Tides at the Moana Surfrider in late April 2017 (photo courtesy of HI Sea Grant King Tides Project)

Summer flooding expected after water levels break 112-year record

by Cindy Knapman, UH News, May 19, 2017

The highest water levels of the summer are expected around the upcoming peak astronomic tides of the year, known as “king tides,” occurring over a few days around May 26, June 23 and July 21 in Hawaiʻi. These may produce flooding events similar to what occurred in late April, and University of Hawaiʻi researchers say summer will provide a glimpse of what will eventually become routine with continued global warming and sea-level rise.

Localized impacts may include coastal erosion, wave over-wash and temporary flooding in low-lying backshore areas around storm drain systems. Impacts may be more severe if the upcoming king tides coincide with an elevated surf event, which occur most often on south and east shores this time of year, and/or during heavy rains. Actual water levels along exposed coasts will largely depend on wave heights during the high tides.

The University of Hawaiʻi Sea Grant Center for Coastal and Climate Science and Resilience (Hawaiʻi Sea Grant CCSR), University of Hawaiʻi Sea Level Center and Pacific Islands Ocean Observing System (PacIOOS) have been tracking high ocean water levels in the region, and are advising that the state likely will continue to experience unusually high tide levels through the summer.

“The oceanic and atmospheric processes that contribute to this prolonged period of high water levels in the Hawaiʻi region occur naturally in cycles. But as sea levels continue to rise with global warming, we will see more and more instances when not just king tides but ordinary high tides combine with high water levels to reach flood stage, with adverse impacts to our beaches, coastal infrastructure, wetlands and low-lying areas of the islands,” said Mark Merrifield, a UH Mānoa oceanography professor, who also serves as director of Hawaiʻi Sea Grant CCSR and UH Sea Level Center.

Help document king tides
The Hawaiʻi Sea Grant CCSR is asking residents to help document high water levels and related impacts through the Hawaiʻi and Pacific Islands King Tides “citizen science” project by submitting photos online. For more information please visit pacificislandskingtides.com.

Water levels above predicted tidal heights

Data from NOAA tide stations around Hawaiʻi show that observed water levels have been 3–6 inches above predicted tidal heights since early 2016. In late April, levels peaked at more than 9 inches above predicted tides at the Honolulu Harbor tide gauge, resulting in the highest daily mean water level ever observed over the 112-year record. The combination of elevated water levels, seasonally high tides and a large south shore surf event resulted in flooding on April 28, 2017.

The elevated water levels are attributed to an unusual combination of ocean eddies with high centers, Pacific-wide climate and sea level variability associated with recent El Ni#241;o events and the Pacific Decadal Oscillation, and sea-level rise from global warming. Based on ocean model forecasts and satellite observations of sea level, UH Sea Level Center researchers indicate that elevated water levels are likely to persist through the summer.

Tides at Ala Moana Regional Park in late April 2017 (photo courtesy of HI Sea Grant King Tides Project)

Public should prepare

Property owners who have experienced flooding and erosion problems in the past, particularly those on south and windward shores, should anticipate impacts similar to those experienced during the high tides of late April. Boating and ocean recreation such as paddling and fishing may also experience unusual water levels and currents in addition to navigation hazards.

Chip Fletcher, associate dean of the School of Ocean and Earth Science and Technology, adds “Water levels may reach more than 1 foot above typical high tide and produce unusual flooding in low-lying regions. Within a few decades this will be the new normal. Hawaiʻi should consider this a practice run, and reevaluate policies and development practices accordingly.”

Community members, businesses, and agencies also are encouraged to regularly check PacIOOS’ six-day high sea level and wave run-up forecasts to help increase preparedness and resiliency. Visit hawaiisealevel.org for more resources and information on how to get involved.