Mudslides are some of the most dangerous phenomena on earth as they cause immense devastation to the landscape in a short amount of time, and they usually occur with little or no warning. They claim thousands of lives every year, displace members of entire communities, cost billions of dollars in damage and there’s little that anyone can do to prevent them. The disastrous mudslides in California in early 2018 represent some of the worst in US history, and a combination of factors contributed to creating the perfect conditions for this tragedy to occur.
The simplest, general explanation of how mudslides occur is when when water saturates ground atop rocky, steep terrain that provides for poor adhesion. The weight and force of water literally dislodges soil, sediment and vegetation which causes it to slide downslope, carrying everything on the surface with it. Nothing downrange is safe from being swept away, and these churning torrents of water, trees, rocks and debris can move at speeds exceeding 35 miles per hour.
They find any available channel to travel through, whether a ravine or street, and nothing can stop these flows except gravity. Consequently, there’s little time to react when a mudslide is occurring, and there are also few places to escape unless people can reach higher ground in very short amount of time. Once the flows stop, all of that mud and debris settles, hardens and buries everything below in muck that can be as much as 20 feet deep in places.
Most people who die in mudslides either get battered by flowing debris within the torrent, or they suffocate after being trapped in thick, hard and heavy mud.
One of the reasons that the mudslides were so severe was that recent forest fires in the area destroyed vegetation that would normally soak up moisture from rain and snow. However, the land over an area roughly the size of Chicago was virtually barren, allowing water to flow freely downslope. Drought was another major factor as the lack of rainfall contributed to the lack of healthy vegetation that could soak up water and slow the rate of flows. After a bout of heavy rain, the ground couldn’t soak up the excess water, and it flowed downhill, saturating the weak soil in low lying communities, and this contributed to the force and severity of the mudslides.
Due to the hilly, rocky topography in the region, the mudslide picked up and transported a lot of large rocks and boulders. This added to the destructive capability of the mudslide as these heavy projectiles battered everything in its path. Rocks also slow down recovery efforts as crews have to dig around them or lift them out of the way with very heavy machinery that may not be able to get to the area.
Finally, steep terrain, hills, valleys and snaking canyons and ravines all channeled the fast-moving mudslides and provided few barriers that could slow their advance.
Why Weren’t Officials Prepared?
Officials have received a lot of criticism in the wake of the devastation for not doing enough to prepare ahead of time. In all fairness, there’s really little that anyone can do to predict when a mudslide will occur, and responding to it is a challenge when local agencies have also been devastated as well. It also takes time to clear roads in order to bring in relief equipment and supplies.
While there are warning signs that residents are taught to observe, such as heavy rains or fast snow-melt that may indicate that a mudslide can occur, most people in the area just returned from a mandatory evacuation associated with wildfires. Consequently, they may not have been as attuned to these indicators as they would have been recouping from the last disaster. In any case, it’s important to remember that there’s little that people can do other than try get out of the way as quickly as possible when a mudslide is imminent.
The best course of action to prevent being victimized by a mudslide is to avoid areas that are vulnerable to mudslides following heavy precipitation until the bulk of the water has flowed away. You should also have easy access to higher ground that will not get swept away by the fast moving torrents. Finally, make sure that you have a bug-out bag as well as an emergency locator just in case you do get trapped. While this isn’t the most-reassuring advice, it’s the best that we can do when dealing with a disaster of this magnitude.