Summary
We call mudslides debris flows because it's not just mud, it's going to have bushes in it, trees, fragments of vegetation, rocks with the mud in a great huge porridge.
Debris flows are the result of this sequence of a events:
1) Rain allows brush to grow
2) Hot sunny dry weather dries out plants
3) Fire accompanied by wind burns brush off the hillside
4) Heavy rain saturates the soil to the point where it gives way and causes a debris flow
1934 New Years Day Disaster in LA Crescenta
NE corner of Rosemont and Fairview
On New Year's Day, 1934 a debris flow destroyed an American Legion outpost. It was raining so hard that some people came to a public building that seemed so sturdy and warm and inviting to shelter from a heavy rainstorm. 12 people where killed at this location, as well as several dozen in La Crescenta. The flow came out of Shields and Goss Canyons in La Crescenta. Heavy rains in preceding days saturated the ground in the hills to the breaking point. When the ground on the hill it had all it could take, it unleashed a catastrophic debris flow that rumbled down the mountain with tremendous force destroying whatever lay in its path.
After this event, The Army Corps of Engineers and other civil engineer groups worked to create a runoff system to help prevent these mud and debris flows. Channels where created to collect the water and control its flow and take it away into a larger drainage network that leads to the ocean. Debris basins, perhaps the most important stop gap, collect the largest objects in the debris flow and strain the water out of it. The water then is directed into channels that follow the natural direction of the canyon slope. Once the water is strained out of that mud porridge, the porridge itself becomes solid and doesn't move thusly removing the threat to life and property.
But in 1978 and 2010 heavy rains produced debris flows that overwhelmed these debris basins. Both events where preceded by fires which took out vegetation that held down the soil. So the debris basin and drainage network is not a perfect cure. Think of ways you can improve this system.
Cycles and conditions for debris flows
Vegetation in our community
On our tour of Deukmejian Wilderness Park, we learned that the vegetation in this community burns regularly. Every 15-35 years it will burn right to the ground. It's biology has adapted to allow it to spring back to life very fast. The Station Fire happened only 4 years ago, but if you look around, unless you were really looking for the charcoal, you wouldn't know there was a recent fire here. The plants in this location a primarily brush. Their distant ancestors may have originated in the tundra or tropics. Thru migration they found themselves here in a common environment. Perhaps 20 different species, that end up looking pretty much the same. It is difficult to tell one from the other as they have similar forms. All the plants have small grey leaves, no big tree trunks, they are all really drought resistant so their leaves get leathery. So when we have different species from different origins that originally looked very different but when they are in similar environment and they all wind up looking the same, we call it convergent evolution. The Mediterranean has a climate just like ours. If you are in Italy or Southern France, it looks just like this! Even though they came from different origins, they converge on a similar form (convergent evolution). In winter, when there is lots of water thats when the growth happens. In summer there is no water and they stop growing. These plants are adapted to drought with small tough leaves. But they are also prone to fire. The leaves contain an oily resin that protects the plants from insects and mold. But it is very flammable, and explodes during fire!
How do these plants come back so quickly after a fire?
Nutrients that where once in the leaves return to the soil in the ash
Dormant buds are present in the base and under ground. Heat from fire knocks out chemicals that have been keeping the buds in suspended animation, and they sprout up immediately after the fire.
Dormant seeds have a similar chemical that keeps them from sprouting. They also contain a proteinaceous compound that is denatured by heat, thus allowing them to sprout. So the seeds may sit on the ground for 4-5 years and do nothing, get switched 'on' by a fire, and as soon as there is a rain they sprout.
Wildfires
The way the fire propagates is thru the wind. It starts to burn as an intense hot fire, and this whispy vegetation sends up sparks and 'Fly Ash' (burning parts). The heat creates a huge updraft that carries these fragments of burning vegetation and ash with them off and other fires start on the next ridge. With wind fires are capable of moving up to 25-30 mph. Fires can leapfrog out running even fire vehicles. This makes these fires very dangerous to fire fighters, as sometimes they are not aware that the fire has moved putting them in danger.
Is there a way to warn fire fighters the immediate location of the fire and their position?
Ariel evacuation?
Heat sensors?
Thermal imaging?
Could ariel drones play a role?
How do we communicate real time information to the fire fighters on the ground?
Wildfires take out the vegetation on hillsides, leaving the soil vulnerable to mudslides and debris flows with the right amount of rain.
Mudslides and Debris Flows
Mudslides and Debris Flows happen after certain conditions. First a wildfire removes vegetation, then heavy rains saturate the soil. When the right amount of water loads in the soil, a mudslide or debris flow may occur.
How do we know if we are at risk for mudslides?
What does the community to warn people about mudslides?
Is there any way of knowing a mudslide is imminent?
In the tsunami world, if there is a submarine earthquake, immediately the international tsunami warning system measures where it is, measures its size and estimates the potential for a tsunami and sends out warnings.
Is there anything comparable where we know the state of a hillside and whether a mudslide is imminent?
If there is, how do we get the warning to people?
This is something you need to research.
If you guys can think of a good technical solution, we could know when the mud is about to give way and send people a warning.
Cameras in the hills?
Seismic activity?
Sensors to measure moisture content in the soil?
Check out this Lego team doing their presentation:
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