Magnitude of flow
There are two main methods of determining the magnitude of the flow for a design event. These are listed below and explained in the following sections:
Method 1. Flood frequency analysis (FFA)
Flood frequency analysis (FFA) enables the magnitude of floods of selected ARI (Average Recurrence Interval) to be estimated by statistical analysis of recorded historical floods.
South Johnstone River
| ARI Event (years) |
Central Mill + Upstream of Central Mill (BoM Rating) Flows (m3/s) |
|---|---|
| 2 | 590 |
| 5 | 1020 |
| 10 | 1330 |
| 20 | 1660 |
| 25 | 1770 |
| 50 | 2120 |
| 100 | 2510 |
| 200 | 2920 |
| 500 | 3520 |
North Johnstone River
| ARI Event (years) |
Goondi (DNRM Rating) + Tung Oil (BoM Rating) Flows (m3/s) |
|---|---|
| 2 | 1280 |
| 5 | 2200 |
| 10 | 2910 |
| 20 | 3660 |
| 25 | 3920 |
| 50 | 4760 |
| 100 | 5700 |
| 200 | 6740 |
| 500 | 8280 |
Method 2. Design rainfalls with the URBS Model
Design flood events are produced using design rainfall events. To determine the intensity and distribution of rainfall that will produce a specified ARI design event, charts developed by the Bureau of Meteorology (BoM) are consulted. These charts are contained in a book called "Australian Rainfall and Runoff"(IEAust, 2001).
General Agreement between the two models
| Major Historical Event |
ARI based on FFI Results |
ARI based on Design Events |
|---|---|---|
| 1932 | 13 | 4 |
| 1935 | 18 | 5 |
| 1967 | 40 | 20 |
| 1979 | 12 | 5 |
| 1982 | 36 | 18 |
| 1986 | 36 | 18 |
| 1994 | 26 | 14 |
| 1997 | 18 | 9 |
| 1999 | 23 | 12 |
Plus historical acedotal evidence
It is really important that whatever model you use, you check it against some real-life flood data. No model is perfect, at best a model is a reasonable approximation of reality. If it does not line up with reak events then the model is of little value.
| Year | Estimated Flow* (m3/s) |
Comment from Alan Dunne |
|---|---|---|
| 1878 | 9000 | About 4.5m higher at Innisfail than 1967 |
| 1894 | 5500 | About 1.6m higher at Innisfail than 1967 |
| 1911 | 5000 | About 0.7m higher at Innisfail than 1967 |
| 1913 | 5800 | About 1.7m higher at Innisfail than 1967 |
| 1927 | 4450 | About equivalent to 1967 |
* rough estimate only
Adopted peak flows - human decision
Based on the models and the real data we must now make an informed human decision as to what flow levels constitute a 2, 5, 10, 20, 50, 100 year flood
| ARI (Years) | Peak Design Flow (m3/s) | |
|---|---|---|
| North Johnstone River | South Johnstone River | |
| 2 | 2100 | 700 |
| 5 | 3060 | 1200 |
| 10 | 3630 | 1600 |
| 20 | 4400 | 2100 |
| 50 | 5340 | 2800 |
| 100 | 6140 | 3330 |
Comparison between historical and defined flood model
| Flood (Year or ARI) |
Flood Height at Innisfail Wharf Gauge (m AHD)* |
Flood Height at Innisfail Wharf Gauge (m Gauge) |
|---|---|---|
| 1878 | 9.0 | 11.0 |
| 1913 | 6.1 | 8.1 |
| 1894 | 6.0 | 8.0 |
| 100 | 5.4 | 7.4 |
| 1911 & 1935 | 5.1 | 7.1 |
| 50 | 4.9 | 6.9 |
| 20 | 4.5 | 6.5 |
| 1999 | 4.4 | 6.4 |
| 1967 & 1927 | 4.35 | 6.35 |
| 10 | 4.1 | 6.1 |
| 1997 | 3.85 | 5.85 |
| 5 | 3.8 | 5.8 |
| 2 | 2.7 | 4.7 |
* Some of the early floods are approximate levels only supplied by Alan Dunne (Dunne, 1999 and pers.comm.2003). The 1913 level is considered to be reasonable reliable as the level supplied by the BoM was independently verified by A. Dunne. In the Innisfail CBD the March 1967 flood levels were higher than the February 1999.
