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When good rivers turn bad: Extreme flooding
Case study - The Johnstone River, Far North Queensland

Design floods

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 Probability (AEP)
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.

Stream Station (a)
Highest Gauge Level
(m)
(b)
Highest Recorded Level
(m)
Date Opened Event (a)(b)
(%)
Nth Johnstone McAvoy Alert
(112908)
Not Gauged 9.80 2000 12/02/1999 -
Nth Johnstone Tung Oil
(112004A)
9.78 10.81 01/10/1966 12/02/1999 78%
Nth Johnstone Nerada
(112905)
Not Gauged 11.35 1989 12/02/1999 -
Nth Johnstone Innisfail
(112900, 112901)
Not Gauged 8.09 1979 30/01/1913 -
Sth Johnstone Central Mill
(12101B)
6.55 10.84 1/10/1974 02/02/1986 34%
Sth Johnstone Corsis Not Gauged 8.63 1989 31/01/1994 -

Calibrating the Flood Model example 1999 flood

Flood ID Recorded Peak Flood Level
(m AHD)
Modeled Peak Flood Level
(m AHD)
Difference (Modeled Recorded)
(mm)
G1 4.23 4.27 40
G2 4.24 4.27 30
G3 4.26 4.27 10
G4 4.25 4.44 190
G5 4.41 4.52 110
G6 4.39 4.53 140
G7 4.27 4.27 0
G8 4.24 4.27 30
G9 4.27 4.27 0
G10 4.24 4.27 30
G11 4.84 4.81 -30
G12 4.21 4.27 60
G13 4.24 4.27 30
G14 4.21 4.27 60
G15 4.73 4.70 -30
G16 4.28 4.27 -10
G17 4.82 4.85 30
G18 4.4 4.38 -20
G19 4.03 3.93 -100
G20 3.87 388 10
G21 4.72 4.49 -230
G22 4.9 4.87 -30
G23 3.94 3.94 0
G24 2.91 3.04 130
G25 2.89 3.04 150
G26 4.25 Not inundated in the model
G27 5.35 5.49 140
G28 3.71 3.73 20
G29 3.33 3.56 230
G30 9.35 9.46 120
G31 4.94 4.88 -60
G32 3.617 3.63 10
G33 4.283 4.27 -10
G34 4.245 4.27 20
G35 4.401 4.52 120
G36 8.4 8.45 50
G37 8.39 8.45 60
G38 6.28 6.34 60
G39 7.45 7.05 -400
G40 8.4 8.44 40
G41 2.1 1.9 -200

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.

Design hydrology

Design Hydrology
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.

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4. The Johnstone River, FNQ


The resources contained in this unit are courtesy of Earth Science Australia http://earthsci.org/