Stormwater is the run-off from rain that falls on a roof or paved area like a driveway, road or footpath that flows into a stormwater drain. Council maintains a complex stormwater drainage network which includes many different types of drainage structures such as:
- Unlined drains (including natural waterways) and concrete-lined open drains;
- Underground pipes and culverts;
- Pump stations and detention basins;
- Gross pollutant traps;
- Tide gates and litter booms;
- Grates, trash arrestors and other inlet structures; and
- Valves, frog flaps and other outlet structures.
The drainage network is designed to be mostly self-managed, with intervention only required in the event of component failure. Find out more about drainage on the Drains and Waterways page.
Managing localised flooding in Cairns
The Cairns CBD and surrounding suburbs are located on low-lying tidal flats at the mouth of the Trinity Inlet. One of the inherent problems we face on a regular basis is localised flooding from the inlet itself or any of the main drainage network of creeks and drains that service the area.
This problem is most common in the summer months when torrential rain falls on a tide with a peak of 2.7 metres or more. There were 174 occurrences of this in 2014. When this happens, the drainage system cannot cope with the influx of stormwater as much of its capacity is already taken by the rising tide. Over the years a series of control measures have been implemented to curb the impact of these events such as:
- Tide gates (also known as 'tide flaps') are installed on culverts within major drainage channels. The gates are lowered into place at the low point preceding these nominated high tides to prevent the ingress of tidal water into the channels, creating invaluable capacity for stormwater runoff. The gates are then opened at the top of the tide to allow the water out as the tide recedes.
- Tide flex's (also known as 'fishtails') are installed on many pipe outlets which allow free flow from the underground pipe system while preventing rising tides from getting in.
- A multi-year program is undertaken to remove built-up silt from major drainage channels to allow them to work at their full capacity. Drainage channels are designated marine environments and an approved Marine Plant Management Strategy is in place to streamline the approval of environmental permits to work in these areas. The Executive Summary of this Strategy is available below.
- An underground stormwater pump station is installed under the southern part of Lake Street (one of the main CBD thoroughfares and lowest point of the city). Most of the CBD’s stormwater system finds its way into the main chamber of the pump station, which is then pumped through a pair of giant tide flex’s under the seawall into the inlet. Automatically activated by sensors in the chamber, the four submersible pumps have a total pumping capacity of 5 cubic metres per second (enough to fill an Olympic sized swimming pool in under 10 minutes). Since its installation in 2009, there has not been an occasion where brackish tidal waters have overloaded the CBD stormwater system and seeped out into the streets. This used to be a regular occurrence in this area.
These measures don't completely avoid tidal flooding problems but the improvements allow better management of the issues in problematic areas. Living in the wet tropics, we are still at the mercy of mother nature.
Flood level calculations
By using historical rainfall, flood level records and hydrological calculations, the likelihood of the occurrence of different sized floods can be determined. The size of a flood is described in terms of how frequently similar sized or larger floods are expected to occur over the very long term (several 100's of years). Small floods are likely to occur frequently, while large floods are possible but far less common.
The probability of a flood event occurring in any year is referred to as Annual Exceedance Probability (AEP) and is expressed as a percentage. For example, a large flood which may be calculated to have a 1% chance to occur in any one year, is described as 1% AEP. The 1% AEP is also known as the 1 in 100 year Average Recurrence Interval (ARI), or Q100 event.
Due to the prohibitive cost of engineering solutions to prevent flooding in extreme circumstances, Council’s stormwater systems are designed to cope with Q2. It is expected that this will result in water over roads and minor localised flooding on occasions however the standing water tends to recede quickly as soon as rainfall eases and tides drop.
A drainage easement is a portion of land that the property owner has granted to Council to carry stormwater to the main drainage system. Illegal dumping of green waste and household rubbish in waterways, easements and drains increases the risk of flooding to properties and costs millions of dollars in property damage across the region. Under the conditions attached to an easement on the land title deeds, a property owner must not obstruct the free flow of water along the drainage pathway. Dumping waste, planting garden beds or building fences across the flow path are all examples of activities that can obstruct stormwater flow. Property owners should be aware that if a drainage easement is deliberately obstructed, they can be held liable for any damage caused to adjacent and neighbouring properties.
To find out more download the Keep Waterways, Easements and Drains Clear Fact Sheet ( PDF, 0.21 MB ) below.
To enable safe passage of vessels, the maintenance dredging of the Half Moon Bay Entrance Channels, Bluewater Harbour, Half Moon Creek and Half Moon Bay Marina, occurs as required.
As part of this activity, a Monitoring and Management Plan has been approved by the Federal Government's Department of Environment and Energy - refer below to download a copy of the plan.
To report any drainage issues
Call Council on 1300 69 22 47 or make an online customer request.