1. Floods

2. How are floods caused?

3. BenefitsPitfalls 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

4. Types of Floods Slow-onset Slow-onset Rapid-onset Rapid-onset Flash flooding Flash flooding Coastal flooding Coastal flooding

5. Slow-Onset Floods Characterised by the slow rise and fall of floodwaters Characterised by the slow rise and fall of floodwaters 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 Dividing Range) – Parts of Western Australia Can last for one or more weeks, or even months on occasion Can last for one or more weeks, or even months on occasion Damage to livestock, infrastructure & crops Damage to livestock, infrastructure & crops Usually time to prepare for these floods and evacuate, however crops and fixed assets are not capable of being moved Usually time to prepare for these floods and evacuate, however crops and fixed assets are not capable of being moved

6. Rapid-Onset Floods Faster rise and fall of floodwaters because usually occur in steeper areas like mountains and valleys Faster rise and fall of floodwaters because usually occur in steeper areas like mountains and valleys The rivers here are steeper and flood more quickly The rivers here are steeper and flood more quickly Floods may only last for 1-2 days Floods may only last for 1-2 days Potentially more damaging as there is less time to take preventative action, and a faster, more dangerous flow of water Potentially more damaging as there is less time to take preventative action, and a faster, more dangerous flow of water This type of flooding can affect many of our major towns and cities This type of flooding can affect many of our major towns and cities

7. Flash Floods Extremely fast rise and fall, often with little or no warning Extremely fast rise and fall, often with little or no warning Results from intense rainfall (often from thunderstorms), falling in a relatively small catchment area Results from intense rainfall (often from thunderstorms), falling in a relatively small catchment area Can occur in almost all parts of Australia – particularly in urban areas where drainage systems cannot cope Can occur in almost all parts of Australia – particularly in urban areas where drainage systems cannot cope Pose the greatest threat of loss of life Pose the greatest threat of loss of life People are often swept away after entering floodwaters on foot or in vehicles People are often swept away after entering floodwaters on foot or in vehicles Can result in significant property damage Can result in significant property damage

8. Coastal Flooding Caused mainly by low pressure systems along the coast (particularly northern Australia) Caused mainly by low pressure systems along the coast (particularly northern Australia) Can also be caused by: Can also be caused by: -Storm surges (raised masses of seawater - For example,50km across and several metres high) -Tidal waves -Tsunamis Potentially dangerous to settlements in coastal areas where the mouth of a river enters the sea Potentially dangerous to settlements in coastal areas where the mouth of a river enters the sea

9. Case Study: Flash Flooding in Melbourne February 2005 February 2005 Series of severe storms (hail, wind, heavy rain) Series of severe storms (hail, wind, heavy rain) 24-hour rainfall of 120.2mm – highest on record for Melbourne (3 x the average for February) 24-hour rainfall of 120.2mm – highest on record for Melbourne (3 x the average for February) Massive low pressure system which remained stationary for 24 hours causing steady and continuous rainfall for a long period of time Massive low pressure system which remained stationary for 24 hours causing steady and continuous rainfall for a long period of time

10. Flash Flooding in Melbourne

13. ARI Average Recurrence Interval Estimate for the likelihood of a flood of similar size occurring again Estimate for the likelihood of a flood of similar size occurring again Eg. An ARI of 100 means a similar flood will occur once again every 100 years Eg. An ARI of 100 means a similar flood will occur once again every 100 years It is important to understand that the ARI of a flood gives no indication of when a flood of that size will next occur It is important to understand that the ARI of a flood gives no indication of when a flood of that size will next occur ARI is a good indication of the severity of a flood event – the larger the ARI, the larger the flood event ARI is a good indication of the severity of a flood event – the larger the ARI, the larger the flood event