About the network
Monitoring coastal flooding from the sea, usually because of storm surge during tropical cyclones, is the role of the Environmental Sciences Division of the department. This work is performed by Coastal Services.
From its base at the Queensland Government Hydraulics Laboratory at Deagon, Coastal Services operates data collection, using a network of 20 storm tide gauges along Queensland's coastline (see figure 1).
Figure 1. Location of storm tide gauges in Queensland
Storm tide gauges
Storm tide gauges are built to withstand severe weather and storm surge. They house instruments to record the rise and fall of the tide automatically at regular time intervals in digital format. Their primary purpose is to record the magnitude of storm tide events. Fortunately these events are quite rare, so for the majority of the time, these gauges act as normal tide gauges recording the regular rise and fall of the tides. The basic components of the typical storm tide gauge (see figure 2) include:
a steel stilling well tube (0.5 metres in diameter) which serves to dampen the effects of wind and swell, and to protect the float from damage caused by debris in the water;
a float that rides up and down inside the stilling well as the tide rises and falls;
a counter weight (in a counter weight tube) connected to the float that drives the recording mechanism as the tide changes. As the tide rises and the float moves up, the counter weight moves down under gravity (and vice versa when the tide falls);
a special beaded stainless-steel line that connects the float to the counter weight. The beads are spaced at regular intervals along the line;
a system of pulley wheels with notches around their perimeters. One pulley is sited vertically above the stilling well tube; the other is sited vertically above the counter weight tube. As the tide rises or falls, the float and counter weight move up or down. The beaded line connecting float and counter weight is wound around the pulley wheels with the beads seated in the notches on the wheels. This provides very accurate movement of the pulley wheels;
a shaft encoder connected to the pulley wheel above the counter weight tube. The shaft encoder converts mechanical values as the pulley turns, into an electrical signal representing water level, which is passed to a data logger;
a data logger that collects and stores water levels obtained from the shaft encoder. The data logger also records the time of the recording. The data logger is connected to the Public Telephone Network so tide information can be transferred to the Coastal Services Branch for inclusion on the web. The data logger is battery-powered as a safeguard to ensure the system will continue to operate during severe weather conditions (when there is an increased likelihood mains power may be unavailable); and
a battery charger connected to mains power is recharging the battery during normal operation.
The instrumentation is contained within a strong aluminium housing to protect it from severe weather conditions.
Figure 2. Components of a storm tide gauge tide network
The value of the storm tide network cannot be overstated, particularly as Queensland's coastal strip is one of the fastest developing urban areas in Australia. This network provides sea level information in near real time. The information, and special storm tide maps that show the extent of coastal land vulnerable to tidal inundation, are vital when Queensland State Counter-Disaster Organisation (SCDO) considers the evacuation of communities during cyclone events. Officers from the department provide advisory services, including projections of the sea level, to SCDO during storm tide threats.
Storm tide plot
Information displayed by storm tide plots is displayed in figure 3. Contained in the plot are:
the predicted tide for the period;
the actual tide recorded;
the residual; and
an indication of Highest Astronomical Tide (HAT) at the site.
Residuals are the differences between predicted and actual tides recorded. Predicted (or astronomical) tides assume normal meteorological conditions prevail, whereas actual tides include influences from atmospheric conditions taking place at the time of recording. Therefore, residuals can serve to measure storm surge during tropical cyclones. Residuals include contributions from other factors including wave setup and wave runup. The indication of Highest Astronomical Tide (HAT) is important because this level is not usually reached by the tide under normal atmospheric conditions. If the HAT is exceeded by the actual tide there is a likelihood of some flooding from the sea. The magnitude of flooding is dictated by local topography and by how much HAT is exceeded. Figure 3, recorded during Tropical Cyclone Steve in February 2000, shows the storm tide gauge at Cairns experienced a storm surge of about 1 metre. However as it occurred during neaps, and on an ebb tide, the actual storm tide recorded was well below Highest Astronomical Tide (and consequently there was little threat from flooding). Had the surge occurred two weeks earlier (during spring tides) there would have been an additional 1.5 metres of tide, and some flooding would have resulted. The datum for heights shown in the diagram is Lowest Astronomical Tide (LAT). Typically, Australian Height Datum (AHD) is the preferred datum, and a correction to the AHD is applied .
Figure 3. Storm surge recorded at Cairns during TC Steve, 17 February 2000
Last updated: 14 February 2007