You are standing on the banks of Martuwarra, the traditional name for the Fitzroy, WA's largest river. This cache asks you to consider the processes that have shaped this spot, in light of the record-breaking flooding of January 2023. For orientation, look towards the SE bank from GZ. Before the flood, the riverbank would have been beneath your feet.
To complete this earthcache, you will need to answer the following questions:
1. Describe the river bank to the south west (right side) of the bridge.
2. Describe the river bank to the north east (left side) of the bridge.
3. Why do you think there is a difference between the two sides? What erosion or stabilisation forces might have contributed?
4: Post a picture of yourself or a personal item including the bridge or the view from along the walkway
To claim a find, answers must be sent to the cache owner within seven days; cachers that do not provide answers will have their logs deleted.
Optional mental exercise: if you have already come from town or are about to go there, think about the volume of water required to reach all the way to flood the Tourist Information Centre and nearby houses...
The 2023 floods reached record breaking levels of 15.81 metres, having risen rapidly over 2 days. and trapping people and wildlife. The existing bridge from 1974 collapsed and a substantial part of the eastern bank eroded.
The headwaters of Martuwarra include the Margaret and Leopold Rivers, with Darnggu / Geike Gorge not far upstream. Gorges are cut into the limestone ranges to the North by longstanding water flow, and any flood arising upstream will be channelled through these narrow spaces. As the landscape transitions to sandy plains, the flooding waters under high force can have an immediate impact on the river banks. A bend in the river is often a key site of erosion at the outside of the curve, with deposition of sediment on the inside of the curve.
River erosion occurs via four processes:
- hydraulic action: the direct force of the water on the bank
- abrasion: the effect of sand & rocks in the water rubbing against the bank
- attrition: breaking down of rocks and sand in the water into smaller pieces, more easily carried away in the water
- corrosion: the chemical breakdown of the riverbed from weakly dissolved chemicals in the water
Stabilisation against erosion can be achieved by a number of means including smoothing out the sediment deposits on the inner curve, adding variation to disrupt flow within the streambed, and the use of vegetation, organic matting or rock layers on the banks to reduce impact. A river with these features naturally will be less susceptible to significant erosion events.
The new bridge on which you are standing was completed in 288 days following a dedicated effort 24 hours per day to ensure the highway could be reopened. The loss of houses is still being felt.
There is a walkway along the bridge, with parking available on the left side of the approach travelling each direction. Please be careful if crossing the road.
https://natureattheconfluence.com/the-anatomy-of-a-river/
https://www.internetgeography.net/topics/how-do-rivers-erode/
https://www.wa.gov.au/system/files/2023-03/River-restoration-manual-Stream-stabilisation.pdf
https://www.mainroads.wa.gov.au/about-main-roads/news-media/288-days-in-fitzroy/