The Wrangell landslide occurred within a matter of seconds.
Atop a hill over Zimovia Highway, the slope began to move. Within a matter of minutes the mud-filled river spread out and ran for almost 4000 feet before traversing the beach before spilling into the ocean. It caused damage to the land to the extent of 37 acres.
Residents had no warning — they heard an eerie sound from the slides. The slide buried two homes with five dead and one person still missing.
According to the state Geologist Barrett Salisbury with the Alaska Department of Natural Resources, it’s very difficult to provide warnings of disasters such as this.
The span style=”font-weight 400 ;”>”We’re always going to be shocked by a landslide,” said Salisbury. “We could provide you with an estimation of where we think the hotspots for future events might be, or could be, but there are no guarantee. .”
The only thing that is certain, Salisbury said, is that hotspots like these exist throughout Southeast Alaska, where many communities live on mountain slopes that are steep. The human-caused climate change may cause more landslides throughout the region.
Landslides are extremely complex. Every one will be shaped according to the specific geography, hydrology and the vegetation that is present on the slope. This is one of the reasons they’re difficult to predict.
However, researchers such as Gabriel Wolken of the Alaska Division of Geological and Geophysical Surveys affirm that Southeast generally lacks the types of surveillance that can ensure that people are safe.
“span style=”font-weight 400 ;”>”We have, as you have this understanding of the many components which can help in the formation of a landslide” said Wolken who is the manager of the state’s climate and cryosphere risks program. “But we’re not able to provide the necessary data .”
How do debris flows begin
Salisbury was part of the geologists who gathered data on this November. 20 slide during the days following it. There’s still lots of unanswered questions.
The span style=”font-weight 400 ;”>”We do not know the exact location where it began. We don’t even know where it started,” Salisbury said. “But we know that the moisture was present .”
Rainy days are almost always the main reason for an “when” in Southeast landslides. Similar to that of Wrangell slide, the deadly slide occurred in Sitka in 2015 and Haines in 2020 occurred during intense rainstorms.
On slopes that are steep the earth is always in opposition to gravity because of the friction between the grains of soil. But soil particles are brittle or jagged and they aren’t arranged precisely.
It’s a span style=”font-weight 400 ;”>”Doesn’t matter how you pack them” Salisbury said. “Those tiny grains touch only a small portion of their size .”
There’s always room between them. In the 24 hours prior to the Wrangell Landslide, a powerful storm poured the area with three inches of water. All that water leaked into the spaces between grains, causing pressure to build up in those pores in the earth.
And, just ahead of the slide accelerated. Within six hours, the storm dropped over 1 centimeter of water. The water continued to push through the pores until they were filled completely.
span style=”font-weight 400 ;”>”Eventually soil particles would be pulled away from each other,” Salisbury said.
Then, the solid earth is transformed into the form of a slurry consisting of water and mud and is primed to be the kind of landslide known as debris flows.
There are many factors that could cause the earth to break. A seismic event could shake the earth or it could slide away from a crack created by the topography of the slope. A falling tree can weaken the soil when it pulls its roots. In Wrangell the area where winds in excess of 60 miles per hour smashed the trees during the storm, this is one possibility.
Geologists are aware that the flow of debris in Wrangell started on a small course that was just 85 feet in width. As it increased speed it widened out to over 700 feet wide at its widest. The dirt became more heavy as it gained momentum as it sucked up debris along its route.
“Trees and shrubs, automobiles, homes — everything in the path becomes part of this flow of debris,” said Salisbury.
All that debris accelerated the slide down Zimovia Highway, where it was deposited on the road in a massive pile that took a few days to remove.
The span style=”font-weight 400 ;”>”It is similar to soup,”” Salisbury said. “The large front end loader tried to get it higher up and then they’d dump it down, and then it would go straight back to the place the original .”
There aren’t sufficient weather stations
In the soils in Southeast Alaska can take on lots of waterthey’ve developed that way because of hundreds of thousands of years with heavy rainfall. The amount of rain that fell on Wrangell prior to the slide was certainly not uncommon.
It’s extremely difficult to identify an exact threshold when enough rain will cause a landslide and Southeast’s dearth of data on weather renders it impractical. In most places that are most comprehensive and reliable weather data is gathered at airports, which are located on level ground close to sea level.
The span style=”font-weight 400 ;”>”We don’t have many weather stations in The Southeast,” Salisbury said. “Especially not on a tiny island like Wrangell in the area where there are a lot of homes spread across different kinds of terrain. .”
Topography in Southeast Alaska creates microclimates, where the weather behaves in a very different way in tiny distances. A microclimate in mile 11.2 of the Zimovia Highway may explain, in part, the reason for the downslope.
span style=”font-weight 400 ;”>”It could be a cloud that is thicker at 10 miles from the town and it was raining even more” Salisbury said. “Or because of the form of the mountains as well as the channels, the winds were focused on triggering the slide. We don’t know the .”
There aren’t any weather records to record what occurred on that ridge. However, it is known that the State Department of Transportation did set up a gauge for rain at that slide area during their investigation. In one night, the gauge recorded twice as much rainfall than the gauge that was in the airfield.
“The “where” of the landslides
The “where” of the landslide could be more difficult. But, according to Wolken his research, the history of landslides can be an excellent guideline.
The span style=”font-weight 400 ;”>”There is a pattern across this landscape” Wolken said. “Where one landslide occurred previously, there’s the possibility that subsequent landslides will be in the same locations. .”
People living on Wrangell Island can recall stories of smaller slides that took place within their own backyards or even in the backyards of their neighbors that left piles of dirt or jumbles rocks. The more overgrown slides’ marks can be observed from planes or vessels, such as a huge mark that the Salisbury team discovered in towards the southwest of the most recent slide. The slide that was in the past stopped just short of homes.
Salisbury said it’s not easy to determine what caused the hill to rise over mile 11.2 has given way. On both sides the slope is similar in terms of topography, geology and cover. However, that November. 20 slide may create new weakness in the slope.
When the slide began to fall it collided with the bedrock ridges, which included the old bench of a log road. These collisions caused tendrils from the liquified soil to split into two parts on the opposite side of the slide – two to the north and another towards the south.
Some of the smaller slides fell into streams that funneled them farther down hillside in which they also stopped in front of houses. These paths are now prone to slide back.
Slides to study in an ever-changing climate
While he is aware of the elements that make up an avalanche, Wolken said predicting them with any accuracy is a long way away with regard to Southeast Alaska.
The span style=”font-weight 400 ;”>”Warning systems are created in locations where instruments and data are in very high quantities,” Wolken said. “But in Alaska this is a truly difficult thing to accomplish. .”
Making more comprehensive inventory of maps and inventories of the past slides could aid in. In Wrangell Wrangell, the Department of Natural Resources did two aerial surveys — one in the summer, prior to the slide, and the second in the latter part of November, following the slide.
The data will be used to create mathematical models for the slopes in Wrangell and will uncover channelized areas, extremely steep slopes, and newly unstable slopes that might be loosened due to the same November. 20 storm.
At the state level the department’s landslides risk program is collaborating with the U.S. Geologic Survey to create a database of landslides across the state that will contain as many historical slides as is possible.
In the near future in the near future, in the near future, Alaska Department of Transportation will be constructing 2 new stations for weather on the location that slide is located – one on the bottom of the slope and the other located on the ridge. These stations will be collecting data about soil humidity, temperature as well as rainfall and wind in the area of the slide.
Wolken stated that data is particularly crucial due to the fact that climate change, which is caused through the combustion of fossil fuels increases the danger of slide.
span style=”font-weight 400 ;”>”We’re living in a dynamic situation right now, due to changing climate conditions that include increases in heavy snowfall, heavy rain and rapid temperature fluctuations,” Wolken said.
Initiatives to create tools to aid people in preparing for the effects of climate change in the slide-prone Southeast are just beginning. After a slide in 2015 that killed three people, residents of Sitka were terrified at the bleak slopes around them.
Researchers from Sitka Sound Science Center Sitka Sound Science Center, together with colleagues from other research agencies, created a basic warning system based on risk in an effort to provide the public security. It took them four years to set up more climate stations and create a forecasting model for the landslide risk — which is a far better goal than forecasting land slides. A lot of Southeast communities such as Wrangell might consider using the system as a reference point.
span style=”font-weight 400 ;”>”And when it comes to forecasting the outcome of a landslide, it’s going to be incredibly difficult, particularly with 100% certainty” Wolken said. “But If we have the data and we do have an opportunity to measure and track.
