Report Details Erosion at Pillar Point Bluff, Safety Plans in Development
Friday, July 15, 2016
San Mateo County Parks will use a geotechnical hazard assessment to develop trail improvement and sign plans for Pillar Point Bluff in Moss Beach. The Department sought the geotechnical information following significant bluff erosion and slumps in the area last winter.
"Views from the bluff trails are spectacular. This is a well-loved area with trails and paths that connect from Pillar Point Marsh to Fitzgerald Marine Reserve," says Marlene Finley, San Mateo County Parks Director. "The dramatic changes that occurred along the cliffs last winter prompted us to seek technical information needed to formulate a plan to reduce erosion and improve visitor safety by taking steps, such as installing barriers and warning signs where necessary to alert walkers and mountain bikers of potential hazards."
Engineers at Cotton, Shires and Associates, Inc. conducted site visits, examined historic and current aerial photographs, base and geological maps, and a 1971 hazard study of the Seal Cove area which is located one-half mile north of Pillar Point Bluff. Pillar Point is bordered on the east by the active Seal Cover-San Gregorio Fault and to the west by a precipitous sea cliffs.
The report zeros in on the current conditions of the bluffs and the geologic factors that contribute to beach and cliff erosion. For example, engineers observed several failures of the upper bluff and a sand deficient coastline that can leave the sea cliffs vulnerable to erosion caused by wind and high surf. It was noted that Pillar Point Bluff erosion rates over the past 23 years have ranged from 0.5 feet per year along the northern trail segment to 1.3 feet per year over the middle trail area. In addition, cracking and fissuring was observed at the top of the bluff which is indicative of gradual ground detachment process that results in large segments of ground falling from the cliffs.
The report concluded by stating that "given the extensive bluff failures that occurred last winter and possible recent acceleration of coastal retreat rates in area, it would be prudent to plan trail alignments based on a conservative average local bluff retreat rate of 1.5 feet per year." This observation is consistent with the San Mateo County Sea Level Rise Vulnerability Assessment, which identified bluff top communities as areas that are vulnerable and require adaptation strategies.
Recommendations called out in the report included:
- Closing off numerous existing trails located in an area designated as the Short Term Hazard Zone.
- Replacing closed trail segments with new locations east of the hazard zone.
- Developing a trail layout strategy for the top of Pillar Point Bluff that would include observation points.
- Installing additional signage advising visitors of potential hazards including rock falls from cliffs.
Moving forward, San Mateo County Parks will develop plans that include trail realignments, safety barriers and possibly observation points that will be reflected in a Coastal Development Permit application.
Find the geotechnical hazard assessment attached below. See the Pillar Point Bluff Trail Improvement Project for more information.