Five questions: What a Cal State Fullerton professor is learning about deadly mudslides

The catastrophic mudslide that inundated houses in Montecito in Santa Barbara County in January, killing 21 people, appeared to hit suddenly. But the disaster, mere weeks after a wildfire scorched the area, didn’t come out of nowhere.

For over two decades, Cal State Fullerton’s Binod Tiwari has studied such mudslides and landslides around the world, including in Southern California, to understand their causes and mitigate their devastation.

In 2014, the civil and environmental engineering professor and his students worked on a regional study on debris flow and mudflow after a series of December storms. The study included areas affected by the Silverado Canyon fire and the 91 Freeway fire, both in September 2014. It found that reports of mudflows and mudslides appeared to be exclusively in areas that burned that year or the year before.

Where the largest debris flows occurred during the December storms, they were generally expected. Where recently burned areas on steep slopes did not experience flows or slides, such as the Silverado fire zones, the biggest and perhaps only reason appeared to be that the rainfall was less intense and prolonged than the known threshold that initiates such events, the study said.

In 2015, Tiwari traveled to Nepal, his homeland, to co-lead an international team that evaluated damage to buildings after an earthquake killed more than 8,600 people. He did something similar after the 2011 quake and tsunami in Japan.

Tiwari, named CSUF’s outstanding professor in 2017, is a vice president of the International Consortium on Landslides and an executive editor of the consortium’s journal Landslides. He is an editor for many of the books, teaching tools and forum proceedings that ICL publishes, and he organizes conferences and collaborates with United Nations organizations on disaster preparedness for landslides.

Tiwari recently answered questions for the CSUF News Service on his latest research efforts to reduce deaths and property destruction caused by catastrophic mudslides:

What is a mudslide versus a landslide?

Mudslides and landslides seem to be synonymous, but they are different. A landslide is the movement of soil or rock mass due to the loss of resistance against the force that is pushing the soil or rock downwards and outwards. Landslides involve a shallow or deep soil/rock mass, which could be dry or saturated with water. As such, they are slides and not a flow. A mudslide is generally a flow of saturated soil mass that has water content high enough for the soil to behave as liquid. Mudslides happen very quickly with fast-moving soil and water.

What causes these natural disasters?

Landslides are caused by the downward seepage of rainwater through the soil mass, which increases the weight of soil, and in most cases, results in raising the groundwater toward the surface and causing soil failure. Landslides can occur due to other causes, such as earthquakes and snowmelt — with soil erosion, in many cases, the primary cause. Mudslides are caused by water saturation of the immediate topsoil layer from rainfall. Wildfires are a triggering factor for mudslides usually due to vegetation loss.

What have you learned through your research efforts?

My research involves landslide and mudslide mitigation in the United States, Japan and Nepal. My work focuses on trying to figure out the rainfall amount per hour, which causes infiltration into the soil to trigger landslides, as well as soil erosion. The intensity of rainfall, as well as slope inclination and lack of vegetation cover, play crucial roles in triggering mudslides. The prevention of landslides requires expensive countermeasures, mostly resistive in nature, such as re-grading the slopes, managing groundwater, restraining slopes with piles, and retaining walls and anchors.

Vegetation and water management are the main preventive measures for mudslides. Barrier dams constructed on gullies and creeks, and retaining walls near properties adjacent to slopes, can also help to reduce the devastating effects of mudslides.

Are you planning any new research efforts on this front?

Together with my students, this spring we will be studying the impact of vegetation loss in triggering mudslides. I have a rain simulator system and a slope model box in my research lab, and we will study how we can minimize post-wildfire mudslides like what happened in the Santa Barbara area. We will be simulating various slopes from Southern California wildfire-affected areas over the past 10 years. We’ll also be evaluating the stability of slopes and the potential for mudslides, using different rainfall intensities and durations, with and without vegetation cover. Additionally, we will explore the effectiveness of various types of mudflow barriers for temporary and long-term protection of properties from potential mudslides.

Do you have any advice to those living in areas prone to mudslides and landslides?

If the risk of mudslide or landslide is high, people need to be ready to evacuate the risk-prone area during intense rainfall.