Researchers at the University of Hawaii at Manoa studied three different sources of sea level rise flooding and found groundwater inundation — a rising of the water table to the surface due to the pressure created by sea level rise — is the major threat to urban Honolulu.
To arrive at this conclusion, the researchers studied the likely causes of sea level rise flooding over the next few decades in Honolulu. They did this by producing flood maps that considered the threats posed by sea level rise-driven groundwater inundation, reverse municipal drainage — where ocean water flows backward through the drainage system and rises up through stormwater drainage grates — and direct marine inundation — where ocean water flows in from the sea. Among their surprising findings, they determined that direct marine inundation represented only 3 percent of the overall near-term threat.
The researchers said their findings clearly indicate that seawalls, one of the most popular approaches to holding back sea level rise flooding from damaging real estate and critical infrastructure, such as roads and water and sewer pipes, won’t be able to stop groundwater inundation. They said the only options in areas impacted by groundwater inundation will be to adapt to the flood waters or abandon the affected area.
“What this means for the future of Honolulu is that we need to consider each different type of flooding individually and really think about how we’re adapting to each one,” said Shellie Habel, a coastal geologist with the University of Hawaii Sea Grant College Program.
The University of Hawaii report should act as a wake-up call to coastal areas everywhere that have high water tables and porous bedrock, like South Florida. The groundwater inundation threat must be considered before enormous amounts of money are invested in sea walls that ultimately won’t be able to protect real estate and critical infrastructure from sea level rise flooding. (Photo courtesy of School of Ocean and Earth Science Technology, University of Hawaii at Manoa)