Predictable, preventable erosion behind 310 Esplanade

Headlines:

  • The crane moved north on Esplanade this afternoon and at this hour is flying rock over the “dog park” and down to the beach at the corner of 310 Esplanade.
  • Good video and info at KTVU.COM, New Problem Emerges At Crumbling Pacifica Cliff.

This post could also be called “No easy answers for Esplanade Ave”. At least I don’t know them.

Commentary and analysis:

First, the latest events.

During the course of monitoring the surf-caused erosion behind the buildings along Esplanade Ave in Pacifica, engineers noticed a new problem. A substantial amount of dirt had fallen away from the northwest corner of 310 Esplanade, and with new storms on the way, the problem would only get worse. Unfortunately, this problem was predictable and should have been prevented. Still worse, solving this problem may just cause it to recur near by.

The short-term solution for 310 Esplanade was to armor the base of the bluff with rock rip rap as a temporary seawall. Luckily, or as the result of prudent planning, engineers still had a huge red crane standing by so they drove it north up Esplanade, planted it in place, and began using it to fly rock over the “dog park” and down to the beach. The crane is much slower than dump trucks and excavators, but the crane can operate safely during high tides and at night. Presumably, if crews can extend the existing temporary rock and sand road far enough, they will use it to move rock in the morning. There’s been a long term solution – a low, permanent seawall at waters edge and stitch piers at the top of the bluff – before the Coastal Commission for some time, and owners had planned (hoped) to have that work done in summer 2010.

Note that last week virtually the same problem arose at the south end of 320 Esplanade, at the end of the existing rip rap seawall. The solution was the same, and despite the recent storm there has been no further erosion.

Second, the implications for the residents of 310 Esplanade.

From past experience with this process at 330 Esplanade and based on the healthy amount of bluff remaining behind 310 Esplanade, we can say:

  • building up and extending the rip rap seawall at the base of the bluff should halt the surf erosion, at least for this winter, and residents should be able to stay.
  • the proximity of the open space “dog park” was fortunate, since the crane can operate in the space above it. If there was no open space then a building would have been evacuated in order for the crane to work. The work crew confirmed this after 330 Esplanade had been red tagged: if it had not been evacuated as unsafe because of the erosion, it would have been evacuated for the crane. (See Santa Rosa Crane Crashes into House)
  • residents in 310 Esplanade may want to stay alert, but may not need to move. If I lived in 310 Esplanade, I would not move – at least not yet.

Third, an incomplete rip rap seawall.

At the risk of stating the obvious, the buildings along Esplanade Ave had an incomplete rip rap sea wall. Last summer the plan was to place rip rap behind the buildings from 310 to 360 Esplanade. South of 360 is an open area that belongs to Pacifica, and north of 310 is a dog park. All of the wall was built except the portion behind 330 Esplanade. (I don’t know why no wall was built behind 330 Esplanade.)

These two images show the rip rap seawall about three months ago, as of 10/1/2009. The right photo is centered on the gap behind 330 Esplanade. The left photo shows the north end of the seawall as it ends behind 310 Esplanade. (Click an image to open the corresponding full page display from the California Coastal Records Project in a separate browser window / tab.)

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And here is today’s image of 310 Esplanade from the KTVU.COM story. It’s easier to see in the video on the KTVU site that bluff top has fallen away just beyond the end of the rip rap.

Fourth, how this was predictable, preventable erosion.

In my post Crumbling coastline: Pacifica’s problems nothing new to the California coast (12/20) I linked to a post with the same title by KNTV meteorologist John Mayeda on Examiner.com.

Rob Mayeda was apparently unsurprised by the erosion behind 330 Esplanade. He explained that partial seawalls do not just block waves, they tend to shift and even focus waves’ erosive energy:

Seawalls are often used to protect structures from the ocean onslaught of storm driven surf. These artificial barriers have been shown to focus the ocean energy in areas around the sides of the seawall structure.

Mayeda illustrated the point with this info-graphic:

If we (I) had been paying attention, we would have been watching for erosion at the ends of the seawalls, not just to see if water over-topped the walls. (If you would like to learn more, the Cal State Monterey site, Monterey Bay Seawalls, has shown that the area behind a seawall may be relatively safe, and the areas at the sides of seawalls may not be.)

Conclusion.

After the major collapse of the bluff behind 330 Esplanade on December 17th action could have been taken to prevent the two significant falls behind 320 Esplanade on January 3rd, and 310 Esplanade on January 13th. In both cases quick work seemed to halted further falls, and for 320 Esplanade at least,  prevented erosion during a substantial storm. It seems possible that equally small, quick efforts would have prevented the additional falls entirely.

(NOTE: This post is part of my coverage of the cliff erosion and collapse at 330 Esplanade Ave, Pacifica. For a complete chronology and links to many more photos see Evacuation underway at 330 Esplanade. You can see all of the pictures I’ve taken along Esplanade on Flickr at Evacuations at 330 Esplanade.)

2 Responses to Predictable, preventable erosion behind 310 Esplanade

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