Promising Solution or Potential Problem?

As we near the end of Palo Alto’s dewatering season, October 31st, we face a dilemma. Do we support the use of cut-off walls for underground construction where the water table is high or not?  We’d like you suggestions and comments.  Save Palo Alto’s Groundwater is hosting a table at this Sunday’s, October 1st, Midtown Ice Cream Social, Hoover Park, 1 – 4 pm. Either in the comments section below or at the Ice Cream Social we would like to know what you would like the City to do about construction dewatering.

The City’s Public Works Department released the chart below that shows the results of three finished 2017 City of Palo Alto dewatering projects. Item number 3 shows a project that used cut-off walls. Not only did this project pump out less than 2% of the amount  of groundwater pumped by the other projects that used conventional dewatering methods, it was able to percolate back all this groundwater so that this process didn’t waste any groundwater! This project also had less impact on neighboring properties, the canopy and infrastructure because of the smaller groundwater drawdown.

Terrific, isn’t it? What’s not to like? One of our concerns about all underground construction is its impact on flooding. Underground construction means the removal of soil – soil that we depend on to retain and slowly release storm water on its way to the Bay. Not only is the removed soil unavailable for storm water management but, also, all those underground basements and garages can act like sticks in a stream. If beavers could speak they would tell us that with a few sticks we can build a dam – a dam to the underground flow of storm water during heavy rains.  With the loss of this soil and underground construction acting as dams, the City will have to build ever more expensive  infrastructure to handle heavy rains and reduce flooding.

But if we are building basements and underground garages already anyway, what’s wrong with using cut-off walls?  To be effective, cut-off walls need to be built down to a non-water bearing layer (clay layer).  In our area this depth is roughly twice the basement’s depth. This means we essentially double the amount of soil unavailable to absorb storm water and block water flow through much of the lot   Do we understand the impacts of basements or cutoff walls on flood risks?  We think not.

Thank you for your interest and your support.

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6 Responses to Promising Solution or Potential Problem?

  1. Hamilton Hitchings says:

    I cannot make the meeting but I believe the draining of our acquifers is much more serious than the flooding blockage for a number of reasons. First, you are not allowed to build a basement in a 100 year flood zone. That means the places most vulnerable to flooding will not be directly impacted. Secondly, during a major flood I do not believe that porousness 10 feet underground is a major factor in the amount of surface runoff. I was here during the 1998 flood and there was 2+ feet high river of water running down channing avenue past my house and 3 feet on the corner of Heather. It only lasted until the creek stopped overbanking and then drained. The idea that basements with deeper cut walls would significantly influence the height of the water seems to be based on someone’s imagination who has not directly experienced how San Francisquito Creek floods. I believe the flood increase from having cut walls on properties near the flood zone is likely negligible. Whereas the water savings is massive.

  2. Bea Cashmore says:

    Hi Keith, Rita and Esther!
    I have given this very quandary a lot of thought over the past several months as you may know! The “best of all possible” solutions is for new constructions to not build basements at all. Although cutoff walls will drastically reduce the amount of groundwater removed, the basements will effectively become floating, cement boats when the rainwater runs off to return to the ground. Neighborhoods could experience flooding. Last winter’s rainfall most surely would produce such an effect, especially in my neighborhood in Triple El, where groundwater can be found as shallow as four feet from the ground surface during the rainy season. Sea level rise is a certainty in our future, and we are a coastal community.

    Since we are not in control of basement construction, and the City continues to approve them, and we reside in one of the most housing impacted cities in the country, I expect we will be seeing more new basement construction. I am encouraged by the fact the our neighboring cities of Mountain View and San Jose do not allow basements to be built. Palo Alto should consider doing the same thing.

  3. Julianne Adams Frizzell says:

    thank you to the “save palos ground water” folks for staying on top of this issue.
    I attended the council meeting in which the cut off wall benefits were explained. It is a sensible and effective technique to stop the wasting of our precious ground water.

    I support the use of cut off walls, and I think that they should be a requirement for every basement project in Palo Alto.

  4. Carl Darling says:

    Palo Alto should not approve basements to be built, as Mountain View also does not allow this construction. the water table in my mid town area is at 7 feet. Basements create several adverse conditions as is stated in your description and in the email responses. This type of construction has become a greater problem in the past 5 years then in the previous years and will be even greater next year. In the lesser of both evil’s water drained for a basement should dictate the answer.

  5. Nicholas Kaposhilin says:

    I would like to commend SPAG for the wonderful community service you have provided to PA. You have presented compelling data, and inspired all of us to pay attention and challenge the status quo. I was very happy with the support you gave to me and my neighbors during our campaign to limit one developers insensible plan to dig 3 basements all at once on adjacent properties at a 7′ water table in Triple El.

    But should we support a full ban on basements in PA? I think not. Why? Because to do so would not be innovative nor would it be realistic in today’s realities of growth and urbanization we are experiencing in the San Francisco Bay Area. Supporting a full ban on basements is a backwards looking move that persists a dream of Palo Alto remaining a small suburban town. We are not and we should accept that.

    As Palo Altans we should focus all our efforts on innovation and leadership as an exemplary city in the SF Bay Area; One that addresses the realities of growth and of becoming a leading global urban residential area of the world similar in stature to the residential boroughs of London or NYC. We should demand that our city officials innovate to solve problems that allow us to grow and urbanize smartly and stand firm against the real estate developer lobby and others who have demonstrated support of reckless, brut force growth at any cost.

    The movement last year lead by SPAG to propose a cut-off wall for new construction of basements is an excellent example of innovative leadership that creates solutions for smart growth. We should continue to focus all our efforts on ensuring that PA City Council permanently ratifies city ordinances for use of these cut off walls for all future basements where ground water pumping is going to be an issue.

    Lets resist the temptation to support broad bans that could be construed as anti-growth by some members of our community. This is not a realistic solution for our future and it does not drive the compromise needed to unite us as Palo Altans towards common goals we can all accept.

    Nicholas Kaposhilin (Palto Alto native and resident in Triple El)

  6. Joseph Rahn - The Purple Pipe Company says:

    Hi everyone,

    We advocate the use of recapturing the water and replacing the potable water use at sports fields and parks nearby the basement dewatering zones.

    We would only be able to recover 10 – 20% of the groundwater going down the storm-drain at a high-flow de-watering sites. However, the diversion of homeowner funds from secant walls to alternative water infrastructure (water tanks/cisterns) at Palo Alto parks and sports fields could lead to a domino effect which could really impact Palo Alto’s water conservation approach, on a more “holistic” level.

    Please see visit the following link at our company website to learn more:

    Best regards,
    Joseph Rahn
    The Purple Pipe Company

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