope with the relatively infrequent but very high-magnitude rainstorms which do most of the geomorphic work within WVP. Selected Rehabilitation TechniquesStepped Infiltration Swale. This application was devised and first employed by Skelly and Loy a few years ago in a gully stabilization project undertaken by PWD within a part of WVP known as Carpenter’s Woods. Here (as in many other areas in the park), large permanent gullies have formed directly below stormwater outfalls where surface runoff collected in storm sewers from adjacent built-up areas was simply unleashed onto unprotected slopes. (These practices date from as much as 100 years ago, when there was little concern for the impacts of surface runoff to the park or its streams. The PWD is today actively engaged in reversing these conditions throughout the City.) Such permanent “wet” gullies (i.e., gullies permanently subject to periodic stormflow discharges) below outfalls can be stabilized with check dams and by regrading and stabilizing oversteepened banks. However, this approach provides no SWM function beyond somewhat lower flow depth and energy dissipation at the grade-control overfalls. In contrast, repairing these gullies using the stepped infiltration swale approach both eliminates erosion and provides runoff attenuation and enhanced infiltration opportunity. This combined erosion control and SWM treatment consists of a series of stacked boulder sills which span the gully cross section. The “cells” between successive boulder sills are filled to near the top of the gully with smaller (but still very coarse) fragmental rock (Figure 1). Finally, the remaining banks adjacent to the filled gully are shaved back and planted to woody vegetation. The boulder sills hold the smaller rock in place and create a naturalistic stair-step channel profile that will dissipate flow energy in the event of a rare storm capable of generating surface flow. In the case of all other storms, discharge from the outfall is routed immediately to the subsurface, where flow through this coarse rock “filter” both slows the flow and encourages infiltration into the floor of the former gully (Figure 1). The net effect of this is the elimination of fluvial erosion and reduced flow rate at the downstream end of the installation. Larger stone is used to armor the “channel” floor just below the outfall and the sills (Figure 1). This is intended to protect against dislodgement of the smaller stone during major surface flow events (the entire surface of the cell can be paved with larger rock if necessary). In the absence of effective (maintained) sediment traps at all contributing storm drains, fine sediment and floated debris will gradually accumulate in the voids within the coarse rock cells. (For example, large quantities of sand used for winter road traction are commonly discharged from stormwater outfalls in this area.) It is therefore anticipated that surface flow through the stepped swale will become more common over time as the structure ages. At that point, the gully will be fully erased and replaced by a rocky stepped-bed channel lined with vegetation. Although reduced, flow attenuation will still be provided by the stepped rocky bed and (ideally) closely encroaching woody vegetation. Trail Closure and Slope Re-Naturalization. Overly steep, gullied trails are currently being closed and replaced by stable, well-drained and properly-aligned paths throughout the WVP. In this project, the closure and obliteration of the retired trail segments conforms to the following general sequence: 1) deep scarification and ridging of the existing trail tread; 2) backfill of the trail incision with clean, locally salvaged soil placed in well-compacted soil lifts; 3) creation of a hummocky, uneven micro-relief on the fill surface; 4) installation of a deep layer of shredded woody mulch overspread with larger branches and logs; and 5) the installation of woody forest understory plantings (Figure 2). Where feasible, closed trail segments should be filled to somewhat above the prevailing grade in order to re-create more or less even hillside contours after the fill has settled.  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. 
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