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Rising seas will change the coast and the groundwater beneath your feet

While concerns over sea-level rise have typically focused on the ocean washing over previously dry land, higher seas also raise the coastal groundwater table — and that could expose far more Californians and their property to climate-change effects than overland flooding.

Miami is already experiencing such groundwater flooding. The Atlantic Ocean has risen enough that it routinely pushes subterranean water levels so they breach the land’s surface in some neighborhoods there on a daily basis, U.S. Geological Survey coastal geologist Patrick Barnard told the state Coastal Commission in a multi-agency presentation on the issue Sept. 8.

“Low-lying areas like ports and reclaimed estuaries — like we have in northern Orange County and the ports of Los Angeles and Long Beach — have very shallow groundwater today and by the end of the century, it will become even more of an issue,” Barnard said.

“It’s going to be a compound issue where it’s not only going to be overland flooding, but the daily impacts of this water table hazard.”

With 6.6 feet of sea-level rise, 600,000 people in the state and $200 billion of property would be in jeopardy from overland flooding during a 100-year storm, according to current modeling. But with the same amount of sea-level rise, 4 million people and $1.1 trillion of property could be exposed to higher groundwater on a daily basis — not just during a storm, he said.

The groundwater rise would threaten seven times as much roadway as overland flooding, and nine times as many critical facilities, including schools, hospitals and police stations, he said. Damage could include flooded basements, disabled drainage, damage to underground pipelines and sewage systems, and undermined roadbeds. Unlike overland flooding, seawalls and similar barriers can’t protect against groundwater rise.

The new modeling presented to the Coastal Commission looks at groundwater levels relative to sea levels, using a color-coded map to show vulnerabilities for each part of the coast. A slider tool allows the online user to see a rough estimate of the degree of susceptibility anywhere from no sea-level rise to 16.4 feet of rise. (Users can also see the same information for overland flooding and related risks.)

However, this early stage of groundwater modeling doesn’t yet identify local geology — particularly the permeability of the earth — or local effects of rainfall. Those factors also play a significant role.

As a result, part of the port as well as some residential neighborhoods of Long Beach, Seal Beach and Huntington Beach are displayed on the modeling map as already subject to groundwater breaching the surface. But such breaching is largely — if not entirely — absent so far. Barnard said the discrepancy is because other local conditions hadn’t yet been gathered and integrated into the models.

Nonetheless, the information now available should raise red flags about the need to prepare for future groundwater rise, he said.

“There’s a lot of research going on now to translate this to what the impacts actually mean,” he said. “This work (presented now), we hope, puts this hazard on the map and we can begin to look to solutions for these different hazards across the state.”

Growing threat

The state’s Ocean Protection Council, which provides sea-level rise benchmarks for agencies to plan by, has recommended preparing for the ocean to rise 3.5 feet by 2050 even though it acknowledges only a 1-in-200 chance of that occurring. It puts those same odds on the state experiencing 6.6 feet of rise by 2100.

For transportation and sewage infrastructure, the Coastal Commission has suggested planning for 10 feet of sea level rise by 2100 because of the long-term planning needed as well as the critical role such infrastructure plays. Both agencies’ benchmarks have been criticized for being too extreme.

But even the estimates of the United Nations’ Intergovernmental Panel on Climate Change provide evidence of the growing threat and were quoted by Barnard in his presentation to the Coastal Commission. He also included input from climate researchers at the commission, Point Blue Conservation Science and the University of Arkansas.

Barnard is among those who’ve called the U.N. estimates conservative, optimistic, best-case scenarios.

Among the U.N. panel’s data is the finding that the rate of sea-level rise has approximately tripled since 1971 and is currently at 0.15 inches per year.

“Global mean sea level has risen faster since 1900 than over any preceding century in at least the last 3,000 years,” according to the panel’s Sixth Assessment Report. “Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years. … Sea level is committed to rise for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and will remain elevated for thousands of years.”

More immediately, the past six years have been the six warmest on record, according to the World Meteorological Organization’s State of the Global Climate 2020, published this year.

Emerging awareness

Cities, ports and other public agencies in the most vulnerable Southern California stretches are becoming aware of the groundwater threat. But if those agencies are alarmed at the prospect, few are letting on.

Climate-change resiliency studies by those entities — in the instances where there are such plans — typically mention the threat of rising groundwater briefly, if at all. A 2018 sea-level adaptation by the Port of Los Angeles does not address the issue.

“However, the port intends to address it in future studies as we continually monitor sea-level rise,” port engineer Adrienne Fedrick Newbold said in an email.

The city of Long Beach’s 192-page proposed Climate Action + Adaptation Plan, completed last year, doesn’t mention the possibility of rising groundwater, although it is brought up in an appendix.

Sea-level rise “causes saline water to intrude into underground reservoirs, raising the historical groundwater elevation ranges beyond what the Long Beach utilities were planned and built to accommodate,” the appendix notes.

Alison Spindler-Ruiz, the city’s advance planning officer, added that future studies will “allow us to study these issues in more detail and come up with more detailed recommendations and implementation projects.”

The Orange County Sanitation District’s 2019 climate resiliency study briefly mentions the potential of groundwater infiltration with a short recommendation to consider the threat to “below grade structures such as dry wells, basement, tanks and tunnels.”

The district’s assistant general manager, Rob Thompson, said that most of his agency’s infrastructure has a 50- to- 75-year life span, and the latest sea-level rise science is taken into account when each is built or rebuilt. He noted that new digesters at its coastal sewage treatment plant in Huntington Beach will sit higher of the ground because of it.

And future groundwater flooding?

“The practical problem we’re facing is relatively minor,” he said. A larger concern is the effect of seismic activity on both underground and above-ground facilities — and fortifying those can also help protect against rising groundwater tables, Thompson said.

“Most of our critical infrastructure is designed with geotechnical considerations in mind,” he said. “But for residential and commercial development, this is something they may want to take into consideration.”

Matt Arms, environmental planning director for the Port of Long Beach, acknowledged the shallow groundwater in the area and said the port takes the most recent data into consideration when building.

“We use the best available science to inform, and adjust when necessary our project designs,” Arms said via email. “As we redevelop and modernize the port, we always consider the expected effects of climate change.”

USGS’s Barnard, meanwhile, said that while the new modeling and assessments were preliminary, they do provide an alert that rising groundwater needs more attention.

“It’s not for engineering,” he said. “It’s not saying, ‘You should design buildings differently.’ It provides potential red flags that need to be investigated.”

To see the map modeling of areas vulnerable to groundwater rise, visit ourcoastourfuture.org/hazard-map/. Under “Scenario Topic,” use the dropdown box and select, “Groundwater.” You can zoom in on the map, select your city of interest, adjust for the amount of sea-level rise and for the permeability of the ground.


Source: Orange County Register

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