Low-Lying Tidal Flats at the Mouth of the Trinity Inlet

Posted on July 8, 2014 · Posted in Cairns Regional Council

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 deal with 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.

The problem is most common in the summer months, when torrential rain falls on a tide with a peak of 2.7m or more (174 occurrences in 2014). 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 regular events.

• A series of tide gates (a.k.a. tide flaps) have been installed on culverts over 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 (a.k.a. fishtails) have been 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 where silt build-up is removed from the major drainage channels to allow them to work at their full capacity. This operation is highly sensitive as the drainage channels are designated marine environments requiring (protected mangrove species, active fish habitats, and the material removed is acid sulphate soil). Cairns Regional Council have led the field by implementing an approved Marine Plant Management Strategy to streamline the approval of permits to work in these areas.

• The Pièce de résistance is the stormwater pump station installed under the aptly-named 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 sea wall 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 one occasion (once a regular event) where brackish tidal waters have overloaded the CBD stormwater system and seeped out into the streets.

These measures haven’t completely nullified the effects of tidal flooding, but have made it possible to manage the problems areas. Future drainage works such as retention basins are in the pipeline to further alleviate potential flooding, but we will always be at the mercy of mother nature.

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