Last Sunday, my partner Wafa and I returned to Glendon College to explore and capture some of the horrific riverbank disasters we observed at our assigned segment 11 of the river.
After countless times of getting lost and walking in circles, we immediately encountered evidence of riverbank erosion and slumping. Multiple regions of segment 11 showed evidence of bank scouring especially in the centre of the segment where a large portion of the bank exhibits signs of mass failure.
Interestingly, our segment did contain a significant amount of rocks as seen in the picture above in the left hand corner. However, it is unclear whether these rocks were placed as part of a previous riverbank stabilization project as the locations of these rocks seem skewed, considering no other part of the segment has rocks, for that matter, any evidence of previous riverbank stabilization.
Bank scouring can be defined as the direct removal of bank materials by the physical action of flowing water and the sediment that it carries (Queensland Government, 2006). Often times, as the flow of the river increases, the erosive power of the flowing water significantly increases, thereby, resulting in scouring as shown in the picture below. Clear cut evidence of undercutting of the bank toe, also seen, in the picture below, suggests obvious signs of scour processes.
Segment 11, as mentioned previously, also showed signs of mass failure. Mass failure can be described as the various mechanisms of bank erosion that result in sections of the bank sliding or toppling into the stream, as seen in the picture below. (Queensland Government, 2006). Mass failure can also be described as collapse or slumping. Obvious signs of mass failure processes includes bare and near-vertical banks of areas of slumped bank materials. Segment 11 exhibited countless acts of mass failure as seen in the three pictures below. Various research suggest that mass failure is often seen in the lower reaches of large streams and is often coupled with scouring, which again, is seen throughout various parts of segment 11.
One of the common issues that our segment displayed was evidence of root exposure. One of things we did notice, was that root exposure was often the most evident in locations where the river bank was extremely close to the land portions of the river. Various other reasons for root exposure could be possible as segment 11 did seem to be show slight variations of hill-sloping.
As Thorne (1990) suggests, there are a variety of processes including hill slope,weathering, fluvial and aeolian that play a major role in determining the interactions between morphology and plant cover, mass and composition. It is important to note that vegetation, on its own, has a very complex relationship with river dynamics and morphology. In particular, it can alter the flow as well as change the physical resistance of the bank material (Coulthard, 2004).
Taking this into mind, establishing various stabilization and remediation strategies to combat riverbank erosion is a must. Florsheim et.al (2008) suggest that combining various bank management goals that focuses on diverse natural bank habitats and vegetation that accommodate erosion processes and watershed-scale sediment cycling contributes to sustainable river management.
Ashraf, M., Bhatti, M.T. & Shakir, A.S.(2016). River bank erosion and channel evolution in sand-bed braided reach of River Chenab: role of floods during different flow regimes. Arab J Geosci. 9:140.
Coulthard, T. J. (2005), Effects of vegetation on braided stream pattern and dynamics. Water Resour. Res., 41.
Florsheim, J., Mount, J., and Chin, A. (2008). Bank Erosion as a desirable attribute of rivers. 58(6): 519-529.
Queensland Government. (2006). Water… securing water for Queensland’s future.
Thorne,C. (1990). Effects of vegetation on river bank erosion and stability.