Where do landslides occur? Landslides in the United States occur in all 50 States. The primary regions of landslide occurrence and potential are the coastal and mountainous areas of California, Oregon, and Washington, the States comprising the intermountain west, and the mountainous and hilly regions of the Eastern United States. Alaska and Hawaii also experience all types of landslides. How fast do landslide travel? Landslides can move slowly, millimeters per year or can move quickly and disastrously, as is the case with debris flows.
Debris flows can travel down a hillside at speeds up to miles per hour more commonly, 30 — 50 miles per hour , depending on the slope angle, water content, volume of debris, and type of earth and debris in the flow. These flows are initiated by heavy periods of rainfall, but sometimes can happen as a result of short bursts of concentrated rainfall or other factors in susceptible areas. Burned areas charred by wildfires are particularly susceptible to debris flows, given certain soil characteristics and slope conditions.
Why study landslides? Landslides are a serious geologic hazard. Globally, landslides cause billions of dollars in damages and thousands of deaths and injuries each year.
Who is most at risk for landslides? As people move into new areas of hilly or mountainous terrain, it is important to understand the nature of their potential exposure to landslide hazards, and how cities, towns, and counties can plan for land-use, engineering of new construction and infrastructure, and other measures which will reduce the costs of living with landslides.
Although the physical causes of many landslides cannot be removed, geologic investigations, good engineering practices, and effective enforcement of land-use management regulations can reduce landslide hazards. Do human activities cause landslides? Yes, in some cases human activities can be a contributing factor in causing landslides. Many human-caused landslides can be avoided or mitigated. They are commonly a result of building roads and structures without adequate grading of slopes, of poorly planned alteration of drainage patterns, and of disturbing old landslides.
Where can I find landslide information for my area? Geological Survey Landslide Hazards Program that collects and distributes all forms of information related to landslides.
The NLIC is designed to serve landslide researchers, geotechnical practitioners engaged in landslide stabilization, and anyone else concerned in any way with landslide education, hazard, safety, and mitigation. Every state in the US has a geoscience agency and most have some landslide information.
What was the most expensive landslide to fix in the United States? The landslide occurred during the spring of , when unseasonably warm weather caused rapid snowmelt to saturate the slope. It also flowed across the Spanish Fork River, forming a dam. The impounded river water inundated the small town of Thistle.
The inhabitants of the town of Thistle, directly upstream from the landslide, were evacuated as the lake began to flood the town, and within a day the town was completely covered with water. Populations downstream from the dam were at risk because of the possible overtopping of the landslide by the lake.
This could cause a catastrophic outburst of the dam with a massive flood downstream. Eventually, a drain system was engineered to drain the lake and avert the potential disaster. How many deaths result from landslides? An average of between 25 and 50 people are killed by landslides each year in the United States. The worldwide death toll per year due to landslides is in the thousands. Most landslide fatalities are from rock fall, debris-flows, or volcanic debris flows.
What should I know about wildfires and debris flows? Wild land fires are inevitable in the western United States. Expansion of human development into forested areas has created a situation where wildfires can adversely affect lives and property, as can the flooding and landslides that occur in the aftermath of the fires.
There is a need to develop tools and methods to identify and quantify the potential hazards posed by landslides produced from burned watersheds. Post-fire landslide hazards include fast-moving, highly destructive debris flows that can occur in the years immediately after wildfires in response to high intensity rainfall events, and those flows that are generated over longer time periods accompanied by root decay and loss of soil strength. Post-fire debris flows are particularly hazardous because they can occur with little warning, can exert great impulsive loads on objects in their paths, and can strip vegetation, block drainage ways, damage structures, and endanger human life.
Wildfires could potentially result in the destabilization of pre-existing deep-seated landslides over long time periods. How do landslides cause tsunamis? Tsunamis are large, potentially deadly and destructive sea waves, most of which are formed as a result of submarine earthquakes. They may also result from the eruption or collapse of island or coastal volcanoes and the formation of giant landslides on marine margins.
These landslides, in turn, are often triggered by earthquakes. Tsunamis can be generated on impact as a rapidly moving landslide mass enters the water or as water displaces behind and ahead of a rapidly moving underwater landslide. What are some examples of landslides that have caused tsunamis?
The Alaska earthquake caused deaths in Alaska alone, with of those due to tsunamis generated by tectonic uplift of the sea floor, and by localized subareal and submarine landslides. The earthquake shaking caused at least 5 local slide-generated tsunamis within minutes after the shaking began.
An eyewitness account of the tsunami caused by the movement and landslides of the Alaska earthquake. Research in the Canary Islands concludes that there have been at least five massive volcano landslides that occurred in the past, and that similar large events may occur in the future.
Giant landslides have the potential of generating large tsunami waves at close and also very great distances and would have the potential to devastate large areas of coastal land as far away as the eastern seaboard of North America.
Rock falls and rock avalanches in coastal inlets, such as those that have occurred in the past at Tidal Inlet, Glacier Bay National Park, Alaska, have the potential to cause regional tsunamis that pose a hazard to coastal ecosystems and human settlements. On July 9, , a magnitude M 7. The landslide generated a wave that ran up m on the opposite shore and sent a m high wave through Lituya Bay, sinking two of three fishing boats and killing two persons.
Due to geological, geotechnical, and geomorphological uncertainties, it is usually difficult to predict where and when a landslide may occur. Nevertheless, it is generally recognized that changes in the water content of the soil imply changes like increase in pore pressure, decrease the effective stress of the soils, and, thus, reduce the shear strength [ 16 ].
Thus, understanding the physical conditions i. Many authors have been working worldwide on the subject of the rainfall-induced landslides, especially in countries like Italy, Switzerland, Spain, Taiwan, Singapore, and China. To do this, they have been proposed in the literature, empirical rainfall thresholds, and physically based models.
Even in countries affected by the effects of intertropical convergence zone, area with high rainfall periods, such as Colombia and Brazil, have been joining forces for the generation of computer applications that allow the evaluation of slope stability based on empirical and physically based methods. Understanding the processes that trigger a landslide is crucial to any successful landslide assessment and zonation. This topic is an active field of research worldwide, and it is required to find out the critical factors that trigger landslides.
Research is required to establish the spatial and temporal prediction of hazardous zones and estimation on the probability and magnitude of future landslide.
According to this, it considers the following research questions: Are the engineers relating the effect of infiltration on the instability of slope processes correctly? Can physically based models be used as a significant tool to evaluate the effect of infiltration process in slope stability?
Proposed models reproduce the real phenomenon in an adequate form? The limitations of some techniques of evaluation and monitoring of landslides triggered by rainfall can be compensated or minimized with the advantages of other techniques used for the same purpose, so to be articulated to establish an integral proposal on this subject?
Some of the answers to the questions aforementioned are well known, also processes involved. However, some mismatches between theory and experiment exist yet. Hence, further investigations are required to understand better, if these gaps correspond to lacks in the theoretical background of the phenomena involved or to the experimental errors in field measurements.
Hence, the current approach involves several fronts to solve the complexity and uncertainty of the addressed problem: A numerical modeling to solve the system of equations describing the failure mechanism due to rainfall.
In this part, it is included numerical modeling of slope stability, considering the effects of infiltration process and spatial variability of geotechnical and hydraulic parameter of soil.
An experimental work in a laboratory with controlled conditions to evaluate how the propagation of water flows inside an unsaturated soil.
An instrumentation of tests field for coupling of proposed theoretical and experimental models. This part will permit the validation of models under realistic conditions of geotechnical, geomorphologic and hydraulic parameters, and rainfall patterns.
These approaches clarify the effects of rainfall and its consequent infiltration in slope stability of unsaturated deposits of tropical mountainous regions. Quantitatively, the risk R can be defined in terms of the hazard P [ T ], understood as the total probability of a threatening event that happens, and the vulnerability P [ C T ], understood as the conditional probability of damage considering that a failure has already occurred and the cost of the consequences C, by the equation:.
This paper emphasizes landslide hazard through a probabilistic methodology for hazard assessment, which uses methods as first order second method FOSM and failure thresholds.
The methodology for the landslide hazard assessment was developed by [ 10 , 18 ] through a calculation model based on FOSM. The methodology shown graphically in Figure 1 allows calculating the total probability of failure TPF according to the theorem of total probability of failure of a slope by the equation:.
Schematic methodology adopted for hazard assessment. The slope probability of failure in both saturated and unsaturated condition usually can be calculated in an independent way. However, determining the probability that the soil is in a saturated condition is complicated, especially due to the complexity of the phenomenon that considers variation of the conditions of soil water content.
The effect of accumulated rainfall and that the occurrence of landslides is possible to be related to the amount of rainfall through so-called failure thresholds or numerical models with physical base to estimate the probability of saturation [ 19 ].
The most common way to assess the slope stability is using limit equilibrium methods with planar or circular surfaces. Particularly, for regional analysis, the concept of infinite slope is often used [ 7 ]. The resulting expression for infinite slope model in this work is presented as below:.
In mountainous tropical regions, landslides occur most often in rainy seasons in which increased soil saturation with consequent decrease in their cohesion and increased pore pressure are presented. The process of decrease in the shear strength due to changes in water content is a highly complex process, which is not considered in the development of this study. Therefore, the effect of saturation is taken into consideration only in the increase of the water pressure, and for purposes of analysis in this study, two situations were considered for the water height measured from the failure surface H w , one where the water level presented in the most critical condition was considered, i.
The eventual saturation condition of the soil is a random phenomenon that must be taken into consideration in the evaluation of the probability of landslides. In this case, it was considering the probability that the soil is saturated or not.
Slope stability in tropical areas is highly affected by rainfall but also depends on soil properties such as shear strength and hydromechanical properties. In the other hand, it has been identified that the relationship between rainfall and landslides is influenced by conditions of preceding rainfall or accumulated rain on the ground before the triggering event [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ], named thresholds of failure.
There are proposed thresholds of failure in terms of the intensity of rainfall and the accumulated rainfall of several days [ 25 ], but in areas where insufficient rainfall information is available, thresholds have been set in terms of accumulated rainfall with precedent rainfall of 15, 30, 60, and 90 days and different durations of triggering events as 1, 3, or 5 days [ 22 , 23 ].
Jaiswal and Van Westen [ 3 , 27 ] conducted research in which empirical thresholds were used to estimate the probability of failure in slopes of roadways of southern India using the Poisson distribution.
The thresholds of failure can be used to assess the triggering effect of rainfall on landslides and the landslide hazard. Later, [ 30 ] studied the relationship between rainfall and landslides in the department of Antioquia, Colombia, for the time period — With a total of landslides, a threshold has been determined according to the equation:. The similarity between Eqs. The threshold proposed by [ 30 ], which is presented in Eq.
In other works, has been identified as the most important constraint for the occurrence of landslides in the Aburra Valley are the long-term accumulative rainfall of around 60 mm in 30 days, mm in 60 days, and mm for 90 days by the seasonal rainfall.
For landslide hazard on roadways, Hidalgo and Assis [ 32 , 33 ] presented a threshold as a relationship between precedent rainfall of 15 days and antecedent rainfall of 5 days. For precedent rainfall of 30 days, they reported exceedance rates greater than in the cases of 15 and 60 days.
Considering that accumulated rainfall for 15 and 30 days are easier to obtain, thresholds based on rainstorms of 5-days duration and previous rainfall of 30 and 15 days are proposed, as shown in Figure 2.
It is important to highlight that this relationship was determined with rainfall data of days when there were landslides of the manmade slopes of the roadway. Due to this, this threshold is lower than those defined by Eqs. Combinations of total accumulated rainfall of 5 and 15 days, for days with landslide events [ 35 ]. In a methodology presented by [ 1 ], the thresholds are used to asses P s. It is accepted that the condition given by the failure threshold represents a saturation condition conducive to landslides, with the already mentioned reduction of shear strength of the material due to the decrease in suction and pressure generation of pores [ 1 , 20 , 32 ].
The probability of saturation is calculated as the probability that the ordered pair accumulated rainfall preceding of 15 days R 15 and, accumulated rainfall antecedent of 3 days R 3 , is above the failure threshold line for study area, i.
Based on the concepts presented above, records for each rainfall station were organized, and mobile windows from accumulated rainfall of 15 and 3 days were calculated for each date. Likewise, for each date, 3-days rainfall value was calculated using a threshold value defined by the threshold equation. The comparison between 3-days rainfall values was established as shown in Eq.
In order to establish the likelihood that the threshold was exceeded, the number of times which the threshold was exceeded along the records was determined, and then, this value of occurrences was divided by the total number of rainfall records, which for the methodology used in this work represents that soil reached the condition of critical saturation Figure 3.
The return period for these events is determined using a Gumbel distribution for the accumulated rainfall. After determining the probability that the soil is saturated according to data from meteorological stations, a geostatistical interpolation process to estimate the probability of saturation in each of the cells, its mean P s is estimated spatially.
Schematic methodology adopted for exceedance probability and rainfall of analysis. The effect of rainfall on slope stability is due to a complex physical process involving the advance of the wet front and the consequent increase in pore pressure and reduction of soil cohesion. Coupled models that consider the gradual advance of the wet front have been developed [ 24 , 33 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 ]; however, obtaining the input parameters is difficult and expensive, so these are not yet in common use.
Consequently, the most rigorous methods are still recommended only for preliminary assessments, and for special cases, when a good amount of measurements of the involved parameters is possible [ 40 ].
In recent years, slope stability analyses have been expanded to include coupled hydromechanical processes under variably saturated conditions. These analyses incorporate the variation of saturation, leading to more accurate assessments of slopes stability under infiltration conditions , and demonstrate that a better physical representation of water flow and stress can be attained in unsaturated soils [ 17 ]. That may seem slow, but over time the movement grinds and breaks up the rock, affecting the stability of surrounding slopes and making them more prone to landslides.
Rainfall in Southern Leyte during this time exceeded sixty-eight centimeters twenty-seven inches , more than twice the normal amount for this time of year. Courtesy NASA. Evans also used his ground observations to validate what he had observed in remotely sensed data. Before departing for Leyte Island, he and his team obtained satellite data that provided topographic information and optical views of pre- and post-landslide Guinsaugon.
In the field, Evans and his team compared the data to their measurements of the vertical landslide descent, the horizontal run-out distance, and the landslide volume. The TRMM image revealed that, in Guinsaugon, more than sixty-eight centimeters twenty-seven inches of rain had fallen between February 4, , and February 17, However, the heaviest rainfall occurred between February 8, , and February 12, , ending four days before the landslide.
He said that landslides occur all the time and that direct triggers do not always exist; however, the excessive rainfall on Leyte Island definitely was a factor.
The TMPA-RT data, currently available online from through the present, are updated in real time, allowing users to determine if an area is currently receiving particularly intense rainfall or has reached a critical level of accumulation. However, Adler and Hong stress that the product is still experimental. Even in its experimental status, researchers and agencies are using the TMPA-RT system to assess landslide and flood hazards.
Making the satellite data available helps supplement conventional ground-based rain-gauge networks that do not provide enough coverage. In addition, a research group at Tennessee Technological University is assessing TMPA-RT data to help gauge precipitation and flooding in more than river basins worldwide.
Many large river systems cross international borders, meaning that downstream countries often need to negotiate with their upstream neighbors to access critical flood hazard information. Using satellite data can be an easier and more cost effective method to observe conditions along an entire river basin, proving critical when upstream nations lack adequate information. Persistent heavy rain during December and January triggered flooding and mudslides across South America, killing more than sixty people in Brazil, Bolivia, and Peru.
In combination with certain types of terrain, soil conditions, and land cover, heavy rain can make certain regions vulnerable to landslides. The top image indicates rainfall in millimeters; the bottom image shows which areas received more rainfall than average for this time of year, in millimeters per day. And for landslide-prone areas like Leyte Island, this research may ultimately save lives.
As with many mountainous areas in the tropics, timely landslide hazard assessment may be difficult to accomplish without satellite data. It can give them quantitative information about where exactly the hazard is and which areas are affected. The online TMPA-RT data provides an easy way for people to download text files or zoom into geographic maps that display three-hour rainfall rates or seven-day accumulations. In addition, Hong is making hourly rainfall data available through Google Earth.
British Broadcasting Company News. What caused the Philippines landslide? Accessed January 16,
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