2018 Hawaii floods

2018 Hawaii floods

Date	April 14–15, 2018
Location	Kauai, Oahu (Hawaii)
Deaths	None reported
Property damage	$20 million (2018 USD)

In April 2018, a series of thunderstorms produced heavy rainfall on the Hawaiian Islands of Kauaʻi and Oahu. An upper-level low moved across the area on April 13, generating a mesoscale convective system that moved over eastern Oahu, producing localized heavy rainfall that reached 5.55 in (141 mm). The heaviest rainfall occurred on northern Kauaʻi. There, a rain gauge owned by the Waipā Foundation, just west of Hanalei, recorded 49.69 in (1,262 mm) of rainfall in the 24 hours ending at 12:45 PM. Pending verification by the National Climatic Extremes Committee, the rainfall total broke the 24 hour rainfall set during Tropical Storm Claudette in Texas in 1979.

The heavy rainfall produced flash flooding and landslides, which covered roads and destroyed several houses. Public damage totaled over $20 million, and 532 houses were affected by the floods.

Meteorological history

On April 11, 2018, an upper-level low was located west of the Hawaiian island of Kauaʻi. The low weakened and opened into a trough, which moved over the Hawaiian islands on April 13, producing heavy rainfall. That day, a mesoscale convective system (MCS) developed northwest of Molokai and intensified while moving westward through the trade winds. The MCS moved over eastern Oahu, dissipating after moving ashore. Another upper-level low developed on April 14, fueling further rainfall through April 16. The first of a series of thunderstorms moved over northern Kauaʻi around midnight (local time) on April 14. After a five hour break in rainfall, another set of thunderstorms hit northern Kauaʻi around midnight on April 15 and lasted for about eight hours. The third set of thunderstorms began around 10:00 AM on April 15 and continued until around 7:00 PM.

Impact and aftermath

The heavy rainfall caused flooding that affected 532 houses. Public damage totaled over $20 million. Despite the damage, there were no deaths or injuries.

After the floods, Hawaiian Governor David Ige declared a State of Emergency. On April 18, the Hawaiian legislature approved $125 million in disaster funding, including $100 million for Kauaʻi.

On May 8, United States President Donald Trump declared a federal disaster area for Kauaʻi and Honolulu counties. Initially, the declaration was limited to federal reimbursement for repairs to public infrastructure, but was expanded to include individual assistance on June 27. Ultimately, the Federal Emergency Management Agency (FEMA) approved 250 individual assistance applications, which totaled about $1.5 million, with another $2.6 million in public assistance grants. FEMA opened three disaster recovery centers, which provided resources for the application process; these centers were closed by July 20.

Oahu

Across eastern Oahu, the mesoscale convective system produced rainfall rates of about 4 in (100 mm) per hour. Over a 15 minute period, a rain gauge in Niu Valley recorded 1.42 in (36 mm) of precipitation, which has a 4–10% annual return period. The same station recorded 5.52 in (140 mm) of rainfall over a two hour period, which has a 0.5–1% annual return period. The rainfall on Oahu was largely limited to the eastern one-third of the island. A station at Daniel K. Inouye International Airport recorded 0.01 in (0.25 mm) of rainfall, just 14 mi (23 km) west of Niu Valley, which recorded 5.55 in (141 mm) during the same 24 hour period.

At Waimanalo Stream north of Niu Valley, the rains caused the waters to rise 3 ft (0.91 m) in 30 minutes, and 5 ft (1.5 m) in an hour. During the rain event, Wailupe Gulch reported an 8 ft (2.4 m) rise in floodwaters. The flooding caused significant runoff, leaving a layer of debris on roads. Flooding entered several houses and cars on the island.

Kauaʻi

On Kauaʻi, the rainfall event occurred at low elevations near the northern coast. This was unusual, as previous rain events on the island usually occurred over higher elevations. The heaviest rainfall occurred early on April 15. A rain gauge owned by the Waipā Foundation, just west of Hanalei on Kauaʻi's northern coast, recorded 49.69 in (1,262 mm) of rainfall in the 24 hours ending at 12:45 PM. Pending verification by the National Climatic Extremes Committee, the rainfall total broke the 24 hour rainfall set during Tropical Storm Claudette on July 26, 1979 in Alvin, Texas. The total would also break the Hawaiʻi record of 38 in (970 mm) set on January 25, 1956 at Kīlauea, also on Kauaʻi. The Waipā rain gauge also recorded 1.77 in (45 mm) over 15 minutes, 5.51 in (140 mm) over an hour, 19.52 in (496 mm) over 6 hours, 33.33 in (847 mm) over 12 hours, and 53.57 in (1,361 mm) over 48 hours. Elsewhere on northern Kauaʻi, a rain gauge in Wainiha recorded 32.35 in (822 mm), and a gauge in Hanalei recorded 28.41 in (722 mm) before it failed. Minimal rainfall occurred on the southern and western coasts of Kauaʻi. The intense rainfall washed out several river gauges. Along the Hanalei River, the United States Geological Survey (USGS) estimated that floodwaters rose 15 ft (4.6 m), representing a peak flow rate of 32,700 cubic feet per second (ft³/s, 926 m³/s), before the station failed. The Wainiha River reported a record flow rate during the storm event.

The sudden rainfall caused flooding and landslides, which washed out several roads and homes. Landslides and flooding closed a portion of Kuhio Highway, which was under 5 to 8 ft (1.5 to 2.4 m) of waters. For about two weeks, residents in Wainiha and Haena lacked road access, which disrupted relief efforts. Local and national emergency crews rescued 273 people from their homes by helicopter, boat, and bus. A Red Cross shelter in Kilauea housed 13 people after the event. The worst erosion occurred in isolated areas. At least four vacant houses were washed away, and dozens of homes had water intrusion.

See also

• List of wettest tropical cyclones in the United States
• Hurricane Lane (2018) - a hurricane that brought heavy rainfall to Hawaii in August 2018