Design floods are hypothetical floods used for planning
and floodplain management investigations. A design flood
is defined by its probability of occurrence. It represents
a flood which has a particular probability of occurring
in any one year. For example, the 1% Annual Exceedence
or 1 in 100 Average Recurrence Interval (ARI) flood is a best estimate of a flood which has 1 chance in 100 of occurring in any one year. It should be noted that planning for the 1 in 100 year ARI flood does not guarantee protection for the next 100 years.
Design flood levels, flows and velocities were determined
for 100, 50, 20, 10, 5 and 2 year ARI floods.
The design floods were used to make an assessment of the financial losses to residential and commercial properties. These financial losses were then used as a basis to do an economic assessment of potential floodplain management measures. Historical damage to public infrastructure was documented where information was available.
Highest Gauge Level
Highest Recorded Level
|Nth Johnstone||McAvoy Alert
|Nth Johnstone||Tung Oil
|Sth Johnstone||Central Mill
|Sth Johnstone||Corsis||Not Gauged||8.63||1989||31/01/1994||-|
Calibrating the Flood Model example 1999 flood
|Flood ID||Recorded Peak Flood Level
|Modeled Peak Flood Level
|Difference (Modeled Recorded)
|G26||4.25||Not inundated in the model|
|Range||Percentage of Calibration
Points within Range (%)
|-400mm to -200mm||5.0|
|-200mm to -50mm||7.5|
|-50mm to +50mm||55.0|
|50mm to 200mm||30.0|
|200mm to 400mm||2.5|
Overall, good agreement between recorded and hydraulic model flood levels was obtained for the calibration events, especially in the most recent February 1999 flood indicating that the model is reliably predicting the flooding behaviour of the current floodplain. It is recommended that results from the southern part of the hydraulic model be used with caution, as this part of the model was not calibrated.
Hydraulic Model Calibration
Design floods are hypothetical floods used for planning and floodplain management investigations. A design flood is defined by its probability of occurrence. It represents a flood that has a particular probability of occurring in any one year. For example, the 1% AEP or 1 in 100 ARI flood is a best estimate of a flood which has 1 chance in 100 of occurring in any one year. It is important to acknowledge that the 100 year ARI event may occur more than once in a 100 year period as the definition of the event is that it occurs once, on average, in 100 years. Therefore, planning for the 1 in 100 year ARI flood does not guarantee protection for the next 100 years. Similarly, the 100 year ARI event may not occur at all within a 100 year period for the same reason. The 2 year, 5 year, 10 year, 20 year, 50 year and 100 year ARI were analysed.