On Nov, 22, 2016 Gregory Milner and I went to York University’s Glendon campus to photograph the eroding riverbanks along our assigned stretch of land. We started our trek at 11:20 on a fairly cold day with temperatures around -2 degrees Celsius and snowfall from the previous night covering much of the ground. We were assigned section 12 and so we made out way through the West trail, where there was an abandoned, broken down truck lodged into the trees. This made me wonder; what other careless human interactions would we encounter ahead and what does this mean in terms of riverbank management?
As we began capturing images it could be seen that there has been and continues to be a significant level of erosion. Erosion is the process of the gradual deterioration of the earth’s crust during which there is transport and detachment of soil particles due to numerous natural factors such as: wind, gravity, water currents (Pinto, n.d.). Sedimentation is the byproduct of erosion where there is settling (heterogeneous mixture between water(solvent) and soil or other particles(solute)) out of particles in which heavier particles such as sand and gravel settle out faster than smaller ones such as clay (Pinto, n.d.). This causes a new layer of sedimentary rocks in the water which may have negative implications on current aquatic habitats (Pinto, n.d.). Studies done in USA have found 30% of sedimentation is due to natural erosion and 70% of sedimentation is due to accelerated human interaction based erosion (Pinto, n.d.).
As can be seen from figures 6 and 7 there is significant erosion resulting in slumping on either side of the riverbank. When looking at figure 7, natural as well as man made litter is visible on the slumped riverbanks such as broken down branches, leaf litter and plastic food packaging and drainage pipes. Although erosion and sedimentation are natural processes, it’s the rate of them and factors causing them which is of greater concern. Common causes of erosion to riverbanks are: increased precipitation, stream bed lowering, and saturation of banks from off-stream sources (Natural Resources, 2006). The drainage pipes although meant to be sanitary also have negative implications such as release of sewage effluents resulting in river eutrophication (loss of oxygen to aquatic species) and higher phosphorus levels that is detrimental to plant species (Jarvie, Neal, Withers, 2005). This leads to higher erosion levels due to lack displaced vegetation (figure 8) causing higher sedimentation rates, that ultimately result in highly concentrated and polluted clogging in the drainage pipes, which again lead to more species deaths (Pinto, n.d.).
As Greg and I approached the end of our study site, the riverbanks there were much less eroded and seemed as though previous restoration work had been done on them.
As seen in figure 9, the right side of the bank looks much more rigid due to work done on it, compared to the left which has significant slumping. The technique implemented here is a retaining wall (circled in red) in figure 9. The retaining wall is a vertical structure constructed of cement and steel that holds back and prevents further erosion (Kosiw, Parks, Besley, 2008). It is also sterile thus having little to no harm on adjacent animal and plant species. Other commonly used preventive measures are Gabion walls which consist of baseball sizes stones forming rock beds and arequite inexpensive to hold up against riverbed erosion . A third method used are Stone Riprap, which consist of large boulders and are more commonly used along shorelines for protection against waves(Kosiw, Parks, Besley, 2008).
All in all, this was a great experience, allowing for both field work and the theoretical learning on riverbed erosion, sedimentation and restoration methods!
Department of Natural Resources, Q. (2006). What causes bank erosion?https://www.qld.gov.au/dsiti/assets/soil/what-causes-bank-erosion.pdf
Jarvie, H. P., Neal, C., & Withers, P. J. A. (2005). Sewage-effluent phosphorus: A greater risk to river eutrophication than agricultural phosphorus?
Kosiw, M., Parks, K., Besley, S., & Cox Eric Sager, R. (2008). Solutions to riverbank erosion: A summary of current shoreline stabilization techniques for the Gull River in Minden, Ontario.
Pinto, C. (n.d.). Erosion and Sediment Control Program watershed plan.http://trca.on.ca/trca-user-uploads/ErosionandSedimentControlProgram.pdf