A Proposal for the River Bank Restoration at York University, Glendon Campus: Vegetation and Techniques That Can Help Stabilize the River Banks

Figure 1. Pictures showing Don River conditions in the York University Glendon Campus area at the coordinates 43°43’41.5″N 79°22’26.7″W. Images are of the east side bank taken from the west side river bank. Image Credit: Wafa Majeed

The Don River passes through the beautiful Glendon Campus of York University, in Toronto. Although encircled by natural landscape in its nearby surrounding, the river bank beds face severe erosion. As it can be seen in the photo evidences taken on 27th November 2016 at 43°43’41.5″N 79°22’26.7″W, vegetation on the river bank, close to the stream have a large amount of their roots exposed. Some are hanging in the air, some have completely no intact soil right underneath much of the root area, generally known as undercutting [3], and others have completely lost anchor into the soil making the tree vulnerable to fall into the stream and further cause issues over time as sediments may build up there blocking the proper flow of the stream.

The photos mainly represent the eastern side of the bank which is steep, but not as steep as the western side, from where the photos were taken. Complete access to the view of the western side was not possible, however at this location, the west side bank was not much affected as the east bank because of the direction of the curve. Thus the primary focus must remain on the ‘shear stress facing side’ of the river bank, i.e. the east side. However, the west side must not be neglected.

Erosion is the process which causes disintegration and removal of soil from a site, which in the case of a stream may be due to the sudden changes in flows, or due to increased run off, or due to storm events [1]. Although the process of river bank restoration must start from solving the root cause, which in many cases may be increased flows due to clearing of vegetation upstream, but since the upstream portion of the river, outside of the York campus, is not being accounted for, efforts to reduce heavy flows must be made by within the campus area, before taking any other restorative action. One such action to be taken could be the installation of barbs, which are dikes that are generally made of large rocks, in to the east side of the stream where the in bend is situated so that scouring effect on the east bank can be reduced as also seen in figure 2 [2]. Local trees that have a strong root structure, may be incorporated into the barb area that extends into the bank for better stability. Barbs not only rule out the negative impacts, but also add in positives. For instance, the installation of barbs helps in increased deposition towards the toe of the bed on the side of the bend [2] in this case it would be on the east side. This decreases the streams width to depth ratio thus having more controlled flows [2] as over time the thalweg, i.e. the deepest part of the river bed, moves away from the bank and into the center as also shown in figure 2 Once that is done, work on the bed can begin so that the result of the measures taken on the banks can be highly efficient and show maximum positive results.

barbs-n-thalwegbarbs-n-scour
Figure 2. Top diagram shows how generally the barbs are installed and how the thalweg changes after installation & the bottom diagram shows the point of scouring when barbs are installed [2]

Next, river bank areas, specifically where soils have been undercut, increasing the risk of slope failure, can be supported by coconut fiber logs on the inside and then then large boulders on the outside that can withstand high flows. Thus, this would allow the bank to get support to stand back in place and would reduce the risk of the bank from collapsing. Then the most important part, that is the slopes of the banks, can be covered with erosion control mats such as those made with either jute or coconut fibers so that the remaining soil and vegetation can stay intact. Right after, without any delay, first the underlying bare soil must be planted with fast growing plants. To start off, grass, which grows very fast, can be planted so that the top soil doesn’t get washed away. Then gradually local plants can be planted that have a good root system and that are able to tolerate high amounts of moisture. Thus the planting of vegetation especially trees, allows the soil to remain intact thus protecting it from erosion as well can prove to slow down flood flows as also illustrated in figure 3 [4,5].

figure-3
Figure 3. This figure shows how the velocity distribution of high flows can be affected with and without vegetation cover on the river bank [5]

It is suggested that a mix of different trees and shrubs be used on the river banks so that a network of a variety of rooting systems may be made to keep the soil in place. Some of the best examples of shrubs and trees that could be used to plant right after grasses have taken root into the soil could include, but not be limited to the following: trees like Common Hackberry, Silver Maple, Tamarack and Shagbark Hickory, & shrub like Highbush Cranberry, Common Elderberry, Bush Honeysuckle and Peachleaf Willow. The reason to propose the use of these plants is because most importantly they are native to our province, Ontario [4,6]. Secondly these can tolerate moist to wet conditions [4,6] as found in the river bank. For instance, Shagbarck Hickory is well adapted to riverbank environments and currently grows on the banks of St. Lawrence River [4]. Willow as well can tolerate wet conditions, and since it grows very fast, it could be one of the first to be planted after grasses. Other plants are no less important; for instance, Cranberry and Elderberry shrubs are both used for controlling erosion by conservation authorities since they grow well in moist conditions and in places like river banks [4].

Although correct vegetation selection can help make an effect in stabilizing the bank, it may still not produce effective results without incorporating other non vegetative techniques as mentioned earlier. Thus together with altering stream flows and mechanically protecting stream banks and planting the banks with the most appropriate species of plants, this river bank can be restored and prevented from further erosion.

References

[1] Iowa Department of Natural Resources. (2006). How to control streambank erosion, 1–52.

[2] USDA. (2007). Technical Supplement 14H Flow Changing Techniques, (August).

[3] Guide, B. (2011). Solutions for Shoreline Erosion A Basic Guide to Bioengineering, 1–16.

[4] Ontario Ministry of Natural Resources. (2013). Ontario’s Tree Atlas.

[5] Donat, M. (1995). Bioengineering Techniques for Streambank Restoration A Review of Central European Practices by Bioengineering Techniques for Streambank Restoration A Review of Central European Practices, (2), 1–92.

[6] TRCA. (n.d.). Native Tree & Shrub Monitoring.

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