By Barbara Sattler and Al Sattler
The Portuguese Bend Landslide Complex is an “ancient” landslide which was reactivated in 1956 during the construction of an extension of Crenshaw Blvd. Today it is characterized as a “creeping” landslide with continual gradual movement, necessitating regular maintenance adjustments to Palos Verdes Drive South.
The RPV City Council has made addressing the landslide one of its priorities, and funded a geotechnical Feasibility Study to examine this issue. The council met on August 7 to “provide direction to begin implementing recommendations from the Feasibility Study.”
The Feasibility Study includes a wide range of actions that might be taken in hopes of slowing the landslide. Groundwater sinking down to the “slippery when wet” bentonite clay layer underlying the landslide has been repeatedly identified as a trigger of sliding, so most efforts to slow the slide, over the past decades, have focused on decreasing the amount of groundwater.
The Sierra Club was especially concerned about one proposal to line the canyons in the landslide area with an impermeable material, since such a measure would permanently destroy prime native habitat within the Portuguese Bend Nature Preserve. We submitted a comment letter to the city expressing our concerns about this potential for destruction of native habitat. We suggested that some of the other options for reducing ground water might offer a more benign approach.
These include eliminating existing septic tanks, and reducing excess irrigation in the watershed. We also stated that the option of horizontal drains, such as those used at White Point Nature Preserve, might be worth investigating further.
The city council opted to start a dialog with the neighboring city of Rolling Hills regarding septic tanks and storm water runoff. They also opted to bring a request for proposals for additional hydrological studies back to the city council for review. These are preliminary steps.
It is important to stay engaged in this issue in order to ensure that any efforts to mitigate land movement are not destructive to natural vegetation and habitat. Additional information can be found at http://www.rpvca.gov/1031/Portuguese-Bend-Landslide