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A landslide is the downward and outward movement of slope-forming soil, rock, and vegetation driven by gravity. Both natural and human-induced changes in the environment can trigger landslides. These changes can include heavy rain, rapid snow melt, steepening of slopes due to construction or erosion, earthquakes, volcanic eruptions, and changes in groundwater levels.
In areas burned by forest and brush fires, a lower threshold of precipitation may initiate landslides. Some landslides move slowly and cause damage gradually, whereas others move so rapidly that they can destroy property and take lives suddenly and unexpectedly.
Among the most destructive types of debris flows are those that accompany volcanic eruptions. A spectacular example in the United States was a massive debris flow resulting from the 1980 eruptions of Mount St. Helens, Washington. Areas near the bases of many volcanoes in the Cascade Mountain Range of California, Oregon, and Washington are at risk from the same types of flows during future volcanic eruptions.
Areas that are generally prone to landslide hazards include previous landslide areas, the bases of steep slopes, the bases of drainage channels, and developed hillsides where leach-field septic systems are used. Areas that are typically considered safe from landslides include areas that have not moved in the past, relatively flat-lying areas away from sudden changes in slope, and areas at the top or along ridges set back from the tops of slopes.
According to USGS, each year landslides cause nearly $6.5 billion (2017 dollars) in damage and between 25 and 50 deaths in the United States.
Rock Fall: Rapid movements of bedrock, which result in bouncing or rolling.
Topple: Section or block of rock that rotates or tilts before falling to the slope below.
Slide: Movements of soil or rock along a distinct surface of rupture, which separates the slide material from the more stable underlying material.
Mudflow: Sometimes referred to as mudslides, mudflows, lahars, or debris avalanches, are fast- moving rivers of rock, earth, and other debris saturated with water. They develop when water rapidly accumulates in the ground, such as heavy rainfall or rapid snowmelt, changing the soil into a flowing river of mud or “slurry.”
Slurry: Can flow rapidly down slopes or through channels and can strike with little or no warning at avalanche speeds. Slurry can travel several miles from its source, growing larger as it picks up trees, cars, and other materials along the way. As the flows reach flatter ground, the mudflow spreads over a broad area where it can accumulate in thick deposits.
Landslides occur along steep slopes when the pull of gravity can no longer be resisted. Heavy rain, especially on wildfire-scarred land where the ground is unstable due to vegetation loss, often catalyzes a landslide event. Human development can also exacerbate risk by building on previously undevelopable steep slopes and constructing roads by cutting through mountains. Maui Island is comprised of two mountainous area – Haleakalā in the southeast and the West Maui Mountains in the northwest. The Islands of Lānaʻi and Molokaʻi also contain steep slopes associated with mountain ranges and extinct shield volcanoes. While susceptibility to landslides varies throughout the county, landslides are possible throughout the planning area.
Previous landslide occurrences were gleaned from previous plans, the NCEI Storm Events Database, previous disaster declarations, and local news sources. In all, 15 landslide events were reported in Maui County between 2004 and May 2020. While all community planning areas have been impacted by landslides, the Hāna community planning area was most affected, with 12 of the 15 events impacting the community planning area.
Extent can be defined using a variety of measures including the USGS Landslide Susceptibility classifications for the State of Hawaiʻi. Landslide size can also be used. For the USGS measure, the greatest extent possible is “very high susceptibly.” The highest category present in Maui County is “very high” susceptibility. Lastly, landslide extent can be measured in terms of size (width of slide or tonnage of debris generated). This data was reported for one of the recorded landslides. This was the May 2014 event on Hāna Highway, which reported a landslide 100 feet long and 20 feet high, which resulted in 2,000 cubic yards of debris being cleared. Larger, more severe landslide events are possible.
Future Probability:
With 15 landslide events occurring in Maui County in the last 16 years, the county experiences roughly one landslide event each year. As landslides are often caused by intense rainfall events or wildfires, the frequency of landslides is often related to the frequency of these other hazards. Therefore, landslide occurrences may increase with climate change as the frequency of intense storms and droughts, which can contribute to wildfire frequency, also increase. Within Maui County, certain community planning areas are more likely to experience landslides than others. The table below presents landslide probabilities for each community planning area.
All current and future buildings, infrastructure, and populations on or at the base of steep slopes, especially those in mapped landslide susceptibility areas, are considered at risk to landslides. Specific impacts to buildings, infrastructure, socially vulnerable populations, life safety, public health, and the economy from landslide hazards are described below. Climate-related impacts on landslides are also described.
Buildings and Infrastructure: All current and future buildings and infrastructure, including critical facilities, located within or downslope of landslide susceptibility areas are considered at risk to landslide. Landslides directly damage structures by disrupting structural foundations caused by deformation of the ground upon which the structure sits, and by the physical impact of debris moving down-slope against structures located in the travel path. As a landslide breaks away from a slope and moves, it deforms the ground into an uneven surface.
When situated on top of a landslide, the deformation distresses structural foundations and the structures themselves by settlement, cracking, and tilting. This can occur slowly, over years, or rapidly within days/hours. A water-saturated, fast-moving debris flow (or “mudslide”) can destroy all in its path, collapsing walls and shifting structures off their foundations.
Health and Safety: Landslides can have severe life safety impacts. People in buildings or cars that are impacted by landslides may suffer death or injury. Landslide movement can occur quickly, without warning, or slowly. Movement can range from feet per second to a slow creep of inches per year. Therefore, no official warning system is in place for landslides.
Economic Impacts: Landslides can result in economic losses in several ways. Direct losses may occur due to damaged destroyed structures, cars or utility lines. Roads may become damaged or undermined. Costs may be incurred by the state or county to dispatch crews in order to clear debris from roadways. Indirectly, landslides may cause business interruptions by cutting off access or disrupting utility services. Even if a small section of a road or utility line is damaged by a landslide, the entire system may be impacted. For instance, a landslide that results in one area of a road being impassible may block accessibility to other roads or locations, preventing evacuations, delivery of goods and services, and disrupting schools and businesses. For example, during the 2006 landslides associated with the Kiholo Bay and Mahukona earthquakes, damage to a truss bridge and debris blocking a road impeding access, isolating approximately 500 Maui residents.
Climate Change Impacts: Climate change may increase the frequency and intensity of storm events, and intense rainfall events often trigger landslides. Further, warming temperatures are projected to increase the frequency and duration of droughts, which in turn could increase wildfire occurrences. Wildfires are able to rapidly move up steep slopes, and fire-scorched land on steep slopes, without the support of rooted vegetation, are more susceptible to landslides. All of these factors would increase the probability for landslide occurrences in Maui County.
