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# 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).

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.

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