Johnstone River
Probable Maximum Flood (PMF)
Estimated probable maximum precipitation
Table 1: Extimated Probable Maximum Precipitation (PMP)
| Storm Duration (hrs) | Total Rainfall Depth (mm) |
|---|---|
| Combined Johnstone River Catchment | |
| 12 | 780 |
| 24 | 1110 |
| 48 | 1650 |
Estimate peak probable maximum flows
Table 2: Estimated Peak Probable Maximum Flows (PMF)
| Design Flood | Peak Design Flow (m3/s) | |
|---|---|---|
| North Johnstone River (McAvoy Bridge: Catchment Area = 971 km2 | South Johnstone River (Central Mill: Catchment Area = 480km2 | |
| PMF | 13,950 | 5,740 |
| 100 | 6,140 | 3,330 |
| PMF/Q100 Ratio | 2.3 | 1.7 |
The peak discharge was within the bounds of rule of thumb calculations for the PMF. For example, the PMF peak discharge is typical 1.5 to 3 times that of the 100 year ARI discharge. The peak PMF discharge in the North Johnstone River at McAvoy Bridge of approximately 14,000 m3/s is about 2.3 times the 100 year ARI discharge at the same location. Another rule of thumb is that for a catchment of about 1000 km2 (North Johnstone River at McAvoy Bridge), the ratio of the peak PMF discharge to the catchment area is typically withing 3 to 25. This gives a PMF discharge range of 3,000 m3/s to 25,000 m3/s.
