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Assessment of Intertidal and Subtidal Impacts of the Alder Creek Landslide

Raimondi, P., and M. Carr (December 2015)

Final Report Submitted to Monterey Bay Sanctuary Foundation, 58pp + appendices.


On April 14, 2011 a landslide occurred just south of Alder Creek, blocking Highway 1 at post mile marker 7.75. The Alder Creek Slide Area (ACSA) buried and overtopped Highway 1, which closed the highway in both directions. This created an emergency requiring immediate action by permitting agencies and the California Department of Transportation (Caltrans).

An emergency coastal development permit (CDP 3-11-032-G) was authorized by the California Coastal Commission (CCC) on April 26, 2011. As noted in the permit, this allowed for "…debris removal and landslide stabilization over a 350-linear foot distance at Highway 1 between post miles (PM) 7.7 and 7.8 on the Big Sur coast just south of Alder Creek, as well as debris placement at the Grey Slip retaining wall (PM 6.8), and the Caltrans Willow Creek site (PM 10.4) and the existing permitted landslide material disposal and rehandling site at Treebones (PM 11.0). The operation will also consist of material placement and spreading at the base of the bluff in the vicinity of the slide (avoiding direct ocean disposal as much as possible) to allow for coastal processes to disburse the material in a manner that mimics natural sloughing (all as more specifically described in the Commission's ECDR file)."

As noted in the Authorization Letter from Monterey Bay National Marine Sanctuary (MBNMS) to the CCC and Caltrans (April 26, 2011), "Caltrans estimates that stabilization of the slide will result in additional slide material seaward of the existing toe of the slide currently located on the adjacent beach. It is expected that approximately 25,000 cubic yards of this landslide material will come to reside on the adjacent beach, resulting in a total of approximately 50,000 cubic yards of material seaward of the highway."

The Alder Creek Slide Area (ACSA) includes the slide above the road and a substantial toe that extended through the intertidal and into the subtidal. On April 22, 2011 NOAA's RV FULMAR and MBNMS science divers conducted a subtidal site visit. Divers investigated two areas, one north of the slide through the persistent kelp bed, and a second area directly into the toe of the slide (Figure 1). Based on aerial images of the area in previous years, staff divers expected the bottom to be mostly sandy with interspersed rocks. To the contrary, the bottom was consolidated reef and boulders with an extensive macroalgae subcanopy (i.e. below the water surface) and extensive coverage by turf (i.e. less than 10 cm tall) macroalgae and invertebrates. The dive headed towards the toe of the slide was aborted at a depth of 20 feet due to poor visibility. The sediment plume generated by the slide was visible from aboard the FULMAR and prevented divers from reaching the actual toe of the slide. Divers stopped about 150 m short of the swash zone at the toe of the slide.

It was recognized early on that the state of knowledge regarding the impact of landslide material to the intertidal and adjacent subtidal was wanting. Clearly, areas buried by the landslide suffered immediate and direct impacts. However, it was less clear to scientists what other direct and indirect impacts were the result of the natural slide and additional engineering and disposal work. For example, it was not known how the sediment plume in the nearshore would affect the benthos (i.e. community of macroalgae and animals inhabiting the seafloor), particularly sedentary and sessile species. Scour, turbidity and subsequent sediment settlement—among others—were likely to increase the zone of influence around the toe of the slide, both in the intertidal and subtidal zones.

MBNMS and Caltrans staff worked with local scientists to develop a monitoring plan that would provide insight on these issues. Specifically, a proposal was submitted by the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) in consultation with MBNMS and Caltrans staff to assess the pattern of intertidal community composition as a function of distance from the ACSA. General questions include:

  1. Does the pattern of intertidal community composition vary as a function of distance from the ACSA?
  2. Does this pattern change over time?
  3. Are the spatial and temporal patterns (questions 1 and 2) consistent with an impact to the community due to the ACSA?
  4. If the answer to question 3 is yes, then what is the expected time to recovery?
  5. Does the relative abundance of substrata (rock versus sand) and vertical relief differ between the shallow subtidal rocky reef site offshore of the ACSA relative to sites to the north and south
  6. Do species abundances of macroalgae and invertebrates on the shallow subtidal rocky reef just offshore of the ACSA differ from sites to the north and south?
  7. Does species composition (identity and relative abundances) of macroalgae and invertebrate assemblages on the shallow subtidal rocky reef just offshore of the ACSA differ from sites to the north and south?
  8. Are the differences in the shallow subtidal rocky reef consistent with what might be associated with changes in the physical environment associated with the ACSA (e.g., sediment burial, scouring, turbidity)?
  9. Do these patterns (5, 6 and 7) vary among depth zones in the shallow subtidal rocky reef and do they change over time?
  10. Does the relationship between giant kelp (Macrocystis pyrifera) biomass (derived from Landsat imagery) and distance from the ACSA differ before and after the slide event?

In this report, we summarize the results of our three-year study of ecological impacts in the rocky intertidal and subtidal zones, including our sampling designs, protocols, analyses and interpretations of our analyses.

URL:    Reviewed: November 20, 2017
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