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.