Photo Essay of Segment 12 of the Don River banks at Glendon College

River bank stabilization policies are currently being reconsidered in many areas, because of the increased awareness of the unsustainable nature of many historic techniques (steel barriers etc.) and also the somewhat recent acknowledgement of the role bank erosion plays in providing important ecosystem services to an area (Piegay et al. 2005). This has left river managers seeking to preserve bank erosion inside what is known as the “defined erodible corridor.” Which is basically a set of guidelines which allows rivers to migrate freely within a defined corridor, limiting any potential serious hazards. This erodible corridor concept uses more of a biotechnical engineering approach, instead of traditional engineering and hard-armored methods, and may be a good solution for the Don River at Glendon Campus (Li and Eddleman 2002).

For this Project, my partner Arnab and I arrived to Glendon Campus around 11:00am in November 22, 2016 one of the coldest days of the year so far. Our 100m section, segment 12 was the most southern and eastern segment of the Don River assigned (Figure 1)

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Figure 1- Segment 12 of the Don River highlight by the red arrow

We started our documentation of the river banks at the most north-west point, and proceeded south-east, downstream. Right from the start of our segment, we found clear signs of erosion. Although this sand/silt section may provide perfect turtle nesting area, it is a clear sign of heavy erosion, and plants cannot survive. Following this section, erosion became more and more evident through various severe bank slumping signs, and severe undercutting of river bank trees. The undercutting will eventually lead to tree and plant mortality, which will remove their root systems that are important for stabilizing the banks, and in turn lead to more bank slumping. There were 2 different techniques of previous bank stabilization: gabions and large rock boulders (Li and Eddleman 2002). However, no signs of vegetation stabilization techniques were observed, such as: live stakes, fascines or crib walls, joint planting or geogrids (Li and Eddleman 2002).

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Start of segment 12- Sand/silt erosion signs with no vegetation

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Trees starting to be undercut by bank erosion, severe slumping

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Severe undercut of entire row of evergreens, much higher than current water level

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Close-up of severe undercutting and slumping

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Woody-debris diverting water-flow into bank, severe slumping

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The start of undercutting and erosion, still some vegetation present though 

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Vegetation still present, however slumping and erosion is starting

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Bridge in the middle of our segment, indicating severe forest decay

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Previous stabilization method (gabions) 

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Previous stabilization method- large rock boulders

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Difference between 2 previous stabilization methods (gabions vs. large rock boulders)

There is no doubt that some action needs to be taken place, as the banks of the Don River at Glendon College are severely slumping, leading to the degradation of the river. There have been past stabilization methods observed in this section which have proven very effective for simply stabilizing the banks. However, they do not provide important habitats for various aquatic and terrestrial organisms and plants. However, the large boulder technique is definitely a step in the right direction, in relation to the gabions. In the future, it is my recommendation that a much more biotechnical engineering approach be taken at stabilizing the Don River banks. This type of approach would incorporate specific planting zones, with specific materials and plants in each zone which would provide the best of both worlds, bank stabilization to prevent slumping, and important habitat to preserve biodiversity.

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Stream bank zones (Li and Eddleman 2002)

Toe zone- bank portion between bed and average water height

Splash zone- bank portion between normal high-water and normal low-water levels

Bank zone- bank portion above normal high-water levels

Terrace zone- bank inland from the bank zone

By incorporating these zones, river bank stabilization can be much more effective and ecologically efficient.

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Example of construction of four zones of vegetated geogrids along a riverbank 

References

Piegay, H., Darby, S.E., Mosselman, E. and Surian, N. (2005). Review of techniques available for delimiting the erodible river corridor: a sustainable approach to managing bank erosion. River Research Applications. 21: 773-789.

Li, M.H, Eddleman K.E. (2002). Biotechnical engineering as an alternative to traditional engineering methods: a biotechnical stream bank stabilization design approach. Landscape and Urban Planning60: 225-242.

 

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