Grewingk Landslide and Tsunami

This article originally appeared in the Homer Tribune.

Imagine hovering in a helicopter above Grewingk Glacier Lake, in Kachemak Bay, fifty years ago. From this safe vantage, you watch as 80 Empire State Buildings worth of material, slowly dislodge from the steep slope above the lake, and then let go all at once. Cleaved from the surface, you see the unfathomable volume of material gain momentum. By the time the deafening roar reaches your ears, the 110 million cubic yards of rock has thundered into the lake, sending a wave hundreds of feet into the air. Craning your neck, you watch this fast moving bulge of water slosh over the outwash plain, uprooting alders and mature trees, carrying everything in its path all the way to Kachemak Bay, more than four miles distant.

You just witnessed a suite of natures most destructive and incredible phenomena—a landslide generated tsunami.

This may seem like a crescendo scene from an apocalyptic Hollywood movie, but last October marked the fifty-year anniversary of the Grewingk landslide and tsunami.

No helicopter hovered in the air and thankfully no one was in the valley that day. However, many in Homer and nearby Halibut Cove heard the crash and later witnessed glacier ice in Kachemak Bay. Homer residents were able that fall to salvage trees for firewood that washed ashore onto the Homer Spit.

This event has captured the attention of two local geologists, who worry another landslide and tsunami could occur in the same area with similar, or worse, devastation.

“The rock all along the mountainside is highly fractured and faulted, as is the rock generally in the Kenai Mountains,” geologist Ed Berg said about the slope above Grewingk Lake. “Major rockfalls or landslides could probably occur anywhere along the steep slope of the mountainside.”

Seldovia resident and tsunami hazards expert Bretwood Higman added, “The combination of steep slopes and weak rock is the perfect recipe for a big landslide. Additionally, the 1967 landslide shows this area has the potential for very large landslides, and this potential may be all the greater now, since the glacier has retreated a lot in the past fifty years.”

Throughout coastal Alaska, landslides, some of which have caused massive tsunamis, are occurring with increased frequency. Steep mountain valleys, fjords, and bays that have, for many thousands of years, been full of glaciers have seen rapid retreat over the last fifty years. This has led to slope instability in many areas. Glaciologists and geologists call this process, glacial debuttressing.

“The slopes above Grewingk Lake are notable because they are much steeper than most slopes in the area, and they're not more strong,” noted Higman. “They probably are so steep because they were supported by the Grewingk Glacier until recently, and because the glacier has been actively undercutting them. The combination of steep slopes and weak rock is the perfect recipe for a big landslide.”

In June, Berg, Higman and the American Packrafting Association organized a human-powered research expedition to Grewingk Glacier Lake, hiking over the trail with a flotilla of packrafts and some low-budget tools.

One of the goals of the group was to better understand the depth of the lake. This information is key to understanding the potential for another tsunami. 

For the trip, Berg engineered a simple 1x4 contraption, which he affixed to the stern of his packraft. From this device he mounted a borrowed sonar fish-finder to measure and record the lake bottom depth profile. Others in the party used less sophisticated instruments to measure lake depth from the platforms of their packrafts, like a handheld sonar depth gauge or weighted lengths of string.

The deepest spot the team discovered—490 feet—was near the glacier face and the average depth below the potential landslide slope averaged 360 feet. “We were surprised that the lake is so deep,” said Berg. For a landslide tsunami, deeper water means greater hazard. The entire volume of the landslide could end up beneath the water‘s surface—creating a much bigger wave. Deeper water absorbs more of the landslides energy, converting it into a bigger, more powerful wave.

But how likely is such a landslide? Berg and Higman both agree that further study is necessary, including a detailed survey of the ridge above the lake. “In particular,” Higman says, “I'd look for roots that are stretched across cracks, or signs of blocks that have shifted down as cracks opened below them. If such signs are apparent up there, that would be a "red alert" situation, suggesting dramatic action like closing trails.”

Higman would also like to see computer modeling of landslide tsunamis to assess areas within the valley, which are most at risk for visitors to the area. Furthermore, computer models could resolve the potential for marine tsunamis in Kachemak Bay. “The marine tsunami risk is likely very minimal,” says Higman “but may be relevant since even a small marine tsunami could be very damaging to the Homer Harbor.”

Thus far, Berg and Higman have been aided with small equipment loans and organizational support from the American Packrafting Association but the two are working without funding or institutional backing.

“We would like to see some monitoring program put in place,” says Berg “that could provide timely warning of a future collapse and tsunami.”

“University researchers could be great, especially if they worked with us locals,” said Higman. Adding, “I'd like to see some funding to support someone to take the next steps, including assessment of the hazard, public outreach, investigating monitoring, [and] coordinating different research efforts.”

Statistically speaking, there is no good reason not to visit Kachemak Bay State Park’s magnificent treasure, the Grewingk Glacier. It’s not advisable to camp on or near the lakeshore but the likelihood of a cataclysmic event occurring during a day hike through the park is quite low. No one can predict when or even if there will be another landslide and tsunami. As our climate continues to warm, the research and study of these interconnected phenomena is wildly important—as important, perhaps, as visiting an Alaskan glacier.

-Bjørn Olson

Icy Bay Mega-Tsunami

This article originally ran in Alaska Magazine. 

On October 17, 2015 a mountain at the head of Taan Fjord, an arm of Icy Bay, Alaska broke in half and crumpled under its own weight. Some 200 million metric tons of material came crashing down in what must have been a deafening roar. Some of the rock slid onto the snout of a tidewater glacier but most slid into the head of Taan Fjord – displacing a massive volume of water. This landslide-generated tsunami had a maximum height of over 600 feet. The wave traveled down the fjord, dislocating sediment and scouring forest for over ten miles.

JUNE 2016: From the foredeck of the MV Seawolf, an aluminum six-pack charter boat, I turn to see Scott Chadwick, the burly Yakutat boat captain, look nervously from his depth sounder to the slowly approaching shore. Chocolate syrup water in the silty glacial fjord obscures hidden rocks. My eyes are peeled; ready to yell stop at the first sign. The bow grinds against the shoreline and gently makes contact. Perfect. The look of apprehension on the captain’s face is gone and I, along with four earth-scientists, disembark. 

On land, the five of us create a gear-chain and hand scientific equipment and my camera gear up the steep and unstable edge of the shore. Directly across the bay is the massive, cleaved-in-half mountain. The boulders and gravel under my feet had traveled over a kilometer from somewhere within that mountain and were deposited here in a feat of unimaginable violence. “Sometimes geology isn’t slow,” Bretwood Higman, one of the scientists observes.

My task is to document what the “crime investigators” learn about the Icy Bay landslide-generated mega-tsunami - to tell their story of geologic and hydrologic discovery.

After shooting video from the shore we wave goodbye to Scott, who will spend the day fishing, while we walk up the valley to find the high watermark of the tsunami.

The National Science Foundation has funded this series of investigative expeditions. Scientific inquiry is why we are here. “It’s like a perfect experiment,” Colin Stark, one of the geologists says. “Find the remotest place in north America where a massive landslide and tsunami occurred, without loss of life or destruction to infrastructure. Run a host of experiments and make observations. Learn from that, and apply it to building some sense of what will happen when a similar event occurs, which will inevitably happen in a much more populated area.”

Landslide tsunamis have happened in Alaska before. Most notably was the 1958 Lituya Bay landslide and tsunami, which created a wave 1720 feet high – the highest wave ever recorded. What is unique about the Icy Bay landslide and tsunami is that scientists are able to study it so soon and with a suite of state of the art equipment.

Alaska’s rapidly warming climate – twice that of the global average - is related to this catastrophe. Icy Bay was as recently as the early 1960s filled with glaciers to the mouth of the bay with ice as thick as 1000 feet. This rapid de-glaciation and de-buttressing of the hill slopes is at least part of the reason for the instability, which led to the landslide. Other areas of Alaska and Norway are experiencing similar phenomena much nearer to tourist destinations and communities.

“This is happening in environments that the Park Service manages,” Eric Bilderback, a National Park Service geologist says. “We should take this opportunity to learn where nobody was hurt so we can maybe be proactive about the things where people could be hurt.”

After gaining 600 feet of elevation in our mile-long hike, I take off my backpack and drink a long slug of water. The day is warm and we’ve all sweat under our loads.  We are standing at the trim-line where the rush of water scoured the brush and left only bare rock. The team looks for evidence of broken branches or out of place sediment above the line – trying to locate the absolute highest run-up. They finally make the call – 187 meters, or 613 feet.

I turn around and face the fjord. The charter boat, anchored in the middle, is now a tiny dot below us. Directly behind the boat is the cleaved mountain. I try to imagine the force, the violence, and I come up short. Geology may not always be slow; thankfully it usually is.

Watch the film, Icy Bay Mega-Tsunami.

Landslide Generated Tsunami in Icy Bay

In October of 2015 a massive landslide occurred in a remote area of coastal Alaska. Seismologists, who usually pick-up and decipher earthquakes, noted a unique low-frequency wave pattern on their instruments – the pattern looked like a landslide, and a big one. They sent the information along to a tsunami expert and within days it was determined that the event had occurred in Taan Fjord – an arm of Icy Bay.

Over the winter experts from around the world began reaching out to each other and eventually it was decided to put together an expedition funded by a National Science Foundation (NSF) grant. The goal was to get as many disciplines from the related fields of study into the field and uncover the mysteries of this fantastic event as soon as possible, before erosion, glacial retreat and other processes sullied the evidence.

I joined the team, as videographer and documentarian, with sedimentologists, tsunami modelers, surveyors, landslide specialists, engineers, and other earth researchers. Like the good burglar Bilbo Bagbins I was the lucky 14th member.

Better understanding of geo-hazards, landslides, and tsunamis leads to better public policy, city planning, engineering and other human-related developments. In this time of rapid de-glaciation and climate change it is very likely we’ll be seeing more of these types of landslides in coastal Alaska and other glacial regions around the world.

Many of the researchers on this expedition have studied earthquake-generated tsunamis in Indonesia, Japan and other places where loss of life and property was profound. To study a landslide generated tsunami in a remote fjord is not without challenges but it also provides a fantastic natural laboratory, where no loss of human life occurred.

Over the next few years, data generated from this expedition and another in August of this year will be crunched and models will be created. This tsunami was the largest (600 feet) anyone has yet to study so soon after the event and the instruments used are state of the art. It seems likely that what was uncovered from this expedition will be useful around the world for many decades to come. 

Photo credit: Bjørn Olson, Ground Truth Trekking, and the National Park Service