Big Island Issues Flood Advisory Until 7 p.m. as Rain Hits 1.5 Inches an Hour

Q: Why does Hawaiʻi's unique volcanic terrain turn heavy rain into a hidden, fast-moving danger before streams even rise?

Hawaiʻi’s volcanic landscape can turn heavy rain into a fast, hidden flood threat because water often runs off steep lava-and-ash slopes and through dry gullies before any stream visibly rises. In places like South Kona and Kaʻū, the terrain is sharply sloped, soils are often thin, and drainage channels may be dry most of the year. When rain falls at rates like the 0.5 to 1.5 inches per hour reported near Kahuku Ranch, the ground can be overwhelmed quickly. Instead of soaking in, water races downslope over rock, ash and compacted soil, collecting in low spots, road crossings and gulches. That makes the danger deceptive. Many affected basins have few perennial streams and limited gauges, so there may be no obvious warning sign such as a steadily rising creek. Floodwater and debris can reach roads and communities suddenly, especially in poorly drained areas like Ocean View, Nāʻālehu and the Kahuku area. By the time people see water on the road, runoff may already be moving fast. Recent storms can make the problem worse. Once volcanic soils become saturated, they lose much of their ability to absorb additional rain, so later showers convert more directly into runoff. Hydrologists and the National Weather Service have noted that in Hawaiʻi, weaker follow-up storms can sometimes produce worse flooding if the ground is already wet. That is why radar-based advisories matter so much here: the flood can begin in the landscape before it becomes visible in the stream.

3 articles · Updated · Big Island Now · Sep 10

Eastern and southeastern parts of Hawaii's Big Island are under a flood advisory until 7 p.m., with minor flooding already occurring or expected soon on roads, in drainage areas and along streams.
Doppler radar around 4 p.m. showed heavy rain over South Kona and Kaʻū, falling at 0.5 to 1.5 inches per hour, with the heaviest rain near Kahuku Ranch.
Additional showers are expected through the evening, extending flood risk for communities including Ocean View, Nāʻālehu, Waiʻōhinu, Discovery Harbour and Miloliʻi.
The National Weather Service urged people to avoid streams, drainage ditches and other low-lying areas prone to flooding.