The afternoon before the flooding, you would have been hard-pressed to find any reason to suspect it was coming. It was an idyllic, mid-April day in Hanalei, Kauaʻi, after all: 79 degrees, a mix of sun and clouds, and with a steady, beach-cooling wind out of the northeast. Over the next 48 hours, though, a truly staggering quantity of water would pummel the northern reaches of the island. The subsequent flooding—the result of meteorological quirks converging with maximal potency—precipitated destruction. Yet, the disaster and recovery has precipitated something else in turn: a response from community that lends a resilient eye toward the future and serves as a model for our islands.

Conditions may have been serene on Kauaʻi that afternoon of April 13th, 2018, but just hours later, the first sign of what was coming touched down an island away. Atmospheric bedlam erupted over eastern Oʻahu around sunset. In a few short hours, five inches of rain had dowsed Hawaiʻi Kai, pouring out of the clouds which crashed with thunder. The downpour engorged waterways on the island’s windward side, raising water in some streams by several feet per hour.

By midnight, the thunderstorms had drifted over northern Kauaʻi, and the intense rain followed. The torrent whipped the island’s lower elevations throughout the day in great pulses, causing landslides that fragmented the Kūhio Highway. The deluge ceased briefly in the evening before a second wave hit that night, with rainfall exceeding seven inches per hour in some locations. Drainages rapidly swelled beyond their normal, steep-sided confines. Hanalei River bloated into a sinuous sea, submerging the highway under a fathom of water. After a third and final dumping of precipitation throughout the 15th of April, it was over.

The level of rainfall that Kauaʻi endured those two days was astonishing. A rain gauge in the ahupuaʻa of Waipā measured more than 53 inches of rain over the 48-hour period. Incredibly, 49.69 inches of that rain fell in a single day, smashing not only a Hawai‘i state rainfall record set in 1956 by some eleven inches, but setting a new single-day rainfall record for the U.S., substantially surpassing the 1979 record of 43 inches set just outside of Houston, Texas during a tropical storm.

In the end, thankfully, no one died from the storm or the flooding. But the damage to the physical environment and property was severe. Between the storms on Oʻahu and Kauaʻi, more than 530 properties are thought to have been damaged from the flash floods. Some houses were completely destroyed, collapsed from the pressure of rushing water. More than a dozen landslides, many of them west of Hanalei, severed highway access to communities like Hā‘ena and Wainiha, making evacuations only possible by air or boat, and complicating efforts to deliver relief supplies. In some sections of the highway, this isolation persisted for weeks. Several state parks and beaches were closed for long periods in the wake of flood damage. For example, the internationally famous Kalalau Trail in Nāpali Coast State Park was closed to hikers for over a year. Estimates place the damage to public properties at around $20 million, with the total toll likely over $125 million.

The flooding was certainly the most extensive and costly natural disaster to hit Kauaʻi since Hurricane Iniki slammed into the island in 1992. But the nameless, April 2018 event was no hurricane. There was no churning cyclone at sea to watch with rapt attention for a week. The April storms seemingly manifested their record-breaking rains on a whim. However, the disaster was rooted in a unique merging of meteorological conditions and landscape features ideal for producing dramatic rainfalls and widespread floods.

Hawaiʻi is no stranger to regular bouts of rain thanks to the islands’ steep, mountainous profiles shaped from the eroded flanks of giant shield volcanoes. The rise of warm, moist air and release of precipitation in mauka (towards the mountains) highlands happen many times daily across the high islands in the state. But the exceptional rainfall of the April 2018 storms wasn’t just an extreme form of this simple process. An extra ingredient—one much higher in the atmosphere—was partially responsible for the downpour.

“People often think about weather being just like the wind or the rain at the ground,” says Alison Nugent, an atmospheric scientist at the University of Hawaiʻi’s School of Ocean and Earth Science and Technology (SOEST). “But sometimes you can have upper level atmosphere features that are different or disconnected from the ground.”

These conditions, says Nugent, exist some 40,000 feet above sea level—about the height that commercial airliners fly. Every so often, a low-pressure system high up can overlap with moist air lower in the atmosphere.

These “upper level lows” are a cold core of air, and when they meet up with a nice, warm mass of air underneath, they can act like a rocket-powered elevator shaft, allowing moist air to rise easily and extremely rapidly.

All that fast, vertical motion of warm, wet air can create incredibly high potential for rainfall once all that water condenses into clouds. Kevin Kodama, a hydrologist with the National Weather Service, likens the system’s components to an engine and its fuel. A change in the wind flow that brings in really wet, tropical moisture from the south is the “gas” and the upper level low drifting over the islands is the “engine,” allowing the wet air to shoot upwards and create clouds.

This is precisely what happened in the days leading up to the April floods, with a “cut-off low” from up north eventually parking over parts of Oʻahu and Kauaʻi and creating a supercharged column of thunderstorms. Nugent says it was this vertical structure that allowed such extreme, localized rainfalls, increasing the proportion of the moisture in the clouds that actually dumps as rain.

“Raindrops fall down and they collect cloud droplets and [more] raindrops,” says Nugent, describing the clouds as a sponge loaded with a finite amount of water. “The setup allowed it to squeeze the sponge more than a normal rainstorm.”

Such efficient wrenching of immense, wet thunderclouds is behind the April 2018 event’s soaking, unprecedented even on Kauaʻi, one of the rainiest locations in the U.S.

“Sometimes … when you have these really unstable conditions, your rainfall intensity can far exceed what you might get out of a tropical cyclone,” says Kodama.

And when all that rain collides with the island’s steep canyons and eroded, volcanic rock, there are few places for the water to go. That’s especially true in urban Hawaiʻi, says Chip Fletcher, a geologist by training and a climate scientist by practice, as well as serving as the associate dean for academic affairs for SOEST.

“In a developed watershed, you can have the waters rushing into a cemented channel,” explains Fletcher. These channels can be loaded with vegetation debris. Even natural streams will send gravel and boulders downhill during a flash flood.

“All of this material […] comes rocketing down onto the flat coastal plain,” says Fletcher, “the center of our socio-economic activities.”

Once that debris creates dams—particularly under bridges and walkways—the rushing floodwaters are forced to jump out of canals and start cutting through people’s homes and properties. Despite quick efforts by government agencies ahead of the “rain bomb” to clear waterways of debris, says Fletcher, debris still accumulated and allowed floodwaters to escape their channels.

It is this particular combination, coastal development on a flat table abutting steep watersheds, that made the intensity of the upper level low’s downpour translate into such debilitating flooding.

Chipper Wichman – President of the National Tropical Botanical Garden – was on the north side of the island at the time and experienced the storm firsthand. On that Saturday morning when the first rains started impacting Kauaʻi, he was at his family home near Hāʻena. It didn’t take long for the nearby Limahuli Stream to swell to the point of blocking road access to the area. By 1 pm, the rainfall had intensified to a degree that neither Wichman nor his 94 year-old father, who was born and raised on Kauaʻi, had ever seen before.

“It started raining so hard that it was unbelievable,” recounts Wichman, adding that the situation was compounded by the weeks of regular, heavy rain that preceded the storm. “Some of the ground was already saturated, and the rain was coming down with such an intensity that there was no ability for any of it to soak into the ground. It all ran off. It was just coming down and running off in such an unbelievable volume.”

As the hillside gushed with brown rapids, a river of water suddenly appeared under the house, says Wichman. The bottom floor home office flooded with two inches of water and had to be promptly bailed out as the storm waters rose. But Wichman is quick to note it could have been much worse.

“That was nothing compared to homes that were right by streams and drainages that were literally washed away or completely filled with mud and debris,” he says.

Just down the road, the NTBG’s Limahuli Garden & Preserve also fared better than they might have. Not a single structure was lost, says Wichman, and thankfully, none of the garden’s collections – some containing some of the rarest plants on Earth – were lost. But roads and culverts had been totally washed away.

“The actual stream channel of Limahuli Stream, which is considered one of the most pristine streams in the state, had been scoured and dropped by about four feet,” says Wichman. Huge boulders had been pushed downstream, replaced by others washed from upstream. Wichman says it was like seeing a geologic event unfold.

“It was kind of chicken skin to feel like you actually witnessed an event like that,” he says.

Since the damage to the garden was relatively minor, Wichman says primary attention was given to community recovery.

“We directed our staff immediately to work on helping the community recover. And I’d say the first month after the storm, we did minimal work in the garden,” he says. “We were more sharing food and checking on families.”

Across the north side of the island, community members rose to the occasion. Mehana Vaughan, an environmental social scientist affiliated with the University of Hawaiʻi’s College of Tropical Agriculture and Human Resources and the university’s Sea Grant College Program (Hawaiʻi Sea Grant), and a member of the flood-impacted community, was a part of the community response shortly after the floods, and describes the systematic methods used to evaluate community members’ needs in the wake of the disaster.

“[The] community response focused on immediately identifying who the elders are in our community. Are they being taken care of?” recounts Vaughan. “[They created] a database of all of the families by going door-to-door and then working in teams, mostly mothers, ahupua’a to ahupuaʻa, and going out and checking on people regularly.”

Getting medicines to people that needed them and addressing the needs of children in impacted areas were also key priorities in the early stages of the response, adds Vaughan. Larger scale, coordinated efforts were also integral to the cleanup effort. She described community members organizing “truck battalions” that could show up wherever people were flooded to help haul away furniture. A crew got together in the Wainiha area to clear out hau bush and other debris that had clogged the river in the flood.

“People also ran amazing, organized, coordinated boat trains to ferry supplies down the coast to people and create packages tailored to different families’ needs, to make sure they got to them,” says Vaughan.

She notes that much of this response grew somewhat organically out of the disaster, and that her community had been thinking about and developing a framework for dealing with natural disasters for years before the floods occurred.

“It wasn’t like the long-term planning was something our community was thinking about for the first time,” she says.

Now, two years on, many of the most visible, physical wounds on the landscape, public spaces, and some properties have been repaired. The events of the April 2018 flooding seem to have had a potent influence on the connections many people on the island are making between natural disasters, community preparedness, and climate change. Ruby Pap, a coastal land-use extension agent for Hawaiʻi Sea Grant, describes a community workshop on vulnerability assessment for coastal hazards on the west side of the island that occurred around the same time as the flooding, and the subject of rainfall kept coming up.

“It was very much on their minds that this could happen here, and that we need to be talking about rainfall,” Pap says, despite the meeting’s focus on sea-level rise. They expressed concerns like “When you add rainfall to the equation, if we have more of these storms, what does that mean?’”

The anxiety about the confluence of flash floods and rising seas is absolutely warranted, given what is already known about how little buffer development Hawaiʻi’s coastal plain currently enjoys. Flash floods, says Chip Fletcher, are starting to run out of coastal space to occupy after flowing downhill towards the shoreline.

“Sea-level rise is bringing the zone where these floodwaters come to a halt from some point that was historically located just off the shoreline to now in our coastal communities,” he says, “and by mid-century or the second half of the century, it will be spread throughout the coastal plain.”

This is, of course, compounded by projections that extreme rainfall events which cause flash flooding are not going away anytime soon. Warmer seas, after all, contribute even more water vapor into the types of weather systems that can cause intense downpours.

“Our expectation is that this process [of flooding] is going to get worse,” says Fletcher.

Preparing for the future, says Fletcher, requires three main steps. First is knowing what the science is telling us about the very near future. The second is modeling the range of possibilities for future flooding scenarios. And third?

“What are the new design parameters and policies that we want to put in place for guiding continued occupancy of the coastal plain?” asks Fletcher. “What are the architectural ramifications? What are the transportation ramifications, for energy and infrastructure? We not only have roads. Underneath each road we have freshwater pipelines. We have sewer pipelines. We have electrical infrastructure. All of this needs to be thought carefully about, and not just repaired ad hoc, but thoughtfully designed in the context of our best understanding of the near-term future.”

Past conditions may have once been useful for informing future planning, but “the past is no longer a key to the future,” argues Fletcher.

However, recent extreme events, like the April 2018 rain bomb, can still yield insights into the physical processes that made such heavy flooding possible, invaluable for weather prediction purposes. That is precisely what’s happening with ongoing research on the rain event between the National Weather Service and the University of Hawaiʻi, says Kevin Kodama. He says the goal is to develop computer simulations of the 2018 rainstorms to better understand the atmospheric conditions that created them.

Now that the clouds have cleared and the waters have abated, and many are looking to the horizon with newly cautious eyes, it’s also particularly important to recognize the recovery’s victories.

During the floods, the river gouged a channel through the Black Pot County Beach Park, sending sand and debris out to sea, and leaving the park peppered with gaping holes filled with saltwater. But upon Ruby Pap’s most recent appraisal of the beloved beach, all seems returned to the state before the storms: a world of laughter and pau hana.

“The beach has been restored, those holes have been filled in with sand from offshore,” she says, “and I see people enjoying the beach, and it looks beautiful.”

 

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