How to restore a slide area successfully By MARCIA WEST SvR Design Co. Over time, all steep slopes are prone to soil erosion and landslides. In the Pacific Northwest we experienced the acceleration of this process during the winter storms of 1997, as slides throughout the region resulted in loss of life and property. The failed slopes also lost valuable habitat in forested ravines and increased the siltation of many streams and tributaries. Over the past century, the region has experienced extraordinary growth in population, economy and related development. Property developers were drawn to the prime coastal bluffs, high banks and the edges of forested ravines. This has caused a rapid increase in failed slopes and potential hazard sites. To minimize harm to the natural and built environments of Puget Sound, local governments have developed standards for protection of critical slopes and for development of structures used to protect shoreline properties. Still, the fragile bluffs and hillsides will continue to fail as nature and human use take their tolls. Repairing and restoring failed slopes require an interdisciplinary approach, blending construction and bioengineering methods. Successful restoration of slope failures requires understanding the regional geology, vegetation conditions, water management and geotechnical engineering issues. To ensure long term success of the slope stabilization, the methods must also consider the surrounding human impacts and address urban runoff and view corridor management. Three critical components are necessary for successful restoration of a failed or threatened slope. The first is a technical understanding of the source of the failure and team evaluation of means and methods to stabilize the condition. The second is an assessment of the natural vegetative conditions of the slope and design of a restoration plan that enhances the stability and mends the habitat. The third and most often neglected element of a successful plan is understanding the human context of the restoration. The most common form of slope failure around the Puget Sound area results from relatively shallow, surficial debris slides or slumps that develop into debris avalanches. Each event often affects a number of home sites and causes downslope deposits of silts and debris. Restoring these slide areas requires geotechnical understanding of the nature of subsurface conditions, evaluation of the soils and hydrology, and development of a restoration method that responds to the unique conditions of the slide location. Potential restoration methods for relatively shallow slides include biotechnical slope protection, mixed systems of soil bioengineeing and planting and inert construction including structural or mechanical systems. The combined skills of an interdisciplinary team selects the integrated package of systems which will be most successful for the site. The goal of slope restoration is to balance structural needs for soil retention with the reestablishment of vegetation for surface cover and habitat restoration. This vegetative cover is critical in maintaining the local stability of the slopes. Vegetation can serve several purposes in stabilization projects, both a primary source of stabilization and as native habitat and greenbelts. The primary ways the establishment of vegetation reduces the potential for slope erosion are: Plant roots, both large and small, provide a fibrous web that stabilizes and anchors the soil; Plant cover intercepts rain, reduces the direct impact of rainwater on the ground surface and protects from surface runoff and erosion. Dense groundcovers and grasses reduce runoff velocity and act as a filter trapping soil that would be washed downslope; Vegetation and associated plant litter filter the transport of soil moisture. Plants can play an important role in dewatering unstable slopes. Soil moisture is drawn up through roots and into plant leaves where transpiration releases it into the atmosphere. The foundation of successful slope vegetation restoration is identifying the composition of the native plant community and carefully considering the mircoclimate of the slope. Plants selected for the revegetation should mimic the native species composition in both type and strata. Within forested slopes and ravines, the planting design should mend the slide area with vegetation identified in the surrounding slopes to provide continuity in the habitat. The vegetation types and methods of slope plantings should vary according to the location of the slide within the landscape of the bluff, hillside or ravine. Separate planting approaches should be used for the crest of the slope, where vegetation acts to strengthen the soils and reduce erosion; the face of the slope, where vegetation is used to resist shallow sliding and moderate surface runoff; and at the toe of the slope, where vegetation is used to resist downslope soil movement. Integral to the slope revegation plan is the planting of the slope setback buffer. The slope setback is often the first zone of conflict between the need for slope stabilization and the desires of property owners. Human use is a large contributor to most landslides in residential areas. Home owners and businesses clear trees and debris to open and maintain views and often deposit cleared debris over the bank of the slope. Trails and stairs are cut into the steep banks for ready access to streams and beaches. Lawns and patios are built up to the edge of the bank, affecting the long-term stability of the slope. Because humans are the prime threat to stable slopes, revegetation plans must consider human use. Even the most viable plant community will be strained to accomodate beach access and view maintenance. The planting design needs to create a long-term plant community that balances the needs of the stabilization with the adjoining neighborhoods. Plant selection should consider mature growth limits and opportunities to incorporate access routes into the stabilization design. To increase the viability, the management plan must be developed with the input of the surrounding neighborhood. Active participation in maintenance of the adjoining slope will not only increase the success of the restoration efforts but also provide an early warning system for any future problems. Mitigation efforts to reduce future slope failure begin with early identification of signs of instability, repair of surface drainage systems that may be creating downslope erosion, relocation of uses that are encroaching into slope setbacks and planting stabilizing vegetation to reduce surface erosion. Management of unstable slopes is dependent on public information and involvement. Many slope stabilization and restoration demonstration projects have been undertaken by Washington State Department of Ecology, King County Surface Water Management, Snohomish County Stormwater Management and the Seattle Parks Department. These projects are valuable resources for approaches to volunteer programs and educational classes. Long-term monitoring of demonstration projects and recently completed field restoration will expand our technical knowledge of a range of restoration approaches. Marcia West is a landscape architect with SvR Design Co., Seattle.