Floods Floods

How are floods caused?How are floods caused?

Benefits Pitfalls

What are the benefits of living on a floodplain? Fertile farm land

Flat land for building

Historically, many settlements were founded near rivers where ports could be developed

Types of FloodsTypes of Floods

• Slow-onsetSlow-onset

• Rapid-onsetRapid-onset

• Flash floodingFlash flooding

• Coastal floodingCoastal flooding

Slow-Onset FloodsSlow-Onset Floods• Characterised by the slow rise and fall of Characterised by the slow rise and fall of

floodwatersfloodwaters• Flooding of rivers in vast flat areas, such as:Flooding of rivers in vast flat areas, such as:

– Central and western NSW/QLD (West of the Great Central and western NSW/QLD (West of the Great Dividing Range)Dividing Range)

– Parts of Western AustraliaParts of Western Australia

• Can last for one or more weeks, or even Can last for one or more weeks, or even months on occasionmonths on occasion

• Damage to livestock, infrastructure & cropsDamage to livestock, infrastructure & crops• Usually time to prepare for these floods and Usually time to prepare for these floods and

evacuate, however crops and fixed assets evacuate, however crops and fixed assets are not capable of being movedare not capable of being moved

Rapid-Onset FloodsRapid-Onset Floods

• Faster rise and fall of floodwaters because Faster rise and fall of floodwaters because usually occur in steeper areas like usually occur in steeper areas like mountains and valleys mountains and valleys

• The rivers here are steeper and flood more The rivers here are steeper and flood more quicklyquickly

• Floods may only last for 1-2 days  Floods may only last for 1-2 days  • Potentially more damaging as there is less Potentially more damaging as there is less

time to take preventative action, and a time to take preventative action, and a faster, more dangerous flow of water faster, more dangerous flow of water

• This type of flooding can affect many of This type of flooding can affect many of our major towns and citiesour major towns and cities

Flash FloodsFlash Floods• Extremely fast rise and fall, often with little or Extremely fast rise and fall, often with little or

no warningno warning• Results from intense rainfall (often from Results from intense rainfall (often from

thunderstorms), falling in a relatively small thunderstorms), falling in a relatively small catchment areacatchment area

• Can occur in almost all parts of Australia – Can occur in almost all parts of Australia – particularly in urban areas where drainage particularly in urban areas where drainage systems cannot copesystems cannot cope

• Pose the greatest threat of loss of lifePose the greatest threat of loss of life• People are often swept away after entering People are often swept away after entering

floodwaters on foot or in vehiclesfloodwaters on foot or in vehicles• Can result in significant property damageCan result in significant property damage

Coastal FloodingCoastal Flooding• Caused mainly by low pressure systems along Caused mainly by low pressure systems along

the coast (particularly northern Australia)the coast (particularly northern Australia)

• Can also be caused by:Can also be caused by:- Storm surges (raised masses of seawater - For Storm surges (raised masses of seawater - For

example,50km across and several metres high)example,50km across and several metres high)- Tidal wavesTidal waves- TsunamisTsunamis

• Potentially dangerous to settlements in Potentially dangerous to settlements in coastal areas where the mouth of a river coastal areas where the mouth of a river enters the seaenters the sea

Case Study: Flash Flooding in Case Study: Flash Flooding in MelbourneMelbourne

• February 2005February 2005

• Series of severe storms (hail, wind, heavy Series of severe storms (hail, wind, heavy rain)rain)

• 24-hour rainfall of 120.2mm – highest on 24-hour rainfall of 120.2mm – highest on record for Melbourne (3 x the average for record for Melbourne (3 x the average for February)February)

• Massive low pressure system which Massive low pressure system which remained stationary for 24 hours causing remained stationary for 24 hours causing steady and continuous rainfall for a long steady and continuous rainfall for a long period of timeperiod of time

Flash Flooding in MelbourneFlash Flooding in Melbourne

Flash Flooding in MelbourneFlash Flooding in Melbourne

Flash Flooding in MelbourneFlash Flooding in Melbourne

ARIARIAverage Recurrence IntervalAverage Recurrence Interval

• Estimate for the likelihood of a flood of Estimate for the likelihood of a flood of similar size occurring againsimilar size occurring again

• Eg. An ARI of 100 means a similar flood Eg. An ARI of 100 means a similar flood will occur once again every 100 yearswill occur once again every 100 years

• It is important to understand that the ARI It is important to understand that the ARI of a flood gives no indication of when a of a flood gives no indication of when a flood of that size will next occurflood of that size will next occur

• ARI is a good indication of the severity of a ARI is a good indication of the severity of a flood event – the larger the ARI, the larger flood event – the larger the ARI, the larger the flood eventthe flood event