Dealing with heavy storms
Melbourne’s drainage and sewerage systems cope extremely well with most situations. But extremely heavy rain can sometimes push them beyond normal limits.
Drainage and sewerage networks are designed with ‘fail-safes’. These fail-safes protect people, property and the system itself from flood damage.
Note: Rivers and creeks can contain a range of contaminants washed in with stormwater. It’s highly recommended to avoid direct contact with waterways for 48 hours after heavy rain.
Sewerage system fail-safes
As well as carrying waste water from your home, the sewerage system sometimes carries stormwater. Very heavy or prolonged rain can fill sewerage pipes. So that sewer levels don’t rise too much the system is designed to automatically overflow stormwater/sewage into rivers. This rarely happens but prevents sewage from backing up and overflowing into your property.
The sewerage system is built to cope with rainfall from a 1-in-5 year storm (which is the national standard). A 1-in-5 year storm means that in any one year, there is a 20% probability of a storm of that magnitude happening.
Drainage system fail-safes
Ever noticed parks and sportsgrounds sometimes flood after big storms? These areas double as ‘retarding basins’ and help take water away from properties and hold it until it can be slowly released into drains. (Holding back the flows protects downstream areas from flooding that might occur if all the runoff rushed downstream unchecked.) These drains then empty into rivers, creeks and the bay.
Local parks and sportsgrounds, like this one in Box Hill, are used to hold back stormwater after extremely heavily rain.
Frequently asked questions
How does stormwater get into the sewage system?
Water can get into the sewerage system by a number of ways including through cracks and joins in terracotta pipes or occasional instances where people fail to properly connect their stormwater (i.e. drainpipes) to the right network. Small amounts of stormwater across the tens of thousands of kilometres of pipes in the sewerage network can add up to a lot.
Why don’t we build a system that can cope with bigger storms?
It's important to strike a balance between the cost of pipes and sewers and capacity. Melbourne’s sewerage and drainage systems are world-class and can deal with the vast majority of storms. But it would cost of a lot of money to completely protect the city against the most extreme events, which is why we balance this by creating a system with fail-safes.
We invest heavily on constantly improving our network, especially in areas that are growing strongly. Two major expansions are the Northern Sewerage Project and Melbourne Main Sewer Replacement.
Why does sewage spill into waterways?
Relief structure overflowing into a waterway.
Sewage can spill into waterways when too much stormwater gets into the sewerage pipes after heavy or prolonged rain. To reduce rising sewer levels they overflow into rivers and creeks. This fail-safe stops sewage and stormwater backing up into people’s homes, most likely through the shower drain as the lowest point in the house.
How is sewage released into waterways?
At certain points in the sewerage system, overflow chambers connect underground sewage mains to stormwater pipes or open waterways. These chambers, called 'relief structures', are designed to automatically release overflows of stormwater/sewage into a river or creek, where it is further diluted.
Relief Structures – Quick Facts
- There are 38 relief structures across the approximately 400 kilometres of sewers managed by Melbourne Water.
- Relief structures are all electronically monitored and send signals to our control room whenever they overflow.
- After a spill each relief structure is checked, cleaned and resealed.
- Relief structures have flap gated outlets to:
- Stop stormwater flowing back up the outlet and into the sewer if the creek has higher flow levels than the sewer.
- Stop odours from the sewer escaping
What is Melbourne Water doing to reduce sewage spills?
Melbourne Water has a number of projects underway to ensure that our sewerage network keeps up with population growth and to reduce spills. Two of the major projects are:
- The Northern Sewerage Project is boosting the capacity in the Northern suburbs.
- The Melbourne Main Sewer Replacement is tripling capacity for the fast-growing Docklands area.
How much of what spills into waterways is sewage?
Sewage is everything that goes down the drain, including shower water, laundry water, toilet waste and kitchen water. In dry weather, the amount of solid human waste in sewerage pipes is about 1%; in significant wet weather there is a further dilution of the sewage by another 4 or 5 times.
If stormwater causes the relief structures to activate, overflows that enter the waterways will be even further diluted by the receiving waterway. During storms, water in rivers and creeks runs a lot faster and more forcefully than normal, meaning if there is a spill, it's washed through the waterway relatively quickly.
It's strongly advisable to avoid contact with floodwaters or waterways after a storm for a number of reasons. There's the chance of contamination from stormwater pollution or spills and danger from hidden debris in the floodwater.
What role does climate change play in all of this?
Climate change projections suggest we can expect more extreme weather events, including heavy storms that deliver a large amount of rain in a short period (2-24 hours). This is despite the fact that annual rainfall volumes are expected to decrease.
Report discussing the effect of climate change on flooding