Thursday, August 6, 2015

Gibralter Road Landslide

Along the south shore of Fidalgo Island is a series of deep-seated landslides. The best known slide is the Gibralter Slide named for the road that crosses the landslide. Dave Wenning has an excellent perspective on the slide at

I have done a few projects on or in the vicinity of this slide complex. My initial projects were done pre LiDAR; however, the LiDAR simply confirms what was alreday generally known about this slide complex. But it sure makes it easier to visualize. 

LiDAR image of the Gibralter Landslide

The LiDAR resolution is not very high, but it is good enough to get a handle on the head scarp of the slide complex and the various somewhat coherent blocks within the landslide complex. The slide complex continues to the northeast but as far as I know the slide activity to the northeast has been minimal or not reported. 

The Washington Coastal Atlas (1978) indicates that the subject property is located on an area of unstable shoreline.  Areas mapped as “recent unstable areas” are indicated to the east and west of the subject property. I have heard verbal reports of some slide movement in the 1960s including slide movement during the Seattle earthquake. 

After slide movement in 1990 and 1991 the slide stability was assessed by GeoEngineers in a report entitled Geotechnical Evaluation, Gibraltar Road Landslide, Fidalgo Island, Skagit County in 1991.  Six borings were drilled through and adjacent to the slide complex. The report concludes that the movement of the slide in 1990/1991 was a re-activation of an ancient landslide complex due to extremely heavy precipitation.  The report indicated that the slide movement of between 1 and 4 feet took place and moved not as a contiguous solid mass but in discrete blocks within the landslide complex. This is consistent with pictures I have seen as well as my own observations of some of the slide blocks within the landslide. The final determination of the report indicated that the slide is slow moving and is unlikely to endanger human life.  Recommendations were made including setbacks from the slope scarps within the landslide complex and preparation for differential settlement on the slide mass.  The report also indicated that reducing stormwater infiltration into the slide and reducing erosion at the toe of the slide would reduce the potential unstable conditions.

Since the GeoEngineers report some additional development has taken place on the landslide area. In addition some additional substantial shoreline armoring has taken place along the shoreline such that the erosion at the toe of the shoreline bluff slope has been essentially eliminated. Stormwater drainage has since been added to the area including drainage improvements on the upland areas above the landslide complex that routes water via tightline pipes to the shoreline versus onto the slide area.   

LiDAR  imagery provides a visual picture of the deep-seated area. The imagery does show that there are discrete blocks within the landslide complex. An aerial view of the site and vicinity is also provided below with edges of blocks. 

Head wall scarp of the landslide north of Gibralter Road

Coherent slide block showing classic back rotation
headwall of slide is to the right and shore area is a few hundred feet to the left

While some homes on the slide have been designed to anticipate the potential fro movement some have not. However, the erosion protection at the toe of the slope along the beach (see DW's post for good pictures) has improved the stability and the stormwater management has likely helped as well. 

The overall stratigraphy consists of older laminated silts and clays overlain by advance outwash overlain by glacial till and then recessional marine deposits. However, these units have been disrupted by the presence of a deep-seated landslide. 

Its the older clays at the base of the slide that are the slide culprit. I suspect some of the clay is pre glacial and the tectonic strain of the clays being loaded with ice and then unloaded may have created failure areas in the clay. I should add this area is also along a line of faulting that could also have fractured the clay beds in a manner that could lead to sliding.

The units and the slide are somewhat similar to the large Ledgewood Slide on Whidbey Island. Only here the shoreline erosion rate is much lower and the overall aspect is lower in elevation and steepness.   

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