Floods and Flooding, Assessing Flood Risk

It is wise to assess the flood risks to the best of your ability both for your home and for areas that you visit frequently. The following guide will give you some pointers, but the specific history of a given area is your best guide. Local residents often know the history. You can also check the Mother Nature Hawaii website for risk and mitigation information. Other risk-assessment information can be derived from Flood Insurance Rate Maps or their equivalents.

Terrain or "the lay of the land" is a strong risk determinant. Your first clue to flood risks at home and away is the land itself. Water naturally flows downhill and, by gravity, seeks the lowest plain or channel.

If the land around you is higher than where you are, that suggests a risk of flooding. If the land around you is well supplied with gulches and channels through which you would logically expect runoff to pass around your immediate area, the risk may be reduced. However, even if there are sufficient channels to carry away rain water, it may be possible for them to overflow if rain continues for a very long time or becomes very intense.

As you look at the lay of the land, include the risk of landslides and mudslide in your assessment. Often, it is possible to see flow patterns from earlier slide events. These will give you a reasonable idea of where mudslides and landslides are most likely. However, changing land uses—open space that has been paved, plowed or developed with structures, for instance—can mean different flood risks. Take current as well as historic facts into consideration.

The Mother Nature page on flood risk says of Flood Insurance Rate Maps, "updates to these maps have not kept up with development… Each year, numerous properties not officially mapped as 'flood prone' are flooded." No resource supplied in any form can be as up-to-date as your informed investigation of your property and its setting.

Sudden downpours and lengthy/continuous rain events can cause floods. Be aware of the usual annual rainfall in your area and its patterns.

It is not enough to know only the average annual rainfall total. A place that has an average annual rainfall total of 20 inches may flood or not, depending on how many rain events those 20 inches are spread over and how much time the ground and ground-covering vegetation have to recover between soakings.

Even relatively light rain can produce floods if it goes on long enough, depending on the ability of the ground to absorb the rainfall.

When rain is forecast or observed, take all watches and warnings seriously. Again, know the terrain because runoff from a rain storm that is not in your immediate vicinity may send water rushing over the ground or down the gulches into your neighborhood.

Make full use of available risk assessment resources. When rain is expected or even after rainfall begins, make a habit of checking weather-monitoring resources frequently. A list of useful links is found on this PDC website.

Dams or retaining basins are meant to prevent uncontrolled water flow, but in extreme conditions, they can overflow or break. Be aware of where the dams and retaining basins in your area are located. Know their intended spillways, and be willing to report apparently dangerous conditions. If the spillway of a dam above your property is blocked with debris or vegetation, for example, that increases the chance of flooding. Report your concern to dlnr@hawaii.gov or by calling DLNR enforcement at 808.643.DLNR (3567).

Flood