Approach
In considering the application of these issues to the specific flood characteristics of the lower Johnstone River floodplain, it is noted that:
- duration of flooding is universally long (in the order of days) across the floodplain;
- warning times can be short (~6 hrs);
- rates of floodwater rise are reasonably fast; and
- flood awareness is generally high and does not vary significantly across the floodplain.
The above four parameters are not significantly variable
across the floodplain to warrant specific treatment and
are therefore not used to define variations in the flood
hazard, but should be included in
development control measures. The flood hazard is therefore
defined on the remaining, varying
characteristics of:
- the size of the flood;
- depth and velocity of floodwaters; and
- evacuation and access.
| Hazard Category | Base Flood Event | Characteristics |
|---|---|---|
| Low | 100yr | - Areas that are inundated in
a 100yr flood, but the floodwaters are relatively
shallow (typically less than 1m deep) and are
not flowing with velocity. - Adult can wade. |
| High - Wading Unsafe | 100yr | - The depth and/or velocity are
sufficiently high that wading is not possible. - risk of drowning. |
| High - Depth | 100yr | - Areas where the floodwaters are
deep (> 1m), but are not flowing with high
velocity. - Damage only to building contents, large trucks able to evacuate. |
| High - Floodway | 100yr | - Typically areas where there is
deep water flowing with high velocity. - Truck evacuation not possible, structural damage to light framed houses, high risk to life. |
| Extreme | 100yr | - Typically areas where the velocity
is > 2m/s. - All buildings likely to be destroyed, high probability of death. |
