DEWATERING WELLS

It is generally accepted by all the geologists who have studied the Portuguese Bend Landslide complex, that water reaching the bentonite layer is the main cause of the sliding.

Water enters the landslide area by means of direct rainfall, rainfall runoff from higher elevations, irrigation (from within and without the landslide), septic tank leach fields, wash down runoff (from within and without the landslide), etc.

An aggressive and effective dewatering program is in effect in the Abalone Cove landslide and geologists generally agree that the dewatering has resulted in slowing the slide.

A program of dewatering in the Portuguese Bend landslide has not been effective and especially during periods of heavy rainfall the slide seems to accelerate.

Dr. Christi A. Hill, who studied groundwater recharge and its relation to landslide movement as her Ph.D. project, made the following observation:

Like many other southern California landslide occurrences, positive correlations between groundwater levels and landslide movement have been observed on the peninsula. The response of the Portuguese Bend landslide to rainfall has been noted to be immediate.

There are also dewatering wells in the Klondike Canyon landslide, however, the effectiveness of the wells is not known (to the preparer of this report). These wells are operated and maintained by the Klondike Canyon Geologic Hazard Abatement District. The wells include a set of “horizontal” wells high up in the canyon and at least one dewatering well below Palos Verdes Drive South. The District contracts with the City to maintain the pipelines that carry water from the horizontal wells, down the canyon, to the ocean.

Abalone Cove Dewatering Wells
There are 15 active dewatering wells in the Abalone Cove landslide area. Ten of these wells are operated and maintained by the Abalone Cove Landslide Abatement District (ACLAD) and five are operated and maintained by the City Improvement Authority (IA). The annual operating and maintenance cost to the IA for these five wells is approximately $37,000. In addition, the IA annually provides ACLAD $60,000 toward their dewatering program. The City wells have recently been removing a total of about 100,000 gallons a day while the ACLAD wells have been yielding about 130,000 gallons a day. The production figures vary significantly as wells clog and are cleaned or re-drilled and groundwater levels vary; probably due to rainfall. One of the ACLAD wells was pumping over 100,000 gallons a day at this time last year. The City contracts with Daphne Clarke to monitor the IA wells and keep them in good operating condition.

Portuguese Bend Dewatering Wells
The dewatering wells in the Portuguese Bend landslide have never produced much water. Apparently the structure of the earth in this area is not conducive to drawing out water and the continual movement quickly destroys or clogs the well casings. Pumping records from 1998 show 23 wells pumping a little over 29,000 gallons a day. Only four of the wells were removing over 1000 gallons a day and only one of those was over 10,000 gallons a day. By comparison, only two of the 15 Abalone Cove dewatering wells regularly produce under 1000 gallons a day and seven regularly produce over 10,000 gallons per day.

The 2006-07 Improvement Authority budget includes about $50,000 for the maintenance and operation of the Portuguese Bend dewatering wells. The City is continuing to operate a number of the wells in Portuguese Bend. Although the City has not been keeping records of production on the Portuguese Bend dewatering wells, we believe only five or six of the wells are producing enough water to continue to maintain. The location of all 23 of the wells is shown on the attached map. Asterisks identify the locations of those wells that have some potential to continue to remove water. These wells are all in need of new equipment and repairs to the discharge lines that carry water to the ocean.

We have asked Daphne Clarke to evaluate the present condition of the six dewatering wells that we believe may provide some benefit of water removal. We will also reinstitute a system of maintaining production records for the wells that produce measurable amounts of water.