River flooding is a frequent risk to high volcanic Pacific island countries due to high rainfall, small river catchment areas and low lying coastal areas. Whilst floods cause considerable damage to people and property, some benefits of flooding include an increase in soil fertility due to sediments being deposited on flood plains, pollutants being washed away and groundwater being replenished.

However, currently our knowledge of Pacific island river system behavior remains poor.

What is a Flood?

Inland flooding results from heavy and prolonged rainfall, when the water level in rivers and streams rises over the banks and inundates the surrounding land.

There are three different types:

  • Flash Floods occur within a few hours of torrential rains with little or no warning and dissipate rapidly. This is the most common form of flooding in Pacific island countries.
  • Rapid-Onset Floods occur within several hours of heavy
  • rainfall, can last several days and are specific to medium-sized river catchments.
  • Slow-Onset Floods occur gradually over a fairly long period of time and are only characteristic of large river systems like the Sepik, PNG. Coastal flooding is a separate hazard which occurs when storm surges, waves and/or extremely high tidal levels inundate low-lying coastal areas.
A simple diagram of the water cycle as it relates to flooding.

What Causes Flooding?

Many factors influence the intensity of a flood:

  • Rainfall intensity and duration.
  • Steepness of terrain.
  • Water levels and moisture conditions preceding the rains.
  • Increased runoff due to deforestation.
  • Capacity of rivers, streams and drainage networks.
  • High tide levels preventing river drainage.

Flood Hazards

Floods disturb fragile island economies by affecting individuals, businesses, insurance companies and governments. The costs of flooding are high. For example, Fiji’s economy suffers annually losses of some FJD 20 million on average due to flooding.

People and property

Floods have tremendous impacts to life and property, with 10 people on average being killed every year in Fiji alone. Buildings, personal belongings and stock get washed away or seriously damaged by muddy water. Businesses and services become disrupted for several days and people need to be evacuated from flooded areas, sometimes for weeks.

Livestock and crops

Animals and crops get drowned and washed away and sometimes soil is saturated for months afterwards, preventing new planting.

Disruption of transport

Floods can seriously affect transport lines with airports closed, roads submerged and bridges washed away.

Health hazards

Spread of epidemics such as cholera is frequently associated with floods due to the flooding of septic tanks and sewage systems contaminating drinking water.