Rising sea level causing groundwater floods in Hawaii

Rising sea level is pushing groundwater up and causing problems in Honolulu and Waikiki, pictured in a 2001 LandSat image via NASA’s Earth Observatory.

Study documents cesspool inundations with possible discharges of effluent to the environment

Staff report

Sea level rise caused by global warming may not be as obvious along the rugged and often steep coast of Hawaii as it is in low-lying areas like Holland, but it’s nonetheless going to present huge challenges in the decades ahead.

Scientists with the University of Hawai’i at Mānoa (UHM) say much of  urban Honolulu and Waikiki face a threat of groundwater inundation, which is flooding that happens when rising sea level pushes groundwater above the ground surface.

To map the problem, UHM geologist Shellie Habel and her team built a computer model to calculate the complex interplay of  ground elevation, groundwater location, monitoring data, estimates of tidal influence, and numerical groundwater-flow modeling to simulate future flood scenarios in the urban core as sea level rises three feet, as is projected for this century under certain climate change scenarios.

Because the water table is within two feet of the surface at high tide, it means groundwater inundation will become a serious concern well before the end of the century. When it rains and infiltration fills this space, it is a problem already.

In a statement announcing release of the study, Habel said, “This flooding will threaten $5 billion of taxable real estate; flood nearly 30 miles of roadway; and impact pedestrians, commercial and recreation activities, tourism, transportation, and infrastructure. The flooding will occur regardless of seawall construction, and thus will require innovative planning and intensive engineering efforts to accommodate standing water in the streets.”

The researchers also found that about 85 percent of the active cesspools in the study area are probably swamped with groundwater, which means that cesspool effluent, including remnant human waste, is  entering coastal groundwater and coastal environments.

Sea level rise of approximately three feet would fully inundate 39 cesspools, introducing effluent at the ground surface where people work and live. This presents a serious health concern that will become progressively more serious as contaminated waters begin breaching the ground surface.

“Our findings suggest that coastal communities in Hawai’i and globally are exposed to complex groundwater flooding hazards associated with sea level rise in addition to the typical concerns of coastal erosion and wave overtopping,” said Chip Fletcher, professor of Geology and Geophysics and associate dean in SOEST and principal investigator on the study.

“Groundwater inundation will require entirely unique adaptation methods if we are to continue to live in and develop the coastal zone. Coastal planners and community stakeholders will need to work with architects, engineers, geologists, ecologists, economists, hydrologists, and other innovative thinkers in order to manage these problems.”

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