1.0  INTRODUCTION

1.1  Purpose

This report presents the results of a comprehensive study of landslides in Seattle, Washington, using data that dates back to 1890.  The first study of landsliding in Seattle was performed by Dr. Donald Tubbs in his doctoral studies at the University of Washington between 1972 and 1973.  The result, Landslides in Seattle, was published in a circular by the Washington Department of Natural Resources in 1974 (Tubbs, 1974).  Tubbs based his study on the wet 1971/72 winter that resulted in the largest number of reported landslides since the winter of 1933/34.  Since then, three major winter storm regimes have heavily impacted Seattle in terms of landslide damage:  1985/86, 1995/96, and 1996/97.

Seattle is unique among cities in the United States in that it instituted a full-time position to gather information on landslides, categorize it in files, and coordinate landslide information among City of Seattle (City) departments.  Since the pioneer efforts in the 1960s, this landslide file has been updated periodically and the files have been open to the public.  Despite some spatial and temporal gaps in the information contained in these files, it is probably one of the most compre­hensive records of landslides in this country.  It is this remarkable record that allows this study to be completed. 

When the city experienced the impact of two succeeding rainy seasons (1995/96 and 1996/97) of abnormally high rainfall, the City, with input from the public, decided to consider new approaches to deal with landsliding in the City.  In order to adopt new City of Seattle policies for landsliding proaction and response, two approaches were solicited:  (1) public outreach and opinion, and (2) a scientific approach to understanding the landsliding and formulating remedial measures to combat it.  The first approach was met by holding a series of five public meetings during which citizen comments were taken.  The second approach is addressed with this report on the landslides in Seattle.

The purpose of this study is four-fold:

1. To inventory and catalog landslides in the City and Shannon & Wilson, Inc. (Shannon & Wilson) files.
2. To better define landslide hazard zones within the City.
3. To aid in the landslide policy decisions by City officials.
4. To increase public knowledge of landslides and landsliding in the City.

The scope of services for this study was developed in an iterative manner with officials from Seattle Public Utilities (SPU).  They also received comments from representatives from Seattle Transportation Department (SEATRAN), Seattle Department of Design Construction and Land Use (DCLU), and Seattle Law Department.  The following are the topics of the scope of work:

This report is presented in two volumes.  Volume 1 contains the report and figures.  Volume 2 contains the Map Folio and the Landslide Database.  Volume 1 is organized into five Parts:

This Part describes the data sources and methods used to develop, field check, and analyze landslides in Seattle.  It includes descriptions of the conditions that lead to landsliding in Seattle, including the topography, geology, groundwater, surface water, climate and cultural features.  This Part also describes the types of landslides that occur in Seattle, when they occurred, and how their locations relate to currently mapped hazard areas.

Part 2 - Geotechnical Evaluation of Landslides Citywide

Part 2 presents a geotechnical engineering evaluation of the landslides described in Part 1.  It describes typical stability improvements that can be made for each of the landslide types and typical details for the stability improvements.  We present unit cost estimates for the typical improvements that could be used for preliminary budgeting purposes.  Finally, this Part evaluates how City utilities and streets can affect stability.

Part 3 - Landslides in Three Study Areas:  West Seattle, Magnolia/Queen Anne, Madrona

Part 3 presents the results of geological and geotechnical studies we made in the original three study areas of the City.  These areas are:  West Seattle, Magnolia/Queen Anne, and Madrona.  The emphasis in this part of the report is on evaluating factors that contribute to slope instability and the remedial measures that could be implemented to improve stability in these areas.

Part 4 - Landslides in North, Central, and South Seattle

Part 4 presents the results of geologic and geotechnical studies we made in four additional study areas in Seattle:  Northwest Seattle, Northeast Seattle, Capitol Hill, and South Seattle.  Similar to Part 3, this section presents measures for improving stability in these areas.

The original scope of work resulted in the preparation of Parts 1, 2, and 3.  In Part 3, three study areas were originally selected for detailed evaluations including field verification of landslide locations.  This resulted in the identification of 26 Stability Improvement Areas where landsliding was prevalent in West Seattle, Magnolia/Queen Anne, and Madrona.  Subsequently, Part 4 was authorized to extend the field verification process to include landslides throughout the City, and to identify additional Stability Improvement Areas.  The result was the delineation of 17 additional Stability Improvement Areas in Northwest Seattle, Northeast Seattle, Capitol Hill, and South Seattle.  Thus, Parts 3 and 4 provide similar results regarding stability improvements in the respective areas; however, Part 3 includes more description and GIS analyses of the landslide inventory data than are presented in Part 4. 

Part 5 - Potential Slide Areas

This part describes the process and results of updating the locations of potential slide areas.

The Figures and Tables for each Part are presented at the end of the Part to which they relate.  They are numbered sequentially in each Part, e.g., Figure 1-1, 1-2, etc., for Part 1, and Figure 2-1, 2-2, etc., for Part 2.  Volume 2 contains the large maps in the Map Folio and the Landslide Database.  Maps in Appendix A relate to Part 1 and maps in Appendix B relate to Part 3.  The maps in Appendix C relate to Part 4, maps in Appendix D related to Part 5, and the Landslide Database is in Appendix E.

This study was performed in general accordance with the scope of work submitted on November 11, 1997, to Mr. Robert Chandler of SPU.  Mr. Chandler orally authorized this study on October 9, 1997.  Authorization by Mr. Chandler to proceed with Part 4 and Part 5 was received in January 1999.

The database information was based on available records and brief, limited field observations.  Differences between the data and actual conditions may exist.  The database entries draw no conclusions regarding the extent to which reported contributing factors caused slope instability.

Based on the database and field observations, geotechnical evaluations were conducted to formulate remedial measures for improving stability.  As a result of this effort, 43 Stability Improvement Areas were developed.  These are areas where previous instability has been prevalent (i.e., a concentration of reported landslides).  For each Stability Improvement Area, we evaluated the conditions that contributed to past instability or that may contribute to potential future instability, and have presented possible remedial measures to be considered by the City and private property owners for improving stability.  The remedial measures presented are intended to be preliminary and are provided to give the City and others information that can be used to prioritize remedial efforts and to develop order-of-magnitude budgets for the work.  The proposed remedial measures are also intended to be illustrative of potential solutions.  Additional evaluation and, in some cases, field reconnaissance, are necessary to prioritize these proposed remedial projects.  To determine final scopes of work and cost estimates, subsurface explorations and/or additional engineering studies are required.

The number of recommended stability improvements are extensive.  It is, thus, obvious that a considerable length of time in years will be needed for conducting further studies, prioritizing the improvements, allocating funds, and implementing the work.  The recommendations are general in nature and provide approximate locations where further analyses could take place for evaluating priorities and scheduling work.  To be effective and to prevent overlapping of remedial measures, the prioritization process must be coordinated with the Needs Assessment of the Drainage Policy Study conducted for SPU by the consulting engineering firm of Black & Veatch. 

Since landslides and potential areas of instability do not obey property boundaries, the improvements recommended in Parts 3 and 4 do not consider the location of property lines and would take place on City property, private properties, or both.  The improvements are those that could be made by the City to protect utilities, drainage features, streets, and other City facilities; and also those measures to be coordinated between the City and private property owners to improve stability of an unstable slope.  Some improvements would be made by the City, while other improvements or protection would be the responsibility of private property owners.  It is anticipated that coordination between the City and private property owners would include expeditious processing of permits; granting of appropriate easements and variances to code requirements where needed to improve stability for private and/or public properties; shared costs, such as by Challenge Grants or Local Improvement Districts (LIDs); or other cooperative efforts.  On private properties, the City may also facilitate the negotiation of easements for stabilizing measures.

The recommendations presented in this report are based on a technical evaluation, and are not intended to set City policy.  The City's landslide responsibilities are a complicated blend of public policies involving public and private responsibilities and partnerships; therefore, the implementation of any or all of the recommendations are solely at the City's discretion.

Improvement of stability involves actions not only by the City, but actions by private property owners.  Such actions by private property owners should include accepting existing conditions and the risks of slope instability, and accordingly controlling drainage, improving stability, providing protection for property and structures, and/or obtaining competent professional advice.  Homeowners or potential property owners should also obtain competent professional advice regarding site selection, property purchase, site improvements, and/or new construction.  The existing conditions to be accepted by private property owners include surface and subsurface drainage conditions, soil conditions and site geology, site topography, and other factors affecting stability as described throughout this report.  The potential adverse conditions that may occur during times of very heavy and/or prolonged precipitation should also be considered.  In addition, private property owners should avoid conducting site work that would jeopardize stability of adjacent property.

None of our studies have considered nor evaluated the specific contributing or predominant causes of any previous landslides.  The stability improvements described in Parts 2, 3, and 4 of this report are general types of action that could be considered by the City and/or private property owners to improve stability and reduce landslide risks. 

There are always risks of damage to property and structures involving landslides, for property located on or adjacent to a slope.  Property owners need to accept those risks.  Although the recommended improvements and homeowner education can lead to immediate or eventual improved slope stability conditions, the risks of damage cannot be completely eliminated.  In addition to natural factors (soil, groundwater, heavy rainfall), other factors that may affect stability are excavations, fills, leaking or broken utility lines, improper drainage, lack of maintenance of drainage facilities or vegetative cover, unwise actions by adjacent property owners, or similar events or unknown conditions that may cause instability.

Property owners should also be aware of the advisability of obtaining insurance in addition to standard homeowner's insurance to specifically cover the risks posed by geologic hazards including earth and debris movement.

Shannon & Wilson has prepared the following "Important Information About Your Geotechnical Report" to assist you and others in understanding the use and limitations of our reports.

IMPORTANT INFORMATION ABOUT YOUR GEOTECHNICAL REPORT