Redwood Reader

The tragic loss of life at La Conchita cannot be undone, but we can learn powerful lessons from the short video captured by the news crew. Let me say that I have witnessed a relatively small sloughing from a vertical bank and was impressed by the speed and power of moving mud. The only one I've seen on film for study was in a Japanese landslide study, where they have built chutes studded with sensors for studying mudflows. These studies showed how far material "rolls out" after sliding and hitting the bottom of the hill. Roll out is a property of material in a liquified state, which is how even dry material acts, once in motion and mixing with air, far further than scientists realized at that time. The proof was that rollout continued for about three times the height of the slide. e can look at the big piece of granite that fell off El Capitan some years ago and see this effect on dry, originally solid material. It takes a lot of force to pulverize granite and turnitinto dust and roll out sediment. In Humboldt County the building code calls for setbacks at least the height of the slide plus forty feet at the top and the height plus 15 feet from the foot. These are insufficient in since we know dry material will move threee times the height of the slide, and wet material may move a little further. While it is difficult to estimate distances on a short video, it is clear houses were closer than the height of the bluff, let alone height plus or three times height.
JIm Marple was a regular contributor to Waterforum@yahoogroups.com. He would rail against massive water projects and their operators as wasteful and unnatural, therefore unsustainable policies. He got in many verbal tiffs with water techs and pros and some tried to ban his posts. Nevertheless his dogged determination as well as his illuminating posts have provided much insight into why my creek was failing. Two outstanding principles were repeated: water must pass through the biological zone, which is at least modified by the action of plants and animals such as beaver, buffalo and gophers. The second is that we have discarded any attempt at sustainability in the greater LA basin simply through political agendas that have created a huge , expensive and unnecessary public works and bureauocracy which sustains itself through inertia and arrogance, which is difficult to dislodge. Using average rainfall, LA needs to capture just 10 percent to supply itself with all the water it uses. Instead, paving and roofing have covered a vast area, creating runoff, publicly channeled into storm drains and shipped to sea as fast as possible. Development, fire and cutting have left the mountains surrounding LA unable to handle the same amount of rain it used to. Remember, landscapes are shaped by peak events and it can be many years between events. So it is in the Southland this year, wettest in 115 years and falling on a landscape considerably diminished in capacity in the interveneing years. Plus we diverted water from all over the Southwest to meet the needs even as we threw their own water away, leading to heated debates about whether it was smarter to build dams or restore the earths natural sponge by returning brush and field back to forest in the upper Colorado watershed. Other hot topics included the costs of pumping water over the Tehachapis, before but especially during the energy crisies, Owens Valley, loss of water in dams from evaporation, use of cisterns and other user friendly water saving ideas. These are big issues, getting bigger by the day. This is science looking at a problem for answers. Marples attackers did not seem to understand this.
In this short video we can see the vegetation ride the slide basically intact. The entire hillside is moving at a depth far below the root zone of the chapparral. As the material moves it is possible to see a clay layer with a small stream in it appear at the bottom of the slide. The moving material is entirely soil. So after record rainfall the soil was saturated throughout the column all the way to the clay layer, which was basically at street lavel and with no weight on the toe, a relatively cheap insurance well known to slide mitigators. What is not clear is the exact trigger. The soil column was probably saturated to the clay. Without roots, hyphae or glomalin at depth the soil lost its adherence, liquifying en masse all at once and sliding on the wet clay. Rollout appears to be at least three times height, bearing out the Japanese studies.
If we were to implement all we have learned in the LA basin there would be a large decrease in the amount of safe land to build on, and a percentage of that would have to be set aside to absorb rain. Insurance regulators, FEMA, and building codes all need to rethink their policies as continuation leads to diminished natural systems and increased human risk and assured losses in heavy events, in addition to increasing fire danger. In this sense the people in LA are at the same risk as the third world debacles in NIcaragua, Haiti, The Phillipines, Venezuela or Colombia, all in the last few years and attributed to logging and development on unstable soils above population centers.