Puzzled #6 – UCSF: Landslides

The sixth in our series of responses to UCSF’s July 9th letter.

This is about potential landslides. We’ve included two maps from the FEMA application which clearly indicate this needs discussion.

Red boundaries shown tree-felling sites. Colored areas indicate landslide risk.

Red boundaries shown tree-felling sites. Colored areas indicate landslide risk.

The letter asks, “Will the projects cause landslides that will put roads and houses at risk?”

It goes on to say, “No. The remaining vegetation will help infiltrate rainfall. On slopes over 30%, vegetation removal will be done selectively and by hand with this in mind. No private homes are downslope from either of the demonstration areas.

Really? Downslope from the South Ridge, just across the street, are the private homes on Christopher and on Crestmont .

Instead of the existing trees and thick bush, the “remaining vegetation” – which could be as little as 10% of what’s there now – plus some new grass and wildflowers are supposed to consolidate the slope above our homes. Twin Peaks and Forest Knolls are proof that they don’t do so well at it.

Landslide under blue tarp. South Ridge at top left.

Landslide under blue tarp. South Ridge at top left.

erosion twin peaksWe accept the project supporters here don’t expect landslides. Neither did whoever stripped vegetation from the nearby slope (Warren Drive) that lay under a blue tarp for months; or the people who built the Forest Knolls home that slid down the mountain; nor whoever did the landscaping that results in rockslides on Twin Peaks every rainy season.

The fact is, this mountain is vulnerable to landslides as the map above shows (it’s produced by Consulting Engineers and is part of the FEMA application). In fact, as the next map shows, there have actually *been* landslides in the past in both sites. (Look for the double arrows.) And there’s a map of landslide risk prepared by state geologists (with FEMA funding) indicating most of the mountain is a landslip zone.

Pink areas and wiggly arrows show landslide risk; double line arrows show past landslides.

Pink areas and wiggly arrows show landslide risk; double line arrows show past landslides.

So, the obvious question follows: Rather than this destructive, controversial project, “Were other options considered?

This is what the letter said: “UCSF does not consider alternatives such as clear cutting and controlled burning to be acceptable. The overall concept of using a mixture of livestock grazing, mechanical equipment, hand labor and limited herbicides appears to be most effective.

Clear-cutting? Controlled burning? We can see why those wouldn’t be good options. Neither, for instance, would pouring concrete over the hillside (which Hong Kong has actually done to stabilize steep slopes). Or leveling the mountain.

Instead of all these drastic and potentially dangerous choices, the best option would be: First, do no harm.

We already have a stable densely-vegetated slope that isn’t covered in thousands of applications of Roundup herbicide. If we strip it off 90% of its vegetation and it lands up in someone’s garage, it won’t console them much to know that it was unexpected.

At a later date, we will make suggestions about caring for this forest which we all love. [ETA: We did.] Except for those who can’t stand eucalyptus and blackberry, of course.

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3 Responses to Puzzled #6 – UCSF: Landslides

  1. Rational Thinker says:

    The soils report for my home when I bought it states that this hill is mostly bedrock with topsoil above it. It is the saftest place to be in the event of earthquake, but the risk for this type of geology is the slide factor. When the soil becomes super-saturated as it can from steady rains and no trees and vegetation to modulate the moisture reaching the soil, the soil simply slips over the bedrock and you have a slide. Removing the vegetation is a reckless idea as most of the hill has been stable for some time. Examples of this type of slide have occurred on Telegraph Hill (bedrock with topsoil) and other locations throughout San Francisco and were well publicized. Does history have to repeat itself over and over again until someone gets the message?

  2. NatureLover says:

    I have not analyzed the second grant application for the Edgewood area (on the north side of Mt. Sutro) in much detail compared to the project on the South Ridge because many of the issues are the same. However, the Edgewood area is much steeper than the South Ridge. According to the “Slope Stability Risk Assessment” (Attachment #47 of Edgewood grant application) some of the project area is “potentially unstable…with evidence of ongoing soil creep and/or past landsliding.” (Map 11 of Attachment #47 to grant application) When the eucalypts are cut down, the deep roots of the eucalypts that presently stabilize this steep hillside will slowly decay over a period of 3 to 5 years, losing their ability to hold the soil in place. Ask any certified arborist for confirmation of this “tree fact.”

    In other words, UCSF paid a soil expert to evaluate the potential for landslides on Mt. Sutro. The expert told UCSF that there IS both a history of landslides and potential for landslides in the future. Yet, UCSF chooses to ignore this danger.

    Next I will post a letter from one of the neighbors of the city’s Natural Areas Program who is suffering the consequences of removing stabilizing vegetation from the sand-mountain in her neighborhood. There is a reason why there is lot of non-native vegetation in San Francisco. One reason is that the native vegetation is not capable of stabilizing the sand which composed about two-thirds of the San Francisco landscape.

  3. NatureLover says:

    The following is a copy of letter that a neighbor of one of the city’s native plant restorations sent to the Planning Dept during the comment period of the environmental review of the so-called Natural Areas Program. I’ve left the woman’s name off because I don’t have her permission to publish her letter here. Please understand that when you make such a comment, your comment becomes public information that can be obtained by people like me. I’ve also deleted her street address, but left her street name so that you determine her neighborhood.

    ————-
    “1800 block of 14th Ave, San Francisco. CA 04122
    May 2. 2009
    Bill Wycko
    S.f. Planning Dept, Natural Areas Management, 1650 Mission St, Suite 400, SF, CA 94103

    Dear Mr. Wycko:

    I am writing concerning the SNRAMP as it relates to the steep sand covered areas of the Rock Outcropping at 14th Ave between Ortega and Pacheco Sts. I cannot attend the meeting on on May 1 2. Therefore I take this opportunity to express concern regarding the very unstable nature of these areas. I have no objection to modifying a different flora in this area, however as a long-term resident of the neighborhood I wish to inform you that any significant denuding of areas in preparation for this will likely have a disastrous effect on sand retention.

    Also, the City does a very poor job of clearing sand in the roadway below, and this causes a traffic hazard on a narrow stretch of 14th. Ave that cannot accommodate crossing traffic as is, despite being a two-way street.

    If you wish to change the plants on the steep portions, it should be done very slowly over a number of years. verifying that new plants are in fact being established. Please do not allow areas more than a few feet across to be denuded of existing plants each year.”
    ———

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