Why Flooding Occurs

Why Flooding Occurs:

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If the rate of precipitation; the amount of rain/irrigation water landing on the soil surface, exceeds the soils infiltration rate, being the rate that moisture can be absorbed into the soil, ponding (water pooling on the surface) will occur. As precipitation continues, run off and flooding will follow unless a physical barrier is produced.

Different soil types have different infiltration rates. Sandy soil will have a higher infiltration rate than clay as more liquid can be absorbed into sandy soil than heavy clay soil. As the precipitation rate increases; so the infiltration rate decreases. Once any type of soil is at a point where no more liquid can be absorbed, it is called saturation point and flooding will occur.

As an example, if you place a dry sponge in a bowl and slowly fill the bowl with water, the sponge will absorb the water, place a soaked sponge into a bowl and fill the bowl with water, the water will simply run over the top of the bowl.

Like anything water takes up physical space and when liquid is forced into a space that is already full, there is no other option other than running over the surface. Following intense, prolonged rainfall, the ground that has become saturated will take time to dissipate the water that it has absorbed, therefore, with each new subsequent rain event, the saturation point will be reached more quickly than before and flooding will occur more rapidly.


Chelsea Public Golf Course experienced ongoing flooding following the 2011 February floods

Put simply; anytime the precipitation rate exceeds the infiltration rate, saturation point will eventually occur due to the soils inability to absorb the moisture quickly enough and flooding is inevitable.


Bonbeach Reserve under water

Precipitation – Rain or Irrigation water.

Infiltration – Moisture entering the ground.

Absorption – Moisture taken in by the soil.

Saturation – The point where no further moisture can be absorbed.

Last Updated (Tuesday, 13 March 2018 10:26)