This part of the report presents an update of the Potential Slide Areas (PSAs), a designation used primarily by the Department of Design Construction and Land Use (DCLU) in the regulation of residential and commercial construction in the City of Seattle (City).  PSAs have been used in conjunction with Steep Slope Areas and Known Slide Areas to identify those areas of the City where additional reconnaissance and engineering work needs to be accomplished when new construction or major remodel work takes place.  However, it became apparent in the course of this Seattle Landslide Study that well-known landslide-prone areas were not represented on the PSA map.  Therefore, this analysis of the PSAs was undertaken to discover and document the inconsistencies between the maps and the features on the ground.

All areas of the City were evaluated in this part of the study.  Areas with high concentrations of landslides were printed on 26 maps at a scale of 1:4800 using Arc View Geographic Information System (GIS) and City of Seattle ArcInfo coverages.  Base map coverage information included topography, property lines, streets, historical landslide initiation locations, trace of the sand/clay contact, and existing PSA boundaries.  Revisions to the PSA boundaries were plotted on the maps in the office, based on landslide concentrations, topography, and general knowledge of geologic conditions.  Based on previous knowledge of certain landslide-prone areas and the historical record, about one-quarter of the sites were delineated with high confidence.

After the preliminary re-delineation of the PSAs in the office, we performed a field review of the proposed revisions to the PSA maps.  The remaining three-fourths of the areas were visited to evaluate the field conditions and to accurately draw the boundaries.  This also included the delineation of runout zones at the toes of hillsides and setback zones at the tops of very steep slopes or bluffs.

Meetings were held with representatives of DCLU and Seattle Public Utilities (SPU) before the revision process to discuss the criteria to be used and after the field verification of the revised boundaries to discuss the results.

20.0  BACKGROUND

The existing PSA maps used by DCLU, primarily in the building permit process, are based on conclusions in the Ph.D. dissertation of Mr. Donald Tubbs, Causes, Mechanisms and Prediction of Landsliding in Seattle, 1975.  Mr. Lloyd Finney of the Seattle Engineering Department started methodical classification of landslide information in the 1960s, but zonation of the landslide-prone areas of the City were not codified until the Environmentally Critical Areas (ECA) were published in a map folio in 1992.  The criteria used for the PSAs were taken directly from the Tubbs dissertation and they included the following factors:

All Class 4 zones, a strip of approximately 200 feet wide along the trace of the Esperance Sand/Lawton Clay or pre-Vashon sediments contact.

All Class 3 zones, areas steeper than 15 percent slope gradient and underlain by the Lawton Clay or pre-Vashon sediments.

Areas with springs or groundwater seepage; however, this criterion was not mapped.

During analysis of the landslide study information, it became apparent that there were many inconsistencies among the actual landslide pattern, the electronic layer of the PSAs on the City's GIS system, and the ECA map folio.  The reasons appear to be:

Differences between the 1958 topographic base map used by Tubbs to construct the original hand-drawn maps and the topographic base map used in the ECA folio.

Large areas of PSA not properly transferred from the original hand-drawn maps to the GIS layer.

Geologic factors not previously considered, such as setbacks from steep bluff, runout zones at the toes of hillsides, instability not related to the Lawton Clay or pre-Vashon sediments, and geologic conditions unknown at the time of Tubbs' work.

21.0  Results

Several criteria were used to revise the boundaries of the PSAs.  They include the following factors:

Areas with historical record of landsliding.

Signs of past landsliding observed in the field, such as landslide scars and deposits.

Signs of potential landsliding observed in the field, such as springs, groundwater seepage, and bowed or backtilted trees.

Topographic expression of runout zones, such as fans and colluvial deposition at the toes of hillsides.

Setbacks from very steep slopes or bluffs.

Extrapolation of the above factors to areas of similar and contiguous topography and geology.

The most important criterion used in this evaluation was the historical record.  The other criteria were used to supplement the analysis.

Twenty-six work maps were prepared with field notes at a scale of 1:4800 showing the original PSA boundaries and the revisions to them.  Two larger scale maps with the same information show the entire City on one map.  Figure D-1 indicates the existing (1992) PSAs based on the current data layer from the City of Seattle GIS department, and Figure D-2 shows the proposed revised PSAs.  In some cases, entirely new PSAs were created; however, in most cases, the boundaries were shifted slightly.  In some parts of the City, areas previously mapped as PSAs were eliminated or reduced because current geologic or topography information could not justify the original boundary.  Absent any new information to change a boundary, no revisions were made.  At the request of DCLU, a map (Figure D-3) was prepared that indicates areas where regulated land, which includes steep slopes (steeper than 40 percent) and PSAs, would be lost or gained due to the proposed revisions to the PSAs.  The following is a brief summary of the significant revisions recommended for the PSA maps.

Northeast Seattle - The original PSA was a strip along the lower portion of the slope overlooking Lake Washington.  The boundary was extended uphill to encompass the 23 landslides that were recorded on the higher elevation slopes.

Inverness/Sand Point - The PSA was extended southward about 4,000 feet because of an apparent error in the transfer of information from the ECA map and the GIS layer.

Windermere - A new PSA was delineated based on the presence of three landslides on very steep slopes that had several signs of past landsliding.

Laurelhurst - A new PSA was created because of 18 reported landslides, very steep topography, and the presence of springs and landslide scars.

Interlaken - The largest new PSA in the City was created in this very steep ravine system, based on the presence of widespread landslide features and 37 reported landslides.

North Capitol Hill - The slope facing Lake Union, along Eastlake, was eliminated as a PSA owing to minimal occurrence of landsliding in the area, and because the steeper portions of the areas would be covered by the steep slope criteria.  The steep slope between Furman Avenue E. and Portage bay Place E. was placed in a PSA because of the high concentration of reported landslides, seepage, and widespread evidence of other landsliding.

St. Mark's Greenbelt - The PSA boundaries were adjusted by eliminating an apparently landslide-benign slope at the north end of the area, and by extending the boundary uphill to the top of the steep slope to encompass 10 reported landslides on the upper portion of the hillside.

Colman Park - The PSA boundaries were extended southward to cover Colman Park, a large amphitheater-like ravine, which contained seepage and widespread evidence of past landsliding.  Two other new PSAs were created to the south of this area where reported landslides, old landslide features, and very steep topography were contiguous.

Rainier Beach/Taylor Creek - A new PSA was created on the steep slopes south and west of Rainier Avenue s. due to the presence of 25 reported landslides on steep ground that included many areas of groundwater seepage, and extended up the Taylor Creek drainage, based on the widespread occurrence of springs on hillsides, old landslide scars, and very steep topography.

Myers way S. - A new PSA was created owing to the presence of widespread springs, signs of landslide topography, and a history of slope stabilization measures along this hillside for SR-509.

Seola Beach Drive S.W. - A new PSA was created to encompass a concentration of seven landslides and very steep slopes in the upper portion of the ravine along Seola Beach Drive.

S.W. 47^th Street - The PSA was extended southward about ¾ miles because of an apparent error in the transfer of information from the ECA map to the GIS layer.

Fauntleroy Creek - An existing PSA was extended up this ravine to encompass the zones of heavy seepage and hummocky ground surface, both indicative of unstable slopes.

Schmitz Park - An existing PSA was extended up this ravine to encompass the zones of heavy seepage and hummocky ground surface, both indicative of unstable slopes.

Alki Avenue/Sunset Avenue - The existing PSA was extended uphill to encompass 29 reported landslides along the upper portion of this steep slope, and downhill to encompass the potential or historical runout zones of landslides or debris flows.

Harbor Avenue/Admiral Way - The existing PSA was extended uphill to encompass 26 reported landslides along the upper portion of this steep slope and widespread signs of past landsliding activity.

East Queen Anne - The original PSA was a strip along the lower portion of the slope west of Lake Union.  The boundary was extended to the very steep slope west of Aurora Avenue to encompass ten landslides that were recorded on the higher elevation slopes.

North Magnolia - The area around discovery Park was encompassed because of very steep slopes that showed signs of past instability.  The area along the entrance to the Government Locks was created because of a concentration of reported and observed landslides on this steep slope.

Northwest Seattle - Several areas of existing PSA were extended uphill to encompass very steep slopes that contained reported landslides, and some were revised due to differences in topographic contours since the 1958 survey.

22.0  REFERENCES

Booth, D.B., 1987, Timing and processes of deglaciation along the southern margin of the Cordilleran ice sheet; in Ruddiman, W.F., and Wright, H.E., Jr., eds., The geology of North America:  Boulder, Colo., Geological Society of America, v. K-3, p. 71-90. 

Galster, R.W. and Laprade, W.T., 1991, Geology of Seattle, Washington, United States of America:  Bulletin of the Association of Engineering Geologists, v. 28, no. 3, p. 235-302.

Tubbs, D.W., 1974, Landslides in Seattle:  Olympia, Wash., Washington Department of Natural Resources Information Circular 52, 15 p.

Tubbs, D.W., 1975, Causes, mechanisms and prediction of landsliding in Seattle:  Seattle, Wash., University of Washington, Ph.D. dissertation, 88 p., 1 plate.

U.S. Army Corps of Engineers, 1997, Post event report:  winter storm of 1996-97, Federal Disaster DR 1159, Western Washington summary:  Seattle, Wash., U.S. Army Corps of Engineers, Seattle District, 39 p. 

Waldron, H.H., Liesch, B.A., Mullineaux, D.R., and Crandell, D.R., 1962, Preliminary geologic map of Seattle and vicinity, Washington:  U.S. Geological Survey Miscellaneous Geologic Investigation Map I-354, scale 1:31,680.