Hawaii First Water

Archives for October 2020

For Honolulu, Rising Seas Deliver Flood Risks Three Ways

OCT 2020/Brett Walton, Circle of Blue/

Build a wall?

The biggest source of flooding linked to sea-level rise for Hawaii’s capital city comes not from the sea itself. It comes from underground.

It seems like an intuitive response for protecting a coastal city from rising seas. Just raise your external defenses. But the intuitive response, in certain cases, is also the wrong response, says Shellie Habel.

If the plan to prevent flooding in a city like Honolulu were to be simply block the ocean, “it’s not going to work,” Habel told Circle of Blue. Water has other, less obvious ways of invading, and that stealth movement has implications for water pollution and transportation in Hawaii’s largest city.

Habel is a coastal geologist with the Hawaii Department of Land and Natural Resources and Hawaii Sea Grant. She is the lead author on a study that investigated three flooding pathways in Honolulu, all of which are a consequence of rising seas. The study is the first to attribute the percent of flooded area in the city that is due to each factor individually and in combination. The likelihood that floods will come from multiple pathways simultaneously — and cause more infrastructure damage — rises along with the seas.

In Honolulu’s case, a sea wall would be a losing strategy against sea-level rise because a wall would not address the most serious flooding problem. The largest source of inundation for the city’s roughly 350,000 residents is not the ocean. It’s groundwater.

Habel and her colleagues found that little more than 2 to 3 percent of flooded area in Honolulu’s urban core is a result only of overland marine flooding. That range holds for the four sea-level elevations that the study looked at.

Groundwater flooding, by contrast, is the predominant individual source of flooding. How does this happen? Groundwater in the coastal region is hydrologically connected to the ocean. When the Pacific swells, so does the inland water table.

A hypothetical wall on the Honolulu waterfront would fail to prevent flooding, Habel said, because water would still bubble up from underground and come through backed up storm drains, which are the third flooding pathway. Groundwater flooding today swamps basements and roadways in low-lying areas and is noticeable in underground parking garages.

“So even if you put in a sea wall, even if you change the type of drainage management, you still have flooding,” she said, adding that a solution that only addresses marine flooding will not work because 97 percent of the total area that is projected to flood will still flood from rising groundwater and storm drain backups. Places like New Orleans and cities in the Netherlands, which also have groundwater flooding problems, employ pumps for that purpose. “In order to mitigate all the flooding you have to adapt to all three pathways.”

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How Efforts To Save Hawaii’s Forests Are Preventing A ‘Freshwater Crisis’

Sept/ Civil Beat; Claire Caulfield.

When Serene Smalley hikes into the Koolau mountains, her goal is to kill as many plants as possible. Armed with a machete and syringes full of herbicide earlier this summer, her sights were set on the mule’s foot fern: a giant Jurassic-looking plant.

Smalley pulled out her cellphone, scrolling through a map app with hundreds of white pins. Each GPS marker pins the suspected location of a mule’s foot fern. A local conservationist spent weeks during the pandemic combing through satellite images and identifying the GPS coordinates of mule’s foot ferns on the mountain range.

Oahu Watersheds Poamoho Wahiawa Serene Smalley Invasive Fern

Serene Smalley uses a machete to fell branches of the fern before injecting a small amount of pesticide into the base of the plant.

Credit: Kuʻu Kauanoe/Civil Beat

To the untrained eye, a mule’s foot fern can look like a native plant, the hapuu fern. “The hapuu is very lacy and pretty,” she said, while the fond of the invasive fern is pointed. “Which reminds me of a snake.”

Some of the offending ferns are right off the Poamoho Ridge Trail near Wahiawa, but to reach others she will have to scale steep slopes in the rain. Smalley invested $60 in specialized shoes that look like hooves and have metal spikes embedded in the soles for traction. They were designed for fishermen and she uses duct tape to stabilize her ankles for the long hike ahead.

For six years Smalley has been scaling mountains, camping on remote peaks and navigating mudslides to kill thousands of non-native plants. She’s one of only about a dozen elite volunteers trusted by the state Department of Land and Natural Resources to assist in the dangerous task of eradicating invasive plants from the nearly 2.2 million acres of protected forests across the state.

“I can definitely see progress,” Smalley said. “It’s very, very rewarding.”

Oahu Watersheds Poamoho Wahiawa Summit Wide

Watersheds provide clean drinking water, carbon sequestration and help prevent floods and droughts. A University of Hawaii study estimated the Koolau Range provides between $7.4 billion and $14 billion in value to the state.

Credit: Kuʻu Kauanoe/Civil Beat

These volunteers, along with hundreds of state employees, dozens of environmental groups and an army of local hunters are fighting an uphill battle to protect Hawaii’s forests — and Hawaii’s drinking water.

The efforts involve coordinating a diverse group of stakeholders that don’t always see eye-to-eye, expensive land acquisitions, millions of dollars in taxpayer-funded fencing and excursions to some of the most remote areas of the island to remove invasive plants.

All this work is amid a backdrop of climate change, which the Honolulu Board of Water supply says is a top threat to drinking water security on the island.

More than 90% of Hawaii’s drinking water comes from aquifers: underground reservoirs of freshwater. Rising sea levels will make some freshwater aquifers turn brackish.

Hawaii’s population has almost doubled since statehood, and pre-pandemic the state was seeing a record number of visitors. But rainfall patterns have taken the opposite trajectory: decreasing at least 18% in the past 30 years, said state sustainability coordinator Danielle Bass.

Healthy forests could provide a one-two punch to the effects of climate change in Hawaii: sequestering carbon and allowing freshwater aquifers to recharge with rainfall.

“Without the necessary coordination and action, we risk a potential freshwater crisis for Hawaii’s future,” Bass said.

 

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