A study on Honolulu’s sea-level rise (SLR) shows that cause of flooding does not only comes directly from the ocean but because of groundwater inundation as the water is shoved to the surface, and stormwater back flow.
As sea-levels continue to rise, Hawaii’s main urban area, Honolulu is experiencing flooding. The study predicts that in the next few decades, sea-level rise will cause inundation on more land area by these three flood mechanisms:
- Direct marine inundation is flooding due to direct surface connection to the marine water and characterised by the land’s elevation below a flooded threshold where the water can enter unimpeded. Preventions of direct marine inundation would flood barriers like those constructed in New Orleans and Netherlands.
- The presence of gravity-flow drainage networks facilitates storm drain backflows. Gravity-flow drainage is common in coastal communities around the world. High-tides in low-lying coastal areas would decrease these elevations and cause storm-backflows. During extreme tides in Honolulu, reverse flow from municipal drainage systems would always affect its low-lying streets and basements. Some municipalities would employ forced drainage which uses check-valves and pumps that would transfer these waters to higher elevation discharge areas.
- Groundwater inundation (GWI) happens when water overflows through the surface and from buried infrastructure. This is the most challenging flood type to deal with because coastal barriers cannot mitigate it as with direct marine flooding. Groundwater is already very close to the ground surface among Honolulu’s low-lying areas and is already affecting submerged infrastructures like cesspools, basements, and others. With continuous sea-level rise, fluctuations in tides and wave set-up, groundwater inundation will increase flooding. In the Netherlands and New Orleans, they prevent GWI though groundwater extractions, however, this method can cause further damages like land subsidence and more chances of flooding if sediments are loose. Subsidence can cause ground fractures, and structural damage to infrastructures, including flood control structures, drainage networks, electric and gas substations, and sewer lines.
To identify vulnerable locations and infrastructures to flooding, researchers have developed flood maps. These flood maps are a result of flood simulations and its depth from the three combined flood types or mechanisms.
Damage assessments using flood maps show that urban infrastructures are already suffering damages especially during long periods of high tides that results in backflow of gravity drainage, traffic slowdowns along submerged roadways, and partial flooding of cesspools.
The study projects that by 2030, sea-level rise will make some stretches of the road impassable. In the coming decades, infrastructure damage will increase fivefold from the combined three mechanisms flooding, according to the study.
Findings from the study show that flood management strategies should require consideration of multiple mechanisms of flooding to make flood management and sea-level rise adaptation effective.
To read the full report, click on the link below: