Hawaii Flooding, Flash Flood Warning, Types of Flood

Hawaii does not have freezes to create the temporary ice dams which produce destructive floods elsewhere, nor does it have spring thaws during which melting ice and snow can result in dangerous flooding. Flooding does happen in Hawaii, however, and it is sometimes very destructive.

Types of Floods in Hawaii

River Flooding or Riverine Flooding: People love to live by running water, but rising water in creeks and rivers can be dangerous. Flooding is a natural event for any river, creek or stream. When the river catchment receives more water than usual, the water level in the river will rise to accommodate the increased flow. At such times, water rises into the flood plain—the area naturally included in the river's territory, often shaped by the fact that it floods regularly (if infrequently). One cause of destructive and potentially dangerous flooding is any kind of development in the flood plain or the reshaping, hardening or other alteration of the flood plain terrain. Homes, barns and outbuildings; berms, walls and fences can all result in exacerbating or even causing destructive flooding.

In times of especially heavy rainfall, the increase in the amount of water coursing through a river channel can be more than the natural flood plain can contain, so the water level rises and land areas not usually—or not frequently—under water will flood. People speak of 100-year floods, for example, meaning the flood level that might be reached once in a century, or that has a 1 percent chance of happening in any given year. Development in 25-year, 50-year and 100-year flood zones is not as strictly controlled as it is in the annual or 10-year flood plain. The nominal infrequency of flooding in a given area might breed an undue sense of security.

Rivers do flood. They will flood at some level in many years, at higher levels in some years, and at surprisingly high levels in rare years. Knowing the terrain, the water sources, the catchment area and its weather patterns will help to understand river floods.

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Flash Flooding: Flash floods occur during extended rainfall events or within a few hours after the rainfall. Flash flooding can also be caused by the breaking of a natural or manmade dam or the suddenly release of water from a reservoir. While alerts of various kinds are usually issued before a river flood, flash flooding often happens so suddenly that no bulletin of any kind is issued. However, Hawaii meteorologists are very familiar with likely flash flood scenarios and locations, and will issue statements to alert the public when possible. Generally, if rainfall is of long enough duration, it will eventually cause flooding along the rivers, creeks and streams draining the catchment area where the rain is being collected, but if the intensity of the rain—the number of inches of rain falling in a given time frame—is too great for the stream system, including flood plains, to carry the water away, flash flooding occurs.

Flash flooding commonly takes place in gulches, valleys and other channels that restrict the spread of the water which, if it could spread widely enough, would become normal flooding or not even amount to flooding at all. The canyons and gulches formed by flowing water are perfect and frequent channels for flash flooding.

Runoff from higher ground (mauka): The natural terrain of Hawaii—basically plains that slope toward the sea, surrounding volcanic mountains—creates a runoff-flood risk in a form called "sheet flooding." If runoff flooding is sudden—usually meaning that it has gathered in a gulch or other channel or has created a channel for itself—it will become flash flooding. If it is gradual—and spread over a flood plain—it will simply be flooding.

Mauka runoff as sheet flooding deserves separate attention in Hawaii because, in some terrain and circumstances, it represents a potential flood hazard that might otherwise go unnoticed, and it is not uncommon. Sheet flooding occurs when storm water collects on the ground, especially already saturated ground, to a depth of several inches, eventually flowing makai, or down-slope, still in a shallow "sheet." The potential for sheet flooding events might be increased when human uses of the up-slope land have disturbed the natural contours of the terrain. When sheet flooding occurs, the danger it presents is increased to some extent because it does not look especially dangerous. Unalarmed, people don't react effectively and quickly.

What starts as sheet flooding can 1) flow harmlessly to the sea (where it may become dangerous to corals and other sea life), 2) collect in relatively low or flattened areas becoming deeper flooding, 3) be caught in a gulch where it may flow away safely or become flash flooding, or 4) it may remain a sheet flood, loosening soil, damaging property and threatening the crops, landscapes and lives as it saturates and passes over the ground.

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Coastal Flooding: The inundation of land areas along the coast caused by sea water above normal tidal actions is called coastal flooding. This can be from a tsunami (formerly called "tidal wave"), but it can also be caused by prolonged strong onshore flow of wind or high astronomical tides, and particularly by the combination of both phenomena. Coastal Flood Warnings are issued when there is widespread coastal flooding expected within 12 hours. Such warnings are not issued if the incoming waves are simply predicted to break onto or to "over-wash" normally dry coastal areas. Information about preparedness for coastal flooding and areas that are most susceptible to the phenomenon can be found in PDC's page of tsunami disaster resources.