The year was 2017. The place was California, and the annual cycle of fire and flood (or drought and deluge) was in full swing at the end of the year. The dry season had triggered wildfires that burned away the vegetation cover, leaving the soils vulnerable and exposed. Seasons changed, and the rains came in full force. Devastating mudslides in Southern California ensued during early 2018 because the conditions were ripe: water-saturated ground and a loss of stabilizing, vegetative ground cover.
Landslides refer to any downslope movement of soil, rock, or slope debris. Mudslides, mudflows, debris flows, rock falls, and slumps are all terms describing landslides. The two most common types of landslides are rotational slides and earthflows.
A rotational slide occurs when a large section of earth is transported downslope by sliding on a discrete detachment surface. The mass of soil and rock will partially disaggregate as it moves downslope. Rotational slides can occur when slopes are too steep or in areas where either natural or human-made processes undercut the base of the slope. An earthflow, or mudslide, occurs when water mixes with soil or debris, and the liquid-like mixture flows rapidly downslope.
Our environment is a matrix of interlocking events and processes. Nothing exists in isolation. If the Arctic gets warmer, then some areas on the planet get colder. If the dry seasons are hotter and longer, then the wet seasons bring a new set of problems.
California’s drought-deluge cycle
The corollary is clear for California’s climate cycle. Dry seasons are lasting longer with higher average temperatures. This sets the stage for more frequent and larger grassland and forest fires. But the rains still come, and intense winter storms dump excessive amounts of water onto landscapes with nothing left to hold the soil in place. Gravity operates independent of climate change, and when the soils are water-saturated, and the slopes are steep enough, the mudslides will flow.
Studies of environmental change are complex because the biosphere is an interconnected web of life that depends on the atmosphere, hydrosphere, and geosphere. Confusion is evident in our news and politics, where extreme weather in one area of the globe is confused with global climate change. Weather is a short-term change in conditions. Climate change is a long-range trajectory into the future. California’s fire-flood cycle is not a set of one-off events. It is a future way of life.