Editorial: City must address Ala Wai flood risk
Honolulu's Flood Crisis Deepens as Climate Change Overwhelms Aging Infrastructure
With the aging sewage system already weakened, the city must do more to keep contaminants from concentrating in the Ala Wai Canal and Honolulu's groundwater, and to improve flood protections so that these contaminants don't wash over Waikiki. Flood risks, exacerbated by climate change and sea level rise, are far higher than was previously assessed - and unless action is taken, flooding and contamination is expected to intensify.
To protect the watershed from repeated, rancid floods, the city must adopt clean-water strategies; act aggressively to end sewage contamination; improve and fortify drainage systems; partner with the state on 'green' methods, including strategic re-foresting and regeneration of land at upper elevations; and develop more stormwater holding capacity, within and upstream of the Ala Wai.
A complete, long-term watershed overhaul and rehabilitation will be complex and costly. In the near term, though, the city and state must prepare for emergencies, with readiness to employ stopgap measures such as pumps and barriers to divert floodwaters, and a plan to address contamination caused by spills. Failing to do so could leave the city facing additional sky-high costs for recovery, and in lost tourism revenue.
The urgency of developing solutions has been underlined by findings in a recent University of Hawaii-backed study overlaying real-world data from past storms and floods with the predicted timeline for rising island water tables. Models developed show that damaging floods are likely to occur sooner, and with more frequency, than maps based solely on sea-level rise would indicate.
Flood risks are magnified by heavy rain and high water tables - and both of these phenomena are made more likely by global warming. Damaging events are also far more likely because of the deteriorating state of Honolulu's drainage and wastewater systems.
Sewage and wastewater systems are 'already failing,' remarked study coauthor Kayla Yamamoto, a climate modeling analyst with UH. The aging system is riddled with choke points and leaks. Under flood conditions, water contaminated with materials from the watershed and the canal fouls surfaces, groundwater, drainage channels and near-shore ocean waters.
Sea-level rise is expected to raise the water table by a full foot by 2050, and research indicates this will lead to repeated storm drain backwash from the Ala Wai Canal into Waikiki, whether or not it rains. That's a problem because the Ala Wai is an 'impaired waterbody' as defined by the federal Clean Water Act - heavily contaminated with sewage and pathogens, heavy metals and agricultural, industrial and household chemicals. The sometimes-deadly bacteria staphylococcus aureus and vibrio have repeatedly been detected in the canal, especially after rains.
Speaking of sewage, another concern is cesspools. Many tourists would be dismayed to learn that properties within Waikiki and Honolulu still dispose of sewage by dumping it into a cesspool - a concrete-lined hole in the ground - but it's true, and studies indicate that nearly 9 in 10 of Waikiki's active cesspools contaminate area groundwater. Ending the continued reliance on polluting cesspools must be a top priority for city and state governments in the years ahead.
After 2050, when the water table has risen by 1 foot, heavy rains could lead to daily spills and flooding, especially in low-lying places like Waikiki and Mapunapuna. But the risk is significant today. The annual probability of rain heavy enough to raise Ala Wai water levels by a foot, which also makes flooding and contamination likely, now stands at from 10% to 20%.
"These aren't 100-year events. These are five- to 10-year events," said study co-author Shellie Habel, a coastal geologist. "It's like a double whammy. You'll have the failure of this system meant to manage stormwater, and ... flooding from the sea level being higher."
