Why We Monitor Lismore's River Flows

1. Where is Lismore?

Geographical Background

Lismore is a city in New South Wales' Northern Rivers region, Australia. It is situated around 730 kilometers north of Sydney and 200 kilometers south of Brisbane, in a valley encircled by the Border Ranges to the west and the Richmond Range to the east. The city is at an elevation of 25 meters above sea level, and the Wilsons River flows centrally through the city. The surrounding landscape comprises fertile floodplains, rolling slopes, and subtropical rainforests, which are responsible for the high precipitation and high vegetation cover.

Human/Cultural

Lismore has a rich cultural heritage, where the native owners of the land have been the Bundjalung people since over 20,000 years ago. European settlement commenced in the 1840s, and Lismore grew as a timber and agricultural center, renowned for its banana plantations and dairy farming. Today, Lismore is a successful regional center with a total population of about 28,000, renowned for its arts culture, alternative lifestyle, and tight community bonding. It nestles Southern Cross University and hosts events like the Lismore Lantern Parade, a reflection of the dynamic and diverse arts and culture.

Overview of River and Hydrology

The Wilsons River is the source of life in Lismore. Originating within the Border Ranges National Park, the river runs 150 kilometers east, flowing through Lismore before it meets the Richmond River at Coraki and drains into the Tasman Sea at Ballina.

Wilsons River is integral to the hydrology, ecology, and economy of the region. It supplies water for drinking, farming, and industry, supporting Lismore's horticulture and dairy industries. Ecologically, the floodplains and river contain diverse ecosystems like rainforest margins, wetlands, and riparian habitat with flora and fauna like the platypus, eastern water dragon, and migratory birds.

In Lismore, the Wilsons River is 50-80 meters in width and 2-6 meters deep in natural condition. Key tributaries include Leycester Creek and Broadwater Creek, which supply its flow. The river floodplain, on which most of Lismore has been built, is occasionally flooded, a natural process that provides nutrients to the soil but which threatens urban settlements. The city utilizes the river as a source of water supply, and a waterworks takes in and screens water from the river, while stormwater outlet overflows drain into the river, whereby there exists a high-graded hydrological connection between the city and the watercourse.

2. What is the River Flow Situation near Lismor?

Influencing Factors

Precipitation and Runoff

Lismore experiences a subtropical climate with heavy rainfall annually that totals 1,750 millimeters, with most occurring during the period from December to March (summer-autumn). Monsoonal troughs and east coast lows bring heavy rain, and up to 300 millimeters in 24 hours is possible. Saturated runoff results in rapid runoff into the Wilsons River since the soils in the region are low infiltration rate clay soils.

The Wilsons River catchment area is 1,950 square kilometers, and water flowing from the ranges arrives in Lismore within 6-12 hours of heavy rain. The river is very susceptible to flash flooding from this, which is a major cause for concern for the city.

Topography and River Morphology

Wilsons River, situated close to Lismore, winds through a scenic valley with steep banks in some sections and broader floodplains in others. The river bed is sandy and gravelly, with submerged logs and boulders as fish habitat. The sinuous course slows water in some sections, but the comparatively steep gradient of 0.3 meters per kilometer accelerates water at high flow.

Lismore's floodplain location means that the river becomes 2 kilometers wide during major floods and the river depth is more than 10 meters in the city center. The natural constrictions such as the "Narrows" upstream of Lismore restrict the water-carrying capacity of the river and worsen floods by slowing discharge.

Reservoir Operation

There are no large storage reservoirs along the Wilsons River but farm dams and small weirs manage flow locally. With no large storage, this guarantees the river flow is rainfall-dominated and there is no artificial flood control during the occurrence of flood events. This guarantees Lismore to be more prone to high flow events.

Historical Hydrological Events

2022 Eastern Australia Floods

Lismore experienced its worst flood in history in February 2022 when the Wilsons River peaked at 14.4 meters. Prolonged coastal low rain quickly swelled the river, covering 90% of the CBD and over 3,000 homes. Two fatalities and AUD 1.5 billion damage, in addition to widespread displacement, resulted. (Source: NSW State Emergency Service, 2022 Flood Report)

1954 Floods

One of the greatest floods was in 1954, with the Wilsons River measuring 12.1 meters, the former record. It devastated 500 homes, damaged infrastructure, and led to the first official flood mapping of Lismore. The tragedy prompted early attempts at flood mitigation, including the building of levees. (Source: Lismore City Council Archives)

Monitoring of these occurrences is necessary to improve flood prediction, update building codes, and preserve lives and property in a city that has experienced severe river flows several times.

3. How Observe River Flows in Lismore?

Traditional Methods

Surface Drift Buoy Method

Traditionally, this involved the release of wooden or foam markers into the Wilsons River and timing their travel between markers. It provided rough surface velocity measurements but no depth variation, with subsurface flows being 30% slower. Wind and trash interference made results unsatisfactory, especially at high flows.

Anchored Boat Method

Between 1960 and 2000, teams used mechanical current meters (e.g., Price AA) from anchored boats. They were lowered at 0.5-meter intervals and measured velocity using rotating cups. The data collection of a single cross-river survey took 4-6 hours, limiting data collection during fast-rising floods. Safety risks during high water led to its replacement by modern methods.

ADCP Introduction: Acoustic Doppler Current Profiler (ADCP)

ADCPs have revolutionized the monitoring of flow in Lismore since the early 2000s. Installed on fixed platforms or vessels, they utilize sound to measure at 100+ depths of velocity simultaneously. It now only takes 30 minutes to conduct a complete river survey, taking data during flood times when conditions are unsafe and traditional methods are not possible. SES and Lismore City Council use ADCPs to measure discharge during floods, critical in updating evacuation warnings.

4. How Does ADCP Work on the Doppler Principle?

ADCPs depend on the Doppler principle: sound waves (300-1200 kHz) from transducers reflect off water sediment particles. When particles are moving towards the ADCP profiler, the frequency coming back is greater; when they are moving away from the ADCP, lower. The frequency difference is proportional to water velocity.

With four transducers, ADCPs construct 3D velocity profiles. With river width/depth measurements (sonar or GPS) added, they estimate discharge (volume flow). In Lismore, ADCPs can distinguish between main channel flows and floodplain overflows, so accurate flood volume measurements are needed for emergency planning.

5. What Is Needed for High-Quality Measurement of River Flows in Lismore?

Equipment Requirements

• Material Robustness: ADCPs must be resistant to flood conditions—water with sediment, colliding debris, and submersion. Enclosures of stainless steel exclude corrosion, and rugged constructions protect electronics from abusive deployment.
• Portability: Hand-portable ADCPs (6-10 kg) are necessary for Lismore's close river bends and flood conditions to allow for deployment off small boats or even kayaks when roads are flooded.
• Cost-Effectiveness: Since constant monitoring is necessary, equipment needs to be long-lasting and minimal maintenance. Long battery life ADCPs (40+ hours) reduce fieldwork costs during extended flood events.

6. What is the Right Current Measurement Equipment?

Deployment Method

• Ship-Borne ADCP: Deployed by the SES in times of flood, installed on vessels to map flow over floodplains and rivers, identifying rapid currents that are threatening infrastructure.
• Fixed ADCP: Installed at Lismore's river gauge station, providing real-time rising/falling level and velocity information, automatically sending out flood warnings when levels are exceeded.
• Cableway ADCP: Installed on the Wilsons River Bridge, across the river to take flows safely at high water without the need for a boat.

Working Frequency

• 600 kHz ADCP: Ideal for normal flows (2-6 meters depth) with high resolution to detect changes in velocity near the riverbed, which is critical for the understanding of sediment transport.
• 300 kHz ADCP: Used in floods where depth exceeds 10 meters, having sufficient range to capture full water column velocities over expanded channels.

Brand Recommendations

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Here is a table with some well known ADCP instrument brands and models.