Detailed technical information on flood plain mapping

Major floods and “flash floods”

The flood studies being reported on here are for the larger watercourse catchments and flooding which may occur generally as the result of an extended period of lighter rainfall, and/or heavy rain. The time from the onset of (continuous) rainfall before flows peak in the watercourse typically varies upwards from 6 hours. 

On the other hand, flash flooding (the term applied to localised flooding occurring in smaller catchments due to short, but very heavy rain) could potentially affect many more properties.  Urban “flash flooding” is very complex to model, and further research and detailed studies are required to identify flood prone land in urban catchments, associated with “flash flood”.

Council’s major flood management service level:

Our service level for major flood flows in urban areas is intended to limit risk in the event of a major (1;100) flood, primarily by conveying flood flows via public land (eg the roadway or Council reserves), or by limiting flows over private land to shallow depth and low velocity.  Stormwater drainage infrastructure in the street is generally not designed to deal with these “flash flood” flows.

To protect dwellings and other buildings from risk of flooding in “flash floods” Development controls applied under our Development Plan will require development to be constructed “to prevent the ingress of floodwaters”.  This typically means setting the floor level of the house sufficiently above the estimated flood level. 

The flood mapping process

The techniques and methodology used for flood mapping studies is used across the nation, and is based on industry standards.

Highly developed computer modelling software used widely throughout the world is utilised for our floodplain modelling and mapping.

Its important to note that flood mapping studies are based on statistical analysis, and require interpretation, interpolation and extrapolation of rainfall and run off data.  In many cases, the available data is limited, either by location, or by length of record.  Accordingly there is a margin for variability in results.

In all our studies, specialist consultants with experience and capability in flood modelling and mapping have been engaged.  The projects are overseen and reviewed by representatives from the Bureau of Meteorology and the Natural Resource Management Board.  Technical review has been provided by the hydrologist at the Department of Planning, Transport and Infrastructure and a former flood hydrologist with the Bureau of Meteorology, both nationally recognised for their competence in this field. 

The process used for our studies involves a number of steps and processes:

  • Identification of rainfall intensity, frequency and duration associated with flood events.  We use either continuous records from rain gauges within or close to the catchment, where available, or estimates provided by the BOM
  • Determination by observation, of the rates of run off from the catchment, (ie: how much and how quickly will rainfall flow across the ground). Estimates are based on assessment of a combination of land use type, gradient, soil type and vegetation cover. 
  • Development of a hydrologic model using widely used and tested software to determine flow time and volumes for overland flow using the rainfall estimates and run off coefficients.
  • Development of a digital terrain model (DTM)or catchment surface model using a combination of aerial photography and ground survey to build a three dimensional model of the catchment ground contours to model overland flows.
  • Creation of a hydraulic model using sophisticated modelling software to simulate stormwater flowing over the catchment surface model, to map overland flows paths and depths, including flows along creeklines and through major bridges and culverts.
  • Wherever possible, the models are “calibrated” ie, comparing modelled flows with actual recorded flows and rainfall, to confirm the model simulates the actual flows as closely as possible.  Unfortunately this relies on having historical information available.  Most of our catchments are “ungauged” and no flow information is available, and even long term rainfall information is not available for all catchments.
  • A number of model simulations are run, using different rainfall intensity and duration, to identify the flood extents.  These are mapped and overlayed on each other, and the outer limit of the combined flood flows is adopted as being representative of the 'design flood'.
  • We model flood extents for the 1;100 ARI – as the generally adopted national risk management standard for flood management
  • Additionally we model “flood hazard” for the 1:100 ARI and 1:500 ARI events.  This is a guide to the degree of hazard associated with flood conditions. It is generally determined based on a combination of flow depth and velocity.  The scale takes into account the ability to wade, or drive through flood waters.  Additionally, flood hazard may also be influenced by other factors including time till peak flow and availability of evacuation routes which impact on our ability to prepare for and respond to a flood emergency.
  • The modelling also determines the capacity of major drains, culverts and bridges to help guide planning for future renewal or improvements.

ARI - Average recurrence interval and flood hazard

Average recurrence Interval

The method of describing the severity of a flood is based on statistical terminology, and the presumption that the larger the flood event, the less likely, or less regularly it is likely to occur.  There will typically be rainfall of short duration or low intensity at various times during a normal year.  However, extremely heavy rain, or long periods of continuous rain which overwhelms a stream’s capacity and result in floods, are much less likely to occur – even though they can, and will occur. 

We use the terms “average recurrence interval” (ARI) To describe the statistical frequency of rainfall and flood events.

A 1:1 ARI event, is a rain event which, based on statistical analysis, is considered likely to occur once per year-ie: there is a 100% chance that it will occur during a year.  It is a reasonably foreseeable and regular event.

A 1:20 ARI event has a statistical probability of occurring once every 20 years, or 5 times every 100 years.  This is a rarer event than a 1:1 ARI event, and therefore, expected to be a more significant event, with higher rainfall, or a longer than usual duration of continuous rain. 

A 1:100 ARI event is a much rarer event, and also a much more severe event, likely to result in widespread rainfall, very heavy rainfall or very long duration rainfall, causing rivers to overtop their banks and result in flooding and possibly even physically alter the path of the river.

A 1:100 ARI event is something that may not occur in a typical lifetime – it is a significant event with only around a 50% chance of one occurring in a typical lifespan!  However, statistically there is still a 1% chance of an event such as this occurring in any year.  This is the standard flood “event” we seek to manage flood impacts for, and is the standard our Development Plan identifies for “Flood management areas”.

The ARI is measured over an infinite time, therefore, the mere fact that there has not been a 1:100 event in our area for 100 years does not imply that there will be one.  Likewise, it is feasible for two or more 1:100 year events to occur consecutively.