The risk assessment approach is based on assessing the risk or likelihood of recurrence of varying floods based on specified rainfall intensities. Flood inundation and high velocity flows are a threat to the local community. The risk of those assets being lost depends on the extent and volume of floodways, flood storages and flood fringes.
Site Hazard Classification
The NSW Floodplain Development Manual defines flood zones into three categories, namely, “floodways”, “flood storage” and “flood fringe”. Each of these is summarised below:
• Floodways are areas of significant flow paths that should be kept free of obstructions, else upstream flood levels may increase.
• Flood storage areas hold significant volumes of water during floods and should not be filled else downstream flood discharges may increase.
• Flood fringe areas are inundated but convey no significant amounts of flood and hold not significant storage. These areas can be developed and filled without adversely affecting flooding.
The Provisional Hydraulic Hazard Categories have been developed as guidelines to be used in conjunction with appropriate site flood studies to determine the likely impact of flooding on property and personnel safety.
An extract of Figure L1 Velocity and Depth relationship has been reproduced below.
Notes
1. At velocities in excess of 2.0 m/s, the stability of foundations and poles can be affected by scour. Also,
grass and earth surfaces rough and unstable
2. The velocity of floodwaters passing between buildings can produce a hazard, which may not be apparent if only
the average velocity is considered. For instance, the velocity of floodwaters in a model test has risen from an
average of 1 m/sec to 3 m/sec between houses.
3. Vehicle instability is initially by buoyancy.
4. At floodwater depths in excess of 2.0 meters and even at low velocities, there can be damage to light-framed buildings from water pressure, flotation and debris impact.
An extract Figure L2 – Provisional Hydraulic Hazard Categories has been reproduced, below.
Notes
The degree of hazard may be either –
reduced by establishment of an effective flood evacuation procedure.
increased if evacuation difficulties exist.
In the transition zone highlight by the median colour, the degree of hazard is dependant on site conditions and the nature of the proposed development.
