Already at major flood stage at 6 m on 17-Dec at 7:00, the river rose another 5 metres in 15 hours to 11.2 m at 22:00 when major damage began, and then rose another 2 metres overnight to 13.3 m damaging property and threatening many lives. The previous maximums were 10.4 m in 1999, 9.5 m in 2004, and 10.05 m in 2014. Other rivers also experienced their flood of record including the Bloomfield and Laura (gauges damaged), and the West Normanby, Normanby at Battle Camp, and Normanby at Kalpowar Crossing. Long-time residents say this is by far the most extreme rainfall and flooding they have witnessed or heard of (but exceptions do exist such as the 1899 Cyclone Mahina at Bathurst Bay).  

CYWP scientists, along with state hydrologists, have been monitoring river height, velocity, discharge, and suspended sediment concentrations at six gauges across the Annan (Big Annan Bridge, Oakey Creek, Trevethan Creek, Scrubby Creek, Little Annan at Beesbike, upper Annan at Collingwood). Surprisingly, four of the gauges were not damaged during the floods, but the Little Annan at Beesbike and Big Annan Bridge gauges were damaged just after peak flood conditions. Data reconstruction will help fill in gaps from this damage. Therefore, the river and tributary discharge (volume of water) and sediment loads can’t be estimated until all the data are quality controlled and analyzed.  

Trevethan Creek was heavily impacted by extreme rain and debris flows in the upper catchment, which pushed water and sediment flood waves past the Mulligan Highway bridge. The creek peaked at 9.2 m, which was 2-3 metres above the bridge deck road surface. Turbidity values maxed out at > 4000+ NTU (nephelometric turbidity unit, a measure of water cloudiness due to particles), which is eight  to 10 times higher than seen during past floods, with previously peaks around 500 NTU and 4m river height.  

Unfortunately, Wallaby Creek was not gauged during Cyclone Jasper when 2970 mm of rain fell in that catchment area. But the extent of the flood damage and high sediment loads from debris flows has instigated the installation of a new CYWP turbidity gauge site and water quality monitoring project starting in early January, which will monitor the recovery of the creek over the coming years.  

The upper Annan River gauge site at Shipton’s Flat, located upstream of the Wallaby confluence, was much less impacted by debris flows and bank erosion than Wallaby Creek, with 1300 mm of rainfall recorded at Collingwood mine. The river at Shipton’s Flat peaked at 8.3 m, which was well above the previous 5.7 m peak recorded at the site in 2019. Turbidity values peaked around 1600 NTU, which were similar to the previous wet season in 2023, but double that of the drier 2022 wet season. The rehabilitation efforts by QDNRME at Collingwood mine during 2023 performed very well during the extreme rainfall. However ongoing surface and batter rill erosion is continuing to contribute continuous turbidity values between 1000 and 4000 NTU into the decant dam creek and Annan River.  

Scrubby Creek is just upstream of the little Annan Bridge at Beesbike Station, and before Cyclone Jasper, it had been identified as the second dirtiest creek in the catchment with major gully and bank erosion. Past years turbidity values peaked between 1000 and 3000 NTU, with maximum turbidity recorded after fires burnt most of the catchment in 2021. However, during Cyclone Jasper, when 1200 mm of rain fell in the Scrubby catchment, turbidity values were 1000 NTU or less during the first flush. Values declined to around 500 NTU after Annan River floodwater backed up Scrubby Creek and diluted sediment concentrations.  

The Oaky Creek catchment received approximately 1200 mm of rain in six days and is the largest and previously dirtiest tributary to the Annan River, with major gully and bank erosion. Oaky peaked at 12.8 m and 800 cubic metres per second, higher than observed by locals in 30 years. Turbidity values were < 1000 NTU, lower than previous year’s first-flush due to major floodwater dilution, despite the catchment being partially burnt in December. However, the suspended sediment loads were substantial, with 36,000 tonnes transported during the event by 172 GL (billion litres) of floodwater.  

Preliminary data indicate that the Annan River estuary was three times dirtier than has been recorded during previous floods. Turbidity peaked at 2000 NTU at the Big Annan Bridge with water velocities at 2.5 m/s and a water height of 7 m, which was 1.5 m over the new bridge road surface. That means extremely high levels of sediment dumped on the seagrass meadows at Walker Bay, and Dawson and Forrester Reefs where gauges record turbidity and chlorophyll. By 24 –December,  satellite images recorded brown and green floodwater from the Daintree, Bloomfield and Annan Rivers reaching the outer reef from Cairns to Lizard Island.  

The water quality implications from the extreme floods are that a new sediment regime has been forced upon Wet Topics catchments including the Annan River. Past hotspots of erosion like Oaky Creek and Scrubby creek are continuing to erode, mainly from gully and bank erosion in dispersive soils. Meanwhile new hotspots such as Wallaby and Trevethan Creek now have elevated coarse and fine sediment loads from mass wasting (landslides), bank erosion, and re-erosion of floodplain sedimentation, These elevated sediment loads could take years to decline to previous conditions. A sediment geochemical tracing study, with samples collected from the Annan tributaries and estuary before, during and after the flood event will help document shifts in the sources of sediment supplied to the river and coast.  

The scale of mass wasting observed was unprecedented in the historic record of knowledge and evidence in the northern Wet Tropics (Annan and Bloomfield catchments). Weathered rock, soil, and organic material built up in headwater hollows over the past century and were released during this event as debris flows, after the stability thresholds were passed. Estimating the associated rainfall and flood magnitude-intensity-frequency will take more analysis of existing and reconstructed data. Rather than a “one in 100 year or one in 200 year event”, a more accurate description is 1% or 0.5% exceedance probability that a given magnitude event or larger will occur in a given year. The probability resets each year, and large events could occur back-to-back or even within the same year. The probability of extreme rainfall is further altered by a warming and changing climate. 

Cape York Water Partnership has been surveying and photographing the extent of mass wasting in the northen Wet Tropics by ground, boat, drone, plane and satellite images since Cyclone Jasper passed. There are literally thousands of large landslides, small landslips, and debris flow scars across the landscape, with the large ones easily visible from roads and towns. The bulk of the landslides and debris flows have been within 10 km of the coast in steep forested terrain underlain by weathered meta-sedimentary rock of Hodgkinson Formation; however landslides into granitic terrain also occurred. The density mass wasting sites declined to the west as both rainfall and geology changed, with the upper Annan catchment west of Mt Finnagan much less impacted.  

Hotspots of mass wasting occurred in the headwaters of Trevethan Creek at Mt Amos, Wallaby Creek and Cedar Bay at Mt Hartley, Gap Creek between Mt Finnagan and Mt Finlay, and Mungumby Creek at the Big Tableland. Preliminary satellite images indicate upper Woobadda Creek above Degarra on the Bloomfield was heavily impacted by many landslides, as was much of the coastal strip around Wujal Wujal. It will take several weeks of clear weather to view and map all of the mass wasting via satellite images. Some preliminary examples are provided below.  

The water quality implications are that a new sediment regime has been forced upon Wet Topics catchments like the Annan River. Past hotspots of erosion like Oaky Creek and Scrubby creek are continuing to erode, mainly from gully and bank erosion into dispersive soils. Meanwhile new hotspots such as Wallaby and Trevethan Creek have elevated coarse and fine sediment loads from mass wasting, riparian bank erosion, and floodplain sedimentation, that could take a decade to decline to previous conditions. This means dirtier water and more sediment flowing to the reef for years to come. 

South Cape York Catchments and Cook Shire Council are working together to manage erosion and sediment runoff to the reef. In October 2023, South Cape York Catchments, in partnership with Cook Shire Council, conducted a large gully erosion control project along Oaky Creek Road. The project goals were to demonstrate the costs and techniques required to control legacy road gullies to guide future Federal, State and local funding of Shire road networks.  

A 0.3 ha and 5 m deep gully was reshaped, compacted, and capped with rock mulch on batters and coarse rock on top of geofabric along rock chute flow paths. 800 tonne of rock was used. The gully was eroding 250 tonnes / year of sediment between 2020 and 2023. Gully control costs were $130,000 and the project saved 116 tonnes of fine sediment (< 20um) to the GBR at a cost effectiveness of $1100 per tonne of fines.  

Cyclone Jasper in December 2023 was a significant test of the gully treatment, with 1100 mm of rainfall in 6 days. The rock chute and batters resisted erosion well, with only a few minor movements of rock. The effectiveness of the rock treatment was ~ 95%, well above comparable gully treatments with organic capping and grass in the same catchment that faired much worse during Cyclone Jasper.  

The Council Roads Project is part of the Eastern Cape York Water Quality Program, funded by the partnership between the Australian Government’s Reef Trust and the Great Barrier Reef Foundation. 

People living on floodplains know they are prone to flooding and prepare accordingly, but predictions of flood height risks for extreme events like these are lacking. Improved rainfall-runoff modeling using high-resolution topography maps (LiDAR) could better assign risks to individual floodplain pockets or properties. LiDAR topography data exist for many of the flood-affected communities south of Cooktown. These areas should be re-flown post-flood to document flood impacts and assess future flood risks. Tide and flood interactions in estuaries could also be better modelled, as could predicted changes with future sea level rise.  

Real time warnings of approaching floodwater could be improved. Queensland Government flood height gauges on the Bloomfield and Annan River were damaged by the extreme floods, but still provided some warning of rising river levels for the area. What was missing was real-time rainfall observations in higher elevation areas above the communities of Rossville and Wujal Wujal. The installation of a rainfall gauge at upper Gap Creek above Rossville could help warn residents of potential floodwater heights and timing. But mobile phone and internet service which is needed to access real time data is currently unreliable. 

Opening blocked roads from landslides and bank erosion is a priority for local Councils, TMR, and community. However, rebuilding back to the same conditions keeps the community vulnerable to future failures. QRA and DRFA disaster recovery guidelines focus on repairing unsealed and sealed roads back to their pre-disaster conditions. Instead of re-building past roads to their pre-disaster conditions, incorporating improved erosion control following Best Management Practices (BMPs) will make roads more resilient for residents and reduce anthropogenic sediment loads to the Great Barrier Reef Lagoon. Examples include using more drainage relief culverts, additional causeways and creek bridges, rock armouring of batters and steep slopes, stabilizing road-caused gullies and landslips, minimizing annual vegetation disturbance, revegetation of bare areas, and sealing gravel roads on unstable soils.  

While no one foresaw the devastation to follow cyclone Jasper, investing in these resilience measures can help our communities be better prepared for future flooding, and improve the health of our local waterways.

Main photo: Wallaby Creek floodplain, after Cyclone Jasper