Assessment of flood potential
Major flooding requires a large scale rainfall situation over the Johnstone River catchment. The following can be used as a rough guide to the likelihood of flooding in the catchment:
Average catchment rainfalls of in excess of 200mm in 24 hours may cause stream rises with moderate to major flooding and traffic disabilities to develop, particularly in the lower reaches downstream of Nerada on the North Johnstone River and Corsis on the South Johnstone River extending to the coastal plain around the Innisfail township and the mouth of the Johnstone River.
Average catchment rainfalls of in excess of 300mm in 24 hours may cause significant stream rises with major flooding and traffic disabilities to develop, particularly in the lower reaches downstream of Nerada on the North Johnstone River and Corsis on the South Johnstone River extending to the coastal plain around the Innisfail township and the mouth of the Johnstone River.
Each river height station has a pre-determined flood classification which details heights on gauges at which minor, moderate and major flooding commences. Other flood heights may also be defined which indicate at what height the local road crossing or town becomes affected by floodwaters.
The table below shows the flood classifications for selected river height stations in the Johnstone River catchment.
| River Height Station | First Report Height | Crossing Height | Minor Flood Level | Crops & Grazing | Moderate Flood Level | Towns and Houses | Major Flood Level |
|---|---|---|---|---|---|---|---|
| Nerada | 6.0 | 7.0 | 8.0 | ||||
| Tung Oil | 7.0 | 8.0 | 8.5 | ||||
| McAvoy Br | 7.1 (B) | 5.0 | 6.0 | 7.1 | |||
| Corsis | 5.0 | 5.5 | 6.0 | ||||
| Central Mill | 3.9 (B) | 7.5 | 8.0 | 8.5 | |||
| Innisfail | 3.5 | 5.0 | 5.5 | 6.0 | 6.0 |
