Mass Wasting

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FALLS  (rock fall and rock avalanche)        

     SLIDES  (rock slide and slump)

             FLOWS  (rock avalanche, debris flow, earth flow, and creep)      

Portuguese Bend Earth Flow

The Portuguese Bend earth flow, widely referred to as the Portuguese Bend landslide, is pictured below.   It originates high on the slopes of Palos Verdes Peninsula, southern California, and moves with variable speed into the Pacific Ocean.   Though the Portuguese Bend area had been mapped as a landslide complex before the 1950's, 100's of homes were built on and above the unstable rock and soil in the early 1950's.   Each home had its own sewage treatment facility (cesspool or septic system) and home owners established lawns and gardens on their properties.   These human activities introduced a lot of ground water beneath the homes, lubricating a layer of bentonite clay formed by the subsurface weathering of volcanic rock called tuff.   Slippage in the Portuguese Bend area began in 1956, coincident with the construction of a road (Crenshaw Boulevard) along the top of the ancient landslide complex.   During this construction, excavated sediment was dumped onto the upper slopes of the complex, initiating new down-slope movement which continues to the present.  A successful suit was filed by area homeowners in 1961, winning $10 million dollars in compensation against Los Angeles County, the responsible party for the road construction.   Strangely, the presiding judge ignored the actions of the homeowners, which almost certainly contributed to the severity of down-slope movement and resulting damage to property.    

Though no one has been directly injured by this earth flow, many people had to abandon their homes due to structural damages caused by the incessant movements of the earth flow.    Due to the natural beauty and wonderful climate of the Portuguese Bend area, many homeowners decided to stay as long as possible before abandoning their homes (picture 1, below).   Note the irregular surface of the earth flow in picture 1.   Now, houses that remain on this active earthflow are equipped with heavy-duty, wall-supporting jacks that can be adjusted to compensate for the sinking or rising of the ground beneath their homes, as is shown in pictures 2 and 3 below.

                               1PortBendLandlsdHomes2002S.JPG (128440 bytes)       2PortBendhousenearestoceanS.JPG (114499 bytes)       3PortBendhouse2S.JPG (151210 bytes)

Efforts, including dewatering wells and improved surface drainage, were begun in the 1980's to remove water from the earth flow as quickly as possible during and after rainy weather have slowed the Portuguese Bend earth flow to a slow crawl - just a few feet per year.   Stopping the movement is the long-term goal, but this is unlikely to occur due to the continued erosion of the toe of the earth flow by wave activity.   Picture 1 shows the drain pipe where it delivers rain runoff to the ocean at the toe of the earth flow.   Here, rocks of all sizes are constantly breaking off from the fractured slopes, falling and avalanching down to the beach - not a good place to lay out your beach towel!   Picture 2 provides a glimpse of Palos Verdes Drive North, the main road crossing the earth flow.   Due to constant horizontal and vertical movement of the earth here, the road needs frequent patching and grading to make it safe for drivers and bicyclers.

                                                 1drainpipePortBendfarS.jpg (70673 bytes)       2PortBendSlideRoadDropS.JPG (150138 bytes)

Below is a composite view of Portuguese Bend earth flow showing a drainage pipe that carries water runoff from near the top of the earth flow complex directly out to the ocean.   Note the numerous slump blocks that comprise part of the earth flow in the right-hand photo, and Palos Verdes Drive North which has to be straightened every few years as the ground beneath it shifts and breaks.   See picture 2 above, for a closer view of the road.  

      

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