The Applied Plant Ecology of York University were sent to the Glendon Campus to review the riverbank of the Don River and provide possible solutions to the current processes happening. Maya, my partner, and I were assigned a section of the riverbank that was situated at a dysfunctional bridge.
There were 4 main segments to our section:
Before the bridge
On one side of the river (north side), there was exposed, fairly packed boulders and slight undercutting from the river which exposed roots of plants and trees.
On the south side of the river, there was less undercutting but more accumulation of sediment, forming what seemed to be the predecessor of a point bar, but not quite. Soil was much more exposed here.
Under/near the bridge
On the north side of the river, there were more large rocks however these were more sparse than the section before. They were not ordered in any fashion. Also, water levels seemed to have dropped as seen on the column supporting the bridge.
The south side of the river was normal, with mud platforms the only characteristic. However this is probably because of lack of vegetation, as this section of the river is shaded by the bridge.
A little after the bridge
On the north side, there was formation of a point bar and many sparse rocks, large and small. Ahead of that was a large cliff that mirrored that of the south side. Another point bar, fallen trees, and debris were subsequently present.
On the south side, there was a tree directly after the bridge which was severely uprooted. Succeeding it was a fairly long mud cliff, with roots hanging over and out of it. All of this was placed above a mud platform.
After the bridge
On the north side (not quite north anymore, but the same side), there was undercutting and sparse vegetation on the exposed soil.
The south section starts with a collection of fallen branches and trees, succeeded by a large point bar, then more undercutting. Along this section the soil was heavily eroded, much more than before. Further ahead there was a long collection of stabilization rocks.
Firstly, it is important to examine each side of the river (north/south) because river dynamics effect each side differently. For example, as you will read, meanders are coupled with pools and riffles on each side of the river, and those elements affect the formation of the riverbank.
Since our segment had many disjointed rocks and boulders, one can assume there was a considerable amount of bedload on the bottom of the stream which would increase turbulence, as water moving over the bedload would be removed from equilibrium. The large amount of erosion and undercutting of the riverbank can also contribute to increased turbulence in the form of suspended load.
Our segment can be defined as a single meander, with the north side having the deep channel and the south side having the shallow channels. The undercutting on the north side, or the cut banks, are a product of meander dynamics. As water approaches a bend, it is forced to the outer side and swings around the bend, where the flow is shortly after redistributed along the entire channel. The force of water swinging around a bend causes erosion along and after the bend until the water is redistributed.
The mud platforms explained above are most likely the product of a below bankfull stage, i.e. low discharge. The river is currently in a below bankfull stage because high discharge that results in bankfull only occurs during the early part of spring. This is because melting of snow and ice after winter inputs great amounts of water into the stream. Bankfull stage would result in the height of the stream reaching the mud cliffs above the mud platforms mentioned.
There was a large amount of erosion due to meander mechanics in our section and probably along the whole river because lower discharge rates increase shear stress on the bed and banks.
One of the most important instances that must be taken to restore river function is the rehabilitation of vegetation. Plants establish root systems which stabilizes the soil and prevents erosion. When the soil is stable, it is tightly packed and avoids degradation. To enhance this process, dominant, native species can be introduced as they can have a positive impact over invasive species, and they establish a deeper root system. Our section of the river was very sparse of vegetation and the soil seemed to be very poor in mineral quality; it almost resembled clay. The roots of trees and grass were heavily exposed on the meander cliffs, a clear sign of weak soil subject to weathering and erosion. However, it would be difficult to establish a strong vegetation system along our riverbank because of the steep slop that makes up the levees. Gravity against the vegetation can put an increased strain on the soil and trees may not be able to stabilize the toe of the bank.
The reason most of the rocks in our section were disoriented is probably due to the weak soil not being able to hold them in place. Also since our section was located at a meander – the force of the stream hitting the rocks may have moved them out of place. This is evident because in the section after our segment, the boulders were not out of place. However the section ahead was also situated along a meander, albeit smaller. Can this mean the rocks in our section were simply placed poorly? Or since the meander bend was larger, hence greater stream force, they were knocked out of place? Or maybe the bridge is a factor? A combination of these may very well be the reason. Obviously we cannot change the landform of the river i.e. it’s meandering processes, however we can simply change it’s surroundings. A combination of gabions, enhanced vegetation, and riprap would greatly enhance our section of the Don River. However examinations of other parts of the river would be useful in finding potent solutions because if the characteristics found in our section is uniform throughout the rest of the river, the most cost efficient technique should probably be used. If our section is an outlier, it can be treated with special circumstances.
The fluvial dynamics, costs, and maintenance of a riverbank is outlined extensively in a restoration and stabilization guide for a stream in Richmond, Virginia. I would recommend a quick skim as it goes into much more detail about protection and stabilization especially. Thank you for reading.
Link to guide: http://www.deq.virginia.gov/Portals/0/DEQ/Water/Publications/BMPGuide.pdf