Introduction
Erosion of coastlines and beaches is a problem that can impact any settlement or city located near a body of water. In both the long and short run, coastal erosion can have enormous detrimental impacts on an area, in terms of both geography and economics. This issue is important the world over, as more than half the world’s population lives within two hundred kilometers of a coastline (Committee). As a result, there are more than 3.5 billion people who could be harmed by coastal erosion and its effects (Committee).
Eroding shorelines can have several negative effects. First, people are forced to evacuate their homes (Pilkey). This results in large costs for both homeowners and the government, which is forced to buy homes from people whose houses are at risk from coastal erosion (Pilkey). Also, there are geographic consequences as eroded land sediment is carried back into the water by destructive waves that gradually decompose the coastline, resulting in polluted water (eurosion).
Toronto, Ontario, Canada is one city whose coastline has truly suffered the effects of coastal erosion. Toronto is Canada’s largest city, with the main metropolis and nearby regions accounting for more than 4 million of Canada’s 35 million people. Because the CBD and immediately surrounding areas become extremely crowded when a city expands, people have chosen to relocate to areas farther away from the downtown region, such as Scarborough, near Lake Ontario (see Map 1).
Today, waterfront areas in Scarborough, and all along the Toronto coastline, are suffering from coastal erosion. The Toronto beaches are in the southeastern portion of the city. The prominent feature of the area is known as the Scarborough Bluffs, which is a long series of cliffs extending along the length of the city adjacent to the water.
In recent years, erosion along the Bluffs has been rampant. Some homeowners have lost more than 30 meters of property over the last decade, putting their homes at risk In fact, some fences which were far away from the waterfront just ten or fifteen years ago are now hanging off the edge of the cliffs, on the verge of falling into the water. These homes are in grave danger of being swept into the Lake.
It is in the light of all these problems facing the city of Toronto that it was decided to conduct a study to determine which areas of the Toronto coastline are at the greatest risk of erosion by measuring cliff heights and angles. This will give a good impression of the area’s risk for coastal erosion in accordance with W. Davis’ 1889 theory of coastal classification. His theory is explained more in-depth in later parts of the study, but in essence, it said that steeper slopes are newer and are at greater risk of being eroded in the future and that more gently sloping land areas have already been eroded considerably, and are therefore not at as much risk as steeper slopes.
Map 1 – Study Area
Study Area
As per the map on the previous page, the study is to be conducted along the Eastern Toronto coast. The highlighted area is the area generally perceived to be at the greatest risk for coastal erosion. It extends from Bluffer’s Park in the north to Ashbridge’s Bay in the south. The reason for this area being chosen is that there are many homes along the coast, making it liable to economic damage as a result of erosion (which varies with cliff height and slope angle).
Hypothesis
It is predicted that the area at greatest risk for coastal erosion is the Scarborough Bluffs.
This is because in this highly residential area, many homes are located right on the cliff which leads down to the water, at a drop of more than 200 feet (66 meters). There is a clear sign that there has been a lot of erosion in recent years: fences erected in the backyards of homeowners near the coast are hanging in midair over the cliffs, held in place only by the few parts of the fence that are still planted in the uneroded ground. This observation, combined with the extensive erosion that has taken place in recent years, is the basis for predicting the Scarborough Bluffs as the region with the greatest risk of being eroded.
On the other side of the spectrum is the Ashbridge’s Bay region, the area at least risk of coastal erosion. Despite also being a developed residential area, Ashbridge’s Bay does not have tall cliffs (the maximum being about 5 meters, as judged by the naked eye) and possesses a very gentle slope, almost making it like a berm. In addition, there are no major constructions in extreme proximity to the water to weaken the ground and simplify erosion. Finally, the beach in this area is very wide (almost 20 meters), so it protects the ground further away from the water from erosion.
The Bluffer’s Park region still does face some erosion risk, but will not be affected by humans, as it has been set aside by the government as parkland. Bluffers Park is a large area spanning much of the area taken up by the eastern beaches. Because it is a parkland, no houses or other such buildings are within a half-kilometer of the cliff next to the water. In addition, there is 25 more meters of land protecting the cliff. Finally, there is thick vegetation growing all over the park, resulting in tightly held, stable soil that will not be eroded easily.
W. Davis’ Coastal Classification Theory
W. Davis was an American geomorphologist who devised a theory to classify coasts based on their slopes. His theory, based on his observations of waterside cliffs in Europe and North America, suggests that cliff slope is time-dependent. He says that slopes are steepest at the beginning of the erosion cycle, and gradually lose height and steepness as time goes on. Gradually, the cliff is eroded (a process known as “down wearing”), giving way to a convex upper and concave lower cliff resulting from faster erosion at the top of the cliff where soil is usually looser. The diagram below illustrates the change in a cliff’s slope over time.
Even though his theory was established in 1898, it is still valid today because erosion is a natural process. Davis would divide cliffs into two different groups: youthful (steep) and mature (gently sloped). Youthful cliffs had not yet undergone erosion, while mature cliffs had already gone through erosion and were therefore not as steep, as the upper part of the cliff had been eroded away. As a result youthful cliffs were more likely to be eroded in the future, while mature cliffs, already having been eroded, would not likely be eroded much farther.
Methodology
Data was collected in mid-October 2004 on the cliff heights and slope angles at various waterfront points along the nine kilometer transect extending from Bluffer’s Park to Ashbridge’s Bay.
Because there were only a few hours available in which to collect data, it was decided in advance that data would be collected using systematic point sampling, which enables data to be collected easily and rapidly. Data was collected every half-kilometer along the 8.5 kilometer transect, resulting in a total of 17 data points.
One drawback to using systematic point sampling is that it is not a true random sample – not all points have an equal opportunity to be considered in the data set, so some patterns may be completely overlooked while others are overemphasized by pure chance. However, this was the most suitable option for this field study, so systematic point sampling was used. The diagram below illustrates systematic point sampling.
Cliff height data was found from Toronto Maps, so instead of using valuable time on the data collection day taking measurements that were already known, it was decided to use the limited time available to just walk the 8.5km transect and measure the slope angles every 500 meters. The angles were measured by using a protractor at the base of the cliff and then lining up the cliff peak with the protractor and recording the angle. For additional precision, 3 measurements were taken and averaged. The average value is recorded in the Data Collection section. The map on the next page shows the locations at which data collection was conducted.
Data Processing & Presentation
Table 1: Quantitative Data Collection
Data Collection Point (see Data Collection Map) Height (meters) Angle (Degrees)
1 – Bluffer’s Park 108 60
2 – Bluffer’s Park 97 67
3 – Scarborough Bluffs 101 96
4 – Scarborough Bluffs 93 77
5 – Scarborough Bluffs 92 65
6 – Scarborough Bluffs 90 62
7- Birch Cliff 96 49
8 – Birch Cliff 87 49
9 – Birch Cliff 76 58
10 – Birch Cliff 64 56
11 – Birch Cliff 69 37
12 – Birch Cliff 63 35
13 – Beach 50 39
14 – Beach 33 34
15 -Ashbridge’s Bay 5 28
16 -Ashbridge’s Bay 5 19
17 -Ashbridge’s Bay 4 8
There are many trends apparent in the graph above. First of all, we can note that the Cliff heights were at a maximum in the Bluffer’s Park/Scarborough Bluffs region, where all cliff heights were at 90 meters or more. After that, there was only 1 cliff height above 90 (the next point). The Birch Cliff area was the transition area, where the cliff heights fell to 63 meters before falling rapidly and reaching as low as 4 meters at Ashbridge’s Bay.
.Much like the cliff height data, the slope angles data decreased along with distance from Bluffer’s Park. One notable point is Data Point #3, where the angle of the cliff is so high that the cliff actually leaned in towards the bottom, so the peak was extending outwards. This is a particularly dangerous situation because if the base of the cliff is eroded enough, the upper part of the cliff will be too heavy for the weak base to support and will collapse into the water. This was the only angle above 77 degrees, making it a clear anomaly. The entire Scarborough Bluffs/Bluffers Park group of cliffs (Data Points #1 – #6) had angles above 60 degrees, and no angles from any other groups were above 58. This group is clearly the group with the largest angles, in addition to having the highest cliff peak heights on average. The Ashbridge’s Bay area had by far the smallest cliff angles, at a minimum of 8 and maximum of 28 degrees, to match with the very small cliff heights.
Qualitative Observation
Some qualitative observation helped to expand the study beyond being solely numerically based. For example, we can see that the cliff in the picture above would be more prone to being eroded than a cliff situated further off the water. The cliff above is a good example to study for this field investigation, as it is extremely steep, indicating it has not been eroded much, and is right next to the water, so we can predict that it will experience a lot of erosion in the near future. Other observations included thick, lush vegetation on the Bluffer’s Park area, indicating very stable soil that will not be eroded.
Statistical Test: Spearman’s Rank Correlation Coefficient
Data Collection Point Height (meters) Height Rank Angle (Degrees) Angle Rank d d2
1 108 1 60 6 -5 25
3 101 2 96 1 1 1
2 97 3 67 3 0 0
7 96 4 49 9.5 -5.5 30.25
4 93 5 77 2 3 9
5 92 6 65 4 2 4
6 90 7 62 5 2 4
8 87 8 49 9.5 -1.5 2.25
9 76 9 58 7 2 4
11 69 10 37 12 -2 4
10 64 11 56 8 3 9
12 63 12 35 13 -1 1
13 50 13 39 11 2 4
14 33 14 34 14 0 0
15 5 15.5 28 15 0.5 0.25
16 5 15.5 19 16 -0.5 0.25
17 4 17 8 17 0 0
The Spearman’s Rank statistical test determines the correlation between two variables and also determines the probability that the correlation occurred by chance. First, all values for the 1st variable are ranked, and using the corresponding 2nd variable values, which are also ranked, we determine the difference between the two ranks (d). Then the differences are squared, to give d2. Then, we use the formula 1 – (6∑d2)/(n3 – n), where ∑d2 represents the sum of all d2 values and n is the number of values in the data set (17 in this case). The value for this set of data is 1 – .12, giving us an r value of .88. Usually, an r value of over .75 means a strong correlation. Therefore this high r value indicates a strong positive correlation (meaning that as cliff heights decreased as we moved southwestward along the coast, slope angles tended to decrease with them). The chart on the next page plots the slope angles against the cliff heights, and we can see that there is a correlation between the two. Anomalous points were point 1, with height rank 1 and angle rank 6, along with point 7, height rank 4 and angle rank 9.5. There were no points whose height rank was abnormally low as compared to angle rank. The degree of significance (probability that the correlation occurred by chance) was less that 0.1 percent (Waugh).
As one can see from the trendline, there is a correlation between slope angle and cliff height.
Data Analysis
After reviewing the data, we see that the Scarborough Bluffs region is clearly at the greatest risk for coastal erosion. This is because the extremely high cliff heights, combined with the great steepness of the slopes, results in a double jeopardy of sorts: steep slopes make it easier for sediment to travel down to the water, while large heights mean that erosion of the base can lead to the upper end of the cliff collapsing. Also, some cliffs in the Bluffs area are situated not even 3 meters from the water, making it very possible that mass erosion will occur in the near future. This result did concur with the hypothesis, although the reasons for prediction were slightly different than the factors actually noted.
In addition the Scarborough Bluffs area can suffer economic damage. As mentioned before, many people have homes located right at the cliffs leading out to the water. Although the residential areas are not at as high of a risk to erosion as the cliff in the picture on page 9, the loose soil resulting from development, combined with the relatively young age, means that it will face some erosion in years to come. Additionally, the loose soil makes it more likely that a storm will cause considerable damage to a property and could cause massive land losses, even over short periods of time.
The area at least risk for erosion is the Ashbridge’s Bay region, as predicted. It was for the same reasons as laid out in the hypothesis – a very small slope, the end result of erosion as per W. Davis’ 1889 theory of coastal classification. The height of the cliff is also minimal, reaching a very low maximum of 5 meters, and the 25 meter beach protects the cliffs from the water of Lake Ontario from reaching them and causing erosion. For these reasons, and as hypothesized, Ashbridge’s Bay is the region that is least likely to face erosion in coming years.
Conclusion
In conclusion, it was found that the area at greatest risk for coastal erosion, in terms of both economics and geographic factors, is the Scarborough Bluffs. The area at least risk is the Ashbridge’s Bay region.
The Spearman’s Rank Coefficient found a strong correlation between the height of the cliffs and the slope angle, namely a positive correlation. There was also a minimal (less than .1%) chance that the correlation was a coincidence, meaning we can accept the results without skepticism.
There would be several ways to improve the study, specifically by broadening the scope. If more time were available, one could conduct a more thorough study – say every 100 meters, instead of 500 – and obtain more exact results.
Also, the study could be expanded to be conducted over a span of several years so that one can track the erosion which took place and derive conclusions as to what natural threats to an area tend to actually cause the damage that was expected.
The hypothesis turned out to be mostly correct, so I would not vary it considerably. However making it more specific, or providing more primary evidence to substantiate a claim, would be desirable as it would increase the credibility of the study. Finally, using more quantitative hypotheses would have been beneficial as objectivity would be greatly decreased – I would not be biased when collecting data, as I would rely simply on the numbers, while in a qualitative hypothesis it is easy to manipulate observations in order to suit the hypotheses.
Taking the steps outlined in the above paragraphs could greatly improve the study, and would provide a valuable insight into what Toronto might need to do to limit the ill effects of erosion.
Works Cited
Committee on Coastal Erosion Management. Managing Coastal Erosion. National Research Council, 1990. New York.
<http://www.eurosion.org/> – A European initiative for sustainable coastal erosion management
Pilkey, Orrin. Coastal Erosion: Has Retreat Sounded? Heldref Publications, 1994. New York.
Waugh, David Geography: An Integrated Approach. Nelson Thornes, 2002. Cheltenham.
