1. Riverine flooding – this is caused by heavy rainfall and run off to waterways such as creeks and rivers that result in high water levels breaching banks and flowing into nearby floodplains and lower lying areas, some areas where this occurs are known and are already mapped within Latrobe City under the Flood and Land Subject to Inundation overlays.
   
    
2. Flash flooding –  is typically caused by high intensity, short duration rainfall events, and can sometimes result in property impacts in areas that haven’t previously been recognised as having a flood risk.  The underground pipe networks can be filled up and overland flow paths engaged resulting in visible flash flood waters travelling down roads and through property that then quickly dissipate.

Figure 1 – Typical layout of minor and major drainage system (ARR 2019)

Current design guidelines provide drainage in two layers:

• Minor drainage system 20% average exceedance probability (AEP) for frequent storm events and are typically conveyed in underground pipes
• Major drainage system 1% AEP for rare storm events and typically include overland flow paths

The chance of a flood of a given or larger size occurring in any one year, usually expressed as a percentage. For example, if a storm event has an AEP of 5% it means that there is a 5% chance (that is a 1 in 20 chance) of the same size or larger storm events occurring in any one year.

Flood risk is determined based on a detailed scientific modelling process that combines current and historical data, survey work and flood modelling. Flood modelling simulates a real-world system by identifying how flood water will travel across the land; how wide it will extend; how deep it will be; and how fast it will travel (rate of flow).

An overland flow path is the route taken by stormwater flowing over the natural ground surface when an existing piped system does not have capacity to carry the size of the rain event. This is different to riverine flooding which is caused when flows discharge from a watercourse such as a creek or river.

Overland flow paths are provided to convey larger size rain events and tend to affect localised areas rather than be widespread. It is normal in these events for water to flow along some sections of urban roads and along the designed overland flow paths in drainage reserves.

A network of pits, underground pipes, open channels and overland flow paths are managed by Council to help convey runoff away from urban areas.

Council is responsible for the management of the stormwater drainage network downstream of private properties (public drains). Property owners are responsible for their drainage connection up to the point of connection with Council’s system.

Drainage standards applied to urban development throughout Victoria changed in the early 1980s - in developments established prior to the 1980s, the underground drainage systems were designed to cope with a 20% AEP rainfall, i.e. drains were designed to convey the runoff resulting from a rainfall event of an intensity which occurs on average once every 5 years.

Today the more frequent minor flows are still catered for using underground drains, but current design standards also consider severe rain events and the major overland flow which occurs when pipes are full and water travels down roads and along overland flow paths.

Latrobe City now applies the 1% AEP flooding standard to all new subdivision developments. This is in an effort to ensure that any development on known flood prone land recognises the risk to public health and safety and is planned accordingly including setting of appropriate finished floor level to minimise the flood risk. Provision must now also be made for severe storm event flows to pass safely overland through any new development.

It is not feasible for all stormwater to be collected and conveyed via underground drainage networks. The pipe sizes required would be huge and the associated cost would be immense.

Piped drainage systems are generally designed to accommodate stormwater runoff for a 20% AEP (1 in 5 year) event.

In a 1% AEP (1 in 100 year) event, the underground piped drainage network quickly fills, and the remaining stormwater runoff flows along the designated overland flow paths (roads, drainage reserves, etc) towards a waterway.

Drainage improvement work is based on known flood problems where houses, commercial or industrial buildings are flooded repeatedly. The key factors are the potential risk to life from flooding, and the cost per property for flood protection works.

To reduce overland flows in established areas it would cost the Council and subsequently the ratepayers, millions of dollars. Therefore, these improvement works once identified, designed and costed, can only be undertaken incrementally over a period of years and as funding becomes available.

You can minimise the impacts of flooding to your home by:

• keeping roof guttering clear
• keeping your stormwater property connections clear
• keeping furniture, fittings, wiring and electrical items above known flood levels
• placing valuables such as photographs, passports and other important documents in high places away from possible flood waters
• moving your car to higher ground if your carport or garage is likely to flood
• securing outdoor furniture, garden equipment and children's toys to prevent them from becoming potential debris or flood hazards.

You can assist Council in reducing the risk of flooding by reporting (via phone, email or Snap/Send/Solve) hazards such as:

• blocked or damaged pits
• blocked or damaged outlet or inlet grates
• blocked stormwater pipes
• water pooling around kerbs and channels
• illegal piping